This concept stays at the base of every cellular system and is named loopback. You need an RSSI (Received Signal Strength Indicator) in your RX downlink station, having steps translated in simple digital words (using an A/D converter) which are sent back to the uplink station. The uplink station will decode that digital information and will increase or decrease its output power, depending by the downlink order.
This loop needs a delay somewhere because is possible to be unstable if the changing of the power levels is too fast.