Performance of Limited Feedback Schemes for Downlink OFDMA with Finite Coherence Time

We consider the capacity of a downlink orthogonal frequency division multiple access (OFDMA) system with limited feedback rate R
_F
per sub-channel and finite coherence time T. The feedback is used to relay channel state information (CSI) from K users to the base station. The order-optimal capacity growth with Rayleigh fading sub-channels is ominus(N log log K) as N and K increase with fixed ratio, where N is the number of sub-channels. However, to achieve this, previous work requires a feedback rate per subchannel that scales linearly with the system size. Here we explicitly include the feedback overhead when calculating the sum capacity, and study the tradeoff between feedback rate and sum capacity. We propose two limited feedback schemes, one based on sequential transmissions across users and the other based on random access, in which the each feedback bit requests the use of a sub-channel group containing multiple subchannels. With fixed R
_F
T, the sum capacity for both schemes with optimized sub-channel groups increases as ominus(N). If R
_F
T grows faster than log K, then both schemes can achieve the order- optimal capacity growth. We also show that when R
_F
T is small, the random access scheme performs better than the sequential transmission scheme, whereas the reverse is true for large R
_F
T.