The Study Of Adaptive Modulation Technique For OFDM

Orthogonal frequency division multiplexing (OFDM) is a novel kind of multi-carriers modulating technique of high efficiency. It develops from MCM (Multi-Carriers Modulation) and it can effectively resist the multipath propagation. Thus we are able to receive the signal disturbed reliably. The quadrature modulation and demodulation of each subchannel can be realized with fast Fourier inverse transformation (IFFT) and fast Fourier transforms (FFT). IFFT and FFT can be easily implemented with the development of large scale integrated circuit technology and the DSP technology. In the wireless communication, as a result of the object moving or other reasons, the channel is time-varying. If we design communications system according to the worst environment, we can not utilize the overall channel capacity completely when channel condition is good. It is a quite important problem to employ limited frequency band resources effectively today, when the wireless frequency resources is more and more sparse day and day. We can use a higher modulation by adaptive modulation technique when the channel condition is good. The overall system frequency utilization efficiency can be improved greatly, and the system capacity can be doubled. Firstly, the basic structure and the application fields of OFDM is introduced, and then the reason of the mobile multipath channel's forming and its influence on signals is elaborated. This letter mainly deals with the base of adaptive modulating technique: Channel estimation technology and the signal to noise ratio vs. error bit rate performance of MQAM. The three channels estimates algorithms: LS criterion, MMSE criterion and the improved version of MMSE are analyzed and compared in the performance and realization complexity. The paper also discusses three kinds of channel estimation interpolation algorithms: Linear interpolation, two steps interpolation and DFT interpolation. The emphasis of this paper is the parameter choice of the adaptive modulation and the detailed deduction of function relation between the signal to noise ratio and error bit rate of MPSK, rectangular MQAM and the circular MQAM modulation under the Gauss channel and the Rayleigh channel. And then it discussed two ways to choose the switch threshold, one is based on the limitation of immediately error bit rate and another is based on the limitation of average error bit rate. On the base of the discussion of OFDM adaptive modulating technique ahead, the Matlab/Simulink simulation model is constructed and the simulation of OFDM baseband system is carried. This letter proposes an efficient approach to realize MQAM modulation based on FPGA, which makes use of the symmetry characteristic of FIR filter coefficients, canonic signed-digit and multiplexer to simplify the design of MQAM modulator. The system needs no multiplier. The result demonstrates the method has some improvements in size and internal fmax. It is an ideal method to design MQAM modulator.