Bandwidth Scaling in Ultra Wideband Communication

Channel uncertainty limits the achievable data rates of power constrained wide band systems, where the data rate penalty depends on the number of channel parameters the receiver has to estimate per unit time. Duty cycle transmission reduces the channel uncertainty because the receiver has to estimate the channel only when transmission takes place. The transmission period should equal the coherence time of the channel in order to minimize the channel uncertainty. The duty cycle parameter (fraction of time used for transmission) should be minimized for the same reason, and its optimal value depends on the spectral efficiency of the signal modulation. The optimal duty cycle parameter decreases as the bandwidth increases, making the signal increasingly bursty. At the limit of infinite bandwidth, direct sequence spread spectrum and pulse position modulation systems with duty cycle achieve AWGN 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 AWGN capacity in the limit, in environments where pulse position modulation cannot be used because of the large number of channel paths.