Research On RF-Optic Transmission And Operation Technologies In Radio-over-Fiber Systems

The wireless access technology has played a more and more important role in the human lives due to its advantages of ubiquitous and flexibility. With the popularity of the personal terminals such as iPhone/iPad, and the rise of the national high-tech industry, the increasing requirement for wireless capacity brings forward grave challenge to the existing wireless access technologies. Furthermore, with the increasing of base stations (BS) implemented by the telecommunication operators, more than70%of the total energy is consumed by the BS and leads to an energy crisis in the cellular mobile communications. At present, C-RAN is intensively developed by China Mobile and aims to construct a clean system based on the centralized processing, collaborative radio and real-time cloud infrastructure, among which the centralized processing requires the separation of remote antenna units (RAUs) from the central office (CO). As one of the most important applications of the Microwave Photonics (MWP), radio-over-fiber (ROF) systems play an important role in the evolution of the convergence between broadband fiber access and wireless access. ROF systems make full use of the advantages of fiber transmission such as low loss and broad bandwidth to realize the separation of RAUs from CO, and build dynamic controllable distributed RAUs systems with wide coverage, low energy and broad bandwidth, thus it becomes an effective solution to the bandwidth crisis and energy crisis existed in the present wireless technologies.In order to meet the application requirement of dynamic controllable ROF systems in the broadband ubiquitous wireless access, and resolve the technical challenge in analog-optic fiber link performance and RF resources controlling, this thesis mainly studies the RF-optic transmission (RFOT) and operation (RFOO) technologies, and most of the work is concentrated on the modeling, RFOT&RFOO technologies and the applications of the ROF systems. The detailed content and contributions are presented as follows:(1) Modeling of the ROF SystemsThe analog-optic fiber link with high performance is necessary for the application of ROF systems in the broadband access and ubiquitous sensing, and the modeling is the priority to analyze and improve the link performance. This thesis builds a mathematic model of the main performance indexes in the direct and external modulated ROF systems based on the microwave amplifier model, and analyzes the impact of the parameters, such as the received optical power of photodetector (PD), relative intensity noise (RIN) of direct modulated laser (DML), half-wave voltage and injected optical power of the external modulator, on the link perfonnance indexes by simulation.Characterization of stimulated Brillouin scattering (SBS) gain and loss spectra is necessary not only in the application but also suppression of the SBS effect in ROF systems. This thesis proposes a simple and precise method to characterize the SBS gain and loss spectra based on the Fresnel reflections and optical carrier suppressed (OCS) modulation, which owns the advantages such as single-ended structure to prevent the error due to laser frequency fluctuation, precise measurement with1MHz or higher resolution due to the small-step sweeping of microwave sources, and low-cost components with bandwidth being half of the Brillouin shifted frequency.(2) RFOT TechnologiesIn order to meet the requirement of networking with complex topology, the thesis proposes a distributed RFOT system based on the coarse wavelength division multiplexing (CWDM) ROF technique and builds a CWDM-ROF based distributed WiFi system with five wavelength channels, which supports5RAUs, and achieves approximate1.5%and2.5%of the error vector magnitude (EVM) for4.5km and22km single mode fiber (SMF) transmission, respectively. The best transmission performance is obtained when the wireless distance is4m. The impact of the laser RIN is also studied and EVM is degraded obviously when RIN>-125dBc/Hz.For application requirement of millimeter (mm) wave band ROF systems, the thesis proposes a gigabit-level mm-wave over fiber technique based on direct modulation and external OCS modulation. The DML is used to generate optical carrier and modulate the baseband data which greatly reduces the system cost. Up-con version of2.4GHz to the32GHz mm-band is demonstrated experimentally and the impact of DML RIN on link performance is studied on the VPI8.7platform.In order to realize broadband RF modulation, the thesis firstly proposes an all-optical RF BPSK modulation and transmission technique based on phase modulation to intensity modulation (PM-IM) conversion. The multi-wavelength single sideband (SSB) signals are generated which can tolerate fiber dispersion and meet the requirement of WDM-ROF applications. Considering the stable issue in the experiment, the frequency modulated (FM) laser and band pass filter (BPF) is employed to accomplish the performance simulation on the Optisystem7.0, which achieves good waveform with wide eye diagram and bit error rates of6×10-9.The impact of laser linewidth on the SBS threshold and link performance is also studied in the thesis. EVM of WiFi signals is measured in the direct and external modulated ROF links, and the optical power is monitored accordingly. The SBS threshold is improved by4dB when the50KHz-linewidth external modulated link is replaced by the lOMHz-linewidth direct modulated link, which confirms the laser linewidth broadening for the suppression of SBS effect in ROF systems.(3) RFOO TechnologiesDynamic controllable ROF systems can improve the efficiency of the RF bandwidth resources, thus the thesis proposes an RF switching technique based on the MEMS optical switch matrix (OSM) in order to realize the dynamic allocation of the RF bandwidth. Switch speed of the4×4MEMS OSM is measured, and the RF switching is observed by monitoring the optical spectra. The impact of the loss of the OSM on the system EVM is studied experimentally with the help of the Erbium doped fiber amplifier (EDFA). This OSM-based RF switching technique can not only realize the RF resources allocation but also provide multicast services which make the ROF system intelligent.RF phase shifter is the key component in the beamfonning of both smart antennas and optical controlled phase-arrayed radars. The thesis firstly proposes a photonic RF phase shifter based on SBS effect and vector-sum theory. PM-IM conversion is completed with the help of SBS functioned on the PM-generated two sidebands. At PD, the two sidebands are mixed with the optical carrier and generate the RF signals with controllable phase shift.(4) Applications of the ROF SystemsWith popularity of the WiFi techniques, a hierarchical architecture of wireless sensor network (WSN) and wireless local area network (WLAN) provides a promising solution to the Internet of Things (IOT). In this thesis, a new hierarchical architecture is proposed for ubiquitous wireless sensing and access with improved coverage using CWDM-ROF links. The demonstration platform is built to provide multi-services, such as broadband access, temperature sensing, video monitoring and RFID location, etc.In order to meet gigabit-level wireless access requirement, the thesis proposes a multi-band (2.4GHz WiFi and60GHz mm-wave band) dynamic controllable ROF system architecture for in-building multi-services integrated access. The ubiquitous WiFi channel is used for the uplink of60GHz-band ROF link which greatly reduces the cost. The dynamic controllable ROF system is used to realize the bandwidth allocation based on the OSM RF switching techniques. Finally, the distribute antenna system with bandwidth dynamic allocation is implemented, which can not only provide gigabit-level un-compressed high definition television (HDTV) services, but also meet the intelligent house services, such as ubiquitous Mb/s-level access, security surveillance, environment monitoring, etc.