Mutual Coupling Matrix Estimation and Null Forming Methods for MBF Antennas

Antenna pattern control techniques for mobile terminals such as adaptive array antenna are expected to improve the characteristic of high-speed radio communications. The drawbacks of adaptive antenna based on DBF (Digital Beam Forming) are the requirement of the same number of RF receivers and A/D (analog-to-digital) converters as the number of array elements, since digital signal processing have to be performed in the baseband. The adaptive antenna also requires high speed signal processing, which is difficult to apply for mobile terminals, since mobile terminals need to be fabricated with low cost and be miniaturized [1]. In addition, mutual coupling effect can not be neglected when array elements are mounted in small space to miniaturize mobile terminals, and no ideal directivity patterns are obtained. This paper focuses on the MBF (Microwave Beam Forming) antenna, consisting of array antenna, phase shifters, and power combiner. Antenna pattern control is presented for mobile terminals, and mutual coupling matrix estimation and null forming methods are also presented in this antenna. We propose the estimation method of coupling matrix. Conventional methods require all of the signals on the antenna element [2], but proposed method does not need each signal and use combined signal of all the input signals. Secondly, we discuss about null forming by phase control including mutual coupling effect. In conventional methods, effect of mutual coupling is compensated by multiply the inverse of coupling matrix to the antenna weight, using both phase and amplitude. It is difficult to apply conventional methods to the MBF antenna and we extend the method by Takanashi et al. [3] to include coupling matrix. Proposed methods can be applied for any shape of array antenna, and a rectangular array of 4 elements of λ/4 spacing is demonstrated in anechoic chamber.