WO2013118996A1 - Method by which stations operating in power save mode in wireless lan systems transmit and receive frames, and apparatus for supporting same - Google Patents

Abstract

The present invention relates to a method by which a station (STA) which operates in power save mode in a wireless LAN system transmits and receives frames. The method includes the steps of: obtaining a channel access authority on the basis of at least one slot time, wherein each slot time is the unit time when a channel is maintained in an idle state for a channel access by the STA; transmitting a power save (PS) poll frame requesting the transmission of a buffered frame to an access point (AP); and receiving a response frame from the AP in response to the PS poll frame. The length of each slot time is set to be longer than the transmission time of the PS poll frame.

Description

Frame transmission / reception method by a station operating in a power save mode in a wireless LAN system and an apparatus supporting the same

The present invention relates to wireless communication, and more particularly, to a method for transmitting / receiving a frame by a station operating in a power save mode in a WLAN system and an apparatus for supporting the same.

Recently, with the development of information and communication technology, various wireless communication technologies have been developed. Wireless Local Area Network (WLAN) is based on radio frequency technology and uses portable terminals such as personal digital assistants (PDAs), laptop computers, and portable multimedia players (PMPs). This is a technology that allows users to access the Internet wirelessly at home, business, or specific service area.

In order to overcome the limitation of communication speed, which has been pointed out as a weak point in WLAN, IEEE 802.11n is a relatively recent technical standard. IEEE 802.11n aims to increase the speed and reliability of networks and to extend the operating range of wireless networks. More specifically, IEEE 802.11n supports High Throughput (HT) with data throughput of up to 540 Mbps and also uses multiple antennas at both the transmitter and receiver to minimize transmission errors and optimize data rates. It is based on Multiple Inputs and Multiple Outputs (MIMO) technology.

In a WLAN system, a station (STA) supports a power save mode. The station may enter and operate in a doze state to prevent unnecessary power consumption. If there is traffic related to data intended to be transmitted to the STA operating in the sleep state, an access point (AP) may indicate this to the STA. The STA may recognize that there is traffic related to data intended for transmission, and may request the AP to transmit it. The AP may transmit a frame in response to a request of the STA.

On the other hand, if the AP can transmit only one frame in response to a request by the STA entering the awake state, it may be inefficient in terms of traffic processing. In addition, since the STA frequently switches the awake state / doze state to receive the buffered frame, efficiency may be lowered in terms of power save operation. Therefore, there is a need for a power save mode operating method that can improve the excellent traffic processing and the power save mode efficiency of the STA.

In addition, when a plurality of STAs combined with one AP are operating in a power save mode and the STAs are in a hidden node relationship, the STAs independently perform channel access to request transmission of their buffered frames. Attempts are made, and thus a collision between requests by each STA may occur. Therefore, there is a need for a frame transmission / reception method that can solve such a problem.

The technical problem to be solved by the present invention is to provide a method for transmitting and receiving a frame by a station operating in a power save mode in a WLAN system and an apparatus supporting the same.

In one aspect, there is provided a frame transmission and reception method performed by a station (STA) operating in a power save mode in a WLAN system. The method acquires a channel access right based on at least one slot time, wherein each slot time is a unit time in which a channel remains idle for channel access of the STA, and a buffered frame Sending a Power Save Poll frame requesting transmission of the AP to an AP and receiving a response frame from the AP in response to the PS-poll frame. The length of each slot time is set longer than the transmission time of the power save poll frame.

The method may further comprise setting a back-off timer that is the number of the at least one slot time.

The method may further include receiving a Traffic Indication Map (TIM) element. The TIM element may include a bitmap sequence. The specific bit of the bitmap sequence may indicate the presence of the buffered frame for the STA.

The value of the backoff timer may be determined based on an order in the bitmap sequence of the specific bit.

The value of the backoff timer may be determined based on an order in the bitmap sequence of the specific bit and a time point when the STA sets the backoff timer.

The value of the backoff timer may be determined based on an order of the specific bit among at least one bit indicating that there is a buffered frame for the specific STA in the bitmap sequence.

The value of the backoff timer is based on an order of the specific bit among the one or more bits indicating that there is a buffered frame for the specific STA in the bitmap sequence and a time point for setting the backoff timer. Can be determined.

The response frame may be the buffered frame.

The response frame may be an acknowledgment (ACK) frame.

In another aspect, a wireless device operating in a power save mode in a WLAN system is provided. The wireless device includes a transceiver for transmitting and receiving wireless signals and a processor operatively coupled to the transceiver. The processor acquires a channel access right based on at least one slot time, wherein each slot time is a unit time at which a channel is idle for channel access of the wireless device and is buffered. A power save poll frame requesting transmission of a frame is transmitted to an access point (AP), and a response frame is received from the AP in response to the PS-poll frame. The length of each slot time is set longer than the transmission time of the power save poll frame.

According to the frame transmission / reception method proposed in the present invention, the length of slot time, which is a unit time for the STA to check whether the channel is idle to access the channel, is longer than the time for transmitting the PS-pole frame by the STA. Through this, the STAs in the hidden node relationship can access the channel during the overlapping or overlapping time interval through the contention to prevent the collision occurs by transmitting the PS-pole frame.

In addition, according to the proposed frame transmission and reception method, an initial backoff timer value set by each STA to access a channel may be uniquely set. This may prevent each STA from accessing the channel at the same time.

By preventing collision of frame transmission by the STAs in the hidden node relationship and preventing the access of the channel during the simultaneous or overlapping period by the STAs, the wireless medium may be prevented from being occupied unnecessarily. Through this, efficient frame transmission and reception is possible, and thus the throughput of the entire WLAN can be improved.

FIG. 1 is a diagram illustrating a configuration of a general wireless local area network (WLAN) system to which an embodiment of the present invention can be applied.

2 is a diagram illustrating an example of a problem that may exist in a WLAN system.

3 is a diagram illustrating an example of a method for solving a problem that may exist in a WLAN system.

4 is a diagram illustrating an example of a power management operation.

5 is a block diagram illustrating an example of a TIM element format.

6 illustrates an example of a bitmap control field and a partial virtual bitmap field according to an embodiment of the present invention.

7 is a flowchart illustrating an example of a response procedure of an AP in a TIM protocol.

8 is a flowchart illustrating another example of a response procedure of an AP in a TIM protocol.

9 is a flowchart showing the procedure of the TIM protocol by DTIM.

10 is a diagram illustrating an example of a frame transmission and reception method based on a TIM protocol and a U-APSD.

11 is a diagram illustrating an example of a collision that may occur when transmitting and receiving a frame.

12 illustrates an example of a channel access method for frame transmission and reception according to an embodiment of the present invention.

13 is a diagram illustrating an example of a method for transmitting / receiving a unique backoff timer based frame according to an embodiment of the present invention.

14 is a diagram illustrating an example of a channel access method according to another embodiment of the present invention.

15 is a block diagram illustrating a wireless device in which an embodiment of the present invention may be implemented.

FIG. 1 is a diagram illustrating a configuration of a general wireless local area network (WLAN) system to which an embodiment of the present invention can be applied.

Referring to FIG. 1, a WLAN system includes one or more basic service sets (BSSs). The BSS is a set of stations (STAs) that can successfully communicate with each other by synchronizing, and is not a concept indicating a specific area.

Infrastructure BSS may include one or more non-AP stations (non-AP STA1 (21), non-AP STA2 (22), non-AP STA3 (23), non-AP STA4 (24), non-AP). STAa 30), an access point (AP) 10 for providing a distribution service, and a distribution system (DS) for connecting a plurality of APs. In the infrastructure BSS, the AP manages non-AP STAs of the BSS.

On the other hand, Independent BSS (IBSS) is a BSS that operates in Ad-Hoc mode. Since IBSS does not include an AP, there is no centralized management entity. That is, in the IBSS, non-AP STAs are managed in a distributed manner. In the IBSS, all STAs may be mobile STAs, and access to the DS is not allowed to form a self-contained network.

A STA is any functional medium that includes a medium access control (MAC) compliant with the IEEE of the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and a physical layer interface to a wireless medium. It includes both AP and Non-AP Stations.

A non-AP STA is an STA, not an AP, and a non-AP STA is a mobile terminal, a wireless device, a wireless transmit / receive unit (WTRU), a user equipment (UE), It may also be called a mobile station (MS), mobile subscriber unit, or simply another name such as user. Hereinafter, for convenience of description, a non-AP STA is referred to as an STA.

An AP is a functional entity that provides access to a DS via a wireless medium for an associated STA to that AP. In an infrastructure BSS including an AP, communication between STAs is performed via an AP. However, when a direct link is established, direct communication between STAs is possible. The AP may also be called a central controller, a base station (BS), a node-B, a base transceiver system (BTS), a site controller, or a management STA.

A plurality of infrastructure BSSs including the BSS shown in FIG. 1 may be interconnected through a distribution system (DS). A plurality of BSSs connected through a DS is called an extended service set (ESS). The AP and / or STA included in the ESS may communicate with each other, and in the same ESS, the STA may move from one BSS to another BSS while communicating seamlessly.

In a WLAN system according to IEEE 802.11, a basic access mechanism of MAC (Medium Access Control) is a carrier sense multiple access with collision avoidance (CSMA / CA) mechanism. The CSMA / CA mechanism is also called the Distributed Coordination Function (DCF) of the IEEE 802.11 MAC, and basically employs a “listen before talk” access mechanism. According to this type of access mechanism, the AP and / or STA senses a radio channel or medium prior to initiating transmission. As a result of sensing, if it is determined that the medium is in an idle state, frame transmission is started through the medium. On the other hand, if the medium is detected as occupied status (occupied status), the AP and / or STA does not start the transmission of its own, but waits by setting a delay period for access to the medium.

The CSMA / CA mechanism also includes virtual carrier sensing in addition to physical carrier sensing in which the AP and / or STA directly sense the medium. Virtual carrier sensing is intended to compensate for problems that may occur in media access, such as a hidden node problem. For virtual carrier sensing, the MAC of the WLAN system uses a network allocation vector (NAV). The NAV is a value that indicates to the other AP and / or STA how long the AP and / or STA currently using or authorized to use the medium remain until the medium becomes available. Therefore, the value set to NAV corresponds to a period during which the use of the medium is scheduled by the AP and / or STA transmitting the frame.

The IEEE 802.11 MAC protocol, along with DCF, is an HCF based on Point Coordination Function (PCF) that periodically polls all receiving APs and / or STAs to receive data packets with a DCF and pollilng-based synchronous access. It provides (Hybrid Coordination Function). HCF uses HCCA Controlled Channel, which uses an enhanced distributed channel access (EDCA) and non-competition-based channel approach that uses polling mechanisms to provide providers with access to data packets to multiple users. Access) The HCF includes a media access mechanism for improving the quality of service (QoS) of the WLAN, and can transmit QoS data in both a contention period (CP) and a contention free period (CFP).

In a WLAN system, there may be two issues related to the above channel access mechanism. The first is the hidden node problem and the second is the exposed node problem. Hereinafter, problems related to the channel access mechanism and a method of solving the same will be described in detail with reference to FIGS. 2 and 3.

2 is a diagram illustrating an example of a problem that may exist in a WLAN system.

FIG. 2 (a) is a diagram illustrating an example of a hidden node problem.

When STA A and STA B are in communication and STA C has information to transmit, there may be a situation where STA A is accessing a channel to transmit information to STA B. When the signal coverages of the STA A and the STA C do not overlap each other, the STA C may not recognize the channel occupation state by the STA A. In this case, STA A may determine that the channel is idle through carrier sensing for the medium. Accordingly, STA C may transmit a radio signal to STA B by accessing a channel, which may cause a collision between a radio signal by STA A and a radio signal by STA C. In such a situation, STA C may be called a hidden node from the viewpoint of STA A.

FIG. 2B is a diagram illustrating an example of an exposed node problem.

STA B is transmitting data to STA A. STA C has data to send to STA D. STA C may determine that the channel is occupied by STA B through carrier sensing. As a result, STA C may determine that the channel is occupied even though it wants to transmit to STA D, and thus may need to wait unnecessarily until the channel returns to the idle state. That is, even though STA A is outside the range of carrier sensing of STA C, data transmission to STA A causes a phenomenon that data transmission to STA D is disturbed. In such a situation, STA C becomes an exposed node of STA B.

Short signaling frames such as Request To Send (RTS) / Clear To Send (CTS) may be introduced in order to prevent the above problems. This may leave room for neighboring STAs to overhear whether to transmit the information of the two STAs. That is, when the STA to transmit the data transmits the RTS frame to the STA that receives the data, the receiving STA may inform that it will receive the data by transmitting the CTS frame to the surrounding terminals. An example of a method of solving the above problem based on the RTS / CTS may refer to FIG. 3.

3 is a diagram illustrating an example of a method for solving a problem that may exist in a WLAN system.

3 is a diagram illustrating an example in which RTS / CTS signaling is applied to a hidden node problem.

When STA A transmits an RTS frame to STA B, STA B transmits the CTS frame to both STA A and STA C around it. As a result, STA C cannot know that the channel is occupied by STA A even if carrier sensing is actually performed, but it can be understood that STA B will receive data through the channel for a specific period through reception of the CTS frame. . Through this, STA C may not perform an operation of accessing a channel to transmit data to STA B during the corresponding interval, thereby preventing a channel access collision.

FIG. 3B is a diagram illustrating an example in which RTS / CTS signaling is applied to an exposed node problem.

STA A, which wants to transmit data to STA B, transmits an RTS frame to STA B, and in response, STA B transmits a CTS frame to both STA A and STA C. STA C may know that STA B receives data through a channel by receiving (or overhearing) a CTS frame transmitted from STA B. However, STA C may know that accessing the channel to transmit data to STA D does not cause collision with STA A and STA B data transmission and reception. Accordingly, STA C accesses the channel and transmits data to STA D. Can be. More specifically, since STA C receives an RTS frame from STA B but does not receive a CTS frame transmitted by STA A, STA C may know that STA A is out of the carrier sensing range of STA C based on this. . Accordingly, STA C may transmit data to STA D.

In the wireless communication system, the existence of the network may not be immediately known when the STA is powered on and starts operation due to the characteristics of the wireless medium. Therefore, any type of STA must perform a network discovery process in order to access the network. The STA that discovers the network through the network discovery process selects a network to join through the network selection process. Thereafter, it joins the selected network and performs a data exchange operation performed at the transmitting end / receiving end.

The network discovery process in a WLAN system is implemented by a scanning procedure. The scanning procedure is divided into passive scanning and active scanning. Passive scanning is performed based on a beacon frame that the AP broadcasts periodically. In general, the AP of a WLAN broadcasts a beacon frame at a specific interval (for example, 100 msec0). The beacon frame contains information about the BSS it manages. The STA passively waits for reception of a beacon frame on a particular channel. The STA acquiring information about the network through reception of a beacon frame ends the scanning procedure on a specific channel. Manual scanning is advantageous because the STA does not need to transmit a separate frame, but only receives a beacon frame, thereby reducing the overall overhead. However, there is a disadvantage in that the scanning execution time increases in proportion to the transmission period of the beacon frame.

In active scanning, an STA actively broadcasts a probe request frame on a specific channel and requests network information from all APs receiving the probe request frame. The AP receiving the probe request frame transmits the network information to the corresponding STA by including the network information in the probe response frame after waiting for a random time to prevent frame collision. The STA terminates the scanning procedure by receiving the probe response frame to obtain network information. Active scanning has the advantage of being able to finish scanning in a relatively fast time. On the other hand, the overall network overhead is increased because a frame sequence based on request-response is required.

After completing the scanning procedure, the STA selects a network according to its specific criteria and performs an authentication procedure with the AP. The authentication process consists of a two-way handshake. After completing the authentication procedure, the STA proceeds with association with the AP.

The joining procedure consists of a two-way handshake. First, the STA transmits an association request frame to the AP. The association request frame includes capability information of the STA. Based on this, the AP determines whether to allow association with the corresponding STA. After determining whether to allow association, the AP transmits an association response frame to the corresponding STA. The association response frame includes information indicating whether to allow the association and information indicating the reason when the association is allowed / failed. The association response frame further includes information on capabilities that the AP can support. If the association is successfully completed, normal frame exchange is performed between the AP and the STA. If the association fails, the association procedure is attempted again based on the information on the failure reason included in the association response frame, or the STA may request the association from another AP.

In order to overcome the limitation of communication speed, which has been pointed out as a weak point in WLAN, IEEE 802.11n is a relatively recent technical standard. IEEE 802.11n aims to increase the speed and reliability of networks and to extend the operating range of wireless networks. More specifically, IEEE 802.11n supports High Throughput (HT) with data throughput of up to 540 Mbps and also uses multiple antennas at both the transmitter and receiver to minimize transmission errors and optimize data rates. It is based on Multiple Inputs and Multiple Outputs (MIMO) technology.

As the spread of the WLAN is activated and the applications using the same are diversified, a need for a new WLAN system for supporting a higher throughput than the data processing speed supported by IEEE 802.11n has recently emerged. The WLAN system that supports Very High Throughput (VHT) is the next version of the IEEE 802.11n WLAN system, and the data processing speed of 1Gbps or more for multiple users at the MAC Service Access Point (SAP), and It is one of the recently proposed IEEE 802.11 WLAN system to support the throughput of 500Mbps or more for a single user.

In addition to the existing wireless LAN system that supported 20MHz and 40MHz, the VHT wireless LAN system supports 80MHz, continuous 160MHz (contiguous 160MHz), continuous 160MHz (non-contiguous 160MHz) bandwidth transmission and / or more bandwidth transmission. do. In addition to the existing WLAN system that supports up to 64QAM (Quadrature Amplitude Modulation), it supports 256QAM.

Since the VHT WLAN system supports a multi user-multiple input multiple output (MU-MIMO) transmission method for higher throughput, the AP may simultaneously transmit data frames to at least one or more STAs paired with MIMO. The number of paired STAs may be up to four, and when the maximum number of spatial streams is eight, up to four spatial streams may be allocated to each STA.

Referring back to FIG. 1, in a given WLAN system as shown in the drawing, the AP 10 includes at least one or more STAs among a plurality of STAs 21, 22, 23, 24, and 30 that are associated with themselves. Data may be simultaneously transmitted to the STA group. In FIG. 1, an AP transmits MU-MIMO to STAs, but transmits data in a WLAN system supporting Tunneled Direct Link Setup (TDLS), Direct Link Setup (DLS), and a mesh network. A desired STA may transmit a PPDU to a plurality of STAs using a MU-MIMO transmission technique. In the following description, an AP transmits a PPDU to a plurality of STAs according to an MU-MIMO transmission scheme.

Data transmitted to each STA may be transmitted through different spatial streams. The data packet transmitted by the AP 10 may be referred to as a frame as a PPDU or a data field included in the PPDU transmitted in the physical layer of the WLAN system. That is, a PPDU or a data field included in the PPDU for single user (SU) -MIMO and / or MU-MIMO may be referred to as a MIMO packet. Among them, the PPDU for the MU may be referred to as an MU packet. In the example of the present invention, it is assumed that the transmission target STA group paired with the AP 10 and the MU-MIMO are STA1 21, STA2 22, STA3 23, and STA4 24. In this case, since a spatial stream is not allocated to a specific STA of a transmission target STA group, data may not be transmitted. Meanwhile, it is assumed that the STAa 30 is an STA coupled with the AP but not included in the transmission target STA group.

In order to support MU-MIMO transmission in a WLAN system, an identifier may be allocated to a transmission target STA group, which is called a group ID. The AP transmits a group ID management frame including group definition information to STAs supporting MU-MIMO transmission for group ID assignment, thereby transmitting the group ID before PPDU transmission. To STAs. One STA may be assigned a plurality of group IDs.

Table 1 below shows information elements included in the group ID management frame.

Order	Information
One	Category
2	VHT Action
3	Membership status
4	Spatial stream position

The category field and the VHT action field are set to identify that the frame corresponds to a management frame and is a group ID management frame used in a next generation WLAN system supporting MU-MIMO.

As shown in Table 1, the group definition information includes membership status information indicating whether it belongs to a specific group ID, and if it belongs to the group ID, the set of spatial streams of the STA is located at a position in the total spatial stream according to MU-MIMO transmission Spatial stream location information indicating whether this is included is included.

Since there are a plurality of group IDs managed by one AP, membership status information provided to one STA needs to indicate whether the STA belongs to each group ID managed by the AP. Accordingly, the membership status information may exist in the form of an array of subfields indicating whether the membership state information belongs to each group ID. Since the spatial stream position information indicates a position for each group ID, the spatial stream position information may exist in the form of an array of subfields indicating the position of the spatial stream set occupied by the STA for each group ID. In addition, membership state information and spatial stream position information for one group ID may be implemented in one subfield.

When the AP transmits the PPDU to the plurality of STAs through the MU-MIMO transmission scheme, the AP includes information indicating a group ID in the PPDU as control information. When the STA receives the PPDU, the STA checks the group ID field to determine whether the STA is a member STA of the transmission target STA group. If it is confirmed that the user is a member of the transmission target STA group, it is possible to check how many positions of the spatial stream set transmitted to the user are located. Since the PPDU includes information on the number of spatial streams allocated to the receiving STA, the STA may find the spatial streams allocated to the STA and receive data.

Sensing the channel at all times for frame transmission and reception causes a constant power consumption of the STA. Since the power consumption in the reception state does not differ significantly from the power consumption in the transmission state, maintaining the reception state causes a relatively high power consumption for the battery operated STA. Therefore, in the WLAN system, the STA continuously maintains a reception state and senses a channel, which may cause inefficient power consumption without a special synergistic effect in terms of WLAN throughput, and thus is not suitable for power management. You may not.

In order to compensate for the above problems, the WLAN system supports a power management (PM) mode of the STA. The power management mode of the STA is divided into an active mode and a power save (PS) mode. The STA basically operates in the active mode. The STA operating in the active mode maintains an awake state. That is, a state in which normal operation such as frame transmission and reception or channel sensing is possible is maintained.

The STA operating in the PS mode operates by switching between a doze state and an awake state. The STA operating in the sleep state operates at the minimum power and does not receive the radio signal transmitted from the AP including the data frame. In addition, the STA operating in the doze state does not perform channel sensing.

As the STA operates in the sleep state for as long as possible, power consumption decreases, so that the STA increases its operating period. However, since the frame transmission and reception is impossible at bedtime, it cannot operate unconditionally long. When there is a frame to be transmitted to the AP by the STA operating in the sleep state, the STA may switch to the awake state and transmit the frame. However, when the AP has a frame to transmit to the STA operating in the sleep state, the STA cannot receive it and does not know that the frame to receive exists. Accordingly, the STA may need to switch to the awake state according to a specific period in order to receive the presence or absence of a frame to be transmitted to the STA. The AP may thus transmit the frame to the STA. This will be described with reference to FIG. 4.

4 is a diagram illustrating an example of a power management operation.

Referring to FIG. 4, the AP 410 transmits a beacon frame to STAs in the BSS at regular intervals (S410). The beacon frame includes a traffic indication map information element. The TIM element includes information indicating that the AP 410 buffers a bufferable frame (BU) for the STAs associated with the AP 410 and transmits the frame. The TIM element includes a TIM used to inform unicast frames and a delivery traffic indication map (DTIM) used to inform multicast or broadcast frames.

The AP 410 transmits a DTIM once every three beacon frames.

STA1 421 and STA2 422 are STAs operating in a PS mode. The STA1 421 and the STA2 422 may be configured to receive a TIM element transmitted by the AP 410 by switching from a sleep state to an awake state at every wakeup interval of a specific period.

A specific wakeup interval may be set such that the STA1 421 may switch to the awake state at every beacon interval to receive the TIM element. Therefore, the STA1 421 switches to the awake state when the AP 410 first transmits the beacon frame (S411) (S421). STA1 421 receives the beacon frame and obtains a TIM element. When the obtained TIM element indicates that the bufferable frame to be transmitted to the STA1 421 is buffered, the STA1 421 receives a PS poll frame requesting the AP 410 to transmit a frame. 410 and transmits (S421a). The AP 210 transmits the frame to the STA1 421 in response to the PS-pole frame (S431). Upon completion of the frame reception, the STA1 421 switches to the sleep state to operate.

When the AP 410 transmits a beacon frame for the second time, since the medium is busy, such as another device accessing the medium, the AP 410 sets the beacon frame at the correct beacon interval. Transmission may be performed at a delayed time without transmission (S412). In this case, the STA1 421 switches the operation mode to the awake state in accordance with the beacon interval, but does not receive the beacon frame transmitted in a delayed state, thereby switching back to the sleep state (S422).

When the AP 410 transmits a beacon frame for the third time, the beacon frame may include a TIM element set to DTIM. However, since the medium is occupied, the AP 410 delays transmission of the beacon frame (S413). The STA1 421 may operate by switching to an awake state according to the beacon interval, and may obtain a DTIM through a beacon frame transmitted by the AP 410. Since the DTIM acquired by the STA1 421 indicates that there is no frame to be transmitted to the STA1 421 and that a frame for another STA exists, the STA1 421 switches to the sleep state and operates. The AP 410 transmits the frame to the STA after the beacon frame transmission (S432).

The AP 410 transmits a beacon frame a fourth time (S414). However, since the STA1 421 cannot obtain information indicating that the bufferable frame for itself is buffered through the previous two times of TIM element reception, the STA1 421 may adjust the wakeup interval for receiving the TIM element. Alternatively, when the signaling information for adjusting the wakeup interval value of the STA1 421 is included in the beacon frame transmitted by the AP 410, the wakeup interval value of the STA1 421 may be adjusted. In this example, the STA1 421 may be configured to switch the operating state once every three beacon intervals to switch the operating state for TIM element reception every beacon interval. Therefore, the STA1 421 may not acquire the corresponding TIM element because the AP 410 transmits a fourth beacon frame (S414) and maintains a sleep state at the time of transmitting the fifth beacon frame (S415).

When the AP 410 transmits a beacon frame for the sixth time (S416), the STA1 421 switches to an awake state to operate and acquires a TIM element included in the beacon frame (S424). Since the TIM element is a DTIM indicating that a broadcast frame exists, the STA1 421 receives a broadcast frame transmitted by the AP 410 without transmitting a PS-poll frame to the AP 410 (S434). ).

Meanwhile, the wakeup interval set in the STA2 422 may be set in a longer period than the STA1 421. Accordingly, the STA2 422 may switch to the awake state and receive the TIM element at a time point S415 when the AP 410 transmits the beacon frame for the fifth time (S425). The STA2 422 transmits a PS-poll frame to the AP 410 in order to request transmission by knowing that a frame to be transmitted thereto exists through the TIM element (S425a). The AP 410 transmits the frame to the STA2 422 in response to the PS-pole frame (S433).

For power save mode operation as shown in FIG. 4, the TIM element includes a TIM indicating whether there is a frame to be transmitted to the STA or a DTIM indicating whether there is a broadcast / multicast frame. DTIM may be implemented through field setting of a TIM element.

5 is a block diagram illustrating an example of a TIM element format.

Referring to FIG. 5, the TIM element 500 includes an element ID field 510, a length field 520, a DTIM count field 530, a DTIM period field 540, and a bitmap. A bitmap control field 550 and a partial virtual bitmap field 560 are included.

The element ID field 510 is a field indicating that the corresponding information element is a TIM element. The length field 520 indicates the total length including the following fields, including itself. The maximum value may be 255 and the unit may be set as an octet value.

The DTIM count field 530 indicates whether the current TIM element is DTIM. If not, the DTIM count field 530 indicates the number of remaining TIMs until the DTIM is transmitted. The DTIM period field 540 indicates a period for transmitting the DTIM, and the period for transmitting the DTIM may be set to a multiple of the number of times the beacon frame is transmitted.

The bitmap control field 550 and the partial virtual bitmap field 560 indicate whether a bufferable frame is buffered in a specific STA. The first bit of the bitmap control field 550 indicates whether there is a multicast / broadcast frame to be transmitted. The remaining bits are set to indicate an offset value for interpreting the partial virtual bitmap field 560 that follows.

The partial virtual bitmap field 560 is set to a value indicating whether there is a bufferable frame to send to each STA. This may be set in a bitmap format in which a bit value corresponding to an AID value of a specific STA is set to one. The AIDs may be allocated in order from 1 to 2007. For example, if the fourth bit is set to 1, it means that the traffic to be sent to the STA having the AID of 4 is buffered in the AP.

On the other hand, in the case of setting the bit sequence of the partial virtual bitmap field 560 in many cases where the bit set to 0 is continuous, it may be inefficient to use all the bit sequences constituting the bitmap. To this end, the bitmap control field 550 may include offset information for the partial virtual bitmap field 560.

6 illustrates an example of a bitmap control field and a partial virtual bitmap field according to an embodiment of the present invention.

Referring to FIG. 6, the bitmap sequence constituting the partial virtual bitmap field 560 indicates whether or not there is a buffered frame in the STA having the AID corresponding to the corresponding bitmap index. The bitmap sequence constitutes indication information for AIDs from 0 to 2007.

In the bitmap sequence, a zero value may be continuously set from the first bit to the kth bit. In addition, a value 0 may be continuously set from another l th bit to the last bit. This indicates that a buffered frame does not exist in each of the STAs assigned 0 to k as AIDs and the respective STAs assigned 1 to 2007. As described above, the continuous 0 sequence from the 0 th to the k th front end of the bitmap sequence is provided with offset information, and thus, the size of the TIM element can be reduced by omitting the subsequent continuous 0 sequence.

To this end, the bitmap control field 550 may include a bitmap offset subfield 551 including offset information of consecutive zero sequences of the bitmap sequence. Bitmap offset subfield 551 may be set to point to k, and partial virtual bitmap field 560 may be set to include k + 1 th bits to l-1 th bits of the original bitmap sequence. Can be.

A detailed response procedure of the STA that has received the TIM element may refer to FIGS. 7 to 9 below.

7 is a flowchart illustrating an example of a response procedure of an AP in a TIM protocol.

Referring to FIG. 7, the STA 720 switches an operation state from a sleep state to an awake state in order to receive a beacon frame including a TIM from the AP 710 (S710). The STA 720 may interpret the received TIM element to know that there is a buffered frame to be transmitted to itself.

The STA 720 contends with other STAs to access a medium for PS-poll frame transmission (S720), and transmits a PS-poll frame to request the AP 710 to transmit a data frame (S720). S730).

The AP 710 receiving the PS-poll frame transmitted by the STA 720 transmits the frame to the STA 720 (S740). STA2 720 receives the data frame and transmits an acknowledgment (ACK) frame to the AP 710 in response to the reception (S750). Thereafter, the STA2 720 switches the operation mode to the sleep state again (S760).

As shown in FIG. 7, the AP may transmit data at a specific time after receiving the PS-poll frame, unlike the immediate response of transmitting the data frame immediately after receiving the PS-poll frame from the STA.

8 is a flowchart illustrating another example of a response procedure of an AP in a TIM protocol.

Referring to FIG. 8, the STA 820 switches an operation state from a sleep state to an awake state in order to receive a beacon frame including a TIM from the AP 810 (S810). The STA 820 interprets the received TIM element to know that there is a buffered frame to be transmitted to the STA.

The STA 820 competes with other STAs for medium access for PS-poll frame transmission (S820), and transmits a PS-poll frame to request the AP 810 to transmit a data frame (S830).

When the AP 810 receives the PS-poll frame but fails to prepare the data frame during a specific temporal interval such as a short interframe space (SIFS), the AP 810 does not transmit the data frame directly but instead sends an ACK frame to the ACK frame. In step S840). This is a characteristic of a delayed response different from step S740 in which the AP 710 of FIG. 7 transmits a data frame directly to the STA 720 in response to the PS-poll frame.

When the data frame is ready after the ACK frame is transmitted, the AP 810 performs competition (S850) and transmits the data frame to the STA 820 (S860).

The STA 820 transmits an ACK frame to the AP 810 in response to the reception of the data frame (S870), and switches the operation mode to the sleep state (S880).

If the AP transmits the DTIM to the STA, then the procedure of the TIM protocol that proceeds later may be different.

9 is a flowchart showing the procedure of the TIM protocol by DTIM.

Referring to FIG. 9, the STAs 920 switch an operation state from a sleep state to an awake state in order to receive a beacon frame including a TIM element from the AP 910 (S910). The STAs 920 may know that a multicast / broadcast frame will be transmitted through the received DTIM.

The AP 910 transmits a multicast / broadcast frame after transmitting the beacon frame including the DTIM (S920). After receiving the multicast / broadcast frame transmitted by the AP 910, the STAs 920 switch the operational state back to the sleep state (S930).

In the method of operating a power save mode based on the TIM protocol described with reference to FIGS. 4 to 9, STAs may determine whether there is a buffered frame to be transmitted due to the buffered traffic through the STA identification information included in the TIM element. . The STA identification information may be information related to an association identifier (AID) that is an identifier assigned when the STA associates with the AP. The STA identification information may be set to directly indicate AIDs of STAs having a buffered frame or may be set to a bitmap type in which a bit order corresponding to an AID value is set to a specific value. STAs may know that there is a buffered frame when the STA identification information indicates their AID.

A power management operation based on APSD (Automatic Power Save Delivery) may also be provided for power saving of the STA.

An AP capable of supporting APSD signals that it can support APSD through the use of the APSD subfield in the capability information field of the beacon frame, probe response frame, and combined response frame. An STA capable of supporting APSD uses a power management field in a frame control field of a frame to indicate whether it is operating in an active mode or a power save mode.

APSD is a mechanism for delivering downlink data and a bufferable management frame to a STA in power save operation. The frame transmitted by the STA in the power save mode using the APSD sets the power management bit of the frame control field to 1, which may cause buffering at the AP side.

APSD defines two delivery mechanisms (Unscheduled-APSD) and U-APSD (Scheduled-APSD). STAs may use U-APSD to allow some or all of their BUs to be delivered during an unscheduled Service Period (SP). The STA may use the S-APSD to allow some or all of their BUs to be delivered during the scheduled SP.

An STA using U-APSD may not receive a frame transmitted by the AP during the service period due to interference. Although the AP may not detect the interference, the AP may determine that the STA did not receive the frame correctly. The U-APSD coexistence capability indicates that the STA instructs the AP to transmit the requested transmission duration so that it can be used as a service interval for the U-APSD. The AP may transmit a frame during the service period, thereby improving the possibility of receiving the frame in a situation where the STA is interrupted. U-APSD can also reduce the likelihood that a frame transmitted by an AP will not be successfully received during a service interval.

The STA transmits an Add Traffic Stream (ADDTS) request frame including a U-APSD Coexistence element to the AP. The U-APSD coexistence element may include information on the requested service interval.

The AP may process the requested service interval and transmit an ADDTS response frame in response to the ADDTS request frame. The ADDTS request frame may include a status code. The status code may indicate response information for the requested service interval. The status code may indicate whether to allow the requested service interval, and may further indicate the reason for rejection when rejecting the requested service interval.

If the requested service interval is allowed by the AP, the AP may transmit a frame to the STA during the service interval. The duration of the service interval may be specified by the U-APSD coexistence element included in the ADDTS request frame. The start of the service interval may be a time point at which the AP normally receives a trigger frame by the STA.

The STA may enter a sleep state when the U-APSD service interval expires.

Meanwhile, as various communication services such as smart grid, e-Health, and ubiquitous have appeared, M2M (Machine to Machine) technology for supporting them has been in the spotlight. Sensors that detect temperature, humidity, and the like, and home appliances such as cameras and TVs, process machines in factories, and large machines such as automobiles, may be one element of the M2M system. Elements constituting the M2M system may transmit and receive data based on WLAN communication. If the devices constituting the M2M system supports the WLAN and the network is configured, this is referred to as M2M WLAN system hereinafter.

In a WLAN system supporting M2M, a frequency band of 1 GHz or more may be used, and the use of a low band frequency may result in wider service coverage. Therefore, the number of wireless devices located in service coverage may be larger than that of existing WLAN systems. In addition, the characteristics of the WLAN system supporting M2M are as follows.

1) Number of STAs: Unlike the existing network, M2M assumes that a large number of STAs exist in the BSS. This is because it considers not only the devices owned by the individual but also the sensors installed in the house, company, and the like. Therefore, a considerable number of STAs may be connected to one AP.

2) Low traffic load per STA: Since the M2M terminal has a traffic pattern that collects and reports the surrounding information, it does not need to be sent often and the amount of information is small.

3) Uplink-oriented communication: M2M has a structure that reports the result data to uplink after receiving a command and taking an action mainly on downlink. Since the main data is generally transmitted in the uplink, the uplink is the center in a system supporting the M2M.

4) Power management of the STA: The M2M terminal is mainly operated by a battery, it is often difficult for the user to charge frequently. Therefore, a power management method for minimizing battery consumption is required.

5) Automatic recovery function: The devices that make up the M2M system need self-recovery functions because it is difficult for a person to directly operate in a specific situation.

According to a server / client structure in a general WLAN system, a client such as an STA requests information from a server, and the server generally transmits information (data) to the STA in response to the request. . At this time, the server providing the information may be viewed as a machine that collects and provides the information mechanically, and the subject receiving the information may be a user using the client. Due to such structural characteristics, communication technology in the downlink direction has been mainly developed in the existing WLAN system.

On the other hand, in the WLAN system that supports M2M, the above structure is reversed. In other words, the client, which is a device, collects and provides information, and the user who manages the server may have a status of requesting information. That is, in the M2M-enabled WLAN system, the M2M server issues a command related to the measurement of the surrounding environment to the M2M STA, and the M2M STAs generally perform an operation according to the command and report the collected information to the server. Unlike in the past, the user accesses the network on the server side and the flow of communication is reversed, which is a structural feature of the M2M supporting WLAN system.

In the WLAN environment as described above, a STA may be provided with a power save mechanism to avoid unnecessary awake state and to switch to the awake state in order to receive the buffered frame when it is confirmed that there is a buffered frame. .

The STA may transmit / receive the frame based on the power save mechanism may be performed based on the TIM protocol as shown in FIGS. 4 to 9. According to the TIM protocol, the AP transmits a data frame after receiving a PS poll frame from the STA. In this case, the AP may transmit one buffered frame, that is, a PSDU in response to the PS poll frame. On the other hand, in an environment where there is a lot of buffered traffic for the STA, it is not efficient for the AP to transmit only one buffered frame in response to the PS poll frame.

As a method for supplementing the above problems, U-APSD may be applied to a frame transmission / reception method based on the TIM protocol. The STA may receive at least one frame from the AP during a service period for itself.

10 is a diagram illustrating an example of a frame transmission and reception method based on a TIM protocol and a U-APSD.

Referring to FIG. 10, an STA in a sleep state enters an awake state to receive a TIM element (S1011).

The STA receives the TIM element (S1012). The TIM element may be included in the beacon frame and transmitted. Upon receiving the TIM element, the terminal may determine whether a bufferable frame for itself is buffered based on the bitmap sequence of the partial virtual bitmap field included in the TIM element and the AID of the STA.

After confirming that the buffered frame exists, the STA enters the sleep state again (S1013).

When the buffered frame is desired to be transmitted, the STA enters into the awake state again and acquires a channel access right through contention (S1021). The STA acquires the channel access right and transmits a trigger frame to inform that the service interval for the STA has been started (S1022).

The AP transmits an ACK frame to the STA in response to the trigger frame (S1023).

The AP may perform an RTS / CTS exchange procedure to transmit the buffered frame within the service interval. The AP acquires a channel access right through competition in order to transmit the RTS frame (S1031). The AP transmits the RTS frame to the STA (S1032), and the STA transmits the CTS frame to the AP in response (S1033).

The AP transmits at least one data frame related to the buffered frame at least once after the RTS / CTS exchange (S1041, S1042, S1043). When the AP finally transmits a frame and transmits by setting (EOSP) in the QoS service field of the frame to '1', the STA may recognize that the last frame is received and the service interval ends.

The STA transmits an ACK frame to the AP in response to at least one frame received at the end of the service interval (S1050). In this case, the ACK frame may be a block ACK as a reception acknowledgment for a plurality of frames. The STA that transmits the ACK frame enters the sleep state (S1060).

According to the frame transmission / reception method described above with reference to FIG. 10, the STA may start a service interval at a desired time point and receive at least one frame during one service interval. Therefore, efficiency can be improved in terms of traffic processing.

In the frame transmission / reception method for an STA operating in the power save mode described above, when STAs intended to receive a buffered frame from an AP have a hidden node relationship, a collision may occur when a channel is accessed for a buffered frame request.

11 is a diagram illustrating an example of a collision that may occur when transmitting and receiving a frame.

Referring to FIG. 11, it is assumed that STA1 and STA2 are associated with an AP, and STA1 and STA2 may transmit and receive frames with the AP, respectively, but are in a hidden node relationship where transmission and reception of radio signals, including frame transmission and reception, are impossible. .

The STA1 and the STA2 operating in the power save mode receive the TIM element from the AP (S1110). The TIM element may be included in the beacon frame and broadcast from the AP. Upon reception of the TIM element, STA1 and STA2 determine whether a bufferable frame for themselves is buffered based on the bitmap sequence of the partial virtual bitmap field included in the TIM element and the AID assigned by each STA from the AP. do. In this example, it is assumed that there are buffered frames for STA1 and STA2.

The STA1 attempts a channel access to request transmission of the buffered frame (S1121). STA2 also attempts channel access to request transmission of the buffered frame (S1122). STA1 and STA2 may perform a channel access attempt after waiting a specific time (e.g. DIFS) after receiving a TIM element.

STA1 and STA2 perform contention for channel access. STA1 and STA2 may respectively set a backoff timer, and the backoff timer may be set randomly. In this example, it is assumed that STA1 sets the backoff timer to '4' and STA2 sets the backoff timer to '6'.

STA1 and STA2 may sense the channel during slot time to reduce the backoff timer by one when the idle state is maintained, and transmit a PS-pole frame when the backoff timer reaches zero. The slot time is a channel idle time unit required to reduce the backoff timer during contention. Therefore, STA1 first accesses the channel and transmits a PS poll frame (S1131).

Even though STA1 transmits a PS poll frame, STA2, which is a hidden node of STA1, cannot receive (or over hi) the PS poll frame transmitted by STA1 and determines that the channel is idle even through channel sensing during slot time. can do. Accordingly, the STA2 may later transmit a PS poll frame to the AP when the value of the backoff timer is reduced to '0' (S1132).

As described above, although STA1 and STA2 perform channel access through contention to prevent collisions, the PS-pole frames transmitted by the STAs collide with each other.

In order to prevent such a collision, a slot time associated with a backoff timer during contention for channel access may be applied to a value larger than the time for PS-pole frame transmission. Such an example may refer to FIG. 12.

12 illustrates an example of a channel access method for frame transmission and reception according to an embodiment of the present invention.

Referring to FIG. 12, it is assumed that STA1 and STA2 are associated with an AP, and STA1 and STA2 may transmit and receive frames with the AP, respectively, but are in a hidden node relationship where transmission and reception of a radio signal, including frame transmission and reception, are impossible. .

The STA1 and the STA2 operating in the power save mode receive the TIM element from the AP (S1210). The TIM element may be included in the beacon frame and broadcast from the AP. Upon reception of the TIM element, STA1 and STA2 determine whether a bufferable frame for itself is buffered based on the bitmap sequence of the partial virtual bitmap field included in the TIM element and the AID to which each STA is assigned APfhqnxj. . In this example, it is assumed that there are buffered frames for STA1 and STA2.

STA1 attempts a channel access to request transmission of a buffered frame (S1221). The channel access attempt by the STA1 may be performed after waiting for a specific time (e.g. DIFS) after receiving the TIM element. STA1 performs contention for channel access. STA1 may set a backoff timer, and the backoff timer may be set randomly. In this example, it is assumed that STA1 sets the backoff timer to '1'. The STA1 senses the channel during slot time to confirm that the channel is in an idle state, and decreases the backoff timer to '0'. Accordingly, the STA1 transmits a PS-poll frame to the AP to request transmission of the buffered frame (S1222).

The AP receives and responds to the PS-poll frame transmitted from STA1 (S1230). The AP may transmit a buffered frame to STA1 or an ACK frame to STA1 in response to the PS-poll frame.

Meanwhile, the STA2 also waits for a specific time (e.g. DIFS) after receiving the TIM element and attempts to access a channel (S1240). STA2 performs contention for channel access. In this example, it is assumed that STA2 sets the backoff timer to '2'. The STA2 senses the channel during the slot time, confirms that the channel is in the idle state, and decreases the backoff timer to '1' (S1241). Subsequently, the STA2 senses the channel during the slot time (S1242). Although STA1 approaches the channel and transmits a PS-poll frame, STA1 determines that the channel is idle because it does not receive the PS-poll frame. However, when the response is initiated by the AP, it is determined that the channel is occupied from the corresponding section. Therefore, STA2 does not reduce the backoff timer for the slot time where STA1 occupies the channel. Therefore, collision between STA1 and STA2 can be prevented.

In the aforementioned example of FIG. 12, the slot time is set longer than the transmission time of the PS-pole frame, thereby preventing collision between two STAs in a hidden node relationship. In this example, the length of the extended slot time may be set as in Equation 1 below.

Here, T _{Slot_Time} represents the length of the slot time, T _PS-poll represents the transmission time of the _PS- pole frame, T _{CCA_of_Response_Frame} represents the CCA detection time for the response frame of the AP, and T _{Air_Propagation_Delay} represents the propagation time.

By extending the length of the slot time as described above, the collision between STAs of the hidden node relationship could be solved. However, the extended length slot time has a problem that the time consumed by the contention process performed by the STA may increase. Therefore, a method for increasing slot time but reducing time consumed by the contention procedure may be additionally required.

The time spent in the contention process may depend on the backoff setting as well as the length of the slot time. STAs that perform contention may set the same backoff timer value. Each STA detects that the channel is in an idle state every slot time, and reduces the backoff timer value. In this case, the backoff timers of the STAs may be simultaneously reduced to '0'. Accordingly, STAs attempt to access a channel at the same time, which causes an STA-to-STA collision. If a collision occurs, the STAs set the backoff timer again to perform the contention, thereby increasing the contention time. That is, due to the same backoff value set by the STAs, the contention execution time of the STAs may be long.

Therefore, in order to apply the extended time slot time, but to prevent a collision due to simultaneous channel access, the value of the initial backoff timer determined by the STA may be uniquely set for each STA. The value of the initial backoff timer set for each STA may be determined according to the position of a bit associated with a specific STA in bitmap type information (virtual bitmap field) of the TIM element. On the other hand, since the location of the bit associated with a particular STA in the bitmap type information is determined by the AID of the STA, the value of the backoff timer may be determined by the AID of the STA. A method of performing contention based on an initial backoff timer set to a unique value as described above may refer to FIG. 13.

13 is a diagram illustrating an example of a method for transmitting / receiving a unique backoff timer based frame according to an embodiment of the present invention.

Referring to FIG. 13, an AP is combined with STA1, STA2, and STA3 and has a buffered frame for each STA. In addition, it is assumed that each STA is in a hidden node relationship with each other, and transmission and reception with the AP is possible.

Each STA sets its own backoff timer value. In this example, STA1 is set to '1', STA2 is set to '2', and STA3 is set to '3' as the backoff timer. The backoff timer setting value may be determined by the location of the bit associated with the corresponding STA included in the virtual bitmap field of the TIM element received by each STA.

The STA1, the STA2, and the STA3 may receive the TIM element transmitted from the AP and recognize that there is a buffered frame for each STA (S1310). In transmitting the TIM element, the AP may configure NAV for unpaged STAs that are not related to the transmission and reception of the buffered frame during the period for the transmission and reception of the buffered frame. In this way, transmission and reception of a buffered frame may be prevented through channel access and / or channel access attempt by non-paged STAs.

The STA1 that sets the initial backoff timer value to '1' determines that the channel is idle through carrier sensing during slot time 0 and decreases the backoff timer value to '0'. Therefore, the STA1 transmits a PS-poll frame requesting transmission of the frame buffered at the slot time 1 to the AP (S1320). The AP transmits the buffered frame for the STA1 to the STA1 in response to the PS-poll frame transmitted from the STA1 (S1330). In this example, it is assumed that the AP transmits a buffered frame to STA1 until slot time3.

STA2 and STA3 having initial backoff timer values set to '2' and '3' determine that the channel is idle during slot time 0, and decrease the backoff timer values to '1' and '2', respectively. STA2 and STA3 cannot receive (or overhear) the PS-poll frame transmitted by STA1 during the slot time1 period and recognize that the channel is idle during the corresponding time, but the buffered frame transmitted from the AP to STA1 By detecting the associated radio signal it is possible to recognize that the channel is occupied. Thus, STA2 and STA3 do not reduce the backoff timer until slot time3.

After the frame buffered from the AP to STA1 ends, the STA2 determines that the channel is idle through carrier sensing during the slot time 4 period, and decreases the backoff timer value to '0'. Therefore, STA2 transmits a PS-poll frame requesting transmission of the frame buffered at slot time 5 to the AP (S1340). The AP may transmit an ACK frame to the STA2 in response to the PS-poll frame transmitted from the STA2 (S1350). This means that the AP's response to STA1 was an immediate response, whereas the AP's response to STA2 was a delayed response.

During the slot time 4 period, the STA3 determines that the channel is idle through carrier sensing and decreases the backoff timer value to '1'. STA3 cannot receive (or overhear) the PS-poll frame transmitted by STA2 during the slot time 5 period, and recognizes that the channel is idle during the corresponding time period, but is associated with the ACK frame transmitted from the AP to STA2. By sensing the signal, we can recognize that the channel is occupied. Accordingly, STA3 does not decrease the backoff timer value for slot time 5 and 6 intervals.

During the slot time7 period, the STA3 determines that the channel is idle through carrier sensing and decreases the backoff timer value to '0'. Accordingly, STA3 transmits a PS-poll frame requesting transmission of the buffered frame at slot time 8 to the AP (S1360). The AP may transmit an ACK frame to the STA3 in response to the PS-poll frame transmitted from the STA3 (S1370). This means that the AP's response to STA3 is a delayed response.

In the situation as illustrated in FIG. 13, when STA2 does not receive a TIM element and thus does not transmit a PS-poll frame, or STA2 receives a TIM element but there is no buffered frame for STA2, STA2 Assume a communication state where no NAV is set. In this case, the STA3 may determine that the channel is idle during the slot time 5 period, reduce the backoff timer to '0', and transmit a PS-pole frame to the AP at slot time 6.

According to the embodiment of the present invention described above with reference to FIG. 13, scheduling of PS-poll frame transmission of paged STAs may be supported unless the environment is overlapped BSS (OBSS). In addition, even when an error occurs in TIM decoding or in other exceptional circumstances, a buffered frame is transmitted through transmission of a scheduled PS-pole frame, but collision between STAs may be avoided.

In the example of FIG. 13, if there is no NAV setting for nonpaged STAs, it is possible for nonpaged STAs to access the channel and transmit data. In this case, the non-paged STAs may access the channel between channel access periods of the paged STAs, thereby delaying transmission of the PS-pol frame of the paged STAs. That is, PS-pol frame transmission of paged STAs may be scheduled as much as possible, and channel access and data transmission of non-paged STAs may be performed based on contention.

The method of setting the initial backoff timer value based on the partial virtual bitmap of the TIM element may be determined based on the ordered AID order index. The detailed method according to this may be divided into a scheme considering both paged STAs and non-paged STAs and a scheme considering only paged STAs.

1) A scheme that considers both paged and nonpaged STAs

The initial backoff timer value may be sequentially set regardless of whether the bit value indicated in the partial virtual bitmap is '1 (paged)' or '0 (not paged)'.

For example, when the bitmap for STA0 to STA9 is set to '1 0 0 1 1 1 0 1 0 0', the initial backoff timers of STA0 to STA9 are 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9 can be set. In this way, if the initial backoff timer is set in consideration of the non-paged STA and the NAV is set for the non-paged STAs, this is due to a dummy initial value that will not be used for the non-paged STAs. There may be transmission waste during the interval. That is, the non-paged STAs do not access the channel during the NAV period, but due to the initial backoff timer values assigned to the STAs, a large initial backoff value may be allocated to a specific paged STA. As a result, even when the channel is continuously idle, the STA may not access the channel until the backoff timer is reduced to '0', and there may be a waste of radio resources. Thus, when the present example is applied, it may be more efficient that NAV is not set for non-paged STAs.

2) Method considering only paged STAs

The initial backoff timer value may be mapped only to STAs whose value indicated in the partial virtual bitmap corresponds to '1 (paged)'.

For example, when the bitmaps for STA0 to STA9 are set to '1 0 0 1 1 1 0 1 0 0', the initial backoff timer values for STA 0, 3, 4, 5, and 7 are respectively 0, It can be set to 1, 2, 3, 4. In case of setting the initial backoff timer considering only the paged STA, setting the NAV to the non-paged STAs may be advantageous in terms of channel access efficiency and buffered frame processing efficiency of the paged STAs.

If the initial backoff timer value is determined based on the order of inclusion in the partial virtual bitmap field indicating whether a buffered frame exists for the corresponding STA, it is always fixed until the AID value of the STA is changed. The timer value is set. Accordingly, a STA assigned a small AID may set a small initial backoff timer to access a channel relatively before other STAs. On the other hand, a STA that has been assigned a large AID may set a large initial backoff timer to access the channel later than other STAs. This can cause fairness issues related to channel access. In order to solve this problem, the initial backoff timer value may be flexibly applied, which may be performed as follows.

First, a method in which the initial backoff timer value is changed according to a specific transmission time, for example, according to a target beacon transmission time (TBTT) or a multiple of the TBTT, may be proposed. The manner in which the initial backoff timer value is changed may include at least one of randomization / permutation, cyclic shift, and reverse. Each process may be a time synchronization function (TSF) given in a beacon frame or a predetermined value. TSF time information may be as shown in Table 2 below.

Name	Type	Effective range	meaning
Result code (Result Code)	List (Enumeration)	Success / failure (SUCCESS / FAILURE)	Report the result of MLME-GETTSFTIME.request primitive
TSF time	essence (integer)	0- (2 64 -1)	TSF timer value, present if result code is SUCCESS

The proposed scheme may be applied only to paged STAs and may be applied to paged STAs and non-paged STAs.

Randomization / permutation

For example, when determining the initial backoff timer based only on the paging STA, STA 0, 3, 4, 5, and 7 when the virtual bitmap field is '1 0 0 1 1 1 0 1 0 0'. The initial backoff timer for may be set as 0, 1, 2, 3, 4 at beacon frame transmission time (eg TSF time) N, respectively. Next N + 1 (e.g. next TSF time) may be set to be substituted as 3, 1, 4, 0, 2. The polynomials and / or matrices applied for the substitution may be implemented through a pseudo-random generator whose seed value is the beacon frame transmission time. For an example of the permutation mat (P) applied in the above example can refer to the following equation (2).

Cyclic shift

In the above example, if the initial backoff timer value is set for STA 0 to STA 4 such as 0, 1, 2, 3, 4 at transmission time N (eg TSF time), 1 at transmission time N + 1 , 2, 3, 4, 0 may be left-cyclic shifted (left-cyclic shifted) or 4, 0, 1, 2, 3 may be right-cyclic shifted (right-cyclic shifted).

More generalizing the above example, if the bitmap sequence length L of the partial virtual bitmap field is referred to as M, and the rearrangement results for M paged STAs are 0 to M-1, the transmission time N (eg TSF). An initial backoff time of M STAs may be expressed as Equations 3 and 4 below.

Reverse operation

In the above example, if the initial backoff timer value is set to STA 0 to STA 5 in the transmission time N (eg TSF time), such as '0, 1, 2, 3, 4', 4, 3, 2, 1 , May be expressed as 0.

More generalized, if the bitmap sequence length of the partial virtual bitmap field is L, and the result of rearranging the M-paged STAs from 0 to M-1 is 0 to M-1, the transmission time N (eg TSF time) An initial backoff timer value of M STAs may be represented as follows.

If N is even: [0 1 2 3 4], if N is odd: [4 3 2 1 0] or if N is even: [4 3 2 1 0], if N is odd: [0 1 2 3 4]

Meanwhile, the initial backoff timer value may be set based on the priority of the STA. The STA may have a different priority according to each class or service type. The STA having a higher priority may be configured to set a large value of the initial backoff timer, and the STA having a higher priority may be configured to set a small value of the initial backoff timer.

In the above, in order to maintain fairness in obtaining channel access rights of STAs having a frame buffered through the TIM element, a mechanism for setting initial backoff timers of respective STAs has been discussed. This method of uniquely setting the initial backoff timer is not only in the environment in which the slot time is set longer than the transmission time of the PS-pole frame as in the above-described embodiment, but also as in the conventional case, the slot time of the PS-pole frame is It can be applied even in an environment that is set shorter than the transmission time. In this case, a time point at which each STA initiates a backoff through contention is uniquely set for each STA, and the aforementioned scheme may be applied. Such an embodiment will be described with reference to FIG. 14.

14 is a diagram illustrating an example of a channel access method according to another embodiment of the present invention.

Referring to FIG. 14, timings at which contented STAs start contention to transmit a PS-poll frame requesting transmission of a buffered frame are different from each other. The contention start time of each STA may be set based on a scheme in which the initial backoff timer value of each STA is uniquely set for each STA. Through this, each of the paged STAs performs contention at the start time set to their own values after receiving the TIM element to acquire channel access authority and then transmits a PS-poll frame to the AP. Meanwhile, before the start time set to a unique value for each paged STA arrives, the corresponding STA may operate in a sleep state. According to this, the time from the reception of the TIM element to the start of contention may be referred to as a sleep interval.

15 is a block diagram illustrating a wireless device in which an embodiment of the present invention may be implemented.

Referring to FIG. 15, a wireless device 1500 includes a processor 1510, a memory 1520, and a transceiver 1530. The transceiver 1530 transmits and / or receives a radio signal, but implements a physical layer of IEEE 802.11. The processor 1510 may be configured to be functionally connected to and operate with the transceiver 1530. The processor 1510 may be configured to perform a power save mode operating method and a frame transmission / reception method according to an embodiment of the present invention. The processor 1510 may be configured to transmit a data by inserting a pilot tone in a symbol as in the embodiment of the present invention described above with reference to FIGS. 11 to 19. The processor 1510 may be set to implement an embodiment according to the accompanying drawings.

The processor 1510 and / or transceiver 1530 may include an application-specific integrated circuit (ASIC), another chipset, a logic circuit, and / or a data processing device. When the embodiment is implemented in software, the above-described technique may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in the memory 1520 and executed by the processor 1510. The memory 1520 may be included in the processor 1510 and may be functionally connected to the processor 1510 through various known means that are separately located outside the processor 1510.

In the exemplary system described above, the methods are described based on a flowchart as a series of steps or blocks, but the invention is not limited to the order of steps, and certain steps may occur in a different order or concurrently with other steps than those described above. Can be. In addition, those skilled in the art will appreciate that the steps shown in the flowcharts are not exclusive and that other steps may be included or one or more steps in the flowcharts may be deleted without affecting the scope of the present invention.

Claims (18)

1. In a frame transmitting and receiving method performed by a station (STA) operating in a power save mode in a WLAN system,
   Acquiring channel access authority based on at least one slot time, wherein each slot time is a unit time for the channel to remain idle for channel access of the STA;
   Send a Power Save Poll frame to the AP to request transmission of the buffered frame; And
   Receiving a response frame from the AP in response to the PS-poll frame,
   The length of each slot time is set longer than the transmission time of the power save poll frame.
2. The method of claim 1 wherein the method
   And setting a back-off timer that is the number of the at least one slot time.
3. The method of claim 2, wherein the method
   Receiving a TIM (Traffic Indication Map) element;
   The TIM element comprises a bitmap sequence,
   The specific bit of the bitmap sequence indicates whether the buffered frame for the STA is present.
4. The method of claim 3, wherein
   And a value of the backoff timer is determined based on an order in the bitmap sequence of the specific bit.
5. The method of claim 3, wherein
   And a value of the backoff timer is determined based on an order in the bitmap sequence of the specific bit and a time point when the STA sets the backoff timer.
6. The method of claim 3, wherein
   The value of the backoff timer is frame transmission and reception, characterized in that determined based on the order (order) of the specific bit of the at least one bit indicating that there is a buffered frame for a particular STA of the bitmap sequence Way.
7. The method of claim 3, wherein
   The value of the backoff timer is based on an order of the specific bit among the one or more bits indicating that there is a buffered frame for the specific STA in the bitmap sequence and a time point for setting the backoff timer. Frame transmission and reception method, characterized in that determined by.
8. The method of claim 1,
   And the response frame is the buffered frame.
9. The method of claim 1,
   And the response frame is an acknowledgment (ACK) frame.
10. In a wireless device operating in a power save mode in a WLAN system, the wireless device,
    A transceiver for transmitting and receiving wireless signals; And
    And a processor operatively coupled with the transceiver to operate,
    The processor,
    Acquire channel access rights based on at least one slot time, wherein each slot time is a unit time for which a channel is idle for channel access of the wireless device,
    Transmit a Power Save Poll frame to the Access Point (AP) requesting transmission of the buffered frame, and
    Set to receive a response frame from the AP in response to the PS-pol frame,
    The length of each slot time is set longer than the transmission time of the power save poll frame.
11. The method of claim 10, wherein the processor
    And set a back-off timer that is a number of the at least one slot time.
12. The method of claim 11, wherein the processor,
    Set to receive TIM (Traffic Indication Map) elements,
    The TIM element comprises a bitmap sequence,
    A specific bit of the bitmap sequence indicates the presence or absence of the buffered frame for the wireless device.
13. The method of claim 12,
    And the value of the backoff timer is determined based on an order in the bitmap sequence of the particular bit.
14. The method of claim 12,
    The value of the backoff timer is determined based on an order in the bitmap sequence of the particular bit, the wireless protocol, and a time point for setting the backoff timer.
15. The method of claim 12,
    The value of the backoff timer is determined based on an order of the specific bit among at least one bit indicating that a buffered frame for at least one wireless device of the bitmap sequence exists. Wireless device.
16. The method of claim 12,
    The value of the backoff timer sets an order of the particular bit and at least one of the at least one bits indicating that a buffered frame for at least one wireless device of the bitmap sequence exists. The wireless device, characterized in that determined based on the viewpoint.
17. The method of claim 10,
    The response frame is the buffered frame.
18. The method of claim 10,
    And the response frame is an acknowledgment (ACK) frame.

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