A question about how to pick the transmitter's IQ data from mixed IQ data

[p577665228]

I have a question about how to pick the transmitter's IQ data from mixed IQ data.

For example, as for zigbee. Firstly, we set a center frequency for USRP, so we can capture the IQ data from zigbee transmitter in the air. But we will also capture the IQ data, whose type is ACK, from zigbee receiver. So the IQ data we captured in air come from transmitter and receiver. How can we pick transmitter's IQ data from mixed IQ data?