With the rapid spread of wireless communication devices, such as smart phones, tablet PCs, and the like, the amount of mobile communication data has also rapidly increased at an average annual rate of 50%˜200%. In response to the rapid increase in the amount of mobile communication data, a variety of technologies have been developed to enhance data transmission speed. One of the most reliable methods for increasing data transmission speed is to use a frequency bandwidth as broad as possible. However, it is difficult to secure a broader frequency bandwidth from the currently used frequency bandwidth for mobile communication systems.
Another method for increasing data transmission speed or capacity of wireless communication systems is to increase the number of antennas between transmitters and receivers for communication. The method employing a number of antennas is a technology using an array of antennas, that is, Multi-user Multiple-Input and Multiple-Output (Multi-user MIMO). The technology makes it possible to transmit data at a high speed, thereby increasing the capacity of systems.
In order to implement a system using a number of antennas, the transmitters require information about channels to communicate with the receivers for receiving data. For example, if a network operates in Frequency Division Duplex (FDD), the receiver receives a Reference Signal (RS) to provide channel information to the transmitter, estimates channel information using the RS, and feeds it back to the transmitter through an uplink. More specifically, the transmitter transmits a preset Reference Signal (RS) to the receiver. The receiver receives the RS from the transmitter, estimates a channel based on the RS, quantizes a state of the estimated channel, and feeds the information about the quantized channel back to the transmitter.
For example, an LTE-Advanced network using a number of antennas, the receiver uses a Channel State Information Reference Signal (CSI-RS) to estimate channel information. In addition, LTE-Advanced network is designed in such a way that CSI-RSs have orthogonality by antennas to achieve precise channel estimation.
This network design increases the amount of resources to transmit CSI-RS with orthogonality for every antenna in proportion to the increased number of antennas in the transmitter.
Although the conventional system increases the number of antennas to step up the capacity, it causes to increase the amount of resources to estimate a channel because of the characteristics of CSI-RS, thereby decreasing the efficiency of resource use.