What do you mean by radio sensing a channel?
The station here is continuously monitoring the energy in the medium. Let us say you want to transmit in channel 1 of 2.4GHz band. Your RF hardware is always in receive mode if you don't want to transmit anything. You are continuously sampling the incoming signal in this particular channel. You use ADC to sample and compute the energy $E = \sum |x[n]|^2 $ of these samples corresponding to window of say, 100us. This is a simplified concept of sensing. You don't need to transmit anything to sense the channel. The value $E$ is computed in terms of $dBm$ unit.
After the above step, the $dBm$ value is compared against a threshold specified in standard. For example, let us say the minimum sensitivity needed for receiving QPSK modeulated data is -60dBm. You need to make sure that your transmission is above this. So you sense the channel. If the RSSI (that is $E$ computed above) is more than -60dBm, then you cannot transmit as the channel is BUSY. If it is less than -60dBm, you can transmit. But to be on safer side you can give a margin of say 5dBm. For higher modulation order (like 256QAM) the threshold will be lesser as you need to have very less ambient noise while transmitting 256QAM signals.