Transmitter-Preprocessing-Assisted Cooperative Downlink Transmission in DS-CDMA Systems Experiencing Propagation Path Loss and Nakagami- $m$ Fading

In this paper, we propose and investigate a relay-diversity transmission scheme for the direct-sequence code-division multiple-access (DS-CDMA) downlink, where each (destination) mobile terminal (MT) is aided by a cluster of relays to achieve the relay diversity. In the considered system, downlink multiuser interference (MUI) is suppressed with the aid of transmitter preprocessing operated at the base station (BS). Two transmitter preprocessing schemes are considered, which are operated in the principles of transmitter zero forcing (TZF) and transmitter minimum mean square error (TMMSE). At the MTs, signals received from the BS and relays are combined based on the principles of maximal ratio combining (MRC) or maximum signal-to-interference-plus-noise ratio (MSINR). In this paper, the bit-error-rate (BER) performance of the relay-assisted DS-CDMA downlink is investigated when the communication channels are assumed to experience both propagation path loss and generalized Nakagami- m fading. Our study and simulation results show that the transmitter preprocessing can help to achieve the relay diversity by efficiently mitigating the MUI present at the relays and MTs. Furthermore, in our proposed relay-diversity scheme, the relays only require low-complexity signal processing to forward information to their served MTs.