Abstract— In this paper we use Multi-carrier code division multiple access (MC-CDMA) with Recursive least square (RLS) equalizer used for channel estimation and equalization then compare it with Orthogonal frequency division multiplexing (OFDM).RLS algorithm is used because its converges faster leading to faster and better results compared to other algorithms. Results are received by simulating the transmission of the signals over Rayleigh fading additive white Gaussian noise (AWGN) channel. Objective is to prove that the bit error rate of MC-CDMA system falls more rapidly as we increase the transmitted signal power compared to the OFDM system.

Index Terms— AWGN channel, MC-CDMA, OFDM,

RLS equalizer, Rayleigh channel.

I. INTRODUCTION The basic principle of OFDM is to split a high-rate data stream into a number of lower rate streams that are transmitted simultaneously over a number of sub-carriers [1].

MC-CDMA refers to a technique for the transmission of multiple users’ data over a set of N narrowband carriers [2]. Beyond 3G (B3G) systems [3], several prevalent candidates for multiple access technology are built on the OFDM structure. One of them is multicarrier code division multiple access (MC-CDMA) [4]–[5], which has garnered significant attention in China and Europe [3], [6] due to the success of CDMA in existing 3G networks. In contrast to direct-sequence CDMA (DS-CDMA), the codes separating different

users or services are spread over the frequency domain across sub-carriers instead of the time domain.

Equalization is an effectual technique to combat ISI (inter symbol interference).Equalizer must be adaptive since the channel is generally unknown and time varying [7].Adaptive equalizer requires a specific algorithm to update the equalizer coefficients and track the channel variations. This leads to a family of powerful, complex, adaptive signal processing technique known as recursive least squares (RLS), which significantly improves the convergence of adaptive equalizers [8].

II. MC-CDMA MC-CDMA is a combination of CDMA with OFDM which

overcomes the disadvantages of single carrier DS-CDMA. In MC-CDMA each symbol is transmitted over N narrowband sub carriers. Multiple users use the same set of sub carriers to transmit their data at the same time but the spreading codes of users are orthogonal to each other that is why the data of different users does not interfere with each other. At the receiver the same code is used to de-spread the data on each sub-carrier that was encoded using the same code at the transmitter side.

The user’s data stream is serial to parallel converted using a serial to parallel converter and then the data streams are spreaded by a spreading code.

Bit Error Rate Comparison of OFDM and MC-CDMA Systems

Muhammad Talha Ahmed ¹, Junaid Khalid ¹, Khalida Noori ¹, Sami Ahmed Haider ²

¹ NUST- School of Electrical Engineering and Computer Science

² NUST - Military College of Signals

48junaid@niit.edu.pk, 48talha@niit.edu.pk

International Conference on Computer Science and Information Technology 2008

978-0-7695-3308-7/08 $25.00 © 2008 IEEE

DOI 10.1109/ICCSIT.2008.96

665

International Conference on Computer Science and Information Technology 2008

978-0-7695-3308-7/08 $25.00 © 2008 IEEE

DOI 10.1109/ICCSIT.2008.96

665

1( ) ( )

1 21

. ( , ... )MC

MC

Gi i T

Gi

S d c S S S−

=

= =∑

Where, thi Users input data is ( )id

thi Users spreading code is ( )ic This is called spreading in the time domain where this

scheme’s name is MC-DS-CDMA (multi carrier direct sequence code division multiple access).

III. OFDM OFDM is a type of multicarrier modulation where a high

rate data stream is divided into lower rate data streams where the substreams are orthogonal and do not interfere with each other. In OFDM, separate symbols are transmitted on each sub-stream whereas in MC-CDMA same symbol is transmitted on each stream. Two periodic signals are orthogonal when the integral of their product over one period is equal to zero. For the case of continuous time:

*0( ) ( ) {

bp for i j

i j for i ja

t t dtφ φ =≠=∫

For discrete time case: 1

0

2 2cos( )cos( ) 0, (y x)N

i

kx ky dtN Nπ π−

=

= ≠∑

The sub-carrier frequencies are spaced

0nf f n f= + Δ Where

1/f N tΔ = Δ Defining the symbol duration at sub carrier level as ST , the transmitted signal s (t ) is written as

2 ( )

1( ) ( )

SCk s

Nj f t iT

ki Si k

s t c e f t iTπ+∞

−

=−∞ =

= −∑ ∑ [1]

where c ki is the i th information symbol at the kth sub carrier, and f (t) is the pulse waveform of the symbol [4].

The effect of ISI is avoided with the usage of a cyclic prefix of length longer than the delay spread of channel. ISI (inter symbol interference) is avoided as the symbol times are longer on each of the low rate data streams.

IV. SIMULATION SETUP The Simulation was done using Matlab according to the following figures.

Figure.1 Block Diagram of our MC-CDMA Transceiver

Figure.2 Block Diagram of our OFDM Transceiver A Black and white image of resolution 64x64 bits was taken

as an input. The input bits were then converted from parallel to serial by a parallel to serial converter. After this, the data is then spreaded using spreading code. The spreading ratio is 8:1 that is, the original data is spreaded eight times after multiplying with the code.Data is then modulated using QPSK modulation. Thirty two (32) point IFFT (Inverse fast Fourier transform) is taken of each substream which means that 32 subcarriers are generated from each substream. First 1/4th portion of the data of each subcarrier is appended at the end of that subcarrier and this process is known as cyclic extention.This avoids ISI (inter symbol interference).Data is converted into a serial stream for transmission over the channel.The serial stream is then passed through Rayleigh fading channel and AWGN channel for a number of different SNR’S (Signal to noise ratio) in ascending order.RLS

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adaptively the incoming signal.The signal in the transmitter is separately provided to the RLS equalizer which acts as a training sequence. The equalized signal is then converted from serial to parallel substreams and guard time is then removed from each substream.FFT(fast fourier transform) is taken of each substream.Each substream is then demodulated and despreaded.After that the image is reconstructed from the received data and compared with the input image for the bit errors.

V. Simulation Results The simulation result in the Figure.3 shows the comparison of the bit error rate (BER) performance of the MC-CDMA system and OFDM system in Rayleigh plus AWGN channel as SNR is increased.The x-axis shows the SNR in dBs (decibels) and y-axis shows the bit error ratio of the transmitted and received signal.

Figure.2 BER performance of our MC-CDMA and

OFDM system The spreading factor used here for MC-CDMA is 8 i.e. the

spreaded data is 8 times the original data. As the Spreading factor is increased further, the performance of MC-CDMA is further increased but the bandwidth required increases too. So we have used spreading factor 8 as an optimal solution. The following picture was transmitted and received both on the MC-CDMA system and OFDM system for the following SNRs.

Figure.3 Transmitted Picture

OFDM MC-CDMA SNR=5

SNR=10

SNR=15

Figure.4 Received pictures at different SNRs

V. CONCLUSION As the spreading factor for MC-CDMA system is increased to 8 and beyond, the MC-CDMA system shows far better performance as compared with OFDM system for the same number of sub-carriers. It can be seen from Figure.2 that with MC-CDMA system we can attain bit error rate of approximately 10^-4 at 15dB which is far better than OFDM system for the same SNR.

REFERENCES [1] Shinsuke Hara, Ramjee Prasad, “Multicarrier technologies for 4G Mobile communications”, Artech House Boston London, 2003, ISBN 1-58053-482-1, [2] Carl R.Nassar, B.Natarajan, Z.Wu, D.Wiegandt, S.A.Zekawat, S.Shattil, “Multicarrier technologies for wireless

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communications”, 2002 Kluwer Academic Publishers, New York, Boston, Dordrecht, London, Moscow, ISBN: 0-792-37618-8. [3] P. Zhang, X. Tao, J. Zhang, Y. Wang, L. Li, and Y. Wang, “A vision from the future beyond 3G TDD,” IEEE Commun. Mag, Jan. 2005 vol. 43, pp.38–44. [4] N. Yee, J. Linnartz, and G. Fettweis, “Multi-carrier CDMA in indoor wireless radio networks,” IEEE PIMRC, Sep. 1993, pp. 109–113. [5] A. Persson, T. Ottosson, and E. Storm, “Time-frequency localized CDMA for downlink multi-carrier systems,” in IEEE ISSTA, Sep. 2002,pp. 118–122. [6] “D1.4 reference scenario specification: Final description,” IST-MATRICE, July 2003. [7]Maleki-Tehrani, A Hassibi, Cioffi, J.M, “Adaptive Equalization of Multiple input Multiple output (MIMO) frequency selective channels”, conference on Signals, Systems, and Computers, 1999, Vol. 1, pp. 547-551. [8] Simon haykin, “Adaptive Filter Theory”, Pearson Education, 3rd Edition, 2003, ISBN: 81 7808 565 8, PP.399-400. .

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