Research and Simulation on the Adaptive Modulation Techniques in OFDM System
|School||Chongqing University of Technology|
|Course||Signal and Information Processing|
|Keywords||Orthogonal Frequency Division Multiplexing Channel estimation Adaptive modulation Resource allocation|
Orthogonal frequency division multiplexing (OFDM) technology is a multi-carrier modulation technique. It can not only improve the frequency utilization, also can be effective against multipath fading, suppress the inter-symbol interference caused by multipath propagation, to get more and more attention in the field of communication. Adaptive technique, of the OFDM system in accordance with the characteristics of the individual subchannels, the adaptive adjustment signal transmission parameters, including modulation scheme, coding, transmit power, etc., to achieve a full utilization of the channel, to improve the transmission performance of the communication system. This paper first introduces the basic principles of the channel characteristics of the wireless communication system and the OFDM technology. On this basis, focus on OFDM system channel estimation, adaptive bit and power allocation algorithm for in-depth study. Channel estimation is an important part of adaptive modulation technology. This article focuses on several channel estimation algorithm, based channel estimation based on the linear minimum mean square error (LMMSE) criterion of least squares (LS) criterion channel estimation and DFT (Discrete Fourier Transform)-based channel estimation algorithm. Principle of algorithm, the algorithm complexity and estimated performance comparison study and simulation. On this basis, for which the channel estimation algorithm for its shortcomings did not consider the effects of noise, an improved channel estimation scheme based on LS criteria. By the guide frequency sequence to obtain the channel response when domain signal energy concentrated while the noise energy dispersion successful introduction of adaptive noise canceling techniques leverage known noise, offset filtered out of the noise mixed in the signal energy of, and further reduce the channel estimation the influence of noise, thereby improving the performance of the estimator. Simulation to verify the correctness and practicality of the improved algorithm. Adaptive modulation theory and optimization criteria for adaptive modulation algorithm, discussed in detail, and conducted in-depth analysis and simulation comparison of typical algorithms such as the Hughes-Hartogs algorithm, Chow algorithm, and Fischer algorithm. And on the basis of the Fischer algorithm, proposed an improved adaptive modulation algorithm. Approximate BER formula modulation derived bit allocation formula to replace the traditional bit allocation formula, take advantage of the water injection threshold parameter, the bit allocation optimal, in the premise of low computational complexity, and improve the algorithm properties. And simulation analysis, with the signal-to-noise ratio increased to improve the effect is more obvious.