2004 RF AND MICROWAVE CONFERENCE, OCTOBER 5 - 6, SUBANG, S E L A N G O K MALAYSIA

Intercarrier Interference (ICI) Analysis Using Correlative Coding OFDM System

A. N. Husna’, S. Y . Sharifah Kamilah’, B. Ameruddin’ and E. Mazlina’

‘Electrical Engineering Department Fusat Pengajian Diploma

Universiti Teknologi Malaysia Jalan Semarak 54100,Kuala Lumpur, Malaysia

’Faculty of Electrical Engineering Universiti Teknologi Malaysia 81310SkudaL Johor, Malaysia

Abstmct - Orthogonal Frequency Division Multiplexing (OFDM) technique is an attractive in wireless communication system. The limitation of OFDM in many applications, however, is very sensitive to frequency errors caused by two destructive effects. One effect is the reduction of signal amplitude and the other effect is the introduction of intercarrier interference (ICI) from the other carriers that are caused by the loss of orthogonality between the subchannels. In order to minimize the intercarrier interference (ICI) in the OFDM system, the correlative coding method was proposed. The normalized frequency offset was introduced in order to analyze the performance of the system where subcarrier frequency offset (SCFO) was used to measure the IC1 in the system using Matlab software. The result shows that correlative coding OFDM (CCOFDM) can improve the level of the IC1 signal apposed to the OFDM without correlative coding (OFDMWOCC) model at about 6.9 dB

1. Introduction

In an OFDM system, the whole available bandwidth is divided into N small paris, and a block of N data symbols are modulated on N corresponding subcarriers which are orthogonal to each other. The spectra of the subcarriers are overlapping; therefore, accurate frequency recovering is needed. However, in the mobile radio environment, the relative movement between transmitter and receiver causes Doppler frequency shifts, in addition, the carriers cannot be perfectly synchronized. These random frequency errors in OFDM system distort orthogonality between subcarriers, as a result intercarrier interference (ICI) occurs. Literature show, that in such systems, the bit error rate (BER) ‘increases rapidly with increasing

frequency offset [1,2,3]. This is main restriction in OFDM systems.

In this paper, the correlative coding between signals modulated on subsequent subcarriers is used to compress IC1 in OFDM system and the IC1 is measured using subcarrier frequenq offset response (SCFO response). Previous study shows that frequency-domain wrrelative coding is a simple solution to IC1 problems, and makes OFDM systems less sensitive to frequency errors, thus it reduces the system complexity and increasing bandwidh efficiency [4].

An outline of this paper is organized as follows. The system model will desmibe about the CCOFDM model where the (I-D) type of correlative coding was chosen and the SCFO response was ineoduced in terms of Doppler shifts in the channel.

In Section 3, we present the analysis on IC1 for the CCOFDM and OFDMWOCC and the discussion of the result was presented in Section 4.

2. System Model

I

Channel ---el (Introduced SCFO)

Y(k y(” YO Conelalive decoding FFT sm

Figure 1: The OFDM system with correlative coding

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A simplified block diagram of the proposed OFDM system with correlative coding is shown in Figure 1. The structure of an OFDM system with correlation coding can be derived from conventional single carrier systems [3]. By using BPSK modulator, the serial modulated signal, ak is coded using correlative coding.

The proposed correlative coding is performed as [51

where L=correlation code length and J = correlation coefficients where i=O,l,. _.., L-I. The coded symbols bk then, are modulated on N subcarriers. The symbol bi takes three possible value which are -2.0.2.

The frequency offset will be introduces in the channel pan normalized to the frequency sampling. Then, the receiver performs the inverse pari of the transmitter. The received signal for the Mh subchannel after FFT processing can he written as

(2) N-1

l=O, /+k Y ( k ) = b ( k ) S ( O ) + b ( l ) S ( / - k )

k = 0,I. .. N - I

where N is the total number of the subcarriers and b(k) denotes the transmitted symbol of the correlative coding for the Mh subcarrier. The first term in the right hand side represents the desired signal. The S(0) takes the maximum value which is equal to one because of the absent of the frequency error (E=O), while the next term is the IC1 components of the systems. The sequence S (I-k) is defined as the IC1 coefficient between the /th and Mh subcarriers caused by SCFO response and given by

3. IC1 analysis

The IC1 analysis consist ofthe simulation from the basic equation of Eq.(3). The IC1 on the k-th subcmier also can he expressed into

N-l i x = b ( / ) S ( / - k )

f = O , l ~ k (4)

From the simulation result, Figure 2 shows that the magnitude of IC1 that will affect the selected subcarrier of /=k=2,14,28 for one symbol data This analysis is on the normalized frequency offset equal to

0.2 with 32-th carriers. At /=k-2,14,28, it gives the highest value because S(0) almost equal to one which means at this point, there is no frequency shifted. This is the desired signal and takes the value of 1.87, which is almost equal to 2 for the maximum value of CCOFDM system.

S I 0 ....."..,"l.L,,"..

Figure 2: The magnitude of IC1 of the CCOFDM system

Figure 3 and Figure 4 shows the total magnitude of the desired signal, (") compared with the IC1 signal, (*) for CCOFDM system and OFDMWOCC system respectively. From the graph, the interference for OFDMWOCC gives almost half of the desired carrier contrast with CCOFDM system; almost quarter of the wanted signal

,"*.. . Figure 3 : The magnitudeof desired and IC1 signal for

CCOFDM system

Figure 4: The magnitude of desired and IC1 signal for OFDMWOCC

4. Discussion

Figure 5 explains about the IC1 level of both system models are proportional to the frequency offset. This result is valid with the theoretical which says that the OFDM system is sensitive to the IC1 cause by the Doppler shifl in the multipath fading channel.

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However, using the CCOFDM, it can improve the level of the IC1 signal apposed to the OFDMWOCC model at about 6.9 dB. When the frequency offset increased the difference of the IC1 level for both systems was declined and will achieved OdB difference at 100% shifted. Its means that both systems achieved the same performance at the normalized frequency offset equal to unity.

5. Conclusion

The correlative coding method has shown that it be able to minimize the IC1 in the OFDM system. The (I-D) type was chosen because it gave the optimum coding that could minimize the IC1 [I]. The simulation shows that, without considering AWGN channel, the OFDMWOCC or basic OFDM gives higher IC1 at about 1.5% difference compared to the CCOFDM system, which means that the OFDMWOCC is more sensitive to the frequency offset.

References

[ I ] Pollet,T. , Van Bladel, M. and Moeneclaey, M. “BER sensitivity of OFDM Systems To Carier Frequency Offset And Wiener Phase Noise.” ir. IEEE Trans. Communication, 1995,43(2/3/4) pp 191-193.

[2] Zhao, Y., Leclercq, J.D. And HHggman, S.G. “Intercarrier Interference Of Compression In OFDM Communication Systems By Using: Correlative Coding”, IEEE Communications Letters, Vol. 2, No. 8,pp. 214-216, 1998

[3] Yuping Zhao And Haggman, S.G , “A Study Of Using Correlative Coding In OFDM Communication Systems,’’ In Proc. Int. conf. On communications (ICT’98), Port0 Carras, Greece, 1998,pp 380-384

[4] Yuping zhao and H@gman, S.G. “BER Analysis Of OFDM Communication Systems With Intercarrier Interference” in proceedings of the International Conference on Communication Technology, ICCT’98, Beijing, China, October 22-24 1998, pp. S3842-1-S3842-5

[5 ] Proakis, J.G, “Digital Communications”, 2”d Edition, McGraw-Hill,l989

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