Computer Simulation, Analysis of Predistortion, Adaptive Equalization of Digital Satellite and Digital Microwave Radio Systems with Nonlinear Transmit Amplifiers and with Multipath Propagation
In this paper, we analyze the performance of predistortion and adaptive equalization to compensate for the nonlinearity of satellite and digital radio channels with multipath propagation (Rummler's model) and additive noise. The study is carried out using 8-PSK, 16-PSK, 32-PSK, 16-QAM, 64-QAM, and 256-QAM modulation. In the transmitter and receiver, Butterworth's digital filters were simulated. Included in the analysis are several types of TWTA tubes (having AM/AM and AM/PM characteristic approximations with up to 7th order polynoms) and predistortion circuits. In the receiver, we simulated the following equalizers: a) linear; b) fractionally spaced; c) decision feedback; d) nonlinear adaptive, proposed by Falconer (for MQAM); and e) nonlinear adaptive, proposed by Benedetto and Biglieri (for MPSK). The nonlinear theory of Norbert Wiener (i.e., Vito Volterra) and an LMS algorithm for the coefficient tap adjust of the equalizer has been proposed. Our research indicates that the Block update algorithm is better for real-time applications because it reduces the processing complexity of the computation of the equalizer coefficients in each symbol interval. In addition, the required number of coefficients of the equalizer can be reduced from that needed for the original Wiener algorithm.
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