Transmitter based inverse filters for MAI and ISI mitigation in a TDD/CDMA downlink

Recently, research has been focused on techniques that simplify the mobile receiver structure in a CDMA system by means of signal precoding. These methods assume knowledge of the channel at the transmitter which is feasible in a time division duplex (TDD) system where the channel is assumed to be the same on uplink and downlink. A novel algorithm based on prefiltering the spread signal in the base station is described. The prefilter taps are determined with an MMSE criterion and the necessary power constraint. We show that this technique increases the system capacity with a low computational cost compared with existing schemes. The portable unit´s structure is reduced to the conventional receiver for a single user in an AWGN channel, a matched filter. Thus no channel estimation, equalisation or multiuser detection is required at the mobile. The theoretical analysis is supported with simulation results where the performance is compared to other recently published methods.