1.  Field of the Invention
The present invention relates to channel information transmission method and, more particularly, to a method for transmitting channel information in a multi-antenna system. 
2.  Description of the Related Art
Due to generalization of information communication services and the advent of various multimedia services and high-quality services, demand for communication services is radically increasing.  To meet the demand, diverse wireless communication technologies are under study in multiple sectors. 
A multi-input multi-output (MIMO) system refers to a system improving a data transmission/reception efficiency by using multiple transmit antennas and multiple receive antennas.  Recently, research on the channel capacity of a multi-user MIMO system for allowing multiple users to effectively use space resources has been actively processing. 
A dirty paper coding (DPC) scheme, in which a base station removes data of other users acting as interference in advance, to thus reduce interference from other users, provides a maximum channel capacity in the multi-user MIMO system.  However, a large amount of channel information and complex calculations (i.e., computations, operations) required at a transmission end make it difficult to implement the DPC scheme in an actual system. 
For a substantial implementation, a PU2RC (Per User Unitary and Rate Control) proposed by Samsung Electronics Inc. in “Downlink MIMO for EUTRA” of 3GPP TSG RAN WG1 #44/R1-060335 is a scheme in which spatial resources are simultaneously allocated to multiple users.  According to this scheme, each user selects a precoding matrix and vector that can maximize their channel transmission rate from among a plurality of precoding matrixes, and feeds back an index of the vector and a signal-to-interference plus noise ratio (SINR) to a base station. 
The Related Art PU2RC scheme uses multi-user diversity in a spatial area.  Thus, it exhibits good performance when the number of users is large, but if there are not many users, the PUR2C scheme cannot ensure sufficient multi-user diversity. 
As a solution to this problem, the method of using a Zero Forcing receiver and an MMSE SIC (Minimum Mean Squared Error Successive Interference Cancellation) receiver, which was proposed in “Adaptive Mode Switching in the Gaussian MIMO Broadcast Channel” by C. Lee, C. B. Chea, S. Vishwanath, R. W. Health, Jr. in Proc.  of the IEEE Radio and Wireless Symposium, Orlando, Fla., in January 2000, adaptively employs a multi-user multi-input/output scheme and a single user multi-input/output scheme.  In this scheme, when a signal-to-noise ratio (SNR) is high and the number of matrix codebook is 1, the fact that two types of transmission rates intersect according to the number of users is checked and the intersection is theoretically calculated.  However, this scheme has a problem in that each user should hold the two types of receivers and it can be hardly applied when there are several precoding matrixes or when the SNR is low. 
In addition, there has been proposed a technique for adaptively changing the number of signals using space resources as disclosed in “Multimode Precoding for MIMO Wireless Systems,” by D. J. Love, R. W. Health Jr. in IEEE Trans.  Signal Process.  Vol.  53, No.  10, pp.  3674-3687 in October 2005.  This technique considers only the single user multi-input/output scheme, and because there is no need to consider channel status of other users to determine an optimum mode, a user can determine an optimum mode by using his channel and only needs to feed back channel information only with the determined mode.  However, when the technique is applied to the multi-user multi-input/output scheme, because modes requested by each user to maximize a transmission rate are different, channel information of each user should be all fed back or channel information with respect to every candidate mode should be fed back to determine an optimum mode in the system.  Thus, if this technique is applied as it is to the multi-user multi-input/output scheme, the amount of feedback information would be hugely increased. 
Therefore, an object of the present invention is to provide a method for transmitting/receiving channel information that exhibits good performance regardless of the number of users or an SNR of users. 
Another object of the present invention is to provide a method for transmitting/receiving channel information capable of reducing the amount of information fed back from a UE to a base station.