The Research On Adaptive Modulation Technique In OFDM System

Data transmission in wireless channels with high speed and reliability is required in future wireless systems. OFDM with the advantage of high spectrum efficiency and easy realization, can strongly combat multi-path and dispersive channel interference, so its application is remarkable in recent years. OFDM which combining adaptive modulation technique is a very important researching field at present . Aiming at this very field, we study the AOFDM technology in this paper.First of all, this paper introduces the basic theory of OFDM briefly .Considering channel estimation is the base of AOFDM, then discussing carefully Three categories of pilot based channel estimation in AOFDM: ML criteria based,MMSE criteria based and LS criteria based. The performance and implementation complexity of these three methods are analyzed and compared. The best position of pilot tones and some implementation issues are also discussed. A simple but effective spread estimation method is also presented. the joint estimation of channel response . Finally, Supposing that channel estimation is complete, a closed form relation between modulation switching levels and target BER is first derived. Then the effects of the more practical issues such as estimation error and the estimation delay are considered, which results in a method that could be used when channel estimation is incomplete. In order to testify the feasibility and validity of these two algorithms, I set up an AOFDM system using simulation tools, Simulation results are given as well.This result shows that the models can improve the variety of multimedia services ,to transimit with faster speed and higher spectrum efficiency.