Asymptotic Analysis of Downlink OFDMA Capacity

We consider asymptotic performance of a downlink OFDMA system as the number of users and sub-channels increase. Specifically, we study the asymptotic growth in the weighted sum capacity, where each user is assigned a weight to reflect its quality of service. We begin by considering a limited feedback scheme, where each user is pre-assigned a threshold and feeds back one bit per sub-channel to indicate whether the channel gain is above the threshold or not. If more than one user requests the same sub-channel, the base station picks the user with the largest weight to transmit. In earlier work we analyzed such a scheme when each user has i.i.d. Rayleigh fading on each sub-channel. Here we consider a larger class of distributions that includes most common fading models. We characterize the asymptotic behavior of the optimal thresholds and the growth of the weighted sum capacity. We then compare the asymptotic capacity achieved by this one bit feedback scheme with the capacity when full CSI is available at the transmitter. We derive upper and lower bounds on the capacity with full CSI. The difference between these bounds asymptotically converges to a constant and the lower bound converges to the capacity of the one-bit feedback scheme.