ANINDYA SANKAR PAUL

Abstract

In this project the main intention is to find out advantages and disadvantages of MC-CDMA system in frequency selective Rayleigh fading channel. Hence the prime objective should be to find out the BER analysis of MC-CDMA system under the Rayleigh fading environment and compare that with DS-CDMA system. For this reason the disadvantages of DS-CDMA system is also discussed and it is clearly mentioned how MC-CDMA is proposed to rectify those faults. As MC-CDMA is developed as a combination of DS-CDMA and OFDM (Orthogonal frequency division Multiplexing), the need for OFDM to increase capacity and to reduce intersymbol interference on binary data communication system is also discussed. In this project it is proposed to find out the BER (Bit error rate) of MC-CDMA system using computer simulation and compare that with other conventional existing CDMA system (DS-CDMA system).

INTRODUCTION:

As MC-CDMA is a combination of DS-CDMA and OFDM, hence the advantages and disadvantages of those systems need to be discussed first.

But the main question is what's the requirement for the OFDM system to be developed. For this reason the BPSK (Binary Phase shift keying) modulation scheme is revisited and it's fault was carefully noted.

In BPSK, the modulating signal is binary data stream and the carrier is a sinusoid. The binary data modulates the carrier, so the information of the message is stored in the phase of the carrier. Hence there is a shift in carrier's phase. This phase shift in the frequency domain can be represented as a sync function.As sync extends upto infinity,there is a huge probability of intersymbol interference between adjacent bits.This scheme is also not suitable for frequency diversity.The period of the sync nulls are separated at a distance of symbol frequency.

Hence OFDM came into picture, OFDM actually converts the high speed data stream into several low speed data streams and transmit them simultaneously in parallel on multiple subcarriers. So OFDM truncates the long data packet into several small data packets and transmit them in parallel on different subcarriers. But we have to choose the carriers suitably placed so that there is no interference between them. This problem can be simply rectified if we separate the carriers at integer multiples of symbol frequency. So that peak power of one subcarrier occures when the power of the other subcarriers are null.

Hence the subcarriers are orthogonal also with respect to one another.

Hence there is very less chance of interference. The intersymbol interference can be removed by introducing certain guard bands between the subcarriers.

So in this scheme we will have lots of subcarrier carried information in a limited frequency band.

· The first advantage is we increase the spectral efficiency compared to other modulation BPSK or QAM.

· The main advantage of OFDM system is it's robustness on frequency selective fading and multipath delay spread. As it truncates the long data packet, now we have symbols with narrow bandwidth. As the time-bandwidth product is constant, the duration of the symbol length is made high, the relative amount of dispersion caused by multipath delay spread is decreased.

· Another obvious advantage is it implements the frequency diversity. As different symbols arrive at the receiver on independent orthogonal subcarriers, they are uncorrelated at the receiver. Hence frequency diversity scheme can be used at the receiver circuitry.

· If we have N no. of subcarriers,we should have N modulator at the transmitter,but instead of that we can do N point FFT on input binary data stream,this will cause the same effect as with N modulator,but here the system becomes faster and the transmitter-receiver complexity also doesn't increase.

There are also certain disadvantages of OFDM:

· The orthogonal subcarriers are not properly synchronized.

· It is more sensitive to frequency offset and phase noise.

Now the Advantages and disadvantages of DS-CDMA system is discussed:

Before CDMA, there was 2 multiple access techniques FDMA (Frequency division multiple access) and TDMA (Time division multiple access). FDMA allows user for simultaneous transmission but here every user gets only a small portion of total bandwidth. In TDMA, though each user can use the total system bandwidth but he has to wait for his turn to come, this will be too irritating, when no. of users are more.

For these reasons, CDMA came into picture. It allows each user in the system to use the total bandwidth as well as simultaneous transmission.hence much attention is paid into it as it revolutionizes the spectral efficiency of multiple access schemes. Each user is given one unique code (PN sequence of period N, N should be large), he spreads his message in time domain with that and transmits. As only the desired receiver knows the correct PN sequence used at the right transmitter, it can only decode, others are not.

This is the great advantage of CDMA.

But it also has certain problems:

· This CDMA scheme is not suitable for frequency diversity.

· In a dispersive multipath channel environment, the CDMA systems having spread N (PN sequence period N) can accommodate N users if and only if it is supported by a highly accurate interference cancellation system. This is quite obvious, if N users are in the system, we have to assign uncorrelated orthogonal PN sequence to each user to reduce interference, but if N becomes very high, it is very difficult to implement. So if the User base in the CDMA system is large, Bit error increases and system performance degrades.

To combat the difficulties faced by the DS-CDMA system, MC-CDMA is proposed. MC-CDMA is a combination of DS-CDMA and OFDM.Much attention is being paid into it and researches are going on worldwide on this.

DEFINITION OF MC-CDMA:

· MC-CDMA is a form of CDMA but here spreading is performed in frequency domain rather than in time domain as in DS-CDMA system.

· MC-CDMA is a form of DS-CDMA, only difference is that after spreading here FFT is implemented.

· MC-CDMA is a form of OFDM, but first we have to apply orthogonal matrix operation (PN sequence) on each user bit.

· MC-CDMA is a form of frequency diversity as each different user bits are transmitted in parallel on different uncorrelated subcarrier ,there will be a constant phase offset on each subcarrier,from the frequency offset of each subcarrier we can detect the user,as each subcarrier has an integral multiple of cycles and there is exactly one cycle difference between 2 successive carrier.

So MC-CDMA has all the good characters of CDMA, it has robustness on frequency selective interference, it uses sprectrum efficiently. In addition to that it can handle N simultaneous users under dispersive multipath condition with good BER.

MC-CDMA is a form of OFDM, it increases symbol duration…combats with delay spread and make frequency diversity scheme feasible. In addition to this the combination of OFDM and DS-CDMA (MC-CDMA) has major advantage that it can lower the symbol rate in each subcarrier so that a longer symbol duration makes it easier to quasi-synchronize the transmissions.

In this project,I was proposed to design a MC-CDMA system,model the frequency selective Rayleigh channel and evaluate the BER of MC-CDMA system under frequency selective Rayleigh environment.

After that it is required to compare the MC-CDMA BER with that of DS-CDMA and OFDM.

All the computer simulations will be done in MATLAB.

REFERENCES:

1. Dimtris Papadopoulos: On the performance of different implementations of Multicarrier CDMA.

2. Vijaya Ramasami: BER performance over fading channels and diversity combining.

3. Jean-Paul Linnartz: Performance Analysis of Synchronous MC-CDMA in Mobile Rayleigh channel with both Doppler and Delay spread, IEEE Transaction on Vehicular technology, November 2001.

4. Gary E Ford and Michael Golanbari: Multi user detection Techniques for interception of IS-95 CDMA signals, final report for Micro project in UC Davis.

5. Wireless Communication group,UC Berkeley: Special issue on Multicarrier Modulation

6. Ramjee Prasad and Shinsuke Hara: Design and performance of MC-CDMA system in Rayleigh fading, IEEE Transaction on vehicular technology, September 1999.

7. Essam A Sourour and Masao Nakagawa: Performance of Orthogonal Multicarrier CDMA in Multipath fading channel, IEEE Transactions on Communications, March 1996.

8. Jean-Paul Linnartz: Synchronous MC-CDMA in Dispersive, Mobile Rayleigh Channel.