BER Degradation of MC-CDMA at High SNR with MMSE Equalization and Residual Frequency Offset

P Harinath Reddy,V U Reddy

doi:10.1155/2009/293264

Abstract

Multicarrier Code Division Multiple Access (MC-CDMA) is an attractive technique for high speed wireless data transmission in view of its advantages over orthogonal frequency division multiplexing. In this paper, we analyze the performance of fully loaded downlink MC-CDMA systems with minimum mean square error (MMSE) equalizer in the presence of residual frequency offset (RFO) in multipath Rayleigh fading channels. We first show that as the SNR is increased beyond a value, referred as threshold SNR, the performance degrades. We then analyze the cause for this behavior and propose a remedy to prevent the degradation by regularizing the coefficient(s) of the equalizer, and use the regularized equalizer for SNRs beyond the threshold value. We suggest two methods for estimating this SNR, one gives close to the true value but requires the knowledge of RFO and the channel state information (CSI), while the other gives an approximate value but requires only CSI. We show that the regularization based on the approximate value also prevents the degradation, but the performance at higher SNRs is slightly poorer compared to that with the better estimate. Numerical and simulation results are provided to support the analysis.

Highlights

Orthogonal Frequency Division Multiplexing (OFDM) employs a number of orthogonal subcarriers to transmit symbols in parallel
We show that though the performance of the minimum mean square error (MMSE) equalizer is significantly better than that of ZF at lower SNRs, it EURASIP Journal on Wireless Communications and Networking starts degrading beyond a SNR value, referred hereafter as threshold SNR, which depends on residual frequency offset (RFO) and the profile of multipath channel, and tends towards that of ZF as SNR is increased further
The above relation, combined with the fact that the bit error rate (BER) performance of MMSE equalizer is significantly better than that of ZF equalizer at low and moderate SNRs in multipath fading channels and that ZF performance has a floor at high SNR due to inter carrier interference (ICI), suggests that the MMSE equalizer performance degrades beyond an SNR value, referred as threshold SNR

Summary

Introduction

Orthogonal Frequency Division Multiplexing (OFDM) employs a number of orthogonal subcarriers to transmit symbols in parallel. In [5], the authors analyze the BER performance of the OFDM in multipath fading channels in the presence of RFO. We analyze the performance of fully loaded downlink MC-CDMA in the presence of RFO with ZF and MMSE equalizers. We show that though the performance of the MMSE equalizer is significantly better than that of ZF at lower SNRs, it EURASIP Journal on Wireless Communications and Networking starts degrading beyond a SNR value, referred hereafter as threshold SNR, which depends on RFO and the profile of multipath channel, and tends towards that of ZF as SNR is increased further.

Brief Background of an MC-CDMA System

Received Signal Model at the FFT Output in the Presence of RFO

BER Performance of MC-CDMA with MMSE and ZF Equalizers in the Presence of RFO

Cause and Remedy for the Degradation

Simulation Results

Conclusions