[Discuss-gnuradio] How to detect collision when two packets are transmitted simultaneously from two transmitters.

I have a setup of one receiver and two transmitters. I am implementing a TDMA structure (using USRPs and GNURADIO) where only one packet is sent in each slot. When both transmitters try to transmit in the same slot, collision occurs. I would like to know

1. How can the receiver detect whether a collision has occurred or not?

2. What happens to the collided packets?

Regards,

Sumedha