Performance of ultrawideband communication systems

The performance of ultra-wideband (UWB) and large-bandwidth communication systems operating in slowly fading channels is examined for different receiver architectures, channel estimation techniques, and diversity combining schemes. Because of cost and complexity constraints, large-bandwidth systems employ suboptimal receivers which must attain sufficient energy capture without incurring significant combining loss from imperfect estimation or noncoherent reception. The performance of rake receivers with non-selective diversity combining and minimum linear mean-square (LMS) estimation or maximum likelihood (ML) estimation is evaluated for frequency-selective, Rician fading channels over a wide range of bandwidths. When the receiver is able to track and combine resolvable paths which contain a specular, or nonfading, component, the combining loss can be smaller than the case in which only diffuse resolvable paths are combined. The corresponding performance gap between these two cases for finite and infinite bandwidths depends upon the power of the specular components relative to the power of the diffuse channel components. The performance of reduced-complexity rake receivers, as well as autocorrelation receivers, is also examined for previously measured UWB indoor channels. Whereas rake receivers perform multiple correlations with a locally generated reference signal to exploit the multipath diversity present in the received signal, autocorrelation receivers simply employ a previously received signal as the reference signal and do not require any diversity combining schemes. The operation of an autocorrelation receiver which averages previously received reference signals to reduce the combining loss resembles a rake receiver with non-selective diversity combining and ML estimation. Furthermore, the combining loss associated with noncoherent reception can be offset by employing M-ary orthogonal modulation at the expense of reduced bandwidth efficiency. The asymptotic (M → ∞) performance of M-ary orthogonal modulation with Reed-Solomon (RS) coding and diversity reception in multichannels, or multiple frequency-nonselective, slowly fading channels, is examined. The analysis indicates that coherent and noncoherent implementations of diversity combining schemes yield the same performance asymptotically.