Designing a simple energy-efficient communication protocol?
You create a "master" that is not reliant on battery power. It transmits RF markers at fixed intervals so that if a sensor needs to transmit it can recognize its time-slot. If the sensor wakes up and listens, it can calculate when to wake up again to transmit at the correct time-slot. This should minimize slave energy usage.

The master needs to transmit the active address for the current active time-slot and also the maximum address so that randomly listening slaves know how long to go to sleep for before their time-slot arrives.

Can you take it from here?