ofdm发送端和接收端，根据短训练字的特性进行相关运算，进行信号到达检测，当检测到相关值大于门限一定次数后，认为有信号到达。-ofdm sending end and receiving end, according to the characteristics of the short training word correlation operation, the signal reaching the detector, when the detected correlation value is greater than the threshold after a certain number of times that a signal arrives.