The effect of inter-user delay and channel phase response on MC-CDMA using WBE codes with application to lower VHF

Reliable, ad hoc communications among multiple users is a crucial objective in complex tactical scenarios. A major challenge is that synchronization mechanisms (such as GPS) are intermittent, and fixed centralized coordinators (e.g., base stations) are infeasible. Classical approaches to multi-user communications relying on tight time and frequency synchronization and/or power control are impractical in such scenarios. In this paper, we study the effect of coarse time synchrony and phase response variation on a class of code division multiple access (CDMA) strategies known as multi-carrier (MC) CDMA. We study this sensitivity when binary codes achieving the Welch bound in a potentially user-overloaded system are used. The codes are selected to minimize the L₂ and the L_∞ bounds at small or negligible inter-user time offsets and phase-response variation. We show that variations in phase response lead to significant loss of code orthogonality; this motivates employing MC-CDMA systems in scenarios with benign channel responses. Of particular interest is the near-ground lower VHF band, which has improved penetration and reduced multipath relative to microwave, in particular in non-line of sight environments, such as through buildings. This paper aims at studying the reduction in the necessary inter-user time synchrony required of bands having a lower phase variation.