A Method for Combating Fading Multipath Conditions in a Direct-Sequence Spread-Spectrum System,

The large bandwidth of a direct-sequence spread-spectrum communications system provides increased time-delay resolution which can resolve multipath components that would not be resolved by the system without spreading. However, normal direct-sequence systems do not fully exploit this capability. In these systems the received signal is despread by correlation with the reference signal the spreading waveform at a single delay. In a multipath situation where there are multiple resolvable delays one of these delays must be chosen for the correlation. If the path with that delay is fading then the signal may be lost at times, even when a strong signal exists at a different delay. In the system proposed here the despreading correlation is performed at a number of delays and the results are combined to provide the improvement expected from a diversity system under fading multipath conditions. The combining is simplified by the use of differential-phase-shift modulation of the data. An added advantage of the proposed system is that the outputs of the multiple-delay correlator are ideal inputs to a simple but effective code-delay tracking system based on keeping the centre of gravity of the values in the correlation window centred in the window. Canada.