An experimental study of quasi-synchronous multiuser communications in cluttered scenarios at low VHF

Reliable, ad hoc communication 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 multiuser communications relying on tight time synchronization and/or power control are impractical in such scenarios. Furthermore, state-of-the-art systems operating at microwave frequencies suffer from lack of penetration and multipath effects causing significant channel variability and exacerbating the near-far effect. In this paper, we investigate the potential advantages of the favorable channel characteristics of the lower VHF band for reliable multiuser communications in highly cluttered environments among near ground nodes. We focus on a class of code division multiple access (CDMA) strategies known as quasi-synchronous (QS) direct sequence (DS) CDMA and use moderate to high QAM constellations. A key aspect of QS CDMA codes is their robustness to coarse inter-user time synchrony and power mismatch, which greatly relaxes the rate that nodes must re-synchronize with each other. Our study experimentally explores the possibilities of multiuser communications in harsh environments among near-ground nodes. Our measurements involve three canonic indoor-to-outdoor, indoor-to-indoor and through-building channels characterized by obstacles and clutter that represent very difficult propagation environments, especially at microwave frequencies. We also study the spatial and temporal variability of our system and characterize the impact of frequency offset on code orthogonality with simulations and experiments. We show that our system has strong potential for excellent multiuser performance in infrastructure-poor regimes.