Retransmission Diversity in Large CDMA Random Access Systems

This paper considers a large CDMA random access system where users randomly access a base station through a common CDMA channel. The number of users and spreading gain tend to infinity with their ratio converging to a constant. An approach to exploitation of retransmission diversity (RD) is proposed that first eliminates the interference signals caused by the previously successfully detected packets and then jointly utilizes all received signals in packet detection. Performance of two schemes is analyzed: (i) random spreading with RD exploitation and (ii) randomly selected but fixed spreading for each packet with RD exploitation, and compared with the conventional scheme that RD is not exploited. Lower bounds on the limit signal-to-interference ratios (SIRs) of the two schemes are obtained. Throughput, packet delay, transmission time, and waiting time as well as spectral efficiency in the limit system are obtained as a function of medium access control. The analytical results are confirmed by simulations. Other schemes that increase RD by changing order of transmitted packets are also discussed. Optimization of media access control (MAC) is studied. In terms of maximization of spectral efficiency and simplicity of MAC, scheme (i) is the most favorable out of all the schemes.