A fluid analysis of a utility-based wireless scheduling policy

In this paper, we consider packet scheduling for the downlink in a wireless network, where each packet\´s service preferences are captured by a utility function that depends on the total delay incurred. The goal is to schedule packet transmissions to maximize the total utility. In this setting, we examine a simple gradient-based scheduling algorithm called the U˙R-rule, which is a type of generalized cμ-rule (Gcμ) that takes into account both a user\´s channel condition and derived utility when making scheduling decisions. We study the performance of this scheduling rule for a draining problem, where there is a given set of initial packets and no further arrivals. We formulate a "large system" fluid model for this draining problem where the number of packets becomes large while the packet-size decreases to zero, and give a complete characterization of the behavior of the U˙R scheduling rule in this limiting regime. Comparison with simulation results show that the fluid limit accurately predicts the corresponding behavior of finite systems of interest. We then give an optimal control formulation for finding the optimal scheduling policy for the fluid draining model. Using Pontryagin\´s minimum principle, we show that, when the user rates are chosen from a TDM-type of capacity region, the U˙R rule is in fact optimal in many cases. Sufficient conditions for optimality are also given. Finally, we consider a general capacity region and show that the U˙R rule is optimal only in special cases.