Low Probability of Detection QS-MC-DS-CDMA for Low VHF

Tactical military ad-hoc networks usually rely on easily and quickly deployable infrastructure. In such systems reliable communication may be compromised due to imperfect synchronization. QS-MC-DS-CDMA is a solution that allows loose synchrony by employing, so called, LS-codes that are characterized by a zero-correlation zone (ZCZ) that enables preserving code orthogonality in the presence of delay within that zone. Another highly desirable characteristic of tactical networks is low probability of detection (LPD). CDMA inherently favors LPD due to its property to spread the signal in the frequency domain thus making it look like noise. However, manmade signals have periodic characteristics that render them detectable using advanced techniques. Our goal is to reduce the patterns in such signals by employing random dithering in time, i.e. by introducing small random delays that do not affect orthogonality as long as they remain within the ZCZ. In addition to the ZCZ size, the amount of dithering that can be performed is also limited by the time uncertainty/synchronization requirement of the system. The periodic characteristic reduction is quantified using the Degree of Cyclostationarity (DCS) and simulation results are presented. We advocate the implementation of such a system in the lower VHF band due to its favorable channel characteristics and and especially its reduced multipath and frequency offset sensitivity. The latter is the key to our strategy towards increasing LPD protection via time dithering under time uncertainty.