Convergence of Proportional-Fair Sharing Algorithms Under General Conditions
BROWN UNIV PROVIDENCE RI DIV OF APPLIED MATHEMATICS
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We are concerned with the allocation of the base station transmitter time in time varying mobile communications with many users who are transmitting data. Time is divided into small scheduling intervals, and the channel rates for the various users are available at the start of the intervals. Since the rates vary randomly, in selecting the current user there is a conflict between full use by selecting the user with the highest current rate and fairness which entails consideration for users with poor throughput to date. The Proportional Fair Scheduler PFS of the Qualcomm High Data Rate HDR system and related algorithms are designed to deal with such conflicts. The aim here is to put such algorithms on a sure mathematical footing and analyze their behavior. The available analysis 6, while obtaining interesting information, does not address the actual convergence for arbitrarily many users under general conditions. Such algorithms are of the stochastic approximation type and results of stochastic approximation are used to analyze the long term properties. It is shown that the limiting behavior of the sample paths of the throughputs converges to the solution of an intuitively reasonable ordinary differential equation, which is akin to a mean flow. We show that the ODE has a unique equilibrium and that it is characterized as optimizing a concave utility function, which shows that PFS is not ad-hoc, but actually corresponds to a reasonable maximization problem. These results may be used to analyze the performance of PFS. The results depend on the fact that the mean ODE has a special form that arises in problems with certain types of competitive behavior. There is a large set of such algorithms, each one corresponding to a concave utility function. This set allows a choice of tradeoffs between the current rate and throughout. Extensions to multiple antenna and frequency systems are given.
- Numerical Mathematics
- Theoretical Mathematics
- Statistics and Probability
- Fluid Mechanics