Method and apparatus for measuring channel on basis of coordination in wireless LAN system

Abstract

The purpose of the present disclosure is to measure a channel on the basis of coordination within a wireless LAN network. According to an embodiment of the present disclosure, a method for operation of a station (STA) in a wireless LAN system comprises the steps of: receiving information regarding a time at which a frame is transmitted from a first AP; receiving a scan response message generated on the basis of the information regarding the time at which the frame is transmitted; receiving a frame from a second AP on the basis of the scan response message; measuring a channel on the basis of the frame received from the second AP; and transmitting a measurement report message to the first AP, wherein the information regarding the time at which the frame is transmitted includes a time at which the frame is transmitted by at least one AP including the second AP.

Description

Method and device for measuring channel based on cooperation in wireless LAN system

[0001] The present invention relates to a wireless local area network (WLAN) system, and more particularly, to a method and device for measuring a channel based on coordination by a wireless LAN terminal in a WLAN system.

[0002] With the recent proliferation of mobile devices, Wireless Local Area Network (WLAN) technology, which can provide fast wireless communication services to these devices, is attracting significant attention. WLAN technology utilizes short-range wireless communication technology to enable mobile devices such as smartphones, tablets, laptops, portable multimedia players, and embedded devices to wirelessly access the Internet.

[0003] As applications requiring higher throughput and real-time transmission emerge, wireless LAN systems can support expanded frequency bandwidth and / or efficient retransmission operations. Furthermore, operations utilizing multiple channels or links simultaneously can be supported.

[0004] Meanwhile, the technology that serves as the background for the invention is written to promote understanding of the background for the invention, and may include content that is not a prior art already known to a person with ordinary skill in the field to which the technology belongs.

[0005] The present disclosure provides a method and device for a terminal to change connections between multiple APs (access points) in a wireless LAN system.

[0006] The present disclosure provides a method and device for a terminal to perform efficient roaming in a wireless LAN system.

[0007] The present disclosure provides a method and device for a terminal to scan for an appropriate roaming target AP in a wireless LAN system.

[0008] The present disclosure provides a method and device for a terminal in a wireless LAN system to change an operating link for a channel scan procedure.

[0009] The present disclosure provides a method and device for reducing delay for a channel search procedure in a wireless LAN system.

[0010] The present disclosure provides a method and device for cooperatively supporting channel access of a wireless LAN terminal by multiple APs in a wireless LAN system.

[0011] The technical problems to be achieved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present disclosure belongs from the description below.

[0012] According to one embodiment of the present disclosure, a method for operation of a STA (station) in a wireless LAN system includes the steps of: receiving information regarding a time at which a frame is transmitted from a first AP; receiving a scan response message generated based on the information regarding the time at which the frame is transmitted; receiving a frame from a second AP based on the scan response message; measuring a channel based on the frame received from the second AP; and transmitting a measurement report message to the first AP, wherein the information regarding the time at which the frame is transmitted includes a time at which the frame is transmitted by at least one AP including the second AP.

[0013] According to one embodiment of the present disclosure, a method for operation of a first AP (access point) in a wireless LAN system includes the steps of: obtaining information about a time at which a frame is transmitted; transmitting information about a time at which the frame is transmitted to an STA; transmitting a scan response message generated based on the information about a time at which the frame is transmitted; and receiving a measurement report message from the STA, wherein the information about the time at which the frame is transmitted includes information about a time at which the frame is transmitted by at least one AP, and the measurement report message includes a channel measurement result for a second AP, which is one of the at least one AP.

[0014] According to one embodiment of the present disclosure, in a wireless LAN system, a station (STA) includes: a transceiver; and a processor connected to the transceiver, wherein the processor is configured to receive information about a time at which a frame is transmitted from a first AP, receive a scan response message generated based on the information about the time at which the frame is transmitted, receive a frame from a second AP based on the scan response message, measure a channel based on the frame received from the second AP, and transmit a measurement report message to the first AP, wherein the information about the time at which the frame is transmitted includes a time at which the frame is transmitted by at least one AP including the second AP.

[0015] According to one embodiment of the present disclosure, in a wireless LAN system, a first AP (access point) includes a transceiver; and a processor connected to the transceiver, wherein the processor is configured to obtain information about a time at which a frame is transmitted, transmit the information about the time at which the frame is transmitted to an STA, transmit a scan response message generated based on the information about the time at which the frame is transmitted, and receive a measurement report message from the STA, wherein the information about the time at which the frame is transmitted includes information about a time at which the frame is transmitted by at least one AP, and the measurement report message includes a channel measurement result for a second AP that is one of the at least one AP.

[0016] According to the present disclosure, in a wireless LAN system, a wireless LAN terminal can operate in a subchannel when the primary channel is busy.

[0017] According to the present disclosure, in a wireless LAN system, if a wireless LAN terminal cannot decode a frame in a primary channel but detects energy exceeding a threshold, it can operate in a subchannel.

[0018] According to the present disclosure, in a wireless LAN system, a wireless LAN terminal can request channel information from a currently connected wireless LAN terminal and perform a channel search procedure based on the provided channel information.

[0019] According to the present disclosure, a wireless LAN terminal in a wireless LAN system can perform rapid channel search.

[0020] According to the present disclosure, roaming operation of a wireless LAN terminal in a wireless LAN system can be performed smoothly.

[0021] The effects that can be obtained from the embodiments of the present disclosure are not limited to the effects mentioned above, and other effects not mentioned can be clearly derived and understood by those skilled in the art to which the technical configuration of the present disclosure is applied, from the description of the embodiments of the present disclosure below. In other words, unintended effects resulting from implementing the configuration described in the present disclosure can also be derived from the embodiments of the present disclosure by those skilled in the art.

[0022] FIG. 1 is a block diagram illustrating a first embodiment of a communication node constituting a wireless LAN system.

[0023] Figure 2 illustrates a conceptual diagram illustrating a first embodiment of a multi-link established between MLDs.

[0024] Figure 3 illustrates a first embodiment of a cooperative channel measurement method in a wireless LAN system.

[0025] Figure 4 illustrates a second embodiment of a cooperative channel measurement method in a wireless LAN system.

[0026] Figure 5 illustrates a third embodiment of a cooperative channel measurement method in a wireless LAN system.

[0027] Fig. 6 illustrates a fourth embodiment of a cooperative channel measurement method in a wireless LAN system.

[0028] Fig. 7 illustrates a fifth embodiment of a cooperative channel measurement method in a wireless LAN system.

[0029] FIG. 8 illustrates a flowchart of a procedure for measuring a channel according to one embodiment of the present disclosure.

[0030] FIG. 9 illustrates a flowchart of a procedure for reporting channel measurements according to one embodiment of the present disclosure.

[0031] The present invention is susceptible to various modifications and embodiments. Specific embodiments are illustrated and described in detail in the drawings. However, this is not intended to limit the present invention to specific embodiments, but rather to encompass all modifications, equivalents, and alternatives falling within the spirit and technical scope of the present invention.

[0032] While terms such as "first" and "second" may be used to describe various components, these components should not be limited by these terms. These terms are used solely to distinguish one component from another. For example, without departing from the scope of the present invention, a first component may be referred to as a "second component," and similarly, a second component may also be referred to as a "first component." The term "and / or" includes a combination of multiple related items described herein or any of multiple related items described herein.

[0033] When a component is referred to as being "connected" or "connected" to another component, it should be understood that it may be directly connected or connected to that other component, but that there may be other components intervening. Conversely, when a component is referred to as being "directly connected" or "connected" to another component, it should be understood that there are no other components intervening.

[0034] The terminology used in this application is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, it should be understood that the terms "comprise" or "have" indicate the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0035] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and will not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0036] Hereinafter, with reference to the attached drawings, preferred embodiments of the present invention will be described in more detail. In order to facilitate an overall understanding in describing the present invention, identical reference numerals are used for identical components in the drawings, and redundant descriptions of identical components are omitted.

[0037] Below, a wireless communication system to which embodiments of the present invention are applied will be described. The wireless communication system to which embodiments of the present invention are applied is not limited to the descriptions below, and embodiments of the present invention can be applied to various wireless communication systems. The wireless communication system may be referred to as a "wireless communication network."

[0038] Figure 1 is a block diagram illustrating a first embodiment of a communication node constituting a wireless LAN system.

[0039] Referring to FIG. 1, a communication node (100) may be an access point, a station, an access point (AP), a multi-link device (MLD), or a non-AP MLD. An access point may refer to an AP, and a station may refer to an STA or a non-AP STA. The operating channel width supported by the access point may be 20 MHz (megahertz), 80 MHz, 160 MHz, etc. The operating channel width supported by the station may be 20 MHz, 80 MHz, etc.

[0040] A communication node (100) may include at least one processor (110), a memory (120), and at least one transceiver (130) that is connected to a network and performs communication. The transceiver (130) may be referred to as a transceiver, an RF (radio frequency) unit, an RF module, etc. In addition, the communication node (100) may further include an input interface device (140), an output interface device (150), a storage device (160), etc. Each component included in the communication node (100) may be connected by a bus (170) and communicate with each other.

[0041] However, each component included in the communication node (100) may be connected through an individual interface or individual bus centered around the processor (110), rather than a common bus (170). For example, the processor (110) may be connected to at least one of a memory (120), a transmission / reception device (130), an input interface device (140), an output interface device (150), and a storage device (160) through a dedicated interface.

[0042] The processor (110) can execute program commands stored in at least one of the memory (120) and the storage device (160). The processor (110) may refer to a central processing unit (CPU), a graphics processing unit (GPU), or a dedicated processor in which methods according to embodiments of the present invention are performed. Each of the memory (120) and the storage device (160) may be configured with at least one of a volatile storage medium and a non-volatile storage medium. For example, the memory (120) may be configured with at least one of a read-only memory (ROM) and a random access memory (RAM).

[0043] FIG. 2 is a conceptual diagram illustrating a first embodiment of a multi-link established between MLDs (multi-link devices).

[0044] Referring to FIG. 2, an MLD may have a single MAC (medium access control) address. In embodiments, the MLD may refer to an AP MLD and / or a non-AP MLD. The MAC address of the MLD may be used in a multi-link setup procedure between the non-AP MLD and the AP MLD. The MAC address of the AP MLD may be different from the MAC address of the non-AP MLD. Access point(s) associated with the AP MLD may have different MAC addresses, and station(s) associated with the non-AP MLD may have different MAC addresses. Access points within the AP MLD with different MAC addresses may be responsible for each link and may function as independent access points (APs).

[0045] Stations within a non-AP MLD with different MAC addresses can be responsible for each link and can act as independent stations (STAs). A non-AP MLD may also be referred to as a STA MLD. An MLD may support STR (simultaneous transmit and receive) operation. In this case, an MLD may perform a transmission operation on link 1 and a reception operation on link 2. An MLD supporting the STR operation may be referred to as an STR MLD (e.g., STR AP MLD, STR non-AP MLD). In embodiments, a link may mean a channel or a band. A device that does not support the STR operation may be referred to as an NSTR (non-STR) AP MLD or an NSTR non-AP MLD (or, an NSTR STA MLD).

[0046] MLD can transmit and receive frames on multiple links by using a non-contiguous bandwidth expansion scheme (e.g., 80 MHz + 80 MHz). Multi-link operation can include multi-band transmission. AP MLD can include multiple access points, and the multiple access points can operate on different links. Each of the multiple access points can perform the function(s) of the lower MAC layer. Each of the multiple access points can be referred to as a "communication node" or a "subordinate entity." The communication node (i.e., access point) can operate under the control of a higher layer (or, the processor (110) illustrated in FIG. 1). Non-AP MLD can include multiple stations, and the multiple stations can operate on different links. Each of the multiple stations can be referred to as a "communication node" or a "subordinate entity." The communication node (i.e., station) can operate under the control of a higher layer (or, the processor (110) illustrated in FIG. 1).

[0047] MLD can perform communications in multiple bands. For example, MLD can perform communications using a 40MHz bandwidth in the 2.4GHz band according to a channel expansion method (e.g., bandwidth expansion method) and can perform communications using a 160MHz bandwidth in the 5GHz band according to a channel expansion method. MLD can perform communications using a 160MHz bandwidth in the 5GHz band and a 160MHz bandwidth in the 6GHz band. One frequency band (e.g., one channel) used by MLD can be defined as one link. Alternatively, multiple links can be established in one frequency band used by MLD. For example, MLD can establish one link in the 2.4GHz band and two links in the 6GHz band. Each link can be referred to as a first link, a second link, a third link, etc. Alternatively, each link can be referred to as link 1, link 2, link 3, etc. The link number can be set by the access point, and an ID (identifier) ​​can be assigned to each link.

[0048] An MLD (e.g., an AP MLD and / or a non-AP MLD) can establish multiple links by performing an access procedure and / or a negotiation procedure for multi-link operation. In this case, the number of links and / or a link to be used among multiple links can be configured. A non-AP MLD (e.g., a station) can check information on a band that can communicate with the AP MLD. In the negotiation procedure for multi-link operation between a non-AP MLD and an AP MLD, the non-AP MLD can configure one or more links among the links supported by the AP MLD to be used for multi-link operation. A station that does not support multi-link operation (e.g., an IEEE 802.11a / b / g / n / ac / ax station) can be connected to one or more links among the multiple links supported by the AP MLD.

[0049] When the bandwidth gap between multiple links (e.g., the bandwidth gap between links 1 and 2 in the frequency domain) is sufficient, the MLD can perform the STR operation. For example, the MLD can transmit PPDU (PLCP (physical layer convergence procedure) protocol data unit) 1 using link 1 among the multiple links, and can receive PPDU 2 using link 2 among the multiple links. On the other hand, if the bandwidth gap between the multiple links is insufficient and the MLD performs the STR operation, in-device coexistence (IDC) interference, which is interference between the multiple links, may occur. Therefore, when the bandwidth gap between the multiple links is insufficient, the MLD may not be able to perform the STR operation. A link pair having the above-described interference relationship may be a Non Simultaneous Transmit and Receive (NSTR) limited link pair. Here, the MLD may be an NSTR AP MLD or an NSTR non-AP MLD.

[0050] For example, multiple links including link 1, link 2, and link 3 can be established between AP MLD and non-AP MLD 1. If the bandwidth gap between link 1 and link 3 is sufficient, the AP MLD can perform STR operation using link 1 and link 3. That is, the AP MLD can transmit frames using link 1 and receive frames using link 3. If the bandwidth gap between link 1 and link 2 is not sufficient, the AP MLD may not be able to perform STR operation using link 1 and link 2. If the bandwidth gap between link 2 and link 3 is not sufficient, the AP MLD may not be able to perform STR operation using link 2 and link 3.

[0051] Meanwhile, in a wireless LAN system, a negotiation procedure for multi-link operation can be performed in the access procedure between a station and an access point.

[0052] A device (e.g., an access point, a station) that supports multiple links may be referred to as a multi-link device (MLD). An access point that supports multiple links may be referred to as an AP MLD, and a station that supports multiple links may be referred to as a non-AP MLD or a STA MLD. An AP MLD may have a physical address (e.g., a MAC address) for each link. An AP MLD may be implemented as if there were separate APs responsible for each link. Multiple APs may be managed within a single AP MLD. Therefore, coordination between multiple APs belonging to the same AP MLD may be possible. An STA MLD may have a physical address (e.g., a MAC address) for each link. An STA MLD may be implemented as if there were separate STAs responsible for each link. Multiple STAs may be managed within a single STA MLD. Therefore, coordination between multiple STAs belonging to the same STA MLD may be possible.

[0053] For example, AP1 of the AP MLD and STA1 of the STA MLD can each be in charge of a first link and can communicate using the first link. AP2 of the AP MLD and STA2 of the STA MLD can each be in charge of a second link and can communicate using the second link. STA2 can receive status change information about the first link from the second link. In this case, the STA MLD can collect information (e.g., status change information) received from each link and control an operation performed by STA1 based on the collected information.

[0054] Next, methods of transmitting and receiving data in a wireless LAN system will be described. Even if a method (e.g., transmitting or receiving a signal) performed by a first communication node among communication nodes is described, a corresponding second communication node can perform a method (e.g., receiving or transmitting a signal) corresponding to the method performed by the first communication node. That is, when an operation of an STA is described, an AP corresponding to it can perform an operation corresponding to the operation of the STA. Conversely, when an operation of an AP is described, an STA corresponding to it can perform an operation corresponding to the operation of the AP. In an embodiment, an operation of an STA can be interpreted as an operation of an STA MLD, an operation of an STA MLD can be interpreted as an operation of an STA, an operation of an AP can be interpreted as an operation of an AP MLD, and an operation of an AP MLD can be interpreted as an operation of an AP.

[0055]

[0056] Figure 3 illustrates a first embodiment of a cooperative channel measurement method in a wireless LAN system.

[0057] Referring to FIG. 3, AP 1 (301), AP 2 (302), AP 3 (305), AP 4 (305), and STA 1 (310) may operate in a wireless LAN network. STA 1 (310) may be associated with AP 1 (301) and may want to discover surrounding APs, namely AP 2 (303), AP 3 (305), and AP 4 (305). STA 1 (310) may discover surrounding APs in order to connect to one of the surrounding APs. A series of operations in which STA 1 (310) stops communicating with an existing connected AP, connects to one of the surrounding APs, and communicates with the surrounding AP may be referred to as a roaming operation. AP 1 (301) and AP 2 (303) may operate in a first link. That is, AP 1 (301) and AP 2 (303) can operate on the same channel. AP 3 (305) can operate on the second link. AP 4 (305) can operate on the third link. The operating frequencies (e.g., center frequencies) of the first link and the second link may be different. The operating frequencies of the first link and the third link may be different. AP 1 (301) to AP 4 (305) can be connected to each other and communicate. For example, AP 1 (301) to AP 4 (305) can communicate with wired Ethernet. Alternatively, AP 1 (301) to AP 4 (305) can communicate with wireless LAN. Alternatively, AP 1 (301) to AP 4 (305) can communicate with each other using a communication standard other than wired Ethernet and wireless LAN. AP 1 (301) to AP 4 (305) can communicate with each other to form an MLD or a part of an MLD layer. For example, AP 1 (301) to AP 4 (305) can form MLD MAC Upper layers. Alternatively, AP 1 (301) to AP 4 (305) can form MLD MAC Lower layers.Alternatively, AP 1 (301) to AP 4 (305) may form both the MLD MAC Upper Layer and the MLD MAC Lower Layer. Alternatively, AP 1 (301) to AP 4 (305) may not form an MLD, but may exchange communication information with each other. For example, the exchange of communication information may include exchanging operation information (e.g., link use and use stop operation information) for roaming operations of STAs connected to APs, or exchanging status information of STAs performing roaming operations (e.g., Sequence Number (SN), Packet Number (PN), BlockAck scoreboard information of downlink and uplink packets for a specific STA) and downlink packets destined for STAs performing roaming operations. As another example, the exchange of communication information may be exchanging operation information of APs.

[0058] Since AP 1 (301) to AP 4 (305) communicate with each other, AP 1 (301) can know information about surrounding APs (AP 2 (303) to AP 4 (305)). Information about surrounding APs includes TBTT (target beacon transmission time) information of the APs. TBTT information may be point-in-time information. That is, TBTT information indicates the expected point-in-time or cycle of beacon frame transmission by each AP. For example, AP 1 (301) can know point-in-time information of when beacon frames of surrounding APs are transmitted from the current time.

[0059] AP 1 (301) can transmit TBTT information of surrounding APs by including it in a beacon frame (320). The TBTT information can be transmitted through an RNR (reduced neighbor report) information element included in the beacon frame (320). STA 1 (310) can receive the beacon frame (320) from AP 1 (301). Based on the RNR information element included in the beacon frame (320), STA 1 (310) can know the TBTT information of surrounding APs. STA 1 (310) can check the TBTT information of surrounding APs in various ways as well as the RNR information element. For example, AP 1 can transmit a beacon frame including an information element such as a multi-link element, and the multi-link element can include operation information including the TBTT information of surrounding APs.

[0060] The received signal strength indication (RSSI), signal-to-noise ratio (SNR), etc. of the beacon frame (320) or / and data frames of AP 1 (301) received by STA 1 (310) may be lower than the roaming threshold of STA 1 (310). The roaming threshold may be a preset value, a value broadcasted through a beacon, or a value set by mutual negotiation. For example, the roaming threshold may be Rth, and the received RSSI may be lower than Rth. If the received strength of the frame received by STA 1 (310) is lower than the roaming threshold, or due to other roaming start conditions, STA 1 (310) may transmit a scan request frame (330). For example, other roaming start conditions may include that the RSSI of the received frame decreases at a rate greater than a certain level. STA 1 (310) may transmit a discovery request frame (330) to AP 1 (301) based on the TBTT information of surrounding APs. Alternatively, STA 1 (310) may not know the TBTT information of surrounding APs because it did not receive the beacon frame (320) or the beacon frame (320) of AP 1 (301) does not include an RNR information element. Even if STA 1 (310) does not know the TBTT information of surrounding APs, it may transmit the discovery request frame (330) to AP 1 (301). The discovery request frame (330) may be a management frame including an action frame, a frame including an information element, a frame including specific information in a control field of a MAC header (e.g., an HT Control field), or other frames. The discovery request frame (330) may include channel discovery schedule information requested by STA 1 (310). Channel search schedule information includes at least one of TBTT information (or TBTT point in time) of a surrounding AP, a link ID of a surrounding AP, a BSSID of a surrounding AP, or an operating frequency information of an AP.The channel search schedule information may indicate that STA 1 (310) performs a search at the TBTT time of the corresponding AP by using the enumeration order of the link ID or BSSID of the surrounding AP. Alternatively, the search request frame (330) may include information for requesting a channel search schedule from AP 1 (301). That is, STA 1 (310) may not transmit the channel search schedule information to AP 1 (301). In this case, STA 1 (310) can receive the channel search schedule information from AP 1 (301) without checking the TBTT information of the surrounding APs, and can search for the surrounding APs.

[0061] AP 1 (301) may transmit a scan response frame (Scan request frame, 340) to STA 1 (310). The scan response frame (340) may be a response to the scan request frame (330) transmitted by STA 1 (310). Alternatively, the scan response frame (340) may be transmitted without receiving the scan request frame (330) even when AP 1 (301) determines that STA 1 (310) requires roaming. The scan response frame (340) may include a response to channel scan schedule information included in the scan request frame (330) transmitted by STA 1 (310). For example, the scan response frame (340) may include information indicating that AP 1 (301) confirms, changes, or rejects the channel scan schedule information transmitted by STA 1 (310). If rejection information is included, a code for the reason for rejection may be included in the search response frame (340). In addition, the search response frame (340) may include channel search schedule information that AP 1 (301) provides to STA 1 (310). If AP 1 (301) confirms the channel search schedule of STA 1 (310), the channel search schedule information may or may not include information transmitted by STA 1 (310). If AP 1 (301) changes the channel search schedule of STA 1 (310), the channel search schedule information may include channel search information proposed by AP 1 (301). If AP 1 (301) rejects the channel search schedule of STA 1 (310), the channel search schedule information may not include the channel search information proposed by AP 1 (301). Alternatively, the channel search schedule information may include the channel search information proposed by AP 1 (301).The channel search schedule that AP 1 (301) provides to STA 1 (310) may include at least one of TBTT information, link ID, BSSID, and operating frequency information of the APs of the surrounding APs including AP 2 (303) to AP 4 (305). The TBTT time points of the surrounding APs may be TBTT aa time point, TBTT bb time point, and TBTT cc time point, and the TBTT aa time point is the TBTT time point of AP 2 (303), the TBTT bb time point is the TBTT time point of AP 3 (305), and the TBTT cc time point is the TBTT time point of AP 4 (305). The TBTT information included in the search request frame (330) and the search response frame (340) may represent the TBTT information of the surrounding APs in units of TUs or / and the Offset of the TSF (Time Synchronization Function). 1 TU is 1024 us.

[0062] STA 1 (310) can receive a search response frame (340) from AP 1 (301), and STA 1 (310) performs channel search based on channel search schedule information provided by AP 1 (301). At TBTT aa, STA 1 (310) can search for AP 2 (303) operating at the same frequency as AP 1 (301). STA 1 (310) receives a beacon frame (351) of AP 2 (303) without changing the operating frequency. AP 1 (301) can terminate transmission of a frame to be transmitted to STA 1 (310) before TBTT aa, which is the TBTT of AP 2 (303). STA 1 (310) can terminate transmission of a frame to be transmitted to AP 1 (301) before TBTT aa. The termination of transmission by AP 1 (301) and STA 1 (310) before each TBTT may mean termination of transmission before the Ts time, which is the time for switching the link in each TBTT. Terminating transmission before a certain time may include completing a transmit opportunity (TXOP) before a certain time. In TBTT bb and TBTT cc, STA 1 (310) may search for AP 3 (305) and AP 4 (305) operating on different frequencies from AP 1 (301), and STA 1 (310) changes its operating frequency to receive beacon frames (353, 355) of AP 3 (305) and AP 4 (305). For example, STA 1 (310) changes its operating frequency to the second link and the third link. After STA 1 (310) completes the search, STA 1 (310) changes the operating frequency again and operates on the first link. The length of the time interval until STA 1 (310) operates by changing the operating frequency in each of TBTT bb and TBTT cc and then operates on the first link again is 2 Ts + (Beacon transmission interval of AP) may be longer than or equal to, and STA 1 (310) cannot receive frames from AP 1 (301) during the interval. Ts is the time required for STA 1 (310) to switch the operating frequency. When STA 1 (310) returns to the first link from the second link or the third link and operates, frame transmission may be impossible or limited during the time Tm, which is the MediumSyncDelay timer or NAVSyncDelay time. AP 1 (301) may terminate transmission of frames to STA 1 (310) before TBTT bb and TBTT cc, which are the times at which STA 1 (310) searches for AP 3 (305) and AP 4 (305). STA 1 (310) may terminate transmission of frames to AP 1 (301) before TBTT bb and TBTT cc. AP 1 (301) and STA 1 (310) terminating transmission before each TBTT may mean terminating transmission before the Ts time, which is the time for switching the link in each TBTT. Terminating transmission before a certain time may include completing a transmit opportunity (TXOP) before a certain time. If AP 1 (301) and STA 1 (310) have completed a channel access operation (e.g., an EDCA backoff operation) close to each TBTT (e.g., a backoff counter reaches 0), AP 1 (301) may not transmit a frame to STA 1 (310), and STA 1 (310) may not transmit a frame to AP 1 (301). That is, the backoff counter of AP 1 (301) and / or the backoff counter of STA 1 (310) may remain at 0. In contrast, even if the backoff counter of AP 1 (301) reaches 0, AP 1 (301) can select a new backoff counter and continue performing the backoff operation.Alternatively, AP 1 (301) may consider that there is no frame destined for STA 1 (310) in the transmission queue of AP 1 (301). If there is a frame destined for an STA other than STA 1 (310), AP 1 (301) may transmit the frame to an STA other than STA 1 (310). After the channel search operation of STA 1 (310) is completed, AP 1 (301) or / and STA 1 (310) may transmit the frame. AP 1 (301) may transmit 2 in TBTT bb and TBTT cc. During a time interval having a length greater than Ts + (Beacon transmission interval of AP), no frame is transmitted to STA 1 (310). That is, no frame is transmitted to STA 1 (310) while STA 1 (310) performs a series of operations for performing a channel search operation. While STA 1 (310) performs a series of operations for performing a channel search operation, STA 1 (310) cannot communicate with AP 1 (301). For example, AP 1 (301) delays a channel access operation for transmission to STA 1 (310), or waits (e.g., maintains the backoff counter at 0 or re-selects the backoff counter) without transmitting a frame to STA 1 (310) even when the EDCA backoff counter reaches 0. AP 1 (301) can transmit a frame to STA 1 (310) after the above interval. The frame that AP 1 (301) transmits to STA 1 (310) may be a trigger frame. For example, the trigger frame that AP 1 (301) transmits to STA 1 (310) may be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1 (310), which restricts frame transmission during the Tm time, and may trigger uplink data of STA 1 (310). Alternatively, the frame that AP 1 (301) transmits to STA 1 (310) may be a downlink frame.

[0063] STA 1 (310) can complete the search based on the search schedule information provided by AP 1 (301). For example, STA 1 (310) can collect information of AP 2 (303) to AP 4 (305). STA 1 (310) transmits a measurement report frame (360) to AP 1 (301). The measurement report frame (360) is a frame that transmits information (e.g., signal strength, SNR, link congestion), etc., of neighboring APs, AP 2 (303) to AP 4 (305). AP 1 (301) can respond to the measurement report frame (360) of STA 1 (310) with a response frame (e.g., ACK frame, BlockAck frame). AP 1 (301) can determine whether to roam and the roaming target based on the surrounding AP information of STA 1 (310).

[0064] In the above embodiment, instead of receiving beacon frames (351, 353, 355) of APs to obtain surrounding AP information, STA 1 (310) may transmit a probe request frame and receive a probe response frame.

[0065]

[0066] Figure 4 illustrates a second embodiment of a cooperative channel measurement method in a wireless LAN system.

[0067] Referring to FIG. 4, AP MLD 1 (401), AP MLD 2 (403), AP MLD 3 (405), and STA MLD 1 (410) can operate in a wireless LAN network. AP MLD 1 (401) has affiliated AP 1-1 (401-1) and AP 1-2 (401-2) operating. AP 1-1 (401-1) can operate in a first link, and AP 1-2 (401-2) can operate in a second link. AP MLD 2 (403) has affiliated AP 2-1 (403-1) and AP 2-2 (403-2) operating. AP 2-1 (403-1) can operate in a third link, and AP 2-2 (403-2) can operate in a fourth link. AP MLD 3 (405) operates with associated AP 3-1 (405-1) and AP 3-2 (405-2). AP 3-1 (405-1) may operate in the first link, and AP 3-2 (405-2) may operate in the fifth link. STA MLD 1 (410) operates with associated STA 1-1 (410-1) and STA 1-2 (410-2). STA MLD 1 (410) may be associated with AP MLD 1 (401) and may attempt to discover surrounding AP MLDs, namely AP MLD 2 (403) and AP MLD 3 (405). That is, STA MLD (410) may attempt to discover APs associated with surrounding AP MLDs. The STA MLD (410) can search for APs associated with a neighboring AP MLD to perform a connection to one of the neighboring APs. A series of operations in which the STA MLD (410) stops communication with the previously connected AP MLD, connects to one of the APs associated with the neighboring AP MLD, and communicates with the neighboring AP may be referred to as a roaming operation. The operating frequencies (e.g., center frequencies) of the first to fifth links may be different. AP MLD 1 (401) to AP MLD 3 (405) may be connected to each other and communicate.For example, AP MLD 1 (401) to AP MLD 3 (405) can communicate with wired Ethernet. Alternatively, AP MLD 1 (401) to AP MLD 3 (405) can communicate with wireless LAN. Alternatively, AP MLD 1 (401) to AP MLD 3 (405) can communicate with each other with a communication standard other than wired Ethernet and wireless LAN. Lower APs of AP MLD 1 (401) to AP MLD 3 (405) or APs below AP MLD 1 (401) to AP MLD 3 (405) can communicate with each other to form a new MLD layer. For example, AP MLD 1 (401) to AP MLD 3 (405) can form a new MLD MAC Upper layer or a new MLD MAC Lower layer. Alternatively, AP MLD 1 (401) to AP MLD 3 (405) may form both a new MLD MAC Upper Layer and a new MLD MAC Lower Layer. Alternatively, AP MLD 1 (401) to AP MLD 3 (405) may not form an MLD layer, but may exchange communication information with each other. For example, the exchange of communication information may include the exchange of operation information for roaming operations of STAs connected to APs (e.g., link use and use stop operation information), or the exchange of status information of STAs performing roaming operations (e.g., Sequence Number (SN), Packet Number (PN), BlockAck scoreboard information of downlink and uplink packets for a specific STA) and downlink packets destined for STAs performing roaming operations. As another example, the exchange of communication information may be the exchange of operation information of APs.

[0068] Since AP MLD 1 (401) to AP MLD 3 (405) communicate with each other, AP MLD 1 (401) can know information of surrounding AP MLDs and lower-level APs (e.g., AP MLD 2 (403), AP 2-1 (403-1), and AP 2-2 (403-2)). Information of surrounding AP MLDs includes TBTT (target beacon transmission time) information of lower-level APs. TBTT information may be point-in-time information. That is, TBTT information indicates the point-in-time or cycle at which beacon frames are expected to be transmitted by each AP. For example, AP MLD 1 (401) can know point-in-time information at which beacon frames (451, 453, 455, 457) of surrounding AP MLDs are transmitted from the current time.

[0069] AP 1-1 (401-1) of AP MLD 1 (401) can transmit TBTT information of surrounding APs by including it in a beacon frame (420). The TBTT information can be transmitted through an RNR (reduced neighbor report) information element included in the beacon frame (420). STA 1-1 (410-1) can receive the beacon frame (420) from AP 1-1 (401-1). Based on the RNR information element included in the beacon frame (420), STA 1-1 (410-1) can know TBTT information of surrounding APs (e.g., TBTT information of AP MLD 2 (403), AP 2-1 (403-1), and AP 2-2 (403-2)).

[0070] The received signal strength indication (RSSI), SNR, etc. of the beacon frame (420) or / and data frames of AP 1-1 (401-1) received by STA 1-1 (410-1) may be lower than the roaming threshold of STA 1-1 (410-1). For example, the roaming threshold may be Rth, and the received RSSI may be lower than Rth. When the received strength of the frame received by STA 1-1 (410-1) is lower than the roaming threshold, or due to other roaming start conditions, STA 1-1 (410-1) may transmit a search request frame (430). For example, another roaming start condition may be that the RSSI of the received frame decreases at a rate greater than a certain level. STA 1-1 (410-1) may transmit a discovery request frame (430) to AP 1-1 (401-1) based on the TBTT information of surrounding APs. Alternatively, STA 1-1 (410-1) may not know the TBTT information of surrounding APs because it did not receive the beacon frame (420) or the beacon frame (420) of AP 1-1 (401-1) does not include an RNR information element. STA 1-1 (410-1) may transmit a discovery request frame (430) to AP 1-1 (401-1) even when it does not know the TBTT information of surrounding APs. The discovery request frame (430) may be a management frame including an action frame, a frame including an information element, a frame including specific information in a control field of a MAC header (e.g., an HT Control field), or other frames. The search request frame (430) may include channel search schedule information requested by STA 1-1 (410-1). The channel search schedule information includes at least one of TBTT information (or TBTT time point) of a surrounding AP, a link ID of a surrounding AP, a BSSID of a surrounding AP, and operating frequency information of the AP.Scheduling information can be provided to search at the TBTT time of the AP in the enumeration order of the link ID or BSSID of the surrounding AP. Alternatively, the search request frame (430) can include information requesting a channel search schedule from AP 1-1 (401-1). That is, STA 1-1 (410-1) may not transmit channel search schedule information to AP 1-1 (401-1). In this case, STA 1-1 (410-1) can receive channel search schedule information from AP 1-1 (401-1) without checking the TBTT information of the surrounding APs, and can search for the surrounding APs.

[0071] AP 1-1 (401-1) may transmit a discovery response frame (440) to STA 1-1 (410-1). The discovery response frame (440) may be a response from AP 1-1 (401-1) to the discovery response frame transmitted by STA 1-1 (410-1). Alternatively, the discovery response frame (440) may be transmitted without receiving the discovery request frame (430) even when AP 1-1 (401-1) determines that STA 1-1 (410-1) or STA MLD 1 (410) requires roaming. The discovery response frame (440) may include a response to channel discovery schedule information included in the discovery request frame (430) transmitted by STA 1-1 (410-1). For example, the search response frame (440) may include information that AP 1-1 (401-1) confirms, changes, or rejects the channel search schedule information transmitted by STA 1-1 (410-1). In addition, the search response frame (440) may include channel search schedule information that AP 1-1 (401-1) provides to STA 1-1 (410-1). When AP 1-1 (401-1) confirms the channel search schedule of STA 1-1 (410-1), the channel search schedule information may or may not include the information transmitted by STA 1-1 (410-1) without change. When AP 1-1 (401-1) changes the channel search schedule of STA 1-1 (410-1), the channel search schedule information may include the channel search information proposed by AP 1-1 (401-1). If AP 1-1 (401-1) rejects the channel search schedule of STA 1-1 (410-1), the channel search schedule information may not include the channel search information proposed by AP 1-1 (401-1). Alternatively, the channel search schedule information may include the channel search information proposed by AP 1-1 (401-1).The channel search schedule provided by AP 1-1 (401-1) to STA 1-1 (410-1) includes TBTT information and link ID or / and operating frequency information of surrounding APs including APs under AP MLD 2 (403) to AP MLD 3 (405). The TBTT time points of surrounding APs may be TBTT aa time point, TBTT bb time point, TBTT cc, and TBTT dd time points. Here, TBTT aa time point is the TBTT time point of AP 2-1 (403-1), TBTT bb time point is the TBTT time point of AP 2-2 (403-2), TBTT cc time point is the TBTT time point of AP 3-1 (405-1), and TBTT dd time point is the TBTT time point of AP 3-2 (405-2). The TBTT information included in the discovery request frame (430) and the discovery response frame (440) can represent the TBTT information of the surrounding AP in TU units or / and the offset of the TSF (Time Synchronization Function). 1 TU is 1024 us.

[0072] The exchange of the above search request frame (430) and search response frame (440) can be performed through multiple links. For example, STA MLD 1 (410) can receive the beacon frame (420) of AP MLD 1 (401) from at least one of the first link and the second link and request the transmission (430) of the search request frame. The exchange of the search request frame (430) and the search response frame (440) can be performed for each link. For example, AP 1-1 (401-1) and STA 1-1 (410-1) operating on the first link can exchange the search request frame (430) and the search response frame (440) for searching for AP 2-1 (403-1) and AP 3-1 (405-1). AP 1-2 (401-2) and STA 1-2 (410-2) operating on the second link can exchange discovery request frames and discovery response frames to discover AP 2-2 (403-2) and AP 3-2 (405-2).

[0073] STA 1-1 (410-1) or STA MLD 1 (410) can receive a discovery response frame (440) from AP 1-1 (401-1) or AP MLD 1 (401), and STA MLD 1 (410) performs channel discovery based on channel discovery schedule information provided by AP MLD 1 (401). In TBTT aa, STA 1-1 (410-1) can discover AP 2-1 (403-1) operating at a different frequency from AP 1-1 (401-1). STA 1-1 (410-1) changes its operating frequency to receive a beacon frame (451) of AP 2-1 (403-1). For example, STA 1-1 (410-1) changes its operating link to a third link. After STA 1-1 (410-1) completes the search, STA 1-1 (410-1) changes the operating frequency again and operates on the first link. The length of the time interval during which STA 1-1 (410-1) changes the operating frequency at TBTT aa and then operates on the first link again is 2Ts + (Beacon transmission interval of AP) may be longer than or equal to, and STA 1-1 (410-1) cannot receive frames from AP 1-1 (401-1) during the interval. Ts is the time required for STA 1-1 (410-1) to switch the operating frequency. When STA 1-1 (410-1) returns to the first link from the third link and operates again, frame transmission may be impossible or restricted during the time Tm, which is the MediumSyncDelay timer or NAVSyncDelay time. AP 1-1 (401-1) may terminate transmission of frames to STA 1-1 (410-1) before TBTT aa, which is the time point at which STA 1-1 (410-1) searches for AP 2-1 (403-1). STA 1-1 (410-1) may terminate transmission of a frame to AP 1-1 (401-1) before TBTT aa. Terminating transmission by AP 1-1 (401-1) and STA 1-1 (410-1) before each TBTT may mean terminating transmission before Ts, which is the time for link switching in each TBTT. Terminating transmission before a specific time may include completing a transmit opportunity (TXOP) before a specific time. When AP 1-1 (401-1) and STA 1-1 (410-1) have completed channel access operations (e.g., EDCA backoff operations) close to their respective TBTTs (e.g., when the backoff counter reaches 0), AP 1-1 (401-1) may not transmit frames to STA 1-1 (410-1), and STA 1-1 (410-1) may not transmit frames to AP 1-1 (401-1). The backoff counter of AP 1-1 (401-1) and / or the backoff counter of STA 1-1 (410-1) may remain at 0. Alternatively, even when the backoff counter of AP 1-1 (401-1) reaches 0, AP 1-1 (401-1) may select a new backoff counter and continue performing the backoff operation.Alternatively, AP 1-1(401-1_ may consider that there is no frame destined for STA 1-1(410-1) in the transmission queue of AP 1. In this case, if there is a frame destined for an STA other than STA 1-1(410-1), the frame may be transmitted to an STA other than STA 1-1(410-1). After the channel search operation of STA 1-1(410-1) is finished, AP 1-1(401-1) or / and STA 1-1(410-1) may transmit frames. AP 1-1(401-1) may transmit frames at TBTT aa of 2. During a time period longer than Ts + (Beacon transmission period of AP), no frame is transmitted to STA 1-1 (410-1). For example, AP 1-1 (401-1) delays a channel access operation for transmission to STA 1-1 (410-1), or waits (e.g., keeps the backoff counter at 0 or re-selects the backoff counter) without transmitting a frame to STA 1-1 (410-1) even when the EDCA backoff counter reaches 0. AP 1-1 (401-1) can transmit a frame to STA 1-1 (410-1) after the above period. The frame that AP 1-1 (401-1) transmits to STA 1-1 (410-1) may be a trigger frame. For example, a trigger frame transmitted by AP 1-1 (401-1) to STA 1-1 (410-1) may be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1-1 (410-1), which restricts frame transmission during the Tm time, and may trigger uplink data of STA 1-1 (410-1). Alternatively, the frame transmitted by AP 1-1 (401-1) to STA 1-1 (410-1) may be a downlink frame. In TBTT cc, STA 1-1 (410-1) may search for AP 3-1 (405-1) operating on the same frequency as AP 1-1 (401-1). AP 1-1 (401-1) terminates transmission of frames to STA 1-1 (410-1) before TBTT cc, which is the TBTT of AP 3-1 (405-1). STA 1-1 (410-1) terminates transmission of frames to AP 1-1 (401-1) before TBTT cc.

[0074] In TBTT bb and TBTT dd, STA 1-2 (410-2) can search for AP 2-2 (403-2) and AP 3-2 (405-2) that operate on different frequencies from AP 1-2 (401-2). STA 1-2 (410-2) changes its operating frequency to receive beacon frames (453, 457) from AP 2-2 (403-2) and AP 3-2 (405-2). For example, STA 1-2 (410-2) changes its operating link to the fourth link and the fifth link. After STA 1-2 (410-2) completes the search, STA 1-2 (410-2) changes its operating frequency again to operate on the second link. The length of the time interval during which STA 1-2 (410-2) operates by changing the operating frequency at TBTT bb and TBTT dd and then operates again on the second link is 2 Ts + (Beacon transmission interval of AP) may be longer than or equal to, and STA 1-2 (410-2) cannot receive frames from AP 1-2 (401-2) during the above time interval. Ts is the time required for STA 1-2 (410-2) to switch the operating frequency. When STA 1-2 (410-2) returns to the second link and operates again from the fourth and fifth links, frame transmission may be impossible or restricted during the time Tm, which is the MediumSyncDelay timer or NAVSyncDelay time. AP 1-2 (401-2) may terminate transmission of frames to STA 1-2 (410-2) before TBTT bb and TBTT dd, which are the time points at which STA 1-2 (410-2) searches for AP 2-2 (403-2) and AP 3-2 (405-2). STA 1-2 (410-2) may terminate transmission of frames to AP 1-2 (401-2) before TBTT bb and TBTT dd. Terminating transmission by AP 1-2 (401-2) and STA 1-2 (410-2) before each TBTT may mean terminating transmission before Ts, which is the time for link switching in each TBTT. Terminating transmission before a certain point in time may include completing a transmit opportunity (TXOP) before a certain point in time. When AP 1-2 (401-2) and STA 1-2 (410-2) have completed channel access operations (e.g., EDCA backoff operations) close to their respective TBTTs (e.g., when the backoff counter reaches 0), AP 1-2 (401-2) may not transmit frames to STA 1-2 (410-2), and STA 1-2 (410-2) may not transmit frames to AP 1-2 (401-2). The backoff counter of AP 1-2 (401-2) and / or the backoff counter of STA 1-2 (410-2) may remain at 0.Alternatively, even if the backoff counter of AP 1-2 (401-2) reaches 0, AP 1-2 (401-2) may select a new backoff counter and continue the backoff operation. Alternatively, AP 1-1 (401-1) may consider that there is no frame destined for STA 1-2 (410-2) in the transmission queue of AP 1-2 (401-2). In this case, if there is a frame destined for an STA other than STA 1-2 (410-2), the frame may be transmitted to an STA other than STA 1-2 (410-2). After the channel search operation of STA 1-2 (410-2) is completed, AP 1-2 (401-2) or / and STA 1-2 (410-2) may transmit frames. AP 1-2 (401-2) is 2 in TBTT bb and TBTT dd. During a time period longer than Ts + (Beacon transmission period of AP), no frame is transmitted to STA 1-2 (410-2). For example, AP 1-1 (401-1) delays a channel access operation for transmission to STA 1-2 (410-2), or waits (e.g., keeps the backoff counter at 0, or re-selects the backoff counter) without transmitting a frame to STA 1-2 (410-2) even when the EDCA backoff counter reaches 0. AP 1-2 (401-2) can transmit a frame to STA 1-2 (410-2) after the above period. The frame that AP 1-2 (401-2) transmits to STA 1-2 (410-2) may be a trigger frame. For example, the trigger frame transmitted by AP 1-2 (401-2) to STA 1-2 (410-2) may be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1-2 (410-2), which restricts frame transmission during the Tm time, and may trigger uplink data of STA 1-2 (410-2). Alternatively, the frame transmitted by AP 1-2 (401-2) to STA 1-2 (410-2) may be a downlink frame.

[0075] STA MLD 1 (410) can complete the search based on the search schedule information provided by AP MLD 1 (401). Through the search, STA MLD 1 (410) can obtain information about APs. For example, STA MLD 1 (410) can collect information about AP MLD 2 (403) to AP MLD 3 (405) (e.g., information about subordinate APs of AP MLD 2 (403) to AP MLD 3 (405)). STA MLD 1 (410) transmits a measurement report frame (460) to AP MLD 1 (401). The measurement report frame (460) is a frame that transmits information (e.g., signal strength, SNR, link congestion of APs operating under AP MLD 2 (403) to AP MLD 3 (405)) of the surrounding AP MLDs, AP MLD 2 (403) to AP MLD 3 (405). AP MLD 1 (401) can respond to the measurement report frame (460) of STA MLD 1 (410) with a response frame (e.g., ACK frame, BlockAck frame). AP MLD 1 (401) can determine whether to roam and the roaming target, etc., based on the surrounding AP MLD information of STA MLD 1 (410).

[0076] In the above embodiment, instead of receiving beacon frames (451, 453, 455, 457) of AP MLDs to obtain surrounding AP MLD information, STA MLD 1 (410) may transmit a probe request frame and receive a probe response frame.

[0077]

[0078] Figure 5 illustrates a third embodiment of a cooperative channel measurement method in a wireless LAN system.

[0079] Referring to FIG. 5, AP 1 (501), AP 2 (503), AP 3 (505), and STA 1 (510) may operate in a wireless LAN network. STA 1 (510) may be associated with AP 1 (501) and may want to search for surrounding APs, such as AP 2 (503) and AP 3 (505). STA 1 (510) may search for surrounding APs to perform connection to one of the surrounding APs. A series of operations in which STA 1 (510) stops communication with an existing connected AP, connects to one of the surrounding APs, and communicates with the surrounding AP may be referred to as a roaming operation. AP 1 (501) may operate in a first link. AP 2 (503) may operate in a second link. AP 3 (505) may operate in a third link. The operating frequencies (e.g., center frequencies) of the first link and the second link may be different. The operating frequencies of the first link and the third link may be different. AP 1 (501) to AP 3 (505) may be connected to each other and communicate. For example, AP 1 (501) to AP 3 (505) may communicate with wired Ethernet. Alternatively, AP 1 (501) to AP 3 (505) may communicate with wireless LAN. Alternatively, AP 1 (501) to AP 3 (505) may communicate with each other using a communication standard other than wired Ethernet and wireless LAN. AP 1 (501) to AP 3 (505) may communicate with each other to form an MLD or a part of a layer of an MLD. For example, AP 1 (501) to AP 3 (505) may form MLD MAC upper layers. Alternatively, AP 1 (501) to AP 3 (505) may form MLD MAC Lower layers. Alternatively, AP 1 (501) to AP 3 (505) may form both MLD MAC Upper Layer and MLD MAC Lower Layer.Alternatively, AP 1 (501) to AP 3 (505) may not form an MLD, but may exchange communication information with each other. For example, the exchange of communication information may include exchanging operation information (e.g., link use and stop operation information) for roaming operations of STAs connected to the APs, or exchanging status information of STAs performing roaming operations (e.g., Sequence Number (SN), Packet Number (PN), BlockAck scoreboard information of downlink and uplink packets for a specific STA) and downlink packets destined for the STA performing the roaming operation. As another example, the exchange of communication information may be exchanging operation information of APs.

[0080] Since AP 1 (501) to AP 3 (505) communicate with each other, AP 1 (501) can know information about surrounding APs (e.g., AP 2 (503) to AP 3 (505)). Information about surrounding APs includes TBTT (target beacon transmission time) information of the APs. TBTT information may be point-in-time information. That is, TBTT indicates the expected point-in-time or cycle of beacon frame transmission by each AP. For example, AP 1 (501) can know the point-in-time information of when beacon frames (551, 553) of surrounding APs are transmitted from the current time.

[0081] AP 1 (501) can transmit TBTT information of surrounding APs by including it in a beacon frame (520). The TBTT information can be transmitted through an RNR (reduced neighbor report) information element included in the beacon frame (520). STA 1 (510) can receive the beacon frame (520) from AP 1 (501). Based on the RNR information element included in the beacon frame (520), STA 1 (510) can know the TBTT information of surrounding APs. STA 1 (510) can check the TBTT information of surrounding APs in various ways as well as the RNR information element. For example, AP 1 can transmit a beacon frame including an information element such as a multi-link element, and the multi-link element can include operation information including the TBTT information of surrounding APs.

[0082] The received signal strength indication (Received Signal Strength Indication), SNR, etc. of the beacon frame (520) or / and data frames of AP 1 (501) received by STA 1 (510) may be lower than the roaming threshold of STA 1 (510). For example, the roaming threshold may be Rth, and the received RSSI may be lower than Rth. STA 1 (510) may transmit a discovery request frame (530) when the received strength of the frame received by STA 1 (510) is lower than the roaming threshold or based on other roaming start conditions. For example, other roaming start conditions may include that the RSSI of the received frame decreases at a rate greater than a certain level. STA 1 (510) may transmit the discovery request frame (530) to AP 1 (501) based on TBTT information of surrounding APs. In contrast, STA 1 (510) may not know the TBTT information of surrounding APs because it did not receive the beacon frame or the beacon frame (520) of AP 1 (501) does not include the RNR information element. Even if STA 1 (510) does not know the TBTT information of surrounding APs, it may transmit a discovery request frame (530) to AP 1 (501). The discovery request frame (530) may be a management frame including an action frame, a frame including an information element, a frame including specific information in a control field (e.g., HT Control field) of a MAC header, or other frames. The discovery request frame (530) may include channel discovery schedule information requested by STA 1 (510). The channel discovery schedule information includes at least one of TBTT information or TBTT time point of the surrounding AP, link ID of the surrounding AP, BSSID of the surrounding AP, or operating frequency information of the AP. STA 1 (510) may inform scheduling information indicating that STA 1 (510) is scheduled to perform discovery at the TBTT time of the AP based on the enumeration order of the link ID or BSSID of the surrounding AP.Alternatively, the search request frame (530) may include information requesting a channel search schedule from AP 1 (501). That is, STA 1 (510) may not transmit channel search schedule information to AP 1 (501). In this case, STA 1 (510) can receive channel search schedule information from AP 1 (501) without checking TBTT information of surrounding APs, and can search surrounding APs.

[0083] AP 1 (501) may transmit a discovery response frame (540) to STA 1 (510). The discovery response frame (540) may be a response to the discovery response frame (540) transmitted by STA 1 (510). Alternatively, the discovery response frame (540) may be transmitted without receiving the discovery request frame (530) even when AP 1 (501) determines that STA 1 (510) requires roaming. The discovery response frame (540) may include a response to channel discovery schedule information included in the discovery request frame (530) transmitted by STA 1 (510). For example, the discovery response frame (540) may include information for AP 1 (501) to confirm, change, or reject the channel discovery schedule information transmitted by STA 1 (510). In addition, the search response frame (540) may include channel search schedule information that AP 1 (501) provides to STA 1 (510). When AP 1 (501) confirms the channel search schedule of STA 1 (510), the channel search schedule information may include or may not include the information transmitted by STA 1 (510) without change. When AP 1 (501) changes the channel search schedule of STA 1 (510), the channel search schedule information may include channel search information proposed by AP 1 (501). When AP 1 (501) rejects the channel search schedule of STA 1 (510), the channel search schedule information may not be included in the search response frame (540). Conversely, even if AP 1 (501) rejects the channel search schedule of STA 1 (510), the search response frame (540) may include the channel search information proposed by AP 1 (501). The channel search schedule provided by AP 1 (501) to STA 1 (510) includes TBTT information and link ID or / and operating frequency information of surrounding APs including AP 2 (503) to AP 3 (505).The TBTT time point of the surrounding AP may be the TBTT aa time point or the TBTT bb time point, and the TBTT aa time point may be the TBTT time point of AP 2 (503), and the TBTT bb time point may be the TBTT time point of AP 3 (505). The TBTT information included in the discovery request frame (530) and the discovery response frame (540) may represent the TBTT information of the surrounding AP in units of TUs or / and the Offset of the TSF (Time Synchronization Function). 1 TU is 1024 us.

[0084] STA 1 (510) can receive a search response frame (540) from AP 1 (501), and STA 1 (510) performs channel search based on channel search schedule information provided by AP 1 (501). In TBTT aa and TBTT bb, STA 1 (510) can search for AP 2 (503) and AP 3 (505) that operate at different frequencies from AP 1 (501), and STA 1 (510) changes its operating frequency to receive beacon frames (551, 553) of AP 2 (503) and AP 3 (505). For example, STA 1 (510) changes its operating frequency to the second link and the third link. Before STA 1 (510) changes the operating frequency, STA 1 (510) transmits a search start frame (561, 563) to AP 1 (501). After STA 1 (510) completes the search, STA 1 (510) changes the operating frequency again and operates on the first link. When STA 1 (510) operates on the first link again, STA 1 (510) transmits a search complete frame (571, 573) to AP 1 (501). The length of the time interval during which STA 1 (510) operates on the first link again after changing the operating frequency in each of TBTT aa and TBTT bb is 2 Ts + (Beacon transmission interval of AP) may be longer than or equal to, and STA 1 (510) cannot receive frames of AP 1 (501) in the interval. Ts is the time required for STA 1 (510) to switch the operating frequency. When STA 1 (510) returns to the first link from the second link or the third link and operates again, frame transmission may be impossible or limited during the MediumSyncDelay timer or NAVSyncDelay time Tm. Accordingly, the search completion frame (571, 573) of STA 1 (510) may not be transmitted immediately. Accordingly, STA 1 (510) may not transmit the search completion frame (571, 573), transmit it after the Tm time, or transmit an RTS frame to AP 1 (501) within the Tm time and transmit it if a response is normally received with a CTS frame. AP 1 (501) may terminate transmission of a frame to STA 1 (510) before TBTT aa and TBTT bb, which are the time points at which STA 1 (510) discovers AP 2 (503) and AP 3 (505). STA 1 (510) may terminate transmission of a frame to AP 1 (501) before TBTT aa and TBTT bb. Terminating transmission by AP 1 (501) and STA 1 (510) before each TBTT may mean terminating transmission before Ts, which is the time for switching the link in each TBTT. Terminating transmission before a certain time point may include completing a transmit opportunity (TXOP) before a certain time point. When AP 1 (501) and STA 1 (510) have completed a channel access operation (e.g., an EDCA backoff operation) close to each TBTT (e.g., a backoff counter reaches 0), AP 1 (501) may not transmit a frame to STA 1 (510), and STA 1 (510) may not transmit a frame to AP 1 (501).The backoff counter of AP 1 (501) and / or the backoff counter of STA 1 (510) may be maintained at 0. Alternatively, even if the backoff counter of AP 1 (501) reaches 0, AP 1 (501) may select a new backoff counter and continue to perform the backoff operation. Alternatively, AP 1 (501) may consider that there is no frame destined for STA 1 (510) in the transmission queue of AP 1 (501). In this case, if there is a frame destined for an STA other than STA 1 (510), the frame may be transmitted to an STA other than STA 1 (510). After the channel search operation of STA 1 (510) is completed, AP 1 (501) and / or STA 1 (510) may transmit a frame. AP 1 (501) receives a search start frame (561, 563) from STA 1 (510) or 2 in TBTT aa and TBTT bb. During a period longer than Ts + (Beacon transmission period of AP), no frame is transmitted to STA 1 (510). For example, AP 1 (501) delays a channel access operation for transmission to STA 1 (510), or waits (e.g., maintains the backoff counter at 0 or re-selects the backoff counter) without transmitting a frame to STA 1 (510) even when the EDCA backoff counter reaches 0. AP 1 (501) can transmit a frame to STA 1 (510) after the above period. The frame that AP 1 (501) transmits to STA 1 (510) may be a trigger frame. For example, the trigger frame that AP 1 (501) transmits to STA 1 (510) may be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1 (510), which restricts frame transmission for the Tm time period, and may trigger the uplink data or search completion frame of STA 1 (510). Alternatively, the frame that AP 1 (501) transmits to STA 1 (510) may be a downlink frame. In a method other than the method in which AP 1 (501) transmits a frame to STA 1 (510) after the above-mentioned section, AP 1 (501) may transmit a frame to STA 1 (510) after receiving a search completion frame from STA 1 (510). While STA 1 (510) performs a series of operations to perform a channel search operation (e.g., during the period in which STA 1 transmits a search start frame to AP 1 and STA 1 completes transmitting a search end frame to AP 1), STA 1 (510) cannot communicate with AP 1 (501).

[0085] STA 1 (510) can complete the search based on the search schedule information provided by AP 1 (501). For example, STA 1 (510) can collect information of AP 2 (503) to AP 3 (505). STA 1 (510) transmits a measurement report frame (580) to AP 1 (501). The measurement report frame (580) is a frame that transmits information (e.g., signal strength, SNR, link congestion), etc., of neighboring APs, AP 2 (503) to AP 3 (505). AP 1 (501) can respond to the measurement report frame (580) of STA 1 (510) with a response frame (e.g., ACK frame, BlockAck frame). AP 1 (501) can determine whether to roam and the roaming target, etc., based on the neighboring AP information of STA 1 (510).

[0086] In the above embodiment, instead of receiving beacon frames (551, 553) of APs to obtain surrounding AP information, STA 1 (510) may transmit a probe request frame and receive a probe response frame.

[0087]

[0088] Fig. 6 illustrates a fourth embodiment of a cooperative channel measurement method in a wireless LAN system.

[0089] Referring to FIG. 6, AP 1 (601), AP 2 (603), AP 3 (605), and STA 1 (610) may operate in a wireless LAN network. STA 1 (610) may be associated with AP 1 (601) and may attempt to discover surrounding APs, AP 2 (603) and AP 3 (605). AP 1 (601) may operate in a first link. AP 2 (603) may operate in a second link. AP 3 (605) may operate in a third link. STA 1 (610) may discover surrounding APs to establish a connection to one of the surrounding APs. A series of operations in which STA 1 (610) discontinues communication with an existing connected AP, connects to one of the surrounding APs, and communicates with the surrounding AP may be referred to as a roaming operation. The operating frequencies (e.g., center frequencies) of the first link and the second link may be different. The operating frequencies of the first link and the third link may be different. AP 1 (601) to AP 3 (605) may be connected to each other and communicate. For example, AP 1 (601) to AP 3 (605) may communicate with wired Ethernet. Alternatively, AP 1 (601) to AP 3 (605) may communicate with wireless LAN. Alternatively, AP 1 (601) to AP 3 (605) may communicate with each other using a communication standard other than wired Ethernet and wireless LAN. AP 1 (601) to AP 3 (605) may communicate with each other to form an MLD or a part of a layer of an MLD. For example, AP 1 (601) to AP 3 (605) may form MLD MAC upper layers. Alternatively, AP 1 (601) to AP 3 (605) may form MLD MAC Lower layers. Alternatively, AP 1 (601) to AP 3 (605) may form both MLD MAC Upper Layer and MLD MAC Lower Layer.Alternatively, AP 1 (601) to AP 3 (605) may not form an MLD, but may exchange communication information with each other.

[0090] For example, the exchange of communication information may include exchanging operation information (e.g., link use and use stop operation information) for roaming operations of STAs connected to APs, or exchanging status information of STAs performing roaming operations (e.g., Sequence Number (SN), Packet Number (PN), BlockAck scoreboard information of downlink and uplink packets for a specific STA) and downlink packets destined for STAs performing roaming operations. As another example, the exchange of communication information may be exchanging operation information of APs.

[0091]

[0092] Since AP 1 (601) to AP 3 (605) communicate with each other, AP 1 (601) can know information about surrounding APs (e.g., AP 2 (603) and AP 3 (605)). Information about surrounding APs includes TBTT (target beacon transmission time) information of the APs. TBTT information may be point-in-time information. That is, TBTT information indicates the expected point-in-time or cycle of beacon frame transmission by each AP. For example, AP 1 (601) can know the point-in-time information of when beacon frames of surrounding APs are transmitted from the current time.

[0093] AP 1 (601) can transmit TBTT information of surrounding APs by including it in a beacon frame (620). The TBTT information can be transmitted through an RNR (reduced neighbor report) information element included in the beacon frame (620). STA 1 (610) can receive the beacon frame (620) from AP 1 (601). Based on the RNR information element included in the beacon frame (620), STA 1 (610) can know the TBTT information of surrounding APs. STA 1 (610) can check the TBTT information of surrounding APs in various ways as well as the RNR information element. For example, AP 1 (601) can transmit a beacon frame including an information element such as a multi-link element, and the multi-link element can include operation information including the TBTT information of surrounding APs.

[0094] The received signal strength indication (SNR), SNR, etc. of the beacon frame (620) or / and data frames of AP 1 (601) received by STA 1 (610) may be lower than the roaming threshold of STA 1 (610). For example, the roaming threshold may be Rth, and the received RSSI may be lower than Rth. If the received strength of the frame received by STA 1 (610) is lower than the roaming threshold, or due to other roaming start conditions, STA 1 (610) may transmit a search request frame (630). For example, other roaming start conditions may include that the RSSI of the received frame decreases at a rate greater than a certain level. The discovery request frame (630) that STA 1 (610) transmits to AP 1 (601) may include a list of surrounding APs that STA 1 (610) obtains from the beacon frame (620) of AP 1 (601). Alternatively, STA 1 (610) may not know surrounding AP information, and the discovery request frame (630) that STA 1 (610) transmits to AP 1 (601) may not include surrounding AP information.

[0095] AP 1 (601) performs coordination for discovery with surrounding APs AP 2 (603) and AP 3 (605) based on the discovery request frame (630) received from STA 1 (610). Based on the cooperation for discovery, a TTT (Time to Trigger) for discovery of AP 2 (603) and AP 3 (605) is set. The TTT for discovery of AP 2 (603) is TTT aa, and the TTT for discovery of AP 3 (605) is TTT bb. AP 1 (601) can include the set TTT information in a discovery response frame (640) and transmit it to STA 1 (610). The discovery response frame (640) further includes link and / or operating frequency information of surrounding APs AP 2 (603) and AP 3 (605). The discovery response frame (640) may be a response to the discovery response frame (640) transmitted by STA 1 (610). The discovery response frame (640) may be transmitted without receiving the discovery request frame (630) even when STA 1 (610) determines that roaming is necessary. The TTT information of the surrounding AP (e.g., TTT aa time point and TTT bb time point) may be expressed as an offset of TU units or / and TSF (Time Synchronization Function) from the time point at which the discovery response frame (640) starts transmitting or completes transmitting. 1 TU is 1024 us.

[0096] STA 1 (610) can receive a search response frame (640) from AP 1 (601), and STA 1 (610) performs channel search based on channel search schedule information provided by AP 1 (601). In TTT aa and TTT bb, STA 1 (610) can search for AP 2 (603) and AP 3 (605) that operate at different frequencies from AP 1 (601), and STA 1 (610) changes its operating frequency to receive frames from AP 2 (603) and AP 3 (605). For example, STA 1 (610) changes its operating frequency to the second link and the third link. AP 2 (603) and AP 3 (605) transmit NDP (Null data PPDU) frames (651, 653) at the configured TTTs, TTT aa and TTT bb. Since AP 1 (601) to AP 3 (605) have negotiated the TTT with each other, AP 2 (603) and AP 3 (605) do not transmit NDPA (NDP announcement) frames before transmitting the NDP frames (651, 653). Conversely, even if AP 1 (601) to AP 3 (605) have negotiated the TTT with each other, AP 2 (603) and AP 3 (605) may transmit NDPA frames before transmitting the NDP frames (651, 653). Instead of transmitting NDP frames (651, 653) in TTT aa and TTT bb, AP 2 (603) and AP 3 (605) can transmit probe response frames. STA 1 (610) can receive frames (651, 653) from AP 2 (603) and AP 3 (605) and obtain information about AP 2 (603) and AP 3 (605).

[0097] After STA 1 (610) completes the search, STA 1 (610) changes the operating frequency again and operates on the first link. When STA 1 (610) operates on the first link again, STA 1 (610) transmits a search completion frame to AP 1 (601). The length of the time interval during which STA 1 (610) operates by changing the operating frequency in each of TTT aa and TTT bb and then operates on the first link again is 2 Ts + (NDP transmission interval of AP) may be longer than or equal to, and STA 1 (610) cannot receive frames from AP 1 (601) in the interval. Ts is the time it takes for STA 1 (610) to switch the operating frequency. When STA 1 (610) returns to the first link and operates again from the second link or the third link, frame transmission may be impossible or limited during the time Tm, which is the MediumSyncDelay timer or NAVSyncDelay time. Therefore, the discovery completion frame of STA 1 (610) may not be transmitted immediately. AP 1 (601) may terminate transmission of the frame transmitted to STA 1 (610) before TTT aa and TTT bb, which are the time points at which STA 1 (610) discovers AP 2 (603) and AP 3 (605). STA 1 (610) may terminate transmission of a frame to AP 1 (601) before TTT aa and TTT bb. Terminating transmission of AP 1 (601) and STA 1 (610) before each TTT may mean terminating transmission before time Ts, which is a time for switching a link in each TTT. Terminating transmission before a specific time point may include finishing a transmit opportunity (TXOP) before a specific time point. When AP 1 (601) and STA 1 (610) have completed a channel access operation (e.g., an EDCA backoff operation) close to each TTT (e.g., when a backoff counter reaches 0), AP 1 (601) may not transmit a frame to STA 1 (610), and STA 1 (610) may not transmit a frame to AP 1 (601). The backoff counter of AP 1 (601) and / or the backoff counter of STA 1 (610) may remain at 0. Alternatively, even if the backoff counter of AP 1 (601) reaches 0, AP 1 (601) may select a new backoff counter and continue performing the backoff operation.Alternatively, AP 1 (601) may consider that there is no frame destined for STA 1 (610) in the transmission queue of AP 1 (601), and if there is a frame destined for an STA other than STA 1 (610), the frame may be transmitted to an STA other than STA 1 (610). After the channel search operation of STA 1 (610) is completed, AP 1 (601) or / and STA 1 (610) may transmit frames. AP 1 (601) may transmit frames at TTT aa and TTT bb times 2. During a period longer than Ts + (the NDP transmission period of the AP), no frame is transmitted to STA 1 (610). For example, AP 1 (601) delays a channel access operation for transmission to STA 1 (610), or waits (e.g., maintains the backoff counter at 0 or re-selects the backoff counter) without transmitting a frame to STA 1 (610) even when the EDCA backoff counter reaches 0. AP 1 (601) can transmit a frame to STA 1 (610) after the above period. The frame that AP 1 (601) transmits to STA 1 (610) may be a trigger frame. For example, the trigger frame that AP 1 (601) transmits to STA 1 (610) may be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1 (610), which restricts frame transmission for the Tm time period, and may trigger the uplink data or search completion frame of STA 1 (610). Alternatively, the frame that AP 1 (601) transmits to STA 1 (610) may be a downlink frame. In a method other than the method in which AP 1 (601) transmits a frame to STA 1 (610) after the above-mentioned section, AP 1 (601) may transmit a frame to STA 1 (610) after receiving a search completion frame from STA 1 (610). While STA 1 (610) performs a series of operations for channel search (while STA 1 (610) switches channels to AP 2 (603) and AP 3 (605) to receive NDP frames), STA 1 (610) cannot communicate with AP 1 (601).

[0098] STA 1 (610) can complete the search based on the search schedule information provided by AP 1 (601). For example, STA 1 (610) can collect information of AP 2 (603) to AP 3 (605). STA 1 (610) transmits a measurement report frame (660) to AP 1 (601). The measurement report frame (660) is a frame that transmits information (e.g., signal strength, SNR, link congestion), etc., of neighboring APs, AP 2 (603) to AP 3 (605). AP 1 (601) can respond to the measurement report frame (660) of STA 1 (610) with a response frame (e.g., ACK frame, BlockAck frame). AP 1 (601) can determine whether to roam and the roaming target, etc., based on the neighboring AP information of STA 1 (610).

[0099]

[0100] Fig. 7 illustrates a fifth embodiment of a cooperative channel measurement method in a wireless LAN system.

[0101] Referring to FIG. 7, AP MLD 1 (701), AP MLD 2 (703), AP MLD 3 (705), and STA MLD 1 (710) can operate in a wireless LAN network. AP 1-1 (701-1) and AP 1-2 (701-2) affiliated to AP MLD 1 (701) operate. AP 1-1 (701-1) can operate in a first link, and AP 1-2 (701-2) can operate in a second link. AP 2-1 (703-1) and AP 2-2 (703-2) affiliated to AP MLD 2 (703) operate. AP 2-1 (703-1) can operate in a third link, and AP 2-2 (703-2) can operate in a fourth link. AP 3-1 (705-1) and AP 3-2 (705-2) associated with AP MLD 3 (705) operate. AP 3-1 (705-1) may operate in the first link, and AP 3-2 (705-2) may operate in the fifth link. STA 1-1 (710-1) and STA 1-2 (710-2) associated with STA MLD 1 (710) operate. STA MLD 1 (710) may be an enhanced multi-link single radio (EMLSR) MLD. That is, STA MLD 1 (710) performs EMLSR operation, and STA 1-1 (710-1) and STA 1-2 (710-2) under STA MLD 1 (710) may perform EMLSR operation. Since STA MLD 1 (710) is an EMLSR MLD, while STA 1-1 (710-1) transmits and receives frames, STA 1-2 (710-2) cannot receive frames and cannot perform channel sensing. While STA 1-2 (710-2) transmits and receives frames, STA 1-1 (710-1) cannot receive frames and cannot perform channel sensing.STA MLD 1 (710) may be connected (associated) to AP MLD 1 (701) and may want to search for surrounding AP MLDs, namely AP MLD 2 (703) and AP MLD 3 (705). That is, STA MLD may want to search for APs associated with surrounding AP MLDs. STA MLD (710) may search for APs associated with surrounding AP MLDs in order to connect to one of the surrounding APs. A series of operations in which STA MLD (710) stops communication with an existing connected AP MLD, connects to one of the APs associated with the surrounding AP MLD, and communicates with the surrounding AP may be referred to as a roaming operation. The operating frequencies (e.g., center frequencies) of the first to fifth links may be different. AP MLD 1 (701) to AP MLD 3 (705) may be connected to each other and communicate with each other. For example, AP MLD 1 (701) to AP MLD 3 (705) can communicate with wired Ethernet. Alternatively, AP MLD 1 (701) to AP MLD 3 (705) can communicate with wireless LAN. Alternatively, AP MLD 1 (701) to AP MLD 3 (705) can communicate with each other with a communication standard other than wired Ethernet and wireless LAN. Lower APs of AP MLD 1 (701) to AP MLD 3 (705) or APs below AP MLD 1 (701) to AP MLD 3 (705) can communicate with each other to form a new MLD layer. For example, AP MLD 1 (701) to AP MLD 3 (705) can form a new MLD MAC Upper layer or a new MLD MAC Lower layer. Alternatively, AP MLD 1 (701) to AP MLD 3 (705) may form both a new MLD MAC Upper Layer and a new MLD MAC Lower Layer.Alternatively, AP MLD 1 (701) to AP MLD 3 (705) may not form an MLD layer, but may exchange communication information with each other. For example, the exchange of communication information may include exchanging operation information (e.g., link use and use stop operation information) for roaming operations of STA MLDs connected to AP MLDs, or exchanging status information (e.g., Sequence Number (SN), Packet Number (PN), BlockAck scoreboard information of downlink and uplink packets for a specific STA MLD) and downlink packets destined for the STA MLD performing the roaming operation. As another example, the exchange of communication information may be exchanging operation information of AP MLDs.

[0102] Since AP MLD 1 (701) to AP MLD 3 (705) communicate with each other, AP MLD 1 (701) can know the information of the surrounding AP MLDs and lower APs (e.g., information of AP MLD 2 (703) and information of AP 2-1 (703-1) and AP 2-2 (703-2) under AP MLD 2 (703)). The information of the surrounding AP MLDs includes TBTT (target beacon transmission time) information of the lower APs. The TBTT information may be point-in-time information. That is, the TBTT information indicates the point-in-time or cycle at which beacon frames are expected to be transmitted by each AP or AP MLD. For example, AP MLD 1 (701) can know the point-in-time information at which the beacon frames (751, 753, 755, 757) of the surrounding AP MLDs are transmitted from the current time.

[0103] AP 1-1 (701-1) of AP MLD 1 (701) can transmit TBTT information of surrounding APs by including it in a beacon frame (720). The TBTT information can be transmitted through an RNR (reduced neighbor report) information element included in the beacon frame (720). STA 1-1 (710-1) can receive the beacon frame (720) from AP 1. Based on the RNR information element included in the beacon frame (720), STA 1-1 (710-1) can know TBTT information of surrounding APs (e.g., TBTT information of APs below AP MLD 2 (703) to AP MLD 3 (705)).

[0104] The received signal strength indication (RSSI), SNR, etc. of the beacon frame (720) or / and data frames of AP 1-1 (701-1) received by STA 1-1 (710-1) may be lower than the roaming threshold of STA 1-1 (710-1). For example, the roaming threshold may be Rth, and the received RSSI may be lower than Rth. When the received strength of the frame received by STA 1-1 (710-1) is lower than the roaming threshold, or due to other roaming start conditions, STA 1-1 (710-1) may transmit a search request frame (730). For example, other roaming start conditions may include that the RSSI of the received frame decreases at a rate greater than a certain level. STA 1-1 (710-1) may transmit a discovery request frame (730) to AP 1-1 (701-1) based on the TBTT information of surrounding APs. Alternatively, STA 1-1 (710-1) may not know the TBTT information of surrounding APs because it did not receive the beacon frame (720) or the beacon frame (720) of AP 1-1 (701-1) does not include an RNR information element. STA 1-1 (710-1) may transmit a discovery request frame (730) to AP 1-1 (701-1) even when it does not know the TBTT information of surrounding APs. The discovery request frame (730) may be a management frame including an action frame, a frame including an information element, a frame including specific information in a control field of a MAC header (e.g., an HT Control field), or other frames. The search request frame (730) may include channel search schedule information requested by STA 1-1 (710-1). The channel search schedule information includes TBTT information (or TBTT time point) of a surrounding AP, a link ID of the surrounding AP, a BSSID of the surrounding AP, and operating frequency information of the AP.The channel search schedule information can indicate scheduling information that the STA performs a search at the TBTT time of the AP by using the enumeration order of the link ID or BSSID of the surrounding AP. Alternatively, the search request frame (730) can include information requesting a channel search schedule from AP 1-1 (701-1). That is, STA 1-1 (710-1) may not transmit the channel search schedule information to AP 1-1 (701-1). In this case, STA 1-1 (710-1) can receive the channel search schedule information from AP 1-1 (701-1) without checking the TBTT information of the surrounding APs, and can search for the surrounding APs.

[0105] AP 1-1 (701-1) may transmit a discovery response frame (740) to STA 1-1 (710-1). The discovery response frame (740) may be a response from AP 1-1 (701-1) to the discovery request frame (730) transmitted by STA 1-1 (710-1). Alternatively, the discovery response frame (740) may be transmitted without receiving the discovery request frame even when AP 1-1 (701-1) determines that STA 1-1 (710-1) or STA MLD 1 (710) requires roaming. The discovery response frame (740) may include a response to channel discovery schedule information included in the discovery request frame (730) transmitted by STA 1-1 (710-1). For example, the search response frame (740) may include information that AP 1-1 (701-1) confirms, changes, or rejects the channel search schedule information transmitted by STA 1-1 (710-1). In addition, the search response frame (740) may include channel search schedule information that AP 1-1 (701-1) provides to STA 1-1 (710-1). When AP 1-1 (701-1) confirms the channel search schedule of STA 1-1 (710-1), the channel search schedule information may include the information transmitted by STA 1-1 (710-1) without change, or may not include the information transmitted by STA 1-1 (710-1). When AP 1-1 (701-1) changes the channel search schedule of STA 1-1 (710-1), the channel search schedule information may include channel search information proposed by AP 1-1 (701-1). When AP 1-1 (701-1) rejects the channel search schedule of STA 1-1 (710-1), the search response frame (740) may not include channel search schedule information. Alternatively, even if AP 1-1 (701-1) rejects the channel search schedule of STA 1-1 (710-1), the search response frame (740) may include the channel search information proposed by AP 1.The channel search schedule provided by AP 1-1 (701-1) to STA 1-1 (710-1) includes TBTT information and link ID or / and operating frequency information of surrounding APs including APs under AP MLD 2 (703) to AP MLD 3 (705). The TBTT time points of surrounding APs may be TBTT aa time point, TBTT bb time point, TBTT cc, and TBTT dd time point, and the TBTT aa time point is the TBTT time point of AP 2-1 (703-1), the TBTT bb time point is the TBTT time point of AP 2-2 (703-2), the TBTT cc time point is the TBTT time point of AP 3-1 (705-1), and the TBTT dd time point is the TBTT time point of AP 3-2 (705-2). The TBTT information included in the discovery request frame (730) and the discovery response frame (740) can represent the TBTT information of the surrounding AP in TU units or / and the offset of the TSF (Time Synchronization Function). 1 TU is 1024 us.

[0106] The exchange of the above-described search request frame (730) and search response frame (740) can be performed through multiple links. For example, STA MLD 1 (710) can receive the beacon frame (720) of AP MLD 1 (701) through at least one of the first link and the second link, and request transmission of the search response frame (740). The exchange of the search request frame (730) and the search response frame (740) can be performed for each link. For example, AP 1-1 (701-1) and STA 1-1 (710-1) operating on the first link can exchange the search request frame (730) and the search response frame (740) for searching for AP 2-1 (703-1) and AP 3-1 (705-1). AP 1-2 (701-2) and STA 1-2 (710-2) operating on the second link can exchange discovery request frames (730) and discovery response frames (740) to discover AP 2-2 (703-2) and AP 3-2 (705-2).

[0107] STA 1-1 (710-1) (STA MLD 1 (710)) can receive a search response frame (740) from AP 1-1 (701-1) or AP MLD 1 (701), and STA MLD 1 (710) performs channel search based on channel search schedule information provided by AP MLD 1 (701). In TBTT aa, TBTT bb, and TBTT dd, STA 1-1 (710-1) and STA 1-2 (710-2) can search for AP 2-1 (703-1), AP 2-2 (703-2), and AP 3-2 (705-2) that operate on a different frequency from AP 1-1 (701-1). STA 1-1 (710-1) can change its operating frequency (e.g., change its operating link to the third link in TBTT aa) to search for AP 2-1 (703-1) in TBTT aa. STA 1-2 (710-2) can change its operating frequency (e.g., change its operating link to the fourth link and the fifth link in TBTT bb and TBTT dd) to search for AP 2-2 (703-2) and AP 3-2 (705-2) in TBTT bb and TBTT dd, respectively. The period during which STA 1-1 (710-1) and STA 1-2 (710-2) operate by changing their operating frequencies in each TBTT and then operate again in the first link and the second link is 2 Ts + (Beacon transmission interval of AP) may be longer than or equal to, and STA 1-1 (710-1) and STA 1-2 (710-2) cannot receive frames from AP 1-1 (701-1) and AP 1-2 (701-2) during the interval. That is, STA 1-2 (710-2) cannot receive frames while STA 1-1 (710-1) performs channel search, and STA 1-1 (710-1) cannot receive frames while STA 1-2 (710-2) performs channel search. Ts is the time it takes for STA 1-1 (710-1) and STA 1-2 (710-2) to switch the operating frequency. The Ts time may be the EMLSR transition delay time or may be greater than or equal to the EMLSR transition delay time. When STA 1-1 (710-1) returns to the first link from a non-primary link and operates, STA 1-1 (710-1) and STA 1-2 (710-2) may be unable to transmit frames or may be restricted from transmitting them during the time Tm, which is the MediumSyncDelay timer or the NAVSyncDelay time. When STA 1-2 (710-2) returns to the second link from a non-secondary link and operates, STA 1-1 (710-1) and STA 1-2 (710-2) may be unable to transmit frames or may be restricted from transmitting them during the time Tm, which is the MediumSyncDelay timer or the NAVSyncDelay time.

[0108] AP 1-1 (701-1) and AP 1-2 (701-2) may terminate transmission of frames to STA 1-1 (710-1) and STA 1-2 (710-2) before TBTT aa, which is the point in time when STA 1-1 (710-1) discovers AP 2-1 (703-1). STA 1-1 (710-1) and STA 1-2 (710-2) may terminate transmission of frames to AP 1-1 (701-1) and AP 1-2 (701-2) before TBTT aa. AP 1-1 (701-1) and AP 1-2 (701-2) can terminate transmission of frames to STA 1-1 (710-1) and STA 1-2 (710-2) before TBTT bb and TBTT dd, which are the time points at which STA 1-2 (710-2) discovers AP 2-2 (703-2) and AP 3-2 (705-2). STA 1-1 (710-1) and STA 1-2 (710-2) can terminate transmission of frames to AP 1-1 (701-1) and AP 1-2 (701-2) before TBTT bb and TBTT dd.

[0109] AP 1-1 (701-1) and AP 1-2 (701-2) and STA 1-1 (710-1) and STA 1-2 (710-2) terminating transmission before each TBTT may mean terminating transmission before the Ts time, which is the time for switching the link in each TBTT. Terminating transmission before a certain point in time may include completing a transmit opportunity (TXOP) before a certain point in time. When AP 1-1 (701-1) and AP 1-2 (701-2) and STA 1-1 (710-1) and STA 1-2 (710-2) have completed channel access operations (e.g., EDCA backoff operations) close to their respective TBTTs (e.g., when the backoff counter reaches 0), AP 1-1 (701-1) and AP 1-2 (701-2) may not transmit frames to STA 1-1 (710-1) and STA 1-2 (710-2), and STA 1-1 (710-1) and STA 1-2 (710-2) may not transmit frames to AP 1-1 (701-1) and AP 1-2 (701-2). The backoff counters of AP 1-1 (701-1) and AP 1-2 (701-2) and / or the backoff counters of STA 1-1 (710-1) and STA 1-2 (710-2) may remain at 0. Alternatively, even if the backoff counters of AP 1-1 (701-1) and AP 1-2 (701-2) reach 0, AP 1-1 (701-1) and AP 1-2 (701-2) may select new backoff counters and continue performing the backoff operation. Alternatively, AP 1-1 (701-1) and AP 1-2 (701-2) may consider that there are no frames destined for STA 1-1 (710-1) and STA 1-2 (710-2) in the transmission queue, and if there are frames destined for STAs other than STA 1-1 (710-1) and STA 1-2 (710-2), the frames may be transmitted to STAs other than STA 1-1 (710-1) and STA 1-2 (710-2).After the channel search operation of STA 1-1 (710-1) and STA 1-2 (710-2) is completed, AP 1-1 (701-1) and AP 1-2 (701-2) or / and STA 1-1 (710-1) and STA 1-2 (710-2) can transmit frames. AP 1-1 (701-1) and AP 1-2 (701-2) can transmit frames at TBTT aa, TBTT bb, or TBTT dd. During the interval having a length greater than Ts + (Beacon transmission interval of AP), frames are not transmitted to STA 1-1 (710-1) and STA 1-2 (710-2). For example, AP 1-1 (701-1) and AP 1-2 (701-2) delay channel access operation for transmission to STA 1-1 (710-1) and STA 1-2 (710-2), or wait (e.g., keep the backoff counter at 0 or re-select the backoff counter) without transmitting frames to STA 1-1 (710-1) and STA 1-2 (710-2) even when the EDCA backoff counter reaches 0. AP 1-1 (701-1) and AP 1-2 (701-2) can transmit frames to STA 1-1 (710-1) and STA 1-2 (710-2) after the above section. The frames that AP 1-1 (701-1) and AP 1-2 (701-2) transmit to STA 1-1 (710-1) and STA 1-2 (710-2) can be trigger frames. For example, the trigger frame that AP 1-1 (701-1) transmits to STA 1-1 (710-1) can be a frame for releasing the MediumSyncDelay timer or NAVSyncDelay of STA 1-1 (710-1), which restricts frame transmission for the Tm time period, and can trigger uplink data of STA 1-1 (710-1). Alternatively, the frame transmitted by AP 1-1 (701-1) to STA 1-1 (710-1) may be a downlink frame.

[0110]

[0111] In TBTT cc, STA 1-1 (710-1) can discover AP 3-1 (705-1) operating on the same frequency as AP 1-1 (701-1). However, STA 1-2 (710-2) may not be able to transmit or receive frames while STA 1-1 (710-1) discovers AP 3-1 (705-1). AP 1-1 (701-1) and AP 1-2 (701-2) terminate transmission of frames to STA 1-1 (710-1) and STA 1-2 (710-2) before TBTT cc, which is the TBTT of AP 3-1 (705-1). STA 1-1 (710-1) and STA 1-2 (710-2) terminate transmission of frames to AP 1-1 (701-1) and AP 1-2 (701-2) before TBTT cc.

[0112] STA MLD 1 (710) can complete the search based on the search schedule information provided by AP MLD 1 (701). For example, STA MLD 1 (710) can collect information of AP MLD 2 (703) to AP MLD 3 (705) (information of lower level APs of AP MLD 2 (703) to AP MLD 3 (705)). STA MLD 1 (710) transmits a measurement report frame (760) to AP MLD 1 (701). The measurement report frame (760) is a frame that transmits information of AP MLD 2 (703) to AP MLD 3 (705), which are neighboring AP MLDs (e.g., signal strength, SNR, link congestion of APs operating under AP MLD 2 (703) to AP MLD 3 (705)). AP MLD 1 (701) can respond to the measurement report frame (760) of STA MLD 1 (710) with a response frame (e.g., ACK frame, BlockAck frame). AP MLD 1 (701) can determine whether to roam and the roaming target, etc., based on the surrounding AP MLD information of STA MLD 1 (710).

[0113] In the above embodiment, instead of receiving beacon frames (751, 753, 755, 757) of AP MLDs to obtain surrounding AP MLD information, STA MLD 1 (710) may transmit a probe request frame and receive a probe response frame.

[0114]

[0115] Hereinafter, embodiments of a channel measurement procedure are illustrated. The channel measurement procedure can be performed between a plurality of APs and a single STA. The plurality of APs include a first AP and remaining APs. The first AP refers to an AP connected to the STA. The remaining APs (remaining APs) refer to the remaining APs among the plurality of APs excluding the first AP. That is, there can be at least one remaining AP. The plurality of APs can be connected to each other. Here, "connection" includes a wired connection and a wireless connection, and the connected APs can communicate with each other. For example, the plurality of APs can be connected to each other through a wired connection or a wireless connection. As another example, at least some APs can be connected to each other wirelessly, and other some APs can be connected to each other through a wired connection.

[0116] Each of the multiple APs may be MLD. Alternatively, each of the multiple APs may not be MLD. If the multiple APs are MLD, the STA may be MLD. If the multiple APs are not MLD, the STA may not be MLD. If the multiple APs and the STA are MLD, the multiple APs and the STA may be EMLSR MLD.

[0117] Each of the multiple APs can operate on a different link. For example, a first AP can operate on a first link, a second AP (one of the remaining APs) can operate on a second link, and another AP can operate on a third link. Each of the links can have a different operating frequency. If each of the multiple APs is an MLD, each AP can operate on multiple links.

[0118]

[0119] FIG. 8 illustrates a flowchart of a channel measurement procedure according to one embodiment of the present disclosure. The channel measurement procedure of FIG. 8 is performed by an STA, not an MLD. An STA (e.g., STA (310) of FIG. 3 ) is connected to a first AP (e.g., AP 1 (301) of FIG. 3 ). That is, the operating link of the STA is the link on which the first AP operates. The link on which the first AP operates may be referred to as the first link.

[0120] Referring to FIG. 8, in step S801, the STA receives information about the time at which a frame (e.g., a beacon frame (351) of FIG. 3) is transmitted on the first link. Here, the time at which the frame is transmitted means the time at which the frame is transmitted by at least one remaining AP. For example, the time at which the frame is transmitted may include the time at which a second AP (e.g., AP 2 (303) of FIG. 3) among the remaining APs transmits the frame, and the time at which another AP (e.g., AP 3 (305) of FIG. 3) among the remaining APs transmits the frame. For example, the time at which the frame is transmitted may be expressed as an absolute time value or a relative time value with respect to the time at which information is received. In addition, the time value may be expressed in a time unit (e.g., ps, s, µs, ms, etc.) or a resource unit (e.g., frame, slot, etc.). The frame may include at least one of a beacon frame and an NDP (null data PPDU) frame. Information regarding the time at which the frame is transmitted can be received from the first AP. Information regarding the time at which the frame is transmitted can be received via a beacon frame from the first AP. Information regarding the time at which the frame is transmitted can be received via the RNR information element of the beacon frame.

[0121] In step S803, the STA transmits a scan request message (e.g., a search request frame (330) of FIG. 3) on the first link. The scan request message may be transmitted based on whether a scan condition is satisfied. For example, the scan condition may be satisfied when the reception strength (RISS) or SNR of a frame transmitted from the first AP is equal to or lower than a threshold. The scan request message may be transmitted in a frame of various formats. The scan request message may include scheduling information of the STA. The scheduling information includes at least one of information regarding the time at which the frame is transmitted, a link ID of at least one remaining AP, a BSSID of at least one remaining AP, or operating frequency information of at least one remaining AP. The scheduling information may be information indicating the time and order in which the STA performs the scan. For example, the STA may perform the scan according to the order of the link IDs included in the scheduling information.

[0122] In step S805, the STA receives a scan response message (e.g., the discovery response frame (340) of FIG. 3). The scan response message may be a message transmitted from the first AP. The scan response message may be a message transmitted in response to a scan request message. Alternatively, the scan response message may be a message transmitted independently of the scan request message. The scan response message may include a response to scheduling information. For example, the scan response message may include a grant, a change, or a rejection of scheduling information. If the scan response message includes a grant of scheduling information, scheduling information transmitted by the STA may be included in the scan response message. If the scan response message includes a change to scheduling information, the scan response message may include scheduling information generated by the first AP. If the scan response message includes a rejection of scheduling information, the scan response message may not include scheduling information or may include scheduling information generated by the first AP. The scheduling information generated by the first AP includes at least one of the time at which the frame is transmitted, the link ID of at least one remaining AP, or the operating frequency information of at least one remaining AP.

[0123] In step S807, the STA measures the channel on the second link. The second link refers to the link on which the second AP operates. The second AP refers to one of the remaining APs. The STA may switch the operating link to the second link at the time when the second AP transmits a frame for channel measurement. For example, the STA may switch the operating link based on the scheduling information transmitted or the scheduling information received from the first AP. Although not shown in FIG. 8, before switching the operating link, the STA may transmit a scan start message (e.g., the search start frame (561) of FIG. 5) to the first AP. After switching the operating link, the STA may receive a frame (e.g., the beacon frame (351) of FIG. 3) transmitted by the second AP for channel measurement. For example, the frame may include a beacon frame or an NDP frame. The STA may measure the channel based on the received frame. For example, the STA may measure the channel by measuring at least one of the signal strength, SNR, and link congestion of the second AP on the operating link. After performing channel measurements for the second AP, channel measurements may be repeated for the remaining APs. For example, channel measurements may be performed for APs other than the second AP among the remaining APs. In other words, channel measurements may be performed for links other than the second link.

[0124] In step S809, the STA transmits a measurement report message (e.g., the measurement report frame (360) of FIG. 3) on the first link. After the channel measurement is completed, the STA may switch the working link to the first link. After the switching of the working link is completed, the STA may transmit a scan end message (e.g., the search end frame (571) of FIG. 5) to the first AP. The STA may transmit a measurement report message to the first AP. The measurement report message may include the result measured in step S807. For example, the measurement report message may include at least one of the signal strength, SNR, and link congestion of the second AP measured in step S807.

[0125] In one embodiment described with reference to FIG. 8, channel measurement is described as being performed for one link. That is, in FIG. 8, a measurement report message may be transmitted after channel measurement for one link. Alternatively, according to another embodiment, a measurement report message may be transmitted after channel measurement for multiple links. The measurement report message may include a measurement result for at least one link. For example, the measurement report message may be transmitted separately for each link, or alternatively, may be transmitted for each group of multiple links. In another example, multiple measurement report messages including at least one measurement result may be transmitted. Steps S807 to S809 may be repeatedly performed even after transmitting the measurement report message. For example, after a channel report message for at least one link is transmitted, channel measurement may be performed for the remaining links, and then the channel report message may be transmitted.

[0126]

[0127] FIG. 9 illustrates a flowchart of a procedure for receiving channel measurements according to one embodiment of the present disclosure. The procedure of FIG. 9 is performed by a first AP (e.g., AP 1 (301) of FIG. 3 ). The first AP is a non-MLD AP. The first AP operates on the first link. The first AP can obtain the timing at which the other APs transmit frames from the other APs.

[0128] Referring to FIG. 9, in step S901, the first AP transmits information regarding the time at which a frame (e.g., a beacon frame (351) of FIG. 3) is transmitted. The time at which the frame is transmitted refers to the time at which the frame is transmitted by at least one remaining AP. For example, the time at which the frame is transmitted may include the time at which a second AP (e.g., AP 2 (303) of FIG. 3) among the remaining APs transmits the frame, and the time at which another AP (e.g., AP 3 (305) of FIG. 3) among the remaining APs transmits the frame. The frame may be, for example, a beacon frame. The APs may be any AP other than the first AP. The information regarding the time at which the frame is transmitted may be transmitted to an STA (e.g., STA (310) of FIG. 3). The information regarding the time at which the frame is transmitted may be received through the beacon frame of the first AP. The information regarding the time at which the frame is transmitted may be received through the RNR information element of the beacon frame.

[0129] In step S903, the first AP receives a scan request message (e.g., the discovery request frame (330) of FIG. 3) from the STA. The scan request message may be generated by the STA based on whether a scan condition is satisfied. For example, the scan condition may be satisfied when the reception strength or SNR of a frame transmitted from the first AP is equal to or lower than a threshold. The scan request message may be transmitted through various formats of frames. The scan request message may include scheduling information of the STA. The scheduling information includes at least one of information regarding a scan time of at least one remaining AP, a link ID of at least one remaining AP, a BSSID of at least one remaining AP, or operating frequency information of at least one remaining AP. The scheduling information may be information indicating an order in which the STA performs a scan. For example, the STA may perform a scan according to the order of the link IDs included in the scheduling information.

[0130] In step S905, the first AP transmits a scan response message (e.g., the discovery response frame (340) of FIG. 3). The scan response message may be transmitted to the STA. The scan response message may be transmitted in response to a scan request message. Alternatively, the scan response message may be transmitted independently of the scan request message. The scan response message may include a response to scheduling information. For example, the scan response message may include a grant, a change, or a rejection of scheduling information. If the scan response message includes a grant of scheduling information, scheduling information transmitted by the STA may be included in the scan response message. If the scan response message includes a change to scheduling information, the scan response message may include scheduling information generated by the first AP. If the scan response message includes a rejection of scheduling information, the scan response message may not include scheduling information or may include scheduling information generated by the first AP. The scheduling information generated by the first AP includes at least one of a time at which a frame is transmitted, a link ID of at least one remaining AP, or operating frequency information of at least one remaining AP. Before transmitting a scan response message, the first AP may share information by coordinating with at least one remaining AP and acquire information about at least one remaining AP. The first AP may transmit a scan response message based on the information about the at least one remaining AP obtained. Based on the scheduling information in the scan response message, the first AP may stop transmitting frames to the STA. In other words, the first AP may set a communication prohibition period based on the scan response message.

[0131] In step S907, the first AP receives a measurement report message (e.g., the measurement report frame (360) of FIG. 3) from the STA. The measurement report message may include information on a link on which at least one remaining AP operates, as measured by the STA. The measurement report message may include at least one of a signal strength, an SNR, and a link congestion of at least one remaining AP, as measured by the STA. The measurement report message may be received after the STA measures a link on which one remaining AP operates. Alternatively, the measurement report message may be received after measurements are made on a plurality of remaining APs. The first AP may determine whether the STA roams or at least one of the roaming targets based on the received measurement report message.

[0132]

[0133] In FIGS. 8 and 9, when the AP and the STA are MLDs, the STA, the first AP, and at least one remaining AP may each operate on multiple links. For example, the STA may perform steps S801 to S805 on one of the links on which the first AP operates (e.g., the first link in FIG. 4 ), and may perform step S807 on at least one of the links on which the remaining AP operates (e.g., the third link in FIG. 4 ). In this case, the measurement result message may be transmitted on the link on which step S801 is performed.

[0134]

[0135] The methods according to the present invention may be implemented in the form of program instructions that can be executed by various computer means and recorded on a computer-readable medium. The computer-readable medium may include program instructions, data files, data structures, etc., either singly or in combination. The program instructions recorded on the computer-readable medium may be those specifically designed and constructed for the present invention, or may be known and available to those skilled in the computer software art.

[0136] Examples of computer-readable media include hardware devices specifically configured to store and execute program instructions, such as ROM, RAM, and flash memory. Examples of program instructions include not only machine language codes generated by a compiler, but also high-level language codes that can be executed by a computer using an interpreter, etc. The hardware devices described above may be configured to operate with at least one software module to perform the operations of the present invention, and vice versa.

[0137] Although the present invention has been described with reference to the above embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

[0138] The present disclosure can be used in devices and recording media in a wireless LAN system.

Claims

1. A method for operation of STA (station) in a wireless LAN system, A step of receiving information about the time at which a frame is transmitted from a first AP; A step of receiving a scan response message generated based on information about the time at which the above frame is transmitted; A step of receiving a frame from a second AP based on the above scan response message; A step of measuring a channel based on a frame received from the second AP; and Including the step of transmitting a measurement report message to the first AP, A method wherein information about the time at which the frame is transmitted includes the time at which the frame is transmitted by at least one AP including the second AP.

2. In paragraph 1, a step of switching the operating link to a link on which the second AP operates, in order to receive a frame from the second AP; and A method further comprising the step of switching the operating link to a link on which the first AP is operating in order to transmit the above measurement report message.

3. In paragraph 2, a step of transmitting a scan start message to the first AP before switching the above-described operating link to a link on which the second AP operates; and A method further comprising the step of transmitting a scan termination message to the first AP after switching the above-described operating link to a link on which the first AP operates.

4. In paragraph 1, Further comprising a step of transmitting a scan request message including scheduling information for measuring a channel of the second AP, The above scheduling information is generated based on information about the time at which the frame is transmitted, A method wherein the scan response message is transmitted based on the scan request message and includes information regarding permission, change, or rejection of the scheduling information.

5. In paragraph 4, The above scan request message is sent based on whether the scan conditions are satisfied. The above scan condition is a method including a case where the reception strength or SNR of a frame transmitted from the first AP is equal to or lower than a predefined threshold.

6. In paragraph 4, A method wherein the scan response message includes scheduling information generated by the first AP, if the scan response message includes information regarding a change or rejection of the scheduling information.

7. In paragraph 4, A method wherein the scheduling information includes at least one of information about a time at which the frame is transmitted, a link ID of the at least one AP, a BSSID of the at least one AP, or an operating frequency of the at least one AP.

8. In paragraph 1, A method in which the above scan response message is generated based on cooperation between the first AP and the at least one AP.

9. In paragraph 1, A method wherein the measurement report message includes at least one of a signal strength, SNR, or link congestion of the second AP measured by the STA.

10. In paragraph 1, A method wherein the above frame comprises at least one of a beacon frame, an NDP frame, or a probe response frame.

11. A method for operating a first AP (access point) in a wireless LAN system, A step of obtaining information about the time at which a frame is transmitted; A step of transmitting information about the time at which the above frame is transmitted to the STA; A step of transmitting a scan response message generated based on information about the time at which the above frame is transmitted; Including a step of receiving a measurement report message from the above STA, Information about the time at which the above frame is transmitted includes information about the time at which the frame is transmitted by at least one AP, A method wherein the above measurement report message includes a channel measurement result for a second AP, which is one of the at least one AP.

12. In paragraph 11, Further comprising the step of receiving a scan request message, The above scan request message includes scheduling information for the STA to perform channel measurement, A method in which the above scheduling information is generated based on information regarding the time at which the frame is transmitted.

13. In paragraph 12, The above scan response message is transmitted based on the above scan request message, A method wherein the above scan response message includes information regarding permission, change or rejection of the scheduling information.

14. In paragraph 11, A method in which the above scan response message is obtained based on negotiation with at least one AP.

15. In paragraph 11, A method wherein the above measurement report message includes a channel measurement result for the second AP, and includes at least one of a signal strength, SNR, or link congestion of the second AP.

16. In paragraph 12, A method further comprising the step of setting a prohibited period during which communication with the STA is not performed based on the above scheduling information.

17. In a wireless LAN system, at STA (station), Transmitter and receiver; and Including a processor connected to the above transceiver, The above processor, Receive information about the time at which a frame is transmitted from the first AP, Receive a scan response message generated based on information about the time at which the above frame is transmitted, Based on the above scan response message, a frame is received from the second AP, Measure the channel based on the frame received from the second AP, configured to transmit a measurement report message to the first AP, Information about the time at which the above frame is transmitted, STA including the time at which the frame is transmitted by at least one AP including the second AP.

18. In the first AP (access point) in a wireless LAN system, Transmitter and receiver; and Including a processor connected to the above transceiver, The above processor, Obtain information about the time at which a frame is transmitted, Transmit information about the time at which the above frame is transmitted to the STA, Transmitting a scan response message generated based on information about the time at which the above frame is transmitted, configured to receive a measurement report message from the above STA, Information about the time at which the above frame is transmitted includes information about the time at which the frame is transmitted by at least one AP, The above measurement report message is a first AP including a channel measurement result for a second AP, which is one of the at least one AP.

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