| Multimode fiber in the high-frequency region is a frequency-selective fading channel. And the multimode fiber communication technology based on OFDM is a new one with very practical prospects, which can overcome the frequency selective fading and transmit high bit rate data using the high frequency region of multimode fiber. So it has become one of the most active research regions. But there are some deep nulls in the channel, the reliability of the system will seriously be degraded if some subcarriers of OFDM located near these nulls. How to solve the effect of the nulls on system is the key point of making OFDM multimode fiber communication technology come true. Studying the methods of solving the deep nulls problem is the major work of this dissertation.Based on deeply studying current international methods of improving communication system and combining OFDM multimode optical fiber communication features, the dissertation has made in-depth research work from using coding techniques and adaptive modulation technique. Turbo Codes, one of the most novel coding techniques, has been introduced in OFDM multimode optical fiber communication and a Turbo Codes module based entirely on the SIMULINK simulation model has been proposed; and adaptive unsaturated technique has been proposed at the first time. The two methods reached relatively good results to solve the deep nulls problem and lower system bit error rate. Especially, the adaptive non-saturated technique has a significant theoretical and practical value. The main works lie:First, implementation of OFDM multimode optical fiber communication system based on SIMULINK has been made, and analysis and testing of the system BER under different conditions has also been done. Composition of each module and methods of realizing system have been described in detail. System BERs of various speeds and subcarriers have been simulated. The simulation results show that system BER is 10-2 with the transmission rate of 10 Gbps, multimode fiber length of 300 m, BPSK modulation and convolutional coding. The simulation results also show that the BER in different number of subcarriers have a big difference, and the system BER become higher when the number of subcarriers increase due to more large probability of subcarriers located near the deep nulls. It is obvious that OFDM multimode optical fiber communication system performance poor, and only using OFDM cannot achieve high reliable and high speed transmission as well as the deep nulls is one of the most important key reasons for system performance.Second, a Turbo Codes module based entirely on the SIMULINK simulation model has been proposed, and its performance was tested. The Turbo Codes module proposed solves the problem that Turbo Codes is hard to simulate because of its complex structure. In the encoder, the components use circulatory system Convolutional Codes to make the odd sequence is the same as the original information. The decoder takes pipeline decoding methods, which is based on SIMULINK model in the posterior probability decoding module (APP Decoder) to make the process of decoding intuitive and convenient. After simulation, the effect of the number of iterations, the length of interleaver, and decoding algorithm on Turbo Codes have been analyzed. The results show that when single-bit signal to noise ratio (Eb/N0) 2dB, the system BER can be close 10-7, seven times of iterations is near to saturated, and the longer the interleaver is, the better the performance of Turbo Codes is.Third, Turbo Codes has been introduced in OFDM multimode optical fiber communication, and the systems BER with Turbo Codes, cascade code (the inner code is convolutional codes and the outer code is RS codes) and convolutional codes have been compared. The simulation results show that the systems BER with Turbo Codes is far less than systems with cascade code and convolutional code, and when the fiber length is 300m, the system BER of transmission 10 Gbps information is lower than 10-6。Then, the principle of adaptive modulation technique to overcome deep nulls has been analyzed in detail, and defects of this technique has also have been revealed. The simulation results say that although adaptive modulation technique can overcome the effect of deep nulls on system by changing bits allocated to poor subcarriers into good subcarrier, the performance of good subcarriers are serious deteriorated due to the modulation order to increase. So the overall system performance is not good as well. The system BER is around 10-3 when transmission 10 Gbps information in 300 meters with 30 dB SNR.Finally, adaptive unsaturated technology has been proposed and the performance of adaptive modulation techniques and adaptive unsaturated technology have been analyzed and compared. The adaptive unsaturated technology overcome the defect that adaptive modulation techniques lead to the shortcoming of the overall system performance dropping for more transmission bit of good subcarrier, by breaking the bottleneck of limited resources to provide additional subcarriers. Moreover, it does not require complex allocation algorithms, and only setting a threshold of SNR will be able to achieve an effective bit allocation. And there is no need to use different modulation mode in different subcarriers and all subcarriers take the same modulation mode. So it is far easier to realize. The simulation results show that the system performance with adaptive unsaturated technology is far superior to the system with adaptive modulation. According to different needs, adaptive unsaturated technology can achieve different BER requirements. Increasing only a small amount of additional subcarriers, 10 Gbps information can reliably transmit in 300 meters multimode fiber with BER of 10-9 when the SNR is 30 dB. When increasing more additional subcarriers, 20 Gbps information can reliably transmit in 300 meters multimode fiber with BER of 10-9.Some of these research results mentioned above have been published in journals at home and abroad or international conference as papers. |
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