I've got a couple of questions for people:
1) What is the shortest preamble only that could be sent in order to guarantee a RSSI interrupt for a RFM69 receiver? Is it a matter of a few bit periods?
and
2) What is the power penalty in waking up a RFM69 from sleep, transmitting this short burst, and immediately going back to sleep again?
I'm wondering whether it's possible to lock on to a RSSI sequence even though there may be noise from RSSI events from other transmitters around. In analogue video for example you can lock on to a sync and extract it from noise even if it has been 'suppressed', i.e. DC level shifted into the video level during the horizontal blanking period. This is done by opening a gate to sample falling edges and locking a PLL to the edges it finds. I wonder whether a similar technique can be used for recovering a known RSSI sequence in hopping when locking a receiver to it.