Code acquisition in advanced CDMA networks

The present dissertation deals with initial synchronization in Code Division Multiple Access (CDMA) networks. In the first part of this thesis an extensive and up-to-date review of the literature is presented.The basic theory of code acquisition as well as different techniques and structures used to achieve the initial synchronization are discussed. A survey of the most common theoretical approaches allowing performance characterization of the acquisition process is included. The effect of noise, interference, carrier Doppler, multipath propagation, fading and data modulation on system performance are reviewed. Advanced code acquisition approaches exploiting interference suppression techniques and multiple antennas are also described. A summary of the results obtained within the area of code acquisition in CDMA networks is also embraced by this thesis. The distinctive assumption is to consider the actual variable effect of multiple access interference on the delay-domain search process, instead of the usual constant approximation. Three directions of research are followed. Models for code acquisition in quasisynchronous and asynchronous CDMA networks are first developed and analysed. Closed-form expressions for the main performance figures of the acquisition process are derived and analysed. Results show a strong dependence of the mean acquisition time with the nature of the multiple access interference. In the second area of research the previous results are extended to consider code acquisition with a multi-branch (Rake) receiver in a multipath channel. A generic model for Rake receiver code acquisition is considered and developed, in which the synchronization takes place in two phases. The first detected path is allocated to the first finger during the initial synchronization phase, whereas the remaining fingers are successively allocated to other available paths in the postinitial synchronization phase. Performance measures for this acquisition process are also derived and analysed. Finally, based on the use of an antenna array and beamforming techniques, conventional delay-domain code acquisition is extended to the angular domain, resulting in a twodimensional (delay-angle) search. This technique is found to be feasible, outperforming the synchronization approach exploiting a single-antenna. It is found that there exists an optimal number of antennas that minimises the mean acquisition time. Two-dimensional code acquisition is studied in a variety of scenarios, including single and multipath channels, fixed and fading channels, and with uniform and nonuniform spatial distributions of interference. Different two-dimensional search strategies are studied. A clear dependence of acquisition performance with the search strategy and the particular distribution of interference is pointed out. The performance of two-dimensional code acquisition is found to be seriously deteriorated by the presence of spatially nonuniform interference. Schemes based on search strategy and adaptive detector structures are considered and analysed to combat the performance degradation in the mentioned case. A comparative study of code acquisition exploiting multiple antennas is also presented.