1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for transmitting a sounding reference signal in a MIMO wireless communication system and an apparatus therefor.
2. Discussion of the Related Art
FIG. 1 is a schematic view illustrating a general MIMO communication system. Multiple-Input Multiple-Output (MIMO) means a scheme that a plurality of transmitting antennas and a plurality of receiving antennas are used. Transmission and reception efficiency can be improved by the MIMO scheme. Namely, a transmitter or receiver of a wireless communication system can enhance capacity and improve throughput by using a plurality of antennas. Hereinafter, MIMO may be referred to as ‘MIMO antenna’.
The MIMO antenna technology does not depend on a signal antenna path to receive a whole message. Instead, in the MIMO antenna technology, data fragments received from a plurality of antennas are incorporated to complete data. If the MIMO antenna technology is used, a data transmission rate can be improved within a specific sized cell region, or system coverage can be enhanced with a specific data transmission rate. Also, the MIMO antenna technology can widely be used for a user equipment for mobile communication and a relay station. According to the MIMO antenna technology, it is possible to overcome limitation of a transmission rate in mobile communication according to the related art where a single antenna is used.
A schematic view of a general MIMO communication system is illustrated in FIG. 1. Referring to FIG. 1, NT number of transmitting antennas are provided at a transmitter while NR number of receiving antennas are provided at a receiver. If a plurality of antennas are used at both the transmitter and the receiver, theoretical channel transmission capacity is more increased than that a plurality of antennas are used at any one of the transmitter and the receiver. Increase of the channel transmission capacity is proportional to the number of antennas. Accordingly, the transmission rate is improved, and frequency efficiency is also improved. Supposing that a maximum transmission rate is RO when a single antenna is used, a transmission rate corresponding to a case where multiple antennas are used can be increased theoretically as expressed by the following Equation 1 as much as a value obtained by multiplying a maximum transmission rate RO by a rate increase Ri. In this case, Ri corresponds to a smaller value of NT and NR.R1=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system that uses four transmitting antennas and four receiving antennas, a transmission rate four times greater than that of a single antenna system can be obtained. After such theoretical capacity increase of the MIMO system has been proved in the middle of 1990, various technologies have been actively studied to substantially improve a data transmission rate. Some of the technologies have been already reflected in the standard of various wireless communications such as third generation mobile communication and next generation wireless LAN.
Upon reviewing the recent trend of studies related to the MIMO system, active studies are ongoing in view of various aspects such as the study of information theoretical aspect related to MIMO communication capacity calculation under various channel environments and multiple access environments, the study of radio channel measurement and model of a MIMO system, and the study of time space signal processing technology for improvement of transmission reliability and transmission rate.
In order to describe a communication method in a MIMO system in more detail, mathematical modeling of the communication method can be expressed as follows. As illustrated in FIG. 1, it is assumed that NT number of transmitting antennas and NR number of receiving antennas exist. First of all, a transmitting signal will be described. If there exist NT number of transmitting antennas, since the number of maximum transmission information is NT, the transmission information can be expressed by a vector shown in Equation 2 as follows.s=┘s1, s2, . . . , sNT└T  [Equation 2]
Meanwhile, different kinds of transmission power can be applied to each of the transmission information s1, s2, . . . sNT. At this time, supposing that each transmission power is P1, P2, . . . PNT, transmission information of which transmission power is controlled can be expressed by a vector shown in Equation 3 as follows.ŝ=[ŝ1, ŝ2, . . . , ŝNT]T=[P1s1, P2s2, . . . , PNTsNT]T  [Equation 3]
Also, Ŝ can be expressed by Equation 4 below using a diagonal matrix P.
                              s          ^                =                                            [                                                                                          P                      `                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =          Ps                                    [                  Equation          ⁢                                          ⁢          4                ]            
Meanwhile, it is considered that a weight matrix W is applied to the information vector Ŝ of which transmission power is controlled, so as to obtain NT transmitting signals x1, x2, . . . , xNT. In this case, the weight matrix serves to properly distribute the transmission information to each antenna depending on a transmission channel status. Such transmitting signals x1, x2, . . . , xNT can be expressed by Equation 5 below using a vector X. In this case, Wij means a weight value between the ith transmitting antenna and the jth information. W may be referred to as a weight matrix or precoding matrix.
                    x        =                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  1                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                i                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                  s                  ^                                            =              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
Meanwhile, in a spatial multiplexing scheme, each of transmission information s1, s2, . . . , sNT has different kinds of values. On the other hand, in a transmission diversity scheme, each of transmission information s1, s2, . . . , sNT has the same value. Examples of a general transmission diversity scheme include a space time block coding (STBC) scheme, a space frequency block coding (SFBC) scheme, and a cyclic delay diversity (CDD) scheme.
Meanwhile, in order to acquire diversity gain, channel measurement is necessarily required. Accordingly, for channel measurement for each of transmission paths through which diversity transmission is performed, sounding reference signals (SRSs) transmitted to each of the transmission paths are required. If a user equipment has two transmitting antennas, diversity can be acquired through channel measurement for transmission of two sounding reference signals. However, if a user equipment transmits sounding reference signals at the same power as that signals are transmitted through two transmitting antennas but has four transmitting antennas, the transmission power of the sounding reference signals is reduced to half, whereby channel measurement may be incorrect. Accordingly, in the case that the number of antennas increases under the state that the transmission power of the user equipment is limited, diversity gain is certainly overflowed at a certain range.
Hereinafter, in order that a user equipment having a plurality of antennas effectively transmits an uplink signal, a method for signaling after selecting a transmitting antenna of a user equipment at a base station and a method for transmitting a sounding reference signal from a user equipment to allow a base station to select an antenna will be suggested.