These are the OIDs you have to use and This signal strength is only for currently-associated clients, and only for the most recent packet received from those clients, it is not a general-purpose scan facility.
awcDot11TpFdbLatestRxSignalStrength 1.3.6.1.4.1.522.3.12.5.1.9
awcDot11TpFdbLatestRxSignalQuality 1.3.6.1.4.1.522.3.12.5.1.10