Designing adaptive filters-based MMSE receivers for asynchronous multicarrier DS-CDMA systems

We investigate various adaptive filters to design a minimum mean square error (MMSE) receiver, when considering asynchronous uplink multi-carrier direct-sequence code division multiple access (MC-DS-CDMA) systems, over a frequency selective fading channel. Two design structures are considered: the so called "separate detection" consists in using a particular adaptive filter structure for each carrier, whereas the so called "joint detection" is based on a joint structure defined by the concatenation of the adaptive filter weights dedicated to each carrier. We carry out a comparative study between both structures with various adaptive filters such as recursive least square (RLS), Normalized least mean square (NLMS) and affirm projection algorithm (APA). The simulation results show that a compromise has to be found between performance and computational cost. The so called "joint detection" scheme provides the best performance in terms of bit error rate (BER). Although NLMS has the lowest computational cost, it is outperformed by APA and RLS when considering the convergence rate or/and excess mean square error. Besides, the RLS provides the best performance but at the price of high computational complexity. Therefore, we propose to use APA in a joint structure to design the receiver.