A fast acquisition technique for a direct sequence spread spectrum signal in the presence of a large Doppler shift

A non-coherent fast acquisition system is described for a DS-SS signal in the presence of a large Doppler shift. This technique utilises N complex correlators of length x such that Nx=m where m is the length of the spreading sequence. These complex correlators produce partial correlations of the DS-SS signal. The outputs of these correlators are fed to an N point FFT. The maximum output of the FFT is then selected and compared against a threshold. In situations with high Doppler shifts the FFT acquisition system described can test all frequency offsets for a given code phase simultaneously. The theoretical probability of detection and false alarm are derived in a Gaussian channel. We determine the mean acquisition time for an 8-point FFT using a single dwell system in the presence of 32 kHz Doppler shift. It is shown by choosing a suitable threshold level that the mean acquisition time is over 16 times lower than a standard single dwell non-coherent decorrelator.