Optimality of greedy power control and variable spreading gain in multi-class CDMA mobile networks

In this paper, we consider a DS-CDMA mobile network supporting real-time and non-real-time services. We study how the delay tolerance of non-real-time traffic can be exploited to allow both transmission power control and variable spreading gain (transmission rate control) as mechanisms to optimally adapt the received energy per bit to the current channel conditions and efficiently manage the multiple access interference so as to optimize performance. We provide the jointly optimal power and spreading gain allocation strategy that maximizes non-real-time throughput subject to constraints on peak transmission power and maximum interference generated by non-real-time sources. We show that under the optimal strategy, the optimal spreading gains are inverse linear in the signal to interference plus noise ratio (SINR), and transmission power is allocated to the non-realtime sources in decreasing order of channel gain according to a greedy control strategy: The sources with the highest quality channels transmit at maximum power, while the sources with the lowest quality channels do not transmit. The number of non-real-time sources permitted to transmit simultaneously decreases as the peak transmission power increases, and there is at most a 3 dB difference in SlNR between permitting all sources to transmit and permitting only one source to transmit. We also present numerical results comparing the throughput and delay performance of the optimal strategy with other common strategies; the optimal strategy can offer substantial performance gains.