High-speed QVLMS-MLSE equalizer for mobile communications

This paper considers the adaptive maximum-likelihood sequence estimation (MLSE) equalizer as a means of delay distortion compensation that is indispensable in high-speed signal transmission of megabit order in mobile communications. Improvement of the processing speed and reduction of the circuit scale are discussed. As a method for channel estimation we propose the quantized variable-gain least-mean-squares (QVLMS) algorithm, in which multiplication processes are eliminated by approximating the gain function in the VLMS algorithm by a power of 2. A method for reducing the circuit scale is also proposed, utilizing symmetry of mapping rotation in the PSK/QAM modulation. Based on the proposed method, a two-tap equalizer for QPSK modulation was constructed in the laboratory, and it is shown that the circuit can be realized at a scale of approximately 40 kgates even at the FPGA level. An equalizer was also constructed for a two-wave independent multipath Rayleigh fading channel with a transmission rate of 1.5 Mbits/s and a delay spread of 650 ns, and excellent performance was realized, reducing the floor error to less than 10^-5 and completing initial convergence in eight symbols.