Implications of time/frequency synchronization tradeoff of quasi-synchronous multi-carrier DS-CDMA for robust communications at lower VHF

A prime objective in tactical environments is reliable, ad hoc communication among multiple users robust to lapses in coordination methods such as GPS or centralized base stations. Classical approaches to multi-user communications relying on tight node synchronization and/or power control are infeasible in ad hoc infrastructure-poor scenarios. In this paper, we study multi-user coding strategies with greatly reduced synchronization requirements (compared to conventional systems such as cellular) ideally suited for mobile near-ground nodes in harsh tactical environments. We exploit quasi-synchronous (QS) direct sequence (DS) code division multiple access (CDMA) codes transmitted over multiple carriers (MC). These codes are characterized by a zero cross-correlation zone (ZCZ), a measure of the delay tolerance among transmitters over which perfect code orthogonality still holds at the receiver. Using multiple carriers extends the ZCZ in time, thereby relaxing the time synchronization requirement, but tightening the frequency synchronization requirement. We study this tradeoff in this paper. We also argue that the QS MC-DS-CDMA system will have superior performance at lower VHF because of reduced multipath, reduced frequency offset, and better penetration relative to UHF or microwave systems in dense urban environments [1]-[4]. We develop an experimental testbed based on software-defined radios (SDR) and analyze the proposed approach at the lower VHF band.