On Higher Order Modulations for OFDM in Frequency-Selective Fading Channels

It has been shown that adaptive modulation in OFDM significantly improves performance with perfect channel state information (CSI) at the transmitter. However, it is often difficult to obtain perfect CSI at the transmitter due to feedback overhead and estimation errors. In this letter, we study the optimal constellation size and coding rate when CSI is not available at the transmitter in frequency-selective fading channels. Our results indicate that increasing the constellation size uniformly for all subcarriers does not decrease the outage capacity. Moreover, we show that, rather than adapting the size of the constellation based on the receiver SNR and estimated root-mean-square delay spread, one should adapt the rate of the error control code.