Orthogonal Frequency and Code Division Multiplexing (OFCDM) is a promising emerging technique for the fourth generation cellular system. In this paper, we propose an adaptive modulation algorithm for OFCDM in order to increase the spectral efficiency without sacrificing the Bit Error Rate (BER) performance. The proposed algorithm is used with an adaptive subcarrier allocation technique which assigns users to subcarriers that produce the best Signal to Interference and Noise Ratio (SINR) characteristics while producing minimal multiple access interference to other users. We examine a fixed threshold adaptation algorithm to switch between modulation levels depending upon the estimated SINR. The performance of adaptive modulation in a Rayleigh fading channel for different BER targets is evaluated. The proposed algorithm is shown to provide an increase in throughput and spectral efficiency than using BPSK only. An increase of 47% and 63% in throughput corresponding to a spectral efficiency of 1.47 and 1.63 bits per symbol can be obtained for a target BER of 1% and 10% respectively without increasing the total transmit power.
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