(1) Definition of Multi-Input Multi-Output (MIMO) Technology
MIMO technology means a method for improving efficiency in data transmission and reception by using multiple transmitting antennas and multiple receiving antennas instead of a single transmitting antenna and a single receiving antenna. Namely, the MIMO technology is to increase capacity or improve throughput by using multiple antennas in a transmitter or receiver of a wireless communication system. Herein, multiple antennas will be referred to as MIMO.
The MIMO technology is an application version of a technology that a single message is completed by receiving data pieces from several antennas without depending on a single antenna path. The MIMO technology can improve a data transmission rate in a specific range or increase a system range for a specific data transmission rate. In this respect, the MIMO technology is a next generation mobile communication technology that can widely be used for a mobile station and a relay station. Also, the MIMO technology has received much attention as a next generation technology that can increase a transmission rate of mobile communication, which has reached the uppermost limit due to extension of data communication.
(2) System Modeling in MIMO
FIG. 1 is a schematic view illustrating a general MIMO communication system. As illustrated in FIG. 1, if the number of transmitting antennas is N7 and at the same time the number of receiving antennas increases to NR, channel transmission capacity increases theoretically in proportion to the number of antennas unlike that a plurality of antennas are used in only a transmitter or receiver. Accordingly, it is possible to improve a transmission rate and remarkably improve frequency efficiency. A transmission rate according to increase of channel transmission capacity can increase theoretically as much as a value obtained by multiplying a maximum transmission rate Ro, which corresponds to a case where one antenna is used, by an increase rate Ri of the following Equation 1.Ri=min(NT,NR)  [Equation 1]
For example, in a MIMO communication system that uses four transmitting antennas and four receiving antennas, a transmission rate that four times bigger than that of a single antenna system can be obtained. After 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 N7 transmitting antennas and NR receiving antennas exist. First of all, a transmitting signal will be described. If there exist N7 transmitting antennas, since the number of maximum transmission information is N7, 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 
Also, ŝ can be expressed by Equation 4 below using a diagonal matrix P.
                    s        =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                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 N7 transmitting signals x1, x2, . . . , xNT. In this case, the weight matrix serves to properly distribute the transmission information to each antenna. 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 i th transmitting antenna and the j th 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                                                      iN                            T                                                                                                                                                              ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            w                                                                                    N                              T                                                        ⁢                            1                                                                                                                                                w                                                                                    N                              T                                                        ⁢                            2                                                                                                                      …                                                                                              W                                                                                    N                              T                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                ⁡                                  [                                                                                                                                          s                            ^                                                    1                                                                                                                                                                                          s                            ^                                                    2                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                    j                                                                                                                                    ⋮                                                                                                                                                                  s                            ^                                                                                N                            T                                                                                                                                ]                                                                                                        =                            ⁢                              W                ⁢                                  s                  ^                                                                                                        =                            ⁢              WPs                                                          [                  Equation          ⁢                                          ⁢          5                ]            
If there exist NR receiving antennas, receiving signals y1, y2, . . . , yNR of the respective antennas can be expressed by a vector of Equation 6 below.y=[y1,y2, . . . ,yNR]  [Equation 6]
Meanwhile, in case of channel modeling in the MIMO communication system, channels can be classified depending on indexes of transmitting and receiving antennas. In this case, a channel that passes from the jth transmitting antenna to the ith receiving antenna will be expressed as hij. It is noted that index of the receiving antenna is prior to index of the transmitting antenna in index of hij.
Several channels can be grouped into one and then can be expressed by a vector type or a matrix type. The channels of a vector type will be described below. FIG. 2 is a diagram illustrating a channel from N7 transmitting antennas to the ith receiving antenna.
As illustrated in FIG. 2, the channel from N7 transmitting antennas to the ith receiving antenna can be expressed as follows.hiT=└hi1,hi2, . . . ,hiNT┘  [Equation 7]
Also, all channels from N7 transmitting antennas to NR receiving antennas can be expressed by Equation 8 below through the matrix of Equation 7.
                    H        =                              [                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                                              ⋱                    ⁢                                                                                                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          iN                      T                                                                                                                    ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Since additive white Gaussian noise (AWGN) is actually added to the channels after the above channel matrix H, AWGN n1, n2, . . . , nNR added to each of the NR receiving antennas can be expressed by a vector of Equation 9 below.n=[n1,n2, . . . ,nNR]  [Equation 9]
The receiving signals obtained using the above equations can be expressed by Equation 10 below.
                                                        y              =                            ⁢                              [                                                                                                    y                        1                                                                                                                                                y                        2                                                                                                                        ⋮                                                                                                                          y                        i                                                                                                                        ⋮                                                                                                                          y                                                  N                          R                                                                                                                    ]                                                                                        =                            ⁢                                                                    [                                                                                                                        h                            11                                                                                                                                h                            12                                                                                                    …                                                                                                      h                                                          1                              ⁢                                                              N                                T                                                                                                                                                                                                                                      h                            21                                                                                                                                h                            22                                                                                                    …                                                                                                      h                                                          2                              ⁢                                                              N                                T                                                                                                                                                                                                          ⋮                                                                                                                                                                                                                                      ⋱                            ⁢                                                                                                                                                                                                                                                                                                                                                                                                  h                                                          i                              ⁢                                                                                                                          ⁢                              1                                                                                                                                                            h                                                          i                              ⁢                                                                                                                          ⁢                              2                                                                                                                                …                                                                                                      h                                                          iN                              T                                                                                                                                                                            ⋮                                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                                                  h                                                                                          N                                R                                                            ⁢                              1                                                                                                                                                            h                                                                                          N                                R                                                            ⁢                              2                                                                                                                                …                                                                                                      h                                                                                          N                                R                                                            ⁢                                                              N                                T                                                                                                                                                                          ]                                    ⁡                                      [                                                                                                                        x                            1                                                                                                                                                                            x                            2                                                                                                                                                ⋮                                                                                                                                                  x                            j                                                                                                                                                ⋮                                                                                                                                                  x                                                          N                              T                                                                                                                                            ]                                                  +                                  [                                                                                                              n                          1                                                                                                                                                              n                          2                                                                                                                                    ⋮                                                                                                                                      n                          i                                                                                                                                    ⋮                                                                                                                                      n                                                      N                            R                                                                                                                                ]                                                                                                        =                            ⁢                              Hx                +                n                                                                        [                  Equation          ⁢                                          ⁢          10                ]            
(3) Definition of Reference Signal
When a mobile communication system transmits packets, since the packets are transmitted through a radio channel, signal distortion may occur during transmission of the packets. In order to normally receive the distorted signal, a receiver should find out channel information and correct distortion of the signal as much as the channel information. In order to find out the channel information, when a signal known by both the transmitter and the receiver is received, the receiver finds out the channel information using a distortion status of the received signal. The signal known by both the transmitter and the receiver will be referred to as a pilot signal a reference signal (RS). The RS includes four types as illustrated in Table 1 depending on its purpose of use.
TABLE 1FeaturesCQI-Common RS for channel measurement. Since UE(UseCRSEquipment) determines CQI(Channel Quality Indicator),PMI(Precoding Matrix Index), RI(Rank Indicator), etcbased on the CQI-CRS, the CQI-CRS is preferablydistributed over all bands.DM-Common RS for demodulation. But DM-CRS may be used forCRSchannel measurement. Since several UEs use the DM-CRSfor channel measurement, a precoding scheme of a specificUE cannot be applied to the DM-CRS. Accordingly, ifPDSCH(Physical Downlink Shared CHannel) is precoded,signaling is required through PDCCH(Physical DownlinkControl CHannel).NDM-Dedicated RS for non-precoded demodulation.DRSPDM-Dedicated RS for precoded demodulation. Since precodingDRSis equally applied to both PDM-DRS and PDSCH, signalingis separately required for PDCCH.
Recently, when most of mobile communication systems transmit and receive data packets using multiple antennas, a separate reference signal for each of transmitting antennas exists, and should know a channel status between each transmitting antenna and each receiving antenna to normally receive a signal.
Meanwhile, FIG. 3A is a diagram illustrating a structure of RS of normal cyclic prefix (CP) in a 3GPP LTE, and FIG. 3B is a diagram illustrating a structure of RS of extended cyclic prefix (CP) in a 3GPP LTE. In the current 3GPP LTE, a structure of a downlink RS is as illustrated in FIG. 3A and FIG. 3B. In one resource block, a horizontal axis means a time axis while a vertical axis means a frequency axis. One subframe includes two slots. FIG. 3A corresponds to a normal CP, wherein each slot includes seven symbols. FIG. 3B corresponds to extended CP, wherein each slot includes six symbols. The extended CP is generally used under the environment that delay is long. The number of base station transmitting antenna ports which are supported is four, and resources used to transmit RS for each of the antenna ports 0˜3 are represented by 0, 1, 2 and 3.
(4) Time/Frequency Spacing of RS
In the 3GPP LTE, when considering 20 MHz system, one symbol includes 2048 samples. In this case, symbol duration is 66.67 μs. 1 μs corresponds to 10−6 seconds. Meanwhile, in case of normal CP, 160 samples are used in the first one of seven symbols as CP, and 144 samples are used in the other six symbols as CP. At this time, each of the symbols can cover channel delay spread equivalent to 5.2 μs and 4.69 μs. In case of the extended CP, since six symbols include 512 samples, they can cover channel delay spread equivalent to 16.67 μs. As seen from FIG. 3A and FIG. 3B, a transmission interval between RS for same antenna ports on the frequency axis of one symbol is 6. However, since staggering is given within one slot, a subcarrier interval between RS on the frequency axis is finally 3. In the present invention, the interval will mean a subcarrier interval on the frequency axis unless specified otherwise. In this case, it is assumed that the subcarrier interval is 15 kHz.
Meanwhile, the transmission interval of the RS and its channel estimation capability will be considered as follows using equations.
For example, if inverse discrete fourier transform is applied to sequence x[i]={x0, x1, x2, . . . , xN-1}, the following Equation 11 can be obtained.
                                          X            ⁡                          [              k              ]                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                          x                ⁡                                  [                  i                  ]                                            ·                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  k                  ⁢                                      ⅈ                    N                                                                                      ,                  k          =          0                ,        1        ,        …        ⁢                                  ,                  N          -          1                                    [                  Equation          ⁢                                          ⁢          11                ]            
Since RS is sent between data at constant intervals, if 0 is filled in N number of data between the respective RSs, sequence
      y    ⁡          [      i      ]        =      {                  x        0            ,                                    0            ,            0            ,            …            ⁢                                                  ,            0                    ︸                          z          ⁢                                          ⁢          zeros                    ,              x        1            ,                                    0            ,            0            ,            …            ⁢                                                  ,            0                    ︸                          z          ⁢                                          ⁢          zeros                    ,              x        2            ,      …      ⁢                          ,              x                  N          -          1                    ,                                    0            ,            0            ,            …            ⁢                                                  ,            0                    ︸                          z          ⁢                                          ⁢          zeros                      }  is obtained. If IDFT is applied to y[i], the following Equation 12 can be obtained.
                                          Y            ⁡                          [              k              ]                                =                                    ∑                              m                =                0                                                              N                  ⁡                                      (                                          Z                      +                      1                                        )                                                  -                1                                      ⁢                                          y                ⁡                                  [                  m                  ]                                            ·                              ⅇ                                  j                  ⁢                                                                          ⁢                  2                  ⁢                  π                  ⁢                                                                          ⁢                  k                  ⁢                                      m                                          N                      ⁡                                              (                                                  Z                          +                          1                                                )                                                                                                                                ,                                  ⁢                  k          =          0                ,        1        ,        …        ⁢                                  ,                              N            ⁡                          (                              Z                +                1                            )                                -          1                                    [                  Equation          ⁢                                          ⁢          12                ]            
In the Equation 12, since y[m] and x[i] are
  {                                                                        y                ⁡                                  [                  m                  ]                                            =                              x                ⁡                                  [                  i                  ]                                                      ,                                                              m              =                                                (                                      Z                    +                    1                                    )                                ·                i                                      ,                                                              i              =              0                        ,            1            ,            …            ⁢                                                  ,                          N              -              1                                                                                                      y                ⁡                                  [                  m                  ]                                            =              0                        ,                                                otherwise            ,                                                                                   the relationship between Y[k] X[k] can be observed using the Equations 11 and 12 in such a manner that signals are repeatedly generated as much as the number of 0 as illustrated in Equation 13.
                              Y          ⁡                      [            k            ]                          =                  {                                                                                          X                    ⁡                                          [                      0                      ]                                                        ,                                      X                    ⁡                                          [                      1                      ]                                                        ,                  …                  ⁢                                                                          ,                                      X                    ⁡                                          [                                              N                        -                        1                                            ]                                                                      ︸                                            N                ⁢                                                                  ⁢                samples                                      ,            …            ⁢                                                  ,                                                                                X                    ⁡                                          [                      0                      ]                                                        ,                                      X                    ⁡                                          [                      1                      ]                                                        ,                  …                  ⁢                                                                          ,                                      X                    ⁡                                          [                                              N                        -                        1                                            ]                                                                      ︸                                            N                ⁢                                                                  ⁢                samples                                              }                                    [                  Equation          ⁢                                          ⁢          13                ]            
For example, if the interval of RS is 6, six signals are repeatedly generated in accordance with the Equation 13 as illustrated in FIG. 4. In this case, if channel delay spread does not exceed 11.11 μs, signal recovery can be performed theoretically. Namely, in case of the normal CP, since the number of CP samples is 160, 144, signal recovery can be performed even in the case that the transmission interval of RS is 6 when channel delay spread of maximum 5.2 μs and 4.69 μs occurs.
Meanwhile, in case of the extended CP, since the number of CP samples is 512, it is supposed that channel delay spread is maximum 16.67 μs. Accordingly, a problem occurs in that interference exists between symbols as illustrated in FIG. 5.
In order to solve the aforementioned problem, in the LTE, staggering is applied to RS of each antenna port, so that the transmission interval of the RS becomes 3. As illustrated in FIG. 6, if the transmission interval of the RS becomes 3, channel delay spread that can be covered reaches 22.22 μs, whereby signal recovery can be performed regardless of the length of the CP.
(5) Cyclic Delay in Time Domain
If sequence x[i]={x0, x1, x2, . . . , xN-1} is cyclic shifted on the time axis, the relationship with the frequency axis will be observed as illustrated in Equation 4 below.
                                                                                                                                    X                      ′                                        ⁡                                          [                                              k                        ′                                            ]                                                        =                                    ⁢                                                            ∑                                              i                        =                        0                                                                    N                        -                        1                                                              ⁢                                                                                            x                          ⁡                                                      [                            i                            ]                                                                          ·                                                  ⅇ                                                      j                            ⁢                                                                                                                  ⁢                            2                            ⁢                            π                            ⁢                                                                                          k                                ′                                                            N                                                        ⁢                            ⅈ                                                                                              ⁢                                              ⅇ                                                  j                          ⁢                                                                                                          ⁢                          2                          ⁢                          π                          ⁢                                                      u                            M                                                    ⁢                          ⅈ                                                                                                                    ,                                                                                                        ⁢                                                      N                    =                    MZ                                    ,                                                            k                      ′                                        =                    0                                    ,                  1                  ,                  …                  ⁢                                                                          ,                                      N                    -                    1                                                                                                                          =                                ⁢                                                      ∑                                          i                      =                      0                                                              N                      -                      1                                                        ⁢                                                            x                      ⁡                                              [                        i                        ]                                                              ·                                          ⅇ                                                                        j2π                          ⁡                                                      (                                                                                                                            k                                  ′                                                                +                                uZ                                                            N                                                        )                                                                          ⁢                        ⅈ                                                                                                                                ⁢                                  ⁢                                            X              ′                        ⁡                          [                                                (                                      k                    -                    uZ                                    )                                ⁢                mod                ⁢                                                                  ⁢                N                            ]                                =                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                          x                ⁡                                  [                  i                  ]                                            ·                              ⅇ                                  j2π                  ⁢                                      k                    N                                    ⁢                  ⅈ                                                                    ⁢                                  ⁢                                            X              ′                        ⁡                          [                                                (                                      k                    -                    uZ                                    )                                ⁢                mod                ⁢                                                                  ⁢                N                            ]                                =                      X            ⁡                          [              k              ]                                                          [                  Equation          ⁢                                          ⁢          14                ]            
In other words, if
  ⅇ      j2π    ⁢          u      M        ⁢    ⅈ  is multiplied on the frequency axis, cyclic delay can occur on the time axis.
If the number of transmitting antennas that can be supported for a system that includes N number of antennas is additionally provided to support M(M>N) number of transmitting antennas, there exist an LTE UE (user equipment) that can recognize the existing N number of transmitting antennas only and an LTE-A UE that can recognize M number of transmitting antennas only. In this case, in addition to RS for supporting the existing N number of transmitting antennas, transmission of additional M-N number of RSs is required. At this time, under the environment that the LTE-A UE that recognizes M number of antennas is added to the LTE UE that recognizes the existing N number of antennas without additional signaling, efficient transmission of data and RS is required.