Optimal QoS tradeoff and power control in CDMA systems

Dynamic power control and scheduling strategies provide efficient mechanisms for improving performance of wireless communications networks. A common objective is to maximize throughput performance of a network or to minimize the total transmission power while satisfying quality-of-service (QoS) requirements of the users. The achievement of these objectives requires the development of medium access control (MAC) strategies that optimally utilize scarce resources in wireless networks. When developing such strategies, a good understanding of the structure of the feasibility region is essential. The feasibility region is defined as a set of all QoS requirements that can be supported by a network with all users active concurrently. Thus, the structure of this set shows when (if at all) scheduling strategies can improve network performance. In particular, if the feasibility region is a convex set, then concurrent transmission strategies are optimal and the optimal power allocation can be obtained efficiently via a convex optimization. Other important problems are how the total transmission power depends on QoS requirements and what the optimal QoS tradeoff is. In this paper, we address all these problems and solve them completely in some important cases. The purpose of this paper is to explore the interrelationship between QoS requirements and physical quantities such as transmission power. Although the results are obtained in the context of a power-controlled CDMA system, they also apply to some other communications systems. A key assumption is that there is a monotonous relationship between a QoS parameter of interest (such as data rate) and the signal-to-interference ratio at the output of a linear receiver.