1

DESIGN AND IMPLEMENTATION OF

OFDM - CDMA TRANSCEIVER ON

SOFTWARE DEFINED RADIO

A PROJECT REPORT

DE-30 (DEE)

Submitted by

NS MUHAMMAD HASSAN IRSHAD

NS HAFIZ AHMAD DANYAL

NS MUHAMMAD RAFAY NADEEM

NS MUHAMMAD NADEEM

BACHELORS

IN

ELECTRICAL ENGINEERING

YEAR

2012

PROJECT SUPERVISORS

Dr. SHOAB A. KHAN

Dr. SHAHZAD AMIN SHEIKH

COLLEGE OF ELECTRICAL AND MECHANICAL ENGINEERING

PESHAWAR ROAD, RAWALPINDI

NUST COLLEGE OF

ELECTRICAL AND MECHANICAL ENGINEERING

DE-3

0 (D

EE)

YEA

R 2

01

2

2

DECLARATION

We hereby declare that no portion of the work referred to in this project thesis has been

submitted in support of an application for another degree or qualification of this of any

other university or other institute of learning. If any act of plagiarism is found, we are

fully responsible for every disciplinary action taken against us depending upon the

seriousness of the proven offence, even the cancellation of our degree.

COPYRIGHT STATEMENT

Copyright in text of this thesis, rest with the student author. Copies (by any

process) either in full, or of extracts, may be made only in accordance with

instructions given by the author and lodged in the library of NUST College of

E&ME. Details may be obtained by the librarian. This page must form part of any

such copies made. Further copies (by any process) of copies made in accordance

with such instructions may not be made without the permission (in writing) of the

author.

The ownership of any intellectual property rights which may be described in this

thesis is vested in NUST College of E&ME, subject to any prior agreement to the

contrary, and may not be made available for use by third parties without the

written permission of the College of E&ME, which will prescribe the terms and

conditions of any such agreement.

Further information on the conditions under which disclosures and exploitations

may take place is available from the library of NUST College of E&ME,

Rawalpindi.

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ACKNOWLEDGEMENTS

By the Grace of Allah All Mighty, we have successfully completed our final

year project work. We are eternally grateful to Almighty Allah for bestowing

us with the strength, potential, the ability, the knowledge, the guidance and

resolve to undertake and complete the project. We acknowledge the efforts

and support of our parents and friends for their love, constant and incessant

support along with the mental peace and strength which they gave us through

their prayers, without which this project would not have been completed.

We would like to acknowledge Dr. Shoab A. Khan for providing us with an

opportunity to work on a highly challenging and interesting project and

also to their technical acumen, precise suggestions, timely discussions

and unwavering support and understanding during the many hours we

dedicated to achieving this milestone in our lives and careers. Special

thanks to our supervisor Dr. Shahzad Amin Sheikh for his continuous

support and guidance throughout this course of work. H is valuable

expe11ise, advice and encouragement made this work possible.

Lastly We would also like to express our gratitude to all the faculty members

specially Sir Hassan Ijaz, Sir Sajid Gul Khawaja , Sir Zeeshan and Madam

Sidra Liaquat for their expertise and advice on various hardware and

software related issues.

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TABLE OF CONTENTS

Declaration and Copyright Certificate………………………………………………….…............1

Acknowledgements.………………………………………………………….……………………2

Table of Contents.…………………………………………………………………….……...……3

List of Figures……………………………………………………….…………………………….6

List of Abbreviations………………………………………………………………………...…....8

Abstract…………………….…………………………………………………………...................9

Error! Bookmark not defined.

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LIST OF FIGURES

Figure 1.1 Communication Systems……………………………………………………..11

Figure 1.2 Digital Communication System………………………………………………12

Figure 2.1 Project layout…………………………………………………………………14

Figure 2.2 Flow Diagram………………………………………………………………...15

Figure 3.1 Modules of SFFSDR platform………………………………………………..17

Figure 3.2 Block Diagram of SFF SDR Platform………………………………………..19

Figure 4.1 OFDM Waveform Representation……………………………………………23

Figure 4.2 Basic OFDM System Model………………………………………………….25

Figure 4.3 Multiple Access Technologies……………………………………………..…26

Figure 4.4 CDMA Representation……………………………………………………….26

Figure 4.5 Figure of Merit………………………………………………………………..27

Figure 4.6 Chips and Symbols…………………………………………………………...27

Figure 4.7 Spread Spectrum……………………………………………………………...28

Figure 4.8 Data Stream Spreading……………………………………………………….29

Figure 4.9 CDMA System Model………………………………………………………..32

Figure 5.1 Transmitter Model……………………………………………………………32

Figure 5.2 Receiver Model……………………………………………………………….33

Figure 5.3 OFDM-CDMA System Model……………………………………………….34

Figure 6.1 Transmiter Block Diagram…………………………………………………...35

Figure 6.2 Receiver Block Diagram……………………………………………………..38

Figure 6.3 SUI Channels Summary………………………………………………………40

Figure 6.4 Scatter Plot……………………………………………………………………41

Figure 6.5 OFDM Time Domain…………………………………………………………41

Figure 6.6 CDMA Spectrum……………………………………………………………..42

Figure 6.7 Equalization…………………………………………………………………..44

Figure 7.1 VPSS Overview………………………………………………………………48

Figure 7.2 VPBE…………………………………………………………………………48

Figure 7.3 VPBE…………………………………………………………………………49

Figure 7.4 Xilinx Simulations……………………………………………………………51

Figure 7.5 ChipScope Pro Results………………………………………………………..52

Figure 8.1 Comparisons in AWGN Channel……………………………………………..53

Figure 8.2 Comparisons in Rayleigh Fading Channel…………………………………...54

Figure 8.3 Comparisons in SUI 6 Channel……………………………………………….55

Figure 8.4 Hardware Utilizations and Trade-Offs………………………………………..56

Figure 9.1 Use of Training Sequence for Estimation of Channel Coefficients…………..57

Figure 9.2 Rake Receiver………………………………………………………………...57

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LIST OF ABBREVIATIONS

OFDM: Orthogonal Frequency Division Multiplexing

CDMA: Code Division Multiple Access

DFT: Discrete Fourier Transform

DSP: Digital Signal Processing

DSSS: Direct Sequence Spread Spectrum

DTFT: Discrete Time Fourier Transform

FFT: Fast Fourier Transform

FPGA: Field Programmable Gate Array

FT: Fourier Transform

HDL: Hardware Description Language

IF: Intermediate Frequency

LUT: Look-Up Table

PN Sequence: Pseudo Noise Sequence

RFD: Ready for Data

SDR: Software Defined Radio

SNR: Sound to Noise Ratio

VHDL: VHSIC Hardware Description Language

CCS: Code Composer Studio

VPBE: Video Processing Back End

VPFE: Video Processing Front End

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SoC: System on Chip

BER: Bit Error Rate

IFFT: Inverse Fast Fourier Transform

SFF: Small Form Factor

UWB: Ultra Wide Band

MLD: Maximum Likelihood Detection

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ABSTRACT

Software defined radio (SDR) is an important element of wireless technology and fast

becoming a hot topic in the telecommunication field. Determining the digital hardware

composition of a software radio is a key design step in its creation. Hybrid

GPP/DSP/FPGA architecture is a viable solution for software defined radio technology.

Software-defined radio (SDR) is a radio communication technology in which the

functionality is defined in the software instead of hardware. This allows for easy, efficient

and low-cost upgrades as no hardware replacement is required. The software radio will

use frequency hopping as the frequency translation scheme for robust and reliable

communication. In Frequency hopping, the radio frequency of transmission continuously

changes according to a pseudorandom pattern known only to the transmitter and receiver.

This scheme makes the communication link more robust to the affects of jammers.

Therefore frequency hopping offers a more secure mode of communication in comparison

to conventional fixed frequency telecommunication links.

The proposed transceiver design is implemented keeping in consideration the

specifications of SDR platform. Therefore, most of the transceiver operations such as

digital signal processing, amplification, equalization and filtering are software based.

SDR implementation makes it easier to improve and redesign the components in

software. The transceiver design is such as to allow all signal processing and data

acquisition to take place at baseband frequencies after elimination of intermediate

frequency (IF). This means there is less power consumption.

The project investigates Orthogonal Frequency Division Multiplexing—Code Division

Multiple Access (OFDM-CDMA) modulation technique. OFDM-CDMA attempts to

combine the advantages of OFDM and CDMA that are complete immunity to multipath

fading and multi user capability respectively.

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CHAPTER 1

Background

Wireless communication has made a huge leap since its first commercial service in the

late 1970s and early 1980s. In the UK, the 1G service was provided by Total Access

Communications Systems (TACS) in 1985. TACS standard is based upon an earlier Bell

Labs system which was developed in the late 1970s and has been deployed in North

America under the name Advanced Mobile Phone System (AMPS).The expend icy is the

biggest benefit which people get from wireless technology. To ensure reliable and secure

transmission the development of wireless technology is still in progress. A lot of

aggravations associated with cords and cables have been removed using wireless

technology.

The first move toward digital wireless communications in Europe began in the early

1980s when the Conference of European Post and Telecommunications (CEPT) initiated

the work for a new digital cellular standard which would provide the capacity for an ever-

increasing demand on the European mobile networks.

New generation is defined by the result of technology changes over a period of 10-15

years’ time frame. Broadband wireless communication systems have gained much

reputation in recent years. The demand for the higher capacity cellular networks has

increased. This demand for higher capacity network has led to the development of third

generation (3G) telecommunication systems. Although, the 3G wireless technology has

not yet been fully implemented, leading companies in the industry are already laying the

networks for 4G technology. The problem with currently used technology (2G) is that its

data rate is too low that it is not possible to use video conferencing etc. The first is 2.5G

(GPRS) technology that allows data transfer at a better rate than GSM and recently 3G

(WCUMNUMTS) technology has come into picture.

4G allows data transfer up to 100 Mbps and stands to be the future standard of Mobile

Wireless Communication. The 4G technology will be able to support Interactive services

like Video conferencing (with more than two sites simultaneously), digital video and

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audio broadcasting, Power line technology, AOSL, Wireless Local Area Networks (LAN)

and Metropolitan Area Networks (MAN), digital radios and Ultra-Wide Band (UWB)

communications.

1.1 Communication System

Communication is basically the transmission of information from one point to another

through a succession of processes as described here:

1. Generation of a message signal i.e. the information e.g. voice, music, picture etc.

2. Description of this signal with a certain measure of precision, by a set of symbols:

electrical, aural, or visual.

3. Modulation of these symbols that is the encoding.

4. Transmission through a medium of the encoded data.

5. Decoding and reproduction of original symbols.

6. Recreation of original message signal.

The above mentioned process is common for any communication process but encoding

techniques and the transmission medium may vary. Wireless communication is the

transmission of data through air. However whatever the form of communication is used

three basic elements are common to every communication system, namely, transmitter,

channel and receiver. Figure 1 shows a basic model of a wireless communication.

Communication System

Message Signal Transmitter

Channel

Reciever User Information

Transmitted Signal Received Signal

Figure 1.1 Communication System

1.2 DIGITAL COMMUNICATION

A digital communication system can be represented by a block diagram shown in Figure

2. The quality of the message signal is improved by source encoder. The resulting

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sequence of symbols is called source code word. The data stream is then processed by the

channel encoder and the resulting data is called the channel code word. Finally the

modulator represents each symbol of the channel code word by a related analog symbol.

The sequence of the analog symbols is called waveform which is fit for transmission over

the channel. At the receiver, the channel output is processed in the reverse order to that in

the transmitter. Design of a digital communication system complex in theory but is easy

to build. Moreover the system is robust in terms of temperature variations, life etc.

Figure 1.2 A Digital Communication System

From this discussion it is apparent that the use of digital communications requires a

considerable amount of electronic circuitry, but nowadays electronics are cheap, due to

the multi-fold increase in the availability of VLSI circuits in the form of silicon chips.

Several digital signal processors are available for the design and implementation of

different communication system. However with the advancements in technology FPGAs

are mostly used for the design of complex communication systems. One such platform for

the design and implementation of digital communication system is the SDR (Software

Defined Radio) platform. Now we describe some basics of SDR platform.

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CHAPTER 2

Introduction

2.1 Aim of the Project

The aim of this research work is to prove that proposed OFDM-CDMA modulation

scheme for 4G technology provides a better performance as compared to other schemes

and to develop an accurate algorithm for implementing OFDM-CDMA which combines

the positive aspects of both basic OFDM and basic CDMA on Software Defined Radio

using Walsh codes as PN sequence for spreading and IFFT to achieve orthogonal

subcarriers in OFDM part.

2.2 Scope of the Project

Following are the challenging goals for the completion of project:

To develop algorithms and designs for CDMA system using Walsh codes.

To develop algorithms and designs for the OFDM system using IFFT.

Integration of the whole system as Hybrid OFDM-CDMA system.

Simulation of the whole system using MATLAB.

Creating a fixed point code for generating random data and NRZ conversion in C

language using Code Composer Studio

Creating a VHDL code for the whole system using Xilinx.

Communication between DSP processor DM6446 and Vertex 4 using VPSS module.

Implementation on SFF Software Defined Radio.

2.3 Project Description

The project deals with the design and implementation of OFDM-CDMA Transceiver on

SDR. The initial goal was to develop the algorithms individually of each basic

modulation scheme as separate module. Basic OFDM and basic CDMA system was

modeled and its algorithm was designed and simulated in MATLAB. Then OFDM-

CDMA system was modeled and its algorithm was designed and implemented in

MATLAB keeping in view different possible methods of combining OFDM and CDMA.

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The next stage was to write a C and VHDL code of OFDM-CDMA transceiver for its

Implementation on DSP and FPGA. The final stage was the implementation of

transceiver on reconfigurable Lyrtech SFF SDR using a wired channel.

2.4 Project Layout

Figure 2.1 Project layout

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2.5 Flow Diagram of OFDM-CDMA System

Figure 2.2 Flow Diagram

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CHAPTER 3

Software Defined Radio

3.1 Importance of Software Defined Radio

With the proliferation of wireless standards—including wide area 3G, 2.5G, and local

area 802.11 networks—future wireless devices will need to support multiple air-interfaces

and modulation formats. Software defined radio (SDR) technology enables such

functionality in wireless devices by using a reconfigurable hardware platform across

multiple standards. With FPGA and data converter technology continuously evolving, the

SDR concept is increasingly becoming a reality.

3.2 Introduction to Software Defined Radio

Software-defined radio (SDR) is a radio communication technology that is based on

software defined wireless communication techniques instead of hardwired realizations.

This can be explained as follows; waveforms are produced as sampled digital signals,

converted from digital to analog signal via a wideband DAC and then possibly up

converted from intermediate frequency (IF) to radio frequency (RF). The receiver,

performs this process in reverse, it uses a wideband Analog to Digital Converter (ADC)

that captures all of the channels of the software radio. The receiver then extracts, down

converts and demodulates the channel waveform using software on a general purpose

processor in our case GPP. In other words, frequency band, air interface protocol and

functionality can be upgraded by updating or downloading new software. This saves the

replacement of complete hardware and reduces the modification cost. SDR provides a

competent and protected solution to the problem of building multi-mode, multi-band and

multifunctional wireless communication devices. An SDR is capable of being re-

programmed or reconfigured to maneuver with different waveforms and modulation

schemes through runtime loading of new waveforms and protocols. These waveforms and

protocols can contain a number of different parts, including modulation techniques and

other important performance parameters defined in software as part of the waveform

itself. Efficient and effective SDR design requires a standard programmable hardware

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platform that allows designers to easily build efficient systems based on tough system

requirements with high computational complexity.

3.3 SFF (Small Form Factor) Software Defined Radio

The traditional SDR concept introduces flexible terminal reconfiguration by replacing

radios completely implemented in hardware by those that are reconfigurable or even

reprogrammable in software to a large degree. This includes reconfiguration of antennas,

the radio transceiver and the baseband. The TI Small Form Factor (SFF) Software

Defined Radio (SDR) Development Platform provides the whole signal chain hardware

from antenna to baseband as well as a software board support package that supports

complete group of software development tools in one integrated development platform.

With the kit, developers can easily design waveforms, create and test single or multi-

protocol radios for applications in military, public safety, commercial etc. The SFF SDR

development platform is designed to be used in the development of applications in the

field of software-defined radio.

3.4 Structure of SFF SDR

The SFF SDR platform is composed of three boards, as illustrated below:

1. A digital processing module

2. A data conversion module

3. An RF module

Figure 3.1 Modules of SFFSDR platform

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Digital processing module

The digital processing module is equipped of a Virtex-4 FPGA and a DM6446 SoC to

offer developers the necessary performance to implement custom IP and acceleration

functions with varying requirements from one protocol to another on the same hardware.

Data conversion module

The data conversion module is equipped with dual-channel analog-to-digital and digital-

to-analog converters

RF module

The RF module covers a variety of frequency ranges in transmission and reception,

allowing it to support a wide range of applications. The RF module is composed of an RX

section-a three-stage super heterodyne receiver with a final IF frequencies of 30 MHz and

a selectable bandwidths of 5 or 20 MHz depending on the application. The TX section of

the RF module is a 2-band (262-438 MHz, 523-876 MHz) quadrature mixer that uses a

divided-by-2 pre-scalar for the lower-band frequencies.

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3.5 Hardware Flow of SFF SDR

The following block diagram represents the hardware layout of the three modules of the

development platform.

Figure 3.2 Block Diagram of SFF SDR Platform

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CHAPTER 4

Research

4.1 Literature Review

The evolution of communication systems has brought a multifold increase in the

efficiency of data transmission. Several techniques have been proposed in recent years

that quantify the efficiency of these systems. Very few hardware modules exist that can

be used with ease for testing and debugging of such complex to implement and efficient

techniques. In this section we present some of the most common and efficient techniques

presented in the past. We have used these modulation schemes in our work on SDR

platform.

We reviewed a number of techniques for different possible combining schemes of Basic

OFDM and Basic CDMA systems to form hybrid OFDM-CDMA system with different

characteristics. In [1] the performance of OFDM-CDMA system was compared with DS-

CDMA for fading channels. The analysis enables a performance comparison between the

DS-CDMA system and the OFDM-CDMA system with respect to the demands of low

complexity receivers which is important for the system design. The results show that

OFDM-CDMA outperforms DS-CDMA in terms of spectral efficiency.

Another interesting document on OFDM-CDMA [2] describes methods to tackle the issue

of the high ratio of the peak power to the average power (PAPR) of the OFDM-CDMA

signal, which is a special drawback of multi-carrier transmission and has prohibited its

wider application. In this paper a new OFDM-CDMA system structure was discussed,

which combines the Time Spreading Structure and Frequency Spreading Structure, called

time-frequency spreading OFDM-CDMA. This system can achieve a much lower BER

and PAPR compared to frequency spreading OFDM-CDMA.

[3] Describes a novel approach of Spreading spectrum in OFDM systems which are

suitable for UWB communication without the need for frequency hopping. The resultant

waveform has the characteristics of a white noise, its power spectrum density is constant

within the desired bandwidth and bandwidth can also be selected flexibly.

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[4] Gives some important insights about Code Spread OFDM (CS-OFDM) which

combines the characteristics of OFDM and Code Division Multiple Access (CDMA) to

create a more robust modulation scheme which provides substantial performance

improvements relative to standard OFDM. The performance of both OFDM and OFDM-

CDMA is evaluated with and without error control coding using the FEC techniques

typically employed in OFDM standards.

Another paper on OFDM-CDMA [5] presents that OFDM-CDMA can be used in a

different manner where instead of many users sharing the same channel the data symbols

can be treated as virtual "users" and spread across the frequency domain before OFDM

modulation as opposed to OFDM where each data symbol modulates one of the available

tones. This form of spreading, which we call Code-Spread OFDM (CS-OFDM), can

effectively reduce the degradation caused by the frequency selective fading to provide

improved performance in multipath fading channels.

In [6], experimental results of OFDM-CDMA system using Walsh-Hadamard Codes and

MLD were discussed. The aim of the study was to analyze the performance of a

convolutionally-coded CDMA system combined with OFDM in a frequency/time

selective fading channel, taking into account the near-far problem. The combination

allows one to perform a maximum-likelihood detection (MLD), to use the available

spectrum in an efficient way, to exploit frequency diversity and time diversity (provided

by channel coding), and to retain many advantages of a CDMA system with a simpler

hardware realization. An example for a mobile communication system using

convolutionally-coded CDMA/OFDM with Walsh-Hadamard code-spreading for the

downlink (base-mobile) was studied. The performance of a MLD is examined by taking

into account the near-far problem. It is shown that the MLD is very robust to the near-far

problem. It is also shown that by using BPSK modulation, in a 1.28 MHz bandwidth, one

can transmit 64 active users at rate of about 10.34kbit/sec.

[7] Proposes a performance analysis for OFDM-CDMA with joint frequency-time

spreading. The average bit error probability of the proposed system using maximum-ratio

combining (MRC) was derived for a frequency-selective fading channel and that of

conventional MC-CDMA was also presented for comparison. Numerical analysis and

simulation results indicated that the proposed system outperformed MC-CDMA system.

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In [8] various OFDM-CDMA schemes have been proposed which can be mainly

categorized into two groups according to codes spreading direction. One is to spread the

original data stream in the frequency domain; and the other is spread in the time domain,

similar to a normal DS-CDMA scheme. Therefore, the frequency Rake receiver or time

Rake receiver will be used, respectively. The former scheme, which is usually referred to

as MC-CDMA, can obtain a good frequency Rake diversity effect through the de-

spreading operation since the fading of each sub-carrier is different. Although the

orthogonality will be degraded due to the frequency selective channel, a proper

combining scheme can be selected to minimize such effect and improve the system

performance. However, such scheme can’t achieve the time diversity gain by itself. The

latter scheme, which is usually called as MC-DS-CDMA, is a good scheme to introduce

the OFDM technology into DS-CDMA systems, especially for the quasi synchronous

mobile communication environment. However, the frequency diversity gain, which is the

main advantage of using such technique, can’t be achieved if good channel coding and

interleaving in the frequency domain are not combined.

[9] Provides an optimal detection scheme when combining OFDM-CDMA with

convolutional and turbo channel coding for the down-link. Especially in the down-link

OFDM-CDMA enables low complex mobile receivers since OF DM can prevent inter-

symbol interference (ISI) and with that, the complexity of a RAKE receiver in a multipath

channel. The various combinations between detection techniques and decoding schemes

enable a comparison between achievable system performance and necessary system

complexity

[10] Discusses the issue of Channel Estimation using Training sequence Design of

OFDM-CDMA Broadband Wireless Access Networks with Diversity Techniques. An

effect of diversity techniques on the performance of OFDM-CDMA based broadband

wireless access networks was investigated and the maximum achievable diversity gain for

a two-path Rayleigh fading environment is evaluated.

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4.2 Introduction to Basic OFDM

OFDM is a combination of modulation and multiplexing. In OFDM the question of

multiplexing is applied to independent signals but these independent signals are a subset

of the one main signal. In OFDM the signal itself is first split into independent channels,

modulated by data and then re-multiplexed to create the OFDM carrier. The basic idea

behind the Multicarrier Modulation (MCM) is very simple and follows from the need for

high rates of data transmission and reception and inter-symbol interference free channel.

OFDM is a special case of Frequency Division Multiplex (FDM). The independent sub­

channels can be multiplexed by frequency division multiplexing (FDM), called multi-

carrier transmission or it can be based on a code division multi-plex (COM), in this case it

is called multi -code transmission.

In OFDM, the carriers are arranged in a special way such that the frequency spectrum of

the individual carriers overlap and the signals are still received without adjacent carrier

interference. To achieve this, the sub-carriers are chosen to be mathematically orthogonal.

The data rate on each of the sub-channel is much less than the total data rate, so the

corresponding sub-channel bandwidth is much less than the total system bandwidth. The

number of sub-carriers is chosen to ensure that each sub-channel has a bandwidth less

than the coherence bandwidth of the channel, so the sub-channels experience relatively

flat fading.

Figure 4.1 OFDM Waveform Representation

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4.3 Characteristics of Basic OFDM

4.3.1 Orthogonality

In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are

orthogonal to each other, meaning that crosstalk between the sub-channels is eliminated

and inter-carrier guard bands are not required. This greatly simplifies the design of

both the transmitter and the receiver unlike conventional FDM, a separate filter for each

sub-channel is not required.

The orthogonality also allows high spectral efficiency, near the Nyquist rate. Almost

the whole available frequency band can be utilized. OFDM generally has a nearly

'white' spectrum, giving it benign electromagnetic interference properties with respect

to other co­channel users.

4.3.2 Simplified Equalization

The effects of frequency-selective channel conditions, for example fading caused

by multipath propagation , can be considered as constant (flat) over an OFDM

sub-channel if the sub-channel is sufficiently narrow-banded, i .e. if the number

of sub-channels is sufficiently large. This makes equalization far simpler at the

receiver in OFDM in comparison to conventional singlee-carrier modulation.

The equalizer on l y has to multiply each sub-carrier by a constant value, or a rarely

changed value.

Some of the sub-carriers in some of the OFDM symbols may carry pilot signals

for measurement of the channel conditions, i.e. the equalizer gain for each

sub-carrier. Pilot signals may also be used for synchronization . If differential

modulation such as DPSK is applied to each sub-carrier, equalization can be

completely omitted , since these schemes are insensitive to slowly changing

amplitude and phase distortion

4.3.3 Advantages of OFDM

Provides efficient use of Spectrum

Avoids Cross Talk and Inter Symbol Interference

24

Deals efficiently with multipath fading

Combats the frequency selectivity of Channel

4.3.4 Disadvantages of OFDM

It has high frequency phase noise

Multiple transmitters and receivers may face small carrier frequency offsets

Has high Peak to Average power Ratio

May have sampling clock offsets

4.4 System Model of Basic OFDM for Baseband

Figure 4.2 Basic OFDM System Model

4.5 Introduction to Basic CDMA

CDMA- Code Division Multiple Access

CDMA is one of the several Multiple Access Techniques as:

25

Figure 4.3 Multiple Access Technologies

4.6 Principles of CDMA

Here are some Basic Principles of CDMA in which:

1) Many Voice Channels use the same frequency band

2) Channels are separated by codes rather time slots

3) All Channels use same frequency at the same time

4) Signaling use a dedicated frequency band

Figure 4.4 CDMA Representation

5) CDMA interference comes from nearby users

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6) Each channel is a small voice in a roaring crowd but with a unique recoverable

code

7) If we have Carrier / Interference ratio as a figure of merit then we have:

Figure 4.5 Figure of Merit

4.7 Brief Description of CDMA

A CDMA signal uses many chips to convey one bit of information in which each user has

a unique chip pattern in effect a code channel. At the receiver side to recover a bit a large

number of chips is integrated with the known user chip pattern while other users code

pattern appear random and integrate toward low values and hence they do not disturb the

decoding process.

Figure 4.6 Chips and Symbols

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4.8 Characteristics of CDMA

4.8.1 CDMA as A Spread Spectrum System

Traditional technologies usually squeeze signal into minimum bandwidth but CDMA

signal uses a large bandwidth and provides increased capacity in terms of processing gain.

Figure 4.7 Spread Spectrum

Sender combines data with a fast spreading sequence providing a fast data stream while

user intercepts the stream and uses the same sequence to recover the original data

Figure 4.8 Data Stream Spreading

4.8.2 CDMA Spreading Sequences

There are three CDMA spreading sequences [11] as:

28

4.8.3 Advantages and Disadvantages of CDMA

Advantages

Increased Capacity, Enhanced Privacy and Security

Reduced Interference to other Electronic Devices

Disadvantages

Wide bandwidth per User Required

Precision Code Synchronization Needed

4.9 CDMA System Model in Baseband

We implemented following model of CDMA system for Baseband:

Figure 4.9 CDMA System Model

29

CHAPTER 5

OFDM-CDMA

Orthogonal frequency Division Multiplexing (OFDM) and Code Division Multiple

Access (CDMA) systems have gained considerable attention due to their use in high

speed wireless communication. Both OFDM and CDMA have distinguishing features, for

example, the former is almost completely immune to multipath fading effects, and the

later has multi-user capability. Orthogonal Frequency Division Multiplexing-Code

Division Multiple Access (OFDM-CDMA) attempts to combine these features, so that we

can achieve higher data rates for multiple users simultaneously.

OFDM-CDMA is a multicarrier multi-user technique, based on a combination of OFDM

and CDMA. There are several ways of making this combination.

5.1 Variants of OFDM-CDMA

There are four variants of OFDM-CDMA [12] [13] which are known in the literature as:

1) MC-CDMA

2) MC-DS-CDMA

3) MT-CDMA

4) TFL-CDMA

5.1.1 MC-CDMA

An MC-CDMA transmits N chips simultaneously by assigning each chip to a separate

carrier so that each input symbol is transmitted on N carriers. Signal spreading in this

scheme is performed purely in the frequency domain. The receiver extracts the

transmitted symbol by correlating the signal samples at the OFDM output with the code

sequence used for signal de-spreading.

5.1.2 MC-DS-CDMA

In MC-DS-CDMA, signal spreading is performed in the time domain so that the first

symbol of each user is transmitted on the first carrier: the second symbol is on the second

carrier, and so on.

30

5.1.3 MT-CDMA

MT-CDMA scheme uses longer spreading codes in proportion to the number

of subcarriers.

5.1.4 TFL-CDMA

In TFL-CDMA the signal is spread both in time and in frequency.

The variant which we choose is MC-DS-CDMA due to its better performance and

advantages over other variances as described next.

5.2 Characteristics of OFDM-CDMA

As described earlier OFDM-CDMA combines good features of both OFDM and CDMA

in which:

OFDM resolves

Frequency Selectivity in Multipath Fading Channels

Provides Efficient use of Spectrum

CDMA provides

Frequency Diversity

Codes Differentiate Users

5.3 Transceiver Model of MC-DS-CDMA

Transmitter and Receiver model of MC-DS-CDMA (OFDM-CDMA) [14] system are

shown as:

31

Figure 5.1 Transmitter Model

Figure 5.2 Receiver Model

32

5.4 System Model of OFDM-CDMA for Baseband

The system model of OFDM-CDMA for baseband communication is:

Figure 5.3 OFDM-CDMA System Model

5.5 Advantages of OFDM-CDMA

1) OFDM-CDMA handles multiple users with good BER using standard receiver

techniques

2) OFDM-CDMA system lowers the symbol rate in each subcarrier increasing the

symbol durations which minimize the multipath fading effects of the channel.

3) In OFDM-CDMA, modulation and demodulation is achieved by using Inverse

Fast Fourier Transform and Fast Fourier Transform (IFFT/ FFT) algorithms

4) Simple Receiver Structure which uses only the

a. Knowledge of its own Walsh Code

b. FFT

c. Equalization

5) As Number of User increases OFDM-CDMA Outperforms other Downlink

Techniques

6) OFDM-CDMA can be scaled relatively easily according to the requirements.

7) In OFDM-CDMA equalization can be under taken on carrier by carrier bases

33

CHAPTER 6

Simulations

Any process of designing and implementation of any particular waveform starts by

simulating it on software tools and obtaining the desired results. The basic aim of this

chapter is to provide an overview of the OFDM-CDMA simulation. The simulations are

carried out on Matlab R2009b. The aim of the simulation is to obtain the bit error rate

plots and compare the performance with CDMA and OFDM alone. The step by step

procedure to simulate the OFDM-CDMA is described below. The commands used in

each step are also described

Transceiver Block Diagram

Figure 6.1 Transmiter Block Diagram

34

Figure 6.2 Receiver Block Diagram

6.1 Random Data Generation

First step in simulations is to generate a random binary data. This data can be of any

distribution. The data can also be generated by converting an audio file into bit-stream.

This is the data that is to transmit.

Matlab Command:

data=Randi([0 1],1,datalength); %This command %generates binary

data of Gaussian distribution.

6.2 Converting binary to bipolar NRZ waveform

The generated binary is converted into bipolar NRZ using a ―for loop‖ or command. This

is equivalent to converting the data into BPSK in baseband. This can be done by the

following command

Matlab Command:

35

NRZ= 2*binary_data -1 ;

6.3 Generating Walsh Hadamard Codes

The walsh-hadamard codes are generated as PN-sequence .These PN-sequence are

required for the spreading of the data. Walsh codes are orthogonal in nature and their

cross correlation is equal to zero.

Matlab Command:

Orthogonal_codes=hadamard(code_length);

%code_length is length of code required

6.4 Spreading

Spreading is done by multiplying a generated code with the original data. Each data bit is

replaced by a chip code in this way a data is spreaded in the frequency domain. The

amount of spreading depends on the spreading gain or length of chip sequence.

Matlab Command:

spread= data'*orthogonal_codes;

In this case the matrix multiplication takes place in such a way that every bit of data is

replaced by the orthogonal code or its inverted value.

6.5 Serial to Parallel Conversion

The stream of bits coming after the CDMA is in serial form. It is converted to the parallel

form before doing OFDM. This serial to parallel conversion can be done by the following

command.

Matlab Command:

Spreaded_serial=reshape(spreaded_data,codelength,datalength);

In this way data is converted from serial to parallel and the number of rows is equal to

length of code and we can vary it according to our own design.

36

6.6 Taking IFFT

The IFFT is used to make orthogonal carriers required to create OFDM waveform. IFFT

modulates the data in the same way as modulating the data bits with individual carrier

frequencies.

Matlab Code:

for i=1:length(totaldata)

ofdmout(:,i)=ifft(totaldata(:,i),8);

end

The data coming is in the form of parallel and the data in a single column is taken as input

to IFFT block. 8 point IFFT is calculated as it is considered that total number of sub

carriers is 8.

6.7 Parallel to Serial Converter

The data output from the IFFT is parallel. Thus it is required to convert it in the serial

form before transmitting. This can be done by using the following command.

Matlab Command:

Spread=reshape (spread, 1, data_length*code_length);

6.8 Channel

The simulations are carried out on SUI, AWGN and Rayleigh channel. The results

obtained from the simulating through these channels are given in the results section of

this chapter.

6.8.1 AWGN (Additive white Gaussian Noise)

This noise is a linear addition of white wideband noise with constant power spectral

density.This noise can be added with the parameters of SNR and signal energy.

Matlab Command:

Ofdm_out=awgn (ofdmout,snr,'measured');

6.8.2 Rayleigh Channel

Rayleigh channel models the effects of propagation on signal in a wireless environment.

37

Matlab Command:

Chan=raylrnd(1,1,3); for the three tap channel

Ofdm_out=filter(chan,1,ofdmout);

6.8.3 SUI channel: (Stanford University Interim)

This channel is more close to the actual model of the environment .It includes the effects

of delays, terrain type , relative motion of transmitter and receiver. Its effect on the signal

is more as compared to the other models like Rayleigh and Rician fading.

Figure 6.3 SUI Channels Summary

6.9 Equalization

Equalization is done by inverse filtering .The coefficients are estimated by sending a

training sequence and using the LMS algorithm.

6.10 Serial to parallel converter

38

The data is converter from parallel to serial using a reshape command.

Matlab Command:

spread=reshape(spread8,1,datalength*codelength);

6.11 Fast Fourier Transform

The demodulation of the OFDM consists of FFT. This Is the inverse of the IFFT on the

transmitter side. The arguments and length of FFT is kept the same as in the IFFT.

6.12 DE spreading

The data is de-spreaded by multiplying and adding it with the same sequence with which

it was spreaded.

Matlab Code:

if(sum(demoddata(1,1+codelength*(i-1):codelength*i).*orthcodes(1,:))>0)

dmoddataa(1,i)=1;

else

dmoddataa(1,i)=-1;

end

6.13 Converting Bipolar NRZ to Binary

The despreaded data is in the NRZ form it is then converted to the binary form.

6.14 BER Plots

The transmitted and received data is compared to make a BER plot. The BER is equal to

the total number of errors in transmission divided by the total number of transmitted bits.

The BER is calculated for different values of SNR .The decibel plot is used to make the

plot.

Matlab command

semilogy(0:SNR_total,BER)

39

6.15 Constellation Diagram

Constellation diagram is a representation of the signal that has been digitally modulated.

Since we have used a BPSK so the there are two message points in our case at 1 and -1.

Matlab Command:

scatterplot(az)

Figure 6.4 Scatter Plot

6.16 Results Obtained in OFDM-CDMA Simulations

In this section we have demonstrated the results of OFDM, CDMA and Equalization.

40

6.16.1 OFDM Time Domain:

Figure 6.5 OFDM Time Domain

6.16.2 CDMA Spectrum:

Figure 6.6 CDMA Spectrum

41

6.16.3 Equalization:

Figure 6.7 Equalization

42

CHAPTER 7

Hardware Implementation

7.1 Hardware Specifications

OFDM-CDMA transceiver after successful simulation in MATLAB was implemented on

Lyrtech SFF SDR platform.

As mentioned before it has three main modules but main module used in this project is

Digital Processing Module in which there is a:

1) Virtex-4 XC4SX35 FPGA from Xilinx

2) TMS320DM6446 DMP SoC

7.2 Implementation Specifications

Implantation specifications used for implementing OFDM-CDMA system model on SFF

SDR are:

Number of Users = 4

Number of Subcarriers = 16

Spreading Gain = 4

Transmission Frequency = 37.5 MHz

7.3 DSP Implementation

A fixed point code in C language using Code Composer Studio v3.3 was written to

generate random binary data as FPGA on SFF DR cannot take inputs efficiently.

This code also has the capability to receive the processed signal data back from FPGA for

plotting in CCS or Matlab.

7.4 Interface using VPSS

After this VPFE and VPBE channels of VPSS module on SDR platform were used to

provide an interface between FPGA and DSP.

43

7.4.1 Introduction to VPSS

The video processing subsystem (VPSS) is a DM6446 16-bit, synchronous video data

transfer port. The VPSS is composed of the video processing front end (VPFE) and the

video processing back end (VPBE). The VPFE is used as an input interface to the DSP

and the VPBE as an output interface from the DSP. The YPSS was adapted to be used on

the digital processing module of the SFF SDR evaluation module/development platform

as an interface to transfer data other than video between the DSP and the FPGA. The

vertical and horizontal synchronization signals (Vsync and Hsync signals, respectively)

are used as the main synchronization signals.

In the FPGA of the digital processing module, a VPSS data port module was

implemented to interface with the DSP VPSS. To emulate video signals, Vsync and

Hsync signals are generated by the VPFE of the FPGA interface. The FPGA VPBE uses

the Ysync and Hsync signals generated by the DSP to synchronize the incoming data

transfer. The block diagram of the VPSS connection between the FPGA and the DSP is

illustrated below.

Figure 7.1 VPSS Overview

The data bus in the FPGA of the digital processing module is 32 bits and the VPSS

module of the DM6446 DSP bus is 16 bits. The VPBE and the VPFE were implemented

44

using a 1024 x 32- bit clock domain crossing and bus width conversion FIFO, as well as

the logic necessary for synchronization and control.

Data on the VPSS is formatted in a video frame buffer structure. Each frame is separated

by a Ysync(VD) pulse. A frame contains one or many lines separated by Hsync(HD)

pulse. The following figure illustrates a frame containing two Jines of four data words.

Each frame can hold up to eight data words. Each frame also contains a blank line not

containing data

7.4.2 Interface at DSP Side

There are quite a few API's given for transfer of data between the FPGA and DSP. But

some of them require that we define the protocol for data transfer by ourselves that

would be a humongous task to do and quite useless to. Instead there are a few API's in

which the protocol for data transfer is already defined.

They are described below:

Int32 VPBE_Jni't (Uint32 aNblinePerFrame, Uint32 aNbDWordPerline, struct

_INTERNAL_BUFFER_HEADER *AllocatedBuffer[20], Uint8 NbBuffer)

This function configures the VPBE channel to communicate with the FPGA. Parameters:

aNbLinePerFrame Line per frame (within the VSYNC period). Valid range is (1- 2).

aNbDWordPerLine Number of 32-bit data words per line (within the HSYNC

period).

The transmitted length is always

NbDWordPerLine + 1 {to account for header). Valid range is (8- 512)

AllocatedBuffer User supplied buffer. Not supported must be NULL.

NbBuffer Number of frame buffer to create. Using many Frame buffer allow the driver

to accept many transfer request that will be automatically sequenced and processed by

the driver

Returns:

45

1: Initialization was succesfull,

0: Initialization failed. Note:

lnt32 VPBE_SendBuffer {Uint32 *Buffer, Uint32 BufferLength)

This function sends data to the VPBE.The data is splitted to fit into frame buffer. The

frame size and line size depends on the parameters used to configure the VPBE.

Parameters:

Buffer pointer to the user buffer containing data to send.

BufferLength Length of user data to send ( 32 bits words unit) Returns:

0: the request was successfully processed.

Note: The function returns when all the data has been transferred.

lnt32 VPFE_GetBuffer {Uint32 Bufferln, lnt32 BufferLength)

This function gets data from the VPFE.

The number of 32 bits read by the function is defined by the function parameter

Bufferlength. The function will not return until there is Bufferlength data in the buffer

pointed by Bufferln.

Parameters:

Bufferln Pointer to the user buffer where to store data. Bufferlength Length of data to

read from the VPFE.

Returns:

0: the request was successfully processed.

Note: The function returns only when all data has been copied into the user buffer.

Uint32 VPFE_Init (Uint32 aNblinePerFrame, Uint32 aNbDWordPerline, struct

S_INTERNAL_BUFFER_HEADER AllocatedBuffer[20L Uint8 NbBuffer)

This function configures the VPFE channel to communicate with the FPGA .

46

Parameters:

aNblinePerFrame Line per frame (within the VSYNC period). Valid range is (1-2) .

aNbDWordPe.rline Number of 32-bit data words per line (within the HSYNC period).

The transmitted length is always

NbDWordPerline + 1 ( to account for header). Valid range is (8- 512)

AllocatedBuffer User supplied buffer. not supported must be NULL.

NbBuffer Number of frame buffer to create. Using many Frame buffer allow the driver

to accept many transfer request that will be automatically sequenced and processed

by the driver.

Returns:

1: Initialization was succesfull, 0: Initialization failed. Note:

See user's manual on how to optimize the VPFE configuration.

7.4.3 VPBE

The first step in sending data is to receive a free frame buffer from the driver. This is

achieved by calling the following function:

C Command: Int32 VPBE_GetFreeFrameBuffer (struct S_FRAME_BUFFER

*FrameBuf)

The second step in sending data is to place data in the frame buffer. Use the line buffer’s

address and length information to perform this step. The final step in sending data is to

send the data by returning the frame buffer to the driver.

C Command: Int32 VPBE_SendFrameBuffer (struct S_FRAME_BUFFER *FrameBuf);

The functions can be used asynchronously. For example, if the driver is configured with

ten buffers, it is possible to call VPBE_GetFreeFrameBuffer ten times before calling

VPBE_SendFrameBuffer.

47

Figure 7.2 VPBE

7.4.4 VPFE

The first step in receiving data from the VPFE is to call the following function:

C Command: Int32 VPFE_GetFrameBuffer (struct S_FRAME_BUFFER *FrameBuf);

The second step in receiving data is to read the data from the frame buffer. Use the line

buffer’s address and length information to perform this step. The final step in receiving

data is to return the frame buffer to the driver with the following function:

C Command: Int32 VPFE_ReleaseFrameBuffer (Uint32 HandleId);

As with the VPBE, the function can be used asynchronously.

Figure 7.3 VPBE

48

7.5 FPGA Implementation

VHDL/Verilog code is written using Xilinx simulation and implementation tool to

acquire data from DSP and its conversion into NRZ line coding. After this a detailed code

of VHDL is written in Custom logic module of complete OFDM-CDMA transceiver for

its implementation on Vertex 4 FPGA. Four different data clocks were used for four

different users. Walsh code was randomly created in VHDL for each user separately.

Built-In cores were used for IFFT and FFT parts of OFDM. Similarly a Built-In core of

multiplication was used for multiplying data signal with Walsh codes for CDMA part to

successfully handle the high data rates of CDMA scheme.

7.6 Xilinx Simulation of Transceiver

A successful simulation of OFDM-CDMA transceiver in Xilinx is shown in the following

figure which clearly shows that transmitted data at the transmitter is successfully

recovered at the receiver end

Figure 7.4 Xilinx Simulations

49

7.7 ChipScope Pro Tool

ChipScope™ Pro tool inserts logic analyzer, system analyzer, and virtual I/O low-profile

software cores directly into design, allowing viewing any internal signal or node,

including embedded hard or soft processors. Signals are captured in the system at the

speed of operation and brought out through the programming interface, freeing up pins

for your design. Captured signals are then displayed and analyzed using the ChipScope

Pro Analyzer tool.

The ChipScope Pro Serial I/O Toolkit provides a fast, easy, and interactive setup and

debug of serial I/O channels in high-speed FPGA designs. The ChipScope Pro Serial I/O

Toolkit allows to take bit-error ratio (BER) measurements on multiple channels and

adjust high-speed serial transceiver parameters in real-time while your serial I/O channels

interact with the rest of the system.

7.7.1 Key Features of ChipScope Pro Tool

Some of the key features of ChipScope Pro tool are:

1) ChipScope core insertion and generation integrated into Project Navigator and

PlanAhead tool flows

2) Add debug probes directly in HDL (VHDL and Verilog) or constraint files

3) Analyze any internal FPGA signal, including embedded processor system buses

4) Inserts low-profile, configurable software cores either during design capture or

after synthesis

5) Analyzer trigger and capture enhancements makes taking repetitive measurements

easy to do

6) Change probe points without re-implementing the design

7) Fast and easy interactive setup and debug of FPGA serial I/O channels

8) Measure bit-error ratios (BER) on multiple channels simultaneously

9) Adjust high-speed serial transceiver parameters in real-time while serial I/O

channels are interacting with the rest of the system

10) Requires only JTAG port access to your board, no extra pins needed for dedicated

high-speed serial debug or setup

50

7.8 Analysis of Implementation Results using ChipScope Pro

ChipScope Pro was used to analyze the results of implementation on SDR platform and a

successful comparison of Simulation and Implementation results was achieved as shown

in following figure:

Figure 7.5 ChipScope Pro Results

As it is clear from the above figure that this figure is very much like that of simulation

results as shown in Simulation part.

51

CHAPTER 8

Analysis and Results

8.1 Comparison on the Basis of BER

To analyze the results obtained both in simulations and hardware implementation a lot of

varying types of comparisons were performed as shown:

8.1.1 Comparison of OFDM-CDMA with OFDM and CDMA in AWGN

Channel

With the SNR varying from 0 to 15 in AWGN channel the results of the

comparison between OFDM-CDMA, OFDM and CDMA are shown in the

following figure:

Figure 8.1 Comparisons in AWGN Channel

In the above figure blue line representing OFDM-CDMA clearly shows that it

outperforms individually OFDM and CDMA in terms of BER in the same conditions.

52

8.1.2 Comparison of OFDM-CDMA with OFDM and CDMA in

Rayleigh Fading Channel

With the SNR varying from 0 to 15 in Rayleigh Fading channel the results of

the comparison between OFDM-CDMA, OFDM and CDMA are shown in

the following figure:

Figure 8.2 Comparisons in Rayleigh Fading Channel

Here again it is clear in the above figure that blue line representing OFDM-CDMA

clearly shows that it outperforms individually OFDM and CDMA in terms of BER in the

same conditions.

8.1.3 Comparison of OFDM-CDMA with OFDM and CDMA in SUI 6

Channel

With the SNR varying from 0 to 15 in SUI 6 channel the results of the

comparison between OFDM-CDMA, OFDM and CDMA are shown in the

following figure:

53

Figure 8.3 Comparisons in SUI 6 Channel

SUI 6 channel provides the same parameters as in the real time environment as discussed

earlier but it is clear from the results shown above that OFDM-CDMA still outperform

individual OFDM and CDMA.

8.2 Comparison on the Basis of Hardware Utilization

As it is clear that OFDM-CDMA shows a much better performance in different type of

noisy channels but just like every case here in this case there is also a tradeoff between

hardware utilization of OFDM-CDMA transceiver and its performance as shown below:

54

Figure 8.4 Hardware Utilizations and Trade-Offs

The bargraph above shows that although BER provided by OFDM-CDMA is much better

but it uses almost 3 to 4 times greater number of hardware resources and its tranceiver is

much complicated as compared to individual OFDM and CDMA.

55

CHAPTER 9

Discussions/Problems Encountered and Solutions

9.1 Equalization

One of the main problems faced during simulations and implementation was the decision

of Equalization which was tackled by the method of Inverse filtering with the use of

channel estimation through training sequence.

9.1.1 Channel Estimation and Training Sequence

First of all a training sequence was sent to estimate the channel coefficients. After getting

the knowledge of channel coefficients 512 points FFT of these coefficients was taken.

After this Inverse of the coefficients was found inverse filtering in frequency domain was

implemented. The received data was multiplied with this inverse filter of the channel and

resulted data was transformed back to the time domain.

Figure 9.1 Use of Training Sequence for Estimation of Channel Coefficients

After this process of Equalization through channel estimation the normal process of

receiver part begins.

56

9.2 Rake Receiver

Rake Receiver is also another alternative of Equalization. By the use of rake receiver the

equalization can be eliminated at all from the receiver part. But for a large number of

subcarriers used in OFDM its usage increases the complexity of receiver so it was not

used in this design.

However, in a fading environment, the receiver may take advantage from the higher

transmission bandwidth of CDMA. A simple structure of Rake receiver is given as:

Figure 9.2 Rake Receiver

9.3 Interface b/w DSP and FPGA

Another problem faced was the communication b/w FPGA and DSP which was mainly

hurdled by different clock speeds of FPGA and DSP. FPGA mounted on SFF SDR

operates on 37.5 MHz while DSP operates on clock speed of 27 Mhz. This difference b/w

their clock speeds presented a real issue in their communication which was solved with

the usage of VPSS port.

9.4 VPSS Port Complications

Although the issue of communication between FPGA and DSP was solved with the help

of VPSS port but understanding of VPSS port, its structure and commands used to

operate this port was itself a big problem which was successfully tackled

57

9.5 Debugging during Process on FPGA and ChipScope

Another main issue was the debugging during process on FPGA to check for errors which

was not possible other than to see the final results which was a time consuming process.

So to tackle this issue of on chip debugging during process ChipScope was used which is

a side product of Xilinx. But to use ChipScope for correct debugging was itself a real

issue which was solved successfully.

58

CHAPTER 10

CONCLUSION

SDR platforms came into existence with their first generation around 2004–2006.

Technology has progressed since then and there have been significant improvements in

signal processing performance, connectivity, and in the quality of RF components such as

mixers and data converters. Now it has become possible to implement most narrowband

communication schemes (e.g., GSM) though not without significant effort and expertise.

Our work has contributed as a module to this extensive work underway on SDR. We have

successfully developed OFDM-CDMA prototype module in simulation and have

implemented it reasonable results on SDR; which can now be used for academic and

industrial research programs.

In recent years technology has moved towards 3G and 4G wireless communication

systems particularly in our country and this research prototype of 4G could be a

reasonable contribution in near future due to successful comparison which we have

gained between simulation and implementation results, because OFDM-CDMA

outperforms other downlink techniques currently being used as shown by the BER plots

and as the number of users increase its performance increases too.

SDR platforms were previously challenged by increasing bandwidths, reducing minimum

signal strengths, and reducing maximum allowable error vector magnitudes. But now

application specific SDR platforms are being constructed with a combination of available

technologies.

59

CHAPTER 11

APPLICATIONS

3G and 4G wireless systems are being driven by the desire to support innovative

broadband multimedia services. Orthogonal Frequency Division Multiplexing Code

Division Multiple Access (OFDM-CDMA) schemes can meet such demand, so they are

broadly considered as effective methods for future wireless multimedia communications.

Since variant OFDM-CDMA schemes will coexist for a long time, reconfigurable

multimode transceivers (SDRs), which are compatible with OFDM-CDMA schemes, are

indispensable for base station and mobile station. Some of the applications are listed

below.

Platform for 4G Communication

Downlink (base to mobile) Communication

High Data Rate UWB Systems such as WPAN

60

CHAPTER 12

Recommendations for Future Work

Following are some recommendations for future work:

1) This OFDM-CDMA system can be implemented on SDR for wireless channel

using RF module

2) This OFDM-CDMA system prototype can also be implemented for Broadband

communication using suitable scheme such as BPSK or QPSK

3) Other implementations of IFFT can be used to increase the speed of the system

4) Hardware should be easily available and technical support must also be available

for students

5) Latest version of Software and Hardware should be bought

6) Technical help of Software is really necessary for any student working on SDR

7) Error correction codes like turbo codes can be used to improve the performance

for increased SNR

8) Other Equalization schemes rather than Training Sequence and Inverse filters can

be used

9) Rake Receiver can be used instead of Equalization to check for better performance

10) Methods to reduce ICI should be used for future projects

11) Synchronization schemes should be adopted for OFDM to develop a system for

real time environment applications

12) Walsh Codes for CDMA can be assigned to users in a random manner to improve

security

13) Reconfigurable transceivers using OFDM-CDMA can be developed for SDR

14) MIMO-OFDM-CDMA Systems can be developed using SDR-SDR

Implementation

61

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