The capacities of ALOHA and power controlled CDMA in the UMTS downlink channel

We compare the performance of data transmission on the downlink channel of a cellular system when two different interference-reducing techniques are used. The first is the conventional CDMA with closed loop power control with FEC coding and the second is S-ALOHA. In the latter, packets are transmitted in random slots at the full channel rate, therefore using bandwidth to achieve a random time spreading of packets. Both techniques use retransmissions when error occurs. The performance evaluation is obtained by simulation in order to consider a realistic cellular environment that takes into account the bursty nature of the inter-cell interference noise and its real statistics, which, in the S-ALOHA case where no spreading is used, is far from being normally distributed. The results presented show that the S-ALOHA approach, in the UMTS environment, achieves better bandwidth efficiency than CDMA.