Research On Decision Feedback Blind Equalization Algorithm Based On Different Error Functions

ISI can be eliminated with blind equalization algorithms that need no training sequence. Of all blind equalization algorithms, the Constant Modulus Algorithm (CMA) has simple structure, but it has slow convergence and large mean square error. Especially to channel of frequency response with obvious fluctuation, CMA has worse equalization performance. But Decision Feedback Filter (DFE) with nonlinear feedback filter is more suitable to channels with different type. Besides the structure of equalizer, influence factor of equalization effectiveness is related to error term of iteration formula for computing weight coefficient. DFE algorithm has different equalization performance when error terms are different.Decision feedback blind equalization algorithms are studied based on the analyse of different error functions and Constant Modulus Algorithm (CMA). Simulation results with channel of frequency response with obvious fluctuation. The research consists of several aspects of as follows:1. Analyzing constant modulus blind equalization algorithms based on different error functionsFrom gradient descent method, the bigger the value of gradient norm of algorithm, the better convergence rate and the larger mean square error is, and vice versa. Hence, an error function with variable weighted coefficient is defined to control value of gradient norm. The characteristic of the proposed algorithm can be changed according with different values of weighted coefficient defined by the author. Thus, a CMA based on the error function of variable weighted coefficient (VCMA) is proposed.2. Analyzing decision feedback blind equalization algorithms based on different error functionsFor channel of frequency response with obvious fluctuation, the constant modulus decision feedback equalizer (CMDFE) has better convergence and less mean square error than CMA algorithm. In order to improve convergence rate of CMDFE, above mentioned error function based on variable weighted coefficient is applied to this equalization system and a decision feedback blind equalization algorithm based on the error function of variable weighted coefficient (VCMDFE) is proposed.3. Analyzing dual-mode decision-feedback blind equalization algorithms based on different error functionsFor making full use of the advantage of each Bussgang blind algorithm and eliminating the disadvantage, we do research on three kinds of dual-mode decision-feedback blind equalization algorithms.1) Combining VCMDFE and Decision Directed (DD), a dual-mode blind equalization algorithm based on the error function of variable weighted coefficient is proposed.2) With the aid of better convergence of CMDFE, an iteration threshold (hereinafter as limit) is introduced to iteration process to modify judging conditions of the dual-mode decision-feedback algorithm based on constant modulus (CMDFE-DD). At the beginning of iteration, iterations adopted CMDFE are equal to iteration threshold, and then switch is carried out based on the decision circle ring. Thus, a dual-mode decision-feedback blind equalization algorithm based on constant modulus and by introducing iteration threshold is proposed.3) VCMDFE algorithm has better convergence than CMDFE algorithm. Similarly, by using above mentioned idea of iteration threshold, a dual-mode decision-feedback blind equalization algorithm of variable weighted coefficient by introducing iteration threshold is proposed.4. Designing different equalizer-modules based on different structures and different algorithmsWith the aid of Simulink library, equalizer-modules in papers based on different structures and different algorithms are designed under simulation modeling environment of Simulink.1) With the aid of linear equalizer (LE) modules with CMA in the library, LE-modules based on frequently used error functions and the error functions defined by the author are designed.2) With the help of decision modules and concatenation modules in the library, DFE-modules based on frequently used error functions and the error functions defined by the author are designed.3) According to the signal that is not constant modulus, a dual-mode DFE-module combining CMDFE and DD is designed. In order to improve convergence rate of the above dual-mode DFE-module, the author has designed two kinds of different dual-mode DFE-modules, based on error function of variable weighted coefficient and introducing iteration threshold.