Pr-sof-1196-Design and Implementation of Ofdm - Cdma Transceiver on Software Defined Radio
-
Upload
ferzia-firdousi -
Category
Documents
-
view
25 -
download
1
description
Transcript of Pr-sof-1196-Design and Implementation of Ofdm - Cdma Transceiver on Software Defined Radio
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.
3
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.
4
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.
5
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
6
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
7
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
8
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.
9
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
10
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
11
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.
12
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.
13
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
14
2.5 Flow Diagram of OFDM-CDMA System
Figure 2.2 Flow Diagram
15
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
16
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
17
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.
18
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
19
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.
20
[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.
21
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.
22
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
23
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
26
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
27
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
REFERENCES
1. OFDM-CDMA versus DS-CDMA: Performance Evaluation for Fading Channels
Stefan Kaiser, German Aerospace Research Establishment (DLR), Institute for
Communications Technology
2. A New System Structure to Reduce PAPR in the OFDM-CDMA
Chen Ying, Ren Lixiang, Long Teng, Beijing Institute of Technology
3. A Novel Approach of Spreading Spectrum in OFDM Systems, Pingzhou Tu
Xiaojing Huang, Eryk Dutkiewicz, University of Wollongong, Australia
4. PERFORMANCE EVALUATION OF CODE-SPREAD OFDM (OFDM-CDMA)
WITH ERROR CONTROL CODING
Muthanna AI-Mahmoud, MichaelD Zoltowski, Purdue University IN 47907-2035
5. I. Perez-Alvarez, I. Raos andetaI, "Interactive Digital Voiceover HF" in 9th
International Conference on HF Radio Systems and
Techniques,vol.493,June2003,pp.31-36.
6. K.Fazel,"Performance of CDMA-OFDM for Mobile Communication Systems" in
International Conference on Universal Personal Communications, vol.2,
October1993, pp.975-979.
7. Performance Analysis for OFDM-CDMA with Joint Frequency-Time Spreading,
IEEE Transactions on Broadcasting, VOL. 51, NO. 1, MARCH 2005
8. “An overview of multi-carrier CDMA,” R. Prasad and S. Hara, IEEE 4th Int.
Symposium Spread Spectrum Techniques and Applications, Mainz, Sep. 22–25,
1996, pp. 107–114
9. Stefan Kaiser and Lutz Papke, ―Optimal Detection when Combining OFDM-
CDMA with Convolutional and Turbo Channel Coding”. German Aerospace
Research Establishment (DLR), Institute for Communications Technology
10. Young-Hwan You, Won-Gi Jeon, Jong-Ho Paik, Dae-Ki Hong, and Hyoung-Kyu
Song “Training Sequence Design and Channel Estimation of OFDM-CDMA
Broadband Wireless Access Networks With Diversity Techniques” IEEE
TRANSACTIONS ON BROADCASTING, VOL. 49, NO. 4, DECEMBER 2003
11. "INTRODUCTION TO CDMA WIRELESS COMMUNICATIONS” By Mosa Ali
Abu-Rgheff
62
12. H. Matsutani, M. Nakagawa, “Multi-Carrier DS-CDMA Using Frequency Spread
Coding” IEICE Trans. Fundamentals, vol.E82-A, no.12, pp.2634-2642, Dec 1999.
13. S. Abeta, H. Atarashi, M. Sawahashi, F. Adachi, “Performance of Coherent
Multi-Carrier/DS-CDMA and MC-CDMA for Broadband Packet Wireless
Access”, ICICE Trans. Comm., vol.E84-B, no.3, pp.406-414, March 2002.
14. Kit Ming Tommy Chee “Hybrid OFDM-CDMA: A Comparison of MC/DS-
CDMA, MC-CDMA and OFCDM” Dept. of Electrical & Electronic, Adelaide
University, SA 5005, Australia