A scheme for throughput maximization in a dual-class CDMA system

In this work, the uplink of a CDMA system with two user classes is considered. One of the classes consists of delay intolerant users requiring support for a constant information bit rate R, while the other consists of delay tolerant users needing support for an information bit rate of at least Rm. It is assumed that when not transmitting information, both classes maintain synchronization contact with the base station at a rate R0. The objective is to maximize the throughput of the delay tolerant users, while ensuring that the interference to other cells is as low as possible by minimizing the sum of all the transmit powers used by the mobiles. Two transmission modes for the delay tolerant users are considered. In the first mode, which is used in conventional CDMA systems, all the users are allowed to transmit information when they wish. In the second mode, the transmissions of the delay tolerant users are scheduled, so that only a limited number of them are transmitting information at any given time instant. It is shown that the second transmission mode, which tends towards a hybrid CDMA/TDMA scheme for the delay tolerant users, affords a better throughput while imposing the same average power requirements as conventional transmission. The results in this paper can be interpreted using the results from previous work based on information theory.