Performance of Repeaters in 3GPP LTE

ANTO SIHOMBING

Master of Science ThesisStockholm, Sweden 2009

Performance of Repeaters in 3GPP LTE

ANTO SIHOMBING

Master of Science Thesis performed at

the Radio Communication Systems Group, KTH.

June 2009

Examiner: Professor Ben Slimane

KTH School of Information and Communications Technology (ICT)Radio Communication Systems (RCS)

TRITA-ICT-EX-2009:67

c© Anto Sihombing, June 2009

Tryck: Universitetsservice AB

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Abstract Repeater communication is one promising candidate solution in future cellular networks because of its ability to increase throughput, data rate and coverage. It is also considered as one candidate technology feature in 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) Advanced. Traditionally repeaters have been active continuously and perform blind forwarding without knowing the signal. However the repeater in LTE Advanced is likely to include some advanced functionalities such as frequency selectivity, gain controllability, multi antenna ability, advanced antenna processing, optimum power control algorithm, etc. In this thesis, on-frequency repeaters with frequency selectivity and gain controllability are analyzed and it is shown that the performance of repeater is highly dependent on the environment. It is necessary that the composite path gain (two-hop link) must be better than direct path gain (direct link) and the interference is attenuated in order to use the repeaters. The repeater directional donor antenna can be employed to further improve these two-hop links. And finally the benefit of advanced repeater functionalities is larger for uplink than downlink especially in heavy interference scenario however power limitation is often a bottleneck in uplink.

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Acknowledgements First of all, I would thank God for His grace and wondrous work in my life and my study. I dedicate this work to my father. I express my gratitude to Peter Moberg and Johan Lundsjö for giving the opportunity to perform this master thesis at Wireless Access Network, Ericsson Research. The experience acquired in the research department, the friendly atmosphere, and the valuable feedbacks have helped me a lot in my studies. I would like also to thank all people in wireless access network department: Aram Antó, Afif Osseiran, Anders Furuskär and Per Skillermark, who gave me essential material and discussions to my thesis work.

I would like to thank Prof. Ben Slimane for his help and his availability during this thesis period. The discussion and feedbacks which I had from him are important to my work. The fruitful discussions I had with Bogdan Timus about problem formulation and his previous research in repeater. Thanks to all my friends in wireless system master program who also give encouragements to me and spend over these two years together in KTH.

Finally I am also grateful for the moral support and understanding given by my mother, brothers, sister and my fiancée in Indonesia.

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Contents Chapter 1 INTRODUCTION 1

1.1 Background ........................................................................................................ 1 1.2 Previous work .................................................................................................... 2 1.3 Problem statement.............................................................................................. 2

Chapter 2 LONG TERM EVOLUTION (LTE) 5

2.1 3GPP LTE Overview ......................................................................................... 5 2.2 LTE Transmission Schemes .............................................................................. 6

2.2.1 OFDM ......................................................................................................... 6 2.2.2 SC-FDMA................................................................................................... 7

2.3 Frame Structure.................................................................................................. 9 2.4 Scheduling in LTE ........................................................................................... 10

2.4.1 Downlink Scheduling................................................................................ 11 2.4.2 Uplink Scheduling .................................................................................... 12

2.5 LTE-Advanced................................................................................................. 12 Chapter 3 REPEATER CONCEPT 15

3.1 Repeater Overview........................................................................................... 15 3.1.1 Basic Repeater Design .............................................................................. 16 3.1.2 Antenna Isolation ...................................................................................... 16 3.1.3 On-Frequency and Frequency Shifting Repeaters .................................... 18 3.1.4 Repeater Delay.......................................................................................... 18 3.1.5 Interference and Capacity ......................................................................... 19 3.1.6 Repeater Applications ............................................................................... 20

3.2 Two-Hop Communication Model.................................................................... 21 3.3 Advanced Repeater .......................................................................................... 22

3.3.1 Frequency Selective Repetition ................................................................ 23 3.3.2 Repeater Gain Controllability ................................................................... 24

Chapter 4 SYSTEM MODEL 27

4.1 General Scenario.............................................................................................. 27 4.2 Propagation and Channel Model...................................................................... 27 4.3 Repeater Model ................................................................................................ 29 4.4 Simulation Models ........................................................................................... 29

4.4.1 Radio Network Simulator ......................................................................... 29 4.4.2 Deployment Scenario................................................................................ 30

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4.4.3 User Generation.........................................................................................30 4.5 Simulation Parameters......................................................................................31 4.6 Performance Evaluation ...................................................................................33

4.6.1 SINR Calculation ......................................................................................33 4.6.2 Throughput ................................................................................................33 4.6.3 Object Bit Rate (OBR) ..............................................................................33

Chapter 5 SIMULATION RESULTS 35

5.1 Propagation Model ...........................................................................................36 5.2 Number of Repeaters per Cell ..........................................................................38 5.3 Repeater Distance from Base Station...............................................................39 5.4 Repeater Gain ...................................................................................................40 5.5 Advanced Repeater...........................................................................................43

Chapter 6 CONCLUSION 49

6.1 Conclusion........................................................................................................49 6.2 Future Work .....................................................................................................50

APPENDIX Appendix A – Simulation time ……………………………………………………. 51 Appendix B – Propagation Model ………………………………………………… 52 Appendix C – Number of Repeaters per Cell ……………………………………… 55 Appendix D – Repeater Distance ………………………………………………….. 57 Appendix E – Repeater Gain ……………………………………………………… 59 Appendix F – Advanced Repeater …………………………………………………. 61 References 83

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List of Tables Table 4-1 Simulation Parameters.............................................................................. 31 Table 5-1 Performance of repeaters in downlink for different propagation models. 45 Table 5-2 Performance of repeaters in uplink for different propagation models...... 47

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List of Figures Figure 2.1 E-UTRAN overall architecture ................................................................ 6 Figure 2.2 A block diagram of SC-FDMA and OFDMA.......................................... 8 Figure 2.3 LTE time-domain frame structure............................................................ 9 Figure 2.4 LTE subframe and slot structure .............................................................. 9 Figure 2.5 LTE frequency-domain structure ........................................................... 10 Figure 2.6 Channel dependent scheduling .............................................................. 11 Figure 2.7 Downlink resource block assuming normal cyclic prefix...................... 12 Figure 3.1 Repeater Block Diagram........................................................................ 16 Figure 3.2 Antenna Isolation ................................................................................... 17 Figure 3.3 On-frequency (a) and Frequency Shifting (b) Repeaters ....................... 18 Figure 3.4 A typical repeater’s installation in outdoor scenario (a) and indoor

scenario (b)............................................................................................. 20 Figure 3.5 Cellular layout of the system.................................................................. 21 Figure 3.6 Simple illustration of two-hop communication model........................... 22 Figure 3.7 An illustration of coordinated frequency selective repetition in

uplink and downlink............................................................................... 23 Figure 3.8 Repeaters with different filters............................................................... 23 Figure 3.9 Controllable filter banks in the repeater................................................. 24 Figure 3.10 Illustration of frequency selective repeater ............................................ 24 Figure 3.11 Example of gain control functionality.................................................... 25 Figure 4.1 Radio Network Simulation..................................................................... 29 Figure 4.2 Repeater deployment illustration ........................................................... 30 Figure 5.1 Illustration of cellular network deployment with repeaters ................... 35 Figure 5.2 A comparison of CDF downlink SINR for different propagation

model ..................................................................................................... 36 Figure 5.3 CDF of uplink SINR for different propagation model........................... 37 Figure 5.4 CDF of object bit rate (OBR) for different propagation model ............. 37 Figure 5.5 Five-percentile downlink object bit rate (OBR) for different

number of repeaters per cell................................................................... 38 Figure 5.6 Mean uplink object bit rate (OBR) for different number of

repeaters per cell .................................................................................... 39 Figure 5.7 Repeater path gain of a moving user for different repeater

deployment distance............................................................................... 40 Figure 5.8 Direct and Composite Path Gain for different downlink

repeater gain in WINNER LOS propagation of BS to RN links .......... 41 Figure 5.9 Mean downlink OBR for different downlink repeater gain ................... 42 Figure 5.10 Mean downlink OBR for different downlink repeater gain ................... 42

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Figure 5.11 Five-percentile downlink SINR vs mean cell downlink throughput ......43 Figure 5.12 Mean downlink SINR vs mean cell downlink throughput .....................43 Figure 5.13 Five-percentile OBR vs mean cell downlink throughput .......................44 Figure 5.14 Mean OBR vs mean cell downlink throughput ......................................44 Figure 5.15 Five-percentile OBR vs mean cell throughput .......................................47 Figure 5.16 Mean OBR vs mean cell throughput ......................................................47 Figure 5.17 Repeater activity in cell radius 166 m and WINNER nlos

propagation model scenario....................................................................48

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List of Abbreviations 3GPP 3rd Generation Partnership Project

AF Amplify and Forward

AP Access Point

BS Base Station

DF Decode and Forward

FDMA Frequency Division Multiple Access

GSM Global System for Mobile Communications

IMT International Mobile Telecommunications

ISI Inter-Symbol Interference

ITU International Telecommunication Union

LOS Line of Sight

LTE Long Term Evolution

MBSFN Multi-Media Broadcast over a Single Frequency Network

MS Mobile Station

OBR Object Bit Rate

OFDM Orthogonal Frequency Division Multiplexing

OTDOA Observed Time Difference of Arrival

RN Repeater Node

RTT Round Trip Time

SCM Spatial Channel Model

SINR Signal to Interference plus Noise Ratio

TDMA Time Division Multiple Access

TTI Transmission Time Interval

UTRAN Universal Terrestrial Radio Access Network

WINNER Wireless World Initiative New Radio

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Chapter 1 INTRODUCTION

1.1 Background In order to ensure the competitiveness of technology, 3rd Generation Partnership Project (3GPP) is considering long term evolution (LTE) as the evolution of third generation (3G) cellular systems. The 3GPP LTE radio-access technology, Evolved Universal Terrestrial Radio Access Network (E-UTRAN), is the future-oriented broadband radio access system whose objective is to obtain higher data rates, low latency, better coverage, improved system capacity, and packet optimized radio access technology [20]. It will also support end to end services with affordable cost by reducing number of nodes and interfaces while providing enhanced performance and capacity.

Meanwhile, the cooperative relay communication for wireless network has recently attracted attention because of its ability to increase the diversity gain in fading environment [1]. The idea behind relay communication model is to improve the cell coverage and capacity in a cost-efficient way. The demand of future cellular networks, especially in densely populated areas, is higher cell capacity than today’s system. This implies a denser deployment by adding more base stations (BSs) with a consequence of potential increase in deployment cost. It is shown in [2] that the deployment cost of a radio network is proportional to the number of Access Point (AP). Alternatively it is promising to reduce cost by deploying Repeater Node (RN) to substitute base station (BS) while keeping the goal to enhance capacity and coverage. The RN acts like base station but without the need of cable or fiber access, and it uses the same radio access technology from BS to RN and from RN to mobile station (MS).

Initial deployment of 3GPP-LTE is expected in 2009 and recent news has been announced that a contract of commercial LTE network in Stockholm is signed between TeliaSonera and Ericsson. An initiative has been taken by 3GPP to plan the future work for LTE, referred as LTE-Advanced [6]. LTE-Advanced will be based on LTE, i.e. reuse the main characteristics of LTE, with a selected set of amendments [21] based on International Telecommunication Union (ITU) “International Mobile Telecommunications (IMT) Advanced” requirements. The context of relaying is also considered as one candidate of technology feature in LTE-Advanced [6]. However, the repeaters considered for LTE-Advanced is likely to include some advanced

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functionalities such as frequency selectivity, on frequency repetition, network controllability and/or observability.

1.2 Previous work The base of relaying concept was already established in the 1970’s and then it has been proposed as one of the methods to improve capacity in cellular network. Various aspects of transmission are developed in [4] where transmission diversity may be employed to improve system performance in term of smaller outage probability. The strategies of diversity transmission include amplifying and forwarding as well as decoding and forwarding. They are also referred as layer-1 (L1) and layer-2 (L2) relays. The deployment strategy of base station (BS) and relay node is also analyzed from a cost perspective [13] [14]. One method was proposed to evaluate the comparison between different deployments which is called iso-performance curve [13] [14].

1.3 Problem statement Repeater which is used to refer L1 type of relay is studied as the focus area in the thesis. They are considered to be simpler than L2/L3 relays and are transparent to the system. Traditionally, repeaters are active all the time and perform "blind" forwarding without knowing the received signals. Repeater in Global System for Mobile communications (GSM) and 3G systems is one example of commercial products which are widely used now [27] [28]. On-frequency repetition, in the other hand, is used at repeaters in order to avoid duplex loss including joint use of signals from the direct link and repeated link.

The study of this thesis involves on learning about LTE and LTE-Advanced system, and how repeaters can be incorporated in such networks. In these systems, repeaters have the advanced ability to select which frequencies used to forward the signals with certain gain. If a resource block is assigned to a user, the repeater will amplify and forward the signal in this resource block.

The aim of this thesis is to quantify the benefits and gain of advanced repeater in 3GPP LTE. There is no doubt that repeaters can be beneficial in specific scenarios, but the question is how large the gain is and how much the system performance is affected by the increased interference that is an inherent characteristic of a repeater. We will see the effects of different propagation models, number of repeaters per cell, repeater distance and repeater gain toward the cell loads. The performance metrics used for the evaluation are signal to interference and noise ratio (SINR), cell throughput, object bit rate (OBR), and repeater activity. It would be more interesting to look at the 5-percentile of SINR and throughput which correspond to target users and the main objective to be improved in repeater deployment scenario. The deployments of repeater could be in a regular pattern with specific radius from the base station, deployed on cell edge, inside buildings [19] or other alternatives. For simplicity, the deployment inside building is assumed to be the same with outside building, i.e. regular pattern, but it is simulated together with

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simple indoor propagation model which is the outdoor propagation model plus wall attenuation loss.

Propagation model used in the system is aligned with 3GPP spatial channel model (SCM) [22] and the backward compatible extension to 3GPP SCM [18]. Parameters used in the propagation are taken from Wireless World Initiative New Radio (WINNER) project and 3GPP TSG-RAN WG1. BS – RN links are modeled as line of sight (LOS) and non-LOS (NLOS). Furthermore the antenna type used in the repeaters is normally assumed to be omni-directional antenna however it is interesting to consider directional antenna directed towards base stations and omni towards users.

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Chapter 2 LONG TERM EVOLUTION (LTE)

2.1 3GPP LTE Overview The growing commercialization of Global System for Mobile Communications (GSM) and its evolution such as Universal Mobile Telecommunications System (UMTS) with High Speed Packet Access (HSPA) have been the focus topic of 3GPP. The GSM / UMTS system is perhaps the most successful communications technology family and its evolution to beyond 3G becomes important issue for the next global mobile-broadband solution. In parallel to evolving HSPA, 3GPP is also specifying a new radio access technology in Release 8 known as LTE in order to ensure the competitiveness of UMTS.

LTE focuses to support the new Packet Switched (PS) capabilities provided by the LTE radio interfaces and targets more complex spectrum situations with fewer restrictions on backwards compatibility. Main targets and requirements for the design of LTE system have been captured in [20] and can be summarized as follows.

• Data Rate: Peak downlink rates of 100 Mbps and Uplink rates up to 50 Mbps for 20 MHz spectrum allocation, assuming 2 receive antennas and 1 transmit antenna at the terminal

• Spectrum: operation in both paired (Frequency Division Duplex / FDD mode) and unpaired spectrum (Time Division Duplex / TDD mode). Enabling deployment in many different spectrum allocations with scalable bandwidth of 5, 10, 15, 20 MHz, and better efficiency (downlink target is 3-4 times better than release 6 and uplink target is 2-3 times better than release 6)

• Throughput: Mean user throughput per MHz is 3-4 times (downlink) and 2-3 times (uplink) better than release 6. Cell-edge user throughput is also expected to be improved by a factor 2 for uplink and downlink

• Latency: Significantly reduced control-plane and user-plane requirements, i.e. less than 5ms in the transmission of an IP packet (user-plane), allow fast transition times of less than 100ms from camped state to active state (control-plane)

• Costs: Reduced CAPEX and OPEX including backhaul for both operators and users, and effective migration from previous release shall be possible.

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One of LTE requirement, as previously described, is to reduce the costs by simplifying the radio architecture. Therefore the number of nodes and interfaces in the network shall be reduced and it means that the 3GPP LTE Radio Access Network architecture need to group user plane functionalities into one network node called evolved Node B (eNB) [23]. The resulting radio architecture is commonly known as System Architecture Evolution (SAE) and is depicted on Figure 2.1 below.

Figure 2.1 E-UTRAN overall architecture [23]

As shown in the figure, the 3GPP LTE Radio Access Network (RAN)

architecture is different from the one of the previous 3GPP releases. The main difference is that a significant part of the radio control functionality has been distributed to the so-called eNBs. Thus, it is possible to reduce latency with fewer hops in the media path and distribution of processing load into multiple eNBs.

2.2 LTE Transmission Schemes 3GPP-LTE introduces the air interface access technologies from the use of orthogonal frequency division multiplexing (OFDM), multiple antenna technologies as well as modifications to the network architecture. OFDM is used in the downlink transmission and Single Carrier FDMA (Frequency Division Multiplex Access) technology is applied in the uplink transmission.

2.2.1 OFDM OFDM is a multi-carrier transmission technique where the available spectrum is divided into multiple carriers, called sub-carriers. It is a modulation technique providing a high degree of robustness against frequency selectivity of transmission channels and achieving high data rate without inter symbol interference (ISI). The idea was proposed in mid 60s and used parallel data transmission and frequency division multiplexing (FDM). OFDM is a current well-established technology, for

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example in standards such as Wireless Local Area Network (WLAN), Worldwide inter-operability for Microwave Access (WiMAX), High Performance Radio LAN (Hyperlan-2), Digital Video Broadcasting (DVB) and Digital Audio Broadcasting (DAB) [9]. The utilization of OFDM in 3GPP LTE downlink transmission scheme enables additional benefits such as access to the frequency domain, i.e. enabling additional degree of freedom to the channel-dependent scheduler compared to HSPA, scheduling, power allocations, flexible bandwidth allocations, broadcast/multicast transmissions [8].

The idea of OFDM is to transform high data rate stream into low data rate streams transmitted in parallel in order to transform a frequency-selective fading channel into a set of frequency non-selective fading channels [9]. It uses relatively large number of narrowband subcarriers which are overlapped and orthogonal to each other. This is enabling OFDM to avoid the use of high speed equalization and to combat impulsive noise, and robustness against multipath fading as well as fully use the available bandwidth.

Any type of non-ideal transmission channels spread the OFDM symbol causing the OFDM blocks to interfere one another. This type of interference where two adjacent blocks overlap causing symbol distortion is called Inter-Symbol Interference (ISI). One possible approach to combat this interference was to introduce a silence period between the transmitted frames, known as zero prefix. The silence period consists of a number of zeros added to the front of each symbol. The effect of ISI is still there however it is affected these prefix and they will be discarded in the receiver before demodulation of useful signal.

Unfortunately, the zero prefix approach will destroy the periodicity of the carrier. Therefore instead of using silence period, one could extend the OFDM block by a cyclic prefix interval. The cyclic prefix interval consists of the last L samples of the OFDM symbol that are copied in the beginning of data block and it should be larger than maximum delay spread of the channel (i.e. Tg ≥ Tm). If the cyclic prefix interval (Tg) spans more than maximum delay spread of the channel (Tm), the interference is entirely absorbed by the cyclic prefix which is then discarded in the receiver. The cyclic prefix facilitates the receiver’s carrier synchronization and maintains the carriers’ periodicity because some signals are transmitted instead of a long silence period in the zero prefix approach.

2.2.2 SC-FDMA The drawbacks of OFDM modulation are the large variations in the instantaneous transmitted signal power, high Peak to Average Power Ratio (PAPR), high sensitivity to frequency offset and a need for an adaptive scheme to overcome spectral nulls in the channels. The smaller PAPR of the transmitted signal, the higher average transmission power can be given for a given power amplifier. Therefore it is required to have expensive and inefficient power amplifier at the transmitter. This is very critical aspect in the uplink, since mobile terminal is power-limited and lower-cost manufactured.

In the other hand, SC-FDMA is basically another way to combat frequency-selective channel which delivers similar performance with essentially the same overall complexity compared to orthogonal frequency division multiple access

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(OFDMA) system. 3GPP LTE proposes SC-FDMA transmission scheme in the uplink.

SC-FDMA is a single carrier transmission based on DFT-spread OFDM where a block of N modulation symbols is applied to N-point DFT. The DFT spreads data symbols between all available subcarriers, combined with pilot symbols in time division multiplexing (TDM) and then mapped to proper subcarriers. After the N-point DFT, a size of M-point IDFT is applied to the signal, where M > N and the unused inputs of the IDFT equals to zero. At the receiver, the process is the opposite way in which after the M-point DFT is applied, the signal is frequency domain equalized and then the signal is finally converted into time domain using N-point IDFT. A comparison of SC-FDMA and OFDMA is drawn in Figure 2.2 which summarizes the difference in block diagram.

Figure 2.2 A block diagram of SC-FDMA ( + ) and OFDMA ( ) SC-FDMA has disadvantages in handling signal due to radio-channel frequency

selectivity, but it can be solved in the eNBs by utilizing more resources. One example of this method is to employ different forms of equalization at receiver however it requires higher receiver complexity. The other disadvantages of SC-FDMA are [11]:

• In SC-FDMA, the noise is averaged over all the bandwidth because the detection is done after equalized signal is reverted to time domain by IDFT, but in contrast, OFDMA performs the detection individually on each subcarrier

• Maximum Likelihood Detector is not feasible for Multiple-Input Multiple-Output (MIMO)

• Additional DFT processing increases mobile station complexity • Unlike OFDMA, Localized SC-FDMA cannot exploit full advantage of

multiuser diversity • Distributed SC-FDMA has some issues, e.g. vulnerability to Doppler and

frequency offset, and pilot design • Only TDM pilots can be supported by SC-FDMA • Low flexibility in multiplexing uplink control and data channels • Degraded link-level performance as compared to OFDMA.

The performance of OFDMA and SC-FDMA has been compared in terms of spectral efficiency (i.e. bits/s/Hz) [7] in which two antenna configurations are used: Single-Input Single-Output (SISO) and 1x2 Single-Input Multiple-Output (SIMO). The simulation results show that SIMO makes the performance of SC-FDMA comparable to OFDMA but in the SISO case OFDMA outperforms SC-FDMA.

Channel

N-point DFT

Add CP / PS

Subcarrier Mapping

M-point IDFT

DAC / RF

Detect

{xn}

N-point IDFT

Remove CP

Subcarrier De-mapping/ Equalization

M-point DFT

RF / ADC

{xn}

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2.3 Frame Structure In the time-domain structure, LTE transmission has frame length of Tframe = 10 ms consisting of ten equally sized subframes of length Tsubframe = 1 ms [8]. One subframe consists of two equally sized slots of length Tslot = 0.5 ms and each slot consists of a number of OFDM symbols including cyclic prefix. To provide consistent and exact timing definitions, different time intervals within the LTE radio access specification can be expressed as multiples of a basic time unit Ts

307200001

= sec. Therefore Tframe

and Tsubframe can be expressed as (307200.Ts) and (30720.Ts) respectively. The illustration of LTE time-domain frame structure is drawn in Figure 2.3.

Figure 2.3 LTE time-domain frame structure

One slot of length 0.5 ms consists of six or seven OFDM symbols depending on

cyclic prefix type. LTE defines two cyclic prefix lengths: normal cyclic prefix and extended cyclic prefix which corresponds to seven and six OFDM symbols per slot respectively shown in Figure 2.4.

Figure 2.4 LTE subframe and slot structure

TC

One subframe = Two slots

TU ≈ 66.7 µs =

Tslot = 0.5 ms

Normal CP

TCP-

Extended CP

TU ≈ 66.7 µs =

#0 #1 #9

1 subframe; Tsubframe = 1 ms

. . . . . .

1 frame; Tframe = 10 ms

1 slot; Tslot = 0.5 ms

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The reasons of defining two cyclic-prefix lengths for LTE are described below [8].

a) Extended cyclic prefix may be beneficial in specific propagation scenarios with very extensive delay spread, for example in very large cells. In this scenario the additional robustness to radio channel dispersion is provided however it is less efficient from an overhead point-of-view. Therefore it is a tradeoff to choose which cyclic prefix length should be used.

b) In case of Multi-Media Broadcast over a Single Frequency Network (MBSFN)-based transmission, the extended cyclic prefix is typically needed to cover both the main time dispersion part from the actual channel and the main timing difference part between the transmissions received from the cells.

In the frequency-domain structure, LTE subcarrier spacing has been chosen to kHzf 15=Δ which corresponds to a useful symbol time Tu ≈ 66.7 µs (2048.Ts). A

group of 12 consecutive subcarriers is called one resource block and it corresponds to 180 kHz. The illustration of LTE frequency-domain structure is shown in Figure 2.5.

Figure 2.5 LTE frequency-domain structure

The basic parameters of the LTE downlink and uplink transmission scheme are

chosen to be aligned as much as possible. In the time-domain structure, the resource block for downlink and uplink is similar to illustration on Figure 2.3 and Figure 2.4. The same basic parameter is also applied in frequency-domain structure as illustrated in Figure 2.5 however there is an unused DC-subcarrier in the center of the spectrum for the downlink case. The reason why it is not used for any transmission is that the transmission may coincide with the local-oscillator frequency at the BS transmitter and/or MS receiver, and may cause to unproportional high interference. In the other hand, uplink transmission has no unused DC-subcarrier because of single-carrier transmission and the presence of a DC-carrier in the center of the spectrum would have made impossible to allocate the entire system bandwidth to a single mobile terminal while still keeping the low-PAR single-carrier property transmission [8]. Thus the total number of subcarriers on a downlink carrier, including the DC-subcarrier, equals: NSC = 12.NRB + 1, where NRB is the number of resource blocks. And in the uplink transmission, the total number of subcarriers is NSC = 12.NRB.

2.4 Scheduling in LTE LTE transmission scheme uses shared-channel transmission in which the time-frequency resource is dynamically shared between users. This is similar to the

. . . . . .

∆f = 15

One resource block (12 subcarriers)

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approach taken in HSDPA, although the realization of the shared resource differs between them, time and frequency in case of LTE and time and channelization codes in case of HSDPA.

Figure 2.6 Channel dependent scheduling [8]

The scheduler controls, for each time instant, to which users the shared resources

should be assigned. It also determines the data rate to be used for each link that is rate adaptation. The scheduler is a key element and to a large extent determines the overall downlink performance, especially in a highly loaded network. Both downlink and uplink transmissions are subject to tight scheduling. It is well known that a substantial gain in the capacity can be achieved if the channel conditions are taken into account in the scheduling decision, so called channel-dependent scheduling. This is exploited already in HSPA where the downlink scheduler transmits to a user when its channel conditions are advantageous to maximize the data rate. Furthermore LTE has also access to the frequency domain in addition to the time domain. Therefore, the scheduler can select the user with the best channel conditions for each frequency region. In other words, scheduling in LTE can take channel variations into account not only in the time domain, but also in the frequency domain. This is illustrated in Figure 2.6.

The possibility for channel dependent scheduling in the frequency domain is particularly useful for low terminal speeds when the channel is varying slowly in time. Channel dependent scheduling relies on channel quality variations between users to obtain a gain in system capacity. In LTE, the scheduling decisions can be taken as often as once every 1 ms and the granularity in the frequency domain is 180 kHz. This allows relatively fast channel variations to be tracked by the scheduler.

2.4.1 Downlink Scheduling In the downlink, each terminal reports an estimate of the instantaneous channel quality to the base station. These estimates are obtained by measuring on the reference signal, transmitted by base station and used also for demodulation purposes. Based on the channel-quality estimate, the downlink scheduler can assign resources to users, taking the channel qualities into account. In principle, a scheduled terminal can be assigned an arbitrary combination of 180 kHz wide resource blocks in each 1 ms scheduling interval.

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Figure 2.7 Downlink resource block assuming normal cyclic prefix1

2.4.2 Uplink Scheduling The LTE uplink is based on orthogonal separation of users and it is the task of the uplink scheduler to assign resources in both time and frequency domain (combined TDMA/FDMA) to different users. Scheduling decisions, taken once per 1 ms, control which mobile terminals are allowed to transmit within a cell during a given time interval, on what frequency resources the transmission is to take place, and what uplink data rate (transport format) to use. Note that only a contiguous frequency region can be assigned to the terminals in the uplink as a consequence of the use of single-carrier transmission on the LTE uplink.

Channel conditions can be taken into account in the uplink scheduling process, similar to downlink scheduling. However, as will be discussed in more detail, obtaining information about the uplink channel conditions is a non-trivial task. Therefore different means to obtain uplink diversity are important as a complement in situations where uplink channel dependent scheduling is not used.

2.5 LTE-Advanced Initial deployment of 3GPP-LTE is coming to the realization and the 3GPP is already planning to the future work for LTE, referred as LTE-Advanced [6]. LTE Advanced will be based on LTE, i.e. improvement of LTE in 3GPP Release 8, with a main driver of ITU requirements so called IMT Advanced 4G wireless system [21]. LTE Advanced targets higher data rates, reduced delay and latency, improved capacity and coverage, and fulfilling or surpassing the requirements for IMT Advanced.

IMT Advanced is a concept used by ITU for radio-access technologies in mobile communication systems with capabilities beyond IMT-2000. The candidate technologies for IMT Advanced have been proposed to ITU and 3GPP already initiated the definition of requirements as well as technology components on LTE Advanced to meet all the requirements of IMT Advanced. 3GPP plans to submit to ITU on September 2009 which will probably be fully specified in 3GPP release 10, and IEEE is likely to submit based on 802.16m standard which is an evolution of

1 Figure is adapted from: http://www.ericsson.com/technology/whitepapers/lte_overview.pdf

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802.16e. The preliminary requirements for IMT Advanced can be found on the ITU’s IMT Advanced website2.

It is quite certain that in terms of spectrum and bandwidth, all technologies beyond 3G such as LTE, LTE Advanced, WiMaX, etc., should have wider bandwidth and be backwards compatible. It means that there is no significant impact at user terminal while deploying LTE Advanced in the spectrum occupied by LTE now and the consequence is that the LTE Advanced should have a similar network as LTE network. The other compatibility is spectrum which is important for a smooth, low-cost transition within the network.

Apart from the requirement on backwards compatibility, LTE-Advanced should fulfill the other requirements of IMT Advanced which are capacity, data rates and low-cost deployment. The final target for peak data rates leaves the possibility to have up to 1 Gbps in the downlink and 500 Mbps in the uplink, 100 Mbps for high mobility and 1 Gbps for low mobility. This is the headline requirements for 4G which is nailed to the same goal as in 3G: the growth in single-user peak data rates. However it is more important to provide high data rates over a larger part of the cell rather than for a single-user data rates.

From the link performance perspective, the current cellular system in LTE is already close to the Shannon bound limit. And from a pure link budget point of view, the peak data rates targeted by LTE Advanced will require a higher Signal to Noise Ratio (SNR) than what is typically experienced in cellular networks. Although some link improvements are still possible, i.e. using additional bandwidth to improve the coding/modulation efficiency, it is also necessary to introduce concepts and tools to improve the SNR. A set of components and technologies being considered for LTE Advances includes: A. Wider-band transmission and spectrum sharing B. Multi-antenna solutions C. Coordinated multi-point transmission D. Repeaters.

The improvement of SNR with denser deployment could be done by deploying denser number of base station or with different types of relaying solutions. The idea is to reduce the transmitter to receiver distance in order to get higher data rates and depending on the schemes, different types of relaying solutions can be employed. The detail of repeater is discussed in the following section.

2 ITU-R IMT-Advanced website: www.itu.int/ITU-R/go/rsg5-imt-advanced

14

15

Chapter 3 REPEATER CONCEPT

3.1 Repeater Overview The fundamental question occurred to wireless system area is how the signals should be distributed to and collected from the user terminal in very efficient manner to have high data rate and system coverage requirements. This developments yield architecture of the present cellular networks which can not meet the requirements of high data rate for the fourth generation (4G) cellular systems. The extreme solution to solve this problem is to use larger bandwidth/spectrum or to significantly increase the density of base stations. Unfortunately, there is no indication that significant new spectrum will be available in the near future. In the other hand, high number of base stations results a considerably high deployment costs and it does not seem economically justifiable [12] [16].

The concept of relaying has been studied as a theoretical problem from a network information theory perspective in 1970’s and in early 1980’s. Capacity regions of simple relaying channels have also been evaluated [15]. However there was no further analytical study most probably due to the fact that there were no foreseeable applications at that time. Recently the concept has been brought up again to be used and proposed in the cellular network system.

Relaying technique is a promising solution to substitute BS because a relay costs considerably lower than a BS but it acts to improve the link budget [13] just as a BS does. It has been used typically for handling areas with low signal coverage such as in radio shadowed areas, tunnels, business and industrial buildings, and where the traffic is too low to justify the installation of a base station. There are many products available in the market for this type of solution, e.g. [27] [28]. Several types of relay which have been developed in cellular network system are:

• Amplify-and-forward (AF) relay or sometimes referred as repeater (Layer 1 relay)

• Decode-and-forward (DF) relay or Layer 2 relay • Self-backhauling relay or Layer 3 relay.

A repeater (L1 relay) is a relay which amplifies the received signal and forwards it to user. The repeaters are transparent to system and they are blind when forwarding the signal without knowing whether it is desired signal, interference or noise.

16

According to what frequency the signals are transmitted, the repeater is classified into two types: on-frequency repeater and frequency shifting repeater. The explanation of these repeaters is given in section 3.1.3. DF relays, in the other hand, decode the received signal first before forwarding it to user. Hence there is additional delay occurred due to signal processing and it needs other resources for the relays to MS links, i.e. first time/frequency slot for BS to MS link and second time/frequency slot for relay to MS link. The advantage of this type of relay is of course no noise forwarded by relay, however it is difficult to receive both the composite and desired signal at the same resource block in LTE therefore it needs more resources. Another type of relay is L3 relay which is often denoted as self-backhauling and performed on layer 3. Self-backhauling is recently attracted researchers because it is very similar with Layer 2 relaying in their basic characteristics and it is proposed to be one of the feature technology in LTE-Advanced [6].

This thesis only studies on-frequency repeater which amplifies the received signal before retransmission.

3.1.1 Basic Repeater Design Repeater basically consists of two way amplifier with duplex filter as shown in Figure 3.1. It has two antennas: one for the connection to the parent base station and the other one for service area to the users. They are called donor antenna and service area antenna respectively. The repeater receives signal from donor antenna, filters the signal, amplifies the signal and directs to the other antenna to be transmitted.

The repeater is located in the cells and is used to improve the cell coverage and cell capacity in certain areas. Physically a repeater could be in one package with two built-in antennas inside or it could be a box with two built-out antennas connected with cable/fiber. The second implementation is sometimes preferable to increase the antenna isolation and to reduce feedback interference by giving more distance to both antennas.

Figure 3.1 Repeater Block Diagram

3.1.2 Antenna Isolation Antenna isolation is an essential issue for the performance of a repeater because the feedback signal from service area antenna to donor antenna acts as interference. An illustration for antenna isolation is drawn in Figure 3.2. In the indoor installation the isolation between the donor antenna, generally mounted outside the building, and the

Adjustable amplifiers

Adjustable amplifiers

Filters Filters

Service Antenna

Donor Antenna

17

service area antennas inside the building is not a big issue. The attenuation path is high enough due to concrete, walls and the usage of feeder cable, but in the outdoor installation the attenuation path from service to donor antenna is relatively low. If the antenna isolation does not meet the requirement, the repeater acts as an oscillator making the repeater itself not to work.

Figure 3.2 Antenna Isolation

Self-oscillation can be avoided if the overall path loss between the repeater

service antenna and donor antenna, referred as antenna isolation, is higher than the maximum repeater gain (GR,max). An additional 15 dB margin is considered to ensure the antenna isolation according to specification for UTRA Repeater [24]. As a rule of thumb, the antenna isolation should fulfill the following inequality:

Antenna isolation ≥ GR,max + 15 dB (3.1)

In the implementation antenna isolation must be resolved before a repeater is installed. If antenna isolation cannot be reached, it is necessary to decrease the maximum repeater gain at the expense of losing repeater coverage area. There are several factors that influence the antenna isolation [17]:

• The distance between antennas: the path loss is higher when the distance between antennas is increased. It is roughly proportional to the square of the distance between the antennas (free space propagation loss) and in the UMTS frequency system, antenna separation up to 10 m is recommended

• Antenna lobe width and front to back ratio: higher antenna isolation can be reached if the antennas have narrow lobe width and higher front to back ratio. The service area antenna is typically an omni directional antenna, but a directional antenna can be employed especially at donor antenna to increase antenna isolation. The typical donor antenna gain ranges from 15 to 18 dBi

• Antenna polarization • Shielding • Surrounding environment: the reflection and attenuation properties of all

materials close to antennas can influence the antenna isolation drastically • Isolation through Self Interference Cancellation3.

3 Peter Larsson, “MIMO On-Frequency Repeater with Self-Interference Cancellation and Mitigation”, VTC2009-Spring, 29 April 2009.

RBS Coverage Antenna

Service Antenna

Donor Antenna

Antenna Isolation = Repeater Gain + 15 dB Margin

Repeater

18

3.1.3 On-Frequency and Frequency Shifting Repeaters An on-frequency repeater, as the name clearly explains, is a repeater which uses the same frequency band on both base station to repeater link and repeater to user terminal link. On-frequency repeater which receives and transmits on the same frequency band sometimes creates problems since it relies on the antenna isolation which effectively limits the repeater gain. Therefore it is important to choose high gain antennas with low side/back lobes and performing a good antenna installation. The resulting coverage from an on-frequency repeater strictly depends on the antenna type and installation skill.

Generally it is difficult to obtain high antenna isolation between donor and service area antenna at the repeaters. Indeed, as shown in Figure 3.3, on-frequency repeater needs to have an additional and costly component to achieve sufficient antenna isolation known as canceller, therefore it makes the entire repeater solution approach more expensive and less desirable. Instead, frequency shifting repeaters can be used. These repeaters use one frequency to communicate with base station and use another frequency to talk with mobile station. For example, it converts frequency F1 to frequency F2 for downlink, and vice versa for the uplink communication. Therefore, sometimes frequency shifting repeaters are also known as frequency translation or conversion repeater.

(a)

(b)

Figure 3.3 On-frequency (a) and Frequency Shifting (b) Repeaters

Frequency shifting repeater uses different frequency for two hops channel which can solve the antenna isolation problem. It is possible to provide up to 100 dB repeater gain with much lower antenna isolation requirement. In general, frequency shift repeaters shall be more cost-effective solution than on-frequency repeaters because they do not have canceller and only have band pass filter in the RF module. The high gain and low antenna isolation requirement of this unit are suitable for coverage extension in rural areas.

However frequency shift between link between repeater to user and link between base stations to repeaters requires more resources in order to allow the repeater to work correctly and this would result a reduced capacity (duplex loss). Obviously more complex frequency planning and guard channels are needed. Therefore frequency-shifting repeaters are not recommended to be used widely in LTE network

Feedback interference

Antenna isolation (30dB)

Filter (45dB)

F2 F1

o

Frequency Shifting Repeater

F2

F1

Base Station Mobile Station

Feedback Interference

On-Frequency Repeater

F1

F1

Base Station Mobile Station

Feedback Interference

Antenna isolation (30dB)

Canceller (45dB)

F1

19

and successful installation enforces high requirements on low adjacent channel signal levels in the area. The other common problem that occurs in the frequency-shifting repeater is synchronization problem. Even though a repeater is equipped with a very high quality local oscillator, a frequency drift in time may not be avoided. A manual frequency re-tuning with an interval of period time is necessary, i.e. less than two years for repeater in GSM.

3.1.4 Repeater Delay In two-hop communication model, the repeater introduces delay due to filtering, processing and feeder links used in application which could deteriorate the signal coming to user terminal. As long as the delay is within cyclic prefix, the signal can be recognized and combined to obtain the original transmitted signal. This happens because OFDM scheme employs cyclic prefix, a circular extension of the data symbol to combat inter-symbol interference introduced by the frequency selectivity of the radio channels. The cyclic prefix is used in order to avoid the effect of time dispersion by multipath propagation which makes two consecutive frames to interfere each other at the receiver. If the frame delay is more than cyclic prefix, the receiver can not combine the signal and they are seen as interference. Therefore it is important to keep the repeater delay as low as possible such that the total delay at user terminal must be within cyclic prefix period. An example of total delay in frequency selective repeater in GSM network is 4 - 6 μs while the extended cylic prefix period in LTE is 14.2857 μs.

Another effect of repeater delay is causing the positioning system to estimate the UE position wrongly. This effect depends on what positioning system type is used. In Round Trip Time (RTT) and Observed Time Difference of Arrival (OTDOA) methods the estimation of UE position strongly depends on propagation delay measurements between the UE and NodeB, however in Cell-ID method the repeater delay will not affect the identification of the serving cell.

3.1.5 Interference and Capacity The deployment of repeaters in cellular network has a main task to extend the cell coverage while keeping the total investment costs lower than the deployment of NodeBs without repeaters. Repeaters have some impacts on the cellular system in term of interference, capacity and coverage especially in outdoor scenarios. In rural areas where the traffic density is low, the number of NodeBs is generally dimensioned according to coverage. In such applications where the capacity is not limited, the repeaters are used to replace one or more NodeBs as long as the service quality is still acceptable. In urban areas capacity is an important factor for determining the number of NodeBs and it is limited by the downlink power. A repeater in this area generally increases the overall downlink power because it amplifies both the desired signal and interference. Thus it actually increases the overall downlink capacity if the desired signal without any interference is amplified. However, in multi-cell environment inter-cell interference exists and it means that there is a tradeoff between the interference and capacity in the system.

20

3.1.6 Repeater Applications Choosing a base station or a repeater to be deployed is not an easy way but it is possible to provide general rules in order to make one choice more sensible than the other one. A base station basically needs high electrical power feeding, a physical transmission link to the core network, and it is relatively expensive. On the other hand a repeater is a cost-effective complementary solution for solving coverage problems, but it does not add any new hardware capacity to the system and it has often somewhat worse radio performance than a base station.

Implementation of a repeater solution starting from the coverage extension issue to a complete working system is a very rapid process. Acceptable signal strength at the donor antenna and a capacity of the donor cell which allows serving the new MSs introduced by the repeater are some prerequisites required for a repeater solution. The repeater can be seen as a natural way in expanding the network coverage, therefore there is an expectation of increased capacity demand in the system. In summary, the cost, required capacity, and limited implementation time are critical factors to this process.

(b)

Figure 3.4 A typical repeater’s installation in outdoor scenario (a) and indoor scenario (b)

Typical repeater applications are to improve cellular system performance in outdoor or indoor scenario. The performance could be system coverage and/or capacity. Figure 3.4 shows a typical outdoor repeater on the building’s rooftop and indoor repeater. The outdoor scenario is typically used for macrocell coverage extension purposes where the direct signal path from BS to MS is obstructed by hills or there are coverage holes in the service areas. Note that in this case the capacity

(a)

Donor antenna

Service area antenna

Repeater

Base Station

User Terminal

21

should not be a limiting factor. The indoor scenario is intensely used for in-building coverage where the external macrocell coverage is not sufficient.

3.2 Two-Hop Communication Model The capability of relaying method is perhaps a promising architecture to be implemented in the wireless network structure as a cost efficient solution. It allows coverage extension with high data rates and can reduce the total number of expensive base stations in the system. The multi-hop feature also allows many units to retransmit the signal to the destination in several hops, but two-hop relaying could decrease the operation cost and simplifies the routing function. Furthermore fixed relaying is utilized to simplify the radio protocols and to improve the efficiency.

Two-hop fixed relaying method is based on fixed repeater node (RN) deployed in the system infrastructure and the number of hops allowed is restricted to two, i.e. BS to RN and RN to MS. This simplification further increases the practical implementation of the multi-hop technology and reduces the complexities. Figure 3.5 shows the layout of our model: hexagonal cells with radius R, one cell site with three sectorized antennas BSs, and repeater node (RN) placed at some distance from the base station to the cell border. Furthermore, 3 repeaters per cell are considered in our work.

Figure 3.5 Cellular layout of the system A simple illustration of two-hop communication model and how the system

works for a single cell is drawn in Figure 3.6. The desired signal is sent from serving base station to the user (black line) and the interference signal comes from other base stations to the user (red line). The blue line represents the desired signal sent through repeaters in other cells. The received signals (with thermal noise) at repeater is amplified and retransmitted to the user, and then all signals received at user terminal are summed together as multipath signals.

: Base Station

: Repeater Node

: Mobile Station

22

1

Figure 3.6 Simple illustration of two-hop communication model

If kiC , is the path gain from base station-k of cell-i, sE is the symbol energy, and

20zσ is the noise variance, the SINR equation of a user in cell-K in this model is:

∑ ∑

∑

≠= =

=

+=Γ

cell repeaters

repeaters

N

Kii

N

kskiz

N

kskK

EC

EC

1 1

2,

2

1

2,

0σ

(3.2)

where thermal noise is assumed to be white.

3.3 Advanced Repeater As being explained in Section 2.5, layer 1 relays or also referred to repeaters are considered to be one of potential technology features of LTE Advanced. Some problems appeared to repeaters in 3GPP Rel-8 are energy consumption, interference problems, lower throughput when served by repeaters, difficult to monitor operation and potential features for improved performance. Practically the conventional repeaters are always on continuously even when there is no data transmitted, so the repeater consumes more energy than what it needs. Moreover in multicells scenario the repeaters may contribute more interference to the other users and could degrade the overall performance. Therefore it is important to have some advanced functionalities, i.e. to switch the repeater on/off in an optimum way, to have frequency selectivity and gain controllability, to use multi antenna ability and advanced antenna processing, etc.

There are two advanced repeater functionalities analyzed in this thesis. First, an advanced repeater may use time and/or frequency selective functionality and it only

Base Station

Base Station

Repeater

Mobile

Desired signal Desired signal from outer repeater Interference from outer cell

23

forwards when there is data to be forwarded and chooses the frequency bands needed. Second, it may control the repeater gain. In these ways, power consumption and interference can be reduced. Furthermore power control at repeaters can give benefits in order to reduce unnecessary power and to increase the overall performance. The simplest power control is to have an on/off switch that will turn the repeater on if there is data to be forwarded from serving base station. More complex power control algorithm can control repeater gain such that all users in the system get the best performance with fairness condition and satisfaction. This power control and other advanced repeater functionalities are not in the scope of this thesis hence they may be considered as future work.

3.3.1 Frequency Selective Repetition Frequency selective repetition is one of the advanced repeater functionalities which can improve the system performance. The frequency selective functionality is used in order to reduce the unwanted signal being forwarded to the users therefore the interference introduced to other cells is reduced. This method is similar to Inter-Cell Interference Cancellation (ICIC) where MSs in the the cell edge served by repeater on coordinated resources and BSs control the repetition of frequencies used by repeater. Hence it can allow an improvement of the user’s SINR and system performance in general. Figure 3.7 illustrates this functionality in a simple case.

Figure 3.7 An illustration of coordinated frequency selective repetition in Uplink and Downlink

The frequency selective repetition is basically implemented in the frequency

selective repeaters by employing filters. According to the bandwidth of the filters implemented, these repeaters can be classified into: broadband repeater, band selective repeater and frequency selective repeater. An illustration of these repeaters with different type of filters is given in Figure 3.8.

Figure 3.8 Repeaters with different filters

f

Broadband

f

Band Selective

f

Frequency Selective

24

Frequency selective repeater has a bank of band-pass filters to amplify multiple channels or resource blocks as shown in Figure 3.9. The bandwidth of a repeater can be made as minimum as possible to achieve the maximum selectivity against adjacent channels, i.e. the minimum schedulable bandwidth in LTE system is one resource block. However sharper filter will cost more and makes the processing time longer. It is sometimes assumed that the filters have the same bandwidth and component. Therefore if the bandwidth of a single filter is narrower, the number of filters in a repeater is higher and the granularity of the filter bank is also finer.

ControllerControllerController

Figure 3.9 Controllable filter banks in the repeater

In LTE the frequency selective repeater can be used to retransmit the data to users scheduled in the resource blocks. Each user can be scheduled in a consecutive resource blocks or in a localized resource blocks. An illustration of repeater frequency selectivity function and the amplification in frequency domain is drawn in Figure 3.10. Note that the repeater is also potentially controllable in time domain, i.e. it may be active when there is resource block assigned to active user served in the cell and can be off when there is no resource block used by active users.

Figure 3.10 Illustration of frequency selective repeater

3.3.2 Repeater Gain Controllability Repeater gain controllability is another feature in the advanced repeater which can be used in order to reduce interference, to reduce power consumption, to improve uplink

Frequency

User 1 User 2

Scheduled resource blocks

Frequency Conventional repeater

Frequency Frequency selective repeater

25

power control and to avoid instability. This feature is motivated from a simple illustration where a repeater could be on/off or continuously transmitting in time. On/off state simply refers to if there are data to be forwarded or not. Furthermore different amplification factor or gain may give different system performance because higher gain contributes higher interference to the system.

There are two granularities that can be controlled which are time granularity and frequency granularity. In the time granularity, the repeater may use long time scale or at scheduling rate to review and to change the gain based on power control algorithm. We define “fast” repeater as a repeater allowed to change the gain drastically at the next time instant and “slow” repeater as a repeater which needs to increase/decrease stepwise. In frequency granularity case, a repeater may amplify the entire bandwidth or in a frequency-selective way. We define “fine resolution” as filters correspond to one resource block and “coarse resolution” as filters correspond to a chunk of resource blocks (e.g. 5 MHz). The tradeoff of this functionality is the need of synchronization and control signaling from base station to repeaters and it makes the interference estimation accuracy decrease.

In our simulation, there are five repeater states which will be considered. They are: “always on”, “slow coarse resolution”, “slow fine resolution”, “fast coarse resolution” and “fast fine resolution”. The example of gain control functionality is given in Figure 3.11.

Figure 3.11 Example of gain control functionality

26

27

Chapter 4 SYSTEM MODEL

4.1 General Scenario 3GPP LTE radio cellular network environment is developed in a system level simulator. In this thesis, a typical urban multi-cell with two-hop transmission link scenario is focused on. The framework developed in this simulator uses a traffic map scenario with some predicted user demand generated in the cellular network area.

The repeaters are placed in fixed locations and the users are generated with uniform distribution throughout the area. The advanced functionalities are implemented in the repeater and are evaluated in term of system performance. Moreover the radio channel is modeled using the 3GPP spatial channel model (SCM) with different parameters and proposed for LTE based on WINNER (Wireless World Initiative New Radio) project and 3GPP TSG-RAN WG1 #54bis R1-084026. Radio resource management for scheduling, link adaptation and power control are also supported by this simulator. Detailed descriptions of the models are given in the following sections.

4.2 Propagation and Channel Model Reflection, diffraction and dispersion due to the obstacles in the environment introduce the multipath property of a radio channel. The received signals from each path have different delays, phase shifts and path losses depending on the path that they cross through. Based on the multipath fading channel model, the impulse response of the frequency selective fading channel can be written as:

( ) ( )∑−

=

−=1

0

P

ppp thth τδ (4.1)

where hp is assumed to be complex Gaussian and represents the channel coefficient of path p, and pτ represents the delay of path p. The frequency response of the channel is therefore given by:

28

( ) ∑−

=

−=1

0

2P

p

fjp

pehfH τπ (4.2)

The channel modeling considered in 3GPP LTE is based on spatial channel model (SCM) [22]. The SCM in the system-level simulations is used to describe the propagation between two nodes when transmitting the data. The propagation path loss occurs due to several factors including among others free-space, penetration and multipath losses. There are several paths modeled for each link and several sub-paths for each path. Each sub-path is modeled with a delay, amplitude, phase and angles at both the transmitter and the receiver sides.

Depending on environments and models used, the path loss dependent equation are generally in the function of carrier frequency, transmitter antenna height, receiver antenna height and distance from transmitter to receiver. Modified COST231 Hata urban propagation model or COST 231 Walfisch-Ikegami model are one example of the model. In a simplified equation, propagation model between two nodes can also be modeled as:

( ) ( )ddL log**10 αβ += [dB] (4.3)

where α is the path loss exponent, β is a constant that depends on the propagation conditions and d is the distance from transmitter and receiver. The path loss exponent varies between 2 and 6 depending on the environment where 2 is used for free-space and 6 is for an environment with many obstacles. Parameter of the propagation model, α and β, used in this thesis is based on WINNER project [26] and 3GPP TSG-RAN WG14. The values of these parameters are given in Table 4-1.

Shadow fading or slow fading is a large variation in the received power due to the sudden appearance or disappearance of obstacles between the transmitter and receiver. This is most often due to the movement of the receiver. In a cellular network this can be modeled by a log-normal distributed random variable σS with a standard deviation of σ . This variable can be added to the path loss equation in (4.3) and then the expression becomes:

( ) ( ) σαβ SddL ++= log**10 [dB] (4.4)

We also consider a simple indoor propagation model. The parameter of indoor propagation model is based on WINNER model with an additional wall attenuation. The wall attenuation is 10 dB as defined in [25] for outside wall loss. It is assumed that all users are indoor users therefore the BS to MS links is always attenuated by 10 dB wall loss. The composite links are defined in 3 cases which are based on repeater location:

1. RN has two separate antennas, one is placed outdoor (donor antenna) and the other one is indoor (service area antenna). No additional changes are made for BS – RN and RN – MS links.

2. RN and its antennas are placed outside the building. BS – RN links are not changed and RN – MS links are attenuated by wall loss.

3. RN and its antennas are placed inside the building. BS – RN links are attenuated by wall loss and RN – MS links are not changed.

We denote Indoor Case 1, Case 2 and Case 3 for these three cases in the results on Chapter 5. 4 3GPP TSG-RAN WG1 #56bis, Seoul, Kroea, March 23- 27, 2009, Doc: R1-091566.

29

4.3 Repeater Model Repeater considered in this thesis amplifies the received signal before retransmission. On-frequency repetition is used in order to avoid duplex loss. Repeaters are deployed in a regular pattern placed at a certain distance from the serving base station. This is the most efficient way to deploy the repeaters because we have homogeneous traffic scenario. Number of repeaters per cell and repeater distance from BS are the variables and they are set to values given in Table 4-1.

Repeater has two basic constraints: maximum repeater gain (GR) and maximum repeater output power (PR). The maximum repeater gain (GR) is set to 90 dB [24] which is 15 dB less than the antenna isolation and the maximum repeater output power is 20 W. It is assumed that the repeater processing delay is shorter than the cyclic prefix of an OFDM symbol. In this case the repeated signal path and the direct signal path do not interfere with each other. Instead they add, in the air, in the same way as normal multi-path does. And it is also assumed that there is no inter-repeater interference due to well-isolation of the repeaters. No MIMO model is considered in this thesis to make the evaluation of advanced repeater functionalities easier.

4.4 Simulation Models

4.4.1 Radio Network Simulator The problems in this thesis are studied and analyzed using software simulator platform. It is implemented in Java and the general concept of this radio network simulation is illustrated in Figure 4.1.

Figure 4.1 Radio Network Simulation

The software simulator platform is built not only for simulating the radio

propagation model, radio resource management, and radio protocol parts but also for modeling the traffic, internet protocols and transport network. The results are taken from real time simulation and simulation time is chosen as low as possible while still keeping low-error compare to results in long simulation time. Reliability of the result can be motivated under some circumstances and assumptions being made. Some assumptions are made in order to decrease the complexity of the simulation, for example: user distribution is assumed to be uniform, repeater deployment is assumed to be regular pattern, antenna isolation in the repeater is assumed to be good enough, etc. Therefore it must be followed by further research, implementation, test bed and measurements.

environment deployment mobility & traffic alg. parameters

...

Radio Network Simulator transmit power

interference error rates

...

throughput service quality block rate drop rate ...

30

4.4.2 Deployment Scenario The cellular network consists of base stations, repeaters and mobile stations. The system is first set up at the beginning of each simulation where a cellular network environment and a predetermined number of sites are created. It is created according to hexagonal pattern and based on inter site distance (ISD). One site has three base stations where each base station uses 1200 directional antenna. Then the repeaters are placed at fixed distance from station (Rrepeater). There are two type of repeater deployment considered in this thesis as shown in Figure 4.2. The deployment in Figure 4.2(a) is regular deployment of repeaters with fixed distance from base station (if Rrepeater < 3cellR ), while in Figure 4.2(b) the repeaters deployment is parallel to

the two sides of the cell edge (if Rrepeater > 3cellR ). The second deployment is considered because the possible arc of circle to deploy the repeaters is smaller if Rrepeater is increased. In this illustration, we show the deployment of system in the cell radius 166m and there are 3 repeaters per cell.

Figure 4.2 Repeater deployment illustration

After deploying base stations and repeaters, the users are created throughout the

simulation area and the channel models as well as the frequency-independent fast fading gain are also initialized. Further explanation of these methods is given in the following section.

4.4.3 User Generation Users are uniformly distributed over the service area. New users are created simultaneously in the beginning of simulation and/or randomly according to Poisson processes at the initialized positions. Each user selects the base station according to cell selection algorithm which is based on the strongest long term path gain. The user can also be connected to a repeater if the composite path gain (the gain from BS through RN to the MS) is larger than direct path gain. A user may stay during entire simulation or may be removed according to some rules, i.e. after random lifetime or finish downloading.

(b) -400 -300 -200 -100 0 100 200 300 400

-400

-300

-200

-100

0

100

200

300

400Scenario deployment

Base StationRepeater

(a) -400 -300 -200 -100 0 100 200 300 400

-400

-300

-200

-100

0

100

200

300

400Scenario deployment

Base StationRepeater

31

The user moves with an average speed 3 km/h. Handover implemented in the system is hard handover, i.e. link to the old eNodeB is removed before the new link with other eNodeB is established. Maximum uplink power in the user equipment is 250 mW which is equivalent to 24 dBm.

4.5 Simulation Parameters The LTE system simulated in this thesis consists of 7 sites with 3 base stations per site which means in total there are 21 hexagonal cells. A wrap-around technique is used to avoid interference-free effects in the border area. This technique makes another virtual tier (6 virtual positions) generated around the original cell. The base stations and repeaters are created according to parameters in Table 4-1. The system is operated at 2 GHz frequency carrier and 5 MHz total bandwidth.

The complete simulation parameters considered in this thesis are provided in the following table.

Table 4-1 Simulation Parameters

Traffic and Mobility Models Traffic model File transfer (download/upload). File size 1 MB,

model includes TCP and protocol overhead User distribution Uniformly distributed in space according to an

intensity (i.e. 5 users per sec) and removed when file transfer completed

User speed 3 km/h Radio Network Models

Cell layout 21 hexagonal cells (7 sites, 3 base stations per site) Cell radius 166m (3GPP case 1), 577 m (3GPP case 3) Repeater

deployment Based on deployment shown in Figure 4.2,

250 m (3GPP case 1) and 800 m (3GPP case 3) from serving BS

Number of repeaters

3 repeaters per cell

Channel model Typical Urban Distance

dependent propagation

According to simplified equation on (4.3): PL = β + α*10log(d) Parameters used in WINNER: WINNER los: β = -40.5, α = -2.35 BS to MS: β = -40.5, α = -3.57 BS to RN: β = -40.5, α = -2.35 RN to MS: β = -40.5, α = -3.57 WINNER nlos: β = -40.5, α = -3.57 BS to MS: β = -40.5, α = -3.57 BS to RN: β = -40.5, α = -3.57 RN to MS: β = -40.5, α = -3.57

32

Parameters used in 3GPP TSG-RAN WG1: BS to MS: β = -30.6, α = -3.67 BS to RN: β = -11.7, α = -3.76 RN to MS: β = -30.6, α = -3.67 Parameters used in indoor model are the same with WINNER nlos with additional attenuation wall loss: Case 1: BS to MS: β = -50.5 Case 2: BS to MS and RN to MS: β = -50.5 Case 3: BS to MS and BS to RN: β = -50.5

Shadow fading Log-normal, dBSF 0=μ and dBSF 8=β Multipath fading SCM Suburban Macro or Urban Micro

AP Maximum Power

Base station: 20 W Repeater: 20 W UE: 250 mW

Maximum repeater gain

90 dB [24]

Repeater Gain Control

1. Time selectivity: Fast/Slow (maximum step up 3 dB, down 0.5 dB)

2. Frequency selectivity: Fine (one resource block granularity) / Coarse (5 MHz granularity)

Antenna configuration and

antenna gains

BS: 1 SCM antenna with sectorized antennas. No MIMO. Max gain: 16 dBi.

RN: 2 antennas (donor and service area antennas) of omni-directional type with 2 dBi gain. In case of directional antenna, the donor antenna has SCM pattern with max gain 10 dBi.

UE: 2 Rx antennas at the terminal. Omni antennas with 0 dBi gain

Scheduler DL: If a UE has enough data buffered it is allocated the entire bandwidth

UL: Both FDM and TDM scheduling (power limitations)

General System Models Simulation time 200 sec

Spectrum 5 MHz DL / 5 MHz UL Carrier Frequency 2 GHz

Number of subbands 25

Number of subcarriers per

subband 12

Number of total subcarriers 300

Number of OFDM symbols used for

PDCCH 3 symbols/subframe

Subframe rate 1000 subframes/sec

33

Modulation and

coding QPSK, 16QAM, and 64QAM

Turbo codes, rates: 0.05, 0.1, 0.14, 0.2, 0.25, 0.33, 0.4, 0.5, 0.6, 0.67, 0.75, 0.8, 0.89, 0.97

4.6 Performance Evaluation Several performance metrics are considered in the evaluation: SINR and cell throughput both in downlink and uplink, and object bitrate. The items are logged during simulation and then post processed. The explanation of these performance metrics is provided below.

4.6.1 SINR Calculation Downlink and Uplink SINR are calculated based on equation (3.2). The signals are measured per subband where combining method of direct path and repeater path are utilized. The total interference depends on repeater type used in the system. Always on repeater creates interference continuously but advanced repeater creates interference when it forwards the data on the same resource blocks. The main interest of evaluation is 5-percentile SINR which corresponds to users in the cell border or area which has limited SINR and mean SINR which corresponds to average SINR in the system.

4.6.2 Cell Throughput Cell downlink throughput is the total number of bits downloaded by users measured at a cell. In the uplink scenario, the definition is the same but it uses uploaded bits instead of received bits. The simulation is done with simulation time period and throughput is logged every 0.1 sec interval.

4.6.3 Object Bit Rate (OBR) The object bit rate is defined as the total number received bits per second received by a user and measured for every object / packet (in this case one TCP block data). In an equation, it is the packet size (including header) divided by time period needed (timeReceived - timeSent).

34

35

Chapter 5 SIMULATION RESULTS The simulation is implemented in the radio simulator platform as the explanation in the Section 4.4.1. This chapter presents the numerical results of several scenarios using parameters in Table 4-1. An example of the cellular network deployed in this thesis is drawn in Figure 5.1. In this figure, there are 7 sites with 3 base stations per site and 3 repeaters per cell. The deployment of repeater follows the pattern shown in Figure 4.2b.

Figure 5.1 Illustration of cellular network deployment with repeaters

The procedure of the simulation is to analyze the effect of one variable by setting

the others to the default parameters, and if it is possible to find the optimum value of this parameter. And then by using these results we analyze the other scenario and use the same procedure again. Simulation is done in 200 sec basis, and it is chosen in order to have a system with stable results. The comparison of simulation with 1000 sec and simulation with 200 sec is given in Appendix A and it is concluded that 200 sec is enough to give necessary results. The rest of this chapter is divided into several subsections. The first scenario is to analyze the impact of several propagation models, and then followed by examining

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Base StationRepeaterUser

36

number of repeaters per cell, repeater distance to base station, repeater gain, and finally the advanced repeater. Performance evaluation metrics used in the simulation are items stated in Section 4.6.

5.1 Propagation Model The evaluation of different propagation model is carried out in this section. The system is set up mainly based on parameters in Table 4-1. There are 7 sites, 3 base stations per site and 3 repeaters per cell in the system. Cell radius is 166 m and repeater distance from base station is 250 m. The users are generated with intensity 10 users/sec. The propagation models used in this scenario are either WINNER or 3GPP TSG-RAN WG1 model. We will use the terms: WINNER nlos and WINNER los to refer WINNER model and the corresponding BS – RN links for the rest of this report. The parameter of distance dependent path loss, α and β, according to equation (4.4) are set to values provided in Table 4-1. Repeater is configured as always on with 90 dB repeater gain. The donor antenna at repeater can be omni-directional antenna (repOmniDonorAntenna) with 2 dBi gain or directional antenna (repDirDonorAntenna) with SCM pattern and max gain of 10 dBi in forward direction.

Figure 5.2 shows the CDF of downlink SINR with different propagation model. Note that nlos and los correspond to propagation of BS – RN links in WINNER model. The reference curves are WINNER noRep and 3GPP noRep which are the performance of system without repeater. From this figure, the SINR in the WINNER los case is worse than WINNER nlos because the path loss exponent for LOS is lower than path loss exponent for NLOS propagation model and then both the desired and interference signals are received with higher signal strength at the repeater. In small cell radius (166 m), the effect of interference is much higher than the desired signal.

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SINR (dB)

CD

F

CDF of downlink SINR

WINNER noRepWINNER, nlos, repOmniDonorAntennaWINNER, nlos, repDirDonorAntennaWINNER, los, repOmniDonorAntennaWINNER, los, repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure 5.2 A comparison of CDF downlink SINR for different propagation model

All scenarios which we simulate in Figure 5.2 show that systems with repeaters

using omni-donor antenna (repOmniDonorAntenna) are worse than systems without repeater (noRep) for both WINNER and 3GPP propagation models. It means that repeaters do not give beneficial gain to the system. In order to have a gain when

37

using repeater, it is necessary to have better path gain in two-hop links than the path gain in the direct link. One way to improve this is by using directional donor antenna at repeater which affects the BS – RN links and another way is to use directional service antenna at repeater which affects the RN – MS links. The intuitive issues for the second approach are the changes of repeater coverage and spatial separation between donor and service antenna if they face the same direction. Since the users in this simulation are generated randomly, we will only consider the first approach and the second approach will not be in the scope of this thesis.

As shown in Figure 5.2, the directional donor antenna at the repeater enhances the SINR. System with repeater directional donor antenna performs better than system without repeater for both in WINNER nlos propagation and 3GPP propagation models. Furthermore repeater directional donor antenna degrades users having low-percentile SINR because it also amplifies the interference. Therefore it is important to know that directional donor antenna at repeater gives additional gain only for specific cases. In WINNER los propagation, it is clear that the repeater does not bring gain even with repeater directional donor antenna. This will be analyzed and explained later in Section 5.5 with other cell loads.

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SINR (dB)

CD

F

CDF of uplink SINR

WINNER noRep

WINNER, nlos, repOmniDonorAntennaWINNER, nlos, repDirDonorAntenna

WINNER, los, repOmniDonorAntenna

WINNER, los, repDirDonorAntenna

3GPP noRep3GPP repOmniDonorAntenna

3GPP repDirDonorAntenna

Figure 5.3 CDF of uplink SINR for different propagation model

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Object bit rate (bps)

CD

F

CDF of Object Bit Rate

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure 5.4 CDF of object bit rate (OBR) for different propagation model

38

Figure 5.3 and Figure 5.4 show the CDF of uplink SINR and the object bit rate (OBR) for this scenario. The results are similar to Figure 5.2. Other results using advanced fast fine resolution repeater, cell radius of 577 m, and simple indoor propagation models can be found in Appendix B. From these results, we can derive a conclusion that system with repeaters performs better/worse than system without repeater depending on propagation model used. Directional donor antenna at repeater can generally give further gain to improve BS – RN links with possibility to lose the performance of users with low-percentile SINR.

5.2 Number of Repeaters per Cell In this section, the effect of number of repeater per cell is analyzed. Most of the parameters are aligned with Table 4-1. First we look into three numbers of repeaters per cell (3, 6 and 9 repeaters per cell) in different cell loads, i.e. 3, 5, 7, 10, 11, 13, and 15 users/sec. Two types of repeaters considered here are always on and advanced fast fine resolution repeaters. Cell radius is 166 m and WINNER nlos propagation model is used. The results are given in Figure 5.5 and Figure 5.6 where the x-axis shows mean cell downlink throughput which also corresponds to cell loads. The same results for cell radius 577 m are also provided in Appendix C.

From Figure 5.5 and Figure 5.6, there is limited gain when we increase number of always on repeaters or it is basically no difference in performance between 6 and 9 repeaters per cell. And with advanced repeaters, some additional gain can be achieved with more repeaters, i.e. from Figure 5.6 at intensity 10 users/sec the gain from 3reps/cell to 6 reps/cell using fast fine repeater is 725 kbps which is higher than 320 kbps, the gain using always on repeater. A threshold also exists if we increase

RN further in which the performance will be worse. In our results, 9 repeaters per cell has a similar performance or slightly worse than 6 repeaters per cell. Therefore the threshold could be in the range of 6 – 9 repeaters per cell.

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no repeateralways on, 3reps/cellalways on, 6reps/cellalways on, 9reps/cellfast-fine, 3reps/cellfast-fine, 6reps/cellfast-fine, 9reps/cell

Figure 5.5 Five-percentile downlink object bit rate (OBR) for different number of repeaters per cell

39

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no repeateralways on, 3reps/cellalways on, 6reps/cellalways on, 9reps/cellfast-fine, 3reps/cellfast-fine, 6reps/cellfast-fine, 9reps/cell

Figure 5.6 Mean uplink object bit rate (OBR) for different number of repeaters per cell

Second, we will analyze several scenarios with numbers of repeaters per cell

RN (0, 1, 2, 3, 4, 5 and 6 repeaters per cell) in one cell load, i.e. 10 users/sec. The results and scenario details are provided in Appendix C. From these results, it is of course possible to optimize the number of repeaters per cell ( )max,RN based on other pre-defined settings such as propagation model, cell loads, repeater distance from base station, repeater types and repeater gain. In this case, we only show WINNER nlos model. Therefore it is quite difficult to conclude max,RN , but it can be seen from results in Appendix C that 3 repeaters per cell is a reasonable number considering all curves in Figure C.1 and Figure C.2. This number is also the number of repeaters used in simulation parameter at [12] and [14].

5.3 Repeater Distance from Base Station In this thesis, a homogeneous traffic system is considered hence the most efficient way to deploy the repeaters is by using a regular pattern with fixed distance from the serving base station. The repeater deployment is based on Figure 4.2 in Section 4.4.2. To emphasize the effect of repeater deployment in a cell and the handover done in the system, a simple case is considered first. In this scenario, the number of site is one with 3 base stations and only 1 repeater per cell is deployed in a fixed distance from BS of 250 m. There is one user in the system placed initially in the origin (x=0 and y=0). This user moves toward repeater (along the green line) with speed of 3 km/hour and continues forward till handed over to the other cells. Wrap around technique is used here therefore this user will always be in the system.

Figure 5.7 shows the path gain for this scenario. The directPathGain shown by solid blue line is the path gain of serving BS to user link and compositePathGain is the path gain of serving BS to user through repeater link. There are five compositePathGain in the Figure 5.7 which correspond to repeater deployment distance: 250 m, 270 m, 290 m, 310 m and 330 m. Handover criterion is based on maximum path gain between directPathGain and compositePathGain. The result shows that the user connects to cell0 first, then at r = 120m (approx.) it uses

40

compositePathGain from repeater0. We can also observe that a repeater works within radius of 110 m (approx.). If the repeater distance from BS is increased, the serving area of repeater will follow its location. Therefore the compositePathGain moves to right direction and follows repeater location when repeater distance from BS is increased. Similar result is shown in Appendix B for 3GPP propagation model.

Figure 5.7 Repeater path gain of a moving user for different repeater deployment distance

The scenario of multiusers environment is now considered. In this scenario, the

users are generated with intensity of 5 users/sec. There are 7 sites, 3 base stations per site and 3 repeaters per cell in the system. Deployment and other related parameters are based on Table 4-1. In addition, BS to RN link is modeled as WINNER nlos propagation model. The result of this simulation case, Figure D.2, is provided in Appendix D. Based on this figure, the SINR of system with repeater deployed around 210-230 m outperforms the other deployments, but they slightly differ one to the others. This happens because the repeater distance only changes the location of the service area. And when the users are generated randomly with uniform distribution in the system, the effect of repeater location is not really significant. In the case of system with other user distribution, repeater location is an important issue and the results could have an optimum repeater distance from BS. Hence repeater planning is recommended. There is a recommendation of repeater location as shown in the result of [12]. It is 1.4*cellradius in the 3-sectorized base stations case. Therefore for the rest of our simulations, we consider the repeater distance from base station as 250 m (3GPP case 1) and 700 m (3GPP case 1).

5.4 Repeater Gain One important issue in the repeater scenario is to control the gain which is used for the amplification when forwarding the signal in the downlink and uplink. If the repeater gain is too large, there is a high probability that repeaters amplify a lot of interference in the system. Therefore it is important to control the repeater gain in order to optimize the overall system performance.

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directPathGain 250m 270m 290m 310m 330m

Repeater Position

cell 0 cell 1

cell 2

41

One simple scenario is considered first to analyze the effect of repeater gain. Most of parameters are the same with the simple case used in the previous section where one user walks towards a repeater in a system which has 1 site and 3 base stations per site. The maximum repeater gain is 90 dB [24] as already explained in the section 4.3, and some other values as well: 80 dB, 70 dB, 60 dB and 50 dB. The result of this scenario is drawn in the Figure 5.8. Additional results of this scenario in WINNER nlos and 3GPP propagation model are provided in Appendix E.

It is clear that the serving area of a repeater depends on repeater gain as shown in Figure 5.8. When the repeater gain is increased, the composite pathgain and the radius of service area are also increased. Therefore increasing the repeater gain can increase the coverage of the repeater. This is true because we assume a rather simple case where there are no other users and cells generating interference. If the multicells and multiusers environment is used, the interference gives more contribution in reducing the received SINR. From this figure, we can observe that a minimum 60 dB repeater gain must be fulfilled in WINNER los in order to utilize repeaters (used/chosen by users) and 80 dB repeater gain is needed in WINNER nlos. In WINNER nlos propagation environment, two-hop links are simply worse than direct link therefore it is necessary to have high repeater gain. To improve these links, one can use directional donor antenna at the repeaters as shown in Figure 5.8.

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Pathgain for different downlink repeater gain in LOS propagation

directPathGain90 dB80 dB70 dB60 dB50 dB

Figure 5.8 Direct and Composite Path Gain for different downlink repeater gain

in WINNER LOS propagation of BS to RN links The effect of repeater gain in multiusers scenario is now analyzed. Deployment

and other related parameters are based on Table 4-1. The users are generated with intensity of 3, 5, 7, 10, 11, 13, and 15 users/sec. Two types of repeaters considered here are always on and advanced fast fine resolution repeaters. In addition, BS to RN link is modeled as WINNER nlos propagation model and cell radius is 166 m. The results are given in Figure 5.9 and Figure 5.10.

Figure 5.9 and Figure 5.10 are similar with Figure 5.5 and Figure 5.6 in some way. In these figures, we can observe that advanced fast fine repeater provide higher additional gain than always on repeater when we increase the repeater gain. From Figure 5.10 with intensity 10 users/sec, increasing the repeater gain from 80 dB to 90 dB results in an object bit rate gain of 935 kbps (fast fine repeater) and 741 kbps (always on repeater). It is also obvious that reducing the gain results in performance

42

degradation and the result goes to system without repeater. Hence there is a minimum repeater gain which must be fulfilled in order to utilize repeaters in the system. In this case (WINNER nlos propagation model with cell radius 166 m), the system with repeater gain of 70 dB performs similarly with system without repeater and 80 dB is the minimum repeater gain needed.

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no repeateralways on, 90 dBalways on, 80 dBalways on, 70 dBfast-fine, 90 dBfast-fine, 80 dBfast-fine, 70 dB

Figure 5.9 Mean downlink OBR for different downlink repeater gain

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Figure 5.10 Mean downlink OBR for different downlink repeater gain

It is important to note that there exist repeaters with 70-80 dB in UMTS [27]

[28], therefore it is reasonable to assume a repeater in LTE working with 80-90 dB. However in order to have worse case, in the simulation the maximum repeater gain should be taken as minimum as possible, i.e. 80 dB.

43

5.5 Advanced Repeater The advanced repeater with frequency selective functionality is evaluated in this section. First the system is set up based on parameters in Table 4-1 with scenario of multiusers environment. The users are generated with intensity of 3, 5, 7, 10, 11, 13, and 15 users/sec. There are 7 sites, 3 base stations per site and 3 repeaters per cell in the system. The cell radius and repeater distance from BS are 166 m and 250 m respectively (3GPP case 1) or 577 m and 700 m (3GPP case 3). Repeaters are placed in fixed distance from BS and they have 90 dB repeater gain. There are 5 different repeaters considered in this simulation as already explained in Section 3.3.1 which are always on, slow coarse, slow fine, fast coarse, and fast fine repeaters. Propagation models used in this simulation are WINNER and 3GPP models. It will be shown later in this section that different propagation models give different results.

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Figure 5.11 Five-percentile downlink SINR vs mean cell downlink throughput

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Figure 5.12 Mean downlink SINR vs mean cell downlink throughput

44

Cell radius considered first is 166 m which corresponds to repeater distance from BS of 250 m. The system is modeled with WINNER NLOS propagation (BS to RN links) and the results are shown in Figure 5.11, Figure 5.12, Figure 5.13, and Figure 5.14. In the x-axis the figure shows mean cell downlink throughput which corresponds to cell load or user intensity.

Figure 5.11 and Figure 5.12 show that the SINR of system with advanced repeater is higher than system with conventional repeater (always on) and system without repeater. The gain in 5-percentile SINR by using fast-fine repeater compare to no repeater case is 2.4 dB at both low load (3 users/sec) and high load (11 users/sec). In the mean downlink SINR, the gain is 2.9 dB and 5.4 dB for low load and high load respectively. We can observe that the SINR will be saturated at high load because at this moment the number of users in the system is high and the period length of a user occupying the system is long. Therefore adding more users in the system gives similar amount of interference.

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Figure 5.13 Five-percentile OBR vs mean cell downlink throughput

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Figure 5.14 Mean OBR vs mean cell downlink throughput

The object bit rate is shown in Figure 5.13 and Figure 5.14. From these figures,

conventional repeater brings additional gain compare to system without repeater and

45

repeater with advanced functionality will enhance the performance further. The gain is generally due to improved geometry. We can observe that “Fine” and “coarse” repeater perform almost identically. In high cell loads, it is obvious if fine and coarse repeater perform similarly because at this point there are a lot of users occupied the system and most of the channels are used. In low cell loads scenario the downlink scheduler allocates the entire bandwidth to the user if there is enough data in the buffer. Therefore “fine” repeater will use and forward the entire bandwidth resulting similar results as “coarse” repeater. Another reason is that the transmissions are rarely power limited.

“Fast” repeater performs slightly better than “slow” repeater as expected because “fast” repeater allows itself to increase the power up to maximum in the next TTI but “slow” repeater needs a ramping to do it. In some cases “fast” repeater performs similar or worse than “slow” repeater probably due to unpredictable interference which neighboring cell terminals might suffer from (e.g. degraded link adaptation).

It has been explained above that system with advanced repeater outperforms

system with conventional repeater (always on) and system without repeater in WINNER nlos propagation environment and cell radius of 166 m. The results of other propagation models (WINNER los propagation, 3GPP propagation and indoor propagation model) in two cell radius (166 m and 577 m) can be found in Appendix D. In addition, a simple conclusion is made in Table 5-1 based on these results where repeaters give significant gain or not. The term “significant” is defined if the gain of object bit rate in one user intensity (10 users/sec) is more than 20% and the system with advanced repeater gives better OBR than system with always-on repeater. The analysis of this table is given below.

Table 5-1 Performance of repeaters in downlink for different propagation models

166 m

(3GPP case 1)577 m

(3GPP case 3)

WINNER model (NLOS for BS to RN links) √ ─

WINNER model (LOS for BS to RN links) ─ √

3GPP model √ limited

Indoor propagation case 1 model √ ─

Indoor propagation case 2 and case 3 model √ ─

√ repeaters bring significant gain (more than 20% OBR) - repeaters do not bring significant gain

In cell radius 166 m, repeaters in system with WINNER LOS propagation model do not give improvement (even with directional donor antenna at repeater). In this scenario, cells are close to each other which make the path gain from BS to RN become higher (or lower path loss). Then all signals coming to repeater, both desired signals and interference, are received with good power reception. Therefore the effect of interference is dominant and the SINR at user terminal is generally bad. This effect is not happened in WINNER NLOS propagation, 3GPP propagation and indoor propagation models because these models have higher path loss than LOS propagation model (in BS to RN links) and they can reduce the interference. To

Cell Radius Propagation Model

46

improve the desired signal the repeater directional donor antenna can be used and it may give more gain than the system with repeater omni-directional antenna.

The interesting results come when we simulate the scenario of 577 m cell radius. In contrary with 166 m cell radius, the repeaters work the other way around. In WINNER nlos propagation and indoor propagation models, the repeaters do not give gain compare to system without repeater (even by employing directional donor antenna at repeater). But the repeaters bring some gain in WINNER los propagation and 3GPP propagation model. The reason can be explained in the following and more details can be found in Appendix F (Table F.1 and F.2). In the WINNER nlos propagation and indoor propagation cases, the composite signal is worse than the direct link therefore the users most likely use the single-hop (direct link). In WINNER los propagation and 3GPP, the composite signal is better than the direct link. Furthermore, it is shown from simple distance dependent analysis where a user is placed at the cell border shows that the BS – RN link in WINNER los is 10 dB better than the BS – RN link in 3GPP link. Therefore the gain of using repeater in WINNER los propagation is higher than in 3GPP propagation model.

From all of these explanations, we can finally give a short review. In two-hop communication downlink model, it is necessary that BS to RN links (with or without repeater directional donor antenna gain) must be good enough both for amplifying the desired signal and attenuating the interference. It has been explained in section 5.3 and 5.4 that the composite pathgain (two-hop links) must be better than direct pathgain (single link) in order to use the repeaters. And then it has also proved that repeater directional donor antenna bring additional gain to the system in general cases.

5.5.1 Upload Scenario We have been looking through the downlink cases and now we will show the result of an upload scenario for system with WINNER propagation model (NLOS for BS to RN links) and cell radius 166 m. The results for this case are given in the Figure 5.15 and Figure 5.16 which are similar to downlink case shown in Figure 5.11 Figure 5.14. Note for this scenario, the cell loads considered are 3, 5, 7, 10 and 11 users/sec.

A simple conclusion for uplink is made in Table 5-2 based on these results where repeaters give significant gain or not. The analysis of these results is similar to downlink cases which are provided before, but more explanation is given below.

The results for uplink case in Table 5-2 show the similarity with the downlink case however repeaters in uplink are more favorable than downlink where power limitation is the bottleneck. The benefit of advanced repeaters is larger for uplink than for downlink as shown in the result of Table F.3 and Table F.4 in Appendix F. We can also observe that uplink case in cell radius 577 m gives slightly different result with downlink case. In uplink, repeater in WINNER LOS propagation model gives less gain than repeater in 3GPP propagation model which is the opposite result of downlink case. A simple distance dependent analysis where a user is placed at the cell border shows that the MS – RN link in 3GPP propagation model is 7 dB better than the MS – RN link in WINNER LOS propagation model. In this case, it might be that desired signal in winner los propagation model is received with very low power at repeater. Furthermore system in WINNER LOS propagation model gives higher interference than system in 3GPP model.

47

1 1.5 2 2.5 3

x 106

0

0.5

1

1.5

2

2.5

3

3.5

4x 10

6

Mean Uplink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean uplink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure 5.15 Five-percentile OBR vs mean cell throughput

1 1.5 2 2.5 3 3.5

x 106

1

2

3

4

5

6

7

8

9x 106

Mean Uplink Throughput (bps/cell)

Mea

n O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean uplink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure 5.16 Mean OBR vs mean cell throughput

Table 5-2 Performance of repeaters in uplink for different propagation models

166 m

(3GPP case 1)577 m

(3GPP case 3)

WINNER model (NLOS for BS to RN links) √ ─

WINNER model (LOS for BS to RN links) ─ limited

3GPP model √ √

Indoor propagation case 1 model √ ─

Indoor propagation case 2 model √ ─ √ repeaters bring significant gain (more than 20% OBR) - repeaters do not bring significant gain

Cell Radius Propagation Model

48

In LTE system, it is preferable to have repeater in uplink scenario because the control signal in downlink is transmitted in the same resource block as the data use and the control signal in uplink is transmitted several timeslot before the data. It means that repeater need to extract the control signal and it must be done less than the cyclic prefix in the downlink. Moreover the downlink scheduler may assign all possible resources to the users if there is available data to be transmitted in the buffer which gives a similar result for both fine resolution repeater and coarse repeater.

5.5.2 Repeater Activity One advantage of using advanced repeaters is the low consumption power or energy efficiency compare to always on repeater which has 100% activity in time domain. Here, we show the repeater activity in one scenario: cell radius 166 m and WINNER nlos propagation model. In downlink, we can reduce the activity of a repeater up to 90% and in uplink we can reduce the repeater activity up to 55% for the high load. Hence it is clear that advanced repeater can decrease power consumption.

1 2 3 4 5 6 70

10

20

30

40

50

60

70

80

90

100

Increasing load

Rep

eate

r act

ivity

in %

Repeater activity logged every 100 ms

always onfast-fine, DLfast-fine, UL

Figure 5.17 Repeater activity in cell radius 166 m and WINNER nlos propagation model scenario

The activity in uplink is higher than the activity in downlink because this is only

the activity of repeater in time domain and the scheduler in LTE uplink may allocate several users

49

Chapter 6 CONCLUSION

6.1 Conclusion Simulation results show that the performance of repeater in 3GPP LTE is highly dependent on propagation model used in the simulation and it depends on real environment when it comes to implementation. In cell radius 166 m (3GPP case 1), repeaters bring significant gain in WINNER nlos propagation model (non-line of sight (NLOS) for base station (BS) to repeater node (RN) links) and 3GPP propagation model, but they degrade the performance in WINNER los propagation model (line of sight (LOS) for BS to RN links). Moreover in cell radius 577 m (3GPP case 3), repeaters work the other way where they bring improvement in both WINNER los propagation model and 3GPP propagation model but they do not give significant gain in WINNER nlos propagation model. This is because in two-hop communication model, it is necessary that BS to RN links (for downlink) or RN to MS links (for uplink) must be good enough for amplifying the desired signal and attenuating the interference. And then the composite path gain (two-hop links) must be better than the direct path gain (direct link) in order to use the repeaters. Furthermore it has also proved that repeater directional donor antenna bring additional gain to the system in general cases, especially for large cell radius.

Simulation results show that having advanced functionalities in the repeater gives additional gain compare to system with conventional repeaters. Fine resolution repeater performs only slightly better than coarse repeater because the downlink scheduler may allocate the entire bandwidth to the user if there is enough data to be transmitted. Fast repeater outperforms slow repeater because fast repeater allows itself to increase the power until maximum in the next TTI but slow repeater needs to increase/decrease stepwise.

Finally the repeater deployment parameters such as number of repeaters per cell and repeater distance from base station depend on several factors, i.e. the propagation model, advanced functionalities at repeater such as repeater frequency selectivity and repeater gain. We have shown through several settings parameter that it is possible to optimize the number of repeaters per cell or repeater distance from base station. However scenarios with different settings give different result. Therefore it is important to define assumptions and model used and to have good planning in deploying repeaters in the real network.

50

6.2 Future Work In this work, we use propagation model based on WINNER project and 3GPP model. The model is intended to be used for system with repeater (LTE Advanced) in outdoor scenario. The standardization for this propagation model is still on the progress and it may change depending on simulation results and measurements in reality. And our simulation results show different propagation model gives different performance result. Therefore more suitable model and indoor propagation scenario can be part of future work. Other future work may include less ideal repeater assumptions (non-ideal isolation, inter-repeater interference) where in our case the repeater is ideal with good isolation. More sophisticated gain and power control algorithms at the repeaters (in collaboration with eNBs and MSs) can be employed in order to improve system performance. And finally the heterogeneous users distribution with heterogeneous repeater deployment would be interesting to analyze.

51

Appendix A – Simulation time This section provides a comparison of result with simulation time 200 sec and 1000 sec. The simulation is done based on parameters in Table 4-1. Propagation model is WINNER los propagation (BS to RN links) and cell radius is 577 m (3GPP case 3). Small difference in the curves is expected but they give similar results. Hence we will do the rest of simulation in 200 sec.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 106

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 106

Mean cell downlink throughput (bps/cell)

5-pe

rc o

bjec

t bit

rate

(bps

)

Obr vs mean cell downlink throughput

no repeater, 200secno repeater, 1000secfast-fine, 200secfast-fine, 1000sec

Figure A.1 Five-percentile OBR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 106

0

1

2

3

4

5

6

7

8

9

10x 106

Mean cell downlink throughput (bps/cell)

Mea

n ob

ject

bit

rate

(bps

)

Obr vs mean cell downlink throughput

no repeater, 200secno repeater, 1000secfast-fine, 200secfast-fine, 1000sec

Figure A.2 Mean OBR vs mean cell downlink throughput

52

Appendix B – Propagation Model In section 5.1, we have simulated different propagation model with always on repeater in cell radius 166 m. This section gives the results when using advanced fast fine resolution repeater in cell radius 166 m and 577 m, and using always on repeater in cell radius 577 m.

-10 0 10 20 30 400

10

20

30

40

50

60

70

80

90

100

SINR (dB)

CD

F

CDF of downlink SINR

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.1 CDF of downlink SINR for different propagation model using advanced fast fine resolution repeater and cell radius of 166 m

0 2 4 6 8 10 12

x 106

0

10

20

30

40

50

60

70

80

90

100

Object bit rate (bps)

CD

F

CDF of Object Bit Rate

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.2 CDF of OBR for different propagation model

using advanced fast fine resolution repeater and cell radius of 166 m

53

-5 0 5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

SINR (dB)

CD

F

CDF of downlink SINR

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.3 CDF of downlink SINR for different propagation model

using always on repeater and cell radius of 577 m

0 2 4 6 8 10 12

x 106

0

10

20

30

40

50

60

70

80

90

100

Object bit rate (bps)

CD

F

CDF of Object Bit Rate

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.4 CDF of OBR for different propagation model

using always on repeater and cell radius of 577 m

-5 0 5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

SINR (dB)

CD

F

CDF of downlink SINR

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.5 CDF of downlink SINR for different propagation model using advanced fast fine resolution repeater and cell radius of 577 m

54

0 2 4 6 8 10 12

x 106

0

10

20

30

40

50

60

70

80

90

100

Object bit rate (bps)

CD

F

CDF of Object Bit Rate

WINNER noRepWINNER nlos repOmniDonorAntennaWINNER nlos repDirDonorAntennaWINNER los repOmniDonorAntennaWINNER los repDirDonorAntenna3GPP noRep3GPP repOmniDonorAntenna3GPP repDirDonorAntenna

Figure B.6 CDF of OBR for different propagation model

using advanced fast fine resolution repeater and cell radius of 577 m

-5 0 5 10 15 20 25 30 350

10

20

30

40

50

60

70

80

90

100

SINR (dB)

CD

F

CDF of downlink SINR

WINNER noRep, Rcell = 166mWINNER noRep, Rcell = 577m3GPP noRep,Rcell = 166m3GPP noRep,Rcell = 577mIndoor, WINNER noRepIndoor case1, repDirDonorAntennaIndoor case2, repDirDonorAntenna

Figure B.7 CDF of downlink SINR for different propagation model

using always on repeater and cell radius of 166 m (except stated in legend)

55

Appendix C – Number of Repeaters per Cell The numbers of repeater per cell that will be analyzed in this simulation are 0, 1, 2, 3, 4, 5 and 6. The simulation is done in WINNER propagation model and the users are generated with intensity 10 users/sec. The results are shown in Figure C.1 and Figure C.2 which correspond to the 5-percentile and mean downlink SINR for different number of repeaters per cell. Table C-1 shows the parameters of scenario a, b, c, d, e and f used in the figures.

0 1 2 3 4 5 6

2

4

6

8

10

12

Number of repeaters per cell vs 5-percentile SINR

Number of repeaters per cell

5-pe

rcen

tile

SIN

R (d

B)

abcdef

Figure C.1 The 5-percentile downlink SINR with different number of repeaters per cell

0 1 2 3 4 5 612

14

16

18

20

22

24

26

28

30

32

34

Number of repeaters per cell

Mea

n S

INR

(dB

)

Number of repeaters per cell vs Mean Downlink SINR

a b c d e f

Figure C.2 Mean downlink SINR with different number of repeaters per cell

56

Table C-1 Parameters used in Figure C.1 and Figure C.2

Curve Directional

Donor Antenna

Propagation model

Intensity (users/sec) Repeater State

a Yes LOS 5.0 Advanced Fast Fine Resolution b Yes LOS 5.0 Always on c No NLOS 5.0 Always on d Yes LOS 10.0 Advanced Fast Fine Resolution e No NLOS 10.0 Always on f Yes LOS 10.0 Always on

Note that: propagation model used is WINNER model. LOS and NLOS correspond to BS to RS links. From Figure C.1 and Figure C.2, we can see that the number of repeaters per cell

( )RN varies on different scenario settings. First, we can observe that system with higher user intensity generally gives lower SINR (the upper three curves are the results from system with intensity 5.0 users/cell and the lower three curves are from 10.0 users/cell). Second, “advanced fast fine resolution” repeater has better performance than “always on” repeater. More explanation of advanced repeater which is the main contribution of this thesis can be read in section 5.5.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

0

1

2

3

4

5

6

7x 106

Mean Downlink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways on, 3reps/cellalways on, 6reps/cellalways on, 9reps/cellfast-fine, 3reps/cellfast-fine, 6reps/cellfast-fine, 9reps/cell

Figure C.3 Five-percentile downlink OBR for different number of repeaters/cell in cell radius 577 m

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

0

2

4

6

8

10

12x 106

Mean Downlink Throughput (bps/cell)

Mea

n O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways on, 3reps/cellalways on, 6reps/cellalways on, 9reps/cellfast-fine, 3reps/cellfast-fine, 6reps/cellfast-fine, 9reps/cell

Figure C.4 Mean downlink OBR for different number of repeaters/cell in cell radius 577 m

57

Appendix D – Repeater Distance This section gives additional results of section 5.3 where we have simulated different repeater distance in WINNER propagation environment. Figure D.1 shows similar result when using 3GPP propagation model and Figure D.2 shows multiusers environment. Detail analysis can be read further in section 5.3.

0 50 100 150 200 250 300 350 400 450 500-120

-110

-100

-90

-80

-70

-60

Distance from base station (m)

Pat

hGai

n (d

B)

Repeater pathgain with different deployment distance in 3GPP propagation model

directPathGain250 m

270 m290 m310 m

330 m

Figure D.1 Repeater Path Gain with different deployment distance in 3GPP propagation model

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

0

2

4

6

8

10

12x 106

Mean cell downlink throughput (bps/cell)

Mea

n ob

ject

bit

rate

(bps

)

Obr vs mean cell downlink throughput

no repeateralways on, 3rep, 190malways on, 3rep, 210malways on, 3rep, 230malways on, 3rep, 250malways on, 3rep, 290m

Figure D.2 Mean OBR vs mean cell downlink throughput

for different repeater distance in cell radius 166 m

58

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 106

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1

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3

4

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7

8

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11x 106

Mean cell downlink throughput (bps/cell)

Mea

n ob

ject

bit

rate

(bps

)

Obr vs mean cell downlink throughput

no repeateralways on, 3rep, 500malways on, 3rep, 600malways on, 3rep, 700malways on, 3rep, 800malways on, 3rep, 900m

Figure D.3 Mean OBR vs mean cell downlink throughput

for different repeater distance in cell radius 577 m

59

Appendix E – Repeater Gain Figure E.1 in 0% load (one user) and WINNER NLOS propagation shows the result which is similar to Figure 5.8.

0 50 100 150 200 250 300 350 400 450 500-200

-180

-160

-140

-120

-100

-80

-60

Pathgain for different downlink repeater gain in NLOS propagationwith directed donor antenna at repeater

Distance from base station (m)

Pat

hgai

n (d

B)

directPathGain90 dB80 dB70 dB60 dB50 dB

Figure E.1 Direct and Composite Path Gain for different downlink repeater gain in 0% load

Multiusers scenario (100% loads) is considered here for different repeater gain in WINNER LOS propagation model and cell radius 577 m. The results are shown in Figure E.2 and E.3.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 106

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5x 10

6

Mean Downlink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways on, 90 dBalways on, 80 dBalways on, 70 dBfast-fine, 90 dBfast-fine, 80 dBfast-fine, 70 dB

Figure E.2 Five-percentile downlink OBR for different downlink repeater gain in cell radius 577 m

60

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

x 106

1

2

3

4

5

6

7

8

9

10x 106

Mean Downlink Throughput (bps/cell)

Mea

n O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways on, 90 dBalways on, 80 dBalways on, 70 dBfast-fine, 90 dBfast-fine, 80 dBfast-fine, 70 dB

Figure E.3 Mean downlink OBR for different downlink repeater gain in cell radius 577 m

Figure E.2 and E.3 show that 80 dB repeater gain is the optimum gain and

increasing the gain to 90 dB gives worse result. The scenario of cell radius 166 m shown in Figure 5.9 and Figure 5.10 where 90 dB repeater gain outperforms 80 dB and 70 dB scenarios. This is because repeater forwards both desired signal and interference and therefore a higher repeater gain does not always give better performance.

61

Appendix F – Advanced Repeater This section gives additional results of section 5.5 where we have simulated and analyzed the performance of advanced repeaters in different propagation environment (WINNER NLOS propagation model, WINNER LOS propagation model, 3GPP model and indoor propagation case 1 and case 2 model) and cell radius (166 m and 577 m). The result of system in WINNER NLOS propagation model (BS to RN links) and cell radius 166 m has been shown in section 5.5. The rest will be shown here (F.1 to F.9) which then gives 10 combination scenarios in total. Note that these are downlink cases, and uplink cases for selected scenarios will be shown after. F.1 Cell radius = 166 m and WINNER LOS propagation model

Figure F.1, F.2, F.3 and F.4 show the result of cellular system with cell radius 166 m and different type of repeaters in WINNER model (LOS propagation for BS to RN links). The simulations are done with several cell loads by generating different user intensities, i.e. 3, 5, 7, 10, 11, 13, 15 users/sec. In the x-axis the figure shows mean cell downlink throughput which is increased as we increase cell load (user intensity) and in the y-axis the figure shows the SINR or object bit rate (OBR).

1 1.5 2 2.5 3 3.5 4 4.5 5

x 106

0

5

10

15

20

25

30

Mean Downlink Throughput (bps/cell)

Mea

n D

ownl

ink

SIN

R (d

B)

Downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.1 Mean downlink SINR vs mean cell downlink throughput

The first impression that we can observe from Figure F.1 is that advanced repeaters generally perform as good as system without repeater. In the always on repeaters, the SINR performance has a large gap compare to system without repeaters. In this case, the users are highly affected by interference from other repeaters and base stations (downlink scenario). Advanced repeater can further improve the SINR which makes the result closer to system without repeater and it is even better in the low load

62

scenario. Note that this figure only shows mean downlink SINR and does not correspond to all users. In fact, users with low-percentile SINR get worse SINR in all repeater type than system without repeater as shown by Figure F.2. The 5-percentile and mean object bit-rate (OBR) of this scenario are given in Figure F.3 and F.4. From these figure, it is showed that increasing the mean downlink SINR by using advanced repeaters does not give direct effect to throughput or OBR. The throughput relies on links adaptation, source coding, etc. From these figures, it can be concluded that repeaters do not give improvement even with directional donor antenna at repeater in system with cell radius 166 m and WINNER model (LOS propagation for BS to RS links).

1 1.5 2 2.5 3 3.5 4 4.5 5

x 106

-8

-6

-4

-2

0

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6

8

10

Mean Downlink Throughput (bps/cell)

5-pe

rc D

ownl

ink

SIN

R (d

B)

Downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.2 Five-percentile downlink SINR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5

x 106

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3

4

5

6x 106

Mean Downlink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.3 Five-percentile OBR vs mean cell downlink throughput

63

1 1.5 2 2.5 3 3.5 4 4.5 5

x 106

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8

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12x 106

Mean Downlink Throughput (bps/cell)

Mea

n O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.4 Mean OBR vs mean cell downlink throughput

F.2 Cell radius = 166 m and 3GPP propagation model

If we use 3GPP propagation model as explained in Table 4-1, the results are similar with WINNER NLOS propagation model (for BS to RS links). Figure F.5, F.6, F.7, and F.8 show the results of this scenario.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

-10

-5

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15

Mean Downlink Throughput (bps/cell)

5-pe

rc D

ownl

ink

SIN

R (d

B)

Downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.5 Five percentile SINR vs mean cell downlink throughput

64

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

5

10

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30

35

Mean Downlink Throughput (bps/cell)

Mea

n D

ownl

ink

SIN

R (d

B)

Mean downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.6 Mean percentile SINR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

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8x 106

Mean Downlink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.7 Five percentile OBR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

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12x 10

6

Mean Downlink Throughput (bps/cell)

Mea

n O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.8 Mean OBR vs mean cell downlink throughput

65

F.3 Cell radius = 166 m and Indoor propagation case 1 model

The results of system in indoor propagation model and cell radius 166 m are given in this section. Figure F.9, F.10, F.11 and F.12 show the results of this scenario.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

-8

-6

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-2

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12

Mean Cell Downlink Throughput (bps/cell)

5-pe

rc D

ownl

ink

SIN

R (d

B)

Downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.9 Five percentile SINR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

5

10

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20

25

30

Mean Cell Downlink Throughput (bps/cell)

Mea

n D

ownl

ink

SIN

R (d

B)

Mean downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.10 Mean percentile SINR vs mean cell downlink throughput

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

x 106

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5

6

7

8x 10

6

Mean Cell Downlink Throughput (bps/cell)

5-pe

rc O

bjec

t Bit

Rat

e (b

ps)

Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.11 Five percentile OBR vs mean cell downlink throughput

66

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Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.12 Mean OBR vs mean cell downlink throughput

F.4 Cell radius = 166 m and Indoor propagation case 2 and case 3 model

Figure F.13, F.14, F.15 and F.16 show the results of system in indoor propagation case 2 model and cell radius 166 m. System with indoor propagation case 3 model gives very similar results to system with indoor propagation case 2 model.

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

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Downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.13 Five percentile SINR vs mean cell downlink throughput

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Mean downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.14 Mean percentile SINR vs mean cell downlink throughput

67

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Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.15 Five percentile OBR vs mean cell downlink throughput

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Figure F.16 Mean OBR vs mean cell downlink throughput

F.5 Cell radius = 577 m and WINNER NLOS propagation model

The results of system with WINNER model (NLOS propagation for BS to RS links) and cell radius of 577 m are given in this section. Figure F.17, F.18, F.19 and F.20 show the results of this scenario. In contrast with results in section 5.5 (cell radius of 166 m and WINNER NLOS propagation model), repeaters with the same propagation model do not give significant gain in this system with cell radius of 577 m. These can be seen from the results in which the gain is very low. The reason is because two-hop links are only slightly better than single-hop link. Even with repeater directional donor antenna, the repeaters still bring limited gain.

68

1 1.5 2 2.5 3 3.5 4 4.5 5

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Downlink SINR vs mean downlink throughput

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Figure F.17 Five percentile SINR vs mean cell downlink throughput

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Mean downlink SINR vs mean downlink throughput

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Figure F.18 Mean SINR vs mean cell downlink throughput

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Figure F.19 Five percentile SINR vs mean cell downlink throughput

69

1 1.5 2 2.5 3 3.5 4 4.5 5

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Figure F.20 Mean OBR vs mean cell downlink throughput

F.6 Cell radius = 577 m and WINNER LOS propagation model

Figure F.21, F.22, F.23 and F.24 show the results of this scenario.

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Downlink SINR vs mean downlink throughput

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Figure F.21 Five percentile SINR vs mean cell downlink throughput

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x 106

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Mean Downlink Throughput (bps/cell)

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Mean downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.22 Mean SINR vs mean cell downlink throughput

70

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Obr vs mean downlink throughput

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Figure F.23 Five percentile OBR vs mean cell downlink throughput

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Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.24 Mean OBR vs mean cell downlink throughput

F.7 Cell radius = 577 m and 3GPP propagation model

Figure F.25, F.26, F.27 and F.28 show the results of this scenario.

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Downlink SINR vs mean downlink throughput

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Figure F.25 Five percentile SINR vs mean cell downlink throughput

71

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Mean downlink SINR vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.26 Mean SINR vs mean cell downlink throughput

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Mean Downlink Throughput (bps/cell)

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Obr vs mean downlink throughput

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Figure F.27 Five percentile OBR vs mean cell downlink throughput

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Mean Cell Downlink Throughput (bps/cell)

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Obr vs mean downlink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.28 Mean OBR vs mean cell downlink throughput

72

F.8 Cell radius = 577 m and Indoor propagation case 1 model

Figure F.29, F.30, F.31 and F.32 show the results of this scenario.

1 1.5 2 2.5 3 3.5 4 4.5

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Downlink SINR vs mean downlink throughput

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Figure F.29 Five percentile SINR vs mean cell downlink throughput

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Mean downlink SINR vs mean downlink throughput

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Figure F.30 Mean SINR vs mean cell downlink throughput

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Figure F.31 Five percentile OBR vs mean cell downlink throughput

73

1 1.5 2 2.5 3 3.5 4 4.5

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Obr vs mean downlink throughput

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Figure F.32 Mean OBR vs mean cell downlink throughput

F.9 Cell radius = 577 m and Indoor propagation case 2 model

Figure F.33, F.34, F.35 and F.36 show the results of this scenario.

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Downlink SINR vs mean downlink throughput

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Figure F.33 Five percentile SINR vs mean cell downlink throughput

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Mean downlink SINR vs mean downlink throughput

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Figure F.34 Mean SINR vs mean cell downlink throughput

74

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Obr vs mean downlink throughput

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Figure F.35 Five percentile OBR vs mean cell downlink throughput

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Mean Cell Downlink Throughput (bps/cell)

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Figure F.36 Mean OBR vs mean cell downlink throughput

Table F.1 and Table F.2 show the summary of downlink case at cell load 10 users/cell which are provided in Appendix F.1 - F.9.

Table F.1 Downlink SINR (at user intensity 10 users/cell)

Downlink SINR 166 m 577 m

NoRep FastFine NoRep FastFine

WINNER model (NLOS for BS to RN links) 7.5 12.6 5.1 5.5

WINNER model (LOS for BS to RN links) 7.5 6.8 5.1 8.4

3GPP model 8 16.6 7.1 9.5

Indoor propagation case 1 model 7 15.6 3.9 4.06

Indoor propagation case 2 and case 3 model 7 12.4 3.9 3.98 Table F.2 Downlink OBR (at user intensity 10 users/cell)

Cell Radius Propagation Model

75

Object Bit Rate 166 m 577 m

NoRep FastFine % NoRep FastFine %

WINNER model (NLOS for BS to RN links) 4.8 7 45.8% 3.15 3.42 8.6%

WINNER model (LOS for BS to RN links) 4.8 3.5 -27.1% 3.15 4.7 49.2%

3GPP model 5 7.7 54.0% 4.4 5.64 28.2%

Indoor propagation case 1 model 4.5 7.7 71.1% 2.5 2.5 0%

Indoor propagation case 2 and case 3 model 4.5 6.9 53.3% 2.55 2.55 0%

We can observe that at cell radius 166 m and without repeater (NoRep), indoor

propagation gives worse downlink SINR and OBR performance than winner nlos propagation. It is obvious because indoor propagation model has wall attenuation loss and the performance of system without repeater in indoor model must be worse than winner nlos propagation. We have shown in the table above the difference at 10 users/cell, but if lower user intensity is used the difference becomes higher, i.e. at 3 users/cell the SINR difference is 4 dB. However having advanced repeater in indoor propagation model gives more gain both in downlink SINR and OBR than in winner nlos propagation. This is also because wall attenuation in the direct link makes the probability of “users connect to composite link” higher. Other observations in this cell radius are that 3GPP model outperforms other models for both without repeater and with repeater cases, and repeater in winner los propagation model reduces the performance due to high interference.

Cell radius 577 m case gives the opposite of cell radius 166 m. We see here that repeater in winner nlos, indoor propagation case 1, case 2 and case 3 models do not give improvement to the system. These three models are basically similar each other which come from winner nlos propagation model. In the other hand, the repeater at system with winner los model gives significant gain. It is clear that system with winner los propagation model has higher received power at repeater both for desired signal and interference due to lower path loss exponent. Moreover a simple distance dependent analysis where a user is placed at the cell border shows that the composite link in winner nlos propagation model is really worse than the direct link, i.e. the composite link is 20 dB less than the direct link, where in winner los propagation model the composite link is 10 dB higher than direct link. Therefore in this case, the users prefer to use the direct link than the composite link.

Cell Radius Propagation Model

76

The following section provides the results for selected uplink scenarios in Table 5-2. The explanation and analysis are similar to downlink cases which can be found in section 5.5. F.10 Cell radius = 166 m and WINNER LOS propagation model (Upload)

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

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Uplink SINR vs mean uplink throughput

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Figure F.37 Five percentile SINR vs mean cell uplink throughput

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Uplink SINRr vs mean uplink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.38 Mean SINR vs mean cell uplink throughput

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

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Figure F.39 Five percentile OBR vs mean cell uplink throughput

77

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

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Figure F.40 Mean OBR vs mean cell uplink throughput

F.11 Cell radius = 166 m and 3GPP model (Upload)

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Uplink SINR vs mean uplink throughput

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Figure F.41 Five percentile SINR vs mean cell uplink throughput

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Uplink SINRr vs mean uplink throughput

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Figure F.42 Mean SINR vs mean cell uplink throughput

78

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Figure F.43 Five percentile OBR vs mean cell uplink throughput

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Figure F.44 Mean OBR vs mean cell uplink throughput

F.12 Cell radius = 577 m and WINNER LOS propagation model (Upload)

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Figure F.45 Five percentile SINR vs mean cell uplink throughput

79

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Uplink SINRr vs mean uplink throughput

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Figure F.46 Mean SINR vs mean cell uplink throughput

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Figure F.47 Five percentile OBR vs mean cell uplink throughput

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Figure F.48 Mean OBR vs mean cell uplink throughput

80

F.13 Cell radius = 577 m and 3GPP model (Upload)

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Uplink SINR vs mean uplink throughput

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Figure F.49 Five percentile SINR vs mean cell uplink throughput

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Uplink SINRr vs mean uplink throughput

no repeateralways onslow-coarseslow-finefast-coarsefast-fine

Figure F.50 Mean SINR vs mean cell uplink throughput

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

x 106

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Obr vs mean uplink throughput

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Figure F.51 Five percentile OBR vs mean cell uplink throughput

81

1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

x 106

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Obr vs mean uplink throughput

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Figure F.52 Mean OBR vs mean cell uplink throughput

82

Table F.3 and Table F.4 show the summary of uplink case at cell load 10 users/cell which are provided in Appendix F.10 - F.12.

Table F.3 Uplink SINR (at user intensity 10 users/cell)

Uplink SINR 166 m 577 m

NoRep FastFine NoRep FastFine

WINNER model (NLOS for BS to RN links) 3.08 5.45 1.88 1.9

WINNER model (LOS for BS to RN links) 3.08 2.51 1.88 2.2

3GPP model 3.26 7.46 2.62 3.68

Indoor propagation case 1 model 2.85 7.1 0.82 0.9

Indoor propagation case 2 and case 3 model 2.85 4.68 0.82 0.835

Table F.4 Uplink OBR (at user intensity 10 users/cell)

Object Bit Rate 166 m 577 m

NoRep FastFine % NoRep FastFine %

WINNER model (NLOS for BS to RN links) 1.68 2.85 69.6% 1.332 1.342 0.75%

WINNER model (LOS for BS to RN links) 1.68 1.6 -4.76% 1.332 1.354 1.65%

3GPP model 1.83 3.8 107.7% 1.59 2.03 27.7%

Indoor propagation case 1 model 1.57 3.6 129.3% 1.227 1.182 -3.7%

Indoor propagation case 2 and case 3 model 1.57 2.36 50.3% 1.227 1.2 -2.2%

We can observe that at cell radius 166 m, the upload case gives similar result

with download case. At cell radius 577 m, the only different result compare to downlink is winner los propagation model which does not give gain if we use repeater. A simple distance dependent analysis where a user is placed at the cell border shows that the RN – MS link in winner los model is 7 dB less than the 3GPP model. Therefore in this case, it might be that desired signal in winner los propagation model is received with very low power at repeater. Furthermore system in winner los propagation model gives higher interference than system in 3GPP model because the distance dependent attenuation of BS – RN link in winner los model is less than in 3GPP model.

Cell Radius Propagation Model

Cell Radius Propagation Model

83

References [1] R. Pabst, B. Walke, D. C. Schultz, and et al, “Relay-Based Deployment

Concepts for Wireless and Mobile Broadband Radio,” IEEE Communications Magazine, Sep 2004.

[2] J. Zander, “On the cost structure of future wideband wireless access”, In Proc. IEEE VTC Spring, 1997.

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