In an LTE-advanced (LTE-A) system, coordinated multi-point (CoMP) transmission is brought into the LTE-A frame work as one of the key technologies. In a coordinated transmission scenario, geographically neighboring transmission points are used to cooperatively transmit signals to user equipment. Signal quality may be improved and coverage may be expanded particularly for cell-edge users. In a CoMP scenario, coordinated transmission points (TPs) participating in data transmission may be divided into a serving point (similar to the serving cell in LTE Rel-8) and a cooperating point.
In coordinated transmission, much attention is paid to a precoding technology based on close-loop feedback. In 3GPP LTE-A standardization, a joint transmission (JT) scheme in a CoMP scenario mainly includes global precoding, MBSFN and local precoding.
As shown in FIG. 1a, the global precoding is to estimate a composite channel matrix of an serving point and cooperating point in an transmission end by user (UE, user equipment), and estimate channel state information (CSI) according to a composite channel of a relatively large dimensional space. The CSI includes a precoding matrix indicator (PMI), a channel quality indicator (CQI) and rank indication (RI). Wherein, the PMI is obtained by search in a codebook of a relatively large dimensional space, corresponding to precoding information of a composite channel. The precoding matrix of each TP in an transmission end is a sub-matrix in the precoding matrix to which the PMI corresponds. The global precoding is able to obtain an optimal coordinated transmission performance, but the UE end needs to search a codebook space of a larger dimension for calculating the precoding matrix, with a complexity in calculation. Furthermore, in such a mode, the UE needs to additionally feed back CSI of the single link from itself to the serving point, so as to support a serving mode where the UE falls back to a conventional signal cell.
As shown in FIG. 1b, the MBSFN is to transmit identical data to the UE by using identical precodes by each TP participating in the data transmission in a coordinated scenario. Such a method is simple in operation, the UE needs only to feed back one piece of precoding matrix information, the searched codebook space is small, and the overhead in feedback is less; however, the coordinated transmission performance is poor.
As shown in FIG. 1c, the local precoding is to transmit identical data to the UE by each TP participating in the data transmission by using respective independent precoding matrix in a coordinated scenario. In comparison with the global precoding and the MBSFN, such a method obtains compromised results with respect to complexity, feedback overhead and performance. In comparison with the global precoding, in feeding back PMIs of multiple single-points, the local precoding needs no extra feedback information for supporting the UE fallback mode.
In the implementation of the present invention, the inventors found that although the local precoding may obtain a compromised system performance, it strictly requires that each TP participating in coordinated transmission in the transmission end transmits data of the same layer and hence complete data interaction between an serving point and a cooperating point in the transmission end is needed. A case as follows is neglected in such conventional local precoding scheme: in data of multiple layers transmitted by the serving point, the channel quality experienced by a certain layer data (such as the 1st layer data) is obviously superior than that of another layer data (such as the 2nd layer data). In such a case, a certain layer data experiencing a better channel quality needs not to be transmitted by the cooperating point. Therefore, other cooperating points than the serving point need not to transmit data of all the layers, and may select data of part of the layers for transmission, such as selecting a layer data with worst quality for transmission.
Thus, in a CoMP transmission scenario, in the process of transmission of data of multiple layers, channel qualities experienced by data of different layers are different, and different precoding methods may be selected, so as to achieve flexible adaptive coordinated transmission in a transmission end.
It should be noted that the above description of the background art is merely provided for clear and complete explanation of the present invention and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background art of the present invention.