A method for requesting a Base Station (BS) to assign sub-channels to a Subscriber Station (SS) in a communication system, The method includes determining a channel quality of sub-channels assigned using a normal sub-channel assignment scheme, transmitting, to the BS, a first codeword of a Channel Quality Indicator CHannel (CQICH) indicating a transition from the normal sub-channel assignment scheme to a band Adaptive Modulation and Coding (AMC) sub-channel assignment scheme, during a sub-channel assignment scheme transition based on the determined channel quality, transmitting a second codeword of the CQICH and a report response message in response to a report request message received from the BS, and transmitting, to the BS, differential Channel Quality Indicators (CQI) of the selected bands.