What are LBT (listen before talk) and AFA, and do they replace duty cycle?
LBT (Listen Before Talk) is the transmitter listening to the channel before starting to transmit and confirming that it is free, while AFA (Adaptive Frequency Agility) is moving to another channel if the channel is busy. In ERC/REC 70-03, LBT+AFA is not an additional condition on top of the duty cycle limit but an alternative to it: in 863-870 MHz, LBT+AFA may be applied instead of the 1% duty cycle.
Duty cycle is the crude but simple way of sharing a band: if everyone talks little, nobody drowns anybody out. LBT+AFA solves the same aim more intelligently.
LBT switches the receiver on immediately before transmitting and measures the energy in the channel. If it is below the defined threshold the channel is considered free and the packet is sent. If it is above the threshold the transmitter waits.
AFA exists so that this wait does not turn into a permanent blockage: if the channel is busy, the device hops to another channel. Applied together, band usage adapts itself to the actual traffic.
The critical point from a regulatory standpoint is that this is an alternative. In the 863-870 MHz sub-bands you either comply with the 1% duty cycle limit or you apply LBT+AFA; you are not required to do both. In 869.4-869.65 MHz the same logic applies for the 10% duty cycle.
In a remote control application LBT has a cost: the listening period adds latency to every packet, and if the channel is continuously busy a command can be delayed. This is why safety-related stopping functions are not tied to LBT; the timeout on the receiver side is arranged so that the machine goes to a safe state when no command arrives.
Full topic: Wireless remote control installation: pairing, relay, safety