说明：  基于MVDR算法，计算两个信号的相关性的函数。
(This program computes the coherence function between 2 signals x1 and x2 with the MVDR method. This algorithm is based on the paper by the same authors: J. Benesty, J. Chen, and Y. Huang, "A generalized MVDR spectrum," IEEE Signal Processing letters, vol. 12, pp. 827-830, Dec. 2005. x1, first signal vector of length n x2, second signal vector of length n L is the length of MVDR filter or window length K is the resolution (the higher K, the better the resolution))