Undersea acoustic channels can exhibit multipath propagation with impulse-r
esponse duration and coherence time both of the order of tens to hundreds o
f milliseconds, Signal reception is further impaired by the presence of tim
e-varying nonwhite ambient-noise spectra having a dynamic range of 30 dB or
more, Acoustic communication requires appropriate waveform design and asso
ciated signal processing to accommodate these adverse transmission characte
ristics while also providing desired performance features such as low-proba
bility-of-detection (LPD) and multi-access networking, Adaptive-equalizatio
n techniques provide good performance only for channels with stable multipa
ths and high signal-to-noise ratios (SNR's) accommodating the signaling rat
es needed to sample and compensate for rapid changes, An alternative approa
ch is to design for robustness against channel fluctuations.
This paper describes a channel-tolerant approach identified as "telesonar t
ype-B signaling.'' The technique has been designed to accommodate network a
rchitectures requiring multiple access to the channel while simultaneously
providing covertness and energy efficiency. Specialized frequency-hopped M-
ary frequency-shift-keg (FH-MFSK) waveforms are combined with related signa
l processing, including nonlinear adaptive techniques to mitigate the effec
ts of all types of interference. This effectively results in a channel that
has uniformly distributed noise in both time and frequency, Powerful error
-correction coding permits low SNR transmissions, Nonbinary, long-constrain
t-length, convolutional coding and related sequential decoding is a classic
al solution for difficult low-rate channels. The probability of bit errors
below 10(-10) is obtainable, even in Rayleigh-faded channels near the compu
tational cutoff rate, and the probability of failure to decode frames of da
ta is extremely small.
Both simulations and analyses of at-sea experiments demonstrate the perform
ance of this noncoherent approach to reliable acoustic communications.