평면구조 기반의 LTE 이동통신용 빔성형 버틀러 매트릭스(4×4) 구현
Implementation of Beamforming Butler Matrix(4×4) Based on Planar Structure for LTE Mobile Communications
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초록
LTE 대역인 2.5-2.68GHz에서 동작하는 빔성형 버틀러 매트릭스를 설계하기 위해 버틀러 매트릭스의 구성요소인 하이브리드 커플러와 크로스오버를 다단 브랜치 선로의 구조로 구성하여 광대역 특성을 갖는 평면 구조의 버틀러 매트릭스를 설계하였다. 버틀러 매트릭스의 구조와 구성 요소들은 3dB 하이브리드 커플러 4개, 크로스오버 2개, 각각 2개의 -45도 위상기와 등위상 조절기로 구성된다. 이들 구성요소 중 3dB 하이브리드 커플러와 크로스오버는 마이크로스트립 기판 내에서 브랜치 선로 구조를 가지고 광대역 특성을 갖도록 설계하였으며, -45도 위상 천이기와 등위상 조정기는 선로의 길이를 조정하여 적절한 위상을 얻을 수 있도록 최적화하였다. 설계된 버틀러 매트릭스는 동작대역에서 23.9dB ~ 34.18dB의 좋은 반사손실 특성과 비교적 작은 5.41˚의 위상오차를 보였다. 그리고 버틀러 매트릭스의 출력신호를 마이크로스트립 패치배열 안테나에 인가하여 신호의 크기와 위상오차가 빔성형 패턴과 빔조향 방향에 미치는 영향을 연구하였다. 그 결과, 버틀러 매트릭스가 최대 5.41˚정도의 위상오차를 가질 때에 성형되는 빔 패턴의 방향은 1˚오차로 크게 변하지 않아 원하는 방향으로 빔을 적절히 조향할 수 있다.
Abstract
In order to design a beamforming Butler matrix operating in the LTE band of 2.5-2.68GHz, the Butler matrix with a broadband structure was designed by constructing a hybrid coupler and crossover, which are components of Butler matrix, with a multistage branch line structure. The structure and components of the Butler matrix consist of four 3dB hybrid couplers, two crossovers, two -45˚ phase shifters and two equiphase controllers. Among these components, the 3dB hybrid coupler and crossover are designed to have broadband characteristics with a branch line structure in the microstrip substrate. The -45˚ phase shifter and equiphase controller adjust the length of the line to obtain the proper phase, respectively. The designed Butler matrix showed good return loss characteristics of 23.9-34.18dB and relatively small phase error of 5.41˚ in the operating band. And also, when the output signal of the Butler matrix was applied to the microstrip patch array antenna, the effect of the signal amplitude and phase error on beamforming pattern and beam steering direction were studied. As a result, when the Butler matrix has a phase error of about 5.41˚, the direction of the formed beam pattern does not largely change by an error of 1˚, and the beam can be properly steered in a desired direction.
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