Stability and control of mobile communication systems with time-varying channels

The authors consider the forward link of a one-cell mobile communications system with a single transmitter, a fixed number (K) of mobiles, and randomly time-varying channels. Data arrives at the base in some random way (which might have a bursty character) and is queued according to the destination until transmitted. We consider CDMA with control over the bit interval and power per bit, TDMA with control over the time allocated, bit interval, and power per bit. The control problem is the allocation of power and time to the K queues to assure stability of the queues. The control decisions are made at the beginning of the (small) scheduling intervals. The Liapunov function based stability methods are used to determine time and power allocations. The system and channel process are scaled by speed. Under a stability assumption on the fluid model obtained from the "mean drift," and some other natural conditions, it is shown that the scaled physical system can be controlled to be stable, uniformly in the speed, for fast enough speeds. Owing to the non-Markov nature of the problem, we use the perturbed Liapunov function method in order to obtain the control allocations. Each such function corresponds loosely to a performance criteria for some optimization problem. Part of the power of the analysis is due to the rather general conditions under which it works.