A spread CDMA slotted ALOHA system with hybrid ARQ for satellite multiple access

In the present paper we propose and investigate a new type of satellite multiple access protocol that combines the characteristics of the spread slotted ALOHA (SSA) protocol with the hybrid-ARQ error controlling and retransmission system in order to increase the throughput, in terms of the number of retransmissions, and to keep the BER of the satellite link low when the channel experiences heavy traffic. The main feature of our proposed systems is the utilization of two different fields in the analysis of the satellite multiple access problem when the traffic is heavy in order to deliver a greater amount of information bits per time slot. A CDMA m-sequence coding for the SSA has been applied together with a type I hybrid ARQ system. The ARQ system utilized was a selective repeat ARQ system together with BCH code intended to perform error detection and correction. The system performance was evaluated according to different amount of parity check bits of the BCH code on an environment with noise (AWGN) and co-channel interference from other users trying to access the hub. Since the hub now possesses the FEC capability to correct errors that appear after the CDMA despreading of the packets, the satellite doesn't need to ask so often for the retransmission of erroneous packets and will ask for retransmission only when the FEC capability is exceeded. Computer simulations show the performance of the proposed multiple access scheme.