Maximum likelihood decoding and code combining for DS/SSMA slotted ALOHA

The paper considers the combination of different copies of a packet to improve the performance of a DS/SSMA ALOHA packet radio system. Both DS/SSMA ALOHA with and without FEC are considered. For the case with FEC, maximum likelihood decoding with code combining is used. Code combining allows for the combination of multiple copies of the same packet (which are typically discarded), to obtain a lower code rate for that specific packet, and therefore an improved probability of successful decoding. Thus, as the number of re-transmissions for a packet is increased, the effective code rate is decreased. In both cases, combining different copies of the same packet results in a throughput which is an increasing function over a broad range of offered load, so that the system is more reliable from the point of view of stability. In addition, combining provides a higher throughput and a smaller time delay for packet transmission. This is illustrated by means of simulation and analytical results.