Research On Compensation Techniques Of Nonlinear Satellite Channel

Channel nonlinearities were the primary factors to deteriorate performances of high speed digital satellite communication systems. Nonlinear distortions were introduced by the saturation effects of satellite amplifiers and group delay distortion. Predistortion and equalization techniques were pragmatic to counteract nonlinear distortions. Accomplishments to compensate nonlinear distortions of satellite channel in this paper were presented as follows:Firstly, amplitude-phase model and quadrature model were reviewed. Group delay distortion model based on finite impulse response filter were proposed. According to group delay parameters, taking advantage of formula method and frequency sampling method, filter coefficients of linear group delay and parabolic group delay were obtained. Influences of system parameters to results were also discussed.Secondly, an efficiency optimization criterion was described to constellation optimization of 16APSK modulation. To compensate the nonlinearity of high power amplifier, an adaptive baseband signal predistorter based on fixed point method was proposed. By virtue of the contracting mapping theorem for nonlinear equations, updating of the table entries could be considered as a fixed point problem. Steffensen's acceleration technique was used to improve the rate of convergence.Then, Volterra equalizer was another crucial technique in mitigating nonlinearity of high power amplifier. However, the equalizer converges slowly. To combat this problem, variable step-size frequency domain LMS algorithm was applied to the Volterra equalizer. Comparing to fixed step-size time domain algorithm, the frequency algorithm can suppress the correlation of the inputs and adjusts the step-size dynamically, so as to increase the convergence process.Finally, constant modulus algorithm was robust in opening the channel eye. Decision directed algorithm had fast convergence rate and low steady state error. Normalized LMS algorithm could adjust the step size dynamically, so as to improve the convergence rate. Combining the advantages mentioned above, a weighted dual-mode blind equalization algorithm was proposed against group delay distortion. The algorithm reduces the complexity of the receivers by avoiding switching over between two different modes. And it improves the stabilities of dual-mode algorithm.