Study On Adaptive Modulation Algorithm In OFDM Multimode Fiber Communication

As the increase of requirement for communication services, people pay more attention to the improvement of short distance networks. Currently, the data rate required for local area networks has increased to over 1 Gbps .On this condition, the optical fiber becomes the perfect medium. Due to it can support high data rate and small attenuation .Fiber to home, fiber to desk is the inevitable tendency in communications. Optical fibers can be categorized by the number of guide mode. That is single-mode fiber and multimode fiber. Due to the relative low costs of multimode fiber, the usage of multimode fiber is very common in short distance networks. Through analysis and simulation, we know that the frequency response of the multimode fiber at low frequencies decreases monotonically, and in high frequency range, the frequency response does not fall off but becomes relatively flat. Since OFDM can be used to overcome the effect of frequency selectivity in the dispersive multimode fiber , it just can be used in the high frequency region with flat frequency response .But, the performance of the system will be effected by the deep nulls in the channel. How to solve the effect of the nulls on system is the key point of making OFDM multimode fiber communication technology come true. And the adaptive modulation technology is one of the effectively method to solve the deep nulls in the channel. Studying and finding a adaptive modulation algorithm suitable for the multimode fiber channel is just the major work of this paper.To simulate the adaptive modulation algorithm in the multimode fiber, a new channel model which is fit for simulation is proposed. Based on deeply studying current adaptive modulation algorithm of improving communication system and combining OFDM multimode optical fiber communication features, In order to make comparisons with other adaptive modulation algorithms, the dissertation uses the current optimization algorithm to the system of OFDM multimode fiber communication. And a improved Greedy algorithm is proposed and use in this system. At last , a new adaptive modulation algorithm which is base on the adaptive unsaturated technique is proposed , both the improved Greedy algorithm and the new algorithm can improve the transmission performance of the system and effectively. Especially, the new algorithm base on the adaptive non-saturated has a significant theoretical and practical value. The main works lie:First, a new multimode fiber model which is fit for simulation is proposed .In this section ,several old multimode fiber models are describe, through function fitting,a simple model which can describe the performance of the multimode fiber channel exactly is found . Through simulating by the QQ map in MATLAB, we get the result that the new model is conforming for the old multimode fiber models very well.Second, the current significant adaptive bit assignment algorithm theories are analyzed, base on these theories, the implement of the algorithm is learnt deeply, and a current best adaptive bit assignment algorithm is used in multimode fiber channel. The simulation result shows that the algorithm can improve the BER effectively. Especially, this algorithm provides a good comparison model with other improved algorithm and new algorithm.Third, a proved adaptive bit assignment algorithm base4 on threshold is proposed. The theory and the flow about the algorithm are described in detail. And the algorithm is used in the OFDM communication system. The algorithm is simulated on different bit rate, different transmission performance. At last, the bit assignment result is compared with the best algorithm. The result shows that adaptive modulation algorithm can improve the transmission performance of the system markedly. the bit assignment result of the proved greedy algorithm and the best algorithm are very close.Finally, a new algorithm base on the adaptive non-saturated is proposed, the theories about the algorithm are described and analyzed in detail. And then, the algorithm is used in the OFDM communication system. The new algorithm is no longer assignment bits on the limited bandwidth 3dB, but assignment bits by the time. So , the bandwidth resources can be used very well. The implement of the algorithm is very simple and the modulation format kinds used is less than the old algorithm. So the new algorithm is very fit for the OFDM multimode fiber communication system. And then , the new algorithm overcome the disadvantage in other algorithm that BER on some of the sub-carriers are extremely worse. The simulation result shows that the BER of every sub-carrier can be improved 10e4 times through sacrificing less bandwidth by using the new algorithm. And it can insure every sub-carrier can reach the target BER.