LTE Uplink Coverage Enhancement Techniques Based On Enhanced TTI Bundling

Yu Zhou, Tiankui Zhang, Zhimin Zeng Beijing Key Laboratory of Network System Architecture

and Convergence Beijing University of Posts and Telecommunications,

Beijing, China yzhoubupt@gmail.com

Yiqun Li, Yunan Han Research Department

China Information Technology Designing & Consulting Institute Co., Ltd.

Beijing, China liyq@dimpt.com

Abstract—In the Long Term Evolution (LTE) system, there is limited on uplink average data rate the cell edge users for the power limitation and the inherent short Transmission Time Interval (TTI) length of the cell edge users. TTI bundling scheme was presented for the VoIP service in the LTE Release 8 specifications to solve the cell coverage problem. But the R8 TTI bundling is restricted to bundles of 4 TTIs, QPSK modulation and allocations up to 3 PRBs, which impose restrictions on the support of the medium data rate service. In this paper, several TTI bundling enhancement schemes with different RTT, HARQ process number, TTI bundled size and the maximum number of HARQ retransmission are discussed in detail and evaluated by the link-level simulation. The simulation analysis proves that TTI bundling enhancement schemes can effectively improve uplink coverage performance in LTE system.

Keywords- LTE, coverage enhancement, medium data service, VoIP, TTI bundling

I. INTRODUCTION The main objective of LTE system is to provide high data

rate service for User Equipment (UE) with low latency and good Quality of Service (QoS). The LTE physical layer transmission time interval (TTI) unit is 1ms, such a small TTI makes the time delay smaller and short HARQ round trip time (RTT) of 8ms. However, in a coverage-limited scenario, due to the limited transmission power of the UEs at the cell edge, even a small packet cannot collect enough energy during one TTI to meet the data transmission block error rate (BLER) requirements. Thus, it is critical to find an effective way which can improve uplink coverage performance. According to the project results of 3GPP TSG-RAN meeting #53, the LTE coverage enhancements item would be discussed in 3GPP TSG-RAN WG1[4]. In order to find the limited link, link-level simulation was done for both the data and control channels in our previous work[5]. The simulation results indicate that Voice over Internet Protocol (VoIP) and medium data rate service on Physical Uplink Shared Channel (PUSCH) has the worst performance.

As a conventional solution to improve the coverage performance, the TTI bundling scheme was presented as an improved scheme for VoIP service in LTE Release 8 specifications, which retransmit a transport block automatically in continuous bundled TTIs with different redundancy version

(RV). Only when the whole bundle of transmissions has decoded, it sends the corresponding HARQ feedback. This approach reduces the head overhead as well as the control signaling when comparing with RLC segmentation. In addition, the packet loss probability of HARQ feedback errors is reduced. The discussion in this paper concentrates on the LTE FDD operation, while TTI bundling has been defined for TDD operation as well.

In the Release 8 specifications, the TTI bundling mechanism is restricted to bundles of 4 TTIs, QPSK modulation and allocations up to 3 PRBs. For VoIP service, these constraints leave some room to further improve the amount of energy transmitted per information bit, and thus the cell coverage. These constraints also impose restrictions on the support of large packet sizes, thus limiting the benefit for medium data rate service. So it is significant to further investigate enhanced TTI bundling schemes with different potential parameters, such as HARQ RTT, the number of HARQ process, the number of bundled TTIs and the maximum number of HARQ retransmission. The link-level simulation results of these schemes are also shown in this paper.

The rest of this paper is organized as follows: Section II explains the principle and problems of conventional R8 TTI bundling; In section III, Several enhanced TTI bundling schemes for medium data rate and VoIP service are discussed in detail; Simulation results are given and analyzed in Section IV before short conclusions are drawn in Section V.

II. CONVENTIONAL R8 TTI BUNDLING SCHEMES Due to RLC Segmentation increases header and control

signaling overhead, the TTI bundling scheme was proposed for VoIP service in LTE Release 8 specifications as an effective coverage enhancement scheme. TTI bundling was also treated as a coverage enhancement scheme for medium data service in 3GPP TSG-RAN WG1 meeting #68[5].

A. Principle of R8 TTI Bundling In R8 TTI bundling solution several redundancy versions

(RVs) corresponding to the entire RLC SDU are transmitted in consecutive TTIs. Thus the amount of accumulated energy within a defined packet delay budget for a single packet has increased. The same HARQ process number is used in each of

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the bundled TTIs. Only when the last redundancy version of the transport block is received by the eNB, the HARQ feedback is sent. The same MCS and frequency bandwidth will be utilized among subframes in a bundle, and only one uplink grant is transmitted for a bundle. The trigger could be for example the UE reporting its transmit power being getting close to the maximum value. So the TTI bundling method avoids the extra header overhead introduced by L2 segmentation, and also avoids additional L1/L2 control channel consumption and increased HARQ feedback channel error.

B. Problems of R8 TTI Bundling Because TTI bundling is presented for VoIP service, there

are many restrictions. As VoIP packets arrive every 20 ms, one VoIP packet can be transmitted for at most 20 subframes, which assumes that all the UL subframes can be assigned for the VoIP transmission in the link budget evaluation. However, all the transmissions for a single VoIP packet should be finished within the packet delay bound of 50ms. So in the R8 specifications, the TTI bundling mechanism is restricted to bundles of 4 TTIs, RTT of 16TTIs, QPSK modulation and to allocations up to 3 PRBs.

Figure 1. Subframe occupancy in R8 TTI bundling.

Fig. 1 shows the subframe occupancy in R8 TTI bundling. Different colors denote different HARQ processes, i.e. different VoIP packets from upper layers. The grid spacing is 4 ms (4 bundled TTIs). The round trip time is 16ms. Therefore, depending on the delay tolerance at air interface, the maximum number of HARQ transmissions is 3 if the delay bound is strictly limited to 50ms.

For the uplink VoIP, the coverage of VoIP still cannot match that of circuit switch voice, even with 4-TTI bundling. The fundamental reason is the discontinuous transmission at physical layer. The TTI of UMTS circuit switch voice is 20ms, whereas the TTI of VoIP with TTI bundling is 4ms. The round trip time (RTT) in the case of 4-TTI bundling is 16ms, which means the entire physical layer transmission should be finished with two HARQ transmissions. So about 8ms is spent on a VoIP packet transmission effectively, which is significantly shorter than 20ms TTI of UMTS circuit switch voice. As for the medium data rate service, because of insensitive to the delay and higher data rate, the application of R8 TTI bundling is more restricted. So we have the incentive to study TTI bundling enhancement schemes for both UL VoIP service and UL medium data rate service by increasing the number of UL subframes used for transmitting a single packet.

III. TTI BUNDLING ENHANCEMENT SCHEMES In order to increasing the maximum transmission time of a

single packet, a straightforward way is to extend the number of bundled TTIs and the maximum number of HARQ

retransmission. In addition, reducing the RTT will have the same effect. Several enhanced TTI bundling schemes will be introduced next.

A. 4-TTI Bundling with reduced RTT[8] If the round trip time can be cut to 8ms, more HARQ

retransmissions are possible within the certain delay requirement. As Fig. 2 shows, the bundling size is still 4 while the max number of HARQ retransmission has been changed to 5. In this case, there are up to 20 TTIs used to transmit a single packet, which may improve the coverage performance. However, the reduced RTT may cause the system does not have enough time to process signal.

Figure 2. 4-TTI bundling with reduced RTT.

B. 4-TTI Bundling with joint coding[10] 4-TTI bundling enhancement with joint coding is another

candidate. As shown as Fig. 3, four subframes will be bundled together and get jointly encoded as a larger transport block, while the data in four subframes get encoded separately in R8 4-TTI bundling. In this way, the efficiency of turbo encoder will be increased by encoding bigger transport block, thus to improve the coverage performance. The HARQ RTT and maximum number of HARQ retransmission for 4-TTI bundling with joint coding is the same as TTI bundling in R8.

Figure 3. 4-TTI bundling with joint coding.

C. 4-TTI bundling with Frequency hoppings[8] It is noted that the above proposed enhancements assume HARQ retransmissions use the same frequency resources as the initial HARQ transmissions. To improve the frequency diversity, different resource blocks could be used for HARQ retransmissions as illustrated in Fig. 4. Note that the slot boundary hopping could be used to achieve the frequency diversity. The drawback of slot boundary hopping is that the demodulation performance would be compromised as the channel in each slot of a subframe has to be estimated independently. Same as scheme A, the reduced trip time is reduced to 8ms, where 5 HARQ retransmissions are allowed within 50ms delay budget in this case.

Figure 4. 4-TTI bundling with reduced RTT and frequency hopping.

D. 8-TTI Bundling[8] In this scheme, the bundling size is extended from 4 to 8,

while HARQ RTT is still 16ms. A maximum of three retransmissions is allowed in the delay bound of 50ms, which makes the maximum transmission time for a single packet is 24ms, thus the coverage performance is improved. However, due to the VoIP packet arrival rate of every 20ms, initial transmission of the second packet would time conflict with the retransmission of the first packet as shown in Fig. 5. The conflict could be resolved by scheduling another resource for the new transmission, while the cost of such parallel scheduling can be high.

Figure 5. 8-TTI bundling.

IV. SIMULATION RESULTS AND ANALYSIS In this section, the performance of UL VoIP and medium

data service with different TTI bundling schemes is given, as well as the performance of pure HARQ retransmission. The simulations are done by a LTE link-level simulation platform. The configuration of one transmitting antenna and two receiving antennas is used in all the simulation scenarios, while the channel model is ePA and the modulation scheme is QPSK. The simulation results will be shown in the following.

A. Medium data rate service The link-level simulation assumptions are summarized in

Table I. The RB allocation number and MCS number are all fixed at 4 and 6. The channel coding scheme is Rel-10 turbo coding, while QoS target is 10%iBLER. Lp of 392 bits is the actual packet length in the simulation, which is obtained according to the number of allocated RB and MCS from Table 7.1.7.2.1-1 in LTE specification [11].

TABLE I. SIMULATION PARAMETERS OF MEDIUM DATA SERVICE

Parameter Value eNB antenna configuration 2 Rx UE antenna configuration 1 Tx

Channel model 3GPP-EPA UE velocity 3 km/h

Channel coding (PUSCH) Rel-10 turbo coding Number of allocated PRB, Lp,

MCS 4 PRB ,392bits,6

Max Number of HARQ retransmission 3

Number of HARQ process 1,4,8 RCL Segmentation OFF

TTI Bundling ON Number of bundled TTI 0,4,8

PUSCH hopping OFF Detector MMSE

Channel estimation for demodulation

Real channel estimation over DM-RS

At the beginning, the simulation scenarios of medium data service we considered are R8 4-TTI bundling and 8-TTI bundling, which can get a very gain when comparing with traditional HARQ process of no bundling. But TTI resource consumption of the three scenarios is different, the 4-TTI bundling scenario occupy 4 times the TTI resources than traditional HARQ process scenario while the 8-TTI bundling scenario occupy 8 times. For fairness, it is reasonable to compare 4 HARQ processes solution with 4 TTIs bundling solution that both take up the same TTI resources. The five scenarios are as follows: 1 HARQ process no bundling, 4 HARQ process no bundling, 1 HARQ process 4-TTI bundling, 1 HARQ process 8-TTI bundling and 8 HARQ process no bundling. In the scenario of 4 HARQ process no bundling, a packet of 392 bits is segmented into 4 packets which transmit separately in 4 TTIs of 4 HARQ process. The situation of 8 HARQ process no bundling is similar. The TTI bundling process is the same as it in R8 specification.

-10 -9 -8 -7 -6 -5 -4 -3 -2 -110

-3

10-2

10-1

100

SNR

iBL

ER

1process no bundling4process no bundling1process 4TTI bundling1process 8TTI bundling8process no bundling

Figure 6. iBLER vs. SNR simulation results for medium data service.

The iBLER vs. SNR simulation results are shown in Fig. 6. In Table II, The simulation result is the required SNR at the QoS target of 10%iBLER, and scenario 1 to scenario 5 are on behalf of 1 HARQ process no bundling, 4 HARQ process no bundling, 1 HARQ process 4-TTI bundling, 1 HARQ process 8-TTI bundling and 8 HARQ process no bundling.

TABLE II. REQUIRED SNR FOR DIFFERENT SCENARIOS

Scenario 1

Scenario 2

Scenario 3

Scenario 4

Scenario 5

Required SNR -1dB -5.8dB -7.1dB -9.2dB -8.0dB

It is obvious that we can get a very large gain for required

SNR by using TTI bundling solution comparing with traditional HARQ process (1 HARQ process no TTI bundling). From the results in Fig. 6 and Table II, we can also observe that the scenario of 1 HARQ process 4-TTI bundling can get a gain of 1.3dB comparing to the scenario of 4 HARQ process no bundling. The gain comes from increased efficiency of turbo encoder by encoding bigger transport block as well as increased probability of successful decoding by 4 times repetition transmission. The same situation happens in 1 HARQ process 4-TTI bundling comparing with 8 HARQ

process no bundling. In addition, using 8-TTI bundling obtained a gain of 2.1dB compared with 4-TTI bundling. This is because more redundant data sent in 8-TTI bundling result in the further increased probability of successful decoding. However, the number of concurrent users may be affected when large number TTIs are bundled.

B. VoIP service The simulation parameters are the same as the parameters

in medium data rate service, except the RB allocation number, Lp and MCS number are changed to 3, 328 bits and 7. The simulation scenarios of VoIP service do not include 8 HARQ process no bundling compared with medium data service. The QoS target is 2%rBLER.

-10 -9 -8 -7 -6 -5 -4 -3 -210

-4

10-3

10-2

10-1

100

SNR

rBL

ER

1process no bundling

4process no bundling

1process 4TTI bundling

1process 8TTI bundling

Figure 7. rBLER vs. SNR simulation results for VoIP service.

The rBLER vs. SNR simulation results are shown in Fig. 7. In Table III., The simulation result is the required SNR at the QoS target of 2%rBLER, and scenario 1 to scenario 4 are on behalf of 1 HARQ process no bundling, 4 HARQ process no bundling, 1 HARQ process 4-TTI bundling and 1 HARQ process 8-TTI bundling.

TABLE III. REQUIRED SNR FOR DIFFERENT SCENARIOS

Scenario 1 Scenario 2 Scenario 3 Scenario 4

Required SNR -2.1dB -7.4dB -8.5dB -10.0dB

The simulation result is the required SNR at the QoS target

of 2%rBLER. As we can see from the results, the basic trend is the same as the medium data service. The scenario of 1 HARQ process 4-TTI bundling can get a gain of 1.1dB comparing to the scenario of 4 HARQ process no bundling, while 8-TTI bundling solution can provide 1.5dB gain compared with 4-TTI bundling. In addition, corresponding to the scenarios in medium data service, the scenarios in VoIP

service can obtain a certain gain. The reduced transmission bits and the QoS target difference may explain this.

V. CONCLUSIONS In this paper we have discussed the different TTI bundling

enhancement schemes for uplink coverage enhancement of LTE FDD system, as well as the evaluation of this schemes by link-level simulation. Instead of conventional R8 TTI bundling, we increase the number of UL subframes used for transmitting a single packet by the change of RTT, the number of bundled TTIs and the maximum number of HARQ retransmission. The simulation results show that the TTI bundling enhancement schemes can achieve a certain gain of SNR and throughput for both medium data rate service and VoIP service. It seems that TTI bundling enhancement schemes can be used in medium data rate PUSCH and VoIP PUSCH for cell-edge users, but maybe system-level simulation is needed for further overall system performance evaluation.

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