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Exact BER analysis is carried out using the characteristic function (CF) method for direct sequence ultra-wide band (DS-UWB) systems in lognormal multipath fading channels. Unlike the Gaussian approximation (GA) method, the CF method deals with exact probability density function (PDF) of the total noise [including multiple access interference, self interference and additive white gaussian noise (AWGN)] instead of making approximation on PDF of the total noise. Exact bit error rate (BER) formula is derived and verified by simulation results. It is shown that the CF method is more accurate than the GA method in BER calculation when SNR is large. Based on the BER formula, the performance of the DS pulse amplitude modulation UWB and DS pulse position modulation UWB systems is accurately compared. Discussions based on CFs provide further insight into the numerical results.