Joint Scale-lag Diversity in Mobile Wideband Communications

We consider the effect of mobility on a wideband direct sequence spread spectrum (DSSS) communication system, and study a scale-lag Rake receiver capable of leveraging the diversity that results from mobility. A wideband signal has a large bandwidthto-center frequency ratio, such that the typical narrowband Doppler spread assumptions do not apply to mobile channels. Instead, we assume a more general temporal scaling phenomenon, i.e., a dilation of the transmitted signal’s time support. Based on a uniform ring of scatterers model, we determine that the wideband scattering function, which quantifies the average scale spreading, has a “bathtub-shaped” scale profile. We investigate, through frame-theoretic tools, the translationand dilationspacing parameters of a scale-lag Rake basis, and compare the performances of a scale-lag Rake and a Doppler-lag Rake, each capable of leveraging the diversity that results from mobility. When the translation spacing of the Rake functions is equal to the minimum resolvable lag, there is no significant performance difference between the receivers. For wider spacings, the receiver is more reliant on dilation diversity; hence, the scale-lag Rake receiver performs relatively better. Such analysis applies, for example, to ultra-wideband (UWB) radio frequency channels and underwater wideband acoustic channels. We study the correlation structure of the scale-lag Rake fingers and show that the normalized scale spread parameter relates directly to the time-variability of the ii channel. We discover that much of the channel energy is concentrated in few eigenmodes and hence propose principal components combining for a reduced-complexity solution. Finally, we perform physical experiments in the air-acoustic channel to demonstrate the applicability of the wideband channel model.