Distributed carrier synchronization for HF cooperative communication employing randomized space time block coding

The HF channel supports the wireless needs of expeditionary forces for both communication between nearby unit members (<5 km) and over-the-horizon reach-back to ship or rear base (OTH > 500 km). Cooperative communication combined with MIMO methods employing randomized space-time block coding (RST-BC) promise significant gain for over-the-horizon transmission which allows lower transmit power and improved warfighter mobility. However, RST-BC requires tight control of RF carrier frequency to be effective, without reliance on GPS or centralized timing protocols. We examined consensus timing created by a distributed first-order frequency locked PLL (FLL) comprised of the ad-hoc network of cooperating nodes. The necessary carrier synchronization can be achieved with distributed first-order FLL using only locally derived carrier frequency offset information. We report the performance of a fast carrier recovery method for the BPSK recruitment link with measured HF groundwave signals. A network simulation of the proposed distributed synchronization method is presented based on measured recruitment link behavior. Fast settling time is demonstrated with adequate tolerance to failure of individual nodes. The method is sufficient to meet the demanding requirements of RST-BC.