The next-generation multimedia wireless communication systems which are recently being actively researched are required to process and transmit various pieces of information, such as video and wireless data as well as the initial voice-centered services. The 4th generation wireless communication systems which are now being developed subsequently to the 3rd generation wireless communication systems are aiming at supporting high-speed data service of downlink 1 Gbps (Gigabits per second) and uplink 500 Mbps (Megabits per second). The object of the wireless communication system is to establish reliable communications between a number of users irrespective of their positions and mobility. However, a wireless channel has abnormal characteristics, such as path loss, noise, a fading phenomenon due to multi-path, Inter-Symbol Interference (ISI), and the Doppler Effect resulting from the mobility of a user equipment. A variety of techniques are being developed in order to overcome the abnormal characteristics of the wireless channel and to increase the reliability of wireless communication.
Technology for supporting reliable and high-speed data service includes Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and so on.
An OFDM system is being considered after the 3rd generation system which is able to attenuate the ISI effect with low complexity. The OFDM system converts symbols, received in series, into N (N is a natural number) parallel symbols and transmits them on respective separated N subcarriers. The subcarriers maintain orthogonality in the frequency domain. It is expected that the market for mobile communication will shift from the existing Code Division Multiple Access (CDMA) system to an OFDM-based system.
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology includes spatial multiplexing, transmit diversity, beam-forming and the like. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
In wireless communication systems, it is necessary to estimate an uplink channel or a downlink channel for the purpose of the transmission and reception of data, the acquisition of system synchronization, and the feedback of channel information. In wireless communication system environments, fading is generated because of multi-path time latency. A process of restoring a transmission signal by compensating for the distortion of the transmission signal resulting from a sudden change in the environment, caused by such fading, is called channel estimation. Channel estimation, in general, is performed using a reference signal (RS) which is known to a transmitter and a receiver.
In OFDM systems, a method of assigning a reference signal includes a method of assigning a reference signal to all subcarriers and a method of assigning a reference signal between data subcarriers. The method of assigning a reference signal to all sub-carriers is performed using a signal comprising only a reference signal, such as a preamble signal, in order to achieve the performance of channel estimation. If this method is used, the performance of channel estimation can be improved as compared with the method of assigning a reference signal between data subcarriers because, in general, the density of reference signals is high. However, the method of assigning a reference signal between data subcarriers is used in order to increase the amount of transmission data because the amount of transmission data is reduced. If such a method is used, the density of reference signals is reduced, thereby deteriorating the performance of channel estimation. To minimize such deterioration, the reference signals should be properly arranged.
Meanwhile, a wireless communication system including relay stations is recently being developed. The relay station functions to expand the cell coverage and improve the performance of transmission. If a base station provides services to a user equipment, placed at the boundary of the coverage of the base station, through a relay station, the cell coverage can be expanded. Furthermore, if a relay station improves reliability in the transmission of signals between a base station and a user equipment, the amount of transmission data can be increased. Although a user equipment is placed within the cell coverage of a base station, it may use a relay station placed in the shadow region.
Uplink and downlink between a base station and a relay station is a backhaul link, and uplink and the downlink between a relay station and a user equipment is an access link. Hereinafter, a signal transmitted through the backhaul link is called a backhaul signal, and a signal transmitted through the access link is an access signal. If a large amount of radio resources are allocated in order to transmit a backhaul signal between a base station and a relay station, radio resources necessary to transmit an access signal between the relay station and a user equipment become short, and the efficiency in the use of the radio resources is lowered.
In the backhaul link, there is a need for a method of efficiently transmitting a reference signal.