Research And Simulation Of Adaptive Modulation Algorithm For OFDM Systems

In the mobile radio channels, the high speed and high quality of communication service are the target and requirement for wireless communication. OFDM has attracted people’s attention because of it’s high spectrum efficiency and the effectively combat the frequency selective fading channels. By combining the adaptive modulation with the OFDM technology,different modulation ways and appropriate transmission power allocation in different sub-carrier can be used to match the channel and improve utilization of resources.In this thesis, we first introduce the wireless channel characteristics, the basic principle and key technology for OFDM system, then study the basic principles and the development of the adaptive modulation techniques.In the adaptive OFDM system,adaptive modulation is also called as bit loading,which means that different modulation approaches can be chosen according to channel conditions in different subcarrier and then adaptive links are realized.According to the OFDM system,water-filling algorithm is a optimal bit loading algorithm as its transimission rate is minimum.Afer the study of the greedy algorithm, Chow algorithm, Fisher algorithm, an improved greedy algorithm is proposed. Under the same bit error rate, the new algorithm reduce the number of iteration and complexity and optimize system performance and improve the system performance.Based on the MIMO-OFDM system, adaptive resource allocation problem is studied firstly.After comparng the two approaches of subcarriers bit power allocation by two step and joint allocation and analizing the influence of the sending and receiving antennas’number to system performance, we propose a improving subcarriers bit power allocation algorithm. In the algorithm we propose a subcarrier complexing approach is offered to two users to send data in the same subcarrier. With the joint allocation of subcarrier and bit power, we improve the procedure of bit re-adjustment. Showed by the simulation results, the algorithm we propose can decrease the computation complexity without influence on bit error of system. After studied the rate optimal (RA criteria) algorithms of sending power and the upper limit withoutchanged of bit error rate, a iterative approach with memory is proposed, which can decrease the number of iteratin caused by the discrete of bits. Based on the approach with memory, we also propose a improving water-filling RA algorithm. The simulation results show that comparing to the optmal RA algorithm, the algorithm we propose achieve approximate optmal transmission rate with the cost of little iteratians’ increasement.