Asymptotic Tradeoff between Cross-Layer Goodput Gain and Outage Diversity in OFDMA Systems with Slow Fading and Delayed CSIT

In this paper, we consider on the asymptotic tradeoff analysis between the system goodput gain and the packet outage diversity gain in cross-layer OFDMA systems with slow frequency selective fading and delayed CSIT. The OFDMA cross-layer design with delayed CSIT is modeled as an optimization problem where the rate adaptation, power adaptation and subcarrier allocation policies are designed to optimize the system goodput (b/s/Hz successfully received by the mobiles). We derived simple closed-form expressions for the power and rate allocations as well as the asymptotic order of growth in system goodput for general CSIT error sigma_e ². We found that the system goodput scales in the order of O(log(1-sigma_e ²/ N_d(log K + N_d log log K))) for large K where Nd is the packet outage diversity and K is the number of user in the cross-layer OFDMA system. Hence, double exponentially larger K is needed to compensate for the penalty in system goodput gain due to CSIT errors sigma_e ² or packet outage diversity N_d for large N_d.