Performance Evaluation of Unslotted Random Access Packet CDMA Channels

Abstract

This paper presents an analytical performance evaluation model for a class of unslotted random access packet CDMA (code division multiple access) channels. In the system considered, many interactive terminals asynchronously transmit (uncoded) spread spectrum modulated variable length data packets on a shared wideband channel. Since unslotted operation at the physical level precludes the application of controlled multiaccess procedures, reliable link level operation is achieved using an ALOHA-Iike procedure in which incorrectly received packets are retransmitted with random delay. In view of the resulting traffic dependent packet success probability, Rich a packet CDMA (or “spread ALOHA”) channel may be expected to exhibit throughput and stability properties qualitatively similar to those of narrowband ALOHA. A flow equilibrium based analytical model for performance evaluation is presented and used to illustrate both throughput-delay and stability characteristics for an example random code direct sequence (DS) unslotted CDMA system. It is shown that a typical uncoded packet DS-CDMA channel achieves a maximum bandwidth normalized throughput (utilization) in the region of 0.15–0.2, and is thus competitive with narrowband ALOHA carrying similar data traffic. Stability issues for finite-population channels are also addressed, leading to the conclusion that relatively long average retransmission delays may often be required to prevent bistable behaviour unless adaptive policies are nsed.