Abstract
Today, an optical access network always acts as one of the most important roles to provide communication services. With the progress of recent services, such as ultrahigh video streams and clouding computing, a laudable goal is to enhance the transmission efficiency of passive optical network (PON) and radio access network (RAN), mainly including the spectral efficiency and cost efficiency. In recent years, orthogonal frequency division multiplexing (OFDM) as a mature technique widely applied in wireless communications has become one of the promising solutions. A direct-detection optical OFDM (DDO-OFDM) signal format is guaranteed effectively enhancing the overall transmission efficiency. Tens and thousands of academic and industrial projects have devoted to the research and development of DDO-OFDM in PONs and RANs. In this dissertation, to further enhance the transmission efficiency with affordable implementation and operation costs, we study and experimentally demonstrate three effective solutions. Firstly, a multiband DDO-OFDM system is proposed to largely relief the electrical bottlenecks in both the transmitter and receivers. With an applied simple optical single-sideband (SSB) filter, the deleterious photo-detection incurred signal-signal beating interference (SSBI) can be mostly avoided. The overall transmission data rate achieves 150 Gb/s with less than 50 GHz transmission bandwidth is used. Secondly, to further improve the spectral efficiency, an iterative DSP for SSBI reduction is employed in each receiver of the PON. By rebuilding the SSBI of the received multiband DDO-OFDM signals, such DSP can gradually wipe out the SSBI contaminations. The results show that the signal performance saturates within only two iteration loops, and the conventional blank bandgap can be narrowed by about 66%. Thirdly, a polarization division multiplexing (PDM) technique is then employed to the above multiband DDO-OFDM system. To follow the trend of centralized RAN architecture in next mobile communications, the receiver oriented complicated feedback polarization tracking mechanism or polarization equalizing DSP is not necessary in our proposed PDM systems. In this topic, a designed PDM signal with two polarization orthogonal optical carriers is generated in the central office, namely the transmitter. A simple optical filtering is enough for PDM demultiplexing. Two solutions based on this polarization-tracking-free mechanism are presented, a PDM multi-service radio-over-fiber (RoF) mobile fronthaul and a PDM intermediate-frequency-over-fiber (IFoF) mobile fronthaul. The conventional training signals for polarization tracking are thus eliminated in these two systems, where the data rates are exactly doubled with respect to those of single polarization systems. Furthermore, the polarization orthogonal optical carriers are reused for an upstream generation without any polarization controlling. Therefore, a true polarization controlling free optical network unit (ONU) and user equipment (UE) has been established. In a demonstration of the PDM multi-service RoF mobile fronthaul a 12-GHz low-frequency wave and a 62.5-GHz millimeter wave are applied. And in the PDM IFoF mobile fronthaul demonstration the noted optical SSB filter is applied for multiband and PDM demultiplexing simultaneously. As a result, by employing the multiband technique, the iterative DSP, and the PDM technique, the spectral efficiencies of a DDO-OFDM system are effectively enhanced with affordable costs. The centralized system architectures also follow the big trend of system complexity centralization in the next generation mobile communications. Therefore, in both PONs and RANs, these presented solutions are proved and guaranteed with high spectral efficiency and cost efficiency, which are greatly benefit to the future researches and developments of optical access network communications.