Adaptive retransmission diversity with packet combining for slotted DS/CDMA packet radio networks

A time diversity automatic repeat-request (ARQ) scheme is investigated for slotted random access direct-sequence code-division multiaccess (DS/CDMA ALOHA) wireless packet radio networks on multipath Rayleigh fading channels. The receiver retains and processes all the retransmissions of a single data block (packet) using predetection diversity combining, instead of discarding those which are detected in error. This effectively improves the system throughput and delay characteristics especially at small values of signal-to-noise ratio (SNR) per bit. A simple and practical selection combining rule is proposed, which lends itself to a low-complexity receiver structure and specifically suitable for high data rate transmissions. Owing to the stochastic nature of the multiple access interference, the new maximum output selection diversity (MO/SD) system yields superior performance in comparison to the traditional maximum SNR selection diversity (SNR/SD) model. The bit error rate performance, throughput and the average number of transmissions required to transmit a packet successfully with and without forward error correction (FEC) are evaluated. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA without incurring a substantial penalty in terms of cost or complexity.