METHOD OF MULTIUSER PRECODING AND SCHEDULING AND BASE STATION FOR IMPLEMENTING THE SAME BACKGROUND OF THE INVENTION FIELD OF INVENTION The present invention relates to radio Multiple Input Multiple Output (MIMO) communication system, and particularly to method of multiuser (MU) precoding and scheduling, which can generate multiuser precoding matrix and scheduling scheme for multiuser precoding and scheduling by using channel state information (CSI, which may be inaccurate due to various factors such as feedback quantization, feedback delay and the like) and statistic properties of CSI error fed back from a user side. DESCRIPTION OF PRIOR ART Recently, MU-MIMO has attracted much attention due to its advantage in capacity as well as the ability to work with single-antenna users (SU) and still maintain MIMO advantages. Compared with SU-MIMO, the transmission processing of MU-MIMO is complicated due to the fact that each user must decode its messages independently without joint operation with other users. The core problem is how to solve co-channel interference (CCI) among users. To solve this problem, multiuser precoding techniques are used in the MU-MIMO system to control or totally avoid CCI among users, so that each user suffers no or only limited interference from other users. For a total avoidance or effective control of CCI, full CSI for all users is required at the transmitter, which is only an assumption impractical to actual systems. In practice, full CSI is difficult to be achieved, and thus imperfect CSI is always used at the transmitter. With such imperfect CSI, the CCI among users cannot be totally avoided even using zero-forcing-type precoding algorithms. The residua! CCI due to imperfect CSI cannot be suppressed at the receiver by common interference-suppression methods such as maximum-likelihood (ML) or minimum-mean-square-error (MMSE) detection. As a result, the CCI can only be regarded as additive noise whose average power grows with an increase in the total transmission power This characteristic of the CCI restricts significantly the performance of MU-MIMO, especially at high SNRs. SUMMARY OF THE INVENTION In order to address deterioration of MU-MIMO performance caused by the residual CCI due to imperfect CSI at the transmitter, the present invention provides a method of multiuser precoding and scheduling in which multiuser precoding matrix and scheduling scheme for multiuser precoding and scheduling is generated by using CSI information and statistic properties of CSI error fed back from a user. The precoding matrix and scheduling scheme thus generated can adapt to a situation that channel information at the transmitter is imperfect, reduce the residue CCI, and thus reduce the deterioration of MU-MIMO performance due to imperfect CSI. An object of the present invention is to provide a method of multiuser precoding and scheduling, comprising: feeding CSI and statistic properties of CSI error back to a base station (BS) from a user equipment (UE); generating, at the BS, multiuser precoding matrix and scheduling scheme according to the feedback CSI and the statistic properties of CSI error; and performing multiuser precoding and scheduling on user data by using the generated multiuser precoding matrix and scheduling scheme. Preferably, the CSI is an estimation of a channel matrix. Preferably, the statistic properties of CSI error comprise a covariance matrix of errors of the estimation of the channel matrix. Preferably, the multiuser precoding uses a minimum-mean-square-error (MMSE)-type algorithm. Preferably, the MMSE-type algorithm is a successive MMSE algorithm. Preferably, the multiuser scheduling uses a capacity-maximization criterion. Preferably, the statistic properties of the CSI error are obtained by measuring channel estimation error, feedback error and quantization error. Preferably, the method is used in a MU-MIMO communication system. The present Invention also provides a base station, comprising: a reception device for receiving CSI and statistic properties of CSI error which are fed back from a UE; a multiuser preceding matrix and scheduling scheme generation device for generating multiuser preceding matrix and scheduling scheme according to the feedback CSI and the statistic properties of CSI error; and a multiuser preceding and scheduling device for performing multiuser preceding and scheduling en user data by using the generated multiuser preceding matrix and scheduling scheme. BRIEF DESCRIPTION OF THE DRAWINGS The objects, advantages and characteristics of the present invention will be more apparent, according to descriptions of preferred embodiments in connection with the drawings, wherein: Fig. 1 is a flow chart of a method of multiuser preceding and scheduling according to the present invention; Fig. 2 is a block diagram of a BS for implementing the method of multiuser preceding and scheduling according to the present invention; and Figs. 3 and 4 are graphs of performance comparison between the method according to the present invention and the prior art method. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With imperfect CSI, the transmitter cannot generate a multiuser preceding matrix that exactly matches the multiuser channel. Such mismatch leads to additional CCI among users, which increases with the transmission power and seriously restricts the performance of MU-MIMO, especially at high SNRs. The CCI caused by CSI error cannot be suppressed at a receiver by common interference-suppression methods such as maximum-likelihood (ML) and MMSE detection. The effect is similar to an increase in additive noise. The basic idea of the present invention is to study the relationship between such CCI level and the preceding/scheduling results, and to use this relationship to adjust the preceding and scheduling algorithms to better suit the imperfect CSI environment. According to the present invention, a method of multiuser preceding and scheduling is provided to better control the CCI level due to CSI error and improve MU-MIMO performance with imperfect CSI at the transmitter. In the present invention, it is assumed that the statistical properties of CSI error are available at the transmitter. For multiuser preceding, a MMSE-type algorithm is generally used, due to its ability to balance interference and noise. Here, the CCI caused by CSI error may be regarded as AWGN(Additive White Gaussian Noise), and a relationship betweenthe average power of CSI error and a preceding/filtering matrix may be established based on a variance matrix of CSI error. Then, the CCI level, preceding and filtering matrixes are optimized jointly according to the MMSE criterion. In the present invention, for multiuser scheduling, we adopt the capacity-maximization criterion and select a user/mode subset with the maximum achievable sum capacity Similarly a relationship between the CSI-error-related CCI level and the user/mode selection may be established and used to adjust scheduling operation, by regarding the CCI as AWGN in estimating the sum capacity for each user/mode subset. Hereinafter, the principle of the method of multiuser preceding and scheduling according to the present invention will be described in detail. Channel model Consider a downlink of a multiuser MIMO system with A/r transmit antennas at the BS and K users receiving service from the same BS over the same time-frequency resource, each with NR receive antennas. (To be noted, K is the number of users served in the same time-slot and frequency band by spatial processing. The total number of users in a cell may be much larger than K). Assuming frequency-flat fading for all users, a channel matrix for user k is denoted by H,=[h['"^], where h["^„\s the fading coefficient between the nth transmit antenna and the mth receive antenna of user k. The number of data streams dedicated to user/c is denoted by Sk. It is always assumed that Sk