说明：  4FSK调制解调的matlab代码。采用相干解调的方式4FSK进行解调，与大多数方案的包络检波不同。基带信号用四种电平来表示，与调制信号相区分。
(Demodulation matlab code 4FSK modulation. Coherent demodulation 4FSK demodulates Unlike most programs envelope detector,. The baseband signal is represented with four level distinguish phase with the modulation signal.)