Second moment adaptation of quadrature amplitude modulation in coded OFDM over fading channel

In this paper, we investigate adaptive QAM in coded OFDM using second moment adaptation of channel SNR. In this scheme the transmitted data is adaptively mapped to M-QAM constellation after a 1/4 rate channel coding, based on the channel state information. The second central moment of the channel impulse response SNR, or its variance, is used as switching thresholds for QAM modulation. With the assumption of perfect channel knowledge, variance of each subband is calculated and compared with the switching thresholds to determine the number of bits per symbol that can be transmitted. The 512 subcarriers of this coded OFDM system is divided into 32 subbands so that adaptation is done on a subband basis. Results show that for adaptation up to 4 bits per symbols, second moment adaptation is about 3 dB more superior to adaptation by mean channel SNR. Comparison has also been made between results of second moment adaptation with fixed 2-, 4- and 8-QAM modulation which shows that BER performance is better (e.g. by as much as 10 db compared to 8QAM) with some compensation on throughput.