Performance of Scalable Feedback Schemes for Downlink OFDMA

In a downlink Orthogonal Frequency Division Multiple Access (OFDMA) system, optimally allocating sub-channels to the users can require excessive feedback. We consider two limited feedback schemes to manage this overhead: a sequential scheme in which each user sends compressed feedback bits over consecutive time slots and a contention scheme in which users send their feedback using a random access protocol. In previous work we characterized the asymptotic performance of these schemes as the number of users and number of sub-channels scale with fixed load ¿. That analysis explicitly models the feedback overhead assuming a finite coherence time and fixed feedback rate per sub-channel. In this paper, we revisit this model, and first characterize the performance of both schemes as a function of ¿. We then study the performance of these schemes for a finite system via simulation. Finally we examine the relative performance of the two schemes as a function of the load, the available feedback rate, and additional overhead per transmission (e.g., due to synchronization).