Receiver synchronisation techniques for CDMA mobile radio communications based on the use of a priori information
Receiver synchronisation can be a major problem in a mobile radio environment where the communication channel is subject to rapid changes. Communication in spread spectrum systems is impossible unless the received spreading waveform and receiver-generated replica of the spreading waveform are initially synchronised in both phase and frequency. Phase and frequency synchronisation is usually accomplished by performing a two-dimensional search in the time/frequency ambiguity area. Generally, this process must be accomplished at very low SNRs, as quickly as possible, using the minimum amount of hardware. This thesis looks into techniques for improving spread spectrum receiver synchronisation in terms of the mean acquisition time. In particular, the thesis is focused on receiver structures that provide and/or use a priori information in order to minimise the mean acquisition time. The first part of this work is applicable to synchronisation scenarios involving LEO satellites. In this case, the receiver faces large Doppler shifts and must be able to search a large Doppler ambiguity area in order to locate the correct cell. A method to calculate the Doppler shift probability density function within a satellite spot-beam is proposed. It is shown that depending on the satellite’s velocity and position as well as the position of the centre of the spot-beam, not all Doppler shifts are equally probable to occur. Under well defined conditions, the Doppler pdf within the spot-beam can be approximated by a parabola-shaped function. Several searching strategies, suitable for the given prior information, are analysed. The effects on the mean frequency searching time are evaluated. In the second part of the thesis a novel acquisition technique, based on a fast preliminary search of the ambiguity area, is described. Every cell of the ambiguity area is examined two times. The first search is a fast straight line serial search, the duration of which is a crucial parameter of the system that must be optimised. The output of the first search is then used as a priori information which determines the search strategy of the second and final search. The system is compared with well known active acquisition systems and results in a large improvement in the mean acquisition time. Its performance is evaluated in Gaussian and fading Rayleigh channels.