I am looking at using a customized version of ESB on the nRF52 where there will be multiple remote devices that share a time period to transmit to a central node (no ACKs). I don't want to use full packet time slots for each remote device as there could be 30 or more, and they will generally transmit infrequently. So I would like to use a series of time slots that are less than the full packet time but long enough that the remote device can check if a different device has started transmitting in a preceding slot. Effectively it would be a combination of clear channel assessment and time slots.
So my question is, what is the earliest indication in receive mode that a packet reception has started? I know there is an event at the end of address reception, but ideally I would like to get notification when the preamble is complete and address reception is starting. Is there any way to do this?
An alternative could be to check the receive power level rather than trying to identify an actual packet reception. I could check the RSSI in the preceding slot, but this will also detect non network interference, so I would prefer to know if a valid packet is actually being received.