Institute of Engineering Pulchowk Campusflipkarma.com/media_dir/main_documents/CDMA%20Network%20Analysis%2C... · This project is an analysis of cellular CDMA and design of a CDMA

Tribhuvan University

Institute of Engineering

Pulchowk Campus

Department of Electronics and Computer Engineering

A FINAL YEAR PROJECT REPORT

ON

CDMA NETWORK Analysis, Design and Simulation

By

Prabhat Man Sainju (060/BEX/428)

Rimesh Man Joshi (060/BEX/433)

Siruja Maharjan (060/BEX/443)

Subin Shrestha (060/BEX/444)

Pratik Joshi (060/BEX/448)

Kathmandu, Nepal

February 2008

A PROJECT REPORT ON


SUBMITTED IN PARTIAL FULFILLMENT OF

THE REQUIREMENTS OF THE DEGREE OF

BACHELORS IN ELECTRONICS AND

COMMUNICATION ENGINEERING

By Prabhat Man Sainju (060/BEX/428)

Rimesh Man Joshi (060/BEX/433)

Siruja Maharjan (060/BEX/443)

Subin Shrestha (060/BEX/444)

Pratik Joshi (060/BEX/448)

Department of Electronics and Computer EngineeringInstitute of Engineering, Pulchowk Campus

Kathmandu, Nepal February 2008

ACKNOWLEDGEMENT

We are grateful to the Department of Electronics and Computer Engineering for

providing us an opportunity to organize a research project in partial fulfillment of the

requirement for Bachelor’s Degree in Electronics and Communication Engineering. Our

sincere gratitude is for Asst. Prof. Ram Krishna Maharjan (Head of the Department of

Electronics and Computer Engineering) and Asst. Prof. Rajendra Lal Rajbhandari (Project

Coordinator).

We are highly indebted to our project supervisors Asst. Prof. Rajendra Lal Rajbhandari,

Asst. Prof. Diwakar Raj Pant and Er. Binit Sharma (Nepal Telecom), without whose

regular guidance and continuous support this project would not have been completed to

its present state.

We are thankful to the engineers at Nepal Telecom Wireless Directorate, Er. Anjil Joshi

and Er. Sujan Kafle, who were more than helpful for guiding us despite their busy

schedule. We are also indebted to our friends for their support and encouragement on the

accomplishment of the project.

Prabhat Man Sainju (060 / BEX / 428)

Rimesh Man Joshi (060 / BEX / 433)

Siruja Maharjan (060 / BEX / 443)

Subin Shrestha (060 / BEX / 444)

Pratik Joshi (060 / BEX / 448)

February 2008

ABSTRACT

This project is an analysis of cellular CDMA and design of a CDMA network based on

the research attributes and also based on the simulation of the network.

The global mobile communication market is booming. The world is demanding more and

more from wireless technologies than ever before as more people around the world are

subscribing to wireless. Today, the CDMA network is spread across the length and

breadth of the country covering the remote areas where communication before was

inaccessible.

Cellular CDMA network works on the third dimension of communication i.e. code, rather

than conventional frequency or time dimension. Hence, the system is unique to model and

analyze. Our research emphasizes on the analysis of the CDMA 2000-1X network of

Nepal Telecom at Madhyapur Thimi. Nepal Telecom has a CDMA network with 242 BSs

spread all over Nepal covering 70 districts.

The network analysis includes the study and analysis of service area topography, handoff

analysis, and coverage/capacity analysis of the sector. Present RF scenario of the BS has

also been studied. Various technical conclusions have been drawn from the studied BS.

We have chosen Gamcha of Dadhikot VDC, Bhaktapur where we observed/identified

Pilot pollution and its major causes from the field survey. Our study focused on

improving the network performance of Gamcha region where optimization of the

interfering BS alone was not sufficient since they had un-intended signal coverage. This

eventually led us in designing a BS at Gamcha.

The changes in various parameters that can bring improvement in the overall system

performance have been studied to incorporate into the design of the system. The BS

design includes the location and specification of the BS to be operated. The design

specification includes BS location, Major coverage regions, Subscriber prediction and

Capacity planning, Handoff allocations and specifies the link to BSC. RF pattern

prediction has been made with simulation model which incorporates Terrain Data and

various losses. Simulation has also been used for determining major coverage regions.

The simulation model that we used for this project has been verified.

TABLE OF CONTENTS

CONTENTS PAGE NO.

List of Figures

Abbreviations

Chapter 1: Introduction 1

1.1 Background 1

1.2 Objectives 2

1.3 Problem Statement 2

1.4 Methodology 3

1.5 Structure of the Report 3

Chapter 2: Literature Review 4

2.1 Code Division Multiple Access 4

2.2 PN sequences and Walsh codes 5

2.3 Features of CDMA System 5

2.4 CDMA Cellular Architecture 6

2.5 Forward and Reverse Links in CDMA 6

2.5.1 Forward Link 6

2.5.2 Reverse link 7

2.6 Power Control Mechanism 8

2.6.1 Reverse link Open Loop Power Control 8

2.6.2 Reverse Link Closed Loop Power Control 8

2.7 Handoff 8

2.8 Diversity Reception 9

2.9 Cellular Traffic 9

2.10 Link Budget 10

Chapter 3: Radio Propagation Model 12

3.1 Radio Propagation 12

3.2 Propagation Models 12

3.2.1 Free Space Propagation Model 12

3.2.2 Two-Ray Ground Reflection Model 13

3.2.3 Knife-Edge Diffraction 14

3.2.4 Durkin’s Model 16

3.2.5 Okumura Hata Model 16

Chapter 4: CDMA NETWORK Analysis 17

4.1 CDMA Network Analysis 17

4.2 PN Offset Coding System 18

4.3 Antennas 19

4.3.1 Antenna Downtilt 19

4.3.2 Transmitted Power 19

4.3.3 Received RF signal strength at MS 20

4.3.4 Pilot EC / I0 Level classification 20

4.4 BTS Types 20

4.5 Link of BTS with BSC 20

4.6 BTS Traffic Calculation 21

4.7 Madhyapur Thimi BTS Analysis 21

4.7.1 Analysis of Madhyapur Thimi (β – sector) 22

4.7.2 Data Analysis in Graphical Form 22

4.7.3 Handoff Analysis for Madhyapur Thimi β sector 23

4.7.4 Link Budget Analysis 24

4.7.5 Coverage/Capacity Analysis of Sector 243 26

Chapter 5 :CDMA NETWORK Design 27

5.1 Field Survey 27

5.1.1 Pilot Pollution at Gamcha 28

5.1.2 Data Analysis in Graphical Form 29

5.2 Base Station Design 30

5.2.1 Base Station Location 30

5.2.2 Major Coverage Region of the BS 30

5.2.3 Subscriber Prediction and Capacity Planning 31

5.2.4 Soft Handoff Channel Allocation 31

5.2.5 Base Station Specifications 32

5.2.6 Link with BSC/MSC 32

5.3 RF Signal Prediction with BS at Gamcha 33

Chapter 6 :SIMULATION 34

6.1 Simulation 34

6.2 Simulation Parameters 34

6.3 Program Structure 35

6.4 Simulation Methodology 35

6.4.1 Terrain Database 36

6.4.2 Interpolation 37

6.4.3 Point-to-Point Calculations 38

6.5 Line of Sight Calculation 38

6.5.1 Cross-over Point 38

6.5.2 Free Space Calculations 39

6.6 Cases of Diffraction 40

6.6.1 1st Fresnel Zone and Diffraction parameter 40

6.6.2 Epstein & Peterson Implementation for Multiple Edge Diffractions

40

6.7 Additional features 41

6.8 Simulation Limitation 42

6.9 Verification of the RF Prediction Model 42

Chapter 7: RESULTS AND CONCLUSION 43

7.1 Future Carry-outs 44

7.2 Recommendations 44

7.3 Conclusion 44

BIBLIOGRAPHY

APPENDIX A, B, C

A.1 Capacity of Cellular CDMA

A.2 Handoff Analysis for the Madhyapur Thimi β sector

A.3 Network Parameters

A.4 Link Safety Study for Madhyapur Base Station

A.5 Forward Link Budget for the designed BS

B.1 Shuttle Radar Topographic Mission 3 (SRTM3)

B.2 3DEM

B.3 GPS (Global Positioning System)

C.1 Drive Test Data of Ch 283 for Gamcha BS Design

C.2 Drive Test Data of Ch 201 for Gamcha BS Design

C.3 Analyzed Data of Madhyapur Thimi β - sector for Ch 283

C.4 Analyzed Data of Madhyapur Thimi β - sector for Ch 201

C.5 Verification of RF Prediction Model with 1C10 BS at Lele

C.6 PN_ID of related Base Stations

LIST OF FIGURES

LIST OF FIGURES PAGE NO.

Figure 2.1: Spreading of a Data signal 4

Figure 2.2: Basic CDMA Cellular Architecture 6

Figure 3.1: Comparison of free-space loss, ground-bounce loss and loss approximation

14

Figure 3.2: Knife-Edge Diffraction 15

Figure 3.3: Graph of Diffraction loss Vs Fresnel zone diffraction parameter 15

Figure 4.1: Cell Sector 18

Figure 4.2: Received power plot of sector 243 for channel 283 22

Figure 4.3: Pilot strength plot of sector 243 channel 283 23

Figure 4.4: Comparative Analysis of User Vs Cell Radius 25

Figure 5.1: CDMA MS in Debug Mode 27

Figure 5.2: Garmin – 12 GPS Receiver 27

Figure 5.3: Received power plot at Gamcha site for channel 283 29

Figure 5.4: Pilot strength plot at Gamcha site for channel 283 29

Figure 5.5: Microwave Link profile from Gamcha to Gatthaghar Exchange 32

Figure 5.6: RSL Prediction with designed BS at Gamcha 33

Figure 6.1: Profile Viewer 35

Figure 6.2: Flowchart of Propagation Modeling 36

Figure 6.3: Top View of interpolated map and line between Tx and Rx 37

Figure 6.4: LOS Check for path loss calculation 38

Figure 6.5: Plot of Free Space Loss and Ground Reflection Loss showing cross- over point

39

Figure 6.6: Two-edge diffraction 41

ABBREAVIATIONS

BS Base Station

BSC Base Station Controller

BTS Base Transceiver Station

CDMA Code Division Multiple Access

DEM Digital Elevation Model

EIRP Effective Isotropic Radiated Power

FDMA Frequency Division Multiple Access

GPS Global Positioning System

GSM Global System for Mobile Communication

HO Handoff

IS-95 Interim Standard-95

LOS Line Of Sight

MS Mobile Station

MSC Mobile Switching Center

NT Nepal Telecom

PN Pseudo-random Noise Sequence

QoS Quality of Service

RC Radio Configuration

RF Radio Frequency

RSL Received Signal Level

RSSI Received Signal Strength Indicator

RTT Radio Transmission Technology

SRTM Shuttle Radar Topographic Mission

TDMA Time Division Multiple Access


1

Chapter 1 INTRODUCTION

1.1 Background There has been a rapid expansion in wireless communication resulting in the need for radio

frequency (RF) planning, link planning and propagation modeling to predict the ever changing

dynamic wireless channel. Most wireless systems must propagate signals through non-ideal

environments and terrains. The wireless or the RF environment has been congested far more

than what was mere 10 years ago.

Cellular concept replaces the broadcast model that has single powerful transceiver using multiple

transceivers (Base Stations) and dividing a large coverage area into cells. Each cell uses a Base

Station Transceiver (BTS) and the users in the cells communicate with each other via the BTS.

Inter-BTS communication is possible with the centralized BSC and MSC. GSM is one of the key

players in the sector with its time division approach and CDMA with code division approach.

CDMA uses a high bandwidth channel of 1.2288 MHz for 2G and 2.5G over its entire network.

Random but distinctive coding is done to identify the users as well as the base stations in CDMA.

Like every other engineering operations, the cellular system has to be planned. GSM cells require

frequency planning with certain frequency reuse factor for cellular planning. CDMA has

frequency reuse factor of 1 meaning no planning in frequency has to be done. Yet in CDMA, the

cellular planning remains the major issue for its RF coverage or in short coverage planning.

Capacity planning is the common issue in all cellular systems including CDMA.

CDMA is interference limited system; the issues are different than GSM. The network is highly

dynamic and hence all parameters are to be managed to operate the network at the optimum

point. For a challenging topography like Nepal, cellular planning is a difficult task. For a network

provider, it is essential to know its weak sectors in terms of radio signal. Theoretical models are

available that are used worldwide to model the propagation. But, these models are empirical and

are applicable only for the types of landscape they were intended. In context of Nepal, the

topography and landscape is jagged. Hence, a newer model has to be developed to account these

important facts. Also, as said earlier, the traffic engineering is also an important part of the

cellular planning.


2

1.2 Objectives• Design a CDMA Base Station at Gamcha, Dadhikot

• Develop a propagation model for Received Signal Level prediction using the Digital

Elevation Map of Nepal

• Based on the model, develop a complete plan of BS at the Gamcha site along with the

predicted RF pattern

• Model the traffic of the planned BS

• Analysis of a CDMA BTS sector

The objectives for designing a CDMA network were absurd earlier. The decision for the design

location at Gamcha has been based on the suggestion by NT engineers. The decision for the

design of a base Station at the specific location has been later verified based on the RF survey of

the area in the design portion of the project. The propagation model design has been based on the

Durkin’s model. Other models such as Epstein Peterson Algorithm for Multiple edge diffraction

have also been applied.

The plan of a BS will include the complete design of a BTS for the CDMA network at the

selected site. The analysis of an earlier established BTS will give a clear picture of on-field RF

pattern of the BS. The analysis will also give the study of other parameters such as Handoff, pilot

levels, call success and traffic analysis. The analysis will be done for the CDMA 2000-1X

Network of Nepal Telecom for Madhyapur Thimi BTS(β – sector) PN_ID 243 forwarding at the

Araniko Highway, South-East region.

1.3 Problem Statement In this project, we seek to identify the poor performance regions of the CDMA Network. We

conduct field survey, drive test and a detail study of the region to identify the causes for the poor

network performance. After the study, the possible and feasible solutions to the problem are

defined.


3

1.4 MethodologyThe project was taken up in three stages

• Analysis of existing CDMA Network

o Selection of site for analysis

o Survey of the area and collection of network parameters

o Analysis of the collected data

• Design of a CDMA BS at the proposed area

o Site selection for the BS

o Acquiring various RF parameters using drive test tools

o Collection of Demographic Statistics of the site

o Conclude on the requirements needed for the BS design based on the results

obtained

• Prediction of received signal level from designed BS

o Obtain terrain data of the design area

o Selection of input parameters required for the modeling

o Choose an appropriate model for the signal level prediction considering various

losses

o Simulation of the model using MATLAB®

1.5 Structure of the Report This report has been divided into six chapters. Chapter 1 has introduced the background of

CDMA and puts forward objectives of the project. Chapter 2 provides the relevant literature

review on CDMA details with a proper explanation of the Access Techniques, Walsh and PN

sequences, Diversity, Handoff, Forward and Reverse Link, Power Control Mechanism and

Traffic Planning. Chapter 3 discusses the different mobile radio propagation models. Chapter 4

deals with basic CDMA Network Analysis and the analysis of CDMA 2000 – 1X BS at

Madhyapur Thimi operated by Nepal Telecom. The analysis includes the channels used, received

power level, pilot strength, link with BS, link budget analysis and BS traffic. Chapter 5 deals

with the CDMA Network Design at Gamcha, Dadhikot which includes data analysis and a

complete BS design specifications. Chapter 6 presents the simulation of the RF signal prediction

modeling incorporating various path losses to produce a realistic prediction model. Finally,

Chapter 7 discusses the results of the project and forwards the conclusions drawn from the

project.


4

Chapter 2 LITERATURE REVIEW

2.1 Code Division Multiple AccessCode Division Multiplexing is a standard digital multiplexing in radio communication that

occupies the entire 1.2288 MHz bandwidth provided every time for every channel. 1.2288 MHz

bandwidth occupancy might seem inefficient in per channel basis but the main theme is that all

the channels use the same bandwidth not only in the same cell but for the entire cluster and entire

network. All the CDMA networks use 1.2288 MHZ bandwidth as well but at different bands

allocated by Spectrum Planning division of the government. In addition to that, all the CDMA

networks use 1.2288 MHz bandwidth all over the world. Hence, the overall efficiency in

spectrum reuse can be made clear. This wide band channel ensures the very most basis of CDMA

communication i.e. transmission of signal much below the noise level or at high Processing Gain.

The phenomenon is also inferred as Spread Spectrum that has been used since Second World

War for safe military communication.

In the CDMA system, each user is given one out of a set of orthogonal codes with which the data

is spread, yielding code orthogonality. The orthogonality property allows the multiple users to be

distinguished from one another. Although users operate on the same frequency at the same time,

the spreading of the baseband signal spectrum allows interference from other users to be

suppressed, which increases the capacity of the CDMA technique.

Figure 2.1: Spreading of a Data signal


5

2.2 PN sequences and Walsh codes The PN sequences have unique property of orthogonality with a near zero cross-correlation and

near zero auto-correlation that is desirable to identify the random users. These sequences are

produced by the dedicated shift registers with the network defined mask. These sequences are of

two variations: Long PNs and Short PNs.

Short PNs are generated from 15 shift registers that are 32768 chips long with chip duration of

26.67 ms. The short PN is used to identify the base station in CDMA network. An offset of 64

chip is used for PN identification that results in 512 combinations. The long PN is produced from

42 shift registers that is 242 -1 chip long. The long chip is used to identify the individual users in

the CDMA.

The Walsh codes are 64 bit orthogonal sequences generated mathematically via Hadamard

matrix. These codes uniquely identify the users in the reverse link of the CDMA that is provided

during the channel assignment. These all chip specifications are the standards of IS-95 and

CDMA2000-1X.

2.3 Features of CDMA System CDMA Mobile communication is a cost-effective technology that requires fewer cell sites and no

costly frequency reuse pattern. The average power transmitted by CDMA mobile station is

significantly lower than the average power required by the GSM mobile station. Transmitting

less power means that average battery life will be longer.

The CDMA system has increased system capacity which is due to an improved coding

gain/modulation scheme, voice activity and reuse of the same spectrum in every cell and all

sectors. CDMA communication is more secure because of its unique coding that provides privacy

and prevents cross-talk. CDMA improves the Quality of Service (QoS) by providing robust

operation in fading and transparent (soft) handoffs. CDMA takes advantage of multi-path fading

to enhance communications and voice quality.


6

2.4 CDMA Cellular Architecture The generalized figure of the CDMA cellular architecture is shown in figure below. Protocols at

various levels of communication have also been specified. The radio interface between the MS

and BS is the Um interface. BSC controls the switching of the Base Stations. MSC is applied in

handoff procedures between the Base Stations. Other units associated with the MSC such as VLR

maintains the roaming subscribers with their RUIMs switched on and HLR maintains all the

RUIMs associated with the MSC. AuC validates the authentic users based on their status

maintained on the HLR. SS7 is used to link the CDMA MSC with other networks such as PSTN

or a GSM network.

Figure 2.2: Basic CDMA Cellular Architecture

MS: Mobile Station VLR: Visitor Location Register

BS: Base Station AuC: Authentication Center

BSC: Base Station Controller HLR: Home Location Register

MSC: Mobile Switching center PSTN: Public Switched Telephone Network

2.5 Forward and Reverse Links in CDMA

All of the communication systems have two way communicating paths. Similarly, the mobile

communication system being a duplex communication tool has two paths namely forward and

reverse link. The forward link in a mobile system is from the BS to MS and the reverse link is

vice-versa. The links are further sub-divided into dedicated channels operating at different rates

with specific purposes.

2.5.1 Forward Link The forward link from BS to MS consists of four channels: Pilot, Sync Paging and Traffic

channel. The data stream in the forward link is convolution coded at half rate, block interleaved

and spread by Walsh code. The channels in the forward link can be separated as:

MSC /

VLR

BSC

To GSM

/ PSTN

AuC /

HLR

Um Interface Abis Interface A Interface

MS BS

�


7

Pilot Channel It contains the W0 Walsh code that doesn’t contain character stream. It is transmitted constantly

by the base station. Mobile station measures the RSSI level of the different pilot signals during

the handoff. EC/IO gives a measure of readability of the received signal where EC is the energy of

the pilot alone and IO is the total interference energy.

Sync ChannelOnce the pilot channel is acquired, the mobile listens to the sync channel for system information.

It carries a data stream of essential system identification and parameter information used by

mobiles during system acquisition stage. The data stream is spread by W32 Walsh code.

Paging ChannelIt is the channel through which the system pages or notifies the mobile stations about the system

messages as call alert. Once the page is accepted, a traffic channel is assigned for the MS to use.

It is the channel where the system messages. The paging channel uses W1-W7 Walsh code.

Forward Traffic ChannelIt is used for user’s data transmission. Maximum number of traffic channels is 55. Any unused

paging channel can be used as a traffic channel. If variable rate vocoders used, data rate can vary

from 1200 bps to 9600 bps.

Power Control Sub-channelAt the interval of 1.25 ms (800 bps), the power level of the received signal is estimated. A power

up/down command is sent at 800 times per second. The power control bit is punctured in the

forward traffic channel and the MS corresponds to the command by fluctuating its transmitting

level up/down by 1 dB.

2.5.2 Reverse linkThe reverse link exists from the MS to the BS. The channels in reverse link are:

Access Channel It is used by the mobile station to initiate communication with the base station (but not yet in a

call such as transmit registration requests, call setup requests/origination message).

Reverse Traffic Channel The channel is used when a call is in progress to send

• Voice traffic from the subscriber

• Response to commands/queries from the base station

• Requests to the base station


8

It supports variable data rate operation for:

• A mobile station using the 8kb vocoders transmitting information on the reverse traffic

channel at variable data rates rate Set 1 - 9600, 4800, 2400 and 1200 bps

• 13 Kbps vocoders rate Set 2 - 14400, 7200, 3600, 1800 bps

2.6 Power Control MechanismCDMA being an interference-limited system, the interference generated internally as well as the

fading RF environment limits the system capacity and quality. Hence, transmission power from

the MS must be controlled to limit the interference. The process of doing this is called power

control. Reverse link power control is an essential in CDMA and is enforced by IS-95 standard.

2.6.1 Reverse link Open Loop Power Control

Near-far effect is pronounced in CDMA system. A mobile closer to BS needs to transmit lesser

power than the one farther. In open loop power control, the MS measures the RX power level on

entire 1.2288 MHz band for all channels (Pilot, Sync, Paging, Traffic channels) from all base

stations on forward links. If the received power is high compared to threshold, the MS reduces its

TX power and if it is low, it increases its TX power. Open loop power control doesn’t involve

BS.

2.6.2 Reverse Link Closed Loop Power Control

Fast fading due to multipath requires much efficient and faster power control. Additional power

adjustments are required for fading losses. Closed loop power control now involves the BS as

well. A power control sub-channel is transmitting continuously at 800 bps i.e. an interval of

1.25ms. At BS’s demodulated output, the power level of the signal is averaged at an interval of

1.25ms. Then, Power Control Bit (PCB-a 2 data bit length code) is punctured in 19.2 ksps 20-ms

frame spread data from the block interleaver through a MUX. A 0 bit PCB indicates that it should

increase its mean power level by 1 dB and a 1 bit PCB means to reduce the level by 1 dB.

2.7 Handoff The process by which the mobile is assigned a new traffic channel of an entirely new BS or a

new sector of BS is called the handoff. Handoffs take place in both idle and conversation state.

During an active call, then the combined Mobile Station, the Base Station and the MSC manage

the communications between the Base Station and the Mobile Station so that the radio link

performance is maintained. In GSM where the MS is assigned a new Absolute Radio Frequency

(ARF) Channel, hard handoff takes place (Break before Make). In CDMA, entire network uses

the same frequency. Hence, the switching takes place only in the channel PN number within the

same frequency i.e. soft and softer handoffs (Make before Break). Connection is established with

utmost 3 possible BSs at the same time during HO. This makes HO rate more successful and the

data communication flawless. The HO in CDMA is a Mobile Assisted HO based on Pilot

Strength measurements. The soft / softer HO increases the capacity of the CDMA system.


9

2.8 Diversity Reception The wireless environment is highly dynamic and widely affected by the changing environment.

Thus, the link between MS and BS in both directions is subjected to both rapid and deep fading

resulting in the signal strength to go below 40-50 dB below the free space power. Fast fading is

more pronounced in a heavily built-up urban environment. Hence, diversity reception techniques

are employed to cope up with the channel fading and improve the reliability of the

communication. Polarization diversity is generally employed in CDMA using the two diversity

reception antennas at 45o.

2.9 Cellular Traffic A network design is never complete without the traffic analysis and design. Every multiple

access technology that provides consumer service has to set its traffic level. Traffic is directly

related to the economy aspect of the network. There lies thin line between over-dimensioning and

under- dimensioning the network; this optimum operating point is Traffic engineering. Over-

dimensioning means estimating greater margin of resources such as channels, servers and other

network components, which is economically unfeasible and the revenue generation is low as

well. Under-dimensioning means estimating the resources mentioned above less than demanded.

This gives poor technical performance in terms of GoS and QoS. This also results in poor

economic efficiency. The blocking level is high as well. Hence, dimensioning the system is an

interactive and iterative process that is dynamic. Traffic engineering decisions trigger resource

acquisitions and further engineering.

Traffic Engineering is an application of science called Queuing Theory. Queuing theory relates

user arrival statistics, number of servers, and various queue strategies, with the probability of a

user receiving service. The Erlang B statistics that is mostly applied for traffic of wireless

systems, assumes that there is no waiting in the user’s queue. If the server channel is not

available, the call is simply blocked. In general, it gives the probability that a call is blocked. It

can be used as a measure of the Grade of Service (GoS) of the trunk. The traffic requests are

assumed to follow Poisson’s Distribution that implies exponentially distributed call arriving

times.

Blocking Blocking is inability to get a circuit when one is needed. Probability of Blocking is the likelihood

that blocking will happen. In principle, blocking can occur anywhere in a wireless system

because of:

• Not enough radios, the cell is full

• Not enough paths between cell site and switch

• Not enough paths through the switching complex

• Not enough trunks from switch to PSTN


10

Blocked calls to the network can’t be estimated, as they never make it to the system as per the

assumption of the Erlang B statistics. Hence, while planning the traffic, these blocked calls are to

be estimated using probability distribution of call rate. The distribution is based on the statistical

data and also depends upon the assumptions of the service provider. Generally, the traffic is

designed to handle the traffic at the busiest hour of a normal day. Other factors such as market

penetration, population density, geography, and economic activity come into play while

designing traffic for a system.

In a CDMA cellular system, there is no fixed number of channels as in an FDMA or TDMA

system, because the capacity (allowable number of users) depends on the degree of interference.

Blocking occurs when the reverse link multiple access interference power reaches a

predetermined level that is set to maintain acceptable signal quality. If the total user interference

at a base station receiver exceeds some threshold, the system blocks (denies access) to the next

user who attempts to place a call. The number of users for which the CDMA blocking

probability, denoted BCDMA, equals a certain value is defined to be the Erlang capacity of the

system and is related to an equivalent number of channels in an FDMA or TDMA cellular

system.

2.10 Link Budget Link budget analysis is the heart for the establishment of every communication unit. Designers

must calculate the maximum allowable path loss that will provide the adequate signal strength for

the acceptable voice quality. The components that determine the path loss are called the Link

Budget. Link Budgets are used to calculate the coverage and the performance for a base station

and a mobile station. The components must include the propagation factors such as TX power,

Noise figure, antenna gains, interference and many other such parameters.

The procedures for radio link budget analysis are:

• Identify the parameters affecting the forward and reverse links

o Access technology specific parameters

o Product specific parameters

o Morphology based parameters

• Determine the maximum allowable path loss to maintain communication in the forward

and reverse links

• Balance the reverse and forward link

A link budget determines the maximum allowable path loss of a given communication link. For

wireless systems, it is simply the difference between the transmitter EIRP and the receiver

sensitivity, plus receiver antenna gain and less fade margin, building penetration loss and body

(or cable) loss (all in decibel scale). The transmitter EIRP is the effective input power to a


11

hypothetical isotropic antenna that achieves such radiation intensity in any direction. In other

words, the transmitter EIRP is calculated as follows,

Transmitter EIRP (dBm) = Transmitter Power (dBm) + TX Antenna Gain (dBi) - Cable (or

Body) Loss (dB)

The receiver sensitivity denotes the minimum signal level at the antenna connector required to

close the communication link at a given data rate and under the worst case fading channel, i.e.,

Receiver Sensitivity (dBm) = Thermal Noise (dBm/Hz) + (Eb/No)req(dB) + Data Rate(dB-Hz)

The conclusion of the radio link budget is the link safety. The decision is whether the link is

feasible for sustainable communication or not. The feasibility of all the channels Pilot, Paging,

Sync and Traffic channels are studied.

The limiting range is given by,

dBI

E

piloto

c 10−>

dBI

E

syncrect

b 7,

>

dBI

E

pagingrect

b 7,

>

dBI

E

trafficrect

b 7,

>


12

Chapter 3 RADIO PROPAGATION MODEL

3.1 Radio Propagation The mobile radio channel places fundamental limitations to the performance of wireless systems.

Communication path between two communicating units may vary from direct line-of-sight to the

ones obstructed by buildings, hills, etc which make things highly dynamic and unpredictable. The

propagation mechanisms are diverse but are mainly governed by three major phenomena;

reflection, diffraction and scattering besides the free space propagation. Small scale fading and

multipath propagation are also described by the physics of these three basic propagation

mechanisms.

If the propagating electromagnetic wave impinges upon an object that has very large dimension

when compared to the wavelength of the propagating wave, then reflection occurs. The reflecting

surface could be buildings, walls & surface of the earth. Diffraction occurs when the radio path

between the transmitter and receiver is obstructed by a surface that has sharp irregularities.

Diffraction causes bending of waves around the obstacle even when a line-of-sight path doesn’t

exist between transmitter and receiver.

3.2 Propagation Models The Propagation Models focus on predicting the average received signal strength at a given

distance from the transmitter as well as the variability of the signal strength in close spatial

proximity to a particular location. The propagation model helps to determine where the cell sites

should be located to achieve an optimal performance in the network. The propagation model is

also used in other system performance aspects including hand off optimization, power-level

adjustments, and antenna placements. Although no propagation model can account for all

perturbations experienced in the real world, it is essential to use one or several models for

determining the path losses in the network.

3.2.1 Free Space Propagation Model The free space propagation model assumes the ideal propagation condition that there is only one

clear line-of-sight path between the transmitter and the receiver. The free space power received

by an antenna which is separated from a radiating transmitter antenna by a distance ‘d’ is given

by the Friis free space equation,

Ld

GGPdP rtt

r 22

2

)4()(

πλ=


13

Where, Pt = Transmitted power Pr = Received power

Gt = Transmitted antenna gain Gr = Receiver antenna gain

L = System loss factor (L>=1) λ = Wavelength (m)

d = Transmitter-Receiver separation distance (m)

The path loss, measured in dB represents the signal attenuation and is given by,

=

r

t

P

PPL log10

Free-Space propagation is applicable if there is only one signal path (no reflections) and the path

is unobstructed (i.e., first Fresnel zone is not penetrated by obstacles)

1st Fresnel Zone clearance radius ( )Dd λ2

1=

Where, D= distance between transmitter and receiver

3.2.2 Two-Ray Ground Reflection Model A single line-of-sight path between two mobile nodes is seldom the only means of propagation.

The two-ray ground reflection model considers both the direct path and a ground reflected

propagation path between transmitter and receiver. The total received signal strength at distance

‘d’ is the resultant of direct line-of-sight component and the ground reflected component which

is:

Ld

hhGGPdP rtrtt

r 4

22

)( =

Where, ht and hr are the heights of the transmitter and receiver antenna respectively.

The two-ray model does not give a good result for a short distance due to the oscillation caused

by the constructive and destructive combination of the two rays. Instead, the free space model is

still used when d is small.

Therefore, a cross-over distance ‘dc’ is calculated in this model. When d<dc, free space model is

used and when d>dc, two-ray model is used. At the cross-over distance, both give the same result.

So dc can be calculated as,

λπ rt

c

hhd

4=


14

The ground reflected wave doesn’t attenuate much but is 180 out of phase causing interference

decaying the signal level by 30-40 dB/decade.

Figure 3.1: Comparison of free-space loss, ground-bounce loss, and the ground-bounce

loss approximation, for ht = 100m, hr = 3m, and l = 0.3m

3.2.3 Knife-Edge Diffraction If a location and height of a single obstructing object in the propagation path to an individual

point is known, it is possible to compute the shadowing that is caused at this point.

Shadowing that is caused by single obstruction can be mathematically analyzed by

approximating the obstruction as a diffracting knife edge (i.e. infinitely narrow perfectly

absorbing half plane that is perpendicular to the propagation path).

The Fresnel zone diffraction parameter is given by,

+=

21

112

ddhv o λ


15

h o

d 1

d 2

O b s tru c t io n

B S

M S

Figure 3.2: Knife-Edge Diffraction

The loss in dB due to diffraction may be approximated by

( )( )

( )( )( )

0.95

2

0 1

20log 0.5 0.62 , 1 0

20log 0.5 , 0 1

20log 0.4 0.1184 0.38 0.1 , 1 2.4

20 log 0.225 / , 2.4

v

d

dB v

v v

e vL

v v

v v

−

≤ −− − − ≤ ≤− ≤ ≤= − − − − ≤ ≤

− >

This loss is added to the previously determined path loss to obtain the total path loss. Other losses

such as free space and ground reflection still apply, but computed independently for the path as if

the obstruction did not exist.

Figure 3.3: Graph of Diffraction loss Vs Fresnel zone diffraction parameter


16

3.2.4 Durkin’s ModelIt is similar to Longley-Rice Model for predicting the field strength contours over irregular

terrain surface and the losses caused by the obstacles in a radio path. The propagation phenomena

modeled is simply LOS and diffraction of obstacles along the radial and excludes reflections

from other surrounding objects and local scatterers. The execution of the Durkin’s Path loss

model consists of two parts:

• Access a topographical database of the proposed area and reconstruct the ground profile

information along the radial joining the Tx to the Rx through interpolation.

• Calculate the expected path loss along the radial

The model makes decisions as to what the expected transmission loss should be. The first step is

to decide whether a LOS path exists between the Tx and Rx. This is done by computing the

difference between the height of the line joining the Tx and Rx antennas from the height of the

ground profile for each point along the radial. If the difference is found to be positive, LOS path

doesn’t exist. Otherwise the LOS path exists. Next, the model checks to see whether the first

Fresnel Zone clearance exists assuming a clear LOS path. If clearance exists, simple free space

transmission formula can be used. If the profile is LOS with inadequate first Fresnel Zone

clearance, diffraction loss is added to the received power. If LOS is not achieved, Knife-edge

diffraction technique is used.

3.2.5 Okumura Hata Model The Okumura model is based on measurements made in Tokyo in 1960, between 200 and 1920

MHz. Okumura model is an empirical relationship derived from the technical report so that the

results could be used in computational tools. Okumura’s report consist a series of charts that have

been used in radio communication modeling.

For the Okumura model, the prediction area is divided into terrain categories: open area,

suburban area, and urban area. The Okumura model uses the urban area as a baseline and then

applies correction factors for conversion to other classifications. A series of terrain types is also

defined. Quasi-smooth terrain is the reference terrain and correction factors are applied for other

types of terrain. The Hata model (sometimes called the Okumura–Hata model) is an empirical

formulation that incorporates the graphical information from the Okumura model.


17

Chapter 4CDMA NETWORK Analysis

4.1 CDMA Network AnalysisThe main purpose of CDMA Network Analysis is to build the background knowledge for

designing a new CDMA Network. By new CDMA Network design, we mean to design a new

CDMA Base Station (BS). Designing a CDMA Network would require us with the detail

knowledge on the CDMA cellular network currently in operation. The study of an existing

CDMA network in operation, first of all, would help us in understanding the CDMA cellular

network. It would help us in gaining an in-depth on the cellular network system. This knowledge

would be used in designing a BS for the CDMA network. The analysis would also help us in

formulating the design criterion for the new network to be designed.

The other purpose of analysis of a CDMA Network is to study the performance and operation of

a specific BS. This study would consist of understanding that particular BS in detail and studying

its working performance and the service that particular BS is providing. This study would be

used in evaluating the performance of that network and find out ways to make its service even

better.

The CDMA Cellular Network under our study is the CDMA Network system of Nepal Telecom

(NT). Our study would focus on the CDMA 2000 1X RTT system that is currently being

operated by Nepal Telecom (NT). This CDMA network has coverage across the length and

breadth of the country. We would analyze and design this 2.5G CDMA 2000 1X RTT Network

system which is operating in the 800 MHz Cellular spectrum.

Nepal Telecom’s CDMA Specification

CDMA 2000 1X RTT (2.5G)

Channels 1. Channel No. 283

2. Channel No. 201

Channel Frequency For Channel No. 283

f Uplink (Reverse Link / MS to BS) = 825 + 0.03 X 283 = 833.49 MHz

f Downlink (Forward Link / BS to MS) = 870 + 0.03 X 283 = 878.49 MHz

For Channel No. 201

f Uplink (Reverse Link / MS to BS) = 825 + 0.03 X 201 = 831.03 MHz

f Downlink (Forward Link / BS to MS) = 870 + 0.03 X 201 = 876.03 MHz


18

Channel No. 283 is the primary channel. For 1 Carrier BTS with only one channel, this primary

channel is used. The Uplink and Downlink frequency are 45 MHz apart.

4.2 PN Offset Coding SystemA PN Offset code is 215 chips long with 64 chip offset assigned for each BS. A total of 512 PN

Offset codes are available for the identification of BS in each LAC (Location Area Code) / BSC.

Each sector of a BS is given a unique PN Offset. For an omni BTS, single PN Offset is assigned.

For a sectorized BTS, each sector is assigned a unique PN Offset. Commonly used sectorized

BTS is the 3 Sector BTS. In the 3 sector BTS, the sectors are named respectively as alpha, beta

and gamma sector according to their orientation with respect to North(0º). The alpha sector is 0º

with North (0º). The beta sector is 120º clockwise from North and the gamma sector is 240º

clockwise from North.

In naming each sector, first, the alpha sector of the BTS is assigned some PN Offset Code. Then,

the PN Offset of the beta and gamma sector of that BTS are derived by adding a constant

number. This constant number is determined by assuming all 3-sector BTS with 512 different

sectors. So, the value of this constant nearly becomes:

512/3=170

A value close to this number can be taken as a constant to be added to determine the beta and

gamma sector offset code. In our case, this value was 168.

The naming of a 3 sector BTS has been illustrated by following example.

Figure 4.1: Cell Sector

Let’s consider the CDMA 3 Sector BTS of Madhyapur Thimi. Here, the alpha sector is assigned

the PN Offset code 75. And, the PN Offset of the beta sector is 75 + 168 = 243. Similarly, the PN

Offset of the gamma sector is 243 + 168 = 411.

120º

120º

120º

N

α-sector PN 75

γ-sector PN 411 β-sector

PN 243


19

IMSI – International Mobile Subscriber Identity R-UIM cards securely store the service-subscriber key (IMSI) used to identify a subscriber.

IMSI Format

429 03 97XXXXXXXXXX |�MCC �|�MNC�|� MIN �|

MCC – Mobile Country Code (3 digits)

MNC – Mobile Network Code

MIN – Mobile Station Identification Number

National Mobile Station Identity (NMSI) = MNC + MIN can be 12 digits long.

4.3 Antennas1. Omni- directional Antenna

Antenna Type : Space Diversity Antenna

Antenna gain : 12 dBi

Beamwidth : 360º Horizontal, 5º - 7º Vertical

2. Sectored Antenna (Dual Polarized)

Antenna Type : ± 45º Slant Dual Polarized Antenna

Antenna gain : 16 dBi

Beamwidth:

Rural : 90º Horizontal, 5º - 7º Vertical

Urban : 65º Horizontal, 5º - 7º Vertical

4.3.1 Antenna DowntiltThe antenna can be downtilted using mechanical or electronic technique. In our case, the

mechanical downtilting of the antenna was used with maximum 10º downtilt possible. The

downtilting of the BTS antenna reduces the cell coverage area and helps in reducing pilot

pollution in the CDMA cellular system.

4.3.2 Transmitted PowerBS Maximum TX Power for Sectoral antenna= 30 W

BS Maximum TX Power for Omni-directional antenna= 20 W

The BS TX Power consists of control power which is fixed and traffic power which is dynamic.

Control power transmits the Pilot, Sync, Paging and Access signals. Traffic power is a dynamic

power which varies with the no. of active users.

BS Receiver sensitivity = -105 dBm

MS Mean TX Power = 2 mW

MS Max. TX Power = 200 mW


20

4.3.3 Received RF signal strength at MS Range: -25 dBm to – 110 dBm

Good Coverage : -25 dBm to -65 dBm

Normal Coverage : -65 dBm to - 85 dBm

Poor Coverage : -85 dBm to -105 dBm

For the Received Signal Level less than -95dBm, Patch Antenna is used to increase the Received

Signal Level by +10dBm

4.3.4 Pilot EC / I0 Level classificationGood Pilot Level : -4 to -7 dB

Normal Pilot Level : -7 to -9 dB

Poor Pilot Level : -9 to -12 dB

Very Poor Pilot Level : > -12dB

4.4 BTS Types1. Micro BTS

1. 1C1O – 1 X 35 simultaneous users � 1 X 880 subscribers

2. 2C1O – 2 X 35 simultaneous users � 2 X 880 subscribers

2. Macro BTS

1. 1C3S – 3 X 35 simultaneous users � 3 X 880 subscribers




1C1O – 1 Carrier 1 Omni 2C1O – 2 Carrier 1 Omni 1C3S –1 Carrier 3 Sector

2C3S – 2 Carrier 3 Sector 1C2S – 1 Carrier 2 Sector 2C2S –2 Carrier 2 Sector

4.5 Link of BTS with BSCA BTS is essentially connected to the BSC/MSC using one or more of the following links:

1. Microwave Link

2. Optical Link

3. E1 Link to the nearest Telecom Exchange

In case of E1 Link, the normal copper pair line with DSL Modem is used to connect the BTS to

the nearest Telecom Exchange. From that exchange, the BTS is connected with the BSC. The

length of copper pair from the BTS location to nearest Exchange should not exceed 3 km. Two

E1 Links are required to connect the 2C3S BTS with the BSC. For 1C1O BTS, a single E1 link is

sufficient.


21

λυ

αβ

×××+

×= 1

1

1

/ 0NE

GM

b

p

4.6 BTS Traffic Calculation

No. of users per cell (M)

Where,

Gp = Processing Gain = 1286.9

2288.1 =kbps

Mcps

Eb / N0 = 6 dB= 3.98

β = Co-channel interference factor = 0.55

α = Power Control Accuracy Factor = 0.8

=ν Voice Activity Factor = 0.4

λ = Sectorization Gain = 2.55

Capacity Calculation

No. of simultaneous users per cell (M) = 105.81

No. of simultaneous users per sector (M / sector) = 353

81.105 = users

NT’s Erlang/Subscriber at Busy Hour = 0.03 Erlang/Subscriber

Blocking Probability, GoS = 2%

Offered traffic = 26.4 (from Erlang-B statistics table for N = 35, GoS = 0.02)

Hence, No. of subscribers supported by the sector 88003.0

4.26

/===

SubErl

fficOfferedTra

Previously, NT was providing 0.051 Erlang per Subscriber that supported 500 approximate

subscribers. But now, due to increased no. of mobile subscribers with less average holding time,

that value has been changed to 0.03 Erlang per Subscriber which has resulted in system to

support 880 approximate subscribers.

4.7 Madhyapur Thimi BTS AnalysisMadhyapur Thimi BTS Details

Location: Madhyapur Thimi, Bhaktapur

Latitude: 27.683413˚ N Longitude: 85.387657˚ E Altitude: 1339 m

BTS Type: 2 Carrier 3 Sector (2C3S)

PN_ID: 75(Alpha), 243(Beta), 411(Gamma)


22

Link with BSC

• DSL Modem (E1 Link) from Madhyapur Thimi BTS to NT Gatthaghar Exchange

• Optical Link from NT Gatthaghar Exchange to BSC0 at NT Bhadrakali

4.7.1 Analysis of Madhyapur Thimi (β – sector) The analysis of Cell1 (120º North clockwise) sector of 2C3S Madhyapur Thimi has been carried

out. The PN Offset code for this sector is 243. The RF Signal Level and Pilot EC / I0 at various

places in this sector’s coverage area have been collected through the field survey. The data has

been collected for both the channels 283 and 201.

4.7.2 Data Analysis in Graphical FormThe data collected from the design survey of sector 243 has been presented in the Appendix and

has been presented below in the graphical form below for the primary channel 283.

Figure 4.2: Received power plot of sector 243 for channel 283


23

Figure 4.3: Pilot strength plot of sector 243 channel 283

4.7.3 Handoff Analysis for Madhyapur Thimi β sector The statistics show that 51.138% channels have been used for the handoff. The interference for

Hard Handoff has the interference factor 2.38. The average Erlang per sector or the capacity can

be compared to that for the Soft Handoff that has an interference factor of 0.55. The table shows

that for 2dB deviation in power control, capacity is nearly double to that of hard handoff. Hence,

hard handoff is found to reduce the capacity of the cell. Thus, soft handoff is more desirable.

Hence, all the BSs in the valley have two carrier configurations. This allows the soft handoff. If

single carrier system is used, the visitor MS would be prompted for hard handoff. Allocating the

51% channels in HO only, the dedicated users at 2% outage is found to be 43.5 together for two

carriers 283 and 201 using Erlang B statistics. The most probable candidate Base stations for the

soft handoffs are

• Bhaktapur γ sector

• Bhaktapur β sector

• Gatthaghar α sector

• Madhyapur Thimi β sector


24

The reason for this high HO could be that the sector monitored is directed towards the Araniko

Highway. This could reduce the numbers of dedicated users of the cell. An approximate of 98%

call success rate is associated with the HO at the Madhyapur Thimi on an average for both the

channels.

A report of drive test done on the coverage area of the 243 sector has been included in the

appendix. The report contains the EC/IO and RSSI scenario of the area. The interfering sectors of

the neighboring Base Stations can be seen from the graph. At the near boundary regions, the

significant interference can be monitored. This results in frequent handoffs. Handoffs are load at

the BSC and causing the reduction in the cell capacity. It is also clear from the handoff analysis

shown in the appendix A.2.

4.7.4 Link Budget Analysis The link budget of the analyzed BS can be used a great deal to study the various parameters of

the BS. The base station is a 2C3S base station with TX power 30 Watts. The calculation gives

the probable radii of the cell. CDMA being interference limited system and the users in CDMA

act as sources of Interference. Hence, the MSs in CDMA that are in active state play decisive role

to determine the cell size.

The ZTE channel power specifications are:

Channel Power=CELL_PWR

( )40

255_

10−

×gainCh

Pilot channel Gain = 225 Pilot Channel Power=17.78%

Sync Channel Gain = 185 Sync Channel Power=1.778%

Paging Channel Gain = 219 Paging Channel Power=12.58%

Traffic Channel Power = 67.849% of total transmit power of BS

Various transmitter losses are given as standard for the BS by the vendor. The losses include

Penetration loss, cable loss, fade margin, Rx noise figure and the system noise floor.

Interference analysis is done for same cell interferences and other cell interferences. The same

cell interference for individual channels is due to the other users in the same cell. The other cell

interference is due to the users in the other cells. The other cell interference here is calculated

accounting the frequency reuse factor of 0.65. The link safety is calculated for the given

specification for all the channels. We specify the limiting parameters for the safe link. If the

acceptable value is obtained, the link is safe. Even for the acceptable pilot channel EC/IO, the

traffic Eb/It could be unacceptable if the number of simultaneous users on the cell increase. The

analysis shows the link being unsafe and resulting in the call drops.


25

The BS analysis was focused on the Madhyapur Thimi BS β sector. The HO data obtained for the

BS are as follows:

Channel No.

Soft

Handoff%

Softer

Handoff%

Call Success Rate

%

283 40.6977 9.4102 98.702

201 40.7143 11.9414 98.814

Thus, the average channel overhead factor is 51.3818%. This gives the dedicated users of the

cells prior to HO customers. The link safety calculation is a complicated operation that requires a

lot of iteration. The result is shown in the appendix A.3. The complete calculation is included in

the soft copy of the report.

The radius of the cell has been calculated using the Hata model. This was done because the loss

model used by us only gives the loss prediction. But, the Hata’s empirical model has a cell radii

prediction. The result of our model has been applied to the Hata model. The deviation from the

actual value is nominal. Rural approximation has been used in the Hata model.

The link budget has been prepared for the voice traffic only and doesn’t account for the data

traffic that the cell carries. The link budget is given in the appendix A.3, 4 and 5. The link safety

region is also calculated for the limiting range stated above. The analysis shows the cell

breathing. As the users increase, the cell radius seems to decrease as for the interference limited

system. A comparative analysis for various user numbers against the cell radius is given by the

graphical view.

User Vs. Cell Radius

3.8843.56

3.252.85

2.128

0.7728

00.5

11.5

2

2.53

3.54

4.5

22 24 26 27 29 30

Users

Rad

ii

Figure 4.4: Comparative Analysis of User Vs Cell Radius (km)


26

4.7.5 Coverage/Capacity Analysis of Sector 243

The coverage area of sector 243 has been determined from drive tests. The major coverage region

of the Madhyapur Thimi BS sector 243 included Madhyapur Thimi – 4, 7, 8, 9, 10, 11, 12, 13,

14, Katunje -2, 9 and Bhaktapur -17. The coverage regions have been determined through field

survey by collecting the RSL, Pilot EC/ IO level and the PN_ID of the BS from where the signal

was received.

The total population in the major coverage area was around 29000. NT’s CDMA Mobile

Telephone Penetration in KTM R. D. is 4.46 %. So, the calculated CDMA subscribers in the

coverage region are 1293 subscribers.

From the previous calculation, we have determined the no. of Subscribers supported per carrier

per sector at 0.03 Erlang / Subscriber is 880 subscribers. And, the subscriber supported by two

carrier BS is equal to 1760 subscribers. Hence, the β – sector of the Madhyapur Thimi BTS is

successful in supporting the subscribers of its coverage region with enough capacity.


27

Chapter 5 CDMA NETWORK Design

5.1 Field SurveyThe field survey was carried out in around 1.5 km periphery of the Gamcha site. The main areas

surveyed were Sirutar VDC, Dadhikot VDC, Gundu VDC, Balkot VDC, Katunje VDC of the

Bhaktapur District and Lubhu VDC in Lalitpur District.

During our field survey, we collected following parameters:

• Latitude / Longitude / Altitude of the places

• Pilot EC / I0 and RF Signal Level of Channel No. 283 and 201

• Pilot PN Offset / Base Station from where the signal is received

GPS Receiver was used to collect the Latitude/Longitude/Altitude information.

We used two CDMA mobile station/mobile sets to monitor the EC / I0 and Received Signal Level

of Channel No. 283 and Channel No. 201 respectively. These data were collected with the MS in

Idle mode.

Figure 5.1: CDMA MS in Debug Mode Figure 5.2: Garmin - 12 GPS Receiver


28

5.1.1 Pilot Pollution at Gamcha The motive of BS design at Gamcha was supported from the survey data collected from the

Gamcha site through field surveys. The CDMA network performance at/around the Gamcha area

was very poor. Pilot Pollution was the major reason for poor CDMA network performance.

During the field survey, we found that, there was network coverage provided at and around the

Gamcha area from near by as well as very far Base Stations. Weak Pilot EC / IO which was less

than -9 dB were observed in most of the area. The low EC / IO was due to the signal present from

many no. of BSs resulting in interference. The no. of Soft Handoff candidates BS were greater

than three in most of the surveyed areas. High altitude BTS such as Nagarkot Transmission at an

altitude of 2040m had very wide cell coverage even with maximum antenna downtilt. Other

interfering included Maitighar BTS and New Baneshwor BTS which was very far (greater than

5km) from the study site.

The determining parameter in CDMA, the pilot EC / I0 was very poor in those regions to an

unacceptable level. The major consequences of poor EC / I0 would be:

• High call drop rate

• Poor network link / Unstable Link

• Frequent Handoffs

• Blocking of originating fresh calls

• Decrease in system capacity

Some of the interference can be reduced by optimization of the interfering BTS while it is

difficult to reduce interference from BTS such as Nagarkot Transmission BTS which is

configured at maximum antenna downtilt. Even though optimization of the interfering BTS can

result in performance improvement by some factor, in our case optimization alone was not

sufficient since the interfering BSs had an un-intended coverage at/around the Gamcha site. This

eventually led us towards designing a BS for the Gamcha region.


29

5.1.2 Data Analysis in Graphical FormThe analyzed data collected from the design survey has been presented in the Appendix and has

been presented in the graphical form below for the primary channel 283.

Figure 5.3: Received power plot at Gamcha site for channel 283

Figure 5.4: Pilot strength plot at Gamcha site for channel 283


30

5.2 Base Station Design

After having a no. of field surveys and drive tests and a detail topographic and demographic

study of the Gamcha region, we have designed a Base Station. The designed Base Station is

based on these studies.

5.2.1 Base Station Location

BS Location: Gamcha – 7, Dadhikot

VDC: Dadhikot District: Bhaktapur

Latitude: 27.65671º N Longitude: 85.39304º E Altitude: 1340 m

The location of the Base Station has been selected from the study of the topographic map, field

survey and the RF Signal Prediction model.

5.2.2 Major Coverage Region of the BS

The major coverage region has been determined by overlapping the predicted RF pattern into the

map of the Gamcha region. The overlapping of the predicted Received Signal Level Pattern into

the topographic map of the concerned area has enabled us in predicting the major coverage

regions with a greater accuracy. The predicted RF pattern is presented later in this chapter and in

the next chapter.

S. No. Coverage Area Population *

1. Dadhikot VDC (90% area) 8043 X 0.90 = 7239

2. Sirutar VDC (95% area) 4532 X 0.95 = 4305

3. Balkot VDC (20% area - West) 8276 X 0.20 = 1655

4. Gundu VDC (20% area - North West) 6392 X 0.20 = 1278

5. Katunje VDC (15% area – West) 14481 X 0.15 = 2172

6. Lubhu VDC (20% area – North East) 8476 X 0.20 = 1695

7. Thimi Municipality (15% area- South) 53017 X 0.15 = 7953

Total Population 26297

* Source: Projected Population in 2005 from Population Census 2001, ‘District Development

Profile Of Nepal 2004 ‘


31

5.2.3 Subscriber Prediction and Capacity Planning

NT’s CDMA Mobile Telephone Penetration inside KTM Valley = 4.46 %

No. of Subscribers supported per carrier per sector at 0.03 Erlang / Subscriber = 880 subscribers

Total population of the coverage region = 26297

Predicted CDMA subscribers in the coverage region,

= 26297 X 0.0446 = 1173 subscribers

From our study, we have found that the major coverage areas of the designed BS have a high

household growth rate. The high growth rate can be accounted for increased households and

urbanization mainly inside the Kathmandu Valley. The no. of subscribers can be expected to

grow at high rate because of this household growth. Also, the CDMA mobile subscriber trend of

the country is increasing at a high rate. For the prediction of the subscriber, the ‘mobile’

subscribers who would be moving inside the coverage region should also be taken into account.

The major coverage area also covers some parts of the Araniko Highway.

Carefully taking into consideration these facts, we are designing a BS which can meet the future

requirements of the service.

From our above study of the Subscriber Prediction, we choose to operate a Two-Carrier Single

Sector BS (2C1O).

Subscribers supported by 2C1O BS = 2 X 880 = 1760 subscribers

Here, we choose to operate a 2 Carrier BS not only to support more subscribers but also to avoid

hard handoff. Most of the CDMA BS operating inside Kathmandu valley is a 2 Carrier BS. So,

the new BS designed to operate with 2 Carriers would help in avoiding the hard handoff. The

hard handoff would imply significant reduction in the capacity of a CDMA system.

5.2.4 Soft Handoff Channel Allocation

Channel allocation for Soft Handoff – 35%

Major Soft Handoff Candidate BS

1. α – sector Gatthaghar BS PN_ID 72

2. β – sector Gatthaghar BS PN_ID 240

3. β – sector Koteshwor BS PN_ID 234


32

5.2.5 Base Station Specifications

BTS Type: 2C10 (Referring to ZTE’s ZXC10-BTS operated by NT)

BS Maximum TX power: 18 - 20 Watts

BS Sensitivity: -105 dBm

Antenna Type: Omni – directional with 12dBi gain

Antenna Tower Height: 30 m

Power consumption: 1100W

5.2.6 Link with BSC/MSC

o BSC/MSC is located at NT Central Office, Bhadrakali

o Microwave Link from Gamcha to NT’s Gatthaghar Exchange using 25m Antenna Tower

o Optical Link from Gatthaghar Exchange to Chabahil Exchange

o Link from Chabahil Exchange to NT Bhadrakali through Kathmandu Valley Optical Ring

Figure 5.5: Microwave Link profile from Gamcha to Gatthaghar Exchange


33

5.3 RF Signal Prediction with BS at Gamcha

The Received Signal Level(RSL) Prediction considering 30 m Antenna height with an Omni-

directional Antenna transmitting at power 20 Watt is been presented below. The color variation

indicates the received signal strength.

Figure 5.6: RSL Prediction with designed BS at Gamcha


34

Chapter 6 SIMULATION

6.1 Simulation Prediction of the designed system is a vital part in system design. The computer simulation thus

is a handy tool which accounts for known issues and predict the versatility of the proposed

system.

MATLAB® has been considered as the tool for the simulation of the CDMA cellular network.

This scripting language has been mainly chosen due to mathematical and graphical resources it

provides. The large base support has also been a criterion for choosing it.

The Received Signal Strength Level is predicted for the proposed Base Station through a

MATLAB program. It uses the outdoor propagation model which includes large scale losses and

is a point to point model. The computationally simpler Durkin’s Model has been implemented in

the program. It includes free space losses and diffraction losses.

6.2 Simulation Parameters The program accepts different input parameters supplied by the user.

� BS and MS antenna height

� BS Transmitter power

� BS and MS location (latitude, longitude)

� Terrain database

The terrain database was based on freely available digital elevation model (DEM) stored as

terrain matrix. The DEM used in this simulation has accuracy of 3 arc second. The missing

elevation data were calculated using the interpolation technique. The program also provides

configurable settings for BS and MS parameters.

The program takes into account the following parameters:

� Free space path loss

� Ground reflection loss

� Fresnel loss

� Multiple edge diffraction

The modeling is done considering an omni-directional antenna as according to the need.


35

6.3 Program Structure The program has been divided into following component:

� RSSI prediction for whole coverage area: The RSSI prediction provides an configurable

transmitter position and height and displays the RSSI level and individual LOS losses and

diffraction losses along with the terrain profile of the area.

� Profile viewer: The profile viewer in the program displays the elevation profile between

the transmitter and the receiver points supplied by the user with the line-of-sight or no

line-of-sight indicated along with the path loss profile.

Figure 6.1: Profile Viewer

6.4 Simulation Methodology The overall prediction of the RSSI level at any point has been done on the basis of multiple loss

additions. The proposed Base Station location is taken as reference point for the power

calculations. Any other point is checked for its terrain status according to its altitude as well as its

visibility as seen by the BS.


36

6.4.1 Terrain Database Initially a terrain database has to be obtained. The terrain database here has been extracted from

NASA’s SRTM3 database that has a resolution of 3 arc which comes about 90m approximate

accuracy.

Is LOS Present ?

Fresnel Clearance?

Calculate Diffraction Loss using Epstein & Peterson

Algorithm

Input: Terrain Map Antenna Parameters BS Location

Find Free Space Path Loss

Interpolate between Tx and Rx points

Total Path Loss = Free Space Path Loss

Calculate Fresnel Loss

Repeat for all the points


+ Fresnel Loss


+ Diffraction Loss

N

Y

Y

N

Figure 6.2: Flowchart of Propagation Modeling


37

6.4.2 Interpolation The linear interpolation technique is applied to calculate the altitude at points of concern where

the data is not available.

Figure 6.3: Top View of interpolated map and line between Tx and Rx

tr

tr

xx

yymSlope

−−=)(

Case 1: 1<=mIncrement x to get the value of y

i.e, )( tptp xxmyy −+=

myt += [ 1=−=∆ tp xxxQ ]

Now, with xp and yp known, we calculate hp

or,

[ ]1

))((

)(

12

1121

112

121

−=−

−−−=

−−−=−

yy

yyhhhh

yyyy

hhhh

pp

pp

Q

Case 2: 1>mIncrement y to get the value of x

i.e, )( tptp xxmyy −=−

mxx tp

1+=∴ [ 1=−=∆ tp yyyQ ]

))(( 1121 xxhhhh pp −−−=∴


38

λπ rt

c

hhd

4=

6.4.3 Point-to-Point Calculations The BS site location is determined then. Now taking the BS as reference, the LOS (Line Of

Sight) feasibility of the link between the BS and the MS is calculated. A 30 m height antenna is

taken as standard.

Figure 6.4: LOS Check for path loss calculation

d

hhSlope tr

LOS

−=

At any point P between the Tx and Rx at the distance d’

'

'*'

hhh

dSlopeh

t

LOS

+==

hhp −=∆

If 0>∆ , then there is no LOS. Else, LOS is present. Then, we have to search for the 1st Fresnel

clearance.

For LOS signals, if the Fresnel clearance is also maintained, then a cross over point has to be

calculated that separates the free-space application and two-ray application.

6.5 Line of Sight Calculation 6.5.1 Cross-over Point The cross-over point is calculated as

d2

d1

P h

Tx

ht

d’

d

Rxhp

hr

dh


39

Figure 6.5: Plot of Free Space Loss and Ground Reflection Loss showing cross-over point

If the cross-over distance is greater than the separation distance between the Tx and Rx, two ray

model is applied that will be discussed earlier. If the distance is inside the cross-over limit, then

free space calculation is done.

6.5.2 Free Space Calculations Friis Equation is employed for free space calculation that is given by,

( ) Ld

GGPdP rtt

r 22

2

4)(

πλ

=

The Friis free space model is valid only for Fraunhofer region that lies beyond the distance ‘df’

given by

( )Dd f λ2

1=

Where,

D = Largest physical linear dimension of the antenna

The dimension taken is the length of the antenna used in CDMA BS.

Two-ray model calculates the received power as

Ld

hhGGPdP rtrtt

r 4

22

)( =


40

6.6 Cases of Diffraction 6.6.1 1st Fresnel Zone and Diffraction parameter As discussed earlier, the assurance of 1st clearance is required before the LOS calculations. The

1st Fresnel radius if not cleared, results in diffraction losses.

Slope of the receiver to transmitter is already calculated.

'*' dSlopeh LOS=The 1st Fresnel radius is given as

dSloper LOSh *=To check whether the LOS has Fresnel clearance, the height difference dh should be greater than

rh.

The difference is calculated as

221 '' hdd +=

1

222 drdd h −+=

'hhh t +=

hhdh p −=

21

21 )(2

dd

dddh

λυ +

×=

υ gives the diffraction parameter. If it is negative, there is LOS. Accordingly, the parameter is

used for cross-over calculations. If it is positive, then there is no LOS and the receiver lies in the

shadow zone. Accordingly, the reception is through diffraction whose diffraction gain depends

upon the diffraction parameter.

6.6.2 Epstein & Peterson Implementation for Multiple Edge Diffractions Multiple edge detection is a continuous angular iteration process. The aim is to find the peaks

between the transmitter and receiver that cause diffraction. The process begins from Tx at first.

Angles of the Tx with the successive points are calculated up to the Rx. The maximum angle at

the specific point is stored as max angle1 for iteration from Tx to Rx.

Now, the iteration begins from Rx towards the Tx. Angles of all the successive points are

calculated and the maximum angle is stored as max angle2 for that specific point. If the maximum

angles are obtained for same point, then the location is the single point indicating the single edge

diffraction. If two angular values are at different points, then there are two or more peaks

between the Tx and Rx.


41

The coordinates of two peaks are taken and checked for their LOS. If there is LOS, then it is a

two-edge diffraction case. If there is no LOS, it means that there is an obstruction between the

peaks and is the case for 3 or more edged diffraction.

The detection has been continued for up to 5 successive edges.

Figure 6.6: Two-edge diffraction

For two-edge diffraction as depicted above, initially the points 1 and 2 are located by angular

iteration. Then LOS is checked to see if there are any further edges. Now, the diffraction

parameter at edge 2 due to edge 1 is calculated according to the diffraction formula as stated

earlier. Then, the corresponding gain is calculated as per the quantified gain for the diffraction

parameter. Similarly, the gain at next edge due to previous edge is calculated. Here, in the above

case the gain at Rx is calculated for the source supposed to be at 1 and edge being 2. The total

gains for both the cases are added. This gives the total gain due to two-edge diffraction.

If three edges are detected, then the gains are added as below:

Diffraction gain due to edge 1 from Tx at edge 2 +

Diffraction gain due to edge 2 at edge 3 with source as 1 +

Diffraction gain due to edge 3 at Rx considering source at edge 2

This approach of adding the gains by superimposing the Tx source at the edges is the Epstein-

Peterson Implementation. The total gains are accumulated. The received power at the Rx now

can be calculated as the summation of Tx power and gains.

BodyLossCableLossGGPTotalPower ndiffractiofresnelLOS −−++=

6.7 Additional features The program has been made MATLAB independent for deployment and further software

development in .NET platform. This feature is implemented using MATLAB Builder for .NET

(also called .NET Builder), an extension to the MATLAB Compiler. The deployment package

includes MATLAB Component Runtime (MCR) and the simulation software.

TxRx

1

2 LOS


42

Requirements1. .NET Framework 2

2. MATLAB 7.4 or MATLAB Component Runtime

6.8 Simulation Limitation The simulation is for a single BS and hence does not consider any interference from other BS.

The attenuation due to atmosphere is not considered.

6.9 Verification of the RF Prediction Model The verification of the model has been done by measuring the received power levels at different

areas due to an isolated BS and then comparing it with the values predicted by the model. For the

verification of the data, Lele was chosen as the test base station. The signal and geographical

isolation from other interfering base stations made it a good choice.

Lele Base Station Details Location: Lele VDC, Lalitpur

Latitude: 27.570105˚ N Longitude: 85.315781˚ E Altitude: 1576 m

BTS Type: 1 Carrier 1 Omni (1C10) PN_ID: 129

The data collected due to time constraint were few and could not be taken as complete

verification of the project. Still it was sufficient to have some reflection on the model. The data

were considerably good for the long distances indicated by lesser predicted signal level than

observed. The data were a little bit wayward for the short distances. This is presumably due to

the large scale propagation model used for the simulation. The terrain map we could obtain was

of 3 arc (approx. 90m). The simulation also does not consider the small scale fading and

multipath effects contributing the error in prediction.

From the verification, the mean error of approx. -57 dBm was obtained. The verification data of

RF Prediction Model with 1C10 BS at Lele has been presented in Appendix C.5.

The simulation will be more predictive if following points are considered:

1. Small scale fading and Multipath effects

2. Scattering and Attenuation due to atmosphere

3. Greater accuracy of the terrain map.

The accumulation of signal and interference level database for different location and weather

conditions will also help in deriving a local correction factor which could be incorporated in

simulation to provide greater accuracy. Overall the simulation has been successful in its purpose

of predicting large scale losses.


43

Chapter 7 RESULTS AND CONCLUSION

The project has been a great window of opportunity to gain an insight on the mobile wireless

communication study. Choosing CDMA, we knew that things were complicated. Unlike other

communication measures, CDMA has more dynamic nature. Predicting things in the dynamic

wireless environment is itself a complex process. Thus, in aggregate the project was a real

challenge.

The analysis and design required an extensive RF survey of the analyzed area and the design site.

Use of the drive test tools was an approach employed for the tests. These results helped us for

testing the design model. This also helped us to have a hands-on experience in the real life

mobile communication scenario. The survey has emphasized on the need for the establishment of

a BS on the proposed site.

The model chosen for the modeling is a point-to-point prediction large scale model. Thus, the

model is more precise than other counterparts and is more practical and realistic. Moreover, the

whole project is based on the aim to provide a useful and applicable approach to design the

CDMA mobile network.

Statistical demographic data have also been used to make the coverage predictions more

accurate. National and regional statistics have been obtained from CBS and other sources. The

data presented are practical on-site data.

A user friendly Graphical User Interface has been built to depict the simulation. It holds various

options relating to the propagation modes of the signal from the base station. The model has been

used to conclude on the probable BS location. The specifications provided by NT have been used

for the transmitter settings. These things make the model more accurate. A wide RF pattern has

been obtained for the selected region and its proximity area. It can be taken as reference for the

design.


44

7.1 Future Carry-outs • Automatic Drive Test Data Logging

• Incorporation Directional Antenna

• Prediction of EC / IO by incorporation of more than one Base Station

• Expansion of Geographic area in the RF Signal Level pattern prediction

7.2 Recommendations • Regular optimization of the Base Stations

• Monitor and minimize any un-intended coverage of the Base Station / Optimization of the

Base Station to provide coverage only to its coverage region

• Optimization of the newly installed Base Station as soon as they are in operation

• Need of new 2C1O Base Station to improve network performance in/around Gamcha area

and to cover the subscribers of that region

• Introduce Electronic downtilting to the Base Station to reduce coverage in cases where

Mechanical downtilting alone is not sufficient

7.3 Conclusion We have completed the analysis of Madhyapur Thimi β – sector and found that the sector was

successful in fulfilling subscriber capacity requirements with a Good/Normal EC/IO observed in

its coverage. We have also completed the BS design for Gamcha, Dadhikot. Major coverage

regions of the designed BS and subscriber prediction have been included in the design. The BS

has been designed to support predicted subscribers concluded from the study and taking into

account future demands and the growing mobile trend. The RF Signal Prediction model

developed in this project can be useful for real world applications. The designed BS for Gamcha

can be implemented practically under supervision of the CDMA system engineers and experts.


BIBLIOGRAPHY

1. Rappaport, T. S., ‘Wireless Communications Principles and Practice’ Prentice Hall 2007, Second Edition

2. Garg , V. K., ‘IS-95 CDMA and cdma2000’ Pearson Education 2002

3. Seybold, J. S., ‘Introduction to RF Propagation’ John Wiley & Sons, Inc. 2006

4. Lee, J. S. and Miller, L. E., ‘CDMA Systems Engineering Handbook’ Arctech House 1998

5. Garg V. K., ‘Wireless Network Evolution 2G to 3G’ Pearson Education 2002

6. ‘CDMA 200 1x Release 0, Student Guide’ Qualcomm 2003

7. ‘Fundamentals of Wireless Communications & CDMA, Student Guide’ Qualcomm January 2000

8. ‘Periodic District Development Plan, Bhaktapur (2059/60 – 2063/64)’ District Development Committee Bhaktapur 2059

9. ‘Population of Nepal, Population Census 2001’ Central Bureau Of Statistics June 2002

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