Spectral Efficiency of Random Time-Hopping CDMA

Abstract

Traditionally paired with impulsive communications, time-hopping code-division multiple access (TH-CDMA) is a multiple access technique that separates users in time by coding their transmissions into pulses occupying a subset of $N_ \mathsf {s}$ chips out of the total $N$ included in a symbol period, in contrast with the traditional direct-sequence CDMA (DS-CDMA), where $N_ \mathsf {s}=N$ . This paper analyzes the TH-CDMA with random spreading, by determining whether peculiar theoretical limits are identifiable, with both optimal and suboptimal receiver structures, in particular in the archetypal case of sparse spreading, that is, $N_ \mathsf {s}=1$ . Results indicate that the TH-CDMA has a fundamentally different behavior than DS-CDMA, where the crucial role played by energy concentration, typical of TH, directly relates with its intrinsic uneven use of degrees of freedom.