Ultra-wideband systems can be used in indoor wireless personal area network (WPAN) or short-range wireless local area network (WLAN) transmission. Yet owing to the effects of indoor dense multipath, it will cause more power consumption. We usually use Rake receiver to improve system performance. However, we should do some compromise between system performance and the design complexity. Thus, the concept of Selective Rake can be used to substitute for the conventional Rake receiver. Selective Rake receiver uses fewer but more powerful paths instead of using all the paths to raise system performance. Hence, we have to precisely detect the multipath components for best performance. Earlier we use CLEAN algorithm to estimate the multipath components. The CLEAN algorithm can be used in selecting the paths with relatively high energy. But as the impact of frequency selective fading makes the transmitted signal distorted, the CLEAN algorithm no longer applies to this situation. Thus, we use Multiscale CLEAN algorithm instead. Multiscale CLEAN algorithm calculate the value of cross-correlation between the received signal and a set of waveforms, and then choose the higher one as the waveform transmitted. Besides, we use Maximal Ratio Combining to weigh the different paths to get the signal with more power. We represent the signal affected by frequency selective fading by using the second derivatives of Gaussian waveform function with different effective widths of pulse. The waveforms corresponding different effective widths have different spectra which represent the different effects of fading. It is seen that that the multiscale CLEAN has better performance than the CLEAN algorithm with more precise estimation of multipath components. In simulation result, we can figure out path searching using Multiscale CLEAN algorithm is more accurate than using CLEAN algorithm. Even the path with smaller energy gain, using multiscale CLEAN algorithm can search successfully, while CLEAN algorithm cannot do.

The Ultra-Wideband (UWB) communication technology has been extensively attended in recent years. In this thesis, we propose the improved selective-Rake receiver and analyze the performance on UWB channels. The UWB transmission channels are modeled with statistical methods and its fading characteristics are discussed. Different impulse radio properties for the UWB communication system are analyzed. The system performance and design complexity issues of selective-Rake receiver (SRake) are studied. Rake receiver has difficulties achieving desired system performance in the dense multipath environment. The main ideas of SRake receiver are to obtain the SNR level on known multipath channel and to determine the desired number of Rake fingers. Matched filters and maximum likelihood detectors are utilized in the implementation of the SRake to estimate the signal time delay. The CLEAN algorithm is then used in selecting the paths with relatively high energy. Furthermore, we also propose a noise cancellation scheme for performance improvement in the SRake receiver. In the noise cancellation scheme, the multiresolution property of wavelet transform is used for filtering the noise interference caused by the rapid fluctuation factor. In addition, a two-stage search is combined with the original CLEAN algorithm to increase the accuracy of path selection. From our simulation results on the UWB channels, the improved SRake receiver, with noise cancellation and two-stage search, indeed has high SRake output SNR and better path accuracy than the original SRake receiver.

In this thesis, we propose a cost-based CLEAN algorithm to accurately find dense multi-path parameters and improve the performance of selective RAKE receiver in indoor UWB systems. RAKE receiver can resolve the dense multi-path interference problems with the multi-path parameters. Because the weak paths are of lower valuable for system performance improvement, selective RAKE receiver combines only the strongest multi-path components and reduce the number of fingers to lower the complexity of RAKE receiver. However, selective RAKE receiver needs accurate multi-path detection to decide the suitable number and parameters of fingers. In order to improve the performance of selective RAKE receiver, the main issue in this thesis is to detect the best paths of channel with the CLEAN algorithm. CLEAN algorithm uses the correlation of the received signal and the template signal as the basis for searching paths. If there are closely adjacent paths, or if one of signal paths is relatively stronger, the detection error of paths may occur and thus affects the performance of the receiver. EP-based CLEAN algorithm uses the cost function and the evolutionary programming (EP) to search the multi-path delay times and gain coefficients for minimizing the cost function. Accurate multi-path detection and high resolution of adjacent paths can be obtained. However, EP-based CLEAN algorithm makes a time-consuming blind search. In the thesis, a CLEAN algorithm based on the cost function is proposed. The proposed cost-based CLEAN algorithm searches the delay times near the peaks of the cross-correlation for local minimum of the cost function, and then uses CLEAN algorithm to extract autocorrelation components and obtain the accurate multi-path detection. By testing the IEEE802.15.3a UWB channel models, and comparing with CLEAN algorithm, the cost-based CLEAN algorithm in the thesis can achieve better detection accuracy in multi-path searching, and improve the performance of selective RAKE receiver.

The Ultra-Wideband (UWB) channel is a dense multipath channel. The system performance and design complexity issues of selective-Rake receiver (SRake) are studied. Rake receiver has difficulties achieving desired system performance in the dense multipath environment. The main ideas of SRake receiver are to obtain the SNR level on known multipath channel and determine the desired number of Rake fingers. In the implementation of the SRake, the CLEAN algorithm is used in selecting the paths with relatively high energy. We can improve the performance of SRake receiver by increasing the accuracy of path selection. By the property of local maximum peak within the smaller partition, Two-Stage CLEAN algorithm acquires the more accurate delay time of multipath. In order to mitigate the sidelobe effect and noise interference, the key assumption in the Deng¡¦s Modified CLEAN algorithm is that using average amplitude around the considered data change as the criterion to determine if the data value is a true path. In this thesis, we investigate CLEAN, Two-Stage CLEAN and Deng¡¦s Modified CLEAN algorithm in three different systems including UWB-Impulse Radio, Pulse Radar and DS-UWB. From the performance comparison, it can be seen that the Two-Stage CLEAN algorithm that has the highest accuracy of path selection in UWB system.