Europfiisches
`Patent: mt
`European
`Patent office
`Office eu ropéen
`
`des brevets
`
`(11)
`
`EP 2 642 781 A1
`
`(12)
`
`EUROPEAN PATENT APPLICATION
`published in accordance with Art. 153(4) EPC
`
`(43) Date of publication:
`25.09.2013 Bulletin 2013139
`
`(21) Application number: 118422363
`
`(22) Date of filing: 15.11.2011
`
`(51) Int CL:
`H04 W 16128 ”“9100
`H04J 11I00 (2055-011
`H04 W 24110 ”"0””
`
`H043 7/04 (2006.01)
`Hou 99:00 (2009.01)
`H04W 23113 (2009.01)
`
`(86) International application number:
`PCTIJ P20111076252
`
`(87) International publication number:
`WO 20121067093 (24.05.2012 Gazette 2012121)
`
`(84) Designated Contracting States:
`AL AT BE BG CH CY CZ DE DK EE ES FI FR GB
`GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO
`PL PT RO RS SE SI SK SM TR
`
`(30) Priority: 16.11.2010 JP 2010256077
`12.04.2011 JP 2011088264
`
`(71) Applicant: Nippon Telegraph And Telephone
`Corporation
`Tokyo 100-8116 (JP)
`
`(72) Inventors:
`- KUDO Riichi
`Musashino—shi
`
`Tokyo 180-8585 (J P)
`- MATSUI Munehiro
`Musashino—shi
`
`Tokyo 180-8585 (JP)
`- MURAKAMI Tomoki
`Musashino-shi
`
`Tokyo 180-8585 (J P)
`
`
`
`-
`
`ICHIKAWA Takeo
`Musashino-shi
`
`Tokyo 180-8585 (JP)
`0 ASAI Yusuke
`Musashino-shi
`
`-
`
`Tokyo 180-8585 (JP)
`ISHIHARA Koichi
`Musashino-shi
`
`Tokyo 180-8585 (JP)
`. YAMADA Tomoyuki
`Musashino-shi
`
`Tokyo 180-8585 (JP)
`- MIZOGUCHI Masato
`Musashino-shi
`
`Tokyo 180-8585 (JP)
`
`(74) Representative: llgart, Jean-Christophe et al
`BREVALEX
`95 rue d’Amsterdam
`
`75378 Paris Cedex 8 (FR)
`
`(54)
`
`WIRELESS COMMUNICATION SYSTEM AND WIRELESS COMMUNICATION METHOD
`
`Communication quality is prevented from being
`(57)
`deteriorated due to interference caused by low accuracy
`of channel information or interference from another base
`
`station or another wireless communication system. A
`wireless communication system includes a first wireless
`station including a plurality of antenna elements, trans-
`mission units oorresponding to the plurality of antenna
`elements. and a transmission directivity control unit that
`controls transmission directivityofthe plurality ofantenna
`elements, and a pluralityofsecond wireless stations each
`including a reception power measurement unit
`that
`measures a reception power. The transmission directivity
`
`control unit determines the transmission directivity of
`each of the plurality of antenna elements so that a signal
`does not reach a particular second wireless station
`among the plurality of second wireless stations. The
`transmission units generate wireless signals using the
`transmission directivity. The plurality of antenna ele-
`ments transmit the generated wireless signals. The re-
`ception power measurement unitmeasures the reception
`power in a period that is set so that a signal does not
`reach the particular second wireless station, and detect
`the presence or absence of interference.
`
`Printed by Jouve, 75001 PARIS (FR)
`
`(Cont. next page)
`
`EP2642781A1
`
`

`

`EP 2 642 781 A1
`
`FIG. 1
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`
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`'TERMINAL
`
` TERM I NAL
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`

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`EP 2 642 781 A1
`
`Description
`
`TECHNICAL FIELD
`
`[0001] The present invention relates to a wireless communication system and a wireless communication method. The
`present invention particularly relates to a wireless communication system and a wireless communication method using
`space division multiple access control technology. Moreover, the present invention particularly relates to transmission
`technology in a wireless communication system that simultaneously communicates with a plurality of communication
`parties using spatial multiplexing.
`Priority is claimed on Japanese Patent Application No. 2010-256077, filed November 16, 2010 and Japanese Patent
`Application No. 2011-088264, filed April 12, 2011, the contents of which are incorporated herein by reference.
`
`BACKGROUND ART
`
`[0002] With recent developments such as those in the Internet, networks have been spreading to both homes and
`offices. Such a network is called a LAN (local area network). A wireless LAN that is a wireless version of the LAN has
`also been spreading. For example, as a high—speed wireless access system using a 2.4 GHz band or a 5 GHz band,
`spread of a wireless LAN or the like of the IEEE (Institute of Electrical and Electronics Engineers) 802.11g standard and
`the IEEE 802.11a standard is remarkable. In these systems, a transmission rate (a transmission rate in a physical layer)
`of a maximum of 54 Mbitls has been achieved using an orthogonal frequency division multiplexing (OFDM) modulation
`scheme, which is technology for stabilizing the property in a multi-path fading environment.
`[0003] However, the transmission rate cited here is a transmission rate on a physical layer. In fact, since the transmission
`efficiency in a MAC (medium access control) layer is about 50 to 70%, an upper limit of the actual throughput is about
`30 Mbps and this property is further deteriorated when communication parties requiring information increase. On the
`other hand, in the world ofa wired LAN, FTTH (fiber to the home) using an optical fiber, including a 100 Base— T interface
`of the Ethernet (registered trademark), has been spreading to individual homes, and thus provision of a high- speed line
`of 100 Mbps has been spreading. Even in the world of wireless LANs, a higher transmission rate is required.
`In this
`[0004] Therefore, as a standard for realizing a higher transmission rate, there is IEEE 802.11n standard.
`standard, a transmission rate ofa maximum of 800 MbiUs can be realized by using MIMO (multiple input multiple output)
`technology as spatial multiplexing transmission technology.
`[0005]
`In recent years, in order to achieve a larger capacity, wireless communication using MU (multi user)—M|MO
`that enables wireless space resources to be effectively used by performing cne-to-many communication through spatial
`multiplexing between a wireless base station and a plurality ofwireless terminal stations using the same wireless channel
`has been studied as awireless system using MIMO technology (see, forexample, Non-Patent Document 1). Forexample,
`an MU-MIMO transmission method has been studied in IEEE 802.11ac (see Non-Patent Document 2).
`[0006]
`In MU—MIMO, channel information between antennas of a base station and each of the terminals is estimated;
`the base station acquires the channel information, controls transmission beams using the acquired channel information,
`and directs the beams toward the terminals in optimal directions, thereby making it possible to simultaneously transmit
`signals to the terminals using the same frequency channel.
`[0007] That is, control has been performed to increase directivity to wireless stations thatare transmission destinations
`and to direct nulls to the other specific wireless stations. Since spatial multiplexing is performed, it is also called a space
`division multiple access (SDMA) control scheme. Since communication can be simultaneously performed in the same
`frequency and at the same time, it is possible to increase the spectral efficiency.
`
`Prior Art Documents
`
`Non-Patent Documents
`
`[0008] Non- Patent Document 1: QH Spencer etc., "An introduction to the multi- user MIMO downlink", lEEE Commu-
`nication Magazine, pp. 60- 67, Oct. 2004.
`Non- Patent Document2: IEEE, "Proposed specification framework for TGac", doc.: IEEE 802.11- 09/0992r21, Jan. 201 1.
`
`SUMMARY OF INVENTION
`
`Problems to be solved by the Invention
`
`[0009] However, although the channel information is essential fortransmission beam control ofwireless communication
`using MU- MIMO, accuracy of once estimated channel information is not guaranteed after that because the channel
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`EP 2 642 781 A1
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`state varies due to a surrounding environment using the wireless communication and/or a state (e.g., mobility) of a
`wireless station.
`
`[0010] There is a problem in that the accuracy of the channel information is deteriorated if the channel information is
`not accurate, so that optimal transmission beam control cannot be performed (a null is not directed to a wireless station
`to which the null should be directed), interference between wireless stations occurs on a receiving end, and deterioration
`in communication quality such as increases in packet retransmissions or an increase in the error rate occurs. It is difficult
`for the wireless station to recognize whether or not the channel information is accurate.
`[0011]
`Furthermore, if another base station or another wireless communication system using the same frequency
`band is present in the vicinity, interference occurs in the same manner as described above. In this case, the communication
`quality is also deteriorated.
`[0012] Here, the configuration of a conventional transmission] reception system is illustrated in FIG. 30. In FIG. 30,
`101 denotes a base station, 102— 1 to 102— i denote terminals, 101— 1 denotes a data selection;t output circuit, 101— 2
`and 102— 1- 4 to 102— i- 4 denote transmission signal generation circuits, 101- 3 and 102-1- 2 to 102- i- 2 denote wireless
`signal transmission! reception circuits, 101- 4-1 to 101- 4- N, 102- 1-1- 1 to 102— 1-1- M, and 102-i-1- 1 to 102— i- 1-
`M, denote transmission! reception antennas, 101- 5 and 102- 1- 3 to 102- i- 3 denote reception signal demodulation
`circuits, 101— 6 denotes a channel information storage circuit, and 101— 7 denotes a transmission method determination
`circuit. Here, 1sisK, where K denotes the number ofterminals, M, denotes the number oftransmission/ reception antennas
`of an ith terminal, and N denotes the number of transmission! reception antennas of the base station.
`[0013] Transmission from the base station to the terminals is considered. In the base station 101, the data selection.t
`output circuit 101-1 outputs information on a communication party to which output can be performed to the transmission
`method determination circuit 101-7’. The transmission method determination circuit 101-7 determines the communication
`
`party using a communication party selection method, which will be described below, determines a spatial multiplexing
`order, transmission weights, a modulation scheme, and a coding scheme for the communication party, and outputs
`them to the data selectionloutput circuit 101-1 and the transmission signal generation circuit 101—2. In this case, the
`transmission weights may not be used. The data selectionloutput circuit 101-1 outputs transmission data destined for
`the corresponding communication party to the transmission signal generation circuit 101-2. The transmission signal
`generation circuit 101-2 performs modulation and coding on the input transmission signal, multiplies a resultant signal
`by the transmission weights, inserts a pilot signal used for signal detection and delivery of communication information,
`and outputsa resultantsignal to the wireless signaltransmissionlreception circuit 1 01-3. The wireless signaltransmissionl
`reception circuit 101-3 up-converts the input signal to a carrier frequency and transmits a resultant signal via the trans-
`mission/reception antennas 101-4-1 to 101-4-N.
`[0014]
`For a method for selecting a communication party, a communication party for which data to be transmitted is
`stored in a memory and transmission is readyto perform may be specified, a communication party corresponding to the
`oldest data among a plurality of pieces of stored data may be selected, a communication party may be selected based
`on 005 (quality of service) of a user, a combination of users previously determined by a group ID (identifier) may be
`selected, or a combination of communication parties having low correlation of channel information may be selected.
`[0015]
`K terminals that are communication parties each input a wireless signal received via antennas provided in each
`terminal to the wireless signal transmission/reception circuit provided in each terminal. In an ith terminal, the wireless
`signal transmission/reception circuit 102—i—2 down—converts the signal received via at least one of the antennas 102—i—
`1-1 to 102-i-1-M) from the carrier frequency, and inputs a resultant signal to the reception signal demodulation circuit
`102-i-3. The reception signal demodulation circuit 102-l-3 establishes synchronization with a received packet, decodes
`a signal, and outputs data.
`[0016] On the other hand, in the case in which transmission is performed from a terminal to the base station, when
`transmission data is input to the terminal, the transmission signal generation circuit 102— i— 4 adds a control frame such
`as a pilot signal to the signal subjected to the modulation and coding and outputs a resultant signal to the wireless
`transmission! reception circuit 102— i- 2. The wireless transmission! reception circuit 102— i- 2 up- converts the signal
`output from the transmission signal generation circuit 102- i- 4 to the carrier frequency, and transmits a resultantsignal
`through at least one of the transmission! reception antennas 102-i-1- 1 to 102— i- 1- M.
`[0017]
`In the base station, the wireless signal transmission/reception circuit 101-3 down-converts the signal received
`via at least one of the antennas 101-4-1 to 101-4-N and outputs a resultant signal to the reception signal demodulation
`circuit 101—5. The channel information used for demodulation or fed-back channel information between the terminal and
`
`the base station included in a demodulation signal is input to the channel information storage circuit 101-6. The channel
`information storage circuit 101-6 performs optimization (correction) of the channel information required to generate the
`transmission weights, such as calibration, on the input channel information, and outputs resultant information to the
`transmission signal generation circuit 101-2 via the transmission method determination circuit 101-7.
`[0018] Hereinafter, a BD (block diagonalization) directivity control method will be shown as an example of communi-
`
`cation with a plurality of communication parties using a spatial multiplexing scheme. A channel response matrix H”- (an
`M,><N matrix) representing channel information for a j‘h frequency channel of the terminal 102- i obtained in the channel
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`

`EP 2 642 781 A1
`
`information storage circuit 101- 6 is divided into a right singular matrix V”- (an NXN matrix), a left singular matrix UH (an
`MiXMi matrix), and a matrix D (an Mi><N matrix) in which diagonal elements are the square roots VAL“ of eigenvalues
`and non— diagonal matrixes are D, by singular value decomposition, as shown in the following equation:
`[0019]
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`[Equation 1]
`
`Hi,j,ll
`‘ Hi.j,21
`Lj‘
`:
`
`H
`
`Hi,j,12
`Hi,j.22
`:
`
`'.
`
`Hi,j,m
`HmN
`:
`
`Ht,j.Mii
`
`Hi,j,w2
`
`Hi,j,M:'N
`
`Ui.j.l]
`= Ui.j.21
`
`Ui,j,12
`Ui,j,22
`
`Uni-m.-
`Ui,j,2Mi'
`
`li.j,1
`0
`
`0
`1/13,];
`
`0
`E
`
`Vi.j,n
`Vi,j,21
`
`0
`
`Vi,j.12
`Vi,j.22
`
`H
`
`Vi.j,1N
`Vi.j.2N
`
`Ui.j,M£l
`
`Ui,j,Mi2
`
`ULJMIME
`
`0
`
`[Equation1]-- wig-M VLj.Nl
`
`Vi.j.N2
`
`Vi,j,NN
`
`= Ui.j(Di,j
`= Ur,1(1):;j
`
`”huff
`”Xvi;
`
`V1".j)”
`
`= ULJ‘DiJVi'JH
`
`[0020] Here, Hi4. Ik denotes a transfer coefficient between the lth antenna of the transmission device and the kth antenna
`ofthe terminal 102-i in the it“ frequency channel. In the right singular matrix Vi,j- V’i'j is a column vector group corresponding
`to the eigenvalues, and W” is a column vector group corresponding to 0. ln eigenvector transmission, which is known
`as a method that enables the maximum spectral efficiency to be obtained in single- user communication, a signal power
`
`represented bya corresponding eigenvalue AW can be obtained by using a column vector ofV’U as transmission weights.
`Here, ELM-Y1 2 Ali. 2 2
`2 it”, M, and a superscript H denotes a complex conjugate matrix.
`[0021] Next, means for the communication party selection method in accordance with a BB method for multi- users
`will be shown. Here, communication with K users (the terminals 102- 1 to 102- K) is considered. A method for calculating
`
`35
`
`transmission weights for the in” terminal 102- i will be shown. First, an aggregate channel matrix Hm corresponding to
`the terminals other than the terminal 102- i is defined as:
`
`[0022]
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`
`[Equation 2]
`
`RIJFIJ
`
`HIj m Ei-IJEi—w
`i+},j'
`i+t,j
`
`RKJHKJ
`
`55
`
`it
`[0023] RaJ is reception weights in the terminal 102- a. If Raj is a diagonal matrix having diagonal elements of 1,
`shows a case in which transmission weights are determined without assumption of reception weights. When singular
`
`value decomposition is performed on H+i_J-. it can be represented as:
`[0024]
`
`

`

`EP 2 642 781 A1
`
`[Equation 3]
`
`H2; =u:;.-<n:.
`
`a w:-
`
`val)”
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`[0025] where V'ti'j is a signal space vector corresponding to eigenvalues 0+”, and V""i!j is a null space vector with no
`eigenvalue or a null space vector corresponding to an eigenvalue 0. Here. when transmission to the null space that is
`
`represented by V"+i,j is performed, interference does not occur for the reception weights of the communication parties
`other than the terminal 102—i. Thus, weights obtained by performing linear calculation on V'HiJ obtained here may be
`used as the transmission weights to be used in the jih frequency channel to perform communication with a plurality of
`communication parties using the spatial multiplexing scheme. For example, weights obtained by multiplying the channel
`
`matrix Hij corresponding to the terminal 102-i by V"“ilj, and multiplying V"+i,j by a basis vector obtained using an orthog-
`onalization method with respect to a row vector of obtained HiJ Vu+i,j or a right singular vector obtained by performing
`singular value decomposition on H” V"+i,j may be used as the transmission weights. If the matrix obtained from Hi,j V""i'j
`is thx a transmission weight vector may be represented as V"+i,j Gil].
`[0026]
`in this way, transmission weights can be calculated for each communication party, and obtained transmission
`
`weights W] for the jth frequency channel for the K users may be represented as:
`[0027]
`
`[Equation 4]
`
`W;
`
`(“’1.1 We;
`(VSGIJ V536,,
`
`it
`
`Wm)
`
`team)
`
`[0028] A reception signal y”- corresponding to the j1h frequency channel in the terminal 102-i when such transmission
`weights are used may be represented as:
`[0029]
`
`[Equation 5]
`
`Via" = RUHMWJXLJ ""' ”a;
`
`r:
`
`= “#3:.”th +IE-ZIRI'JHLJW3JKLJ + “m
`
`be:
`
`[0030] Here, xH- denotes a transmission signal destined for the terminal 102- i transmitted using the jth frequency
`channel. and ni‘j denotes a thermal noise vector in the terminal 102- i in the jth frequency channel. If there is no error in
`the channel information, Rii HM and Whi are orthogonal to each other and R” Hi’j W|,j is 0 (I a: i). However, if there is an
`error in the channel information of the base station, the second term at the right side in the second line in Equation 5 is
`not 0, which is inter— user interference, and deteriorates transmission quality. Therefore, it is necessary to estimate SINR
`taking the quality deterioration into consideration and determine an appropriate modulation scheme, a coding rate. and
`the number of streams to be subjected to spatial multiplexing.
`[0031] As described above. when the transmission to the plurality of communication parties using the spatial multi—
`plexing scheme is performed, the inter-user interference occurs due to the channel estimation error, and thus there is
`
`

`

`EP 2 642 781 A1
`
`a problem in that an appropriate modulation scheme, a coding rate, and the number of spatial multiplexing streams
`cannot be determined and the throughput is greatly deteriorated.
`[0032] The present invention has been made in consideration of such circumstances, and an object thereof is to
`provide awireless communication system and a wireless communication method capable of preventing communication
`quality from being deteriorated due to interference caused by low accuracy of channel information or interference caused
`by another base station or another wireless communication system.
`[0033] Moreover, an object ofthe present invention is to provide awireless communication system capable of evaluating
`inter- user interference and appropriately selecting a modulation scheme, a coding rate, and the number of spatial
`streams for each terminal when transmission to a plurality of communication parties using a spatial multiplexing scheme
`is performed at the same time and in the same frequency.
`
`Means for Solving the Problems
`
`in order to solve the problems described above, the present invention is a wireless communication system
`[0034]
`including: afirstwireless station including a plurality ofantenna elements, transmission units corresponding to the plurality
`of antenna elements, and a transmission directivity control unit that controls transmission directivity of the plurality of
`antenna elements; and a plurality of second wireless stations each including a reception power measurement unit that
`measures a reception power, wherein the transmission directivity control unit of the first wireless station determines the
`transmission directivity of each of the plurality of antenna elements so that a signal does not reach a particular second
`wireless station among the plurality of second wireless stations, the transmission units generate wireless signals using
`the transmission directivity, the plurality of antenna elements transmit the generated wireless signals, and the reception
`power measurement unit of the particular second wireless station measures the reception power in a period that is set
`so that a signal does not reach the particular second wireless station, and detects the presence or absence of interference.
`[0035]
`in the wireless communication system described above, the transmission directivity control unit may periodically
`switch the particular second wireless station.
`[0036]
`In the wireless communication system described above, the transmission directivity control unit may determine
`the particular second wireless station based on the numbers of times a packet was not delivered in a given period in the
`plurality of second wireless stations.
`[0037]
`in the wireless communication system described above, the transmission directivity control unit may determine
`the particular second wireless station based on error rates in the plurality of second wireless stations.
`[0038]
`In the wireless communication system described above, the transmission directivity control unit may determine,
`as the particular second wireless station, a second wireless station for which application data to be transmitted is not
`stored among the plurality of second wireless stations.
`[0039]
`In the wireless communication system described above, the particular second wireless station may further
`include: an interference estimation unit that estimates an interference amount; and a transmission unit that transmits
`the interference amount estimated by the interference estimation unit to the first wireless station, and the first wireless
`station may determine at least one of a modulation scheme, a coding rate, or wireless stations with which transmission
`and reception are performed simultaneously through spatial multiplexing, and a frequency channel to be used, based
`on the interference amount.
`
`In the wireless communication system described above, the plurality of second wireless stations each may
`[0040]
`include: a pluralityof antenna elements; and reception power measurement units corresponding to the plurality of antenna
`elements.
`
`Furthermore, the present invention is a wireless communication method in which wireless communication is
`[0041]
`performed between a first wireless station including a plurality of antenna elements, transmission units corresponding
`to the plurality of antenna elements, and a transmission directivity control unit that controls transmission directivity of
`the plurality ofantenna element, and a plurality of second wireless stations each including a reception power measurement
`unit that measures a reception power, and the method includes: a step of determining, using the transmission directivity
`control unit in the first wireless station, the transmission directivity of each of the plurality of antenna elements so that a
`signal does not reach a particular second wireless station among the plurality of second wireless stations; a step of
`generating, using the transmission units in the first wireless station, wireless signals using the determined transmission
`directivity, and transmitting the generated wireless signals from the plurality of antenna elements; anda step of measuring,
`using the reception power measurement unit in the particular second wireless station, the reception power in a period
`that is set so that a signal does not reach the particular second wireless station, and detecting the presence or absence
`of interference.
`
`Furthermore, the present invention is a wireless communication system in which a base station performs
`[0042]
`simultaneous communication cfdata with a plurality of terminals using the same frequency, and the base station includes:
`a data selection.l generation unit that generates the data; a null signal terminal determination unit that determines, when
`there is a terminal for which an interference power needs to be evaluated, the terminal as a null signal terminal; a first
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`EP 2 642 781 A1
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`reception unit that receives a signal transmitted from the null signal terminal and acquire interference power information;
`a transmission method determination unit that determines a transmission weight, a modulation scheme, and a coding
`scheme based on channel information of a terminal that is a communication party and the interference power information
`acquired from the null signal terminal; a data selection/ output unit that selects transmission data destined forthe terminal
`that is the communication party from among data to be transmitted and outputs the transmission data; and a first
`transmission unit that transmits the transmission data using the determined modulation scheme, coding scheme, and
`transmission weight, and each of the terminals including: a second reception unit that receives the transmission data
`as a reception signal; a reception signal demodulation unit that performs detection, synchronization, and decoding of
`the reception signal; a null signal determination unit that determines that each of the terminals itself is the null signal
`terminal based on an output ofthe reception signal demodulation unit; an interference power evaluation unitthat measures
`a reception power of a null signal portion and records the reception power as an interference power when each of the
`terminals itself is determined to be the null signal terminal; and a second transmission unit that modulates, codes, and
`transmits information on the interference power.
`[0043]
`Furthermore, in the wireless communication system described above, the null signal terminal determination
`unit in the base station may determine, as the null signal terminal, a terminal communicating with another base station
`adjacent to the base station, and the second transmission unit in each of the terminals may modulate and code the
`information on the interference power and may transmit resultant information to the base station communicating with
`each of the terminals.
`
`Furthermore, in the wireless communication system described above, the null signal determination unit may
`[0044]
`determine that each of the terminals itself is the null signal terminal when a reception signal power of a data portion is
`smaller than a reception power ofa pilot portion by an amount that is greater than or equal to a predetermined reference.
`[0045]
`Furthermore, in the wireless communication system described above, the base station may include: a null
`signal ID notification unit that assigns a bit for specifying the null signal terminal as a null signal ID to a pilot signal in
`advance, and notifies a terminal as a candidate for the null signal terminal ofthe corresponding null signal ID; a downlink
`transmission unit that specifies the null signal ID when the pilot signal is added to the transmission data; and a null signal
`determination unit that determines that each of the terminals itself is the null signal terminal based on the null signal ID
`specified by the pilot signal of the reception signal.
`[0046]
`Furthermore, in the wireless communication system described above, the null signal terminal determination
`unit may specify another base station as the null signal terminal.
`[0047]
`Furthermore, in the wireless communication system described above, the null signal terminal determination
`unit may specify, as the null signal terminal, a terminal that has not been selected as the null signal terminal for a given
`period, a terminal that has not performed multiplexing communication with a plurality of terminals using the same fre-
`quency fora given period, ora terminal that has not been able to successfully perform communication when performing
`multiplexing communication with a plurality of terminals using the same frequency.
`[0048]
`Furthermore, in the wireless communication system described above, the interference power evaluation unit
`may measure the reception power ofthe null signal portion at two or more timings, and record information on an increase
`amount of the interference power, and the second transmission unit may modulate, code, and transmit information on
`the interference power and the information on the increase amount of the interference power or information on an
`interference power obtained by applying a function to the interference power and the increase amount of the interference
`power.
`in the wireless communication system described above, the data selection] output unit may
`Furthermore,
`[0049]
`calculate a duration in which the interference power does not exceed a predetermined maximum interference power
`based on the information on the interference power and the information on the increase amount of the interference
`power estimated for a terminal, and may determine the maximum data length that is able to be set for the terminal.
`[0050]
`In multi-user MIMO beamforming, channel information between terminals and a base station is estimated,
`transmission weights are calculated so that inter-user interference does not occur between terminals to which simulta-
`neous transmission from the base station is performed, signal processing is performed, and then a transmission process
`is performed. However, despite inter-user interference occurs because there is an error in the estimated channel infor-
`mation, a modulation scheme, a coding rate, and a multiplexing order are selected without taking this fact intc consid-
`eration, and thus there is a problem in that communication quality is deteriorated.
`In the present invention, when transmission from a base station is performed, a null signal terminal which is the target
`of measurement of inter-user interference is set, and transmission is performed with transmission weights applied so
`that a null is directed to the null signal terminal. On the other hand, ifthe null signal terminal receives a signal despite
`the null being directed to the null signal terminal, the null signal terminal notifies the base station of a reception power
`as the interference. The base station receiving the notification calculates the optimal modulation scheme, the optimal
`coding rate, and the optimal multiplexing rate based on the notified interference power, and performs communication
`with the null signal terminal.
`As a result, it is possible to select the optimal modulation scheme, the optimal coding rate, and the optimal multiplexing
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`EP 2 642 781 A1
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`order in consideration of the channel estimation error in multi-user MIMO.
`
`Furthermore, the present invention may be applied to a case in which there is a plurality of base stations (communication
`cells). In this case, a first base station performs transmission to a terminal that is connected to a second base station
`and serves as the null signal terminal, and the terminal notifies the second base station ofthe interference.
`
`Advantageous Effects of Invention
`
`[0051] As described above, in accordance with the present invention, even if there is interference due to low accuracy
`of channel information, a null is directed to a terminal, interference in the terminal is detected, and channel estimation
`is performed again to re-acquire channel information when the terminal is determined to suffer from the interference,
`thereby improving the accuracy of the channel information and mitigating the interference.
`[0052] Moreover, even if there is interference due to low accuracy of channel information, interference due to another
`base station or another wireless communication system, or the like, the interference amount is measured and commu-
`nication with increasing interference resistance in accordance with the interference amount is