Characterization of stability margins of the Foschini-Miljanic power allocation strategy under constant and time-varying delays

This work studies the power control algorithm proposed by Foschini-Miljanic (1993), standardized for universal mobile telecommunication systems (UMTS). Continuous and discrete time versions of this algorithm are analyzed. First, it is shown that the distributed power allocation scheme can be visualized as a tracking problem of an equivalent power reference that specifies the desired quality-of-service (QoS). For this purpose, a proportional controller is then employed under integral action in order to achieve good tracking despite time-varying and unknown channel gains. Next, the effects of constant and time-varying time delays in the closed-loop structure are studied. Explicit stability regions for the control gain in the Foschini-Miljanic scheme are derived for both the continuous and discrete-time versions of the algorithm, under constant and time-varying delays. A comprehensive evaluation using simulations is performed to validate the analytical derivations described in the paper.