Combined Matched-Filter/Serial-Search Acquisition Concept for Direct-Sequence Systems

A commonly used method for the code acquisition phase in direct-sequence spread spectrum systems is serial-search of the relative positions of received signal and locally generated reference waveform in discrete steps of a fraction of a code chip. However, if no a priori timing information is available to the receiver, uniform search over the long pseudonoise code periods generally employed in anti-jam and anti-intercept communications may lead to acquisition times that are prohibitive. The initial uncertainty can be reduced with the aid of shorter code segments, which are processed in matched filters. In order to maintain low probability of exploitation by unintended parties, these code segments should be easily variable. Thus programmable matched filtering is required, which advantageously can be performed by surface acoustic wave (SAW) convolvers. This paper analyses the performance of a combined matched-filter/serial search rapid acquisition strategy, which is based on the application of SAW convolvers to preamble processing. Correct detection of the preamble reduces the uncertainty region for the subsequent serial-search to a few chips. The measure of performance for the combined search strategy is the overall mean acquisition time, which is calculated for an example system.