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In this paper, an adaptive modulation technique is proposed in a MultiBand Orthogonal Frequency Division Multiplex Ultra Wide Band (MB-OFDM UWB) communication system, to improve the BER performance in a frequency selective fading channel. The modulating bits are varied across the sub-channels depending on the signal to noise ratio (SNR). Sub-channels experiencing severe fading employ lower or no bit-loading while subchannels with little or no fading utilise higher bit-loading to maintain a constant system data rate. Simulation results show that using the adaptive modulation to maintain a constant data rate features a better performance than using only QPSK modulation in a frequency selective fading channel.