Channel Uncertainty in Ultra Wideband Communication Systems

TL;DR

This paper investigates how duty cycle transmission can mitigate channel uncertainty in ultra wideband systems, enabling capacity achievement under certain conditions, and compares modulation schemes like spread spectrum and pulse position modulation.

Contribution

It demonstrates the conditions under which duty cycle transmission allows wideband systems to reach channel capacity, highlighting the advantages of spread spectrum modulation in complex multipath environments.

Findings

Duty cycle reduces channel uncertainty in wideband systems.

Spread spectrum modulation can achieve capacity when channel paths grow slowly.

Pulse position modulation is limited by large numbers of channel paths.

Abstract

Wide band systems operating over multipath channels may spread their power over bandwidth if they use duty cycle. Channel uncertainty limits the achievable data rates of power constrained wide band systems; Duty cycle transmission reduces the channel uncertainty because the receiver has to estimate the channel only when transmission takes place. The optimal choice of the fraction of time used for transmission depends on the spectral efficiency of the signal modulation. The general principle is demonstrated by comparing the channel conditions that allow different modulations to achieve the capacity in the limit. Direct sequence spread spectrum and pulse position modulation systems with duty cycle achieve the channel capacity, if the increase of the number of channel paths with the bandwidth is not too rapid. The higher spectral efficiency of the spread spectrum modulation lets it achieve the channel capacity in the limit, in environments where pulse position modulation with non-vanishing symbol time cannot be used because of the large number of channel paths.