Patent Publication Number: US-2022240339-A1

Title: Methods and systems for low power reconnection

Description:
BACKGROUND 
     Artificial reality such as a virtual reality (VR), an augmented reality (AR), or a mixed reality (MR) provides immersive experience to a user. In one example, a user wearing a head wearable display (HWD) can turn the user&#39;s head, and an image of a virtual object corresponding to a location of the HWD and a gaze direction of the user can be displayed on the HWD to allow the user to feel as if the user is moving within a space of artificial reality (e.g., a VR space, an AR space, or a MR space). 
     In one implementation, an image of a virtual object is generated by a computing device communicatively coupled to the HWD. In one example, the HWD includes various sensors that detect a location and/or orientation of the HWD, and transmits the detected location and/or orientation of the HWD to the computing device. The computing device can determine a user&#39;s view of the space of the artificial reality according to the detected location and/or orientation of the HWD, and generate image data indicating an image of the space of the artificial reality corresponding to the user&#39;s view. The computing device can transmit the image data to the HWD, according to which the image of the space of the artificial reality corresponding to the user&#39;s view can be presented to the user. In one aspect, the process of detecting the location of the HWD and the gaze direction of the user wearing the HWD, and rendering the image to the user should be performed within a frame time (e.g., 11 ms or 16 ms). Any latency between a movement of the user wearing the HWD and an image displayed corresponding to the user movement can cause judder, which may result in motion sickness and can degrade the user experience. 
     SUMMARY 
     Various embodiments disclosed herein are related to a method of resuming wireless communication between a first device and a second device. In some embodiments, the method includes entering, by a first device, a wake up mode from a sleep mode. In some embodiments, the method includes obtaining, by the first device, in response to entering the wake up mode, stored information indicating a wireless channel previously used to communicate with a second device, to monitor for a beacon frame from the second device. In some embodiments, the method includes receiving, by the first device, the beacon frame from the second device, according to the stored information. In some embodiments, the method includes transmitting, by the first device to the second device, an association frame in response to receiving the beacon frame. 
     In some embodiments, the association frame allows the second device to perform association with the first device to resume wireless communication. In some embodiments, the stored information further indicates a target beacon transmission time of the second device. In some embodiments, the method includes predicting, by the first device according to the stored information, a first time at which the second device is scheduled to transmit the beacon frame. In some embodiments, the first device enters the wake up mode at a second time before the predicted first time. 
     In some embodiments, the first device is configured to monitor for the association frame while bypassing channel scanning. In some embodiments, the method includes generating the stored information based on the wireless channel previously used to communicate with the second device. In some embodiments, the first device comprises a head wearable device, and the second device comprises a soft access point. 
     Various embodiments disclosed herein are related to a first device comprising a wireless interface and one or more processors coupled to the wireless interface. In some embodiments, the wireless interface is configured to communicate with a second device through a wireless communication link. In some embodiments, the one or more processors are configured to cause the wireless interface to enter a wake up mode from a sleep mode. In some embodiments, the one or more processors are configured to obtain, in response to entering the wake up mode, stored information indicating a wireless channel previously used to communicate with a second device to monitor for a beacon frame from the second device. In some embodiments, the one or more processors are configured to cause the wireless interface to receive the beacon frame from the second device, according to the stored information. In some embodiments, the one or more processors are configured to cause the wireless interface to transmit an association frame to the second device, in response to receiving the beacon frame. 
     In some embodiments, the association frame allows the second device to perform association with the first device to resume wireless communication. In some embodiments, the stored information further indicates a target beacon transmission time of the second device. In some embodiments, the one or more processors are configured to predict, according to the stored information, a first time at which the second device is scheduled to transmit the beacon frame. In some embodiments, the one or more processors are configured to cause the wireless interface to enter the wake up mode at a second time before the predicted first time. 
     In some embodiments, the one or more processors are configured to monitor for the association frame while bypassing a channel scanning. In some embodiments, the one or more processors are configured to generate the stored information based on the wireless channel previously used to communicate with the second device. In some embodiments, the first device comprises a head wearable display, and the second device comprises a soft access point. 
     Various embodiments disclosed herein are related to a first device including a wireless interface and one or more processors coupled to the wireless interface. In some embodiments, the wireless interface is configured to communicate with a second device through a wireless communication link. In some embodiments, the one or more processors are configured to determine a time period, and cause the wireless interface to periodically switch between a wake up mode and a sleep mode according to the time period, until an association is completed with the second device to resume wireless communication of data. In some embodiments, the time period is set according to a listening interval of the second device. In some embodiments, the one or more processors are configured to cause, for each switch to the wake up mode from the sleep mode according to the time period, the wireless interface to transmit an association frame to the second device according to stored information to establish the wireless communication link with the second device. 
     In some embodiments, the second device is configured to switch between another wake up mode and another sleep mode of the second device. In some embodiments, the second device is configured to receive the association frame while the second device is in the another wake up mode during the listening interval. In some embodiments, the one or more processors are configured to determine the time period to allow at least one switch to the wake up mode from the sleep mode to occur while the second device is in the another wake up mode. In some embodiments, the second device is configured to receive the association frame, while bypassing use of beacon frames. 
     In some embodiments, the stored information indicates a wireless channel previously used to communicate with the second device. In some embodiments, the one or more processors are configured to cause, for each switch to the wake up mode from the sleep mode according to the time period, the wireless interface to transmit the association frame to the second device through the wireless channel previously used to communicate with the second device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. Like reference numbers and designations in the various drawings indicate like elements. For purposes of clarity, not every component can be labeled in every drawing. 
         FIG. 1  is a diagram of a system environment including an artificial reality system, according to an example implementation of the present disclosure. 
         FIG. 2  is a diagram of a head wearable display, according to an example implementation of the present disclosure. 
         FIG. 3  is a timing diagram showing a process of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. 
         FIG. 4  is a flowchart showing a process of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. 
         FIG. 5  is a timing diagram showing a process of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. 
         FIG. 6  is a flowchart showing a process of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. 
         FIG. 7  is a block diagram of a computing environment according to an example implementation of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Before turning to the figures, which illustrate certain embodiments in detail, it should be understood that the present disclosure is not limited to the details or methodology set forth in the description or illustrated in the figures. It should also be understood that the terminology used herein is for the purpose of description only and should not be regarded as limiting. 
     Disclosed herein are related to systems and methods for resuming a wireless communication between a first device and a second device. The first device and the second device may be mobile devices. In one aspect, the first device and the second device may periodically enter (e.g., switch between) a sleep mode and a wake up mode. In the wake up mode, the first device, the second device, or both may communicate with each other and generate and store the re-association information. The re-association information may indicate, for example, a channel (e.g., frequency band) and/or timing (e.g., synchronization, clock information) of the communication between the first device and the second device. In the sleep mode, the first device and the second device may be disabled (e.g., in inactive or low power operation) to conserve power. The first device, the second device, or both may attempt to resume communication in a subsequent wake up mode, according to the re-association information. For example, the first device, the second device, or both may initiate communication according to the channel used in the previous communication session. For example, the first device, the second device, or both may schedule and may enter the subsequent wake up mode, according to the timing of the previous communication session. In one aspect, the first device and the second device may resume communication based on the re-association information without performing a scan procedure. Scan procedure may involve searching for available channels and/or performing handshake and/or authentication. Bypassing the scan procedure allows the first device and the second device to resume communication with a reduced latency and can achieve power savings. 
     In one aspect, the first device may be a HWD, and the second device may be an intermediate device between an access point and the first device. The first device may operate as a station device for a communication link (interlink) between the second device and the access point. The second device may receive content for artificial reality from the access point. The second device may also operate as a soft access point (e.g., a software enabled access point) for a communication link (intralink) between the second device and the first device. In one example, the first device detects a location and/or orientation of the first device, and transmits the detected location and/or orientation of the first device to the second device through a wireless connection. The second device can determine a user&#39;s view of the space of the artificial reality according to the detected location and/or orientation of the first device, and can generate image data indicating an image of the space of the artificial reality corresponding to the user&#39;s view. The second device can transmit the image data to the first device, by which the image of the space of the artificial reality corresponding to the user&#39;s view can be presented to the user. After transmitting the image data, the first device and the second device may enter a sleep mode to conserve power, until a next frame time (e.g., 11 ms or 16 ms). In one aspect, the first device and the second device may resume communication based on the re-association information (e.g., retrieved from storage) without performing a scan procedure, such that time/latency for communication between the first device and the second device to render artificial reality can be reduced. Hence, artificial reality can be presented in a seamless manner with reduced latency by obviating the scan procedure. Moreover, time duration for the first device and the second device operating in the sleep mode can be increased to achieve power savings. 
       FIG. 1  is a block diagram of an example artificial reality system environment  100 . In some embodiments, the artificial reality system environment  100  includes an access point (AP)  105 , one or more HWDs  150  (e.g., HWD  150 A,  150 B), and one or more computing devices  110  (computing devices  110 A,  110 B) providing data for artificial reality to the one or more HWDs  150 . The access point  105  may be a router or any network device allowing one or more computing devices  110  and/or one or more HWDs  150  to access a network (e.g., the Internet). The access point  105  may be replaced by any communication device (cell site). A computing device  110  may be a computing device or a mobile device that can retrieve content from the access point  105 , and provide image data of artificial reality to a corresponding HWD  150 . Each HWD  150  may present the image of the artificial reality to a user according to the image data. In some embodiments, the artificial reality system environment  100  includes more, fewer, or different components than shown in  FIG. 1 . In some embodiments, the computing devices  110 A,  110 B communicate with the access point  105  through wireless links  102 A,  102 B (e.g., interlinks), respectively. In some embodiments, the computing device  110 A communicates with the HWD  150 A through a wireless link  125 A (e.g., intralink), and the computing device  110 B communicates with the HWD  150 B through a wireless link  125 B (e.g., intralink). In some embodiments, functionality of one or more components of the artificial reality system environment  100  can be distributed among the components in a different manner than is described here. For example, some of the functionality of the computing device  110  may be performed by the HWD  150 . For example, some of the functionality of the HWD  150  may be performed by the computing device  110 . 
     In some embodiments, the HWD  150  is an electronic component that can be worn by a user and can present or provide an artificial reality experience to the user. The HWD  150  may be referred to as, include, or be part of a head mounted display (HMD), head mounted device (HMD), head wearable device (HWD), head worn display (HWD) or head worn device (HWD). The HWD  150  may render one or more images, video, audio, or some combination thereof to provide the artificial reality experience to the user. In some embodiments, audio is presented via an external device (e.g., speakers and/or headphones) that receives audio information from the HWD  150 , the computing device  110 , or both, and presents audio based on the audio information. In some embodiments, the HWD  150  includes sensors  155 , a wireless interface  165 , a processor  170 , and a display  175 . These components may operate together to detect a location of the HWD  150  and a gaze direction of the user wearing the HWD  150 , and render an image of a view within the artificial reality corresponding to the detected location and/or orientation of the HWD  150 . In other embodiments, the HWD  150  includes more, fewer, or different components than shown in  FIG. 1 . 
     In some embodiments, the sensors  155  include electronic components or a combination of electronic components and software components that detects a location and an orientation of the HWD  150 . Examples of the sensors  155  can include: one or more imaging sensors, one or more accelerometers, one or more gyroscopes, one or more magnetometers, or another suitable type of sensor that detects motion and/or location. For example, one or more accelerometers can measure translational movement (e.g., forward/back, up/down, left/right) and one or more gyroscopes can measure rotational movement (e.g., pitch, yaw, roll). In some embodiments, the sensors  155  detect the translational movement and the rotational movement, and determine an orientation and location of the HWD  150 . In one aspect, the sensors  155  can detect the translational movement and the rotational movement with respect to a previous orientation and location of the HWD  150 , and determine a new orientation and/or location of the HWD  150  by accumulating or integrating the detected translational movement and/or the rotational movement. Assuming for an example that the HWD  150  is oriented in a direction 25 degrees from a reference direction, in response to detecting that the HWD  150  has rotated 20 degrees, the sensors  155  may determine that the HWD  150  now faces or is oriented in a direction 45 degrees from the reference direction. Assuming for another example that the HWD  150  was located two feet away from a reference point in a first direction, in response to detecting that the HWD  150  has moved three feet in a second direction, the sensors  155  may determine that the HWD  150  is now located at a vector multiplication of the two feet in the first direction and the three feet in the second direction. 
     In some embodiments, the wireless interface  165  includes an electronic component or a combination of an electronic component and a software component that communicates with the computing device  110 . In some embodiments, the wireless interface  165  includes or is embodied as a transceiver for transmitting and receiving data through a wireless medium. The wireless interface  165  may communicate with a wireless interface  115  of a corresponding computing device  110  through a wireless link  125  (e.g., intralink). The wireless interface  165  may also communicate with the access point  105  through a wireless link (e.g., interlink). Examples of the wireless link  125  include a near field communication link, Wi-Fi direct, Bluetooth, or any wireless communication link. Through the wireless link  125 , the wireless interface  165  may transmit to the computing device  110  data indicating the determined location and/or orientation of the HWD  150 , the determined gaze direction of the user, and/or hand tracking measurement. Moreover, through the wireless link  125 , the wireless interface  165  may receive from the computing device  110  image data indicating or corresponding to an image to be rendered. 
     In some embodiments, the processor  170  includes an electronic component or a combination of an electronic component and a software component that generates one or more images for display, for example, according to a change in view of the space of the artificial reality. In some embodiments, the processor  170  is implemented as one or more graphical processing units (GPUs), one or more central processing unit (CPUs), or a combination of them that can execute instructions to perform various functions described herein. The processor  170  may receive, through the wireless interface  165 , image data describing an image of artificial reality to be rendered, and render the image through the display  175 . In some embodiments, the image data from the computing device  110  may be encoded, and the processor  170  may decode the image data to render the image. In some embodiments, the processor  170  receives, from the computing device  110  through the wireless interface  165 , object information indicating virtual objects in the artificial reality space and depth information indicating depth (or distances from the HWD  150 ) of the virtual objects. In one aspect, according to the image of the artificial reality, object information, depth information from the computing device  110 , and/or updated sensor measurements from the sensors  155 , the processor  170  may perform shading, reprojection, and/or blending to update the image of the artificial reality to correspond to the updated location and/or orientation of the HWD  150 . 
     In some embodiments, the display  175  is an electronic component that displays an image. The display  175  may, for example, be a liquid crystal display or an organic light emitting diode display. The display  175  may be a transparent display that allows the user to see through. In some embodiments, when the HWD  150  is worn by a user, the display  175  is located proximate (e.g., less than  3  inches) to the user&#39;s eyes. In one aspect, the display  175  emits or projects light towards the user&#39;s eyes according to image generated by the processor  170 . The HWD  150  may include a lens that allows the user to see the display  175  in a close proximity. 
     In some embodiments, the processor  170  performs compensation to compensate for any distortions or aberrations. In one aspect, the lens introduces optical aberrations such as a chromatic aberration, a pin-cushion distortion, barrel distortion, etc. The processor  170  may determine a compensation (e.g., predistortion) to apply to the image to be rendered to compensate for the distortions caused by the lens, and apply the determined compensation to the image from the processor  170 . The processor  170  may provide the predistorted image to the display  175 . 
     In some embodiments, the computing device  110  (sometimes referred to as a stage device, or an AR/VR computing device) is an electronic component or a combination of an electronic component and a software component that provides content to be rendered to the HWD  150 . The computing device  110  may be embodied as a mobile device (e.g., smart phone, tablet PC, laptop, etc.). The computing device  110  may operate as a soft access point. In one aspect, the computing device  110  includes a wireless interface  115  and a processor  118 . These components may operate together to determine a view (e.g., a FOV of the user) of the artificial reality corresponding to the location of the HWD  150  and the gaze direction of the user of the HWD  150 , and can generate image data indicating an image of the artificial reality corresponding to the determined view. The computing device  110  may also communicate with the access point  105 , and may obtain AR/VR content from the access point  105 , for example, through the wireless link  102  (e.g., interlink). The computing device  110  may receive sensor measurement indicating location and the gaze direction of the user of the HWD  150  and provide the image data to the HWD  150  for presentation of the artificial reality, for example, through the wireless link  125  (e.g., intralink). In other embodiments, the computing device  110  includes more, fewer, or different components than shown in  FIG. 1 . 
     In some embodiments, the wireless interface  115  is an electronic component or a combination of an electronic component and a software component that communicates with the HWD  150 , the access point  105 , other computing device  110 , or any combination of them. In some embodiments, the wireless interface  115  includes or is embodied as a transceiver for transmitting and receiving data through a wireless medium. The wireless interface  115  may be a counterpart component to the wireless interface  165  to communicate with the HWD  150  through a wireless link  125  (e.g., intralink). The wireless interface  115  may also include a component to communicate with the access point  105  through a wireless link  102  (e.g., interlink). Examples of wireless link  102  include a cellular communication link, a near field communication link, Wi-Fi, Bluetooth, 60 GHz wireless link, or any wireless communication link. The wireless interface  115  may also include a component to communicate with a different computing device  110  through a wireless link  185 . Examples of the wireless link  185  include a near field communication link, Wi-Fi direct, Bluetooth, or any wireless communication link. Through the wireless link  102  (e.g., interlink), the wireless interface  115  may obtain AR/VR content, or other content from the access point  105 . Through the wireless link  125  (e.g., intralink), the wireless interface  115  may receive from the HWD  150  data indicating the determined location and/or orientation of the HWD  150 , the determined gaze direction of the user, and/or the hand tracking measurement. Moreover, through the wireless link  125  (e.g., intralink), the wireless interface  115  may transmit to the HWD  150  image data describing an image to be rendered. Through the wireless link  185 , the wireless interface  115  may receive or transmit information indicating the wireless link  125  (e.g., channel, timing) between the computing device  110  and the HWD  150 . According to the information indicating the wireless link  125 , computing devices  110  may coordinate or schedule operations to avoid interference or collisions. 
     The processor  118  can include or correspond to a component that generates content to be rendered according to the location and/or orientation of the HWD  150 . In some embodiments, the processor  118  includes or is embodied as one or more central processing units, graphics processing units, image processors, or any processors for generating images of the artificial reality. In some embodiments, the processor  118  may incorporate the gaze direction of the user of the HWD  150  and a user interaction in the artificial reality to generate the content to be rendered. In one aspect, the processor  118  determines a view of the artificial reality according to the location and/or orientation of the HWD  150 . For example, the processor  118  maps the location of the HWD  150  in a physical space to a location within an artificial reality space, and determines a view of the artificial reality space along a direction corresponding to the mapped orientation from the mapped location in the artificial reality space. The processor  118  may generate image data describing an image of the determined view of the artificial reality space, and transmit the image data to the HWD  150  through the wireless interface  115 . The processor  118  may encode the image data describing the image, and can transmit the encoded data to the HWD  150 . In some embodiments, the processor  118  generates and provides the image data to the HWD  150  periodically (e.g., every 11 ms or 16 ms). 
     In some embodiments, the processors  118 ,  170  may configure or cause the wireless interfaces  115 ,  165  to toggle, transition, cycle or switch between a sleep mode and a wake up mode. In the wake up mode, the processor  118  may enable the wireless interface  115  and the processor  170  may enable the wireless interface  165 , such that the wireless interfaces  115 ,  165  may exchange data. In the sleep mode, the processor  118  may disable (e.g., implement low power operation in) the wireless interface  115  and the processor  170  may disable the wireless interface  165 , such that the wireless interfaces  115 ,  165  may not consume power or may reduce power consumption. The processors  118 ,  170  may schedule the wireless interfaces  115 ,  165  to switch between the sleep mode and the wake up mode periodically every frame time (e.g., 11 ms or 16 ms). For example, the wireless interfaces  115 ,  165  may operate in the wake up mode for 2 ms of the frame time, and the wireless interfaces  115 ,  165  may operate in the sleep mode for the remainder (e.g., 9 ms) of the frame time. By disabling the wireless interfaces  115 ,  165  in the sleep mode, power consumption of the computing device  110  and the HWD  150  can be reduced. In some embodiments, the processors  118 ,  170  may configure or cause the wireless interfaces  115 ,  165  to resume communication based on stored information indicating communication between the computing device  110  and the HWD  150 . In the wake up mode, the processors  118 ,  170  may generate and store information (e.g., channel, timing) of the communication between the computing device  110  and the HWD  150 . The processors  118 ,  170  may schedule the wireless interfaces  115 ,  165  to enter a subsequent wake up mode according to timing of the previous communication indicated by the stored information. For example, the wireless interfaces  115 ,  165  may predict when to enter the subsequent wake up mode, according to timing of the previous wake up mode, and schedule to enter the subsequent wake up mode at the predicted time. After generating and storing the information and scheduling the subsequent wake up mode, the processors  118 ,  170  may configure or cause the wireless interfaces  115 ,  165  to enter the sleep mode. When entering the wake up mode, the processors  118 ,  170  may cause or configure the wireless interfaces  115 ,  165  to resume communication via the channel or frequency band of the previous communication indicated by the stored information. Accordingly, the wireless interfaces  115 ,  165  entering the wake up mode from the sleep mode may resume communication, while bypassing a scan procedure to search for available channels and performing handshake or authentication. Bypassing the scan procedure allows extension of a duration of the wireless interfaces  115 ,  165  operating in the sleep mode, such that the computing device  110  and the HWD  150  can reduce power consumption. Detailed descriptions on resuming communication based on stored information are provided below with respect to  FIGS. 3 through 6  below. 
     In some embodiments, the computing devices  110 A,  110 B may coordinate operations to reduce collisions or interferences. In one approach, the computing device  110 A may transmit a beacon frame periodically to announce or advertise a presence of a wireless link  125 A between the computing device  110 A and the HWD  150 A and coordinate the communication between the computing device  110 A and the HWD  150 A. The computing device  110 B may receive the beacon frame from the computing device  110 A, and may schedule communication with the HWD  150 B to avoid collision or interference with the computing device  110 A and the HWD  150 A. For example, the computing device  110 B may schedule the computing device  110 B and the HWD  150 B to enter a wake up mode, when the computing device  110 A and the HWD  150 A operate in the sleep mode. For example, the computing device  110 B may schedule the computing device  110 B and the HWD  150 B to enter a sleep up mode, when the computing device  110 A and the HWD  150 A operate in the wake up mode. Accordingly, multiple computing devices  110  and HWDs  150  in proximity (e.g., within  10  feet) may coexist and operate with reduced interference. 
       FIG. 2  is a diagram of a HWD  150 , in accordance with an example embodiment. In some embodiments, the HWD  150  includes a front rigid body  205  and a band  210 . The front rigid body  205  includes the display  175  (not shown in  FIG. 2 ), the lens (not shown in  FIG. 2 ), the sensors  155 , the wireless interface  165 , and the processor  170 . In the embodiment shown by  FIG. 2 , the wireless interface  165 , the processor  170 , and the sensors  155  are located within the front rigid body  205 , and may not visible to the user. In other embodiments, the HWD  150  has a different configuration than shown in  FIG. 2 . For example, the wireless interface  165 , the processor  170 , and/or the sensors  155  may be in different locations than shown in  FIG. 2 . 
       FIG. 3  is a timing diagram  300  showing a process of resuming communication between a computing device  310  and a station device  320 , according to an example implementation of the present disclosure. The computing device  310  may be the computing device  110  and the station device  320  may be the HWD  150  of  FIG. 1 . The computing device  310  and the station device  320  may be mobile devices. In one approach, the computing device  310  may periodically enter or switch between a sleep mode and a wake up mode. In the wake up mode, the computing device  310  may enable wireless interface (e.g., wireless interface  115 ) for association with the station device  320 . In one aspect, the computing device  310  and the station device  320  can exchange data after performing device association. In the sleep mode, the computing device  310  may disable the wireless interface (e.g., wireless interfaces  115 ) to conserve power. 
     In one approach, during beacon transmission time intervals  330 A,  330 B,  330 C, the computing device  310  may operate in a wake up mode periodically (e.g., every 300 ms) and can transmit beacon frames. A beacon frame may announce/advertise/provide a SSID and/or data rate of a wireless link, and can indicate a timing (e.g., target beacon transmission time (TBTT) and/or a duration) of transmission of the beacon frame. A duration of each beacon transmission time interval  330  may be 1˜2 ms. 
     In one approach, when in communication with the computing device  310 , the station device  320  may generate and store re-association information. Re-association information may indicate a channel and/or timing of the communication with the computing device  310 . In one example, the station device  320  may configure, predict or estimate a timing of a subsequent beacon transmission time interval  330 , according to the re-association information, and can schedule to enter a wake up mode at or before the predicted timing of the subsequent beacon transmission time interval  330 . In the wake up mode, the station device  320  may monitor for a beacon frame from the computing device  310 . The station device  320  may monitor for the beacon frame through a channel or a frequency band specified by the re-association information. In response to receiving the beacon frame, the station device  320  may transmit an association frame. The association frame may include SSID of the wireless link, MAC address of the station device  320 , and security settings. 
     After transmitting the beacon frame, the computing device  310  may operate in the wake up mode and monitor for an association frame from the station device  320  for a listening time interval  335 . Duration of each listening time interval  335  may be 2˜3 ms. In response to receiving the association frame within the listening time interval  335 , the computing device  310  may perform association with the station device  320  to exchange data. In response to not receiving an association frame within the listening time interval, the computing device  310  may enter the sleep mode and disable the wireless interface until the subsequent beacon transmission time interval  330 . 
     Advantageously, the station device  320  may resume communication with the computing device  310  based on the re-association information without performing a scan procedure. A scan procedure may involve searching for available channels, and performing handshake and/or authentication. By bypassing the scan procedure, the computing device  310  and the station device  320  may resume communication with a reduced latency and achieve power savings. 
     In the example shown in  FIG. 3 , the computing device  310  may transmit a beacon frame during the beacon transmission time interval  330 A, and can monitor for an association frame from the station device  320  during the listening time interval  335 A. In the example shown in  FIG. 3 , the station device  320  may be disabled or may operate in the sleep mode during the beacon transmission time interval  330 A, and may not receive the beacon frame transmitted during the beacon transmission time interval  330 A. Because the station device  320  in the sleep mode may not receive the beacon frame during the beacon transmission time interval  330 A, the station device  320  may not transmit an association frame during the listening time interval  335 A. Accordingly, the computing device  310  may not receive the association frame from the station device  320  during the listening time interval  335 A, and enter the sleep mode after the listening time interval  335 A. 
     In the example shown in  FIG. 3 , the station device  320  may be scheduled to enter a wake up mode and can monitor, during a beacon detection period  340 , for a beacon frame from the computing device  310 . The station device  320  may predict the beacon transmission time interval  330 B according to the stored re-association information, and can schedule the beacon detection period  340  according to the predicted beacon transmission time interval  330 B to receive the beacon frame. For example, the beacon detection period  340  may be determined to begin a predetermined time interval (e.g., 1 ms) before the predicted timing of the start of the beacon transmission time interval  330 B. In response to receiving the beacon frame from the computing device  310 , the station device  320  may transmit an association frame during the association frame transmission time interval  345 . In response to the beacon frame transmitted during the beacon transmission time interval  330 B, the computing device  310  may receive the association frame from the station device  320  during the listening time interval  335 B, and may perform association with station device  320  according to the association frame. According to the association with the station device  320 , the computing device  310  may operate in the wake up mode during the communication time interval  370  until the subsequent beacon transmission time interval  330 C to exchange data with the station device  320 . For example, the computing device  310  may receive, from the station device  320 , sensor measurements indicating location and orientation of the station device  320 , and can provide image data indicating a view of artificial reality corresponding to the location and orientation of the station device  320 . The station device  320  may render artificial reality according to the image data. 
       FIG. 4  is a flowchart showing a process  400  of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. In some embodiments, the process  400  is performed by the station device (e.g., station device  320  or HWD  150 ). In some embodiments, the process  400  is performed by other entities. In some embodiments, the process  400  includes more, fewer, or different steps than shown in  FIG. 4 . 
     In one approach, the station device schedules  410  a wake up. In one approach, the station device predicts/determines when the subsequent beacon transmission will occur, based on re-association information. The station device may predict/determine when the subsequent beacon transmission will/can occur based on timing of the previous beacon transmission time. For example, the station device may predict the subsequent beacon transmission will occur at a beacon transmission time period (e.g., 300 ms) or a multiple of the beacon transmission time period from the timing of the previous beacon transmission time. The station device may schedule to enter the wake up mode at the predicted time or a predetermined time (e.g., 1˜2 ms) before the predicted time. After scheduling the wake up, the station device may enter  420  a sleep mode. In the sleep mode, the station device may disable (e.g., reduce operations in) a wireless interface (e.g., wireless interface  165 ) to reduce power consumption. The station device may enter  430  a wake up mode at the scheduled time. 
     In one approach, the station device receives  440  a beacon frame from the computing device  110  during a listening time interval after entering the wake up mode. A beacon frame may announce a SSID and/or a data rate of a wireless link, and/or may indicate a timing (e.g., target beacon transmission time (TBTT) and/or a duration) of transmission of the beacon frame. The station device may obtain re-association information, and can monitor for the beacon frame according to the re-association information. The re-association information may indicate a TBTT and/or channel of previous communication with the computing device. The station device may generate and can store the re-association information during the previous communication with the computing device. The station device may monitor a channel indicated by the re-association information to receive the beacon frame. In one aspect, the station device may receive the beacon frame without a scan procedure. A scan procedure may involve searching for available channels and/or performing handshake or authentication. Bypassing the scan procedure allows the computing device and the station device to resume communication with a reduced latency and can achieve power savings. 
     In one approach, the station device transmits  450  an association frame in response to the beacon frame. The association frame may include a SSID of the wireless link, a MAC address of the station device  320 , and/or security settings. In one approach, the station device performs  460  association (e.g., handshaking and/or connection/linking) with the computing device and can exchange data with the computing device according to the association. For example, the computing device and the station device may exchange data for presenting artificial reality according to the association between the computing device and the station device. For example, the computing device may receive, from the station device, sensor measurements indicating location and orientation of the station device, and can provide image data indicating a view of artificial reality corresponding to the location and orientation of the station device according to the association between the computing device and the station device. The station device  320  may render artificial reality according to the image data. 
       FIG. 5  is a timing diagram showing a process  500  of resuming communication between a computing device  510  and a station device  520 , according to an example implementation of the present disclosure. The computing device  510  may be the computing device  110  and the station device  520  may be the HWD  150  of  FIG. 1 . The computing device  510  and the station device  520  may be mobile devices. In one approach, the computing device  510  may periodically enter a sleep mode and a wake up mode. In the wake up mode, the computing device  510  may enable wireless interface (e.g., wireless interface  115 ) for an association with the station device  520 . In the sleep mode, the computing device  510  may disable the wireless interface (e.g., wireless interface  115 ) to conserve power. 
     In one approach, the computing device  510  may periodically enter a wake up mode and a sleep mode every association frame listening time interval. The association frame may include a SSID of the wireless link, a MAC address of the station device  520 , and/or security settings. For example, the computing device  510  may periodically (e.g., every 100 ms) enable the wireless interface  115  during association frame listening time intervals  535 A,  535 B,  555 C to monitor for an association frame from the station device  520 . Duration of each association frame listening time interval  535  may be 10˜20 ms. The computing device  510  may enable the wireless interface  115  during the association frame listening time intervals  535 A,  535 B,  555 C without transmitting any beacon frame. 
     In one example, the station device  520  may determine a time period, and can schedule the wireless interface (e.g., wireless interface  165 ) of the station device  520  to periodically switch between a wake up mode and a sleep mode according to the time period. In the wake up mode, the station device  520  may transmit an association frame during an association frame transmission time interval  540 , and can monitor for an acknowledge frame from the computing device  510  for an acknowledgement frame listening time interval  545  after the association frame transmission time interval  540 . Duration of the association frame transmission time interval  540  and the acknowledgement frame listening time interval  545  may be 1˜2 ms. In response to receiving the acknowledgement frame, the station device  520  may perform association with the computing device  510 , and can exchange data with the computing device  510  according to the association. In response to not receiving the acknowledgement frame, the station device  520  may enter a sleep mode until a subsequent association frame transmission time interval  540  according to the determined time period. 
     In one aspect, the station device  520  may configure, set or determine the time period to be equal to or less than the duration of the association frame listening time interval  535  (e.g., 10˜20 ms). Hence, at least one switch to the wake up mode from the sleep mode of the station device  520  may occur while the computing device  510  is in the wake up mode in the association frame listening time interval  535 . Accordingly, the computing device  510  may receive an association frame from the station device  520  during the association frame listening time interval  535 , and can perform association with the station device  520  to resume communication with the station device  520 . 
     In one aspect, the station device  520  may resume communication with the computing device  510  based on the re-association information without performing a scan procedure. For example, the computing device  510  may transmit an association frame, according to a channel or a frequency band specified by the re-association information. The scan procedure may involve searching for available channels and/or performing handshake or authentication. By bypassing the scan procedure, the computing device  510  and the station device  520  may resume communication with a reduced latency and achieve power savings. 
     In the example shown in  FIG. 5 , the computing device  510  may enter a wake up mode to monitor for an association frame during the association frame listening time interval  535 A. In the example shown in  FIG. 5 , the station device  520  may be disabled or may operate in the sleep mode during the association frame listening time interval  535 A, and may not transmit the association frame during the association frame listening time interval  535 A. Accordingly, the computing device  510  may not receive the association frame from the station device  520  during the association frame listening time interval  535 A, and may enter the sleep mode after the association frame listening time interval  535 A. 
     In the example shown in  FIG. 5 , the station device  520  may be scheduled to enter a wake up mode and transmit, during an association frame transmission time interval  540 , an association frame. After transmitting the association frame, the station device  520  may monitor, for an acknowledgement frame from the computing device  510  during the acknowledgement frame listening time interval  545 . The station device  520  may switch between the sleep mode and the wake up mode periodically (e.g., every time period or every 10˜20 ms), until receiving the acknowledgement frame from the computing device  510 . For example, the computing device  510  may receive the association frame from the station device  520  during the association frame listening time interval  535 B, and can transmit an acknowledgement frame to confirm receipt of the association frame. The station device  520  may receive the acknowledgement frame from the computing device  510  during an acknowledgement frame listening time interval  545 , and can perform association with the computing device in response to the acknowledgement frame. According to the association, the computing device  510  and the station device  520  may resume communication to exchange data. For example, the computing device  510  may receive, from the station device  520 , sensor measurements indicating location and orientation of the station device  520 , and provide image data indicating a view of artificial reality corresponding to the location and orientation of the station device  320  according to the association. The station device  320  may render artificial reality according to the image data. 
       FIG. 6  is a flowchart showing a process  600  of resuming communication between a computing device and a station device, according to an example implementation of the present disclosure. In some embodiments, the process  600  is performed by the station device (e.g., station device  520  or HWD  150 ). In some embodiments, the process  600  is performed by other entities. In some embodiments, the process  600  includes more, fewer, or different steps than shown in  FIG. 6 . 
     In one approach, the station device detects  610  a loss of association or loss of connection with the computing device. The station device may detect the loss of association or loss of connection with the computing device, in response to detecting unsuccessful communication with the computing device. 
     In response to detecting the loss of association with the computing device, the station device may enter  620  or schedule to enter a wake up mode. The station device may operate in the wake up mode for an association frame transmission time interval to transmit an association frame. Duration of the association frame transmission time interval may be 1˜2 ms. In one example, the station device may schedule to enter the wake up mode, according to a time period. The time period may be equal to or less than a duration (e.g., 10˜20 ms) of a listening time interval of the computing device. The computing device may periodically switch between the wake up mode and the sleep mode, and monitor for an association frame from the station device. 
     In one approach, the station device may transmit an association frame during the association frame transmission time interval. The association frame may include a SSID of the wireless link, a MAC address of the station device  320 , and/or security settings. In response to the association frame, the computing device may perform association with the computing device, and can transmit an acknowledge frame in response to receipt of the association frame. 
     In one approach, the station device may determine  640  whether association with the computing device is successful or not. For example, the station device may monitor for an acknowledgement frame from the computing device for an acknowledgement frame listening time period after transmitting the association frame. Duration of the acknowledgement frame listening time period may be 1˜2 ms. 
     In response to not receiving the acknowledgement frame during the acknowledge frame listening time period, the station device may determine that the association with the computing device is unsuccessful, and may enter  650  a sleep mode until the subsequent association frame transmission time interval according to the time period. When the subsequent association frame transmission time interval occurs, the station device may proceed to the step  620 . 
     In response to receiving the acknowledgement frame during the acknowledge frame listening time period, the station device may determine that the association with the computing device is successful, and can resume  660  communication with the computing device according to the association. For example, the station device may transmit sensor measurements indicating a location and/or an orientation of the station device and receive image data indicating a view of artificial reality corresponding to the location and orientation of the station device according to the association. 
     In one aspect, the station device may resume communication with the computing device based on the re-association information without performing a scan procedure. For example, the computing device may transmit an association frame, according to a channel or a frequency band specified by the re-association information. The scan procedure may involve searching for available channels and/or performing handshake or authentication. By bypassing the scan procedure, the computing device and the station device may resume communication with a reduced latency and achieve power savings. 
     Various operations described herein can be implemented on computer systems.  FIG. 7  shows a block diagram of a representative computing system  714  usable to implement the present disclosure. In some embodiments, the computing device  110 , the HWD  150  or both of  FIG. 1  are implemented by the computing system  714 . Computing system  714  can be implemented, for example, as a consumer device such as a smartphone, other mobile phone, tablet computer, wearable computing device (e.g., smart watch, eyeglasses, head wearable display), desktop computer, laptop computer, or implemented with distributed computing devices. The computing system  714  can be implemented to provide VR, AR, MR experience. In some embodiments, the computing system  714  can include conventional computer components such as processors  716 , storage device  718 , network interface  720 , user input device  722 , and user output device  724 . 
     Network interface  720  can provide a connection to a wide area network (e.g., the Internet) to which WAN interface of a remote server system is also connected. Network interface  720  can include a wired interface (e.g., Ethernet) and/or a wireless interface implementing various RF data communication standards such as Wi-Fi, Bluetooth, or cellular data network standards (e.g., 3G, 4G, 5G, 60 GHz, LTE, etc.). 
     User input device  722  can include any device (or devices) via which a user can provide signals to computing system  714 ; computing system  714  can interpret the signals as indicative of particular user requests or information. User input device  722  can include any or all of a keyboard, touch pad, touch screen, mouse or other pointing device, scroll wheel, click wheel, dial, button, switch, keypad, microphone, sensors (e.g., a motion sensor, an eye tracking sensor, etc.), and so on. 
     User output device  724  can include any device via which computing system  714  can provide information to a user. For example, user output device  724  can include a display to display images generated by or delivered to computing system  714 . The display can incorporate various image generation technologies, e.g., a liquid crystal display (LCD), light-emitting diode (LED) including organic light-emitting diodes (OLED), projection system, cathode ray tube (CRT), or the like, together with supporting electronics (e.g., digital-to-analog or analog-to-digital converters, signal processors, or the like). A device such as a touchscreen that function as both input and output device can be used. Output devices  724  can be provided in addition to or instead of a display. Examples include indicator lights, speakers, tactile “display” devices, printers, and so on. 
     Some implementations include electronic components, such as microprocessors, storage and memory that store computer program instructions in a computer readable storage medium (e.g., non-transitory computer readable medium). Many of the features described in this specification can be implemented as processes that are specified as a set of program instructions encoded on a computer readable storage medium. When these program instructions are executed by one or more processors, they cause the processors to perform various operation indicated in the program instructions. Examples of program instructions or computer code include machine code, such as is produced by a compiler, and files including higher-level code that are executed by a computer, an electronic component, or a microprocessor using an interpreter. Through suitable programming, processor  716  can provide various functionality for computing system  714 , including any of the functionality described herein as being performed by a server or client, or other functionality associated with message management services. 
     It will be appreciated that computing system  714  is illustrative and that variations and modifications are possible. Computer systems used in connection with the present disclosure can have other capabilities not specifically described here. Further, while computing system  714  is described with reference to particular blocks, it is to be understood that these blocks are defined for convenience of description and are not intended to imply a particular physical arrangement of component parts. For instance, different blocks can be located in the same facility, in the same server rack, or on the same motherboard. Further, the blocks need not correspond to physically distinct components. Blocks can be configured to perform various operations, e.g., by programming a processor or providing appropriate control circuitry, and various blocks might or might not be reconfigurable depending on how the initial configuration is obtained. Implementations of the present disclosure can be realized in a variety of apparatus including electronic devices implemented using any combination of circuitry and software. 
     Having now described some illustrative implementations, it is apparent that the foregoing is illustrative and not limiting, having been presented by way of example. In particular, although many of the examples presented herein involve specific combinations of method acts or system elements, those acts and those elements can be combined in other ways to accomplish the same objectives. Acts, elements and features discussed in connection with one implementation are not intended to be excluded from a similar role in other implementations or implementations. 
     The hardware and data processing components used to implement the various processes, operations, illustrative logics, logical blocks, modules and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. A processor also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some embodiments, particular processes and methods may be performed by circuitry that is specific to a given function. The memory (e.g., memory, memory unit, storage device, etc.) may include one or more devices (e.g., RAM, ROM, Flash memory, hard disk storage, etc.) for storing data and/or computer code for completing or facilitating the various processes, layers and modules described in the present disclosure. The memory may be or include volatile memory or non-volatile memory, and may include database components, object code components, script components, or any other type of information structure for supporting the various activities and information structures described in the present disclosure. According to an exemplary embodiment, the memory is communicably connected to the processor via a processing circuit and includes computer code for executing (e.g., by the processing circuit and/or the processor) the one or more processes described herein. 
     The present disclosure contemplates methods, systems and program products on any machine-readable media for accomplishing various operations. The embodiments of the present disclosure may be implemented using existing computer processors, or by a special purpose computer processor for an appropriate system, incorporated for this or another purpose, or by a hardwired system. Embodiments within the scope of the present disclosure include program products comprising machine-readable media for carrying or having machine-executable instructions or data structures stored thereon. Such machine-readable media can be any available media that can be accessed by a general purpose or special purpose computer or other machine with a processor. By way of example, such machine-readable media can comprise RAM, ROM, EPROM, EEPROM, or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to carry or store desired program code in the form of machine-executable instructions or data structures and which can be accessed by a general purpose or special purpose computer or other machine with a processor. Combinations of the above are also included within the scope of machine-readable media. Machine-executable instructions include, for example, instructions and data which cause a general purpose computer, special purpose computer, or special purpose processing machines to perform a certain function or group of functions. 
     The phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” “comprising” “having” “containing” “involving” “characterized by” “characterized in that” and variations thereof herein, is meant to encompass the items listed thereafter, equivalents thereof, and additional items, as well as alternate implementations consisting of the items listed thereafter exclusively. In one implementation, the systems and methods described herein consist of one, each combination of more than one, or all of the described elements, acts, or components. 
     Any references to implementations or elements or acts of the systems and methods herein referred to in the singular can also embrace implementations including a plurality of these elements, and any references in plural to any implementation or element or act herein can also embrace implementations including only a single element. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements to single or plural configurations. References to any act or element being based on any information, act or element can include implementations where the act or element is based at least in part on any information, act, or element. 
     Any implementation disclosed herein can be combined with any other implementation or embodiment, and references to “an implementation,” “some implementations,” “one implementation” or the like are not necessarily mutually exclusive and are intended to indicate that a particular feature, structure, or characteristic described in connection with the implementation can be included in at least one implementation or embodiment. Such terms as used herein are not necessarily all referring to the same implementation. Any implementation can be combined with any other implementation, inclusively or exclusively, in any manner consistent with the aspects and implementations disclosed herein. 
     Where technical features in the drawings, detailed description or any claim are followed by reference signs, the reference signs have been included to increase the intelligibility of the drawings, detailed description, and claims. Accordingly, neither the reference signs nor their absence have any limiting effect on the scope of any claim elements. 
     Systems and methods described herein may be embodied in other specific forms without departing from the characteristics thereof. References to “approximately,” “about” “substantially” or other terms of degree include variations of +/−10% from the given measurement, unit, or range unless explicitly indicated otherwise. Coupled elements can be electrically, mechanically, or physically coupled with one another directly or with intervening elements. Scope of the systems and methods described herein is thus indicated by the appended claims, rather than the foregoing description, and changes that come within the meaning and range of equivalency of the claims are embraced therein. 
     The term “coupled” and variations thereof includes the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent or fixed) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members coupled directly with or to each other, with the two members coupled with each other using a separate intervening member and any additional intermediate members coupled with one another, or with the two members coupled with each other using an intervening member that is integrally formed as a single unitary body with one of the two members. If “coupled” or variations thereof are modified by an additional term (e.g., directly coupled), the generic definition of “coupled” provided above is modified by the plain language meaning of the additional term (e.g., “directly coupled” means the joining of two members without any separate intervening member), resulting in a narrower definition than the generic definition of “coupled” provided above. Such coupling may be mechanical, electrical, or fluidic. 
     References to “or” can be construed as inclusive so that any terms described using “or” can indicate any of a single, more than one, and all of the described terms. A reference to “at least one of ‘A’ and  13 ” can include only ‘A’, only  13 ′, as well as both ‘A’ and  13 ′. Such references used in conjunction with “comprising” or other open terminology can include additional items. 
     Modifications of described elements and acts such as variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations can occur without materially departing from the teachings and advantages of the subject matter disclosed herein. For example, elements shown as integrally formed can be constructed of multiple parts or elements, the position of elements can be reversed or otherwise varied, and the nature or number of discrete elements or positions can be altered or varied. Other substitutions, modifications, changes and omissions can also be made in the design, operating conditions and arrangement of the disclosed elements and operations without departing from the scope of the present disclosure. 
     References herein to the positions of elements (e.g., “top,” “bottom,” “above,” “below”) are merely used to describe the orientation of various elements in the FIGURES. The orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure.