Indeed, taken from Jaap Braam’s explanation of his single channel gateway:
Detecting a signal using CAD takes less than two symbols. So when a SF7 CAD returns, two symbols of the preamble are ‘used’, leaving 6 more to synchronize the reading of the message. When SF7 CAD does not detect a preamble, two SF7 symbols are gone which acounts for ONE SF8 symbol. So there are 7 preamble symbols available to detect a SF8 signal.
For a SF9 signal there will be 8-(1+0.5) = 6.5 symbols available
For a SF10 signal there will be 8-(1+0.5+0.25) = 6.25 symbols available
For a SF11 signal there will be 8-(1+0.5+0.25+0.125) = 6.125 symbols available
For a SF12 signal there will be 8-(1+0.5+0.25+0.125+0.0625) = 6.0625 symbols available.So there are always enough preamble symbols left to try to detect a higher SF signal, if the lower SF detection fails.
In order to make this work it is important to detect the presence of a signal on the channel as soon as possible, the RSSI detection in FSK mode can be used to do that.
A drawback of this approach is that the range of this gateway will be less of that of a ‘real’ gateway; it can only receive messages that can be detected by RSSI.