Analysis of code division random multiple access systems with packet combining

A reverse link random access system is considered where CDMA with random spreading is used for user separation. The receiver consists of either a matched filter (MF) or a Minimum Mean Squared Error (MMSE) detector followed by autonomous forward error correction (FEC) decoders for each user. The random access strategy combines slotted ALOHA with incremental redundancy (IR). Such a system is defined as a Code Division Random Multiple Access (CDRMA) system. Two types of IR, namely code combining and maximal ratio combining (MRC) are considered. Bounds on the throughput of a CDRMA system are obtained for different detectors and IR schemes, when the number of users K and the spreading factor N, are both large (K, N → ∞, K/N = alpha;). These bounds are derived using known results on the information theoretic capacity for a user within a slot. The bound on the throughput of a CDRMA system is shown to be equal to the bound on the throughput of an equivalent fixed access (conventional) CDMA system.