The present invention provides a subband determining method, a base station, user equipment, and a communications system, each of which is used for information feedback. 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 (i) selects some subbands to form a subband set and assign the subband set to user equipment (UE) and (ii) notifies the user equipment (UE) of information about the subband set. The user equipment (UE) 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 (UE) further (i) divides the subband set into its corresponding subbands on the basis of a result found and (ii) 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, fed back from the user equipment (UE), so as to not only secure a quality of service of a wireless cell, but also improve system efficiency. This method allows feedback to be carried out for downlink information with ease and high efficiency. This in turn reduces load for uplink wireless transmissions and improves feedback precision so as to save wireless resources and improve system efficiency.