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Ultra-wideband (UWB) communications is a new technology being developed for high-speed, short distance wireless communications. One possible modulation scheme is pulse amplitude and position modulation (PAPM), which has a flat power spectral density that is highly suited to UWB. Closed-form expressions for the error rate performance of a PAPM UWB receiver based on maximal ratio combining (MRC) in a lognormal fading channel have been derived using Wilkinson's approximation for lognormal sums. The receiver is analyzed using a more accurate approximation to the lognormal sum distribution. Our approach is extended to obtain accurate closed-form expressions for the performance of equal gain combining (EGC). The performances of MRC and EGC diversity are calculated and compared to the results obtained using Wilkinson's approximation.