Communication systems always suffer time distortion. At the physical layer asynchrony between clocks and motion-induced Doppler effects warp the time scale, while at higher layers there are packet delays. Current wireless underwater modems suffer a significant performance degradation when communication platforms are mobile and Doppler effects corrupt the transmitted signals. They are advertised with data rates of a few kbps, but the oil and gas industry has found them useful only to around 100 bps. In our work, time-varying Doppler is explicitly modeled, tracked and compensated. Integrated into an iterative turbo equalization based receiver, this novel Doppler compensation technique has demonstrated unprecedented communication performance in US Navy sponsored field tests and simulations. We achieved a data rate of 39kbps at a distance of 2.7km and a data rate of 1.2Mbps at a distance of 12m. The latter link is capable of streaming video in real-time, a first in wireless underwater communication.Time distortion can also be intentional and be used for communication. We explore how much information can be conveyed by controlling the timing of packets when sent from their source towards their destination in a packet-switched network. By using Markov chain analysis, we prove a lower bound on the maximal channel coding rate achievable at a given blocklength and error probability. Finally, we propose an easy-to-deploy censorship-resistant infrastructure, called FreeWave. FreeWave modulates a client's Internet traffic into acoustic signals that are carried overVoIP connections. The use of actual VoIP connections allows FreeWave to relay its VoIP connections through oblivious VoIP nodes, hence keeping the FreeWave server(s) unobservable and unblockable. When the VoIP channel suffers packet transfer delays, the transmitted acoustic signals are time distorted. We address this challenge and prototypeFreeWave over Skype, the most popular VoIP system.
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