Interference Reduction in CDMA Systems Using Antenna Arrays

Abstract

The field of wireless communications is growing at an explosive rate, covering many technical areas. The deployment of cellular systems is a success and as the subscriber population continues to grow at a rapid pace, service providers may get capacity problems due to limited available frequency spectrum. Consequently, service providers are forced to find methods of enhancing the coverage and the capacity of their cellular networks. A promising future method suggested is to utilize adaptive antennas, which has also been recognized as one potential economic approach. Adaptive base station antennas have been proposed as a means to achieve the requested longer range and higher capacity in cellular systems. In WCDMA, there is large number of interferers present in the system due to frequency reuse factor of one. The large numbers of interferers in these systems require a beamforming technique and other methods like null forming that would efficiently suppress the power in the undesired direction at the same time maximizing the power in the desired direction. For these systems side lobe adjustment and null forming would be a simple solution towards suppressing intercell and intracell interference and hence improving CIR performance and placing the nulls in the direction of individual interferers. In this dissertation, a beamforming algorithm is analyzed for flexible side lobe adjustment. The algorithm places a very low side lobe level over a large width in the beam pattern. The convergence of this algorithm is fast and requires only two to three iterations. This algorithm can also form nulls in the desired direction. This beamforming algorithm is suitable for uplink and downlink since it is based on the DOAs estimated in uplink. The null forming method, based on constrained mean square approximation is discussed, which forms the null in the desired direction.