MAXIMUM-LIKELIHOOD DECODING AND CODE COMBINING FOR DS SSMA SLOTTED ALOHA/

This paper considers the combination of multiple copies of a packet to improve the performance of a slotted direct-sequence spread-spectrum multiple-access (DS/SSMA) ALOHA packet radio system with coherent bina ry phase-shift keying (BPSK) modulation, Both slotted DS/SSMA ALOHA wi th and without forward error correction (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 ra te for that specific packet, and therefore an improved probability of successful decoding, In both cases, combining multiple copies of the s ame packet results in a throughput which is an increasing function ove r a broad range of offered load, so that the system is more reliable f rom the point of view of stability. In addition, combining provides a higher throughput and a smaller time delay for packet transmission, Th is is illustrated by means of analytical and simulation results.