Research On Adaptive Modulation Algorithm For OFDM Systems

As the wireless mobile communications and the rapid development of wireless LAN, wireless communications need to use limited spectrum resources to achieve high-speed data transmission and multimedia services.With shortage of spectrum today, reasonably making use of spectrum is becoming increasingly important, so it is necessary to take the high spectrum efficiency and good anti-fading ability of technology to improve system performance.Orthogonal frequency division multiplexing (OFDM) as a special multi-carrier transmission technology has good resistance to multipath propagation and frequency selective fading ability. So adaptive modulation technology as one of the best ways to improve spectrum efficiency can be as much as possible to improve transmission efficiency and system performance.Adaptive modulation technology provides higher spectrum efficiency and the technology of OFDM can effectively combat multipath fading,so the adaptive modulation technology used in OFDM system can constitute complementary technology.It is suitable for future high-speed and high quality of service requirement mobile communication systems.The paper introduced the development status of the wireless communication and the history and research on the technology of adaptive modulation in OFDM system.Systematica-lly introduces several classical radio channel transmission model.In-depth study of the Greedy,Fischer,Chow and SBLA algorithm,and analyze the performance of these traditional algorithms.This paper studies two kinds of adaptive modulation algorithms for link transmission system. The first algorithm is based on Campello algorithm, not considering target bit error rate and fixed-step single-bit defects. A flexible and optimal bit allocation algorithm is proposed, which added in the preassigned bit error rate factor, and improved system flexibility.Then using the way of variable bit-step adjusts the sub-carrier transmission bits and power,speeds up the convergence rate.Simulation results show that the algorithm to ensure performance, while reducing the complexity of the algorithm.The second algorithm direct at the problem of power margin to fixed SNR adaptive modulation threshold,proposing the maximum throughput algorithm,which take full use of the power margin,adjust the sub-carrier modulation,the simulation results show that the new algorithm ensure the quality of information transmission while improving transmission efficiency of the system.