The most important requirement of a next-generation wireless access system is to support a high data transfer rate. To achieve this, various technologies such as Multiple Input Multiple Output (MIMO), Cooperative Multiple Point Transmission (CoMP), relay, etc. have been developed and studied.
Joint Processing (JT) is a representative base station (BS) coordinated scheme. However, JT requires a high rate backhaul between the base stations (BSs) to implement data sharing, so that there is a limitation to use the JT scheme when there is no high rate backhaul. If the base stations (BSs) share only channel information, Interference Alignment (IA) and Coordinated Beamforming (CBF) may be used. Whereas IA can implement the degree of freedom (DoF) that linearly increases in proportion to the number of users (i.e., user equipments) in the same manner as in JT, the IA has a disadvantage in that all UEs must recognize all channel information and must perform symbol extension to a time axis or a frequency axis. CBF need not perform symbol extension, and feeds back a smaller amount of channel information than the IA, so that the CBF can be relatively easily implemented as compared to the IA scheme.
The above-mentioned BS operation schemes (JT, IA, CBF) are characterized in that all receivers must feed back a variety of CSI (Channel State Information). However, according to a frequency division duplex (FDD) system in which a total of feedback bits are limited, as a feedback link increases, CSI accuracy of each feedback link is deteriorated, so that the number of feedback links significantly affects system performance.