A subband determining method, a base station, user equipment, and a communications system are provided. The base station of the present invention finds the subband size on the basis of a system bandwidth and divides the system bandwidth into subbands. The base station further selects some subbands to form a subband set and assign the subband set to user equipment and notifies the user equipment of information about the subband set. The user equipment in turn finds its corresponding subband size and the number of subbands for use in feedback on the basis of the size of the assigned subband set. The user equipment further divides the subband set into its corresponding subbands on the basis of a result found and feeds back information about a predetermined number of subbands to the base station. The base station carries out optimization of its transmitter on the basis of the information.