Research And Design Of OFDM-ROF Access System

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 radio over fiber (ROF) access technology which combines the advantages of optical communication and wireless communication has become a new access method and the focus of the research. A ROF 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 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 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 this paper, the ROF technology is researched, and the application of OFDM in ROF system is introduced and analyzed. The major research contents and the achievements are listed as following:(1) The ROF basic theories and key technologies are introduced, and a bidirectional ROF system is proposed. In this system the erbium-doped fiber laser (EDFL) and the Bragg grating are used as the multi-wavelength laser source, and the remote heterodyne detection (RHD) is used to generate the60GHz mm-wave which is DPSK modulated. At the BS, the downlink mm-wave is OOK remodulated to transmit the uplink data, and this guarantees the simplicity and economy theory of the BS. In this paper, the transmission performance is analyzed and the feasibility of the system is proved.(2) The theories and signal processing measures of OFDM with the applications and the structures in optical communication of OFDM are introduced in this paper. A bidirectional OFDM-ROF system is proposed, where two uplink and downlink structures are applied: the downlink direct method ROF structure and coherent method ROF structure, the uplink direct-detection optical OFDM (DDO-OFDM) and coherent optical OFDM (CO-OFDM). The transmission performances of the downlink and uplink are analyzed. The channel estimating property of different interpolation algorithm and the transmission performances of the uplink and downlink structures are compared.