In a wireless OFDM multi-carrier communication system, a mobile station obtains measurement result of a first received signal strength indication (RSSI) of a first preamble signal transmitted from a base station over a first radio frequency (RF) carrier. The mobile station also obtains signal offset information between the first RF carrier and a second RF carrier of the base station. In response to the measurement result and the signal offset information, the mobile station estimates a second RSSI of a second preamble signal transmitted by the base station over the second RF carrier without performing scanning over the second RF carrier. In one novel aspect, the mobile station identifies a carrier group containing multiple carriers that belong to the same base station. Measurement optimization, initial cell selection and cell reselection optimization, report reduction, and other carrier group based optimization can be achieved in the multi-carrier system based on identified carrier group.