Research And Simulation On The Adaptive Modulation Techniques In OFDM System

Orthogonal frequency division multiplexing (OFDM) is an effective multiple carrier modulation technology, which can not only enhance the system frequency utilization, but also can effectively against multipath fading, restrain inter-symbol interference caused by multipath spread in communication system, It has got widespread concern in recent. The adaptive technology of OFDM systems is based on the characteristics of each child channel, adaptive adjustments of signal transmission parameters includes modulation mode, coding mode and transmit power. Using the method of adaptive modulation, the system could improve the utility ratio of frequency spectrum and reduce launching power, so it could use the frequency spectrum more effectively.Firstly, this paper introduces channel characteristics of wireless communication system and the basic principles of OFDM technology. On this basis, emphasis was put on OFDM channel estimation, single user bit power allocation algorithm and multi-user resource allocation algorithm were analyzed for further discussion. The channel estimation is important segment to adaptive technology. The principle and the steps of bit allocation algorithm based on single user OFDM system is introduced in detail. Performance of some classic bit allocation algorithms such as Hughes-Hartogs algorithm, Chow algorithm and Fischer algorithm are analyzed and compared. Based on the LS algorithm, channel estimation algorithm without considering the shortcoming of the noise affect, we propose an improved optimization scheme algorithm. Because of the channel response by training sequence get in frequency-domain, signal energy concentration and noise energy scattered in their time domain, separating out the noise energy using RLS algorithm. Further reduces the noise influence, improving performance.For adaptive modulation algorithm, the principle and optimization criterion of adaptive modulation is discussed in detail. Analysis of the classic algorithm, such as Hughes - Hartogs algorithm, Chow algorithm and Fischer algorithm was made. On the basis of Fischer algorithm,deduces new bits formulas using the approximation BER formula of modulation mode. The simulation results show that this algorithm is better than Fischer algorithm.