Multiuser subcarrier allocation for OFDM transmission using adaptive modulation

A multiuser OFDM (orthogonal frequency division multiplexing) downlink transmission with adaptive modulation is considered. Assuming that the base station knows the instantaneous channel gains for all users, we propose an algorithm which minimizes the total transmit power by assigning, to each user, a set of subcarriers within an OFDM symbol and by determining, for each sub-carrier, the number of bits to be transmitted and the power level used. We obtained the performance of our proposed algorithm in a multiuser frequency selective fading environment with various root-mean-square (RMS) delay spread. The results show that our proposed algorithm outperforms conventional OFDM systems with or without optimal bit allocation using TDMA or FDMA as multiaccess techniques. We have also quantified the improvement in terms of the required transmit power, for fixed bit error rate.