Optical IM/DD transmissions and networking for mobile network services

Abstract

Recent years have witnessed a tremendous growth of data traffic due to expansions of various applications like internet-of-things (IoT), ubiquitous mobile internet, smartphones, and cloud technologies. Optical links can offer an enormous capacity, and optical communication systems are often seen as the only candidate that can meet today’s demand for transmitting vast amounts of data over a long distance. An optical communication system can be realized by employing two types of detection methods: coherent detection and direct detection. The first technique enables the detection of both amplitude and phase by using a local oscillator laser. The second technique only detects the signal intensity. A system that utilizes direct detection technology is also labeled as intensity modulation and direct detection (IM/DD) system. Although a coherent detection system possesses numerous advantages over a direction detection system, the primary benefits of employing IM/DD systems are its low implement cost and complexity. Therefore, an IM/DD optical system is still favorable and widely used in the field of optical communications. In this dissertation, we perform the transmission of a multi-level signal in an IM/DD optical communication system for the application in optical access networks. Realizing a multi-level signal transmission in a directly modulated laser (DML)-based IM/DD system can be critically challenging due to bandwidth constraints and nonlinearity of the electro-optic components, even though it can increase the spectral efficiency of the system. Several works have been proposed in order to mitigate such an impairment. Among them, a nonlinear Volterra equalizer and a nonlinear lookup table (LUT) are the most popular approach. However, the former approach requires a large number of arithmetic operations and memory size, which is the major limiting factor. Nonetheless, the performance of a nonlinear LUT is reliant on digital-to-analog converter (DAC) resolution, the width of the LUT and the number of input bits, which exponentially increases as the size of the LUT increases

hence memory requirements. In this dissertation, we propose a novel nonlinear equalizer based on a machine learning algorithm called an artificial neural network (ANN). The proposed scheme is low cost as it can significantly reduce the number of arithmetic operations. Experimental results for a DML-based pulse amplitude modulated (PAM) signal transmission over a standard single mode fiber confirm that the proposed nonlinear equalizer can mitigate the impact of bandwidth constraints and nonlinear penalties during multi-level signal transmissions in an IM/DD optical communication system. The future fifth-generation (5G) radio access networks (RANs) are expected to provide data rates of at least 10 Gbps. The small-cell approach is no longer a viable solution for increasing transmission capacity due to inter-cell interference, capital expenditure (CAPEX), and operational expenditure (OPEX). A novel mobile architecture, called cloud radio access network (C-RAN) proposed by China Mobile is currently considered the most cost-effective way to meet the capacity requirements for 5G RAN. However, the current C-RAN architecture requires a dedicated high-capacity optical fronthaul link between a baseband unit (BBU) and a remote radio head (RRH), which can be eased if the present C-RAN architecture can be configured with an orthogonal frequency division multiplexed passive optical network (OFDM-PON) technology. It is worth mentioning that the OFDM-PON is often seen as the main candidate for the next-generation optical access networks. Nonetheless, an OFDM-PON system suffers from two major limitations in the uplink include optical beating interference (OBI), and symbol timing offset. In this dissertation, we provide a Zadoff-Chu (ZC) sequence-based low-complexity upstream time synchronization scheme for an OFDM-PON with C-RAN fronthaul applications. The proposed scheme can be implemented with a direct detection OFDM-PON system. In addition, the proposed scheme is hitless, as it does not require any guardband in the spectrum. Furthermore, simulation studies and experimental results confirm that the proposed scheme can perform upstream synchronization without interrupting the subcarriers that are already in service.