Adaptive Array Antenna Processing

This paper, specifically written for GradCon ’99, presents an overview of the field of adaptive arrayprocessing. Beginning with a brief review of array synthesis theory, the concept of an adaptive array is introducedand defined. The broad field of adaptive array applications can be divided into two categories: adaptive signalprocessing for multiple access communication systems, and adaptive beamforming for the suppression ofinterference. In commercial wireless systems, the azimuthal plane is divided into sectors, typically three 120°regions, and the spacing of the antennas is used to provide diversity gain in each sector. In general, constant gain isdesired across the sector. Pre-processing is then applied to the antenna outputs to separate signals pertaining to eachuser. Adaptive filtering algorithms, with correlation techniques are used to recover the signal from the distortionscaused by frequency-selective fading and multipath signal components. In general, the adaptive processing isapplied at the base station.In adaptive beamforming applications, the objective is often to maximize the gain in a particular direction whilesteering the nulls to the directions of interference. In an environment where the noise characteristics change, thesystem uses adaptive processing techniques to adjust the complex weighting (amplitude and phase) of each antennaelement to optimize the overall array pattern.The goal of adaptive processing is to increase the Signal-to-Noise Ratio (SNR) of the communication in order toreduce error rates and, in wireless communications, better utilize the capacity of the channel. This paper presentsexamples of current techniques being tested in both the commercial wireless field and in adaptive nullingapplications.Finally, a framework is presented outlining how an adaptive antenna array at the mobile user could beimplemented using currently available components. In this model, the base station is omni-directional, and themobile unit tracks the angle of arrival of the strongest signal. In a wireless system, direct line-of-sight is the optimalpath for the signal, but due to reflections off of objects in the environment, signals arrive at the receiver alongmultiple paths. If the receiving system cannot differentiate the multi-path components, they are combined at thereceiver and result in a spreading of the signal. If the angles of arrival can be resolved, the multi-path componentsmay be synchronized and summed to produce a stronger signal. This proposed method uses Butler matrices with aplanar array to produce a multiple beam antenna covering the spatial region of interest. Each beam is considered achannel for signal propagation. The beam receiving the strongest signal is identified. By correlating this signal withthose received by the other beams, the angle of arrival and delay of desirable multi-path components are identified.The remaining channels are not used. The selected channels can be synchronized and combined using a RAKEreceiver to obtain an improved signal. The motivation for this project is to build a unit that is simple and robust,using lowest-cost available components. References[1] Pozar, D.M., Microwave Engineering, 2 Ed., John Wiley & Sons, Inc., New York, 1998. [2] Mailloux, Robert J., Phased array antenna handbook, Artech House, Boston, 1994. [3] Applebaum, Sidney, “Adaptive Arrays”, IEEE Trans. Antennas & Prop., vol. AP-24, No. 5, pp.585 – 598,Sept. 1976. [4] Haykin, Simon, Adaptive Filter Theory, 3 Ed., Prentice-Hall, 1996. [5] La Porta, Thomas F. (Editor), et al., IEEE Personal Communications Special Edition on Smart Antennas,Vol. 5, No. 1, Feb. 1998. ADVISOR: Dr. A. Sebak