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Abstract

A spread-spectrum transmission system wherein a plurality of spread-spectrum signals are frequency division multiplexed by offsetting the center or carrier frequencies of the spread-spectrum signals by a fraction of the spectral-line spacing of the signals. The signals are generated by modulating a carrier with a pseudo-noise (PN) code signal. At a receiver, demultiplexing is carried out by mixing (42) the received multiplexed signal with a plurality of local oscillator frequencies (43) which are offset from each other in the same manner as the centre or carrier frequencies of the multiplexed spread-spectrum signals. The mixed signals are then further mixed (45) with generated PN code signals (47) corresponding to those used to generate the multiplexed spread-spectrum signals. The epoch of each generated PN code signal is then advanced or retarded in response to a respective epoch control signal (48) generated by a respective controller (46). After mixing (45) the signals are passed through respective narrowband filters (49) to select the baseband signals which are then fed to the respective controllers (46) to enable the generation of the epoch control signals (48 ). When applied to a vehicle tracking system a number of spaced receivers are utilized.

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