说明：  M2M4-该代码是经典的信噪比估计算法，用到了统计学中基于矩估计的思想，利用信号的2、4阶矩来估计接收信号的信噪比
(M2M4-the code is the classic SNR estimation algorithm used in the statistical moment estimation based on the idea, the signal of 2,4-order moments to estimate the signal to noise ratio of the received signal)