Study On Adaptive Unsaturated OFDM For Muitimode Fiber Communication Technology

Nowadays, as the widespread use of a large number of intelligent and integrateservices, network flow is growing explosively and the demands for bandwidth will beextremely tremendous. In the case of high-speed and large-capacity, the advantages ofoptical fiber communication are obvious to all. So far, core networks of optical fibersare common and the transmission rates of core network are fast. That is to say, accessnetworks will be the bottlenecks of the whole network systems. In order to realizehigher speed services, access networks of optical fibers have become an irresistibletrend. In short distance networks, a lot of adapter connectors, joints and tortuous pathsare the characteristics. Multimode fiber（MMF） because of the low price of thecomponents fit for it is the most popular transmission medium for short distancecommunication. By a mass of simulations we know that high frequency regions ofmultimode fiber are relatively flat and approximate Rayleigh distribution, theamplitude responses of them do not decrease as the frequency increases. It has beenstudied that Orthogonal Frequency Division Multiplexing （OFDM） can overcome theeffects of frequency-selective fading, so it is suitable for the high frequency regions ofMMF. But there are several deep fading nulls in high frequency regions of MMF,OFDM can overcome the effects of frequency-selective fading, it cannot solve theproblem caused by the deep fading nulls. Adaptive modulation technology is regardedas an effective method to solve the deep fading nulls. But its complex modulationformats leading to high hardware requirements and performances of good carriersdeclining limit the improving of the performances of multimode fibercommunications. Proposing a new technology that can overcome the effects of deepfading nulls as well as the shortcomings of adaptive modulation technology is just themajor work of this paper.In order to show that the deep fading nulls in high frequency regions of MMFhave great impact on system performances, we first simulated the performances of communication systems with different multimode fibers and then compared them. Theresults showed that it is important to overcome the deep fading nulls. Based on deeplystudying adaptive modulation technology, adaptive unsaturated technology based onthe characteristic ’unsaturated’ of MMF was proposed, the technology is suitable forOFDM multimode fiber communications. The principle of the technology wasdeduced and verified by simulations in MATLAB. Then the technology was appliedto the system of OFDM multimode fiber communication, the results showed that,adaptive unsaturated technology can improve the performances of system better anddoes not need complex modulation formats, and it can be realized simply. An OFDMadaptive unsaturated multimode fiber communication system was set up in optisystem7.0and the radio frequency spectrum and optical spectrum were observed. The mainworks lie:First, a number of amplitude response curves of multimode fibers were simulatedto prove that the amplitude responses of the high frequency regions of multimodefibers do not decrease as the frequency increases. The high frequency regions arecomprised of some band-pass regions and approximate Rayleigh distribution. Thereare also several deep fading nulls in high frequency regions. We compared differentmultimode fibers and got that the deep fading nulls have great impact on systemperformances.Second, adaptive modulation technology and its principle of overcoming thedeep fading nulls were introduced. Some shortcomings of adaptive modulationtechnology were found by simulations. OFDM adaptive unsaturated technology wasproposed and its principle was deduced. The realization method and realizationprocess of OFDM adaptive unsaturated technology were verified by simulations:additional subcarriers are obtained by increase the number of IFFT and they transmitbits which have been allocated to poor subcarriers （subcarriers located at frequencyresponse with low gains）.Third, bit allocation results of adaptive modulation technology and adaptiveunsaturated technology in the same multimode fiber were compared. BERcomparisons among equal bits allocation, adaptive modulation and adaptive unsaturated technique were compared. The results show that adaptive unsaturatedtechnique can overcome the effect of deep fading nulls better and improve theperformance of system more, and it also can overcome the shortcomings of adaptivemodulation technology whose complex modulation formats leading to high hardwarerequirements and performances of good carriers declining. When SNR is higher than30dB the BER of system with adaptive unsaturated technique will be lower than10^-9.Finally, An OFDM adaptive unsaturated multimode fiber communication systemwas set up in optisystem7.0and the radio frequency spectrum and optical spectrumwere observed. These are useful foundations for the realization of physical experimentsystems. Constellation diagrams of OFDM multimode fiber commutations with andwithout adaptive unsaturated technology were compared. The results show that theconstellation diagram with adaptive unsaturated technology converges obviously.