STABILITY AND THROUGHPUT ANALYSIS OF UNSLOTTED CDMA-ALOHA WITH FINITENUMBER OF USERS AND CODE SHARING

We consider a system comprising a finite number of nodes, with infinit e packet buffers, that use unslotted ALOHA with Code Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition for the stability of the packet buffers; we inter pret this as the capacity of the access method. We calculate and compa re the capacities of CDMA-ALOHA (with and without code sharing) and TD MA-ALOHA; we also consider carrier sensing and collision detection ver sions of these protocols. In each case, saturation throughput can be o btained via analysis pf a continuous time Markov chain. Our results sh ow how saturation throughput degrades with code-sharing. Finally, we a lso present some simulation results for mean packet delay. Our work is motivated by optical CDMA in which ''chips'' can be optically generat ed, and hence the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful i n the optical CDMA context as it reduces the number of optical correla tors at the receivers. Our throughput results help to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA-A LOHA yields better capacity than TDMA-ALOHA.