Performance of limited-state sequence estimation under land mobile frequency-selective fading channels

The frequency-selective fading is one of the problems in the high-speed transmission in land mobile communications. A solution is to use the Viterbi equalizer. When this method is applied to the highly efficient modulation system such as QPSK, however, the computational complexity increases rapidly with the increase of the delay time of the delayed wave to be compensated. This makes the instrumentation impractical. This paper proposes a method in which the computational complexity is reduced drastically by the limited-state sequence estimation. In the proposed method, limited-state sequence estimation (Viterbi algorithm) is applied where two waves with the largest powers are selected based on the estimated impulse response. Based on the tentative decision obtained beforehand by a simple method, the effects of other waves are eliminated. The number of states is estimated considering only the selected two waves. The performance of the proposed method is examined by a computer simulation. It is shown as a result that the range of compensation can be enlarged and the computational complexity can be reduced, compared to the conventional simplified method. In other words, the effectiveness of the proposed method is demonstrated.