||The potential strength of ultra-wideband (UWB) radio technology lies in its extremely wide transmission bandwidth, which results in desirable capabilities including high channel capacity, resistance against fading, and high multiple access capability. However, there are still challenges in making this technology live up to its full potential. The main challenges are on account of the very low allowed average power spectral density (PSD) and the very fine multipath delay resolution. In this thesis, we will focus on the design of UWB direct-sequence impulse radio wireless communication systems. Since UWB wireless communication is very power-limited, we will investigate the potential of improving its power-efficiency by increasing the dimensionality of the signal waveforms. Using N orthogonal pulse shapes and M orthogonal code-words, we will propose a 2MN-ary biorthogonal keying modulation and will show its power-efficiency improvement for direct-sequence multiple-access impulse radio systems, compared with bipolar pulse amplitude modulation and biorthogonal-code keying. The overlap between pulses arriving at the receiver is generally ignored in impulse radio studies, but it occurs when the pulse duration is greater than the fine multipath delay resolution of the UWB channel, thereby reducing the power-efficiency of correlator-based UWB receivers. We will provide a performance analysis incorporating the effect of channel-induced pulse overlap for direct-sequence multiple-access impulse radio systems. The detailed analysis as well as extensive simulations will reveal that channel-induced pulse overlap has a significant impact on the system performance and our analytical formula shows its accuracy and importance in performance analysis for direct-sequence impulse radio communications. UWB communications with limited transmission power while signal energy dispersed by a large number of multipath components, require a practical Rake receiver that can provide a desirable output signal-to-noise ratio (SNR). We will propose a pilot-channel assisted (PCA) generalized selection combining (GSC) with log-likelihood ratio (LLR) threshold test per path (PCA- │LLR│-T-GSC) for UWB Rake receivers with limited number of fingers. We will show the good of PCA- │LLR│-T-GSC in providing desirable performance under channel time-variations and multi-user interference, while without sacrificing the system data rate. Our future work will focus on investigating methods for increasing the achievable distance of UWB systems for a fixed average PSD, by trading lower spectral efficiency for increased power efficiency to achieve a desired rate/range operating point. We will also investigate practical algorithms for initiating and maintaining synchronization at the UWB receiver, to combat the challenges in timing synchronization induced by the stringent power constraints and the fine delay resolution of UWB signals.