Study Of Adaptive Bit Allocation Algorithm For OFDM Systems

The orthogonal frequency division multiplexing was a novel and effective multiple carrier modulation technology, which was good at anti-noise and resisting the route disturbance from multiple paths as well as having high use ratio of frequency spectrum. So it was regarded as one kind key technique of the next generation mobile communication system, and it got more and more attention. In broad band OFDM system, different subcarrier has different transmission ability since the channel fade influence from different communication channels. If all the subcarrier channels adopt identical modulation methods, the system parameters need be designed according to the channel which had the worst transmission ability in order to ensure the system was able to work normally. That method could not use the system resource fully. By the method of adaptive modulation to each subcarrier wave carriers for OFDM system, the system could improve the utility ratio of frequency spectrum and reduce launching power, so it could use the wireless resource more effectively.Now we first introduced the wireless channel transfer characteristic and the basic principle of OFDM system, and then discussed the technology of channel estimation and the information theory basis of OFDM system. This provided a theoretical principle to the algorithm of adaptive bit allocation for OFDM system.And then we concluded all the algorithms of adaptive bit allocation for OFDM system of the recent 20 years, also we compared the classical algorithms which were Hughers-Hartogs algorithm, P.S.Chow algorithm and Fischer algorithm, and we carried out simulated analysis on the platform of OFDM system. On the basis of that we proposed an improved algorithm of adaptive bit allocation based on M-QAM modulation system. Through the detailed theoretical analysis and simulated experiment on that algorithm, the final result proved: the property of that algorithm approached the greedy algorithm on the function, but the complicated degree of our method was much lower than it obviously, so it could be used in the wireless data transmission of fixed velocity. This progress had pragmatic significance. Last we discussed the factors which affected adaptive modulation, which provided more theoretical principles for our realistic systematic design.