Impulse radio (IR) ultra wide-band (UWB) is one of the most promising wireless technologies to develop both indoor and outdoor location and tracking applications. Due to its high bandwidth and short duration pulses, UWB potentially allows a great accuracy in distance measurements based on time of arrival (TOA) estimations. The main objective of this paper is to compare and to provide a qualitative and quantitative evaluation of different TOA-based distance estimation mechanisms joint to receiver architectures proposed for UWB systems. This comparison has been done in terms of accuracy and the effect of characteristic parameters of the transmitted signal, propagation conditions and signal-to-noise ratio has been analyzed. With this purpose a simulation tool has been developed, where the different receiver architectures, estimation mechanisms and channel models proposed on 802.15.4a have been implemented.