This thesis covers two topics. The first one is signal design for accurate Time-of-Arrival estimation using a number of frequency separated signals. Rather than use a full UWB band, we will use sparse subband signals spanning the full band to construct a new virtual UWB signal. To evaluate the performance of the constructed signal, Cramer-Rao lower bound and auto-correlation are used. And given a given fixed bandwidth the number of subbands within a 1 GHz UWB channel, and optimal subbands' allocation will be found based on the evaluation results. Our results show that when three 50 MHz subbands are used to construct a virtual 1GHz UWB signal, a lower CRLB and better auto-correlation performance can be reached when subbands are close to the edges of the virtual band. However, the auto-correlation still has multiple peaks, which poses a serious challenge for accurate time estimation.The second topic is to investigate the frequency dependence of the channel impulse response of subbands with different frequency separations. We propose a covariance calculation method to determine the frequency dependence which changes with frequency separation.To validate the method, different artificial UWB channels with distinct paths are given. The results show that covariances between the subband CIRs stay at a high level when measured at the direct path and the majority of interference caused by other paths can be eliminated by a wider bandwidth subband. Given UWB channels measured from 5 to 10 GHz with a link-budget of 120dB, the frequency dependence of the direct path and reflections are determined, different bandwidths and frequency separations are used, and the results show that the channel impulse response of the subbands will become different when measured at different center frequencies, where the difference increases with an increased frequency separation of the subbands. The correlation of the direct path is maintained over larger frequency distances than that of reflected paths.