Code shift keyed spread spectrum for ocean acoustic telemetry

Code shift keyed spread spectrum (CSK-SS) transmits information by sending symbols consisting of various phases of a finite length spread spectrum code. In its rudimentary form, a symbol interval of length T is partioned into N chips, modulated with cyclic shift of an underlying length N code sequence. The information is demodulated using an FFT matched filter, which realizes periodic correlation. The information rate of the basic system is (1/T)log/sub 2/N bits/second. We discuss the application of CSK-SS to acoustic telemetry, with an aim towards improving the spectral efficiency by overlaying multiple codes. The demodulator is a mismatched filter (equalizer) that trades off SNR for a reduction in the ISI self-noise caused by the overlay. In this way, increased spectral efficiency is possible at the expense of poorer performance in the presence of channel impairments such as phase offset and timing jitter.