Packet combining error control for CDMA systems

Two examples of the application of packet combining to CDMA are presented and analyzed: an asynchronous DS/CDMA system supported by an averaged diversity combining (ADC) scheme and a similar system using code combining (CC). The ADC scheme uses a Viterbi decoder combined with a CRC code to generate retransmission requests, while the CC scheme employs a sequential decoder operating with the time-out algorithm. Multiple received packets are combined to form a single, more reliable packet. The error correcting decoder operates on the combined packet, as opposed to the most recently received individual packet (e.g. as in a type-I hybrid ARQ protocol), substantially increasing the probability of acceptance with each additional transmission. The authors show through numerical analysis that both techniques provide a significant increase in the CDMA system capacity, throughput and reliability