CODE DIVISION MULTIPLE ACCESS (CDMA) FOR MOBILE COMMUNICATIONS (PSK MODULATION)

IDASUZANA BT. IDRIS

Tesis Dikemukakan Kepada Fakulti Kejuruteraan, Universiti Malaysia Sarawak

Sebagai Memenuhi Sebahagian daripada Syarat Penganugerahan Sarjana Muda Kejuruteraan

Dengan Kepujian (Kejuruteraan Elektronik dan Telekomunikasi) 1999

To my beloved parents, brothers and friends.

ACKNOWLEDGEMENT

I would like to express my gratitude to the people who have helped me in the

completion of this thesis. I would like to convey my deepest gratitude to my thesis

supervisor Madam Park Young Soon for her guidance, expertise and knowledge.

Special thanks to Encik Wan Abu Bakar for providing me with the necessary

equipment, parts and tools that were needed to complete this thesis.

Last but not least thanks to those who are involved directly or indirectly towards in

completing this thesis.

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ABSTRACT

The aim of this project is to study the general concept of digital radio communications

focused on modulation techniques. To encode information onto a RF carrier signal, it

needs to be modulated. Several methods of doing these digital methods will be

discussed. 8-PSK modulation is one of the modulation techniques used in the CDMA

multiple access method. Experiments of an 8-PSK modulator are conducted to

understand the theory. An 8-PSK modulator block diagram are used as a guidance to

hardware design. Basic knowledge in Spread spectrum Communications and CDMA

are being introduced in the first and second chapter. In chapter three, digital

modulation techniques are studied. In chapter four, additional knowledge in cellular

communications is introduced. Experiments on the hardware implementation are

discussed in chapter five. In the final chapter, conclusion and future

recommendations are presented. This study discovers that while doing the

modulation process, a type of filters has to be considered. In reality, the nonideal

practical filters reduce the theoretical maximum values of bandwidth efficiency.

During the time this project is being conducted, several types of problems have arisen

such as results obtained different from theoretical value and a serial input source is

needed in order to produce serial data bit. The recommendations suggest how to

improve the system's performance in the future.

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AB STRAK

Tujuan utama kajian ini ialah untuk menentukan konsep umum fokus komunikasi

radio digital ke atas teknik-teknik modulasi. Untuk mengekodkan maklumat kepada

isyarat pembawa RF, ia perlu dimodulasikan. Beberapa kaedah digital ini akan

dibincangkan. Modulasi 8-PSK ialah salah satu teknik modulasi yang digunakan

dalam kaedah "CDMA Multiple Access". Satu eksperimen berkaitan modulator 8-PSK

telah dilakukan untuk memahami maklumat. Kekotak diagram modulator 8-PSK

dijadikan panduan di dalam menyediakan rekaan kakasan. Pengetahuan asas dalam

komunikasi sebaran spektrum dan CDMA dibincangkan dalam bab pertama dan

kedua. Bab tiga mendalami kerja lapangan teknik modulasi digital. Bab keempat,

maklumat tambahan di dalam komunikasi selular diperkenalkan. Kajian ke atas

pelaksanaan kakasan dibincangkan dalam bab kelima. Bab keenam membincangkan

kesimpulan dan cadangan di masa hadapan. Kajian ini menemui bahawa sewaktu

proses modulasi, sejenis penapis perlu diberi pertimbangan. Sementara hakikatnya

pula, penapis praktikal mengurangkan nilai-nilai maksimum kecekapan jarak

gelombang secara teorinya. Semasa projek ini dijalankan, beberapa masalah telah

timbul seperti nilai eksperimen yang diperolehi berbeza dari nilai teori dan sumber

kemasukan sesiri diperlukan di dalam penghasilan data bit sesiri. Oleh itu, cadangan

di masa hadapan adalah penting untuk memperbaiki persembahan sistem.

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TABLE OF CONTENTS

APPROVAL LETTER

APPROVAL SHEET

PROJECT TITLE

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENTS

LIST OF FIGURES

GLOSSARY OF ABBREVIATIONS

Chapter

INTRODUCTION

1.1 Background

1.2 Introduction to Spread Spectrum

Communications

1.3 Direct Sequence and Frequency Hopping

Systems

1.4 Pseudo Random Sequences

1.5 Applications Of Spread Spectrum

Communications

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V

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1

1

3

4

4

5

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1.6 Access Techniques For Mobile 5

Communications

1.6.1 Frequency Division Multiple Access (FDMA) 6

1.6.2 Time Division Multiple Access. (TDMA) 6

1.7 Implementation Overview 7

1.8 Concerns 8

1.9 Scope of Works 9

2 THE CONCEPT OF CODE DIVISION MULTIPLE 10

ACCESS (CDMA)

2.1 Background 10

2.2 Traditional Multiple Access Communications 10

2.3 The Magic of CDMA 13

2.4 Near Far Problem And Power Control 16

2.5 Embedded Cell Capacity 18

2.6 So what is the point of using CDMA? 19

2.7 Voice Coding 20

2.8 Multipath Propagation 21

2.8.1 When does multipath cause fading and when 22

does it not?

2.9 Coverage versus Capacity 22

23

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3 DIGITAL MODULATION TECHNIQUES SURVEY

3.1 Introduction

3.2 Digital Modulation ASK, FSK and PSK

3.2.1 Amplitude Shift Keying

3.2.2 Frequency Shift Keying

3.2.3 Phase Shift Keying

3.. 3 Quadrature Overlapped Raised Cosine

Modulation

3.3.1 Quadrature Phase Shift Keying

3.3.2 A GMSK Structure

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24

24

25

26

29

31

32

36

4 CELLULAR COMMUNICATIONS 40

4.1 Historical Background

4.2 Analog To Digital

4.3 Global Market & Providers

40

44

44

5 EXPERIMENTS ON 8-PSK MODULATOR 46

5.1 Summary

5.2 Operational Amplifiers

5.2.1 Practical Operational Amplifier Circuit

5.2.2 Results

5.3 Noninverting Op-Amp

5.4 Differential Amplifier

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46

47

49

50

51

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5.5 Hardware Implementation

5.5.1 Dual Four Level Converter

5.6 Experiment Results on the Hardware

5.7 Components of the 8-PSK Modulator

5.7.1 Carrier Oscillator 1700Hz

5.7.2 900 Delay

5.7.3 I and Q Balanced Modulator

5.7.4 Linear Adder and Output

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54

59

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61

62

64

65

6 CONCLUSION AND RECOMMENDATIONS 66

6.1 Conclusion

6.2 Recommendations

66

68

BIBLIOGRAPHY 70

APPENDICES 72

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LIST OF FIGURES

FIGURE

1.2 Spread Spectrum Systems

1.7 8-PSK Modulator Block Diagram

2.2 Reuse Channel Patterns

3.2.1(a) Amplitude Shift Keying

3.2.1(b) Amplitude Shift Keying - frequency domain

3.2.2(a) Frequency Shift Keying - frequency domain

3.2.2(b) Frequency Shift Keying

3.2.3 Binary Phase Shift Keying

3.3.1(a) Overlapped Raise Cosine Shaped Pulse

3.3.1(b) Power Spectral Densities of QORC, QPSK and

MSK

3.3.1(c) PSD comparison of QORC, QPSK and MSK

3.3.2(a) Symbols and Phase (in radians) of MSK & GMSK

signals samples.

3.3.2(b) Baseband (I, Q) MSK and GMSK signal vs

samples for f9T=36

3.3.2(c) MSK and GMSK signal vs samples for f5T=36

4.3 U. S Providers Standards

5.2.1(a) Inverting Amplifier

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7

13

25

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5.2.1(b) Noninverting Amplifier

5.5(a) Dual Input To Four Level Converters

5.5(b) Dual Output To Four Level Converters

5.5(c) Demodulator Section of an 8-PSK modem

5.6 Truth Table

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56

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GLOSSARY OF ABBREVIATIONS

A/D Analog to Digital Converter

ASK Amplitude Shift Keying

AMPS Advanced Mobile Phone Service

BPSK Binary Phase Shift Keying

CDMA Code Division Multiple Access

CMOS Complementary Metal Oxide Semiconductor

DBPSK Differential Binary Phase Shift Keying

DS Direct Sequence

FCC Federal Communications Commissions

FM Frequency Modulation

FDMA Frequency Division Multiple Access

GSM Global System For Mobile Communications

GMSK Gaussian Minimum Shift Keying

IC Integrated Circuit

MSK Minimum Shift Keying

PCS Personal Communications Systems

PN Pseudo-noise

PSD Power Spectral Densities

PSK Phase Shift Keying

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PSTN Public Switch Telephone Network

PPM Pulse Position Modulation

QAM Quadrature Amplitude Modulation

SCPC Single Channel Per Carrier

SNR Signal To Noise Ratio

TDMA Time Division Multiple Access

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CHAPTER 1

INTRODUCTION

1.1 Background

An important concern in the study of digital communications is that of

providing efficient utilization of signal bandwidth and power. Notwithstanding the

importance of these two primary communication resources, there are situations where

it is necessary to sacrifice their efficient utilization in order to meet other design

objectives. A system may be required to provide a form of secure communication in a

hostile environment such that the transmitted signal is not easily detected or

recognized by unwanted listeners. A class of signaling techniques known as spread

spectrum communications caters to this requirement.

1.2 Introduction to Spread Spectrum Communications

The primary advantage of a spread spectrum communication system is its

ability to reject interference whether it be unintentional interference by another user

simultaneously attempting to transmit through the channel, or the intentional

interference by a hostile transmitter attempting to jam the transmission.

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It also provides excellent narrow-band noise rejection characteristics. The

fundamental concept of spread spectrum is to spread the baseband digital signal

with a periodic binary sequence, noise-Like in nature, called a pseudo random

noise (PN) sequence. Through this spreading technique, the relatively narrow-

band digital baseband signal is made to appear as wide band noise. Furthermore,

the receiver must know the pseudo random noise sequence used by the

transmitter in order to properly recover the transmitted signal. Any other

additional receiver listening on the channel will not be capable of recovering the

transmitted message without the correct pseudo random noise sequence, hence

the secure nature of this type of communication. The spread spectrum systems

are shown in Figure 1.2.

f

i _ý

Data Modulator

Data

Carrier

De spreading

t Spreading sequence BPF

Data demodulator

Synchronization

Figure 1.2 Spread Spectrum Systems

f

ý

Carrier

ý

The same pseudo random sequence used to de-spread the received signal,

that is, convert the wide band signal to a narrow band signal, will spread any narrow

band noise, such as jamming signals, to a wide band signal. In effect, this makes

narrow band noise appear as wideband noise at the receiver input, improving

performance.

Spread spectrum communication was originally developed for military

applications, where resistance to jamming (interference) was of major concern.

However, commercial applications also benefit from the unique characteristics of

spread spectrum communication. It can be used in multiple-access communications,

in which a number of independent users are required to share a common channel.

1.3 Direct Sequence and Frequency Hopping Systems

There are two common spread spectrum techniques used to transmit

baseband digital signals. They are Direct Sequence (DS) Spread Spectrum and

Frequency Hopping (FH) Spread Spectrum.

In a Direct Sequence Spread Spectrum system, a pseudo random sequence

is used to convert a narrow-band digital signal to a larger bandwidth signal, referred

to as a spread signal. To transmit the spread signal through a channel such as air,

Binary Phase Shift Keying (BPSK) or Differential Binary Phase Shift Keying

(DBPSK) techniques are applied to the spread signal. A sinusoidal carrier is

multiplied by the spread data to produce BPSK modulated data or the carrier is

multiplied by differentially encoded spread data to produce DBPSK modulated data.

The received signal may be recovered by using coherent detection, a phase lock loop

and a matched filter. 3

Synchronization is of concern with the recovery of the baseband digital

signal. For proper operation, a spread spectrum system requires that the locally

generated pseudo random sequence used to de-spread the received signal be

synchronized to the same pseudo random sequence for the transmitted signal.

When a locally generated pseudo random sequence is compared to an

interval of the received signal, a measure of correlation is used to determine when the

two signals are satisfactorily aligned. After alignment, the remaining received signal

is then correlated with the pseudo random sequence and the received signal is

properly de-spread using a matched filter and the baseband digital data is properly

recovered.

In Frequency Hopping Spread Spectrum systems, the data modulated

carrier is randomly hopped from one frequency to another over a predetermined period

so that its spectrum is relatively sequential rather than instantaneous compared to

that of Direct Sequence Spread Spectrum.

1.4 Pseudo Random Sequences

Frequency Hopping and Direct Sequence Spread Spectrum apply the

principle of spreading the spectrum through the use of pseudo random sequences.

Pseudo random means to be deterministic. For unwanted listeners, it seems random.

But for wanted listeners it's predictable. Pseudo noise sequence is generated by shift

registers. The generator is a set of feedback shift registers operated by a single clock.

During a pulse of the clock, the state of flip-flop is shifted to the next one and result is

fed back as the input to the first flip-flop. This sequence is then employed in the

transmitter and receiver for spreading and de-spreading.

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1.5 Applications of Spread Spectrum Communications

Spread spectrum technology is used in many areas of telecommunications.

Some applications of this technology are outlined below. The rapid growth in Digital

Cellular Mobile Radio Systems has resulted in increasing congestion in the cellular

bands. One way to get around this problem is with Code Division Multiple Access

(CDMA) Direct Sequence Spread Spectrum Systems. With such a system, users are

able to share a common channel simultaneously. Added security is also achieved since

each user is assigned a unique code sequence, without which the transmitted message

intended for the user would appear as noise to all others.

Another area where a more secure communications link is required is with

cordless telephones. A relatively new application of spread spectrum technology has

appeared in Wireless Local Area Networks. Operating in the 2.4 GHz, these network

cards are capable of operating at two or four Mbps.

1.6 Access Techniques for Mobile Communications

Besides CDMA, they are two access techniques for mobile communications

which is called Frequency Division Multiple Access (FDMA) and Time Division

Multiple Access (TDMA). Both of these methods are widely used for digital

transmission. Very brief distinctions between these two methods are now being

clarified for the mobile communications applications.

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1.6.1 Frequency Division Multiple Access (FDMA)

Roughly speaking, FDMA simply means splitting up an available frequency

band into a specific number of channels. Each channel is used for duplex operation.

The bandwidth of each channel depends on the type of information signals to be

transmitted. This information is superimposed on a carrier at the channel center

frequency. The information can be a made up of several information signals. Either

multiplexed prior to being superimposed on the carrier or a single information signal

can be placed on the carrier. These types would be called a single channel per carrier

(SCPC) system, which is widely used in satellite technology. At first, the analog

information was superimposed on the carriers using FM. Then the analog signals

have been converted to digital pulse streams and the PSK and QAM techniques

employed.

1.6.2 Time Division Multiple Access (TDMA)

In TDMA it uses only one frequency band. Transmitting information for

each channel in allocated time slots creates many channels. In TDMA mobile radio

system, each base station is allocated a 25kHz or 30kHz channel, and users share this

same channel on a time allotted basis. The maximum number of users of each

channel depends on how many bits per second are required to digitize the voice of each

user. For example if a digital modulation technique such as 8-PSK is used, each voice

channel can be digitized with a bandwidth efficiency of at least 3b/s/Hz. In reality, the

nonideal practical filters reduce these theoretical maximum values.

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1.7 Implementation Overview

The figure below is the diagram of the 8-PSK modulator block diagram

which is intended to be implemented.

Figure 1.7 8-PSK modulator block diagram

The first step should be take into account in process of completing this

design are based on the above block diagram. For the second steps try to find out

what is the circuit diagram involved for each block diagram. The circuit in figure

shows that the input half of the 8PSK modulator differs from the QPSK modulator but

that the modulators and carrier generator are exactly the same. The 3 input bits are

applied to the shift register in a, b or c order. The a and c bits are unloaded to the dual

input, four level converter and I-balanced modulator for the sine wave output. The b

bit and an inverted c bit (c) are applied to the dual input, four-level converter and the

Q balanced modulator for the cosine wave output. The sine and cosine waves are then

vectorially added and produce an 8PSK output. These outputs are put onto the phone

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lines. In the phase shift modulator, two identical, balanced modulators are used in

parallel. The message signal to each modulator is identical in all respects except

phase. The signal is fed directly to one modulator and delayed by 90degree to the

second modulator. Both modulators use the same carrier signal frequency, but the

carrier signal is delayed by 90 degree when applied to the second modulator. The

output voltages from the modulators are then added in summing amplifier. The

summing of these two signals results in the cancellation of one sideband.

1.8 Concerns

There are several areas of concern in this project. Outlined below are some

of these concerns.

" For the successful operation of this system, it is imperative that the carrier

oscillator signal be in phase with local carrier signal at the balanced

modulator.

" The delivery of components is a major concern that can seriously affect the

time line and prevent successful completion of this project.

" Noise within the channel is another concern that which may prevent the

output part from properly detecting the signal. The amount of noise in the

channel will affect the phase shift of sine wave and cosine wave that the

system will operate, if at all.

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1.9 Scope of works

Part 1

" Study on Spread Spectrum Communications.

" General review through the suggested reference books.

" Solve problems by doing some exercise.

Part 2

" Study on modulation technique area.

" Doing a survey on digital modulation techniques.

" Majoring on Phase Shift Keying modulation area.

" Try to design any type of phase shift keying modulator.

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CHAPTER 2

THE CONCEPT OF CODE DWISION MULTIPLE ACCESS (CDMA)

2.1 Background

The great attraction of CDMA technology from the beginning has been the

promise of extraordinary capacity increases over narrowband multiple access wireless

technologies. Simple models suggest that the capacity improvement may be more

than 20 times that of the existing narrowband cellular standards, such as AMPS in

North America, NMT in Scandinavia, TACS in the United Kingdom. Historically, the

capacity was calculated using simple arguments. Reality, of course, is much more

complicated than the idealized models. Real cell coverage areas are highly irregular,

not the neat hexagons found in textbook models. Offered load is not spatially uniform,

changes dramatically with time-of-day, and is often subject to other uncontrollable

influences.

An idealized multiple access mobile radio system consists of a family of base

stations, or "cells, " geographically distributed over the service area, and mobile

stations. The term "mobile station" refers to any subscriber station. The majority of

new cellular sales are now in fact hand held portable units, and the market outlook is

for that trend to continue for the foreseeable future

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Non-traditional uses, such as wireless data modems in laptops, are also expected to

grow dramatically in the near future.

Spectrum for mobile wireless is normally allocated in frequency division

duplex (FDD) paired bands. Cellular systems are separated by 45 MHz, PCS bands by

80 MHz. Although there have been some proposals for the use of time division duplex

(TDD), such operation inherently limits the coverage area, and have not achieved

widespread acceptance.

Communication between base stations and mobile stations is established by

a negotiation upon call origination. Once communication is established between base

and mobile, movement of the mobile is detected and the service is handed over from

one base station to another. One cell at a time services each mobile in the narrowband

services. The concept of handoff is extended to a multi-way simultaneous "soft"

handoff in the CDMA standards.

2.2 Traditional Multiple Access Communication

Traditionally radio communication systems have separated users by

frequency channels, time slots, or both. These concepts date from the earliest days of

radio. Even spark transmitters used resonant circuits to narrow the spectrum of their

radiation. Scheduled net operation was probably the first manifestation of time

slotting. Modern cellular systems began with the use of channelized analog FM. More

recently several hybrid FDM-TDM digital systems have been developed, ostensibly to

enhance service quality and capacity. In all these systems, each user is assigned a

particular time-frequency slot.

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