| The communication network is composed of the core network and the access network. The core network has the functions of exchange and transmission, and the access network connects with the users directly and applies multiple services. The core network has developed rapidly, but the developing of the access network is relatively slow and cannot meet the public need of the high speed and multiple service of access network. As the transmission media is the optical fiber which has lots of advantages, the optical access network has become the promising access method. The system of RoF-PON combines the advantages of passive optical network and radio over fiber technology, which has become the hot point of the access network. RoF-PON system is composed with the center station (CS), the base station (BS) and the user end. The center station and the base station are connected with the optical fiber, and the base station and the user end are connected with the millimeter wave (mm-wave). The center station which plays the role of data exchanging and processing has complicated facilities and is shared by many base stations. The base station has the responsibility of the data transmission of downlink and uplink:converting the light received from the center station to mm-wave which is sent to the user end next, and the uplink data received from the user end is transmitted by the optical fiber from the base station to the center station. The base station facility is simpler and amount is larger.RoF-PON system takes the advantages of the low loss and high capacity of the optical fiber, and at the same time it suffers from the dispersion and other disadvantage factors. To lighten the dispersion of the optical fiber in RoF-PON system, the orthogonal frequency division multiplexing (OFDM) is used, which can use the digital processing method to solve the analogy aspect problem. On the other hand, as the quadrature amplitude modulation (QAM) is used in OFDM, so the signal bandwidth is narrower. In order to improve the transmission efficiency, RoF-PON system always combine with wavelength division multiplexing technology, the traditional laser array has not satisfy its requirements. Now, RoF-PON system mainly uses intensity modulation to produce RF signals, which can cause nonlinear crosstalk because of phase shift between different frequencies, it is necessary to design a novel modulation to reduce the nonlinear crosstalk.In this paper, we introduce OFDM technology to RoF-PON system, design a multi wavelength light source, broadband wave De-multiplexer and optical coupling SSB modulation. The contents and achievements of this paper are as following:(1) We explore the principle of OFDM technology and its application in optical communication. Pilot, cyclic prefix and window synchronization technology are verified by simulation, so laid the foundation in the application of OFDM technology in RoF-PON system.(2) We explore the principle of Zach-Zehnder Modulator producing all orders of optical sideband, and design a multiple wavelength light source applied in RoF-PON system of. The result of simulation shows that it has high output, good flatness and less crosstalk. We also analyze the features of Array Waveguide Grating, and design a broadband flat wave de-multiplexer used in multiple wavelength light source.(3) In order to reduce the harmonic distortion and intermodulation distortion, we design a novel optical coupling SSB modulation, its feasibility is verified by theoretical analysis and simulation. By related experiment, we use the signal to noise ratio, error vector magnitude, constellation diagram, eye diagram and bit error rate to verify its performance. The experimental results show that the optical coupling SSB modulation can effectively reduce the nonlinear distortion and double its transmission efficiency. |
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