MMSE acquisition of DSSS acoustic communications signals

A method for acquisition of direct sequence spread spectrum (DSSS) signals based on a minimum mean-square error (MMSE) receiver is presented. An adaptive decision feedback equalizer (DFE), operating at the chip rate, uses the known spreading code as training data. A fixed number of unmodulated code periods is transmitted for acquisition purposes. An arbitrarily chosen initial code timing is hypothesized, and this timing is used to adapt the DFE and despread the chip estimates at its output. When the despreader output exceeds a threshold, lock is declared, and the receiver switches from acquisition to tracking mode. If the threshold is not exceeded during a pre-specified amount of time, a different initial timing is hypothesized, and the receiver is re-initialized. The locally generated code can be advanced by a number of chips that is proportional to the length of the feedforward equalizer filter. The MMSE receiver also automatically acquires and tracks the Doppler shift that results from source-receiver motion using a phase-locked loop and integrated sample-rate interpolation. Code acquisition is demonstrated using experimental data with length 63 Kasami sequences transmitted at a chip rate of 10 kHz for a resulting data rate of 150 bps. Up to 8 simultaneous users are reliably acquired and demodulated using the recursive least-squares (RLS) update algorithm.