Slotted DS/SSMA ALOHA with packet combining in a Rayleigh fading channel

A DS/SSMA slotted ALOHA packet radio system using BPSK modulation and a RAKE receiver is considered in a mobile environment. We assume a Rayleigh fading channel, with an exponential multi-path intensity profile (MIP) for all users which in conjunction with CDMA results in a frequency selective channel. Average power control is assumed, which means that on average the same power is received from each user. The receiver retains and combines all received copies of a packet, instead of discarding those which are detected in error. This effectively results in a multiplication of the diversity order at the RAKE receiver, by a factor d, where d is the number of packet copies being combined. The throughput and the average number of transmissions to transmit a packet successfully with FEC are evaluated. It is shown that combining the received copies of a packet results in a throughput which is an increasing function of the offered load (at least for a broad range of the offered load). This ensures a stable (or at least more stable) system. It is also shown that this combining results in a higher throughput and a lower average number of transmissions to successfully obtain a packet.