Some Simulation Results on the Error Rate Performance of Spread Aloha

ALOHA is a random multiple access technique useful for interactive computer working. It, however, suffers from the limitation that it requires a high power output from the earth station that is accessing the channel. Spread ALOHA, a variation of ALOHA proposed by Abramson eliminates this problem by using the technique of bit spreading. In this paper the results of a simulation study on the error rate performance of spread ALOHA in the context of simultaneous transmissions of packets from earth stations are presented. The effect of the following parameters on chip error rate (CER) and bit error rate (BER) have been evaluated. It is shown that the use of longer spreading sequences bring down the BER to facilitate the use of smaller earth stations. Simultaneous access is possible with the use of spread ALOHA, however, while the CER increases slowly as the number of interfering carriers is increased, the BER shoots up after a certain number of interfering carriers are reached. This suggests a limit on the number of simultaneous transmissions on a spread ALOHA channel and this limit is independent of the limitation imposed by the acquisition circuit.