ICI mitigation for mobile OFDM receivers - Communications, 2003. ICC '03. IEEE International Conference on

Orthogonal Frequency Division Multiplexing (OFDM) is robust against Inter-Symbol-Interference (ISI) due to the increase of the symbol duration. However, for mobile applications, channel variations during one OFDM symbol introduce Inter-Carrier-Interference (ICI), which degrades the performance. This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. We introduce two new methods to mitigate ICI in an OFDM system with coherent channel estimation. Both methods use a piece-wise linear model to approximate channel variations. The first method extracts channel variations information from the cyclic prefix. The second method estimates these variations utilizing the next symbol. Since mathematical analysis of the whole system becomes intractable in a long delay spread environment, we provide a mathematical analysis that investigates the effect of linearization for the case of a time-variant one path channel. Then our simulation results show how these methods would improve the performance in a highly time-variant environment with a long delay spread. INTRODUCTION Orthogonal Frequency Division Multiplexing (OFDM) handles channel frequency selectivity, resulted from the delay spread, by expanding the symbol duration [1]. By adding a guard interval1 to the beginning of each OFDM symbol, the effect of delay spread (provided that there is perfect synchronization) would appear as a multiplication in the frequency domain, for a time-invariant channel. This allows for higher data rates and has resulted in the recognition of OFDM as a standard for Digital Audio and Video Broadcasting (DAB and DVB). However, a transmission in a mobile communication environment is impaired by both delay and Doppler spread. Due to the increase of the symbol duration, time variations degrade the performance of an OFDM system more severely [2]-[4]. It will introduce Inter-Carrier-Interference (ICI) which results in the loss of orthogonality among carriers. Therefore, to improve the performance of an OFDM system in a time-variant environment, there is a need for ICI mitigation. In this paper, we propose two new methods for ICI mitigation in a pilot-aided OFDM system. For coherent channel estimation pilot tones should be transmitted. If maximum channel delay spans ν sampling periods, Negi [5] ∗Authors would like to thank Sirius Satellite Radio for its financial support 1This will also prevent inter-symbol-interference has shown that only L = ν + 1 pilots are needed for perfect channel estimation in the absence of AWGN and mobility. However, in the presence of Doppler spread, these pilots are not enough to estimate the channel. Then we show how to estimate channel variations using either the cyclic prefix or the next OFDM symbol. Finally, our mathematical analysis and simulation results show the performance of the proposed methods, indicating how they can reduce the required amount of transmitted power. SYSTEM MODEL Fig. 1 shows the discrete baseband equivalent system model. We assume perfect synchronization in this paper. The available bandwidth is divided into N sub-channels and the length of the guard interval2 is G. Xi represents the transmitted data in the ith sub-channel and is related to xi as Xi = ∑N−1 k=0 xke − j2πki N . xp is the added cyclic prefix vector with length G and is related to x as follows: xp(i) = xN−G+i 0 ≤ i ≤ G − 1 (1) Let T be the time duration of one OFDM symbol after adding the guard interval. Then, h k represents the kth channel path at time t = i×Ts where Ts = T N+G . In our notation h (i) k for −G ≤ i ≤ −1 and 0 ≤ i ≤ N − 1 represents kth channel path in the guard and data interval respectively3. Then the data part of the channel output can be expressed as follows: