Study On Multimode Fiber Broad-Band Communication Technology Based On OFDM

Optical communication has been used widely and has been undergoing swift and violent development since it was proposed, because of its wide band-width,big communication capability,low transmission dispersion and the resistance of electromagnetism disturbance. Optical communication has become the communication back-bone network's basal transmission technology by its nearly 50Tbit/s band-width; Optical communication satisfies the band-width needs which is used for the data service,image service,phonetic service and all kinds of rush services. The maturation of optical back-bone network's transmission technology,the enhance of its transmission ability and the needs for the band-width make the limitation of the short-distance communication network more obvious. Under this background, optical communication needs to be developed deeply and the development of all kinds of optical communication network is also necessary. The short-distance communication networks such as connection network,ethernet backbones,local area network should be supported by the optical fiber is the basic of wide-band communication network.Multimode fiber is the preferred transmission medium of short-distance,multi-node,multi-link network, because of multimode fiber has large core medium and NA,the apparatus which is fit for the multimode fiber are cheaper, at the same time the connection of different multimode fiber is easier. Amounts of criterions make multimode fiber as the local area network medium. IEEE802.3ae said that 10Gbit/s signal should be transmitted over 300m on the installed multimode fiber. All the things mentioned above make the study of enhancing the transmission ability of multimode fiber to be the hot-spot. A lot of solutions have been proposed such as coarse wavelength division multiplexing,offset launch,electrical equalization. All the solutions make use of the base-band of the multimode fiber. The high frequency region of the multimode fiber consists of lots of band-pass regions and its band-width is large, if this region can be used to transmit data, the transmission ability of the multimode fiber would be enhanced greatly.The high frequency region of multimode fiber has band-width, however, this region is a frequency-selective fading channel and there are lots of deep-nulls there.OFDM can restrain the frequency-selective fading, used the OFDM in multimode fiber can solve the frequency-selective fading of the high frequency region and the multimode fiber can be used to transmit high bit rate signal.Adaptively modulation can load the data according to the channel characteristic, it would transmit more data when the channel is good and transmit less or no data when the channel is bad. Using the adaptively modulation in multimode fiber communication can solve the problem caused by the deep nulls in high frequency region.In this thesis, the characteristic of the multimode fiber is analysed. Designed a multimode fiber communication system which is based on SCM to validate the usability of the multimode fiber's high frequency region. To solve the frequency-selective fading, OFDM is used in multimode fiber communication and the communication system is designed, the system's transmission performance and transmission ability was analysed. At last, the adaptively modulation is used in the OFDM multimode fiber communication system to solve the problem caused by the deep nulls in high frequency region. The AMOOFDM communication system's transmission ability and transmission performance are analysed.The details of the thesis is given as follows:1. the transmission characteristic of the multimode fiber is researched ,the channel modal is proposed. High frequency region of multimode fiber is analysed using mathematics .Designed a multimode fiber communication system which is based on SCM to validate the usability of the multimode fiber's high frequency region. Analyzing the transmission performance of the system and find the pivotal factor which affects the system's performance.2. Introducing the OFDM and designing OFDM-multimode fiber communication system to solve the frequency-selective fading problem. In the thesis all the parameters of the system are given and the performance and transmission ability are given by simulation.3. Based on the OFDM-multimode fiber communication system, an adaptively modulation OFDM multimode-fiber communication system is designed which is used to solve the deep nulls problem. At the same time, based on the multimode fiber channel characteristic, two adaptively bit loading algorithm is proposed. Used the two algorithms in the AMOOFDM communication system, giving the system's transmission performance and transmission ability.In this thesis the OFDM is used in optical communication and solve the frequency-selective fading problem of multimode fiber. For the problem caused by the deep nulls of high frequency region, adaptively modulation is used. It can get the result that when the channel performance is good more data is transmitted; when the channel performance is bad, less data is transmitted. According to the multimode fiber's characteristic, two kinds of bit loading algorithm are proposed.