Abstract:

Undersea acoustic telemetry and ranging (telesonar) requires a realistic evaluation of the available physical transmission channel. Benign channels support high-bit-rate coherent transmissions, provided that a suitably powerful channel equalizer is available, and provided that covert signaling is not required. Effective signaling in adverse channels requires accommodation of temporal and spectral dispersion and distortion, mitigation of interference, and asynchronous multiple access by many users. This is attained with frequency-hopped, M-ary FSK signals in which the elemental waveform ``chips'' are designed to tolerate channel spreading and nonstationarity. The M-ary nature of the signal reduces the needed SNR, while the frequency-hopping patterns inherently provide interference mitigation and multiple access. Specialized signal processing of the received signal effectively yields a channel with uniformly distributed noise in both time and frequency. Residual interference is removed by powerful nonbinary convolutional encoding and related sequential decoding. The sensitivity of signal acquisition and demodulation to errors in estimates of channel parameters is explored. Overly pessimistic estimates of spreading result in reduced channel access and increased interference at the processor output, while overly optimistic estimates result in signal loss and intersymbol interference. [Work supported by ONR-321 and NRaD telesonar Contract No. N66001-96-C-6003.]