Analytical Performance of a Frequency Offset Multi-user Multi-array System

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) modulation is a technique commonly used to deal with scattering environment. This is mainly due to the fact that it does not involve any complicated channel equalization, since each sub-carrier undergoes a flat fading channel. However this modulation is very sensitivity to frequency offset in the channel. The frequency offset causes two bad effects: the first is the reduction of the signal amplitude of each sub-carrier and the second is introduction of Inter Carrier Interferences (ICI) from the others carriers which are now no longer orthogonal. In a multi-users system the different transmitters got a independent local oscillator, then each transmitter is affected by a different frequency offset (in comparison to the receiver). The usual solution to deal with this problem in the single user case is to lock the frequency of the transmitter and the receiver using a phase lock loop. But in the case of multi-users transmission we cannot do it this way. The aim of this paper are twofold. First we first give an improved multi-users model including the frequency offset effects. Then using this model we derive the analytical performance at the channel decoder output. This paper is subdivided in three parts. First we describe our system. This model takes into account the effect of the frequency offset. Then by derivation of this model we describe the effect of the frequency offset as a noise floor, and we explain the analytical performance of our system according to the frequency offset of the different transmitters. In a