The Cramer-Rao Bound (CRB) analysis shows that ultra-wideband (UWB) can provide high resolution timing and positioning, thanks to its huge bandwidth. However, in practical implementation, the CRB can not be achieved because the perfect channel information may not be available at the receiver. In this paper, we investigate how to exploit the huge bandwidth for high resolution ranging in practical scenarios. We will compare the coherent and noncoherent combining of the multi-band signal for ranging. The comparisons show that a high timing resolution can be achieved by combining the channel information from multiple sub-bands in a coherent manner. However, when the channel is independent for different sub-bands, the multi-band signals can only be combined in a noncoherent manner to improve the mistiming performance. In particular, the slope of the probability of mis-timing curve becomes steeper as the number of sub-bands increases. This is known in communications as diversity. Simulations are also carried out to corroborate our theoretical analysis.