Iterative Multiuser Decoding of Narrowband Signals in Fading Channels

In this paper we investigate multiuser detection techniques for coded narrowband signals in fading channels. In [1], Moher proposed a suboptimal multiuser detector and its performance is demonstrated for the AWGN channel. In this paper, this multiuser detector is modified for fading channels, such as the Rayleigh fading channel model. The fading channels described and simulated are flat narrowband channels. The receiver is assumed to be synchronous. I. Multiuser Channel Model A. Discrete Time Synchronous Model Multiuser transmission means that K users share one resource for transmission of data. The shared resource can be the spectrum, time, or power allocations. The common code multiplex technique CDMA is used where it makes sense to have a concrete example, for motivating the model equation and for giving an example how to calculate the crosscorrelations. The model itself can be used with all types of multiuser transmission and is not specific for CDMA. In a CDMA system, every user k is assigned a unique signature waveform sk (t) known by the receiver. If the length of the waveforms is not larger than the symbol duration T , there is no ISI and a synchronous model can be used. That is, the received signal y (t) depends only on the mapped bits bi, 1 ≤ i ≤ K of the actual point of time. If the used channel is an AWGN channel the difference between the both signal after the channel y (t) and before the channel s (t) is a white Gaussian distributed noise process: y (t) = s (t) + n (t) (1) where n (t) represents white Gaussian noise with variance σ. The signal at the input of the receiver can be described for a symbol duration T as follows: