Wireless communications system, wireless communications method, and wireless communications apparatuses

BACKGROUND OF THE INVENTION

Field of the Invention

0001The present invention relates to a system, apparatus, method and computer program for a wireless communication among a plurality of wireless stations, such as a communication by a wireless LAN (Local Area Network). In particular, the invention relates to such a system, method and apparatus which realize a broadband wireless transmission in home or other similar communication environments.

0002More specifically, this invention relates to a system, apparatus, method and computer program which enhance the transmission capacity by employing a communication where a transmitter and a receiver each having a plurality of antennas communicate with each other using space division multiplexing, that is, MIMO communication; in particular, the invention relates to such a system, method and apparatus, which are adapted to perform MIMO transmission using a singular value decomposition (SVD) of a channel information matrix each element in which represents propagation information of one of sub-channels each linking a pair of an antenna of the transmitter and an antenna of the receiver.

Description of Related Art

0003Computer networking such as LAN efficiently enables to share information and apparatus resources. Wireless LAN is attracting attention of people as a system for relieving users from the conventional wired LAN construction. In a working space such as an office, most of cables and wires can be dispensed with by employing a wireless LAN, facilitating relocation of a communication terminal such as a personal computer.

0004Recently, demand for wireless LAN has increased with the speed improvement and price-reduction of wireless LAN. In particular, to establish a small wireless network of a plurality of electronic devices present around people so as to enable communications thereamong, introduction of Personal Area Network (PAN) is considered. For instance, there are defined various wireless communications systems and apparatuses using respective frequency bands, e.g. 2.4GHz and 5GHz bands, which are permitted for use without a license from the supervisory authority.

0005One of the standards related to wireless networking is IEEE (the Institute of Electrical and Electronics Engineers) 802.11 (see nonpatent literature 1). IEEE 802.11 standard is further divided, depending upon the employed methods and used frequencies, into IEEE 802.11a, IEEE 802.11b...etc., defining respective wireless communications methods.

0006IEEE 802.11a standard supports a modulation method achieving a communication speed of up to 54Mbps. However, there is demand for a wireless standard capable of realizing a higher bit rate as the communication speed. In this situation, MIMO (Multi-Input Multi-Output) communication technology has recently attracted increased attention. This technology is for enhancing the communication speed by providing both of the transmitter and receiver with a plurality of antennas, so as to realize space division multiplexing, i.e., a plurality of sub-channels which are logically independent of one another, to increase the transmission capacity. Using the space division multiplexing, MIMO is bandwidth-efficient.

0007<figref idref="f0006">Fig. 7</figref> schematically shows a MIMO communications system, where each of a transmitter and a receiver is equipped with a plurality of antennas. The transmitter space-time encodes N data for transmission to be multiplexed, and distributes the encoded data to M antennas of the transmitter from which the data are sent over a channel to the receiver in a multiplexed fashion. The receiver receives and space-time decodes the data received through N antennas thereof via the channel, to obtain received data. Thus, a MIMO communication is not the same as a communication by a simple transmission/reception adaptive array. In MIMO, the channel model involves an RF environment (transfer function) on the side of the transmitter, a construction (transfer function) of the channel space, and an RF environment (transfer function) on the side of the receiver. When a signal is transmitted from antennas in a multiplexed fashion, crosstalk occurs; by signal processing performed on the part of the receiver, the multiplexed signal is retrieved correctly.

0008In brief, a MIMO system is such that the transmitter sends out the transmitted data or signal by distributing components of the data to the plural antennas thereof (hereinafter referred to as "transmit antennas"), and the receiver obtains received data by processing the signal components received through the plural antennas thereof (hereinafter referred to as "receive antennas"), and is a communications system utilizing a characteristic of the channel. Although there are various applications of the MIMO transmission technology, SVD-MIMO system as one of ideal modes of MIMO is known that uses SVD (Singular Value Decomposition) of a propagation function. See Patent Document 2, and Nonpatent Literature 2, for instance.

0009<figref idref="f0006">Fig. 8</figref> schematically shows a SVD-MIMO transmission system, where a matrix of numbers, i.e., a channel information matrix H, each of whose elements represents information on each of sub-channels linking respective antenna pairs, is subjected to a singular value decomposition to obtain UDV^H, and an antenna weighting coefficient matrix V on the part of the transmitter (hereinafter referred to as "transmit antenna weighting coefficient matrix V") and an antenna weighting coefficient matrix U^H" on the part of the receiver (hereinafter referred to as "receive antenna weighting coefficient matrix U^H") are provided. Accordingly, the channel information is expressed by a diagonal matrix whose diagonal elements are square roots of respective eigenvalues λ_i. Thus, a signal can be transmitted in a multiplexed fashion without suffering from crosstalk at all. However, in the SVD-MIMO transmission system, it is not easy to perform the operation of the SVD in real time, and the set-up procedure such that the derived V or U^H is beforehand communicated to the other part of the communication is essential.

0010It is possible to achieve the theoretically maximum communication capacity by the SVD-MIMO transmission system. For instance, where the transmitter and receiver respectively have two antennas, a transmission capacity of two times large at maximum can be achieved.

0011There will now be described the scheme of the SVD-MIMO transmission system. Where the numbers of antennas of the transmitter and receiver are M and N, respectively, transmitted signal x is represented as vector (M × 1) while the received signal y is represented by vector (N × 1). In this case, the channel information can be represented as a matrix H of N × M. An entry h_ij of the channel information matrix H represents a transfer function with respect to a sub-channel from a j-th transmit antenna to an i-th receive antenna. A vector y representing the received signal equals to a multiplication of the matrix H by the vector of the transmitted signal, plus a noise vector n, and is expressed by the following equation (1):<maths id="math0001" num="(1)"><math display="block"><mrow><mi>y</mi><mo>=</mo><mi mathvariant="italic">Hx</mi><mo>+</mo><mi>n</mi></mrow></math><img file="EP1530305B1_D0001.tif" /></maths>

0012The channel information matrix H subjected to the singular value decomposition as described above, is expressed by the following equation (2):<maths id="math0002" num="(2)"><math display="block"><mrow><mi>H</mi><mo>=</mo><msup><mi mathvariant="italic">UDV</mi><mi>H</mi></msup></mrow></math><img file="EP1530305B1_D0002.tif" /></maths>

0013In equation (2), the transmit antenna weighting coefficient matrix V and receive antenna weighting coefficient matrix U are unitary matrices which respectively satisfy the following equations (3) and (4):<maths id="math0003" num="(3)"><math display="block"><mrow><msup><mi>U</mi><mi>H</mi></msup><mi>U</mi><mo>=</mo><mi>I</mi></mrow></math><img file="EP1530305B1_D0003.tif" /></maths><maths id="math0004" num="(4)"><math display="block"><mrow><msup><mi>V</mi><mi>H</mi></msup><mi>V</mi><mo>=</mo><mi>I</mi></mrow></math><img file="EP1530305B1_D0004.tif" /></maths>

0014That is, the receive antenna weighting coefficient matrix U^H is an array of normalized eigenvectors of HH^H, while the transmit antenna weighting coefficient matrix V is an array of normalized eigenvectors of H^HH. Further, D represents a diagonal matrix whose diagonal elements are square roots of respective eigenvalues of H^HH or HH^H. The size of the matrix D corresponds to the smaller one of the numbers M and N of the transmit antennas and receive antennas, that is, the matrix D is a square diagonal matrix having a rank of min(M, N).<maths id="math0005" num="(5)"><math display="block"><mrow><mi>D</mi><mo>=</mo><mfenced open="[" close="]"><mtable><mtr><mtd><msqrt><mrow><msub><mi>λ</mi><mn>1</mn></msub></mrow></msqrt></mtd><mtd><mo>⋯</mo></mtd><mtd><mspace width="1em" /></mtd><mtd><mn>0</mn></mtd></mtr><mtr><mtd><mo>⋮</mo></mtd><mtd><msqrt><mrow><msub><mi>λ</mi><mn>2</mn></msub></mrow></msqrt></mtd><mtd><mspace width="1em" /></mtd><mtd><mspace width="1em" /></mtd></mtr><mtr><mtd><mspace width="1em" /></mtd><mtd><mspace width="1em" /></mtd><mtd><mo>⋱</mo></mtd><mtd><mspace width="1em" /></mtd></mtr><mtr><mtd><mn>0</mn></mtd><mtd><mspace width="1em" /></mtd><mtd><mspace width="1em" /></mtd><mtd><msqrt><mrow><msub><mi>λ</mi><mrow><mi>min</mi><mfenced separators=","><mi>M</mi><mi>N</mi></mfenced></mrow></msub></mrow></msqrt></mtd></mtr></mtable></mfenced></mrow></math><img file="EP1530305B1_D0005.tif" /></maths>

0015In the above description related to the singular value decomposition, a case where only real numbers are involved is assumed. It is noted that in the case where imaginary numbers are also involved, even where eigenvectors of the matrices U and V, each of which is a matrix of eigenvectors, are manipulated so that the norm of each matrix is 1, that is, normalized, an infinite number of eigenvectors having respective phases, not a single eigenvector, exist. In some cases, the equation (2) can not be established depending upon the phase difference between U and V, namely, where U and V are correct but have different phases. To completely synchronize the phases, V is obtained as a matrix of eigenvectors of H^HH as ordinary, while U is obtained by multiplying both terms of the equation (2) by V, as expressed by the following equation (6): <maths id="math0006" num="(6)"><math display="block"><mrow><mtable><mtr><mtd><mi mathvariant="italic">HV</mi><mo>=</mo><msup><mi mathvariant="italic">UDV</mi><mi>H</mi></msup><mi>V</mi><mo>=</mo><mi mathvariant="italic">UDI</mi><mo>=</mo><mi mathvariant="italic">UD</mi></mtd></mtr><mtr><mtd columnalign="left"><mi>U</mi><mo>=</mo><msup><mi mathvariant="italic">HVD</mi><mrow><mo>−</mo><mn>1</mn></mrow></msup></mtd></mtr></mtable></mrow></math><img file="EP1530305B1_D0006.tif" /></maths>

0016The transmitter weights the components of the signal for respective sub-channels by the transmit antenna weighting coefficient matrix V, while the receiver receives the signal with weighting the components by an inverse weighting coefficient matrix U^H; since each of U and V is a unitary matrix (U is a matrix of N × min(M, N) while V is a matrix of M × min(M, N)), the following expression is obtained:<maths id="math0007" num="(7)"><math display="block"><mrow><mtable><mtr><mtd columnalign="left"><mi>y</mi><mo>=</mo><msup><mi>U</mi><mi>H</mi></msup><mi mathvariant="italic">HVx</mi><mo>+</mo><msup><mi>U</mi><mi>H</mi></msup><mi>n</mi></mtd></mtr><mtr><mtd columnalign="left"><mtable><mtr><mtd columnalign="left"><mo>=</mo><msup><mi>U</mi><mi>H</mi></msup><mfenced><msup><mi mathvariant="italic">UDV</mi><mi>H</mi></msup></mfenced><mi mathvariant="italic">Vx</mi><mo>+</mo><msup><mi>U</mi><mi>H</mi></msup><mi>n</mi></mtd></mtr><mtr><mtd columnalign="left"><mo>=</mo><mfenced separators=""><msup><mi>U</mi><mi>H</mi></msup><mi>U</mi></mfenced><mi>D</mi><mfenced separators=""><msup><mi>V</mi><mi>H</mi></msup><mi>V</mi></mfenced><mi>x</mi><mo>+</mo><msup><mi>U</mi><mi>H</mi></msup><mi>n</mi></mtd></mtr><mtr><mtd columnalign="left"><mo>=</mo><mi mathvariant="italic">IDIx</mi><mo>+</mo><msup><mi>U</mi><mi>H</mi></msup><mi>n</mi></mtd></mtr></mtable></mtd></mtr><mtr><mtd columnalign="left"><mi>y</mi><mo>=</mo><mi mathvariant="italic">Dx</mi><mo>+</mo><msup><mi>U</mi><mi>H</mi></msup><mi>n</mi></mtd></mtr></mtable></mrow></math><img file="EP1530305B1_D0007.tif" /></maths>

0017The vectors y and × are not determined by the numbers of the antennas of the transmitter and the receiver, but are respectively expressed by (min(M, N) × 1).

0018Since D is a diagonal matrix, each transmitted signal can be received without suffering from the crosstalk. The amplitude of each of the sub-channels which are independent from one another is proportional to the square root of the eigenvalue λ, and thus the power of each sub-channel is proportional to λ.

0019As to the noise component n, since the columns of U are the eigenvectors normalized so that the norm is 1, U^Hn does not affect the noise power of the received signal. U^Hn is a vector whose size is min(M, N), which is the same size as y and x.

0020As described above, in the SVD-MIMO transmission, plural independent logical sub-channels free from crosstalk even where occupying the same frequency band and the same time period can be obtained. This means that it is enabled to simultaneously transmit plural data using a same frequency band, improving the transmission speed.

0021In the SVD-MIMO system, the receiver must obtain the channel information matrix H, implement the singular value decomposition, and communicate V^H as a factor of UDV^H obtained as the result of the decomposition to the transmitter. In effect, the transmitter uses V and therefore V must be communicated to the transmitter.

0022An amount of information carried by the transmit antenna coefficient matrix V will be now discussed, by taking for example IEEE 802.11a which defines one of LAN systems where the SVD-MIMO transmission is applicable, namely, OFDM (Orthogonal Frequency Division Multiplexing) of 5GHz band.

0023Where each of the transmitter and receiver has three antennas, the transmit antenna weighting coefficient matrix V is a 3 × 3 matrix, having nine elements. In this case, when each element is a complex number represented using 10 bits, and 52 carriers are provided, a total of 9360 bits of information, i.e., 9 (the number of elements of the matrix) × 2 (the real and imaginary part of a complex number) × 10 × 52 (the number of OFDM sub-carriers), has to be fed back to the transmitter from the receiver.

0024The MIMO requiring such feedback is called closed-loop MIMO, while the opposite thereof is open-loop MIMO. A closed-loop SVD-MIMO system must feedback information of that much (9360 bits) to the transmitter, upon initiation of a communication. Let us assume that the information is fed back where the most reliable one in the modulation schemes provided by IEEE 802.11a, i.e., BPSK is employed as a first modulation method, the coding rate is 1/2, and OFDM is employed as a second modulation method. Since 1 OFDM symbol can carry only 24 bits, 390 OFDM symbols are required for the transmission of the information, making the SVD-MIMO unpractical.

0025As one of embodiments for realizing the above-described set-up processing in the MIMO transmission by a relatively simple mechanism, a technique called V-BLAST is known. V-BLAST is an acronym of "Vertical Bell Laboratories Layered Space Time" and refers to a technology originally developed by the now-defunct AT &amp; T Bell Laboratories. See Patent Documents 1, for instance.

0026<figref idref="f0007">Fig. 9</figref> schematically shows a structure of a V-BLAST communications system. The major difference between the V-BLAST and SVD-MIMO systems is that the transmitter in the V-BLAST does not provide the antenna weighting coefficient matrix V, but simply multiplexes a signal with respect to the transmit antennas and the feedback processing for beforehand providing the antenna weighting coefficient matrix V is all omitted. The transmitter inserts, prior to sending the multiplexed signal, training signals to be used in channel estimation by the receiver, in the multiplexed signal. For instance, the training signals for respective antennas are inserted in the signal in a time division fashion. In the example of <figref idref="f0007">Fig. 9</figref>, the training signals are sent included in the data packet such that a training signal Training-1 corresponding to an antenna #1 is sent following a preamble signal and a training signal Training-2 corresponding to an antenna #2 is subsequently sent, in a time division fashion.

0027On the part of the receiver, a channel estimator thereof performs a channel estimation using the training signals, to calculate the channel information matrix H representing information on the sub-channels linking respective antenna pairs. A first antenna weighting coefficient matrix calculator performs zero-forcing or others for each of signals corresponding to the respective transmit antennas so as to cancel unnecessary signals, i.e., signals other than those for the respective receive antennas, and obtains a receive antenna weighting coefficient matrix Z_R. The transmitted signal having the highest S/N ratio among the signals retrieved after Z_R is provided, is first decoded to obtain a signal x₁.

0028Next, the decoded signal is encoded again by an encoder to generate a replica (duplicate) of the transmitted signal x₁, which is canceled from the signals just received by the receive antennas. A second receive antenna weighting coefficient matrix calculator excludes the transmit antenna corresponding to the transmitted signal x₁ as canceled, and again applies zero-forcing criteria to each of the other signals, to calculate a receive antenna weighting coefficient matrix Z_R'. The signal x₂ exhibiting the highest S/N ratio among the remaining received signals is decoded by the decoder.

0029In the second decoding, since the transmitted signal as decoded first is eliminated, the degree of freedom of the receive antennas is enhanced and the effect of maximal ratio combining is accordingly improved. Thereafter, all transmitted signals as multiplexed are sequentially decoded by iteration of the above-described processing.

0030As described above, a characteristic of the V-BLAST resides in that zero-forcing and canceling are sophisticatedly combined so that even a signal whose S/N ratio can not be made sufficiently high only by application of zero-forcing criteria can be improved in S/N ratio by taking advantage of the degree of freedom of the antennas which is provided by the canceling, and thus the accuracy of the decoding is enhanced. Thus, the V-BLAST can realize an efficient MIMO transmission system by a combination of relatively simple mechanisms.

0031However, since the transmitter does not perform the weighting before the data transmission, the receiver is required to implement the first decoding only by zero-forcing, without performing the canceling operation. Thus, the number of receive antennas is made larger than that of the transmit antennas so as to obtain a redundancy in degree of freedom of the receive antennas. In the example shown in <figref idref="f0007">Fig. 9</figref>, two transmit antennas and three receive antennas are provided.

0032Document <patcit id="pcit0001" dnum="US2003130003A"><text>US-A-2003130003</text></patcit> (Patent Document 3) discloses a method and apparatus for allocating a power in a multiple-input multiple output (MIMO) communication system.

0033<ul id="ul0001" list-style="none" compact="compact"><li><b>[Patent Document 1]</b><patcit id="pcit0002" dnum="JP10084324A"><text>JP-A-10-84324</text></patcit></li><li><b>[Patent Document 2]</b><patcit id="pcit0003" dnum="US6058105A"><text>U. S. Pat. No. 6058105</text></patcit></li><li><b>[Patent Document 3]</b><patcit id="pcit0004" dnum="US2003130003A"><text>US-A 2003130003</text></patcit></li><li><b>[Nonpatent Literature 1]</b> International Standard ISO/IEC 8802-11:1999 (E) ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications</li><li><b>[Nonpatent Literature 2]</b> http://radio3.ee.uec.ac.jp/MIMO(IEICE_TS).pdf ( as of October 24, 2003 )</li></ul>

SUMMARY OF THE INVENTION

0034A first object of the present invention is to provide an excellent wireless communications system, wireless communication method and wireless communications apparatus, which realizes a broadband wireless transmission under a communication environment such as in a home.

0035A second object of the invention is to provide an excellent wireless communications system, wireless communication method and wireless communications apparatus, which can enhance the transmission capacity by implementing a communication (MIMO communication) using space division multiplexing between a transmitter and a receiver each having a plurality of antennas.

0036A third object of the invention is to provide an excellent wireless communications system, wireless communication method and wireless communications apparatus, which can suitably implement a MIMO communication using a singular value decomposition (SVD) of a channel information matrix whose elements respectively represent a gain on each sub-channel linking a pair of a transmitter and a receiver.

0037A fourth object of the invention is to provide an excellent wireless communications system, wireless communication method and wireless communications apparatus, which can reduce an amount of information fed back from a receiver to a transmitter in a SVD-MIMO transmission.

0038The present invention has been developed for attaining the above-described objects, and aspects of the invention provide a wireless communications system, a wireless communications method, a transmitter and a receiver according to the appended claims.

0039It is noted that the term "system" means a logical assembly of a plurality of apparatuses (or functional modules for realizing respective specific functions); the apparatuses or functional modules may or may not be enclosed in a single housing.

0040According to the present invention, instead of feeding back the antenna weighting coefficient matrix V of the transmitter, which is obtained by performing a singular value decomposition of the channel information matrix obtained by the receiver, the receiver sends the reference signals or symbols to the transmitter on which part a singular value decomposition is performed to obtain a transmit antenna weighting coefficient matrix V necessary when transmitting data. Thus, an amount of information fed back to the transmitter from the receiver can be reduced.

0041Further, even though the amount of the information fed back is reduced, the transmitter sends data where the transmit antenna weighting coefficient matrix V is applied, the receiver can obtain a satisfactory decoding capability with a reduced number of antennas.

0042The invention can provide the excellent wireless communications system, wireless communication method and wireless communications apparatus, where the pair of the transmitter and receiver each having the plural antennas is capable of making a communication using space division multiplexing (MIMO communication) in which the transmission capacity is enhanced.

0043Further, the invention can provide the excellent wireless communications system, wireless communication method and wireless communications apparatus, which are capable of performing a MIMO transmission using the singular value decomposition (SVD) of the channel information matrix each of whose elements corresponds to the characteristic of each sub-channel linking each pair of a transmit antenna and a receive antenna.

0044The invention can also provide the excellent wireless communications system, wireless communications method and wireless communications apparatus, which are capable of reducing the amount of information fed back from the receiver to the transmitter in performing the SVD-MIMO transmission.

0045Incidently, in addition to the above-mentioned related art, there have been proposed by the present inventors techniques related to the present invention as disclosed in unpublished Japanese Patent Application Nos. <patcit id="pcit0005" dnum="JP2003426294A"><text>2003-426294</text></patcit>, <patcit id="pcit0006" dnum="JP2004040934A"><text>2004-040934</text></patcit>, <patcit id="pcit0007" dnum="JP2004140485A"><text>2004-140485</text></patcit>, <patcit id="pcit0008" dnum="JP2004140488A"><text>2004-140488</text></patcit> and <patcit id="pcit0009" dnum="JP2004140486A"><text>2004-140486</text></patcit>.

0046The further objects, features and advantages of the invention will be clarified by the more detailed illustration of the invention based on embodiments of the invention as described below and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

0047<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> schematically shows a construction of a MIMO communications system according to a first embodiment of the invention.</li><li><figref idref="f0002">Fig. 2</figref> schematically shows a construction of a MIMO communications system according to a second embodiment of the invention.</li><li><figref idref="f0003">Fig. 3</figref> is a view for illustrating a reversibility of transfer functions of a communications channel in the uplink and downlink directions.</li><li><figref idref="f0004">Fig. 4</figref> schematically shows a structure of a communications apparatus having a plurality of antenna elements.</li><li><figref idref="f0004">Fig. 5</figref> is a view for illustrating a procedure of obtaining a loopback transfer function implemented in the apparatus of <figref idref="f0004">Fig. 4</figref>.</li><li><figref idref="f0005">Fig. 6</figref> is a flowchart illustrating steps of a procedure of a calibration for compensating an error in characteristics of transfer functions of transmitting and receiving analog circuits.</li><li><figref idref="f0006">Fig. 7</figref> illustrates a concept of MIMO communications system.</li><li><figref idref="f0006">Fig. 8</figref> illustrates a concept of SVD-MIMO transmission system.</li><li><figref idref="f0007">Fig. 9</figref> illustrates a concept of V-BLAST communications system.</li></ul>

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

0048There will be described embodiments of the invention by reference to the drawings.

0049In this invention, a receiver does not feed back to a transmitter a transmit antenna weighting coefficient matrix V as obtained by performing a singular value decomposition of a channel information matrix H obtained by the receiver to yield UDV^H, but sends reference signals or symbols to a transmitter, so that the transmitter performs a singular value decomposition to obtain the transmit antenna weighting coefficient matrix V necessary when transmitting data. The amount of information fed back to the transmitter from the receiver can be thus reduced.

0050<figref idref="f0001">Fig. 1</figref> schematically shows a construction of an SVD-MIMO communications system according to a first embodiment of the invention.

0051A transmitter space-time encodes each transmitted signal to multiplex the signal and distributes the multiplexed signal to three antennas to send the signal therefrom to a receiver over a channel. The receiver receives the multiplexed signal via the channel through two antennas and space-time decodes the signal to obtain received signal or data.

0052The communications system shown resembles the V-BLAST system shown in <figref idref="f0007">Fig. 9</figref> in general. However, the transmitter, not the receiver, provides an antenna weighting coefficient when transmitting the data, and the antenna configuration of the transmitter and receiver is such that the number of the transmit antennas is larger than that of the receive antennas. The number of the receive antennas corresponds to the number of signal sub-channels.

0053In the system shown in <figref idref="f0001">Fig. 1</figref>, the part of the transmitter has a redundancy in the degree of freedom of the antennas. To take advantage of this redundancy for improving the S/N ratio of the received signal, the transmitter sends a signal weighted by MSN (Maximum Signal-to-Noise ratio) which is criteria for maximizing the S/N ratio of signal of self, by zero-forcing, or by a combination of the MSN and zero-forcing. As a result, even where a redundancy in the degree of freedom of the antennas on the part of the receiver is not available (that is, the number of receive antennas is relatively small), the degree of freedom on the part of the transmitter can compensate this, to assure a satisfactory decoding capability.

0054The operational procedure in the present communications system will be described.

0055As a preparatory step, a training signal "Pre-training Signal" as a reference symbol with respect to each antenna is sent from the receiver 20 in a time division fashion. In the specific example of <figref idref="f0001">Fig. 1</figref>, the receiver has two receive antennas, and therefore two Pre-training Signals are sent. A preamble "Preamble" prefixed to the "Pre-training Signal" is an additive signal for serving a signal detection, a timing synchronization and an adjustment of receiver gain.

0056The transmitter 10 receives the training signal from the receiver 20 as a reference signal, calculates the channel information matrix H by a channel estimator 11 of the transmitter 10, and determines a transmit antenna weighting coefficient matrix Z_T by a transmit antenna weighting coefficient matrix calculator 13 by applying the MSN, zero-forcing, or combination of these, with respect to each antenna.

0057Subsequently, the transmitter 10 sends a concatenation of training signals and a signal as a component of the signal indicative of the data of interest, which is obtained by multiplexing the signal by space division. The training signals are weighted for reflecting the characteristics of the respective corresponding antennas by using the matrix Z_T obtained as described above. It is particularly noted that even in the period where the training signals are sent out, the weighting for reflecting the characteristics of the corresponding antennas is performed for each signal multiplexed. In the example of <figref idref="f0001">Fig. 1</figref>, training signals Training-1 and Training-2, which are respectively subjected to weighting by element vectors w₁ and w₂ of the antenna weighting coefficient matrix Z_T (=[w₁, w₂]), are sent by time division.

0058On the other hand, a channel estimator 21 of the receiver 20 calculates a channel information matrix H' each element of which corresponds to a pair of one of the transmit weighting coefficient vectors and a corresponding receive antenna, based on the training signals Training-1 and -2 as weighted with respect to respective signal components sent in a multiplexed fashion.

0059A first receive antenna weighting coefficient matrix calculator 22 performs zero-forcing for each transmit antenna to cancel the unnecessary signals other than a signal related to the receive antenna itself, so as to obtain a receive antenna weighting coefficient matrix Z_R. Among the signals retrieved after the matrix Z_R is provided, the signal exhibiting the highest S/N ratio is first decoded by a decoder 23 into x₁.

0060Next, the encoder 24 encodes the signal as decoded once again to produce a replica (duplicate) of the transmitted signal, which is canceled from a signal just received by the antenna. A second receive antenna weighting coefficient matrix calculator 25 excludes the corresponding transmit antenna and performs again zero-forcing for the signal to calculate a receive antenna weighting coefficient matrix Z_R'. The signal x₂ exhibiting the highest S/N ratio among the remaining received signals is retrieved to be decoded by the decoder 23. In the second decoding operation, since the transmitted signal as first decoded is eliminated, the degree of freedom of the receive antennas is increased, accordingly enhancing the effect of maximal ratio combining. By iterating the above-described processing, all the multiplexed transmitted signals are decoded in sequence.

0061The first embodiment is such that the transmitter 10 performs transmission of signals by using the MSN, zero-forcing, or combination of these, in weighting the signals. Thus, the degree of freedom of the transmit antennas is fully exploited, enhancing the S/N ratio of the received signals. Hence, even where there is no redundancy in the degree of freedom of antennas on the part of the receiver 20, the redundancy of the degree of freedom on the part of the transmitter can compensate this.

0062<figref idref="f0002">Fig. 2</figref> is a diagram illustrating a construction of a communications system according to a second embodiment of the invention.

0063The system of <figref idref="f0002">Fig. 2</figref> is identical with the system of <figref idref="f0001">Fig. 1</figref> in that each transmitted signal multiplexed on the part of the transmitter is space-time decoded to be distributed to plural antennas through which the signal components are sent to the receiver over respective sub-channels of a channel in a multiplexed fashion, and the receiver space-time decodes the signal components received through plural antennas via the sub-channels to obtain a received signal or data.

0064In the embodiment of <figref idref="f0001">Fig. 1</figref>, the transmit antenna weighting coefficient matrix calculator 13 determines, for each antenna, the transmit antenna weighting coefficient matrix Z_T by the MSN, zero-forcing, or combination of these, based on the channel information matrix H obtained by a calculation using the training signals from the receiver 20. On the other hand, the second embodiment shown in <figref idref="f0002">Fig. 2</figref> is such that a singular value decomposition unit 15 employs the SVD (Singular Value Decomposition) in calculating the transmit antenna weighting coefficient, and weights the signal by the weighting coefficient matrix V before transmission of the signal.

0065When zero-forcing criteria is applied to the training signals sent with weighted by V, the weighting coefficient matrix on the part of the receiver 20 necessarily becomes U^H. Therefore, it is obvious that if the SVD calculation on the part of the transmitter 10 is allowed, a SVD-MIMO transmission without communication of U^H to the receiver 20 is enabled, omitting the necessity to perform the singular value decomposition on the part of the receiver 20. That is, according to the present embodiment, a MIMO system with 2 × 2 antennas can be relatively easily realized.

0066On the part of the receiver 20, the channel estimator 21 calculates a channel information matrix H' each element of which corresponds to a pair of one of the transmit weighting coefficient vectors and a corresponding receive antenna. The first receive antenna weighting coefficient matrix calculator 22 performs zero-forcing for each transmit antenna to cancel unnecessary signals other than the signal related to the receiver itself, to obtain a receive antenna weighting coefficient matrix U^H. The signal exhibiting the highest S/N ratio among the received signals retrieved after U^H is provided is decoded by a decoder 23 to obtain a signal x₁.

0067Thereafter, the decoded signal is again encoded by an encoder 24, to produce a replica (duplicate) of the transmitted signal which is canceled from a received signal just received by the antenna. A second receive antenna weighting coefficient matrix calculator 25 excludes the transmit antenna corresponding to the transmitted signal subjected to the canceling, and again applies zero-forcing to the signal to calculate a receive antenna weighting coefficient matrix U^H. The signal x₂ exhibiting the highest S/N ratio among the remaining received signals is retrieved and decoded by the decoder 23. Or alternatively, the second multiplexed signal x₂ may be directly retrieved from each received signal retrieved after the first receive antenna weighting coefficient matrix calculator 22 has provided U^H.

0068Meanwhile, there is a problem that there is a noncoincidence between channel characteristics of a transmitting circuit and a receiving circuit of the transmitter 10. This is because of the following fact: Although the spatial transfer function shows a reversibility, the channel information matrix H, which is a function of the following factors: an RF environment (transfer function) on the side of the transmitter 10, a construction (transfer function) of the channel space, and an RF environment (transfer function) on the side of the receiver 20, where the transfer functions related to the transmitter 10 and receiver 20 show variation due to variation in characteristics of the RF transmitting and receiving analog circuits, is not assured of a reversibility between the uplink and downlink directions.

0069A channel transfer function as measured in the direction from the transmitter to the receiver has factors including a spatial transfer function showing reversibility, and a transmitter transfer function involving variation in the characteristic of the RF analog transmitting portion of the transmitter and a receiver transfer function involving variation in the characteristic of the RF analog receiving portion of the receiver, as irreversibility components. On the other hand, the channel transfer function measured in the opposite direction (from the receiver to the transmitter) has factors including the spatial transfer function showing reversibility, and a transmitter transfer function involving variation in the characteristic of the RF analog transmitting portion of the receiver and a receiver transfer function involving variation in the characteristic of the RF analog receiving portion of the transmitter, as irreversibility components. Hence, the channel transfer function as measured in the direction opposite to the direction of an actual data transmission is affected by the transfer functions of the transmitting RF analog circuit of the receiver and of the receiving RF analog circuit of the transmitter. See <figref idref="f0003">Fig. 3</figref>.

0070This irreversibility does not matter when the transmit antenna weighting coefficient matrix V obtained by the singular value decomposition of the channel information matrix as acquired by the receiver is fed back to the transmitter. However, where the antenna weighting coefficient matrix V' which is necessary for the transmitter in sending the data is obtained such that the receiver sends the reference signals or symbols to the transmitter which then performs the singular value decomposition, the matrix V' does not coincide with the antenna weighting coefficient matrix V for the direction from the transmitter to the receiver. Thus, a calibration is required.

0071The calibration is more specifically described. The entry h_ij of the channel information matrix H represents a transfer function of a sub-channel linking the j-th transmit antenna to the i-th receive antenna, and is expressed by the following equation (8):<maths id="math0008" num="(8)"><math display="block"><mrow><msub><mi mathvariant="italic">h</mi><mi mathvariant="italic">ij</mi></msub><mo>=</mo><mi mathvariant="italic">Transfer function of an j</mi><mo>−</mo><mi mathvariant="italic">th transmitting RF analog circuit of the transmitter</mi><mo>−</mo><mi mathvariant="italic">Spatial transfer function</mi><mo>×</mo><mi mathvariant="italic">Transfer function of a i</mi><mo>−</mo><mi mathvariant="italic">th receiving RF analog circuit of the receiver</mi></mrow></math><img file="EP1530305B1_D0008.tif" /></maths>

0072On the other hand, a transfer function h_ij' of the sub-channel in the opposite direction, namely, a channel linking the i-th receive antenna to the j-th transmit antenna, is expressed by the following equation (9).<maths id="math0009" num="(9)"><math display="block"><mrow><msub><mi mathvariant="italic">h</mi><mi mathvariant="italic">ij</mi></msub><mo>′</mo><mo>=</mo><mi mathvariant="italic">Transfer function of an i</mi><mo>−</mo><mi mathvariant="italic">th transmitting RF analog circuit of the receiver</mi><mo>−</mo><mi mathvariant="italic">Spatial transfer function</mi><mo>×</mo><mi mathvariant="italic">Transfer function of a j</mi><mo>−</mo><mi mathvariant="italic">th receiving RF analog circuit of the transmitter</mi></mrow></math><img file="EP1530305B1_D0009.tif" /></maths>

0073It is typical that the transfer functions of respective analog circuits are different from one another, due to the manufacturing error and depending upon the temperature, for instance. Accordingly, in the sub-channel linking the j-th antenna of the transmitter and the i-th antenna of the receiver, the transfer function h_ij in the forward direction (i.e., from the transmitter to the receiver) and the transfer function h_ij' in the reverse direction are different. Thus, in the method where the transmitter performs a singular value decomposition of H' to derive an equation H' = U'D'V'^H, while the receiver performs a singular value decomposition of H to derive an equation H = UDV^H, so that the transmitter weights the signal of interest by the antenna weighting coefficient matrix V' and sends the weighted signal to the receiver which decodes the received signal with weighting the signal by the matrix U^H, logically independent sub-channels of a channel can not be provided.

0074To solve the above-described problem, in the invention both the transmitter and receiver performs the calibration for compensating the error in the characteristic of the transfer functions of the transmitting and receiving analog circuits, so that a correct transmit antenna weighting coefficient matrix V' can be derived from the channel information matrix H' of the reverse direction.

0075The calibration referred to here corresponds to, for instance, a technique used for making the directivity in the downlink identical with that in the uplink in an adaptive array antennas, and is a method for compensating an error related to transmitting and receiving analog devices. It is known in the field that in the case of a device capable of 5GHz performance, once in a few hours is a sufficient frequency of the calibration.

0076The transmitter performs the calibration at a frequency, e.g., once in a few hours, and holds the calibration coefficient. The receiver also performs the calibration once in a few hours and holds the calibration coefficient.

0077When the reference signals or symbols for yielding the channel information matrix H' in the reverse direction, i.e. the channel information matrix of the uplink, is sent from the receiver to the transmitter, the receiver first implements the calibration using the calibration coefficient and sends the calibrated reference symbols. The transmitter implements the calibration using the calibration coefficient for the received reference symbols (see equation (9)) and performs the singular value decomposition of the channel information matrix H' constituted by the calibrated transfer functions, to obtain the transmit antenna weighting coefficient matrix V'.

0078There will be next described a method of the calibration. The transmitter and receiver independently perform the calibration.

0079<figref idref="f0004">Fig. 4</figref> schematically shows a structure of a communications apparatus having a plurality of antennas, focusing on the antennas and its vicinity.

0080A transmitting analog circuit Tx1 and a receiving analog circuit Rx1 belong to an antenna #1. Since the characteristics of the respective analog circuits Tx and Rx are different from each other, the calibration is required.

0081In one of methods of the calibration, a coupler is provided on the output side of the transmitting analog circuit belonging to an antenna of one of a plurality of antenna systems or elements, so that a loopback path, as shown in <figref idref="f0004">Fig. 4</figref>, which is connected to a receiving analog circuit belonging to another antenna is made.

0082A loopback transfer function is obtained as follows. As shown in <figref idref="f0004">Fig. 5</figref>, a coupler is provided on the output side of a transmitting analog circuit belonging to the antenna #2, to enable acquisition of the reverse loopback transfer function.

0083With employing one of transmitting and receiving RF circuits as a reference, a ratio of a transfer function of each RF circuit to the reference is obtained. Where the RF circuit belonging to the antenna #1 is employed as a reference, the following transfer function can be obtained from the loopback shown in <figref idref="f0004">Fig. 4</figref>:<maths id="math0010" num="(10)"><math display="block"><mrow><msub><mi>T</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jμ</mi><mn>1</mn></msub></mfenced><mi>R</mi></mrow><mi>i</mi></msub><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mi>i</mi></msub></mfenced></mrow></math><img file="EP1530305B1_D0010.tif" /></maths>

0084Similarly, the following transfer function can be obtained from the loopback shown in <figref idref="f0004">Fig. 5</figref>:<maths id="math0011" num="(11)"><math display="block"><mrow><msub><mi>R</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mn>1</mn></msub></mfenced><mi>T</mi></mrow><mi>i</mi></msub><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jμ</mi><mi>i</mi></msub></mfenced></mrow></math><img file="EP1530305B1_D0011.tif" /></maths>

0085The calibration coefficient is a ratio of the former transfer function to the latter one and expressed by the following equation:<maths id="math0012" num="(12)"><math display="block"><mrow><msub><mi>C</mi><mi>i</mi></msub><mo>=</mo><mfrac><mrow><msub><mi>T</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">iμ</mi><mn>1</mn></msub></mfenced><mi>R</mi></mrow><mi>i</mi></msub><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mi>i</mi></msub></mfenced></mrow><mrow><msub><mi>R</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mn>1</mn></msub></mfenced><mi>T</mi></mrow><mi>i</mi></msub><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jμ</mi><mi>ι</mi></msub></mfenced></mrow></mfrac></mrow></math><img file="EP1530305B1_D0012.tif" /></maths>

0086In a 2 × 2 MIMO communications system where the number of antenna elements of both of the transmitter and the receiver is two, the calibration coefficient of the transmitter is represented by CT(i) while the calibration coefficient of the receiver is represented by CR(i), where i indicates the antenna number.

0087A procedure of the calibration for compensating the error of the characteristics of the transfer functions of the transmitting and receiving analog circuits will be described referring to a flowchart shown in <figref idref="f0005">Fig. 6</figref>.

Step 0:

0088The transmitter obtains the calibration coefficient CT(j) with respect to the j-th antenna of the transmitter, while the receiver obtains the calibration coefficient CR(i) with respect to the i-th antenna of the receiver.

Step 1:

0089The transmitter sends an OFDM symbol from each antenna j, by time division. The OFDM symbols are modulated by BPSK (Binary Phase Shift Keying).

Step 2:

0090The receiver receives a reference signal for each antenna j from the transmitter, and calculates a transfer function h_ij for each antenna pair or sub-channel. In the specific example shown in <figref idref="f0002">Fig. 2</figref>, each of two antennas of the transmitter sends out the reference signal twice, and the signals are received by two antennas of the receiver. Thus, a total of four transfer functions can be obtained. That is, in the case of 2 × 2 MIMO transmission, a 2 × 2 channel information matrix H is obtained. The entry of the matrix H represents a transfer function having a value of a complex number.

Step 3:

0091The receiver performs a singular value decomposition of the channel information matrix H to yield the expression H = UDV^H. The receiver obtains a matrix U necessary when decoding the received signals, and a receive antenna weighting coefficient matrix U^H for the decoding.

Step 4:

0092This time the receiver sends out an OFDM symbol as a reference signal from each antenna i, by time division. The OFDM symbols are modulated by the BPSK. A reference signal sent from an antenna 0 is compensated by a calibration coefficient CR(0), while a reference signal sent from an antenna 1 is compensated by a calibration coefficient CR(1). The calibration coefficient is held in the form of a complex transfer function, the compensation is completed by multiplying the transmitted signal by the calibration coefficient. By using CR(i) in the calibrating compensation, the transfer function expressed by the equation (9) can be made free from the influence of the variation related to the RF analog transmitting portion of the receiver.

Step 5:

0093The transmitter receives a reference signal for each antenna i sent from the receiver, and calculates a transfer function h_ij' for each antenna pair or sub-channel. A reference signal received by the antenna 0 is compensated by the calibration coefficient CT(0), while a reference signal received by the antenna 1 is compensated by the calibration coefficient CT(1). By using CT(j) in the calibrating compensation, the transfer function expressed by the equation (9) can be made free from the influence of the variation related to the RF analog receiving portion of the transmitter. Then, the channel information matrix H' in the reverse direction is obtained from the transfer functions as compensated by the calibration.

0094The transmitter performs a singular value decomposition of the obtained channel matrix H' to yield the expression H' = U'D'V'^H. Then, the transmitter can obtain the transmit antenna weighting coefficient matrix V' whose entry is weighting vector [w₁, w₂] for each antenna and which is necessary when transmitting signals or data of interest.

Step 7:

0095An actual data communication is initiated. When transmitting the data, the transmitter weights a plurality of logically independent bit data by respective weighting vectors [w₁, w₂] as entries of the transmit antenna weighting coefficient matrix V', and sends the weighted bit data out from the respective antennas, by space-time multiplexing. On the other hand, the receiver decodes the data or signals received by the respective antennas, with using the receive antenna weighting coefficient matrix U^H.

0096The transmitter and the receiver perform the calibration beforehand according to the processing procedure as described above. When a SVD-MIMO transmission is actually performed, the weights used by the receiver are obtained on the basis of the obtained channel information matrix in regard to the direction from the transmitter to the receiver, while the weights required for data transmission by the transmitter are obtained by using the channel information matrix which is obtained by receiving from the receiver the reference signals as compensated by the calibration coefficient related to the receiver, and compensating the received reference signals by using the calibration coefficient related to the transmitter.

0097According to the above-described method, the transmit antenna weighting coefficient matrix V which is directly obtained from the channel matrix H acquired in regard to the direction from the transmitter to the receiver, and the weighting coefficient matrix V' on the part of the transmitter which is directly obtained from the channel matrix H for the direction from the transmitter to the receiver, and the other transmit antenna weighting coefficient matrix V' obtained from H' acquired by the calibration based on the signals sent from the receiver to the transmitter, do not in effect completely coincide with each other. Each of vectors of V is identical with a vector which corresponds to the counterpart of V' but each of whose components is rotated by an angle.

0098This is because of that the calibration coefficient is a value determined on the basis of a particular antenna as a reference, the value of the calibration coefficient is not an absolute value, but a relative calibration coefficient among a plurality of antenna elements.<maths id="math0013" num="(13)"><math display="block"><mrow><msub><mi>C</mi><mi>i</mi></msub><mo>=</mo><mfrac><mrow><msub><mi>T</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">iμ</mi><mn>1</mn></msub></mfenced><mi>R</mi></mrow><mi>i</mi></msub><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mi>i</mi></msub></mfenced></mrow><mrow><msub><mi>R</mi><mn>1</mn></msub><msub><mrow><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jθ</mi><mn>1</mn></msub></mfenced><mi>T</mi></mrow><mi>i</mi></msub><mspace width="1em" /><mi>exp</mi><mfenced><msub><mi mathvariant="italic">jμ</mi><mi>i</mi></msub></mfenced></mrow></mfrac></mrow></math><img file="EP1530305B1_D0013.tif" /></maths>

0099In the above described embodiment, the calibration coefficient of each antenna i is defined on the basis of the transfer functions of the transmitting and receiving analog portions of the antenna 1 as references. Thus, the calibration coefficient is not an absolute value directly derived from the transmission transfer function of the antenna i and the reception transfer function of the antenna i, but is a relative calibration coefficient. In general, the calibration of antennas in a multi-antenna system is such a relative calibration. It is noted that the principle of the invention operates in combination with the relative calibration.

0100Where the weights are used for weighting data transmitted in an SVD-MIMO system, the phase rotation mentioned above does not matter at all, in effect. This is because that the optimum transmit antenna weighting coefficient matrix V is always rotating equivalently, due to a slight clock error between the transmitting and receiving devices. That is, there is no point in having V and V' identical with each other, but it is sufficient to have every component of every vector of V as rotated by a same angle identical with the corresponding component of the corresponding vector of V'. Further, since V' and V are unitary matrices, the norm of each vector of V' is identical with the norm of corresponding vector of V.