说明：  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
(The code is a classic signal to noise ratio estimation algorithm, using the statistical moment estimation based on the idea of the use of 2,4-order moment of signals to estimate the received signal to noise ratio)