Title of Invention

METHOD FOR ALLOCATING REFERENCE SIGNALS IN MIMO SYSTEM

Abstract

There is provided a method for placing reference signals in a wireless communication system. The method includes preparing a plurality of sub-frames for a plurality of antennas, placing a reference signal for one sub-frame and placing a reference signal for another sub-frame not to overlap with the reference signal for one sub-frame, wherein the reference signal for one sub-frame and the reference signal for another sub-frame are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. Channel estimation or data demodulation can be prevented from performance degradation.

Full Text

Description METHOD FOR ALLOCATING REFERENCE SIGNALS IN MIMO SYSTEM Technical Field [ 1 ] The present invention relates to wireless communication, and more particularly, to a method for allocating reference signals in a multiple-input multiple-output (MIMO) antenna system. Background Art [2] A multiple-input multiple-output (MIMO) system is defined as a system that improves data communication efficiency by the use of multiple transmit antennas and multiple receive antennas, The MIMO system may be realized using a MIMO scheme such as a spatial multiplexing and a spatial diversity. According to the spatial mul- tiplexing, different streams are concurrently transmitted through the multiple transmit antennas, and thus fast transmission is achieved without having to increase a system bandwidth. According to the spatial diversity, same streams are transmitted through the multiple transmit antennas to obtain diversity. [3] In order to reproduce a signal transmitted from a transmitter, channel estimation has to be carried out by a receiver. Channel estimation is defined as a process in which a distorted signal is restored by compensating for signal distortion due to fading. In general, for the channel estimation, reference signals are required which are known by both the transmitter and the receiver. [4] The reference signals may be allocated using either a first scheme in which the reference signals are allocated over the entire frequency band or a second scheme in which the reference signals are allocated over a part of the frequency band. The reference signals are further densely allocated in the first scheme rather than the second scheme. The channel estimation can be further accurately performed when the first scheme is used. On the other hand, a higher data rate can be achieved in the second scheme rather than the first scheme. In the second scheme, the reference signals are scarcely allocated, and thus the channel estimation may degrade. [5] In the MIMO system, multiple channels are independently provided for multiple antennas. The reference signals need to be allocated in consideration of the multiple channels. In addition, the MIMO system may operate in either a single-codeword mode or a multiple-codeword mode according to a rank. The number of reference signals may increase along with the increase in the number of transmit antennas. But, this may adversely affect the data rate. [6] Therefore, there is a need for a technique in which the reference signals can be ef- fectively allocated in consideration of the multiple antennas. Disclosure of Invention Technical Solution [7] The present invention provides a method of allocating reference signals for a multiple-input multiple-output antenna (MIMO) antenna system over wireless com- munication. [8] According to an aspect of the invention, there is provided a method for allocating reference signals for a sub-frame in a wireless multiple-input multiple-output (MIMO) communication system. The sub-frame includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in a time domain and a plurality of sub- carriers in a frequency domain. The method includes allocating a plurality of first reference signals for a first antenna on a first OFDM symbol over a sub-frame for the first antenna at regular intervals in the frequency domain, allocating a plurality of second reference signals for a second antenna on the first OFDM symbol over a sub- frame for the second antenna at regular intervals in the frequency domain such that the plurality of second reference signals does not overlap with the plurality of first reference signals, allocating a plurality of third reference signals for a third antenna on a second OFDM symbol over a sub-frame for the third antenna at regular intervals in the frequency domain, wherein the second OFDM symbol is contiguous with the first OFDM symbol and allocating a plurality of fourth reference signals for a fourth antenna on the second OFDM symbol over a sub-frame for the fourth antenna such that the plurality of fourth reference signals does not overlap with the plurality of third reference signals. [9] According to another aspect of the invention, there is provided a method for placing reference signals in a wireless communication system. The method includes preparing a plurality of sub-frames for a plurality of antennas, one sub-frame comprising a plurality of OFDM symbols in a time domain and a plurality of sub-carriers in a frequency domain, placing a reference signal for one sub-frame and placing a reference signal for another sub-frame not to overlap with the reference signal for one sub-frame, wherein the reference signal for one sub-frame and the reference signal for another sub-frame are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. [10] According to still another aspect of the invention, there is provided a method for placing reference signals in a wireless communication system. The method comprising placing a plurality of reference signals for dedicated signal and placing a plurality of reference signals for multi-user signal such that intervals in the frequency domain of the plurality of reference signals for multi-user signal are shorter than that of the plurality of reference signals for dedicated signal. [11] According to still another aspect of the invention, there is provided an apparatus for an OFDM based wireless MIMO communication system.. The apparatus includes a plurality of transmit antennas, a multiplexer for allocating a plurality of reference signals for the plurality of transmit antennas not to overlap with each other, wherein at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers and an OFDM modulator for modulating the plurality of reference signals. [12] According to still another aspect of the invention, there is provided an apparatus for an OFDM based wireless communication system. The apparatus includes at least one receive antennas and a channel estimator for estimating a channel using a plurality of reference signals for the plurality of transmit antennas;, wherein the plurality of reference signals does not overlap with each other and at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. [13] According to still another aspect of the invention, there is provided a reference signal structure to provide information for channel estimation in an OFDM based wireless MIMO system. The reference signal structure includes a plurality of reference signals for a plurality of antennas not to overlap with each other, wherein at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. Brief Description of the Drawings [14] The features, nature, and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: [15] FIG. 1 is a block diagram of a transmitter having multiple antennas; [16] FIG. 2 is a block diagram of a receiver having multiple antennas; [ 17] FIG. 3 illustrates an example of a reference signal allocation when two transmit antennas are used; [ 18] FIG. 4 illustrates an example of a reference signal allocation when four transmit antennas are used; [19] FIG. 5 illustrates an example of a reference signal allocation; [20] FIG. 6 illustrates another example of a reference signal allocation; [21] FIGS. 7 through 19 illustrate examples of a reference signal allocation for a multi- user signal; [22] FIGS. 20 to 82 illustrate examples of a reference signal allocation according to an embodiment of the present invention; and [23] FIGS. 83 to 91 illustrate examples of a reference signal allocation for a multi-user signal. Mode for the Invention [24] Additional features and advantages of the invention will be set forth in the de- scription which follows, and in part will be apparent from the description, or may be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. [25] The technique to be described below may be used in various communication systems. The communication systems are widely distributed so as to provide various communication services (e.g. voice, packet data, etc). The technique may be used for downlink or uplink. In general, downlink means communication from a base station (BS) to user equipment (UE), and uplink means communication from the UE to the BS. The BS is generally referred to a fixed station that communicates with the UE and may also be referred to as another terminology such as a node-B, a base transceiver system (BTS) and an access point. The UE may be fixedly located or may have mobility. The UE may also be referred to as another terminology such as a mobile station (MS), a user terminal (UT), a subscriber station (SS) and a wireless device. [26] A communication system may be either a multiple-input multiple-output (MIMO) system or a multiple-input single-output (MISO) system. The MIMO system includes a plurality of transmit antennas and a plurality of receive antennas. The MISO system includes a plurality of transmit antennas and a single receive antenna. [27] There is no limit in a multiple access modulation scheme. The multiple access modulation scheme may be a well-known single carrier modulation scheme (e.g. time division multiple access (TDMA), frequency division multiple access (FDMA), code division multiple access (CDMA), single carrier-frequency division multiple access (SC-FDMA)) or a multiple carrier modulation method (e.g. orthogonal frequency division multiplexing (OFDM)). [28] The channel estimation can be effectively performed by a receiver when reference signals are allocated under the following conditions. [29] First, the reference signals have to be allocated so that the receiver can distinguish the reference signals transmitted from multiple transmit antennas. This is because the reference signals are used by the receiver for the channel estimation. The reference signals can be allocated not to overlap one another in a time and/or frequency domain for the respective transmit antennas, so that the receiver can distinguish the reference signals. Alternatively, when the reference signals are orthogonal to each other in a code domain, the reference signals can overlap one another in the time and/or frequency domain. To achieve orthogonality in the code domain, the reference signals may use an orthogonal code having excellent auto-correlation or cross-correlation. Examples of the orthogonal code include a constant amplitude zero auto-correlation (CAZAC) sequence and a Walsh code. [30] Second, a channel variance has to be negligibly small in a region where the reference signals are placed. A channel in this region is used to decode data allocated adjacent to the reference signals. If the channel significantly changes in this region, a channel estimation error may become significant. [31] In exemplary embodiments, reference signals may be shifted by a specific interval on the time axis or by a specific interval on the frequency axis. That is, for each sub-frame for respective transmit antennas, the reference signals may be generally shifted by a specific time interval and/or by a specific frequency interval while the interval between reference signals is maintained. [32] A reference signal may be a reference signal for an user or a reference signal for a multi-user signal. The multi-user signal may be a broadcast signal and/or a multicast signal. The broadcast signal is sent to all users within a specific area (e.g. cell and/or sector). The multicast signal is sent to a specific group of users. A unicast signal is sent to a specific user. One example of the multi-user signal may be a mobile broadcast/ multicast service (MBMS) signal. When transmitting the MBMS signal, the same signal is transmitted from all cells (or base stations). [33] Hereinafter, various examples of a reference signal allocation for an MIMO system having four transmit antennas will be described. The reference signals will be allocated according to the following principles. First, the number of reference signals for first and second antennas in a sub-frame is larger than that of reference signals for third and fourth antennas in the sub- frame. Second, the percentage occupied by the entire reference signals in the sub-frame is below a predetermined value. Third, Reference signals for each transmit antenna do not overlap one another. [34] A sub-frame includes a plurality of OFDM symbols in a time domain and a plurality of sub-carrier in a frequency domain. The sub-frame is a resource grid which is defined for each transmit antenna. A transmission time interval (TTI) can be defined as a time required for transmitting a single sub-frame. A frame may include a plurality of sub- frames. For example, one frame may include ten sub-frames. [35] The sub-frame can be divided by two regions, a control channel and a data channel. The control channel is the region carrying control data. The data channel is the region carrying user data. For example, a first OFDM symbol, a second OFDM symbol and a third OFDM symbol may be allocated for the control channel and the other OFDM symbols mat be allocated for the data channel. Although the number of OFDM symbols for the control channel is smaller than that of OFDM symbols for the control channel, the reliability for the control channel has to be higher than that of the data channel. Only a part of multiple antennas can be assigned for transmitting the control channel. A first antenna and a second antenna can be used for the control channel. In this case, reference signals for a third antenna and reference signals for a fourth antenna may not be assigned for the OFDM symbols for the control channel because the third antenna and the fourth antenna are not used for the control channel. [36] FIG. 1 is a block diagram of a transmitter having multiple antennas. [37] Referring to FIG. 1, a transmitter 100 includes a channel encoder 120, a mapper 130, an MIMO processor 140, a multiplexer 150 and an OFDM modulator 160. The channel encoder 120 encodes an input stream according to a predetermined coding scheme and then generates a coded word. The mapper 130 maps the coded word to a symbol that represents a position on signal constellation. Since there is no limit in a modulation scheme of the mapper 130, the modulation scheme may be m-phase shift keying (m-PSK) or m-quadrature amplitude modulation (m-QAM). Examples of the m-PSK include BPSK, QPSK, and 8-PSK. Examples of the m-QAM include 16-QAM, 64-QAM, and 256-QAM. The MIMO processor 140 processes an mapped symbol by using an MIMO scheme according to transmit antennas 190-1, ..., 190-Nt (Nt>l). For example, the MIMO processor 140 may handle codebook-based pre-coding. [38] The multiplexer 150 allocates an input symbol and reference signals to a sub-carrier. The reference signals are allocated for the respective transmit antennas 190-1,..., 190-Nt. The reference signals, also referred to as pilots, are used for channel estimation or data demodulation and are known by both the transmitter 100 and a receiver 200 of FIG. 2. The OFDM modulator 160 modulates a multiplexed symbol and thus outputs an OFDM symbol. The OFDM modulator 160 may perform inverse fast Fourier transform (IFFT) on the multiplexed symbol, and may further insert a cyclic prefix (CP) therein after IFFT is performed. The OFDM symbol is transmitted through the respective transmit antennas 190-1, ..., 190-Nt. [39] FIG. 2 is a block diagram of a receiver having multiple antennas. [40] Referring to FIG. 2, a receiver 200 includes an OFDM demodulator 210, a channel estimator 220, an MIMO post-processor 230, a de-mapper 240, and a channel decoder 250. Signals received from receive antennas 290-1, ..., 290-Nr are subject to fast Fourier transform (FFT) by the OFDM demodulator 210. The channel estimator 220 obtains an estimated channel by using reference signals. The MIMO post-processor 230 performs post-processing equivalent to the MIMO processor 140. The de-mapper 240 de-maps the input symbol to a coded word. The channel decoder 250 decodes the coded word so as to be restored to original data. [41] Now, allocation of reference signals will be described. [42] FIG. 3 illustrates an example of a reference signal allocation when two transmit antennas are used. In general, data transmission can be achieved in the unit of a sub- frame for respective transmit antennas according to a OFDM modulation scheme. For example, the sub-frame shown in FIG. 3 includes seven OFDM symbols where a TTI is 0.5 milli-second (msec.). However, the present inventive concept is not limited thereto, and thus the sub-frame and the TTI may be configured in various forms. [43] Referring to FIG. 3, reference signals are respectively allocated for a sub-frame of a first antenna and a sub-frame of a second antenna. D denotes a data symbol for carrying data, R denotes a first reference signal for the first antenna and R denotes a second reference signal for the second antenna. The first reference signal R may be equal to or different from the second reference signal R . [44] Each of elements over a resource grid constituting a sub-frame is referred to as a resource element. For example, a resource element q(k,l) is placed at a k-th OFDM symbol and an 1-th sub-carrier. The data symbol D, the first reference signal R , and the second reference signal R2 are carried in one resource element. [45] Regarding the sub-frame of the first antenna, the reference signals are allocated over seven OFDM symbols. For clarity of description, hereinafter, the seven OFDM symbols will be respectively referred to as a first OFDM symbol, a second OFDM symbol, ..., and a seventh OFDM symbol from the beginning of a TTI. [46] In the first OFDM symbol, the first reference signals R may be allocated at the interval of six sub-carriers. Likewise, in the fifth OFDM symbol, the second reference signals R may be allocated at the interval of six sub-carriers. In the fifth OFDM symbol, the second reference signals R are each shifted by the size of three sub- carriers from positions where the first reference signals Rl in the first OFDM symbol are placed. In the sub-frame, an array of (R , D, D, D, D, D) is repeated in the first OFDM symbol and an array of (D, D, D, R , D, D) is repeated in the fifth OFDM symbol. [47] Regarding the second antenna, reference signals are allocated in the same pattern as in the first antenna. In the first OFDM symbol, the first reference signals R are allocated at the interval of six sub-carriers. In the fifth OFDM symbol, negative second reference signals -R are allocated at the interval of six sub-carriers. The negative second reference signals -R are obtained by negating the second reference signals R . In the fifth OFDM symbol, the negative second reference signals -R are each shifted by the size of three sub-carriers from positions where the first reference signals Rl in the first OFDM are placed. That is, an array of (R , D, D, D, D, D) is repeated in the first OFDM symbol and an array of (D, D, D, -R2, D, D) is repeated in the fifth OFDM symbol. [48] Since the reference signals are allocated in the same pattern in both the first and second antennas, an orthogonal code can be used so that the receiver can distinguish the reference signals for the respective transmit antennas. The orthogonal code may be a CAZAC sequence or a Walsh sequence having excellent auto-correlation or cross- correlation. [49] FIG. 4 illustrates an example of a reference signal allocation when four transmit antennas are used. The reference signals are allocated for each sub-frame for the respective transmit antennas. Here, N denotes a null symbol, R denotes a first reference signal, R2 denotes a second reference signal and D denotes a data symbol. The null symbol can be defined as a symbol that does not carry data. The null symbol may be generated when no data is allocated to a sub-carrier or when the sub-carrier allocated with data is punctured later. [50] Regarding the first antenna, reference signals are allocated at the interval of six sub- carriers. In other words, the reference signals are placed with five sub-carriers therebetween. The five sub-carriers may include four data symbols D and one null symbol. Therefore, the first OFDM symbol is repeated with an array of (R , D, D, N, D, D). The null symbol is allocated to a resource element where reference signals for the third and fourth antennas to be described below are placed. Reference signals are not allocated in the second, third, and fourth OFDM symbols. Instead, data symbols D are allocated therein. Reference signals may be allocated at the interval of six sub- carriers in the fifth OFDM symbol. The reference signals in the fifth OFDM symbol are each shifted by the size of three sub-carriers from positions where the reference signals in the first OFDM symbol are placed. The sixth and seventh OFDM symbols are allocated with data symbols instead of reference signals. [51] Regarding the second antenna, reference signals are allocated in the same pattern as those of the first antenna. The reference signals for the first and second antennas are allocated to overlap each other in the same OFDM symbols and sub-carriers. The receiver may use an orthogonal code having an excellent auto-correlation or cross- correlation in order to distinguish the reference signals for the first antenna from the reference signals for the second antenna. By using orthogonality of the reference signals R and R transmitted through the first antenna and the reference signals R and -R transmitted through the second antenna, the receiver may separate these reference signals from one another. [52] Regarding the third antenna, reference signals are allocated as follows. T'he reference signals R are allocated at the interval of six sub-carriers in the first OFDM symbol. Likewise, the reference signals R are allocated at the interval of six sub-carriers in the fifth OFDM symbol. The reference signals R in the fifth OFDM symbol are each shifted by the size of three sub-carriers from positions where the reference signals R in the first OFDM symbol are placed. Therefore, an array of (N, D, D, R , D, D) is repeated in the first OFDM symbol, and an array of (R2, D, D, N, D, D) is repeated in the fifth OFDM symbol. Regarding the fourth antenna, reference signals are allocated in the same pattern as those of the third antenna. Reference signals are allocated at the interval of six sub-carriers in the first and fifth OFDM symbols. The receiver may use an orthogonal code in order to distinguish the reference signals for the third antenna from the reference signals for the fourth antenna. [53] Although the aforementioned reference signal allocation is exemplified, the present inventive concept is not limited thereto, and thus reference signals may be shifted by a specific interval on the time axis or by a specific interval on the frequency axis. That is, for each sub-frame for respective transmit antennas, the reference signals may be generally shifted by a specific lime interval and/or by a specific frequency interval while the interval between reference signals is maintained. Since the reference signals can be generally shifted as described above without having to reallocate the reference signals, channel estimation can be achieved for multiple cells, multiple sectors and multiple users. [54] In the mean time, reference signals for a specific antenna may be partially or entirely used (or not used) according to time-varying channel variation in a multiple of the number of sub-frames. [55] In the aforementioned descriptions, the reference signals overlap one another when at least two transmit antennas are used. The overlapping reference signals maintain their orthogonality in the code domain by using an orthogonal code. [56] FIG. 5 illustrates an example of a reference signal allocation. R denotes a reference signal and a blank of a resource element denotes a data symbol or a null symbol. [57] Referring to FIG. 5, a plurality of reference signals R is allocated at the interval of two sub-carriers in the third OFDM symbol. A plurality of reference signals R is also allocated at the interval of two sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. A plurality of reference signals R is allocated at the interval of two sub-carriers in the eleventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the seventh OFDM symbol. [58] Each reference signal R may be a reference signal for a multi-user signal. Here, the multi-user signal may be a broadcast signal and/or a multicast signal. The broadcast signal is sent to all users within a specific area (e.g. cell and/or sector). The multicast signal is sent to a specific group of users. A unicast signal is sent to a specific user. One example of the multi-user signal may be a mobile broadcast/multicast service (MBMS) signal. When transmitting the MBMS signal, the same signal is transmitted from all cells (or base stations). Therefore, all base stations use the same reference signal. [59] When using the MBMS signal, the reference signals R can be placed with a narrow interval therebetween so as to minimize frequency selectivity due to delay spread. In addition, the reference signals are densely arranged on the time axis so as to minimize time selectivity. [60] According to some MIMO technique such as cyclic delay diversity (CDD) and beam- forming, an UE seems to receive reference signals through single transmit antenna. Therefore, a BS does not need to transmit the reference signals by classifying the reference signals for each transmit antennas. [61] FIG. 6 illustrates another example of a reference signal allocation. [62] Referring to FIG. 6, reference signals R are placed with a relatively wider interval of the frqeucny domain therebetween than that of FIG. 5. By dosing so, it is advantageous when frequency selectivity is relatively low or when bandwidth of sub-carrier is relatively small. The bandwidth of sub-carrier may be half that of sub-carrier shown in FIG, 5. [63] FIG. 7 illustrates an example of a reference signal allocation for a multi-user signal. Herein, R is a reference signal for the first antenna. R2 is a reference signal for the second antenna. [64] Referring to FIG. 7, the reference signals R1 are allocated at the interval of two sub- carriers in the third OFDM symbol. That is, the reference signals Rl are placed with one sub-carrier therebetween. Therefore, an array of (R1, N) is repeated in the third OFDM symbol, where N denotes a null symbol. The reference signals Rl are allocated at the interval of two sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. [65] The reference signals R2 are alternately allocated in the same OFDM symbol with respect to the reference signal R1. That is, one reference signal R2 is placed between two reference signals R1 with the same interval in the frequency domain. [66] FIG. 8 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [67] Referring to FIG. 8, the reference signals R1 for the first antenna are allocated at the interval of four sub-carriers in the third OFDM symbol. That is, the reference signals R are placed with three sub-carriers therebetween. Therefore, an array of (R1, D, N, D) is repeated in the third OFDM symbol where a blank of a resource element denotes D and N. The reference signals R are allocated at the interval of four sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. [68] The reference signals R2 for the second antenna are alternately arranged with respect to the reference signal R1 in the same OFDM symbols at the same interval as the reference signals R1. That is, one reference signal R2 is placed between two reference signals R with the same interval in the frequency domain. [69] FIG. 9 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [70] Referring to FIG. 9, the reference signals Rl for the first antenna are allocated at the interval of two sub-carriers in the third OFDM symbol. That is, the reference signals R are placed with one sub-carrier therebetween. Therefore, an array of (R1, N) is repeated in the third OFDM symbol. The reference signals R are allocated at the interval of two sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R1 in the third and seventh OFDM symbols are staggered from each other in the frequency domain. [71] The reference signals R2 for the second antenna are allocated in the same frequency domain as in the case of the reference signals R1 in OFDM symbols (e.g. fourth OFDM symbol, eighth OFDM symbol, etc) which are adjacent to the OFDM symbols where the reference signals R1 are allocated. That is, the frequency signals R2 are allocated at the same interval as the frequency signals R2. [72] FIG. 10 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [73] Referring to FIG. 10, the reference signals R1 for the first antenna are allocated at the interval of four sub-carriers in the third OFDM symbol. That is, the reference signals R 1 are placed with three sub-carriers therebetween. Therefore, an array of (R1, D, N, D) is repeated in the third OFDM symbol. The reference signals R are allocated at the interval of four sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. [74] The reference signals R2 for the second antenna are allocated in the same frequency domain as in the case of the reference signals R1 in OFDM symbols (e.g. fourth OFDM symbol, eighth OFDM symbol, etc) which are adjacent to the OFDM symbols where the reference signals R1 are allocated. That is, the frequency signals R2 are allocated at the same interval as the frequency signals R2 in the frequency domain. [75] FIG. 11 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [76] Referring to FIG. 11, the reference signals R for the first antenna are allocated at the interval of two sub-carriers in the third OFDM symbol. That is, the reference signals R1 are placed with one sub-carrier therebetween. Therefore, an array of (R1, N) is repeated in the third OFDM symbol. The reference signals R1 are allocated at the interval of two sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. [77] The reference signals R2 for the second antenna overlap the reference signals R in the same OFDM symbols in the same frequency domain. The reference signals R and R2 can maintain orthogonality in the code domain by using the orthogonal code. [78] FIG. 12 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [79] Referring to FIG. 12, the reference signals R for the first antenna are allocated at the interval of four sub-carriers in the third OFDM symbol. That is, the reference signals R are placed with three sub-carriers therebetween. Therefore, an array of (R1, D, N, D) is repeated in the third OFDM symbol. The reference signals R1 are allocated at the interval of four sub-carriers in the seventh OFDM symbol which is spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R1 in the third and seventh OFDM symbols are staggered from each other. [80] The reference signals R2 for the second antenna overlap the reference signals R1 in the same OFDM symbols in the same frequency domain. The reference signals R1 and R2 can maintain orthogonality in the code domain by using the orthogonal code. [81] FIG. 13 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [82] Referring to FIG. 13, the reference signals R1 for the first antenna are allocated at the interval of three sub-carriers in the third OFDM symbol. The reference signals R2 for the second antenna are adjacent to the reference signals R1 and are allocated at the same interval as the reference signals R1. Therefore, an array of (R1, R2, D) is repeated in the third OFDM symbol. [83] Both of the reference signals R1 and R2 are allocated in OFDM symbols which are spaced apart by the size of three OFDM symbols starting from the third OFDM symbol. [84] FIG. 14 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. Throughout FIGS. 14 to 19, R denotes a reference signal for a multi-user signal, and T denotes a reference signal for a dedicated user signal. That is, hereinafter, two heterogeneous reference signals will be exemplified. [85] Referring to FIG. 14, the reference signals T1 for the first antenna and the reference signals T2 for the second antenna are allocated in the first OFDM symbol. In addition, the reference signals T1 and T2 are also allocated in the fourth OFDM symbol. The reference signals R1 for the first antenna and the reference signals R2 for the second antenna are allocated in OFDM symbols which are spaced apart by the size of four OFDM symbols from the third OFDM symbol. [86] FIG. 15 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [87] Referring to FIG. 15, the reference signals R1 for the first antenna and the reference signals R2 for the second antenna are allocated at a wider interval than those in the example of FIG. 14. [88] FIG. 16 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [89] In comparison with the example of FIG. 4, referring to FIG. 16, the reference signals R1 for the first antenna and the reference signals R2 for the second antenna are re- spectively allocated at the interval of three sub-carriers in the third OFDM symbol. The reference signals R2 are adjacent to the reference signals R1 and are allocated at the same interval as the reference signals R1. [90] FIG. 17 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [91] In comparison with the example of FIG. 14, referring to FIG. 17, the reference signals T1 for the first antenna and the reference signals T2 for the second antenna are allocated only in the first OFDM. [92] FIG. 18 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [93] Referring to FIG. 18, the reference signals R1 for the first antenna and the reference signals R2 for the second antenna are allocated at a wider interval than those in the example of FIG. 17. [94] FIG. 19 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [95] In comparison with the example of FIG. 16, referring to FIG. 19, the reference signals T1 for the first antenna and the reference signals T2 for the second antenna are allocated only in the first OFDM. [96] In embodiments of FIGS. 14 to 19, reference signals for multi-users can be transmitted through single transmit antenna. Since an UE seems to receive reference signals through single transmit antenna in CDD or beam-forming, a BS does not need to transmit the reference signals after classifying the reference signals for each transmit antennas. [97] Hereinafter, various examples of a reference signal allocation for an MIMO system having four transmit antennas will be described. The reference signals will be allocated according to the following principles. [98] (1) The reference signals R1 for the first antenna described in the example of FIG. 3 remain their positions also in the MIMO system having four transmit antennas. [99] (2) Among the whole signals used, the percentage occupied by the entire reference signals is below a predetermined value. When the percentage of the entire reference signals increases, the receiver can relatively perform accurate channel estimation by receiving a plurality of reference signals. However, the higher the percentage, the lower the data rate is. It will be assumed that the percentage is below about 15 percent or 20 percent. In this case, if the reference signals are effectively allocated, performance degradation of the channel estimation can be minimized. [100] (3) Reference signals for each transmit antenna do not overlap one another. That is, the reference signals for each transmit antenna do not overlap one another in both the time domain and the frequency domain. [101] FIG. 20 illustrates an example of a reference signal allocation according to an embodiment of the present invention. T1 is a reference signal for the first antenna, T2 is a reference signal for the second antenna, T3 is a reference signal for the third antenna, and T4 is a reference signal for the fourth antenna. A blank resource element may be a data symbol or a null symbol. [102] Referring to FIG. 20, one sub-frame comprises fourteen OFDM symbols. However, this is only an example, and thus the number of OFDM symbols constituting one sub- frame may vary. Although one sub-frame is illustrated for convenience, reference signals for each antenna are allocated for each sub-frame for respective antennas. That is, the reference signals T1 are allocated in the sub-frame for the first antenna. The reference signals T2 are allocated in the sub-frame for the second antenna. The reference signals T3 are allocated in the sub-frame for the third antenna. The reference signals T are allocated in the sub-frame for the fourth antenna. For clarity of de- 4 scription, it will be assumed that fourteen OFDM symbols are defined as a first OFDM symbol, a second OFDM symbol, ..., and a fourteenth OFDM symbol from a beginning of a TTI. [103] The reference signals T are allocated at the interval of six sub-carriers in the first and eighth OFDM symbols. In addition, the reference signals T are also allocated at the interval of six sub-carriers in the fifth and twelfth OFDM symbols. The reference signals T1 allocated in the fifth and twelfth OFDM symbols are each shifted by the size of three sub-carries from those allocated in the first and eighth OFDM symbols. [104] The reference signals T2 are allocated at the interval of six sub-carriers in the first, fifth, eighth and twelfth OFDM symbols. The reference signals T2 in the first and fifth OFDM symbols are each shifted by the size of three sub-carriers from positions where the reference signals T1 are placed. The reference signals T2 in the fifth and twelfth OFDM symbols are each placed in the same positions as the reference signals T1. [105] The reference signals T3 are allocated at the interval of 12 sub-carriers in the first, fifth, eighth and twelfth OFDM symbols. The reference signals T3 in the first OFDM symbol are each shifted by the size of one sub-carrier from positions where the reference signals T1 are placed. The reference signals T3 in the fifth, eighth and twelfth OFDM symbols are allocated at the interval of twelve sub-carriers and are shifted by the size of one sub-carrier where reference signals of other antennas are placed. [106] The reference signals T4 are allocated at the interval of twelve sub-carriers in the first, fifth, eighth and twelfth OFDM symbols. The reference signals T4 are each shifted by one sub-carrier from positions where the reference signals T3 are placed. [107] The reference signals T1 and T2 are more densely allocated than the reference signals T3 and T4 so that the first and second antennas which are more frequently used than other antennas can have better channel estimation performance. [108] In general, more control signals are carried in OFDM symbols located prior to the third OFDM symbol. The reference signals T1 to T4 are allocated such that an array of (T1, T3, T4, T2, D, D, T1, D, D, T2, D, D) is repeated in the first OFDM symbol, and an array of (T2, D, D, T1, D, D, T2, T3, T4, T1, D, D) is repeated in the fifth OFDM symbol. An array of (T1, D, D, T2, T3, T4, T1, D, D, T2, D, D) is repeated in the eighth OFDM symbol, and an array of (T2, D, D, T1, D, D, T2, D, D, T1, T3, T4, T2) is repeated in the twelfth OFDM symbol. Data symbols D may be allocated where these reference signals are not placed. In this case, the percentage occupied by the data symbols D is about 86 percent. [109] The percentage occupied by the data symbols in a sub-frame may be different according to characteristics of the system. Hereinafter, we exemplarily illustrate 14 OFDM symbols per a TTI but it's not limited. One TTI may include 12 or more OFDM symbols. [110] The depicted reference signal allocation pattern is shown in relatively positions, and thus this does not indicate absolute positions. The reference signal pattern may be shifted in the time domain and/or the frequency domain while the reference signals maintained each interval. [111] In a sub-frame, a null symbol may be allocated to a resource element where reference signals of other antennas are placed. For example, in the sub-frame for the first antenna, the null symbol may be allocated to a resource element where reference signals for the second to fourth antennas are placed. [112] At least one of the reference signals for respective antennas may be a reference signal for a multi-user signal. In a sub-frame, the reference signal for a multi-user signal may not be allocated in OFDM symbols including a dedicated control signal but be allocated in the rest of OFDM symbols. For example, if the first and second OFDM symbols include the dedicated control signals, the reference signals for the multi-user signal may be allocated starting from the third OFDM symbol. [113] FIG. 21 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [114] Referring to FIG. 21, the reference signals T1 and T3 are sequentially allocated in the first OFDM symbol together with one data symbol. A blank resource element may be a data symbol or a null symbol. The reference signals T2 and T4 are sequentially allocated, following the data symbol D. Accordingly, an array of (T1, T3, D, T2, T4, D) may be repeated. [115] In the fifth OFDM symbol, the reference signal T2 is placed followed by two data symbols D and the reference signal T1. Two data symbols D are placed again, followed by the reference signal T2. Accordingly, an array of (T2, D, D, T1, D, D) may be repeated. [116] The eighth OFDM symbol may have the similar pattern to the first OFDM symbol, and thus an array of (T1, T4, D, T2, T3, D) may be repeated. The twelfth OFDM symbol may have the same pattern to the fifth OFDM symbol. [117] The percentage occupied by the data symbols D is about 86%. Therefore, the percentage occupied by the reference signals is about 14%. Accordingly, the reference signals do not overlap one another for respective transmit antennas, and thus the receiver can estimate respective channels. [118] FIG. 22 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [119] Referring to FIG. 22, in the first OFDM symbol, the first, third, and fourth reference signals T1, T3, T4 and T2 are each allocated. A blank resource element may be a data symbol or a null symbol. Then, two data symbols D are placed, followed by another reference signal T1. Two data symbols are placed again, followed by another reference signal T2. Then, two data symbols are placed, followed by the reference signals T1, T3, T4 and T2, in that order. Accordingly, an array of (T1, T3, T4, T2, D, D, T1, D, D, T2, D, D) may be repeated in the first OFDM symbol. [120] In the fifth OFDM symbol, the reference signal T2 is placed, followed by two data symbols and the reference signal T1. Then, two data symbol are placed, followed by the reference signals T2, T3, T4 and T1, in that order. Then, two data symbols are placed again, and this arrangement may be repeated. Accordingly, an array of (T2, D, D, T1, D, D, T2, T3, T4, T1, D, D) may be repeated in the fifth OFDM symbol. [121] The eight OFDM symbol has the same reference signal allocation as the first OFDM symbol. The twelfth OFDM symbol has the same reference signal allocation as the fifth OFDM symbol. [122] Data symbols occupy about 85% of the entire area. Thus, reference signals occupy about 15%. Accordingly, the receiver can estimate channels by using the reference signals transmitted from the respective transmit antennas. [123] FIG. 23 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [124] An array of (D, T1, T3, T4, T2, D) is repeated in the first OFDM symbol. An array of (T4, T1, D, D, T2, T3) is repeated in the eighth OFDM symbol. An array of (D, T2, D, D, T1, D) is repeated in the fifth and twelfth OFDM symbols. [125] FIG. 24 illustrates an example of a reference signal allocation according to an embodiment of the. present invention. [126] An array of (D, T1, D, D, T2, D) is repeated in the first and eighth OFDM symbols. An array of (D, T2, T3, T4, T1, D) is repeated in the fifth OFDM symbol. An array of (T 4, T2, D, D, T1, T3) is repeated in the twelfth OFDM symbol. [127] FIG. 25 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [128] An array of (D, T1, T3, T4, T2, D) is repeated in the first and eighth OFDM symbols. An array of (T4, T2, D, D, T1, T3) is repeated in the fifth and twelfth OFDM symbols. [129] FIG. 26 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [130] An array of (T1, T3, D, T2, T4, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T4, D, T1, T3, D) is repeated in the fifth and twelfth OFDM symbols. [131] FIG. 27 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [132] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the second and ninth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T4, D, D, T3, D, D) is repeated in the sixth and thirteenth OFDM symbols. [133] FIG. 28 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [134] An array of (T1, D, T3, T2, D, T4) is repeated in the first and eighth OFDM symbols. An array of (T2, D, T4, T1, D, T3) is repeated in the fifth and twelfth OFDM symbols. [135] FIG. 29 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [136] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fifth and twelfth OFDM symbols. [137] FIG. 30 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [138] An array of (T1, D, D, T2, D, D, T3, D, D, T4, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3, D, D, T4, D, D, T1, D, D, T2, D, D) is repeated in the fifth and twelfth OFDM symbols. [139] FIG. 31 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [140] An array of (T1, T3, D, T2, D, D, T1, T4, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T4, D, T1, D, D, T2, T3, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [141] FIG. 32 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [ 142] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3, D, D, D, D, D, T4, D, D, D, D, D) is repeated in the second and ninth OFDM symbols. [143] An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T4, D, D, D, D, D, T3, D, D, D, D, D) is repeated in the sixth and thirteenth OFDM symbols. [144] FIG. 33 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [145] An array of (T1, D, D, T2, D, D) is repeated in the first OFDM symbol. An array of (T 2, D, D, T1, D, D) is repeated in the eighth OFDM symbol. An array of (T3, D, D, T4, D, D) is repeated in the fifth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the twelfth OFDM symbol. [146] FIG. 34 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [147] An array of (T1, D, D, T3, D, D, T2, D, D, T4, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T4, D, D, T1, D, D, T3, D, D) is repeated in the fifth and twelfth OFDM symbols. [148] FIG. 35 illustrates an example of a reference signal allocation according to an embodiment of the present invention, [ 149] An array of (T1, D, T3, T2, D, D, T1, D, T4, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, T4, T1, D, D, T2, D, T3, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [150] FIG. 36 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [151] An array of (T1/T3, D, D, T2/T4, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2/T4, D, D, T1/T3, D, D) is repeated in the fifth and twelfth OFDM symbols. Here, the reference signals T1 and T3 are allocated in the same sub- carrier in the same time domain. The reference signals T1 and T3 maintain their or- thogonality by using an orthogonal code having the features of auto-correlation and cross-correlation. This may be also applied to reference signals T2/T4. [152] FIG. 37 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [153] An array of (T1, T3/T4, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, T3/T4, D) is repeated in the fifth and twelfth OFDM symbols. Here, the reference signals T3 and T4 are allocated in the same sub-carrier in the same time domain and maintain their orthogonality by using an orthogonal code having the features of auto-correlation and cross-correlation. [154] FIG. 38 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [155] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3/T4, D, D, D, D, D) is repeated in the second and ninth OFDM symbols. 3 4 An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (D, D, D, T3/T4, D, D) is repeated in the sixth and thirteenth OFDM symbols. [156] FIG. 39 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [157] An array of (D, T1, D, T3/T4, T2, D) is repeated in the first and eighth OFDM symbols. An array of (T3/T4, T2, D, D, T1, D) is repeated in the fifth and twelfth OFDM symbols. [158] FIG. 40 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [159] An array of (T1, T3, D, T2, T4, D) is repeated in the first OFDM symbol. An array of (T1, T4, D, T2, T3, D) is repeated in the eighth OFDM symbol. An array of (T2, D, D, T1 , D, D) is repeated in the fifth and twelfth OFDM symbols. [160] FIG. 41 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [161] An array of (T4, T1, D, T3, T2, D) is repeated in the first OFDM symbol. An array of (T3, T1, D, T4, T2, D) is repeated in the eighth OFDM symbol. An array of (D, T2, D, D, T1, D) is repeated in the fifth and twelfth OFDM symbols. [162] FIG. 42 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [163] An array of (T1, T3, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T4, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [164] FIG. 43 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [ 165] An array of (T1, T3, T4, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [166] FIG. 44 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [167] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T3, D, T1, T4, D) is repeated in the fifth OFDM symbol. An array of (T 2, T4, D, T1, T3, D) is repeated in the twelfth OFDM symbol. [168] FIG. 45 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [169] An array of (D, T1, D, D, T2, D) is repeated in the first and eighth OFDM symbols. An array of (T4, T2, D, T3, T1, D) is repeated in the fifth OFDM symbol. An array of (T 3, T2, D, T4, T1, D) is repeated in the twelfth OFDM symbol. [170] FIG. 46 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [171] An array of (T1, D, T3, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, T4, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [172] FIG. 47 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [173] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T3, T4, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [174] FIG. 48 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [175] An array of (T1, T3, D, T2, T4, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [176] FIG. 49 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [177] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, T3, D, T1, T4, D) is repeated in the fifth and twelfth OFDM symbols. [178] FIG. 50 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [179] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the second and ninth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [180] FIG. 51 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [181] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the second OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the ninth OFDM symbol. [182] In the first, fifth and twelfth OFDM symbols, the reference signals T1 and T2 are staggered from each other for each antenna in the frequency domain. In the second and ninth OFDM symbols, the reference signals T3 and T4 are staggered from each other for each antenna in the frequency domain. Accordingly, selectivity can be ensured in the frequency domain. [183] The reference signals T1 and T2 are allocated in the first OFDM symbol. The reference signals T3 and T4 are allocated in the second OFDM symbol adjacent to the first OFDM symbol. When reference signals for multiple antennas are allocated over two consecutive OFDM symbols, the lower the rank, the higher the effectiveness is. For example, if the rank is one in some MIMO techniques, the same data is transmitted through four antennas. In this case, channel estimation can be further effectively achieved when the reference signals are allocated in the two consecutive OFDM symbols. [184] Furthermore, reference signals for at least two antennas are transmitted across the same frequency domain in the two consecutive OFDM symbols. Therefore, channel estimation can be achieved less erroneously than the case where reference signals are excessively staggered when the reference signals are concentrated in the frequency domain and the time domain. [185] Only a part of reference signals for all antennas are allocated in one OFDM symbol. For example, among reference signals for four antennas, only reference signals for two antennas may be allocated. Thus, power can be further boosted for each antenna, where power is allocated to the reference signals. As the power of reference signals increases, channel estimation can be further effectively carried out by the receiver. [186] In some receivers, the first some OFDM symbols (e.g. three OFDM symbols) are decoded. If the decoding result does not coincide with data stored in the receiver, the OFDM symbols transmitted thereafter are not buffered. This is referred to as a micro- sleep mode. In this case, the first some OFDM symbols have to include reference signals for all antennas. The micro-sleep mode may also be implemented when the reference signals for all antennas are allocated in the first and second OFDM symbols. [187] FIG. 52 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [188] An array of (T1, D, T3, T2, D, T4) is allocated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. [189] FIG. 53 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [190] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, T3, T1, D, T4) is allocated in the fifth and twelfth OFDM symbols. [191] FIG. 54 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [192] An array of (T1, D, D, T2, D, D) is allocated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the sixth and thirteenth OFDM symbols. [193] FIG. 55 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [194] An array of (T1, D, D, T2, D, D) is allocated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is allocated in the sixth OFDM symbol. An array of (T 4, D, D, T3, D, D) is allocated in the thirteenth OFDM symbol. [195] FIG. 56 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [196] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. An array of (T3/T4, D, D) is allocated in the second and ninth OFDM symbols. [197] FIG. 57 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [198] An array of (T1, T3/T4, D, T2, T3/T4, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. [199] FIG. 58 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [200] An array of (T1, T3/T4, D, T2, D, D) is allocated in the first and eighth OFDM symbols. An array of (T2, T3/T4, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [201] FIG. 59 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [202] An array of (T1, D, D, T2, D, D) is allocated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. An array of (T3/T4, D, D, D, D, D) is allocated in the second, sixth, ninth, and thirteenth OFDM symbols. [203] FIG. 60 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [204] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3/T4, D, D) is repeated in the sixth and thirteenth OFDM symbols. [205] FIG. 61 illustrates an example of a reference signal allocation according to an embodiment of the present, invention. [206] An array of (T1, D, T3/T4, T2, D, T3/T4) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [207] FIG. 62 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [208] An array of (T1, D, T3/T4, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, T3/T4, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. [209] FIG. 63 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [210] An array of (T , D, D, T , D, D) is repeated in the first and eighth OFDM symbols. An array of (T , D, D, T , D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T , D, D, T , D, D) is repeated in the third and tenth OFDM symbols. An 3 4 array of (T , D, D, T , D, D) is repeated in the seventh OFDM and fourteenth OFDM symbols. [211] FIG. 64 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [212] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is allocated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the third OFDM symbol. An array of (T4 , D, D, T3, D, D) is repeated in the tenth OFDM symbol. [213] FIG. 65 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [214] An array of (D, T1, D, D, D, D) is repeated in the first OFDM symbol. An array of (T 4, T1, D, D, T2, T3) is repeated in the eighth OFDM symbol. An array of (D, T2, D, D, T 1, D) is repeated in the fifth and twelfth OFDM symbols. [215] When a micro-sleep mode is applied in which a control signal is allocated in an OFDM symbol positioned in an initial time sequence on the time axis, the control signal may be transmitted through one or two antennas. If the control signal is transmitted through the first antenna, reference signals for the first antennas may be allocated in the first OFDM symbol. [216] FIG. 66 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [217] An array of (T1, D, D, D, D, D) is repeated in the first OFDM symbol. An array of (T 2, D, D, T1, D, D, T2, T3, T4, T1, D, D) is repeated in the fifth OFDM symbol. An array of (T1, D, D, T2, T3, T4, T1, D, D, T2, D, D) is repeated in the eighth OFDM symbol. An array of (T2, D, D, T1, D, D, T2, D, D, T1, T3, T4) is repeated in the twelfth OFDM symbol. [218] If the control signal is transmitted through the first antenna in the micro-sleep mode, the reference signals for the first antenna are allocated in OFDM symbols positioned in the initial time sequence on the time axis. For example, if the control signal is transmitted through the first antenna, the reference signals for the first antennas may be allocated in the first OFDM symbol. [219] FIG. 67 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [220] An array of (D, T1, D, D, T2, D) is allocated in the first OFDM symbol. An array of (T4, T1, D, D, T2, T3) is allocated in the eighth OFDM symbol. An array of (D, T2, D, D, T1, D) is allocated in the fifth and twelfth OFDM symbols. If the control signal is transmitted through the first and second antennas in the micro-sleep mode, the reference signals for the first and second antennas are allocated in OFDM symbols positioned in the initial time sequence on the time axis. [221] FIG. 68 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [222] If the control signal is transmitted through two antennas in the micro-sleep mode, reference signals are allocated as follows. An array of (T1, D, D, T2, D, D) is repeated in the first OFDM symbol. An array of (T2, D, D, T1, D, D, T2, T3, T4, T1, D, D) is allocated in the fifth OFDM symbol. An array of (T1, D, D, T2, T3, T4, T1, D, D, T2, D, D) is allocated in the eight OFDM symbol. An array of (T2, D, D, T1, D, D, T2, D, D, T 1, T3, T4) is allocated in the twelfth OFDM symbol. If the control signal is transmitted through the first and second antennas in the micro-sleep mode, reference signals for the first and second antennas are allocated in OFDM symbols positioned in the initial time sequence on the time axis. [223] FIG. 69 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [224] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fourth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the ninth OFDM symbol. [225] A sub-frame can be divided by two regions, a control channel and a data channel. T he control channel is the region carrying control data. The data channel is the region carrying user data. For example, a first OFDM symbol, a second OFDM symbol and a third OFDM symbol may be allocated for the control channel and the other OFDM symbols mat be allocated for the data channel. Although the number of OFDM symbols for the control channel is smaller than that of OFDM symbols for the control channel, the reliability for the control channel has to be higher than that of the data channel. Only a part of multiple antennas can be assigned for transmitting the control channel. A first antenna and a second antenna can be used for the control channel. In this case, reference signals for a third antenna and reference signals for a fourth antenna may not be assigned for the OFDM symbols for the control channel because the third antenna and the fourth antenna are not used for the control channel. [226] FIG. 70 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [227] An anray of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the sixth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the ninth OFDM symbol. [228] At the channel, reference signals for a third antenna and reference signals for a fourth antenna is next to reference signals for a first antenna and reference signals for a sencond antenna so as to improve accuracy for channel estimation. [229] FIG. 71 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [230] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fourth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the eleventh OFDM symbol. [231] In consecutive sub-frames, the interval for reference signals for a third antenna and a fourth antenna can constantly be maintained. [232] FIG. 72 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [233] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the sixth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the tenth OFDM symbol. [234] FIG. 73 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [235] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fourth OFDM symbol An array of (T4, D, D, T3, D, D) is repeated in the tenth OFDM symbol. [236] FIG. 74 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [237] An array of (T 1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the sixth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the eleventh OFDM symbol. [238] FIG. 75 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [239] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fifth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the sixth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the thirdteenth OFDM symbol. [240] FIG. 76 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [241] An array of (T1, D, D, T2, D, D) is repeated in the fust and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the third OFDM symbol- An array of (T4 , D, D, T3, D, D) is repeated in the ninth OFDM symbol. [242] FIG. 77 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [243] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fifth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the ninth OFDM symbol. [244] FIG. 78 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [245] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the third OFDM symbol. An array of (T4 , D, D, T3, D, D) is repeated in the eleventh OFDM symbol. [246] FIG. 79 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [247] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fifth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the tenth OFDM symbol. [248] FIG. 80 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [249] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the third OFDM symbol. An array of (T4 , D, D, T3, D, D) is repeated in the tenth OFDM symbol. [250] FIG. 81 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [251] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fifth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the eleventh OFDM symbol. [252] FIG. 82 illustrates an example of a reference signal allocation according to an embodiment of the present invention. [253] An array of (T1, D, D, T2, D, D) is repeated in the first and eighth OFDM symbols. An array of (T2, D, D, T1, D, D) is repeated in the fourth and twelfth OFDM symbols. An array of (T3, D, D, T4, D, D) is repeated in the fifth OFDM symbol. An array of (T4, D, D, T3, D, D) is repeated in the thirteenth OFDM symbol. [254] FIGS. 65 to 82 illustrate examples of a reference signal allocation where reference signals are allocated in the first OFDM symbol. If the number of OFDM symbols applied in the micro-sleep mode increases, the reference signals may be allocated in other OFDM symbols such as the second and third OFDM symbols. [255] FIG. 83 illustrates an example of a reference signal allocation for a multi-user signal. [256] Referring to FIG. 83, R denotes a reference signal for a multi-user signal. The reference signal R may be used for any antenna. In the case of using two antennas, R may denote either a reference signal for the first antenna or a reference signal for the second antenna. A blank resource element may be a data symbol or a null symbol. [257] In one sub-frame, the reference signals T1 for the first antenna are allocated at the interval of six sub-carriers in the first OFDM symbol. That is, the reference signals T1 are allocated with five sub-carriers therebetween. The reference signals T2 for the second antenna are allocated at the same interval as the first reference signals T1 so as not to overlap the first reference signals T1 in the same OFDM symbols. That is, the reference signals T2 are placed between the two references signals T1 at the same interval as the first reference signals T1. [258] The reference signals R are allocated staring from positions where dedicated control signals are not allocated, for example, from the third OFDM symbol. That is, the reference signals R are allocated at the interval of two sub-carriers in the third OFDM symbol. The reference signals R are allocated at the interval of two sub-carriers in the seventh OFDM symbol spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. The reference signals R are allocated at the interval of two sub-carriers in the eleventh OFDM symbol spaced apart by the size of four OFDM symbols from the seventh OFDM symbol. [259] FIG. 84 illustrates another example of a reference signal allocation for a multi-user signal. [260] In comparison with the example of FIG. 83, referring to FIG. 84, the reference signals T1 and T2 for the first and second antennas are allocated in the fourth OFDM symbol. When a dedicated control signal is allocated in a region where a multi-user signal is transmitted, an error rate of the dedicated control signal can be reduced. [261] FIG. 85 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. This is the case where four antennas are used, and at least one antenna among them transmits a multi-user signal. [262] Referring to FIG. 85, in one sub-frame, the reference signals T1 are allocated at the interval of six sub-carriers in the first OFDM symbol. The reference signals T2 are allocated at the same interval as the first reference signals T1 so as not to overlap the first reference signals T1 in the same OFDM symbols. That is, the reference signals T2 are placed between the two references signals T1 at the same interval as the first reference signals T1. Furthermore, in one sub-frame, the reference signals T3 are allocated at the interval of six sub-carriers in the first OFDM symbol. The reference signals T4 are allocated at the same interval as the third reference signals T3 so as not to overlap the third reference signals T3 in the same OFDM symbols. [263] The reference signals R are allocated staring from positions where a dedicated control signal is not allocated, for example, from the third OFDM symbol. The reference signals R may be transmitted through at least one of four antennas, the first to fourth antennas. The reference signals R are allocated at the interval of two sub- carriers in the third OFDM symbol. The reference signals R are allocated at the interval of two sub-carriers in the seventh OFDM symbol spaced apart by the size of four OFDM symbols from the third OFDM symbol. The reference signals R in the third and seventh OFDM symbols are staggered from each other. [264] The reference signals T1 and T2 are allocated in the fourth OFDM symbol. When a dedicated control signal is allocated in a region where a multi-user signal is transmitted, an error rate of the dedicated control signal can be reduced. [265] FIG. 86 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. This is the. case where four antennas are used, and at least one antenna among them transmits a multi-user signal. [266] Referring to FIG. 86, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. [267] FIG. 87 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [268] Referring to FIG. 87, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. The first to fourth reference signals R1 to R4 are each allocated in the frequency domain at the interval of six sub-carriers. [269] FIG. 88 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [270] Referring to FIG. 88, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. [271] The reference signals T1 to T4 for the first to fourth antennas are allocated in the first and second OFDM symbols. Also, the reference signals T1 and T2 are allocated in the fourth OFDM symbol. [272] FIG. 89 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [273] Referring to FIG. 89, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. The first to fourth reference signals R1 to R4 are allocated in the frequency domain at the interval of six sub-carriers. [274] The reference signals T1 to T4 for the first to fourth antennas are allocated in the first and second OFDM symbols. Also, the reference signals T1 and T2 are allocated in the fourth OFDM symbol. [275] FIG. 90 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [276] Referring to FIG. 90, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. [277] The reference signals T1 to T4 for the first to fourth antennas are allocated only in the first and second OFDM symbols. [278] FIG. 91 illustrates another example of a reference signal allocation for a multi-user signal when multiple antennas are used. [279] Referring to FIG. 91, first and second reference signals R1 and R2 are allocated in OFDM symbols starting from the third OFDM symbol at the interval of four OFDM symbols. Third and fourth reference signals R3 and R4 are allocated in OFDM symbols starting from the fifth OFDM symbol at the interval of four OFDM symbols. The first to fourth reference signals R1 to R4 are allocated in the frequency domain at the interval of six sub-carriers. [280] The reference signals T1 to T4 for the first to fourth antennas are allocated only in the first and second OFDM symbols. [281] Reference signals for multiple antennas are effectively allocated. Channel estimation or data demodulation can be prevented from performance degradation. [282] As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing de- scription, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims. Therefore, all changes and modi- fications that fall within the metes and bounds of the claims, or equivalence of such metes and bounds are intended to be embraced by the appended claims. [283] Claims [ 1 ] A method for allocating reference signals for a sub-frame in a wireless multiple- input multiple-output (MIMO) communication system, the sub-frame comprising a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols in a time domain and a plurality of sub-carriers in a frequency domain, the method comprising: allocating a plurality of first reference signals for a first antenna on a first OFDM symbol over a sub-frame for the first antenna at regular intervals in the frequency domain; allocating a plurality of second reference signals for a second antenna on the first OFDM symbol over a sub-frame for the second antenna at regular intervals in the frequency domain such that the plurality of second reference signals does not overlap with the plurality of first reference signals; allocating a plurality of third reference signals for a third antenna on a second OFDM symbol over a sub-frame for the third antenna at regular intervals in the frequency domain, wherein the second OFDM symbol is contiguous with the first OFDM symbol; and allocating a plurality of fourth reference signals for a fourth antenna on the second OFDM symbol over a sub-frame for the fourth antenna such that the plurality of fourth reference signals does not overlap with the plurality of third reference signals. [2] The method of claim 1, wherein intervals in the frequency domain of the plurality of first reference: signals, the plurality of second reference signals, the plurality of third reference signals and the plurality of first reference signals are same. [3] The method of claim 1, wherein the locations in the frequency domain of the plurality of third reference signals are same as those of the plurality of first reference signals and the locations in the frequency domain for the plurality of fourth reference signals are same as those of the plurality of second reference signals. [4] The method of claim 1, wherein the locations in the frequency domain of the plurality of fourth reference signals are same as those of the plurality of first reference signals and the locations in the frequency domain for the plurality of third reference signals are same as those of the plurality of second reference signals. [5] The method of claim 1, further comprising: allocating a plurality of additional first reference signals for the first antenna on a third OFDM symbol over the sub-frame for the first antenna at same intervals for the plurality of first reference signals in the frequency domain, wherein the third OFDM symbol is non-contiguous with the first OFDM symbol and the second OFDM symbol; and allocating a plurality of additional second reference signals for the second antenna on the third OFDM symbol over the sub-frame for the second antenna at same intervals for the plurality of second reference signals in the frequency domain such that the plurality of additional second reference signals does not overlap with the plurality of additional first reference signals, [ 6] The method of claim 5, wherein the locations in the frequency domain of the plurality of additional first reference signals are same as those of the plurality of second reference signals and the locations in the frequency domain for the plurality of additional second reference signals are same as those of the plurality of first reference signals. [7] The method of claim 5, further comprising allocating a plurality of additional third reference signals for the third antenna on a fourth OFDM symbol over the sub-frame for the third antenna at same intervals for the plurality of third reference signals in the frequency domain, wherein the fourth OFDM symbol is non-contiguous with the first OFDM symbol and the second OFDM symbol and is contiguous with the third OFDM symbol; and allocating a plurality of additional fourth reference signals for the fourth antenna on the fourth OFDM symbol over the sub-frame for the fourth antenna at same intervals for the plurality of fourth reference signals in the frequency domain such that the plurality of additional fourth reference signals does not overlap with the plurality of additional third reference signals. [8] The method of claim 7, wherein the locations in the frequency domain of the plurality of additional first reference signals are same as those of the plurality of second reference signals and the locations in the frequency domain for the plurality of additional second reference signals are same as those of the plurality of first reference signals. [9] The method of claim 7, wherein the locations in the frequency domain of the plurality of additional third reference signals are same as those of the plurality of fourth reference signals and the locations in the frequency domain for the plurality of additional fourth reference signals are same as those of the plurality of third reference signals. [10] The method of claim 1, further comprising: allocating a plurality of additional third reference signals for the third antenna on a third OFDM symbol over the sub-frame for the third antenna at same intervals for the plurality of third reference signals in the frequency domain, wherein the third OFDM symbol is non-contiguous with the first OFDM symbol and the second OFDM symbol; and allocating a plurality of additional fourth reference signals for the fourth antenna on the third OFDM symbol over the sub-frame for the fourth antenna at same intervals for the plurality of fourth reference signals in the frequency domain such that the plurality of additional fourth reference signals does not overlap with the plurality of additional third reference signals. [11] The method of claim 10, wherein the locations in the frequency domain of the plurality of additional third reference signals are same as those of the plurality of fourth reference signals and the locations in the frequency domain for the plurality of additional fourth reference signals are same as those of the plurality of third reference signals. [12] The method of claim 1, wherein the first OFDM symbol is near the beginning of a transmission time interval (TTI), the TTI comprising at least two OFDM symbols. [ 13] The method of claim. 1, further comprising: allocating a plurality of reference signals for the multi-user signal on a third OFDM symbol, wherein the third OFDM symbol is contiguous with the first OFDM symbol or the second OFDM symbol. [14] A method for placing reference signals in a wireless communication system, comprising: preparing a plurality of sub-frames for a plurality of antennas, one sub-frame comprising a plurality of OFDM symbols in a time domain and a plurality of sub-carriers in a frequency domain; placing a reference signal for one sub-frame; and placing a reference signal for another sub-frame not to overlap with the reference signal for one sub-frame, wherein the reference signal for one sub-frame and the reference signal for another sub-frame are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. [15] The method of claim 14, further comprising placing a null symbol for another sub-frame to overlap with the reference signal for one sub-frame. [16] The method of claim 14, further comprising placing a null symbol for one sub- frame to overlap with the reference signal for another sub-frame. [17] A method for placing reference signals in a wireless communication system, comprising: placing a plurality of reference signals for dedicated signal; and placing a plurality of reference signals for multi-user signal such that intervals in the frequency domain of the plurality of reference signals for multi-user signal are shorter than that of the plurality of reference signals for dedicated signal. f 18] An apparatus for an OFDM based wireless MIMO communication system, the apparatus comprising: a plurality of transmit antennas; a multiplexer for allocating a plurality of reference signals for the plurality of transmit antennas not to overlap with each other, wherein at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers; and an OFDM modulator for modulating the plurality of reference signals. [19] An apparatus for an OFDM based wireless communication system, the apparatus comprising: at least one receive antennas; and a channel estimator for estimating a channel using a plurality of reference signals for the plurality of transmit antennas, wherein the plurality of reference signals does not overlap with each other and at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. [20] A reference signal structure to provide information for channel estimation in an OFDM based wireless MIMO system, the reference signal structure comprising a plurality of reference signals for a plurality of antennas not to overlap with each other, wherein at least two reference signals among the plurality of reference signals are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. There is provided a method for placing reference signals in a wireless communication system. The method includes preparing a plurality of sub-frames for a plurality of antennas, placing a reference signal for one sub-frame and placing a reference signal for another sub-frame not to overlap with the reference signal for one sub-frame, wherein the reference signal for one sub-frame and the reference signal for another sub-frame are successively placed on contiguous OFDM symbols or on the contiguous sub-carriers. Channel estimation or data demodulation can be prevented from performance degradation.

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3943-KOLNP-2008-(16-01-2014)-ANNEXURE TO FORM 3.pdf

3943-KOLNP-2008-(16-01-2014)-ASSIGNMENT.pdf

3943-KOLNP-2008-(16-01-2014)-CORRESPONDENCE.pdf

3943-KOLNP-2008-(17-10-2008)-FORM-13.pdf

3943-KOLNP-2008-(20-08-2014)-CORRESPONDENCE.pdf

3943-KOLNP-2008-(20-08-2014)-OTHERS.pdf

3943-KOLNP-2008-(26-09-2014)-ANNEXURE TO FORM 3.pdf

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3943-kolnp-2008-abstract.pdf

3943-kolnp-2008-claims.pdf

3943-kolnp-2008-correspondence.pdf

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