Wireless communication device and wireless communication method

This applicationMore than one reissue application has been filed for the reissue of U.S. Pat. No. 10,063,342. The applications are the present application and Application Serial No. 17/006,357, filed Aug. 28, 2020. The present application is a reissue continuation of reissue application Serial No. 17/006,357, which is a reissue application of U.S. Serial No. 15/192,403, filed Jun. 24, 2016 (now U.S. Patent No. 10,063,342), whichis a Continuation of International Application No. PCT/JP2015/050420, filed on Jan. 5, 2015, the entire contents of whichisarehereby incorporated by reference.

Embodiments described herein relate to a communication controlling device, a wireless terminal, a memory card, an integrated circuit, and a wireless communication method.

As a wireless communication system in which a wireless access point and a wireless terminal communicate with each other, the wireless LAN (Local Area Network) based on the IEEE 802.11 standard that uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is widely known. IEEE Std 802.11™-2012 and IEEE Std 802.11ac™-2013 are known as wireless LAN specification. In the IEEE 802.11n standard, MIMO (Multi-Input Multi-Output) technology is used, which achieves an increased speed of wireless communication.

In the MIMO technology, a transmission-side apparatus and a reception-side apparatus each include a plurality of antennas installed therein. The transmission-side apparatus simultaneously transmits a plurality of pieces of data with the plurality of antennas. The reception-side apparatus simultaneously receives the plurality of pieces of data with the plurality of antennas, and separates the plurality of received pieces of data from one another. This allows a plurality of data streams to be transmitted and received. The higher the number of data streams is, the higher throughput can be obtained as compared with conventional SISO (Single-Input Single-Output).

In addition, in an effort to increase the speed, the IEEE 802.11ac standard is now being formulated. The IEEE 802.11ac standard uses a downlink multiuser MIMO (DL-MU-MIMO) technology, which is an expansion of the MIMO technology. In the downlink multiuser MIMO, an access point simultaneously transmits different pieces of data to a plurality of wireless terminals from a plurality of antennas. This allows the access point to simultaneously communicate with the plurality of wireless terminals.

The downlink multiuser MIMO uses a technique called beamforming. In the beamforming, a beam that causes data streams addressed to the wireless terminals to least interfere with one another, that is, a beam that makes data streams addressed to the wireless terminals spatially orthogonal, are formed and transmitted for each wireless terminal. This makes spatial multiplexing available, which in turn allows the access point to simultaneously transmit different pieces of data to the plurality of wireless terminals.

To implement the downlink multiuser MIMO, the access point needs to obtain information on channels between the antennas thereof and the wireless terminals. That is, the access point needs to detect how the wireless terminals receive signals transmitted from the antennas thereof. Using information on the channels allows the access point to form an optimal beam that does not interfere with other wireless terminals. The IEEE 802.11ac standard defines such a protocol that allows for estimation of the channels between the access point and the wireless terminals.

According to the protocol defined in the IEEE 802.11ac standard, the access point causes the wireless terminals to estimate channel responses and to feed them back in turn, so as to detect a state of the channels with respect to the wireless terminals. As a result, the higher the number of the wireless terminals performing channel estimation, the longer it takes for the estimation to be performed. This increases an overhead time taken to start the downlink multiuser MIMO. In addition, this decreases a correlation between a state of the channels when the wireless terminals estimated them and a state of the channels when the downlink multiuser MIMO is actually started. Thus, in the downlink multiuser MIMO transmission using the channel information obtained from the wireless terminals, an adequate beam may not be formed due to temporal fluctuations of the channels. Also, an inadequately formed beam may disadvantageously interfere with other wireless terminals.

According to one embodiment, a wireless communication device includes a transmitter and a receiver. The transmitter is configured to transmit a first frame containing notification information to specify a first wireless terminal and a second wireless terminal and a second frame for channel estimation. The receiver is configured to receive a third frame transmitter from the first wireless terminal and a fourth frame transmitted from the second wireless terminal. The third frame and the fourth frame are transmitted in a multiplexing scheme by the first wireless terminal and the second wireless terminal. The third frame contains first channel information estimated based on the second frame. The fourth frame contains second channel information estimated based on the second frame. The first frame contains at least either information to specify a preamble of the third frame and a preamble of the fourth frame or information to specify a subcarrier used for transmission of the third frame and a subcarrier used for transmission of the fourth frame.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Assume that the entire contents of IEEE Std 802.11™-2012 and IEEE Std 802.11ac™-2013, known as the wireless LAN specification, are incorporated by reference in the present specification.

(First Embodiment)

shows a wireless communication system according to a first embodiment.

The wireless communication system ofis a wireless network including an access point (AP)and a plurality of wireless terminals (stations),,, and. The access pointis one form of a wireless terminal. The access pointand the wireless terminalstoperform wireless communication based on any wireless communication method. As one example, the access pointand the wireless terminalstoperform the wireless communication based on the IEEE 802.11 standard. Although the following description will be mainly made assuming that a wireless LAN complies with the IEEE 802.11 standard, the present embodiment is not limited thereto.

The access pointincludes a plurality of antennas. In the example of, the access pointincludes four antennasA,B,C, andD. The access pointincludes a wireless communication device (refer toto be described hereafter) installed therein. The wireless communication device includes a wireless communication unit and a communication controlling device that controls communication with the wireless terminalstobeing a plurality of communication devices.

The wireless terminalstoeach include one or more antennas. In the example of, the wireless terminalstoinclude one antennaA,A,A, andA, respectively. The wireless terminals each include a wireless communication device (refer toto be described hereafter) installed therein. The wireless communication device includes a wireless communication unit and a communication controlling device that controls communication with the access pointbeing a communication device.

The access pointforms the wireless network (referred to as first network) that is formed with the wireless terminals, and may be connected to another wired or wireless network (referred to as second network) in addition to the first network. The access pointrelays communication between the first network and the second network, or among the wireless terminals. The access pointreceives frames addressed to the wireless terminalstofrom the second network or a wireless terminal in the first network, and holds the frames in an internal buffer thereof. The access pointtransmits the held frames addressed to the wireless terminalstoto the wireless terminalstoby spatial multiplexing. The transmission by spatial multiplexing means that a plurality of data streams are transmitted at the same time in the same frequency band. Specifically, the access pointtransmits the frames addressed to the wireless terminals in the downlink multiuser MIMO. The downlink multiuser MIMO transmission can increase the throughput. Note that the frames in the present embodiment may be, for example, those called packets, not only those called frames in the IEEE 802.11 standards.

To perform the downlink multiuser MIMO transmission, the access pointobtains in advance channel information that represents a state of the channels of downlinks from the antennasA toD to the wireless terminalsto, from the wireless terminalsto. One of features of the present embodiment is obtaining the channel information from these wireless terminalstoin a short time. A method of channel estimation in the wireless terminal will be described thereafter in detail.

The access pointcalculates a directional beam pattern on the basis of the pieces of channel information obtained from the wireless terminals, in such a manner as to suppress interference among signals addressed to the plurality of the wireless terminalsto. The access pointforms beams,,, andin conformity with such a directional beam pattern. The data frames addressed to the wireless terminals are thereby transmitted. In such a manner, the access pointcan transmit data frames to the wireless terminals at the same time in the same frequency band. That is, the downlink multiuser MIMO transmission is possible.

In the example of, although the number of the directional beams is four and the number of wireless terminals simultaneously communicating is also four, the present embodiment is not limited thereto. For example, the number of wireless terminals simultaneously communicating may be three while the number of the directional beams is four. In this case, one of the three wireless terminals may include a plurality of antennas, and the access pointmay form two directional beams toward the wireless terminal in question. It is thereby possible to transmit two data streams to the one wireless terminal in question, and to transmit one data stream to each of the remaining two wireless terminals. Note that, in, a terminal other than wireless terminalstomay establish a wireless link with the access point.

In the example of, the number of antennas included in the access pointis four, but the number of antennas may be any number higher than one. Note that a maximum of the number of data streams that the access pointcan transmit is equivalent to the number of the included antennas. In the case where the wireless terminals each include one antenna, the number of data streams that each wireless terminal can transmit is one, and the number of antennas included in the access pointmay be therefore equal to or higher than the number of wireless terminals with which the downlink multiuser MIMO transmission is performed.

Note that, in the example of, in the case where the downlink multiuser MIMO transmission is performed from the access pointto the wireless terminals, the access pointforms the beams, and the wireless terminals receive the beams formed. In terms of these roles, the access pointmay be called a Beamformer, and the wireless terminals may be called Beamformees.

is a diagram illustrating an outline of a manner the access pointobtains the pieces of channel information from the wireless terminals. Signals (frames) transmitted by the access pointand the wireless terminals are shown as rectangles. The horizontal axis is a time axis, and a right direction thereof along the drawing is a direction of time passage.

As a precondition, the access pointhas established in advance a wireless link with a plurality of wireless terminals including the wireless terminalsto, and holds the frames addressed to the wireless terminalstoin the internal buffer thereof. Let us assume a situation in which the access pointwill now transmit the frames to the wireless terminalstoin the downlink multiuser MIMO manner.

First, the access pointdetermines a plurality of wireless terminals that should perform channel estimation, and transmits, through any one of the antennas, a notification framecontaining notification information that specifies the plurality of wireless terminals in question. The notification framemay contain, as will be described hereafter, additional information on a method of transmitting channel information frames from the wireless terminalsto. The plurality of wireless terminals that should perform channel estimation are, for example, wireless terminals to which frames to be transmitted exist in the buffer of the access point. Note that the access pointcan transmit the notification framealso from a plurality of antennas.

The notification frameis transmitted, as one example, in a broadcasting manner. Note that the notification framemay be transmitted in a manner other than broadcasting, such as multicasting or unicasting. The notification frameis configured by, for example, a Control Frame defined in the IEEE 802.11 standards. As a specific example of the notification frame, an NDP (Null Data Packet) Announcement (NDPA) frame, which is defined in the IEEE 802.11 standard, may be used. Alternatively, the NDPA frame defines an expanded frame, and the frame defined thus may be used.

Note that, it is assumed that the access pointperforms carrier sensing based on the CSMA/CA to obtain a right to transmission, before transmitting the notification frame. The right to transmission is obtained since a result of carrier sensing comes back as idle if a signal of a certain level or more is not received during carrier sensing. If a signal of a certain level or more is received during carrier sensing, the right to transmission is not obtained since the carrier sensing result comes back as busy. In this case, carrier sensing may be performed again after a backoff time.

Next, the access pointtransmits a framefor channel estimation after the lapse of a certain period of time Tfrom transmitting the notification frame. The access pointtransmits the channel estimation framesat the same time with signals that are orthogonal to one another, from the antennas of the access point. By the simultaneous transmission of the frame signals orthogonal to one another from the antennas, the reception side can correctly separate the frames even when the signals from the antennas are received in a duplicate manner. A configuration of the channel estimation frames (bit streams) transmitted from the antennas is known to the wireless terminals. The configuration of the channel estimation frames may be notified in advance, when associating or with a pilot signal, to the wireless terminals with which the wireless links are established, or can be notified to the wireless terminals being targets of channel estimation, with the notification frame. The method of transmitting the signals orthogonal to one another from the antennas will be described in detail in the description of transmitting the channel information frames from the wireless terminals to the access point. Note that, as a modification of this operation, the access pointmay consecutively transmit the channel estimation framesusing the antennas in turn, instead of transmitting the channel estimation frames at the same time through the antennas.

The channel estimation framemay be in any format. The propagation channel estimation framemay belong to neither the Control Frame nor a Management Frame, nor a Data Frame, which are defined in the IEEE 802.11 standards. For example, the propagation channel estimation framemay be a frame including only a PHY part of a multiuser VHT packet structure (MU VHT Packet Structure) defined in the IEEE 802.11 standards, not information such as MAC. In this case, the propagation channel estimation frameincludes neither the MAC address of an access point to be a transmission source nor the MAC address of a wireless terminal to be a transmission destination. As an example of such a frame, an NDP (Null Data Packet) defined in the IEEE 802.11 standard may be used. Alternatively, a frame defined as an expanded NDP may be used. The certain period of time Tmay be any value as long as it is a predetermined certain period of time. As one example, a SIFS (Short Inter-frame Space) time (=16 μs), which is a time interval between frames regulated in the MAC protocol specification of the IEEE 802.11 wireless LAN can be used.

The plurality of wireless terminalstoeach receive the notification frametransmitted from the access point. The wireless terminalstoeach analyze notification information in the notification frameto recognize that the terminals themselves are specified as targets of channel estimation.

Each of the wireless terminalsto, which are specified as the targets of channel estimation, performs thereafter channel estimation on the basis of the channel estimation framereceived from the access point. Here, the channels to be estimated are downlink channels from the antennas of the access pointto the wireless terminals. The wireless terminal estimates the downlink channel to obtain the channel information representing a state of the channel from the access point to the wireless terminal.

The channel information may be, for example, channel state information (CSI: Channel State Information) (channel response matrix), may be components of a V matrix that is obtained through Singular Value Decomposition (hereafter referred to as SVD) of the channel state information (channel response matrix), or may be other information.

Any well-known method may be used as a method of estimating the channel information in each wireless terminal. For example, as will be described with reference to, the channel information such as the channel response matrix may be estimated by a model similar to that for channel responses of uplinks from the wireless terminals to the access point.

The wireless terminalstogenerate channel information frames,,, andcontaining the obtained pieces of channel information, respectively. The wireless terminalstotransmit the channel information frames,,, andto the access pointin the same frequency band, respectively, after a certain period of time Tfrom receiving the channel estimation frame. That is, the channel information frames transmitted by the wireless terminals are transmitted in an uplink multiuser MIMO manner that is spatially multiplexed. If the notification framecontains information on the method of transmitting the channel information frames, the method of transmitting the channel information frames is determined according to the relevant information and the transmission is performed. A specific method of implementing the uplink multiuser MIMO manner of transmission will be described hereafter in detail. Note that, in the case of transmitting the channel state information (channel response matrix) to the access point as channel information, the pieces of channel state information (channel response matrices) may be decomposed in singular values on the access point side to obtain the V matrix.

Since, in the uplink multiuser MIMO, the wireless terminals do not share information on timings of transmission of the channel information frames, it is required to time the timings of the transmission by some method. In the first embodiment, the wireless terminals transmit the channel information frames after the lapse of SIFS from receiving the channel estimation frames from the access point. For this reason, it can be said that the transmission of the channel estimation frame from the access point practically serves as a trigger to time the timings of transmission in the uplink multiuser MIMO transmission.

Here, the propagation channel information frames,,, andmay be configured by, for example, the Management Frames defined in the IEEE 802.11 standards. In this case, the configuration of the Management Frame may have the configuration of a typical MAC frame, and may be, for example, a frame including a Frame Control field, a Duration field, an RA field, a TA field, a Frame Body field, an FCS field, and the like. In this case, the propagation channel information is stored in the Frame Body field. In addition, the RA field stores the MAC address of the access point, and the TA field stores the MAC address of the own terminal. The MAC address of the access pointto be set to the RA field may be an address that is set to the TA field in the notification frame, or may be the MAC address of the access pointthat is known in advance. As a specific example of the channel information frame, the Compressed Beamforming frame defined in the IEEE 802.11 standard may be used. Alternatively, a frame defined by expanding the Compressed Beamforming frame may be used.

In addition, the certain period of time Tmay be any value as long as it is a predetermined certain period of time. As one example, the period of SIFS (Short Inter-frame Space) time (=16 μs), which is a time interval between frames regulated in the MAC protocol specification of the IEEE 802.11 wireless LAN can be used. The certain period of time Tmay have the same length as that of the certain period of time T, or may have a different length therefrom.

The access pointreceives the channel information framestotransmitted from the wireless terminals in the uplink multiuser MIMO transmission. The access pointcalculates a beam pattern used for the downlink multiuser MIMO transmission by making use of the pieces of channel information on the wireless terminals contained in the channel information framesto. For example, if obtaining the components of the V matrices from the wireless terminals as the pieces of channel information, the access pointmay calculate a beam pattern for each antenna using the components as transmission weights. If obtaining the channel response matrices as the pieces of channel information, the access pointmay decompose into singular values the channel response matrices to obtain the V matrices, and calculate a beam pattern for each antenna using the components of the V matrices as the transmission weights. The calculation of the beam pattern is not limited to the example here described, and any well-known method can be used therefor.

The access pointbeam-transmits frames to the wireless terminalstobased on the calculated beam patterns after the lapse of a certain period of time Tfrom receiving the channel information framesto. The downlink multiuser MIMO transmission is thereby realized. The access pointmay once stop the transmission when the downlink multiuser MIMO transmission to the wireless terminalstocontinues for a predetermined period of time, or a certain amount of transmission is made, and may restart the process with the transmission of the notification frame. Alternatively, the access pointmay restart the process with the transmission of the notification frameif there is a change in the wireless terminals to be targets for transmission.

Here, the frames transmitted from the access pointto the wireless terminalstomay be, for example, Data Frames defined in the IEEE 802.11 standards. In this case, the configuration of the Data Frames may have the configuration of a typical MAC frame, and may be, for example, a frame including a Frame Control field, a Duration field, an RA field, a TA field, a Frame Body field, an FCS field, and the like. In this case, data to be transmitted to the wireless terminal is stored in the Frame Body field. In addition, the RA field stores the MAC address of the wireless terminal, and the TA field stores the MAC address of the access point. The MAC addresses of the wireless terminalstoto be set to the respective RA fields may be the MAC addresses that are set to the TA fields in the propagation channel information framesto. The certain period of time Tmay be any value as long as it is a predetermined certain period of time. As one example, the period of SIFS (Short Inter-frame Space) time (=16 μs), which is a time interval between frames regulated in the MAC protocol specification of the IEEE 802.11 wireless LAN can be used. The certain period of time Tmay have the same length as that of the certain period of time Tor T, or may have a different length therefrom. Alternatively, the beam transmission may be made after obtaining right to transmission on the basis of CSMA/CA with carrier sensing, instead of using the certain period of time T.

There will be described below in detail a manner the wireless terminalstoperform the uplink multiuser MIMO transmission of the channel information frames to the access point.

shows an example of a frame format of the notification frame.

The notification framecontains, for example, a Frame Control field, a Duration field, an RA field, a TA field, a common information field, a terminal information field, and an FCS field.

In the Frame Control field, information representing a type of the frame or the like is set.

In the Duration field, a time set as a virtual carrier sense is stored. A device that receives a frame containing the Duration field having a value set counts down until the time set to this field reaches zero, and recognizes that the channel is busy until the time set reaches zero. This is called the virtual carrier sense.

In the RA (Receiver Address) field, a MACaddress of a transmission destination of the frame is normally set. Since the notification frameis transmitted addressed to a plurality of wireless terminals, a broadcast address may be set to the RA field. Note that a multicast address or a MAC address of the wireless terminal (unicast address) may be set to the RA instead of the broadcast address. In the case of setting the unicast address, a plurality of RA fields may be set to specify a plurality of addresses.

In the TA (Transmitter Address) field, a MAC address of a transmission source of the frame is contained. In the present embodiment, for example, a MAC address of the access point is set thereto.

In the common information field, information that should be notified commonly to the wireless terminals to be targets of channel estimation (i.e., the wireless terminals to be the targets of the downlink multiuser MIMO transmission) is set, as information needed to perform the uplink multiuser MIMO transmission. If the notification frameis formed by expanding the NDPA frame defined in the IEEE 802.11 standard, it can be realized by adding the common information field to any place in the NDPA frame.

The terminal information field (individual information field) is provided for each wireless terminal to be a target of channel estimation. The number of the terminal information fields can vary depending on the number of wireless terminals to be targets of channel estimation. In the example of, the number of the provided terminal information fields is four. That is, a terminal information field, a terminal information field, a terminal information field, and a terminal information fieldare provided.

The terminal information field contains identification information on the wireless terminal to be a target of channel estimation, and individual information unique to the wireless terminal in question as information needed for performing the uplink multiuser MIMO transmission of the channel information frame. If the notification frameis formed by expanding the NDPA frame regulated in the IEEE 802.11 standard, it may be realized by the terminal information field being a subfield in an STA info field in the NDPA frame. It is conceivable as a modification that the pieces of identification information of the wireless terminals to be targets of channel estimation are set to the common information fields instead of the terminal information fields. The identification information on the wireless terminal is not limited to specific information as long as the identification information allows the wireless terminal to be distinguished, and for example, any ID can be used that allows a terminal to be distinguished, such as a MAC address and an Association ID (AID).

In the FCS (Frame Check Sequence) field, FCS information on the notification frameis set. The FCS information is used for frame error detection on the reception device side.