Spread spectrum communication system employing composite spreading codes

Abstract

A hybrid despread and demodulation receiver for low symbol rate communications employs a passive (SAW) matched filter to remove a 'short' coding portion of a composite spreading code that has been used to spread the data signal. The composite spreading sequence is formed by multiplying different length coding sequences, thereby obtaining an overall signal processing operator the duration or symbol span of which is sufficient to maintain a high signal processing gain, but is considerably longer than can be processed using a practical sized passive (e.g. SAW) filter design. The design of the receiver takes advantage of the fact that the relatively short sequence can be despread using a practical SAW structure and is comprised of a hybrid signal processor, the front end of which contains a compact SAW matched filter and the downstream end of which is implemented using analog processing components. The matched filter removes the relatively short spreading sequence from the received signal and feeds its output to a mixer, which combines the output of the matched filter with the longer coding sequence to complete the despreading process. The despread signal is then differentially coherently decoded and coupled to an integrate and dump circuit, which accumulates the energy in successive long code symbol intervals in order to determine the value of the respective data bits.