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This paper proposes an adaptive modulation algorithm for orthogonal frequency and code division multiplexing (OFCDM) system to increase the spectral efficiency without sacrificing the BER performance under different spreading factors. The proposed algorithm is used with an adaptive subcarrier allocation technique which assigns users to subcarriers to minimize the overall BER of the system. A fixed threshold is used to switch between modulation levels depending on the estimated SINR in each group. A spectral efficiency of 3.2 bits per symbol is obtained for a target BER of 10-2. BCH (511, 385) coding with rate 3/4 used to accommodate a lower target BER of 10-3 yields a spectral efficiency of 2.8 bits per symbol. The proposed algorithm provides an increase in spectral efficiency than using BPSK only, without increasing the total transmit power.
Date of Publication: July 2008