Improving the throughput of DS-CDMA systems using adaptive rate transmissions based on variable spreading factors

In this contribution adaptive rate transmissions are investigated in the context of DS-CDMA systems using variable spreading factors (VSF). In contrast to conventional adaptive rate transmission schemes, where the transmission rate is usually adapted in response to the channel fading, in this contribution the transmission rate is adapted in response to the multiuser interference (MUI) level. We present the philosophy of the proposed adaptive rate transmission scheme and analyze the effective throughput as well as the resultant bit error rate (BER) performance, when communicating over additive white Gaussian noise (AWGN) channels. Our study shows that when the number of active users in a DS-CDMA system can be modelled with the aid of a Markov chain and when the conventional matched filter based receiver is employed, the VSF-assisted adaptive rate transmission scheme is capable of substantially increasing the system´s effective throughput. Specifically, our results show that the effective throughput may be increased by up to 40%, when compared to that of DS-CDMA systems using constant spreading factors. This increased throughput is achieved without wasting power, without imposing extra interference upon other users and without increasing the BER.