Subband Spreading Technique Using Orthogonal Code Multiplexing in OFDM Systems

We propose a subband spreading technique to average a frequency selective fading channel which produces different signal-to-noise ratios (SNR) per subcarrier in orthogonal frequency division multiplexing (OFDM) systems. Data symbols are spread over a subband using orthogonal codes and transmitted. Each subband is composed of a group of subcarriers. Received symbols in a subband have the same reliability even in a frequency selective fading channel due to the subband spreading at the transmitter. As the number of subcarriers per subband increases, the averaging effect increases even though the complexity to multiplex the symbols increases. We compare the bit error rate (BER) performance of the proposed scheme with that of the conventional OFDM systems including the pre-equalization scheme. Furthermore, we propose an adaptive modulation scheme using the subband spreading technique in order to reduce signaling overhead and complexity. The proposed scheme can outperform the conventional subcarrier-by-subcarrier adaptation scheme in terms of throughput when the channel varies quickly.