Design of turbo-MUD receivers with density evolution in overloaded CDMA systems

An original turbo multiuser detector for overloaded coded DS-CDMA systems is presented and studied; the performance of the proposed system is tested by Monte Carlo simulations and by means of the density evolution theory. The turbo-MUD receiver is based on the use a combination of a linear MMSE detector in the first stage and an interference cancellation scheme in the successive ones. The inputs of both receivers are the soft information from a bank of turbo decoders. The performance of the proposed receiver can be effectively predicted in overloaded communication systems by means of the density evolution technique; the use of this technique also permits the proper number of MMSE iterations to be evaluated, making the design of the receiver easier.