PN code synchronisers for direct-sequence spread-spectrum systems-a comparative evaluation

Direct-sequence spread-spectrum modulation is being used increasingly in applications such as satellite and digital cellular radio systems where the levels of interference relative to the signal are often very high. In addition, the received signal may be also subject to multipath fading, shadow loss and Doppler frequency offsets and it is under these conditions that acquisition of coarse PN code synchronisation is required to take place in the receiver prior to obtaining accurate phase lock with a tracking loop. Such is the importance of code synchronisation, that much of the complexity of the spread-spectrum receiver lies in the code acquisition and tracking subsystems. Code acquisition strategies such as: serial search, parallel search with maximum likelihood detection and sequential estimation have been used successfully. However, in conditions of very poor SIR, sequential estimation techniques such as RASE and RARASE fail and parallel search strategies, although extremely fast, are expensive. As a consequence, many of the proposed synchronisers are based on a serial search strategy. This paper compares the acquisition performance of some of the more popular serial synchronisers under similar conditions of AWGN