Optimization of CDMA systems with respect to transmission probability, part I: Mutual information rate optimization

Direct sequence code division multiple access (CDMA) systems that use non-continuous transmission were considered throughout the history of spread spectrum systems, but gained renewed interest with the emergence of impulse radio (IR) technology. Recently, several works had shown that in non-continuous CDMA, the transmission duty cycle (or transmission probability) has a significant effect on system performance. In this work we address the optimization of the performance of CDMA systems with adjustable transmission probabilities. We consider CDMA systems that implement non-continuous transmission by random puncturing, and study the system optimization with respect to both transmission powers (termed power control) and transmission probabilities (termed probability control). We show that the joint optimization has a significant performance advantage over the optimization with respect to transmissions powers only. For some cases we even show that the optimization with respect to transmission probabilities alone is sufficient to achieve optimal performance. In this part, we demonstrate the importance of probability control, by studying the case of frequency-flat slow-fading multiple access channel (MAC) and no spreading. We prove, for this special case, that mutual information rate optimization is achieved by probability control only, while power control is redundant. The theory is supported by simulation results, which show that the achievable rates of all users are better in a system that uses probability control instead of power control.