In this paper, Pre-Rake scheme for pulse based Ultra Wideband impulse radio (UWB-IR) communications system is proposed considering a standardized UWB channel model from IEEE 802.15 SG3a, which is based on an extensive set of short-range indoor channel measurements. Two kinds of pulse waveforms are taken in account here, namely, Gaussian mono-pulse and newly designed Prolate Spheroidal Wave Functions (PSWF) pulses corresponding to FCC Spectrum Mask. The Pre-Rake scheme is shown to contribute to the low-power, cost-efficient UWB system designing as well as Rake combining gain. Instead of building a Rake receiver at the receiving side (e.g. portable unit), the transmitter (e.g. access point) can pre-combine the UWB signal before transmission in the forward link by estimating the channel impulse response from the reverse link. While the Pre-Raked signal is convolved with the estimated channel impulse response, the function of Rake combination at the receiver is automatically performed. Meanwhile, in order to defeat inter-pulse-interferences (IPI) caused by severe multipath fading conditions, adaptive guard-time scheme for consecutive pulses is proposed as well. Monte-Carlo simulations are carried out to compare the Pre-Rake with Rake results and show that Pre-Rake scheme is as good as Rake combining for both types of pulse waveforms. Then the mobile or portable unit with a conventional receiver can still achieve the diversity gain of Rake combination. Moreover, the effects of placing guard-time between pulses are also verified.