Handshake protocols and adaptive modulation for underwater communications networks

The US Navy telesonar RDT&E effort is advancing methods for asynchronous, multi-access, networked communications in littoral waters. The goal is to provide wireless, battery-powered connectivity in adverse, shallow-water channels while maintaining a low probability of counter-detection. Telesonar type-B signaling combines powerful, non-binary convolutional error-correction coding, M-ary frequency-shift-keying (MFSK) modulation, and sophisticated frequency hopping. Non-coherent demodulation and sequential decoding provides reception at low signal-to-noise levels. The link protocol includes low-bit-rate handshaking to accommodate energy-conserving sleep modes and to achieve secure addressing, power control, channel characterization, and prioritization. Once the terminals have synchronized at the low bit rate, the message is transmitted with a bit rate and modulation appropriate for the prevailing channel. Power control ensures that messages are received with sufficient, but not excessive SNR. Frequency hopped spread-spectrum transmission supports asynchronous, code-division multiple access (CDMA) networking. Our analyses indicate that unauthorized observers require at least 10 dB more signal energy for detection than will the intended receiver