In this paper, the performance of RAKE receiver on channel shortening in ultra-wideband (UWB) systems is studied by analyzing the impact of inter-symbol interference (ISI) on the bit error rate (BER) error floor in dense multipath channel environments. A closed form expression for BER with Nakagami channel model is developed. The equivalent channel length after RAKE receiver is derived by comparing the different error floors of UWB systems with all RAKE (ARAKE) or selective RAKE (SRAKE) with one tap receiver. Numerical results are shown to demonstrate the partial equalization ability of RAKE receiver. For high data rate or large delay spread, we find that the length of equivalent channel after ARAKE receiver approximately equals the length of multipath channel divided by the spreading gain, which will provide guideline for designing high data rate UWB receivers.
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