Performance analysis of link adaptation in wireless data networks

We analyze the performance of a link adaptation scheme in which a user, based on his SIR estimate, either transmits using a given modulation and coding scheme or does not transmit (backs off). The impact of channel correlation on the optimal back off SIR threshold is studied for a co-channel interference limited cellular system. Expressions are derived for the average packet waiting time given the basic system parameters like packet arrival statistics, channel fade statistics, number of users per cell, and link adaptation thresholds. Analysis results are shown to be in good agreement with the simulation results. We show that the optimal back-off threshold crucially depends on the channel correlation, a fact not considered by the formulae suggested in literature for determining the link adaptation thresholds. For channel correlation /spl rho/=0.82, the back-off mechanism could reduce average packet delay by up to 15%, as compared to the no back-off case. No such improvement was found for /spl rho/=0.41.