광 링크를 통한 CDMA 신호 전송시 비선형 특성

Abstract

The number of subscribers to mobile communication systems is increasing rapidly because of a strong demand for communication at any place and time. So, it is necessary to provide systems satisfying such a demand in future personal communication. To meet this demand, a microcellular communication system has been investigated. This system has such advantages as effective frequency utilization, accommodation of a large number of subscribers and the small power launched. However, this system has such a disadvantage that a large number of base stations are required because the number of microcells rapidly increases due to the inverse proportionality to the square of the radius of microcells. To reduce the number of the base stations, distributed remote antenna systems replace and connect to the base station using optical links which are an economical and flexible way to deliver the radio signals. In this paper, I measured and analyzes the third-order intemodulation distortion(IMD3) mostly appeared in a laser diode of an optical link for CDMA signals(IS95). First, I analyzed and mearsured power spectrum and ACPR of transmitted CDMA signals over the optical link with 1, 2 and 4 frequency assinments(FAs). Secondary, I measured adjacent channel power ratio(ACPR) of CDMA signals as a function of FAs, which negativerly affected to ACPR. And I measured the back reflection effect to the nonlinearity of the optical link. Finally, I estimated the conditions for the laser diodes to satisfy less than -45 dB ACPR with the Wideband CDMA signals.;이동 통신 환경에 광 전송 기술을 적용한 유무선 엑세스 망의 구현이 점차 증가되고 있다. CDMA 방식 유무선 복합 엑세스망을 위한 고아 전송 시스템의 성능은 수신 감도(sensitivity), 파형 품질(waveform quality), in-band/out-of-channel power 그리고 코드 영역 전력(code domain power) 등을 측정함으로써 평가할 수 있다. CDMA 신호가 광 링크를 통과함에 따라 이러한 시스템의 성능은 비선형 특성 때문에 저하된다. 비선형 효과는 신호의 dynamic range를 감소시켜 신호 왜곡을 초래하며 또한 상호 채널 간에도 영향을 미치게 된다. 본 논문에서는 CDMA 신호가 광 전송로를 통해 전송되었을 때 생기는 비선형 효과를 CDMA signal generator 같은 고가의 장비 없이 변조되지 않은 두 개의 반송파 실험(two-tone experiment)만을 이용하여 분석한다. 또한 차세대 통신인 IMT-2000의 Wideband-CDMA 신호를 광 링크로 전송할 때 요구되는 시스템 사향을 제안한다.