Abstract
We investigate in this paper the implications of scheduling and adaptive modulation and coding (AMC) on the performance of downlink traffic in OFDMA-based IEEE 802.16 cells. To this end, we model the downlink traffic as a discrete-time Markov chain by assuming exponential interarrival times of packets at the upstream of the unidirectional unbuffered connection maintained by the base station with each associated subscriber station (SS). Various performance metrics are derived by considering two usual scheduling policies: equal-slot sharing (ESS) and equal-throughput sharing (ETS). In particular, the estimated packet access delays provide a lower bound on the achieved delays in case of buffering. Besides, our model considers the adaptivity of the modulation and coding schemes (MCSs) used by the base station. By assuming uniform distribution of the SSs over the cell and shadowing log-normal radio propagation, we derive the probability density function of the signal-to-noise ratios measured at the level of the SSs. Albeit defined in the context of WiMAX networks, this second contribution can be exploited in more generic wireless settings. Several numerical results are presented and discussed to expose the effects of system parameters and scheduling policies on performance.