Delta-sigma-digitized IFoF and RoF transmission systems

Abstract

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.