On the lowsource power regime of the DS/CDMA relay channel

This paper considers a DS/CDMA relay system where random spreading sequences are employed at the relays to allow multiple access to the channel between relays and destination. Two different protocols are considered and compared, the difference between them being whether the source remains silent or sends new information symbols during the relaying. For both protocols, we provide a low source power approximation for the spectral efficiency when employing, at the destination, either the optimal or the linear minimum mean square error receiver. The new expression is studied for the single relay case to identify under which conditions relaying results convenient over direct source-destination communication. Furthermore, we investigate the optimum percentage of relaying time in order to maximize the global spectral efficiency.