Underwater acoustic single- and multi-user differential frequency hopping communications

Differential frequency hopping (DFH) is a fast frequency hopping, digital signaling technology that achieves the desirable performance features of non-interfering spread spectrum operation, spectral re-use, fading mitigation, and interference resistance. Therefore, DFH coding provides the critical capability for multiple users to seamlessly communicate in the bandwidth-limited acoustic channel. In previous work, DFH coding has been shown to be superior to other coding schemes in additive Gaussian white noise and Rayleigh-fading environments when considering the joint constraints of multiple user access, detectability mitigation, and the presence of jamming. In this paper, we describe the auto-synchronizing single-user DFH decoder we have developed for a single hydrophone receiver. We present the performance of this decoder on multi-user simulated data and on multi-user data collected at sea during the Rescheduled Acoustic Communications Experiment (RACE08).We use the Sonar Simulation Toolset (SST) to produce the simulated data for soft through hard bottom compositions to provide a range of multipath severity to gain insight into DFH performance across environments. Based on these initial results, the DFH waveform shows considerable promise for computationally minimal, high reliability communications among uncoordinated users in an underwater acoustic channel.