  In this paper, we investigate the impact of mistiming on the performance of
Rake receivers in direct-sequence ultra-wideband (DS-UWB) systems from the
perspective of the achievable information rate. A generalized expression for
the performance degradation due to mistiming is derived. Monte Carlo
simulations based on this expression are then conducted, which demonstrate that
the performance loss has little relationship with the target achievable
information rate, but varies significantly with the system bandwidth and the
multipath diversity order, which reflects design trade-offs among the system
timing requirement, the bandwidth and the implementation complexity. In
addition, the performance degradations of Rake receivers with different
multipath component selection schemes and combining techniques are compared.
Among these receivers, the widely used maximal ratio combining (MRC)
selective-Rake (S-Rake) suffers the largest performance loss in the presence of
mistiming.
