Generally, data transmitted through a radio channel in a wireless communication system may have signal distortion. In order to normally decode the distorted signal by a receiving side, information on a transmission channel has to be provided, and the distortion of the received signal has to be compensated based on the information on the transmission channel. In order for the receiving side to check the information on a transmission channel, a transmitting side has to transmit a signal recognized by both the transmitting side and the receiving side. The receiving side can check the information on the transmission channel with reference to a distorted degree of the signal received through the transmission channel. Here, the signal recognized by both the transmitting side and the receiving side is called a ‘pilot signal’ or a ‘reference signal (RS)’.
Recently, a wireless communication system enhances the efficiency of data transmission and reception by adopting a multiple input multiple output (MIMO) scheme for transmitting data with using multiple transmit antennas and multiple reception antennas. In the communication system based on the MIMO scheme, each transmit antenna is provided with an independent channel. Accordingly, a channel state between each transmit antenna and each reception antenna can be checked by transmitting a reference signal independent from each transmit antenna.
Generally, a reference signal may be classified into a reference signal for acquiring channel information, and a reference signal for demodulating data.
The reference signal for acquiring channel information is used for a user equipment (UE, or terminal) to acquire downlink channel information. Since even a UE which does not receive downlink data in a specific subframe has to receive and measure the reference signal, the reference signal for acquiring channel information is transmitted to a broadband. The reference signal for acquiring channel information is also used to measure a handover, etc.
On the other hand, the reference signal for demodulating data is transmitted by being included in a corresponding resource when a base station transmits data to a UE through a downlink. A UE receives the reference signal for demodulating data, thereby performing channel estimation and demodulating received data. Accordingly, the reference signal for demodulating data is transmitted by being included in a data transmission region.
In a long term evolution (LTE) system and an LTE-Advanced system where a standardization operation is being performed, it is required to support data transmission and reception on a downlink through a plurality of transmit antennas of a base station. However, when the base station supports 4˜8 transmit antennas, an efficient method for allocating and transmitting a reference signal has not been defined.
In order to reduce inter-cell interference and to enhance a communication performance of a UE positioned at the edge of a cell in a multi-cell environment, a coordinated multi-point transmission/reception (CoMP) method has been proposed. The CoMP method indicates a communication method for two or more base stations or cells with a corresponding UE in a coordinated (cooperative) manner so as to more smoothly perform a communication between a specific UE and a base station (eNB, access point) or a cell.
The CoMP method may be classified into two types according to a data transmission configuration from each base station or each cell. According to a joint processing (CoMP-JP) method, data is simultaneously transmitted to a UE from each base station which performs a CoMP operation, and the UE couples the signals received from the respective base stations to each other thus to enhance a reception performance. On the other hand, according to a cooperative scheduling/beamforming (CoMP-CS) method, data is instantaneously transmitted from one base station to one UE, and scheduling or beamforming is performed so as to minimize interference on the UE resulting from other base station.
According to the CoMP method, the UE commonly receives data information and control information from a multi-cell base station. This may enhance a communication performance of the UE positioned at the edge of a cell. In order to perform a CoMP operation, a CoMP cluster, a cluster of a base station such as BS, eNB, cell and Access Point capable of substantially performing a CoMP operation has to be firstly determined. A cluster configuration method may be classified into a method for configuring and managing a cluster by a system or an additional base station controller, and a method for configuring and managing a cluster by each UE according to a subject of a configuration.
FIG. 12 is a view schematically showing a method for configuring a CoMP cluster.
FIG. 12(a) shows a method for configuring a cluster by a system. Referring to FIG. 12(a), each UE measures interference amounts from adjacent base stations, and obtains an average of the measured interference amounts for a predetermined time period. Then, the UE transmits the average information to a system. Based on the received information, the system configures a cluster by collecting eNBs expected to have the most enhanced transmission performance owing to a large amount of mutual interference when using a CoMP operation. The number of base stations which belong to one cluster may be variable according to an environment, and each cluster may have a different number of base stations. For convenience, FIG. 12(a) shows a case that each cluster has three base stations.
FIG. 12(b) shows a case that a UE configures a cluster. UE1 and UE2 measure interference amounts from adjacent base stations, and obtains an average of the measured interference amounts for a predetermined time period, respectively. Then, the UE1 and UE2 directly configure a cluster by collecting eNBs expected to have the most enhanced transmission performance owing to a large amount of mutual interference when using a CoMP operation. Then, the UE1 and UE2 exchange information on the configured cluster with the system. Like in FIG. 12(a), it is assumed that one cluster consists of three base stations in FIG. 12(b). The number of base stations which belong to one cluster may be variable, and each cluster may have a different number of base stations.
In the CoMP system, a reference signal has to be allocated and transmitted so as to perform channel measurement and data demodulation. However, an efficient method to allocate and transmit a reference signal has not been defined in the CoMP system.