Research On The Application Of Adaptive Modulation In OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology for high data rate transmission in wide band wireless system due to its ability to mitigate the effects of frequency selective and combat inter-symbol interference. Adaptive modulation can significantly improve the system spectral efficiency by changing its modulation constellation and transmit power according to the instantaneous Channel State Information (CSI). In frequency selective fading channel, the combination of OFDM and adaptive modulation can utilize the merits of both technologies and significantly improve the frequency efficiency and transmit rate. Therefore the adaptive OFDM system is attracting more and more research attention.When the adaptive modulation is applied in the OFDM system, there are lots of problems to be solved. How to reduce the transmit power while each user's rates and Quality of Service (QoS) being satisfied? How to keep the performance of the system when the channel state information is imperfect? How to further improve the performance of the adaptive OFDM system so that the utilization of the frequency band is more efficient? These problems are some important issue in the study of the OFDM system and should be well solved.This dissertation majorly focuses on the application of adaptive modulation in OFDM system. Our study can be summarized in the following six aspects:1. The bit and subcarrier allocation problem in the multiuser OFDM sytem is a non-linear optimization problem. Based on the consideration that the genetic algorithm has strong ability to solve such problems, we propose a genetic-based algorithm to allocate bit and subcarrier in adaptive multiuser OFDM system. We also improve the convergence of the genetic algorithm through the method of adding some individuals with 'good' genes to the initial population. Simulation results show that the proposed algorithm has good performance.2. For the bit and subcarrier allocation problem in the multiuser OFDM sytem, we use compound constellation to eliminate the restriction in most studies that a subcarrier cannot be shared by different users.We also delvelop an equation to calculate the required power for the compound constellations. Then we propose a multiuser water-filling algorithm for the bit and subcarrier allocation based on compound constellation. The...