Analog Radio-over-Fiber (RoF) link deteriorates in proportion to the transmission distance. Moreover, it is considerably affected by the nonlinearity of both electrical and optical components in the system. Therefore, transmission systems that prevent the signal quality deterioration and thereby enable more robust and reliable operation need to be developed. Among several candidates, $\Delta\Sigma$ modulation has recently drawn attention. This is because the $\Delta\Sigma$-digitized DRoF system does not require a DAC in a remote unit (RU) and the signal reconstruction is easily performed by a filter. This in turn considerably simplifies the RU structure and reduce the installation and maintenance cost of the RU. In this effort, we propose and experimentally demonstrate a $\Delta\Sigma$-digitized intermediate-frequency-over-fiber (IFoF) and multi-pulse Manchester-encoded digitized radio-over-fiber (DRoF) systems. The digitized IFoF (DIoF) transmission system has been evaluated with a realized 130 nm CMOS 2-level 3rd-order low-pass $\Delta\Sigma$ modulator chip operating at 622.08 MHz to digitize the analog IF signal. Moreover, two $\Delta\Sigma$-modulated digital radio-over-fiber (DRoF) transmission systems that employ a multi-pulse Manchester encoder are also proposed and experimentally evaluated. With a two-step modulation process comprised of $\Delta\Sigma$ modulation and the multi-pulse Manchester encoding, a high frequency replica or image of a $\Delta\Sigma$-digitized analog communication signal can be successfully transmitted without significant power loss. This is achieved by exploiting the spectral characteristics of the modified Manchester code. Thirdly, three important non-idealities (jitter, rise/fall time mismatch and amplitude level non-linearity) affecting the proposed DIoF and DRoF systems are introduced and evaluated. From the evaluation results, the requirements of the non-idealities can be determined. Finally, a correlative code-assisted $\Delta\Sigma$-digitized IFoF system is proposed to increase the transmission capacity. With a proposed two-step coding process comprised of $\Delta\Sigma$ modulation at a high sampling rate and correlative encoding for the $\Delta\Sigma$-digitized signal for bandwidth reduction, the transmission capacity of the $\Delta\Sigma$-digitized IFoF system is considerably enhanced. According to experimental results, the proposed correlatively-coded $\Delta\Sigma$-IFoF system shows at least three times higher capacity than that of CPRI. In addition, it is estimated that the transmission signal bandwidth up to 1440 MHz (=LTE 20M × 72) in the proposed correlatively-coded $\Delta\Sigma$-IFoF with K=1 is possible with a 14.4 GHz optic link.
아날로그 radio-over-fiber (RoF) 기반 시스템의 전송 신호 품질은 전송 거리에 비례하여 거리가 늘어날수록 열화 될 뿐만 아니라, 전송 시스템 내 전기 및 광학 구성 요소의 비선형성에 의해 상당한 영향을 받는다. 따라서 신호 품질 저하를 방지하고 좀 더 신뢰할 수 있는 전송 품질을 가능하게 하는 전송 시스템을 개발 요구가 증대되고 있다. 상기 요구를 만족하기 위한 몇 가지 후보 중에서 $\Delta\Sigma$ 변조가 최근 주목을 끌고 있다. 그 이유는 다음과 같다. 첫째, 아날로그 통신 신호를 $\Delta\Sigma$ 변조 방식을 사용하여 디지털 신호로 변환한 후에 광전송 시스템을 통하여 전송함으로서 상기 아날로그 RoF 기반 문제점을 해결할 수 있다. 둘 째, $\Delta\Sigma$ 디지털 변조된 신호를 Remote Unit (RU)에서 복원할 때 Digital-to-Analog Converter (DAC)를 필요 하지 않고 필터로 쉽게 수행할 수 있기 때문이다. 따라서, 기존의 다른 디지털 전송 구조에 비해서 RU 구조를 상당히 단순화할 수 있다는 장점을 갖는다. 이러한 노력의 일환으로, 본 논문에서는 $\Delta\Sigma$-IFoF, Multi-pulse Manchester-assisted $\Delta\Sigma$-RoF 및 Correlative-code-assisted $\Delta\Sigma$-IFoF 시스템을 제안하고 실험적으로 제안 구조를 검증하였다.