(12) United States Patent
`Utsunomiya et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,369,511 B2
`May 6, 2008
`
`USOO7369511B2
`
`(54) COMMUNICATION METHOD,
`COMMUNICATION APPARATUS, AND
`COMMUNICATION SYSTEM
`(75) Inventors: Yoriko Utsunomiya, Tokyo (JP);
`Masahiro Takagi, Tokyo (JP)
`Assignee: Kabushiki Kaisha Toshiba, Tokyo (JP)
`Notice:
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 764 days.
`Appl. No.: 10/948,542
`Filed:
`Sep. 24, 2004
`
`(73)
`(*)
`
`(21)
`(22)
`(65)
`
`Prior Publication Data
`US 2005/O152387 A1
`Jul. 14, 2005
`
`(30)
`Jan.
`
`Foreign Application Priority Data
`9, 2004
`(JP)
`............................. 2004-004846
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`Int. C.
`(2006.01)
`H04L 2/26
`U.S. Cl. ....................................... 370/252; 370/328
`Field of Classification Search ..................... None
`See application file for complete search history.
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`6,760,882 B1*
`2002fOO398.84 A1*
`2003.0036359 A1
`2005.0007947 A1*
`
`7/2004 Gesbert et al. ............. 714/774
`4/2002 Raynes et al. ............. 455,13.3
`2/2003 Dent et al.
`1/2005 Abe et al. ................... 370,206
`
`FOREIGN PATENT DOCUMENTS
`
`JP
`
`2002-290246
`
`10, 2002
`
`
`
`MAC layer
`management
`entity (MLME)
`
`10
`
`JP
`WO
`
`2003-69531
`WO O1/33761 A1
`
`3, 2003
`5, 2001
`
`OTHER PUBLICATIONS
`
`Lau et al., “On the Design of MIMO Block-Fading Channels With
`Feedback-Link Capacity Constraint.” IEEE Transactions on Com
`munications (Jan. 2004), 52:62-70.
`European Search Report, issued by European Patent Office, dated
`Apr. 13, 2005, in European Application No. 0525.0025.3-1237.
`Notification of the First Office Action dated Apr. 13, 2007, from
`Chinese Patent Office in Chinese Patent Application No.
`2005100.04.1337.
`* cited by examiner
`Primary Examiner Chau Nguyen
`Assistant Examiner—Dargaye H Churnet
`(74) Attorney, Agent, or Firm—Finnegan, Henderson,
`Farabow, Garrett & Dunner, L.L.P.
`
`(57)
`
`ABSTRACT
`
`A method for exchanging channel information between a
`transmitter and a receiver each including antennas. A request
`frame including a first preamble, second preamble, and
`channel information feedback request is transmitted to the
`receiver, wherein the first preamble is transmitted from one
`of the antennas of the transmitter, the second preamble is
`transmitted from each of the antennas, and the channel
`information feedback request is transmitted from the one of
`the antennas. The request frame is received at the receiver,
`and the channel information feedback request is detected by
`a decoding based on the first preamble. Channel information
`is estimated using the second preamble, and a notification
`frame including the estimated information is constructed and
`transmitted to the transmitter.
`
`20 Claims, 5 Drawing Sheets
`
`100
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Channel information
`feedback request
`
`Channel estimation
`result notification
`frame creation
`
`
`
`7
`
`Preamble
`Data
`
`R1
`
`Channel estimation
`
`Decoding g
`
`Preamble addition
`
`Physical layer
`
`LG Elecs. Ex. 1024
`LG Elecs. v. Pantech Corp.
`IPR2023-01273 Page 1
`
`

`

`U.S. Patent
`
`May 6, 2008
`
`Sheet 1 of 5
`
`US 7,369,511 B2
`
`101
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`100
`
`MAC layer
`
`Channel information
`feedback request
`frame Creation
`
`
`
`instruction
`
`MAC layer
`management
`entity (MLME)
`R5
`Channel estimation result Yes Antenna COnfiguration
`notification frame 2
`determination
`
`R4
`
`Channel information
`feedback request
`
`
`
`Channel estimation
`result notification
`frame Creation
`
`
`
`
`
`
`
`a r.
`Channel estimation
`Decoding g
`
`/
`/
`
`Preamble
`Data
`
`R1
`
`Preamble addition
`
`Physical layer
`
`200
`
`Transmitter
`
`Receiver
`
`
`
`
`
`5-2
`
`Channel estimation
`proCeSS
`
`F. G. 2
`
`F. G. 3
`
`IPR2023-01273 Page 2
`
`

`

`Sheet 2 of 5
`
`US 7,369,511 B2
`
`lly ~ 90/91, OVW
`
`7 '0 | -
`
`U.S. Patent
`
`May 6, 2008
`
`607807107907907707807Z07|07
`
`IPR2023-01273 Page 3
`
`

`

`U.S. Patent
`
`May 6, 2008
`
`Sheet 3 of 5
`
`US 7,369,511 B2
`
`Preamble (11a)
`
`Preamble (MIMO)
`
`MAC frame
`Destination Source
`Channel information
`address
`address
`request data
`(11a)
`
`F. G.5
`
`S1, hl
`S2, h21
`
`F. G. 6
`
`S1, hl. 1
`
`S2, h22
`S3, h32
`
`it is
`
`TX
`
`
`
`Tx 2
`
`S2, h23
`
`
`
`S3, h33
`
`X 2
`
`X 3
`
`X 1
`
`X 2
`
`x1
`
`X 2
`
`X 3
`
`IPR2023-01273 Page 4
`
`

`

`U.S. Patent
`
`May 6, 2008
`
`Sheet 4 of 5
`
`US 7,369,511 B2
`
`MAC frame
`Destination SOUrce
`Channel information
`address
`address
`request data
`(11a)
`
`Preamble (MIMO)
`
`F. G.7
`
`Preamble (11a)
`
`Preamble (MIMO)
`
`MAC frame
`Destination Source
`Channel estimation
`address
`address
`result information
`(11a)
`
`MAC frame
`Destination Source
`Channel estimation
`address
`address
`result information
`(11a)
`
`Preamble (MIMO)
`
`G 9
`
`Preamble (11a)
`
`Preamble (MIMO)
`
`Eators:
`address
`address
`
`MAC frame
`Channel estimation
`result information
`(MIMO)
`
`MAC frame
`Destination SOUrce
`Channel estimation
`address
`address
`result information
`(11a)
`
`Preamble (11a)
`
`F. G. 11
`
`IPR2023-01273 Page 5
`
`

`

`U.S. Patent
`
`May 6, 2008
`
`Sheet S of 5
`
`US 7,369,511 B2
`
`Transmitter
`
`Receiver
`
`Channel estimation
`proCeSS
`
`6-2
`
`
`
`Channel estimation
`proCeSS
`
`F.G. 12
`
`Preamble (11a)
`
`Preamble (MIMO)
`
`MAC frame
`Destination SOurce
`address
`address
`(11a)
`
`MAC frame
`Destination Source
`Physical layer Capability
`address
`address
`information
`(11a)
`
`Preamble (11a)
`
`F. G. 14
`
`IPR2023-01273 Page 6
`
`

`

`US 7,369,511 B2
`
`1.
`COMMUNICATION METHOD,
`COMMUNICATION APPARATUS, AND
`COMMUNICATION SYSTEM
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application is based upon and claims the benefit of
`priority from prior Japanese Patent Application No. 2004- 10
`004846, filed Jan. 9, 2004, the entire contents of which are
`incorporated herein by reference.
`
`BACKGROUND OF THE INVENTION
`
`2
`BRIEF SUMMARY OF THE INVENTION
`
`According to one aspect of the present invention, there is
`provided a method for exchanging channel information
`between a transmitter and a receiver each including anten
`nas. A request frame including a first preamble, second
`preamble, and channel information feedback request is
`transmitted to the receiver, wherein the first preamble is
`transmitted from one of the antennas of the transmitter, the
`second preamble is transmitted from each of the antennas,
`and the channel information feedback request is transmitted
`from the one of the antennas. The request frame is received
`at the receiver, and the channel information feedback request
`is detected by a decoding based on the first preamble.
`Channel information is estimated using the second pre
`amble, and a notification frame including the estimated
`information is constructed and transmitted to the transmitter.
`
`BRIEF DESCRIPTION OF THE SEVERAL
`VIEWS OF THE DRAWING
`
`FIG. 1 is a block diagram showing a communication
`apparatus according to embodiments of the present inven
`tion;
`FIG. 2 is a view showing an example of a MIMO channel
`in a communication system according to the embodiments of
`the present invention;
`FIG. 3 is a chart showing an example of a communication
`sequence according to the first embodiment of the present
`invention;
`FIG. 4 is a view showing an example of the format of a
`physical frame according to the first embodiment of the
`present invention;
`FIG. 5 is a view showing an example of the format of
`channel information feedback request frame 1 according to
`the first embodiment of the present invention;
`FIG. 6 is a chart for explaining a case in which data may
`not be reassembled in MIMO depending on the transmission
`channel environment;
`FIG. 7 is a view showing another example of the format
`of channel information feedback request frame 1:
`FIG. 8 is a view showing an example of the format of
`channel estimation result notification frame 2-1 according to
`the first embodiment of the present invention;
`FIG. 9 is a view showing an example of the format of
`channel estimation result notification frame 3-1 according to
`the first embodiment of the present invention;
`FIG. 10 is a view showing an example of the format of
`channel estimation result notification frame 3-2 according to
`the first embodiment of the present invention;
`FIG. 11 is a view showing an example of the format of
`channel estimation result notification frame 3-3 according to
`the first embodiment of the present invention;
`FIG. 12 is a chart showing an example of a communica
`tion sequence according to the second embodiment of the
`present invention;
`FIG. 13 is a view showing an example of the format of
`channel estimation result frame 2-2 according to the second
`embodiment of the present invention; and
`FIG. 14 is a view showing an example of the format of a
`physical layer capability information notification frame
`according to the third embodiment of the present invention.
`
`1. Field of the Invention
`The present invention relates to a communication method,
`apparatus, and system which feedback channel estimation
`information to a counterpart apparatus and, more particu
`larly, to exchange of channel estimation information asso
`ciated with MIMO (Multi Input Multi Output).
`2. Description of the Related Art
`A MIMO technique of transmitting frames by using a
`plurality of antennas has recently received a great deal of
`attention. A wireless communication system using the
`MIMO technique can realize high speed transmission with
`out extending frequency bandwidth. MIMO is likely to be
`employed also in a wireless LAN standard IEEE802.11n
`which has currently been standardized. Particularly in the
`LOS (Line Of Sight) propagation environment, the channel
`state of each path may not be estimated even by a signal
`process on the receiver in a wireless communication system
`using MIMO. In this case, a data frame that is transmitted by
`using a plurality of antennas may not be able to be decoded
`on the receiver. One of the causes is that the channel state
`from the transmitter to the receiver is not obtained on the
`transmitter. If the transmitter does not know the condition of
`the received data on the receiver, the transmitter may trans
`mit a frame using antennas by which the receiver may not
`correctly decode information. As a result, the band is wasted,
`which inhibits another communication.
`To solve this problem, the transmitter may specify an
`antenna configuration used for transmission according to the
`channel state, and then transmits data so as to allow the
`receiver to decode information correctly.
`For example, there is known a technique of feeding back
`information on a channel state estimated on the receiver to
`the transmitter and selecting a transmission method by the
`transmitter according to the channel state. This technique
`includes a method of Switching a modulation scheme on the
`transmitter on the basis of channel information fed back
`from the receiver (see Jpn. Pat. Appln. KOKAI Publication
`No. 2002-290246), and a method of controlling the coding
`ratio (see Jpn. Pat. Appln. KOKAI Publication No. 2003
`69531).
`Feedback of channel estimation information for proper
`control of transmission using MIMO on the transmitter
`requires a technique unlike conventional communication
`using one antenna. For example, when a conventional tech
`nique is simply applied, frames are sequentially transmitted
`from a plurality of antennas one by one on the transmitter.
`The receiver measures the channel state for each antenna,
`and feeds it back to the transmitter. However, according to
`this method, the channel resource is greatly wasted depend
`ing on the number of antennas, and this feedback method is
`too complicated.
`
`15
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`IPR2023-01273 Page 7
`
`

`

`US 7,369,511 B2
`
`3
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`First Embodiment
`
`4
`300. The receiver receives signals multiplexed by the anten
`nas 300, and decodes the received information by a signal
`process.
`The first embodiment assumes that the matrix channel is
`a wireless channel. It is therefore difficult for the transmitter
`to know the condition of the received data on the receiver
`without feeding back channel estimation result information
`from the receiver because the channel state greatly varies.
`A mechanism of feeding back channel estimation result
`information to the transmitter in the wireless communication
`system using a MIMO terminal will be explained in accor
`dance with the communication sequence of FIG. 3. Channel
`information is fed back from the receiver to the transmitter
`mainly when communication starts or the communication
`quality degrades. Channel information may be fed back
`periodically.
`As shown in FIG. 3, the communication sequence com
`prises a first step 5-1 of transmitting channel information
`feedback request frame 1 from the transmitter to the
`receiver, a second step 5-2 of estimating a channel by the
`receiver on the basis of channel information feedback
`request frame 1, a third step 5-3 of notifying the transmitter
`by frame 2-1 of a channel estimation result on the receiver,
`a fourth step 5-4 of estimating a channel by the transmitter
`on the basis of frame 2-1, and a fifth step 5-5 of transmitting
`any one of channel estimation result notification frames 3-1,
`3-2, and 3-3 as the channel estimation result of the trans
`mitter from the transmitter to the receiver.
`FIG. 4 is a view showing a basic example of the physical
`frame in the first embodiment. The first embodiment of the
`present invention appropriately modifies and adopts the
`physical frame shown in FIG. 4. A physical frame 400 is
`roughly divided into a preamble and MAC frame 411. In the
`first embodiment, the preamble includes two types of pre
`ambles 401 and 402. The preamble 401 is transmitted by the
`IEEE802.11a communication mode, and the preamble 402 is
`transmitted by MIMO communication mode. The MAC
`frame 411 basically has the same frame format as the
`IEEE802.11a frame format, and is transmitted by the
`IEEE802.11a communication mode, unless otherwise speci
`fied. The MAC frame 411 is different from the IEEE802.11a
`physical frame in that the MAC frame 411 does not have a
`field corresponding to the preamble 402. A channel infor
`mation feedback request frame and channel estimation result
`information frame (to be described later) are basically
`generated on the basis of the format of the physical frame
`400.
`FIG. 5 is a view showing the format of channel informa
`tion feedback request frame 1. The MAC layer processing
`unit 100 creates the MAC frame part of a channel informa
`tion feedback request frame upon reception of a channel
`information feedback request instruction from the MAC
`layer management entity 101 (T1). The frame part is sent to
`the physical layer processing unit 200 which executes
`encoding defined by IEEE802.11a or encoding used for
`MIMO transmission (T2). An IEEE802.11a preamble and
`MIMO preamble are added at the head of the MAC frame
`(T3), and the PHY frame is transmitted from the antenna
`3OO.
`The MAC frame part is transmitted by the IEEE802.11a
`communication mode, as shown in FIG. 5. FIG. 6 is a chart
`for explaining a case in which data may not be decoded in
`MIMO depending on the transmission channel environment.
`MIMO in which data S1, S2, and S3 are transmitted using
`three transmission antennas (Tx1, TX2, and TX3) as shown
`in FIG. 6 will be considered. The transmission antenna Tx1
`transmits the data S1. Similarly, the transmission antennas
`
`10
`
`15
`
`30
`
`FIG. 1 is a block diagram showing a communication
`apparatus according to the first embodiment of the present
`invention. The communication apparatus comprising a com
`munication system according to the first embodiment, in
`turn comprises a MAC layer processing unit 100 and physi
`cal layer processing unit 200. These processing units are
`implemented as analog or digital electronic circuits in accor
`dance with the packaging design, or firmware executed by a
`CPU assembled into an LSI. In this communication appa
`ratus, the physical layer processing unit 200 is coupled to,
`e.g., three antennas 300. As shown in FIG. 1, the MAC layer
`processing unit 100 has a MAC layer management entity
`(MLME) 101. The MAC layer processing unit 100 also has
`a function T1 of creating a channel information feedback
`request frame to be transmitted to a counterpart apparatus
`according to an instruction from the MAC layer manage
`ment entity 101. A MAC frame such as the channel infor
`mation feedback request frame created in the MAC layer
`processing unit 100 is sent to the physical layer processing
`unit 200. The physical layer processing unit 200 has a
`25
`function T2 of encoding a MAC frame, and a function T3 of
`adding a preamble to the encoded data to generate a physical
`frame. The generated physical frame is transmitted from the
`antennas 300.
`The physical layer processing unit 200 has a function R1
`of estimating the channel state on the basis of a preamble
`received by the antenna 300 upon reception, and a function
`R3 of decoding data on the basis of the channel estimation.
`The channel estimation result is used by a channel estima
`tion result notification frame creation function R2 in the
`MAC layer processing unit 100. The MAC layer processing
`unit 100 has a function R4 of determining, for data decoded
`by the function R3, whether the received frame is a channel
`information feedback request frame, and a function R5 of
`determining whether the received frame is a channel esti
`40
`mation result notification frame. If the received frame is a
`channel information feedback request frame, a channel
`estimation result notification frame is created by the function
`R2 and transmitted to the channel information requesting
`side. If the received frame is a channel estimation result
`notification frame, an antenna configuration used to transmit
`data to at least a communication apparatus which has
`transmitted this frame are determined (function R6). The
`antenna configuration determined by R6 is sent to the
`encoding function T2 to control transmission operation. An
`50
`example of a method of determining antenna configuration
`by function R6 will be described later.
`The communication system according to the first embodi
`ment of the present invention assumes a wireless LAN
`system employing a MIMO communication mode. A com
`55
`munication terminal (to be referred to as a MIMO terminal
`hereinafter) adopted in this communication system can
`process a format used in the IEEE802.11a communication
`mode. The IEEE802.11a communication mode corresponds
`to a first physical layer protocol, and the MIMO communi
`cation mode correspond to a second physical layer protocol.
`FIG. 2 is a view showing an example of a MIMO channel
`in the communication system according to the first embodi
`ment of the present invention. As shown in FIG. 2, the first
`embodiment of the present invention prepares three antennas
`300 to form a matrix channel (MIMO channel). The trans
`mitter receives different information from the three antennas
`
`35
`
`45
`
`60
`
`65
`
`IPR2023-01273 Page 8
`
`

`

`5
`Tx2 and Tx3 transmit the data S2 and S3, respectively. The
`receiver receives the data S1, S2, and S3 by using three
`reception antennas (RX1, RX2, and RX3). The reception
`antennas RX1, RX2, and RX3 receive a signal in which the
`three data S1, S2, and S3 are mixed. When attention is paid
`to the reception antenna RX1, a signal r1 received by the
`reception antenna RX1 is given by
`
`10
`
`15
`
`where h11, h21, and h31 are the channel statement
`information. The channel state between the transmission
`antenna Tx1 and the reception antenna RX1 is represented by
`h1 1. The data S1 transmitted by the transmission antenna
`Tx1 changes under the influence of the channel state h1 1
`during transmission, and is received as S1h1 1 by the
`reception antenna RX1. Similarly, the data S2 and S3 are
`received as S2*h2 1 and S3*h3 1, respectively.
`Similarly, a signal r2 received by the reception antenna
`RX2 is given by
`Equation (2)
`r2=h1 2*S1-h2 2*S2+h3 2*S3
`A signal r3 received by the reception antenna RX3 is given
`by
`
`r3=h1 3*S1-h2 3*S2+h3 3*S3
`Equations (1) to (3) are summarized into:
`
`Equation (3)
`
`25
`
`US 7,369,511 B2
`
`6
`having the high correlation is selected. An unselected path
`having the high correlation is not used. Accordingly, an
`antenna used for transmission via the selected path can be
`determined.
`As an example of another method, the CNR (Carrier vs.
`Noise Ratio) of each path is obtained, and a transmission
`antenna is selected so that a path having a low CNR is not
`used. In some cases, it is difficult to reassemble, for respec
`tive paths, signals received via paths having low CNRS. For
`this reason, when the CNR of a given path is lower than a
`preset threshold CNR th, the path is set not to be used. The
`CNR of each path can be obtained by equation (4) and the
`channel state estimation result hX y (X=1, 2, 3, y=1, 2, 3) in
`FIG. 6. Since a path having a large channel estimation result
`value hx y also exhibits a high CNR, an antenna configu
`ration can be determined from the channel estimation result
`value hx y without actually calculating the CNR. The same
`determination can be used if the CNR used for determination
`is replaced with an SNR (Signal vs. Noise Ratio).
`Under LOS environment, the signals S1, S2, and S3
`transmitted from the transmission antennas TX1, TX2, and
`Tx3 pass through almost the same channel, and the corre
`lation between the reception signals r1, r2, and r3 becomes
`high. More specifically,
`
`r1
`
`|
`
`r3
`
`h1 1 h 1 h31 S1
`
`h2 2
`
`h13 h23 h33) S3
`
`S1
`
`-
`
`Equation (4)
`
`30
`
`The channel state matrix H is given by
`
`In this case, equation (4) is rewritten as:
`
`35
`
`The receiver can separate and extract S1, S2, and S3 from
`the reception signals r1, r2, and r3 by Solving simultaneous
`equation system (4) as far as a channel state matrix H can be
`estimated. Estimation of H utilizes a preamble which is
`added and transmitted together with data. When a MIMO
`preamble is added to data, the channel state matrix H can be
`estimated by the receiver. Thus, S1, S2, and S3 can be
`separated and extracted from the reception signals r1, r2, and
`r3.
`By using the channel state matrix H estimated by the
`receiver, the transmitter determines an antenna configuration
`to use when communicating with the receiver (function R6).
`An example of a method of determining the antenna con
`figuration will be explained.
`A method using a correlation coefficient C. between paths
`is conceivable. The correlation coefficient C. between paths
`can be obtained by equation (4) and a channel State estima
`tion result hx y(x=1, 2, 3, y=1, 2, 3) in FIG. 6. For example,
`if the hx y values of two paths are almost equal to each
`other, the correlation coefficient C. between the two paths is
`high. To the contrary, if the hx y values of two paths are
`greatly different from each other, the correlation coefficient
`C. between the two paths is low. In MIMO communication,
`the receiver may not reassemble a plurality of signals which
`have been received via paths having a high correlation
`coefficient C. When the correlation between paths is high,
`only one path is desirably used. That is, the correlation
`coefficient C. between paths is obtained from the channel
`estimation result hx y, and if the correlation coefficient C. is
`higher than a predetermined threshold C. th, one of paths
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`r1
`
`r3
`
`| = (h1 1 h2 1 s
`
`S1
`
`S3
`
`Equation (4)
`
`Since the equation may not be solved, the transmission
`signals S1, S2, and S3 may not be calculated. Hence, when
`MIMO is used in the LOS environment, S1, S2, and S3 may
`not be separated and extracted from the reception signals r1,
`r2, and r3.
`As described above, since there may be a case wherein
`data may not be reassembled in MIMO depending on the
`environment, the MAC frame part of channel information
`feedback request frame 1 is transmitted by the IEEE802.11a
`instead of MIMO.
`Note that channel information feedback request frame 1
`shown in FIG. 5 may be configured as shown in FIG. 7. In
`FIG. 7, a format transmitted by the IEEE802.11a commu
`nication mode is decoded by using a preamble transmitted
`by MIMO. When a preamble transmitted by MIMO is used,
`the channel state can be estimated for possible combinations
`of antennas, and the channel state of a channel used to
`transmit a MAC frame can also be obtained. That is, a MAC
`frame part of a format transmitted by the IEEE802.11a
`communication mode can be decoded. By using a preamble
`transmitted by MIMO, a MAC frame of the format trans
`mitted by the IEEE802.11a communication mode can be
`decoded. The frame as shown in FIG. 7 can be employed in
`place of channel information feedback request frame 1 in
`FIG.S.
`
`IPR2023-01273 Page 9
`
`

`

`7
`The steps of the feedback procedure shown in FIG. 4 will
`be explained in detail.
`<First Step (Transmitter->Receiver)>
`Channel information feedback request frame 1 shown in
`FIG. 5 is transmitted from the transmitter to the receiver. The
`transmitter transmits a preamble by the IEEE802.11a com
`munication mode using one antenna. The transmitter then
`transmits a preamble by MIMO communication mode using
`three antennas. Data which requests feedback of channel
`information is stored in the MAC frame, and transmitted by
`using one antenna which has been used to transmit the
`preamble based on the IEEE802.11a communication mode.
`In this case, data which requests feedback of channel infor
`mation may be stored in the header or payload of the MAC
`15
`frame.
`Alternatively, data which requests feedback of channel
`estimation result information may be added to general
`application data and transmitted by MIMO by using a data
`frame instead of channel information feedback request
`frame 1.
`<Second Step (Channel Estimation Process on Receiver)>
`The receiver receives channel information feedback
`request frame 1. The receiver estimates a channel by using
`the preamble transmitted by the IEEE802.11a communica
`tion mode. The receiver then estimates a channel in MIMO
`communication by using the preamble transmitted by
`MIMO communication mode. The receiver decodes the
`MAC frame part on the basis of the channel estimation result
`30
`using the preamble transmitted by the IEEE802.11a com
`munication mode. This decoded data shows that the trans
`mitter requests feedback of channel estimation result infor
`mation in MIMO communication. The receiver creates
`channel estimation result notification frame 2-1 for notifying
`35
`the transmitter of the channel estimation result in MIMO
`communication. The channel estimation result is generally
`expressed by a complex number including the phase and
`amplitude information. Channel estimation result informa
`tion to be fed back to the transmitter may be not a complex
`number itself but simple information. For example, a round
`number prepared by quantizing a complex number, or a
`bitmap expressing a good/bad channel state by a true/false
`value (1/O) may be fed back as channel estimation result
`information. When a communication apparatus which esti
`mates a channel converts a channel estimation result into
`Summarized information, the process of a communication
`apparatus which receives the information can be simplified.
`<Third Step (Receiver->Transmitter)>
`50
`Channel estimation result notification frame 2-2 in MIMO
`communication is sent back from the receiver to the trans
`mitter. FIG. 8 shows an example of the format. In this step,
`the receiver transmits a preamble by the IEEE802.11a
`communication mode using one antenna. The receiver then
`transmits a preamble by MIMO using three antennas. The
`receiver stores, in the MAC frame, channel estimation result
`information in MIMO communication that is obtained in the
`second step, and transmits the information by using one
`antenna which has been used to transmit the preamble by the
`IEEE802.11a communication mode. In this case, data which
`requests channel estimation result information may be stored
`in the header or payload of the MAC frame.
`In transmitting a preamble and MAC frame, one antenna
`in a good channel state can also be selected on the basis of
`the channel estimation result in the second step, and trans
`mitted by the IEEE802.11a communication mode.
`
`45
`
`8
`In this step, channel estimation result information may be
`added to general transmission data and transmitted by
`MIMO by using a data frame instead of channel estimation
`result notification frame 2-1.
`
`<Fourth Step (Channel Estimation Process on Transmitter)>
`The transmitter receives channel estimation result notifi
`cation frame 2-1. The transmitter estimates a channel by
`using the preamble transmitted by the IEEE802.11a com
`munication mode. The transmitter then estimates a channel
`in MIMO communication by using the preamble transmitted
`by MIMO. The transmitter decodes the MAC frame part by
`using the channel estimation result based on the preamble of
`the IEEE802.11a communication mode. This decoded data
`shows that the receiver has fed back channel estimation
`result information in MIMO communication. The transmit
`ter determines an antenna configuration in MIMO data
`transmission. At this time, the channel estimation result in
`MIMO communication and the channel estimation result in
`MIMO communication on the receiver that has been fed
`back from the receiver can be compared to confirm whether
`these channel estimation results coincide with each other.
`This process is executed because a channel state from the
`transmitter to the receiver and a channel state from the
`receiver to the transmitter may not always coincide with
`each other due to the difference between the transmitter and
`the receiver in their configuration. With this process, mis
`matching in recognition of the channel State between the
`transmitting and receivers can be recognized before the start
`of data communication. A communication parameter per
`missible between the transmitting and receivers can be set.
`<Fifth Step (Transmitter->Receiver)>
`In the fifth step, channel estimation result information
`obtained on the transmitter in the fourth step is transmitted
`from the transmitter to the receiver. If it is determined from
`the channel estimation result fed back from the receiver in
`the third step that the signal transmitted by MIMO can be
`correctly decoded by the receiver, channel estimation result
`notification frame 3-1 or 3-2 is employed. FIG. 9 shows an
`example of the format of channel estimation result notifi
`cation frame 3-1, and FIG. 10 shows an example of the
`format of channel estimation result notification frame 3-2.
`When channel estimation result notification frame 3-1 or 3-2
`is used, the antenna configuration which is determined in the
`fourth step is adopted. Channel estimation result information
`obtained on the transmitter in the fourth step are multiplexed
`to transmit the MAC frame part. If it is determined from the
`channel estimation result fed back from the receiver in the
`third step that the signal transmitted by MIMO may not be
`correctly reassembled by the receiver and information may
`not be multiplexed and transmitted from a plurality of
`antennas, channel estimation result notification frame 3-3 is
`employed. FIG. 11 shows an example of the format of frame
`3-3. In this case, MIMO communication may not be
`executed and only transmission/reception using one antenna
`can be done. Thus, the MAC frame part which stores
`channel estimation result information obtained on the trans
`mitter in the fourth step is transmitted from one antenna.
`Channel estimation result information on the transmitter in
`the fourth step may be transmitted using frame 2-1 similar
`to the third step regardless of whether the receiver can
`correctly reassemble the signal transmitted by MIMO. In the
`fifth step, channel estimation result information may be
`added to general transmission data and transmitted by
`MIMO by using a data frame instead of channel estimation
`result notification frames 3-1, 3-2, and 3-3.
`
`US 7,369,511 B2
`
`5
`
`10
`
`25
`
`40
`
`55
`
`60
`
`65
`
`IPR2023-01273 Page 10
`
`

`

`US 7,369,511 B2
`
`9
`By executing the first to fifth steps described above,
`transmitting and receiving communication terminals can
`recognize their channel estimation results. The transmitter
`can transmit data by specifying one or more antennas used
`for transmission so as to allow the receiver to correctly 5
`decode information.
`According to the first embodiment, in the fourth step, the