Channel-Dependent Adaptive Spreading Code Selection in Downlink MC-CDMA

This paper proposes a channel-dependent adaptive spreading code selection method in downlink multicarrier code division multiple access (MC-CDMA). The proposed method is compared to the conventional MC-CDMA and orthogonal frequency division multiple access (OFDMA). First we show that the signal-to-noise and interference power ratio (SINR) after minimum mean squared error (MMSE)-based equalization is represented as a weighted average of the instantaneous SNR of the respective subcarriers to which the spread symbols are mapped, where the weighting factor is determined by the respective chip power of the spreading sequence. Based on this, we propose a method for channel-dependent adaptive spreading code selection in which the minimum SINR among code multiplexed users is maximized. The simulation results show that the proposed adaptive spreading code matrix selection increases the minimum SINR at the outage probability of 10-2 by approximately 1 dB compared to the OFDMA with channel-dependent subcarrier allocation (frequency-domain scheduling).