Subband bit and power loading for adaptive OFDM

The adaptive bit and power loading is a very promising technique to optimize the orthogonal frequency division multiplexing (OFDM) performance especially in frequency selective fading channels. However, because every subcarrier may be allocated different number of bit or power, the signaling overhead may be too large to bear for systems with large number of subcarriers. In this paper, we discuss using the subband bit and power loading methods to decrease the signaling overhead. We modify Fischer's loading algorithm for subband bit and power loading, and we group the subcarriers into subbands by two means: One is based on successive subband grouping and the other one is based on sorted subband grouping. The later one is expected to decrease the difference between the averaged channel gain of the subband and the actual channel gains of the affiliated subcarriers. The simulations are carried out on a 512-subcarrier OFDM platform and the results show that the signaling overhead can be reduced by 75% at the cost of performance loss less than 1 dB, if the number of subcarriers per subband is properly chosen.