On the problem of channel mismatch in constant-bit-rate adaptive modulation for OFDM

We investigate the performance of adaptive modulation in OFDM under a constant-bit-rate requirement When the transmitter knows the channel state information perfectly, a large gain over a non-adaptive system is possible. However, channel estimation errors and delay in feedback may cause channel mismatch, which diminishes the gain of adaptive modulation. The mechanism of how channel mismatch leads to performance degradation is also investigated. Based on the insights we obtain, two approaches are proposed to mitigate the adverse effect: a coding technique assisted by differential channel information, and a robust loading scheme.