Two-channel zero forcing equalization on CDMA forward link: trade-offs between multi-user access interference and diversity gains

A zero-forcing (ZF) equalization criterion is developed for a dual antenna receiver on the forward link in a CDMA based multi-user communication system. In the forward link, all users are broadcasted synchronously and the multi-user access interference (MAI) traverses the same channel as the desired user. Thus, effecting ZF equalisation (with two antennas) for the desired user also also effects ZF equalization for the MAI. Assuming the use of orthogonal variable spreading factor (OSVF) codes on transmit, one may correlate with the desired user´s code at the output of the ZF equalizer and ideally have zero MAI in the symbol decision statistics obtained by sampling once per symbol after synchronization. Simulations are presented demonstrating the advantages of the proposed two-channel ZF equalizer, followed by a conventional correlator with symbol rate sampling, over a RAKE receiver.