Patent Publication Number: US-8982774-B2

Title: Method for ranging to a station in power saving mode

Description:
TECHNICAL FIELD 
     The present embodiments relate generally to wireless networks, and specifically to determining the location of Wi-Fi enabled wireless devices. 
     BACKGROUND OF RELATED ART 
     The recent proliferation of Wi-Fi access points in wireless local area networks (WLANs) has made it possible for navigation systems to use these access points for position determination, especially in areas where there are a large concentration of active Wi-Fi access points (e.g., urban cores, shopping centers, office buildings, and so on). In WLAN positioning systems, the locations of Wi-Fi access points (APs) are used as reference points from which well-known trilateration techniques can determine the location of a mobile wireless device or station (e.g., a Wi-Fi-enabled cell phone, laptop, or tablet computer). For example, the wireless station (STA) can use the received signal strength indicators (RSSI) associated with a number of visible APs as indications of the distances between the mobile device and each of the detected APs, where a stronger RSSI means that the mobile device is closer to the AP and a weaker RSSI means that the mobile device is further from the AP. The STA can also use the round trip time (RTT) of signals transmitted to and from the APs to estimate the distances between the STA and the APs. Once the distances between the STA and at least three APs are calculated, the location of the STA relative to the APs can be determined using trilateration techniques. 
     WLAN positioning systems are typically controlled by a central server that can instruct APs associated with the WLAN to perform ranging operations with a STA and then report the resulting ranging measurements back to the server. For example, an AP can initiate a ranging operation with the STA by sending a NULL frame to the STA, which in response thereto sends an acknowledgement (ACK) frame back to the AP. The AP can use the difference between the time of departure (TOD) of the NULL frame and the time of arrival (TOA) of the ACK frame to calculate a round trip time (RTT) value of the exchanged NULL and ACK frames. Then, the RTT value can be correlated to a distance. 
     However, if the AP probes the STA when the STA is in a power save mode, the STA may not receive the NULL frame sent by the AP, in which case the STA will not respond with an ACK frame sent back to the AP. Thus, ranging operations may be difficult to initiate when the STA is in the power save mode. However, it is even more difficult for other APs not associated with the STA to initiate such ranging operations with the STA when the STA is in the power save mode. For example, according to the IEEE 802.11 family of standards, a STA can be “associated” with only one AP at any given time. Thus, the AP with which the STA currently has an established wireless communication channel or link is commonly referred to as the associated AP, and all other APs (which do not have currently have an established wireless communication channel or link with the STA) are commonly referred to as “non-associated” APs. Although these “non-associated” APs can spoof the MAC address of the associated AP and then send spoofed NULL frames to the STA to initiate ranging operations, these non-associated APs typically do not know whether the STA is in the power save mode. Thus, unless the STA coincidentally wakes up from power save mode precisely when these non-associated APs send spoofed NULL frames to the STA, the STA will not receive the spoofed NULL frames and, therefore, will not respond with ACK frames sent back to the spoofing AP. Accordingly, such spoofing techniques frequently fail to successfully initiate ranging operations with a STA that is in power save mode. 
     Thus, there is a need for WLAN positioning system to allow any of its APs to initiate ranging operations with the STA, regardless of whether the STA is in power save mode and regardless of whether the ranging AP is currently associated with the STA. 
     SUMMARY 
     A network based positioning (NBP) system is disclosed that solves the above-mentioned problems by allowing any of the system&#39;s access points (APs) to initiate ranging operations with a STA within range of the APs, even when the STA is in power save mode. In accordance with the present embodiments, the NBP system includes a server connected to a plurality of APs that together form a wireless local area network (WLAN). Each AP forms a basic service set (BSS) that provides one or more associated STAs with access to one or more networks (e.g., LAN, intranet, the Internet, and so on) connected to the WLAN. To initiate ranging operations with a STA that is associated with one of the WLAN&#39;s APs, the server can instruct one or more of the APs not associated with the STA to synchronize themselves with the timing and/or beacon transmission schedules of the associated AP. Once the non-associated APs are synchronized with the associated AP, the non-associated APs can determine when the associated AP is scheduled to transmit beacon frames to the STA and, perhaps more importantly, can also determine when the STA is scheduled to wake up from power save mode to listen for such beacon frames. Thereafter, the non-associated APs can probe the STA during the time periods that the STA is awake from power save mode, thereby ensuring that the STA is able to receive and respond to probes sent from the non-associated APs. Once ranging operations between the non-associated AP(s) and the STA are complete, the non-associated AP(s) can send the resulting ranging measurements to the WLAN server, which in turn can use trilateration techniques to determine the location of the STA. In this manner, the non-associated APs can initiate and successfully complete ranging operations with the STA even when the STA periodically enters and exits the power save mode. 
     Further, for some embodiments, the server can instruct (e.g., via one or more of its connected APs) the STA to stay awake for longer periods of time upon waking up from power save mode, thereby providing the STA with a longer time period to receive and respond to multiple probes sent to the STA from one or more different APs. For one embodiment, the associated AP can instruct the STA via its transmitted beacon frames that it has additional data frames buffered for the STA (e.g., even if it does not), thereby effectively keeping the STA awake longer than normal. For another embodiment, the associated AP can assert the “more data” bit in any of the data frame sent to the STA, which also causes the STA to stay awake for a longer period of time. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present embodiments are illustrated by way of example and are not intended to be limited by the figures of the accompanying drawings, where: 
         FIG. 1  is a block diagram of a WLAN positioning system within which the present embodiments can be implemented; 
         FIG. 2  is a block diagram of a wireless station (STA) in accordance with some embodiments; 
         FIG. 3  is a block diagram of a WLAN server in accordance with some embodiments; 
         FIG. 4  is a block diagram of an access point (AP) in accordance with some embodiments; 
         FIG. 5A  is a sequence diagram depicting an exemplary ranging operation in accordance with some embodiments; 
         FIG. 5B  is a timing diagram for the exemplary ranging operation depicted in  FIG. 5A ; and 
         FIGS. 6A-6D  are illustrative flow charts depicting ranging operations in accordance with the present embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present embodiments are described below in the context of ranging operations performed by and between Wi-Fi enabled devices for simplicity only. It is to be understood that the present embodiments are equally applicable for performing ranging operations using signals of other various wireless standards or protocols. As used herein, the terms WLAN and Wi-Fi can include communications governed by the IEEE 802.11 standards, Bluetooth, HiperLAN (a set of wireless standards, comparable to the IEEE 802.11 standards, used primarily in Europe), and other technologies having relatively short radio propagation range. Further, the term “associated AP” refers to an AP of a WLAN that currently has an established communication channel or link with a STA within range of the WLAN, and the term “non-associated AP” refers to an AP of the WLAN that does not currently have an established communication channel or link with the STA. 
     In the following description, numerous specific details are set forth such as examples of specific components, circuits, and processes to provide a thorough understanding of the present disclosure. The term “coupled” as used herein means connected directly to or connected through one or more intervening components or circuits. Also, in the following description and for purposes of explanation, specific nomenclature is set forth to provide a thorough understanding of the present embodiments. However, it will be apparent to one skilled in the art that these specific details may not be required to practice the present embodiments. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present disclosure. Any of the signals provided over various buses described herein may be time-multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit elements or software blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be a single signal line, and each of the single signal lines may alternatively be buses, and a single line or bus might represent any one or more of a myriad of physical or logical mechanisms for communication between components. The present embodiments are not to be construed as limited to specific examples described herein but rather to include within their scopes all embodiments defined by the appended claims. 
       FIG. 1  is a block diagram of a wireless network based positioning (NBP) system  100  in accordance with the present embodiments. The NBP system  100  is shown to include a wireless STA, a wireless local area network (WLAN)  120 , a WLAN server  130 , and an access point location server (APLS)  140 . The WLAN  120  is formed by a plurality of Wi-Fi access points (APs) that may operate according to the IEEE 802.11 family of standards (or according to other suitable wireless protocols). Although only three access points AP 1 -AP 3  are shown in  FIG. 1  for simplicity, it is to be understood that WLAN  120  can be formed by any number of access points. Each of access points AP 1 -AP 3  is assigned a unique MAC address (i.e., MAC 1 -MAC 3 , respectively) that is programmed therein by, for example, the manufacturer of the access point. Similarly, the STA is also assigned a unique MAC address (MAC_STA). Each MAC address, which may be commonly referred to as the “burned-in address,” the organizationally unique identifier (OUI), or the BSSID, in one embodiment includes six bytes (and thus 12 nibbles) of data. The first 3 bytes of the MAC address may identify which organization manufactured the device, and may be assigned to such organizations by the Institute of Electrical and Electronic Engineers (IEEE). The second 3 bytes of the MAC address, which may be referred to as the network interface controller (NIC) specific bytes, may be used to uniquely identify the individual device. 
     The WLAN server  130 , which is coupled to AP 1 -AP 3  over wired and/or wireless connections to control the operation of the APs, can instruct any of its associated APs to initiate ranging operations with the STA (e.g., to determine the location of the STA), regardless of whether a particular AP is currently associated with the STA and/or regardless of whether the STA avails itself of power save mode. More specifically, as described in more detail below, the WLAN server  130  can synchronize the transmission of probes from non-associated APs with the transmission of beacon frames from the associated AP so that the probes arrive at the STA during the time period that the STA wakes-up from power save mode to listen for the beacon frames broadcast by the associated AP. In addition, the WLAN server  130  can also coordinate the operation of one or more of its APs to keep the STA awake for longer time periods, thereby increasing the time period during which the STA is responsive to probes sent from one or more non-associated APs. 
     The APLS  140 , which may be accessible by the STA and/or the WLAN server  130 , includes a database that stores the MAC addresses and location coordinates of a plurality of deployed access points (e.g., not just access points AP 1 -AP 3  of  FIG. 1 ). The database (not shown for simplicity) associated with the APLS  140  may be provided by companies such as Google, Skyhook, Devicescape, and/or WiGLE. The APLS  140  may also store other information associated with the access points including, for example, the accuracy of the location coordinates of each access point, the last location update for each access point, the last time each access point was visible, the protocol version of each access point, and so on. For some embodiments, selected portions of the APLS  140  can be retrieved and stored within the STA. For other embodiments, the APLS  140  may be omitted. 
     The STA can be any suitable Wi-Fi enabled wireless device including, for example, a cell phone, PDA, tablet computer, laptop computer, or the like. For the embodiments described herein, the STA may include radio frequency (RF) ranging circuitry (e.g., formed using well-known software modules, hardware components, and/or a suitable combination thereof) that can be used to estimate the distance between itself and one or more visible access points (AP) using suitable ranging techniques. For example, the STA can use received signal strength indicator (RSSI) and/or round trip time (RTT) techniques to estimate the distance between itself and the access points AP 1 -AP 3 , for example, by correlating each RSSI or RTT value with a distance. In addition, the STA may include a local memory that stores a cache of Wi-Fi access point location data, and includes a processor that may execute WLAN positioning software, ranging software, APLS data retrieval software, and/or power save mode software, as described in more detail below. 
       FIG. 2  shows a STA  200  that is one embodiment of the STA of  FIG. 1 . The STA  200  includes a global navigation satellite system (GNSS) module  210 , a transmitter/receiver circuit  220 , a processor  230 , a memory  240 , and a scanner  250 . The transmitter/receiver circuit  220  can be used to transmit signals to and receive signals from access points AP 1 -AP 3  and/or APLS  140  (see also  FIG. 1 ). Scanner  250 , which is well-known, can be used to scan the surrounding environment to detect and identify nearby access points (e.g., access points within range of STA  200 ). For some embodiments, the scanner  250  can search for nearby access points by periodically transmitting MAC address request frames. An AP within range of STA  200  receives one or more of the requests and responds by transmitting its MAC address to the STA  200 . If the STA  200  has line-of-sight with a suitable number (e.g., 3 or more) of navigation satellites, the GNSS module  210  can determine the current location of the STA  200  using triangulation techniques, and can then provide the location information to processor  230  for storage in memory  240 . 
     Memory  240  may include an AP location table  242  that can be used as a local cache to store the MAC addresses of a plurality of APs, the location coordinates of such APs, and other suitable location or configuration information of the APs. For some embodiments, each entry of the AP location table  242  includes an access point field to store the name of the associated AP, a BSSID field to store the MAC address of the AP, a coordinate field to store the location coordinates of the AP, and an uncertainty field to store a location uncertainty value for the AP. 
     Memory  240  may also include a non-transitory computer-readable medium (e.g., one or more nonvolatile memory elements, such as EPROM, EEPROM, Flash memory, a hard drive, and so on) that can store the following software modules:
         a positioning and ranging software module  244  to determine the location of the STA based on locations of APs using, for example, trilateration techniques and/or to enable ranging operations with one or more APs of a corresponding WLAN (e.g., as described for operation  654  of  FIG. 6B );   a data retrieval software module  246  to query the APLS  140  for access point information; and   a power save software module  248  to selectively enter power save mode to reduce power consumption and to selectively wake-up from power save mode to listen for and receive beacon frames, probes, and/or other signals transmitted to the STA from one or more APs (e.g., as described for operations  664  and  674  of  FIGS. 6C and 6D , respectively).
 
Each software module includes instructions that, when executed by processor  230 , can cause the STA  200  to perform the corresponding functions. Thus, the non-transitory computer-readable medium of memory  240  can include instructions for performing all or a portion of the station-side operations of methods  650 ,  660 , and  670  of  FIGS. 6B ,  6 C, and  6 D, respectively.
       

     Processor  230 , which is coupled to transmitter/receiver circuit  220 , GNSS module  210 , memory  240 , and scanner  250 , can be any suitable processor capable of executing scripts or instructions of one or more software programs stored in the STA  200  (e.g., within memory  240 ). For example, processor  230  can execute WLAN positioning and ranging software module  244 , data retrieval software module  246 , and/or power save software module  248 . The positioning and ranging software module  244  can be executed by processor  230  to determine the location of the STA  200  using nearby APs as reference points. For example, to determine the position of the STA  200 , the precise locations of three selected APs (e.g., access points AP 1 -AP 3 ) are first determined, either by accessing their location coordinates from location table  242 , by retrieving their location coordinates from the APLS  140 , or by parsing AP location information embedded within signals transmitted by the APs. Then, positioning and ranging software module  244  as executed by processor  230  can estimate the distance between the STA  200  and each of the selected APs using suitable RF ranging techniques (e.g., RSSI and/or RTT techniques), and thereafter can use the location coordinates of the selected APs and the estimated distances between them and the STA  200  to calculate the position of the STA  200  using, for example, trilateration techniques. The positioning and ranging software module  244  can also be executed by processor  230  to initiate, respond to, and/or enable the STA  200  to perform or otherwise participate in ranging operations with the APs. 
     The data retrieval software module  246  can be executed by processor  230  to retrieve the location coordinates of one or more APs of interest from the APLS  140 , and to provide such location coordinates to location table  242  for storage and/or to positioning and ranging software module  244  for determining the location of the STA  200 . 
     The power save software module  248  can be executed by processor  230  to cause the STA  200  to enter power save mode, and to wake-up from power save mode (e.g., to listen for and receive beacon frames, probes, NULL frames, management frames, and/or other signals transmitted to the STA from one or more the APs). 
       FIG. 3  shows a WLAN server  300  that is one embodiment of WLAN server  130  of  FIG. 1 . WLAN server  300  includes a network interface  310 , a processor  320 , and a memory  330 . The network interface  310  can be used to communicate with APs associated with WLAN  120  (e.g., access points AP 1 -AP 3 ) either directly or via one or more intervening networks and to transmit signals, and can be used to communicate with an associated distributed service (DS) network including, for example, a private LAN, a virtual LAN, an intranet, the Internet, and the like. Processor  320 , which is coupled to network interface  310  and memory  330 , can be any suitable processor capable of executing scripts or instructions of one or more software programs stored in WLAN server  300  (e.g., within memory  330 ). 
     Memory  330  includes an AP location database  332  that stores the MAC addresses (e.g., BSSIDs) of a plurality of APs, the location coordinates of such APs, and other suitable location or configuration information of the APs. Memory  330  also includes a non-transitory computer-readable medium (e.g., one or more nonvolatile memory elements, such as EPROM, EEPROM, Flash memory, a hard drive, and so on) that can store the following software modules:
         a ranging software module  334  to instruct one or more AP(s) associated with WLAN  120  to perform ranging operations with the STA (e.g., as described for operation  606  of  FIG. 6A );   a positioning software module  336  to determine the location of the STA based on known locations of the APs and calculated RTT values and/or distances between the STA and the APs using, for example, trilateration techniques (e.g., as described for operation  618  of  FIG. 6A );   an AP timing synchronization software module  338  to facilitate the timing synchronization of multiple APs associated with the WLAN  120  (e.g., as described for operation  606  of  FIG. 6A );   a power save software module  340  to instruct one or more APs to request the STA to selectively enter power save mode, to selectively exit power save mode, and/or to stay awake for longer periods of time upon waking up from power save mode (e.g., as described for operations  602  and  620  of  FIG. 6A ).
 
Each software module includes instructions that, when executed by processor  320 , cause the WLAN server  300  to perform the corresponding functions. The non-transitory computer-readable medium of memory  330  thus includes instructions for performing all or a portion of the server-side operations of method  600  of  FIG. 6A .
       

     Processor  320 , which is coupled to network interface  310  and memory  330 , can be any suitable processor capable of executing scripts or instructions of one or more software programs stored in the WLAN server  300  (e.g., within memory  330 ). For example, processor  320  can execute ranging software module  334 , positioning software module  336 , AP timing synchronization software module  338 , and/or power save software module  340 . 
       FIG. 4  shows an AP  400  that is one embodiment of the access points AP 1 -AP 3  of  FIG. 1 . AP  400  includes a network interface  410 , a processor  420 , and a memory  430 . The network interface  410  can be used to communicate with WLAN server  300  of  FIG. 3  either directly or via one or more intervening networks and to transmit signals. Processor  420 , which is coupled to network interface  410  and memory  430 , can be any suitable processor capable of executing scripts or instructions of one or more software programs stored in AP  400  (e.g., within memory  430 ). 
     Memory  430  includes an AP location database  432  that stores the MAC addresses (e.g., BSSIDs) of a plurality of APs, the location coordinates of such APs, and other suitable location or configuration information of the APs. Memory  430  also includes a non-transitory computer-readable medium (e.g., one or more nonvolatile memory elements, such as EPROM, EEPROM, Flash memory, a hard drive, and so on) that can store the following software modules:
         a ranging software module  434  to initiate and perform ranging operations with one or more STAs (e.g., as described for operations  614  and  616  of  FIG. 6A , and/or for operations  652  and  656  of  FIG. 6B );   a positioning software module  436  to determine the location of the STA based on known locations of the APs and calculated RTT values and/or distances between the STA and the APs using, for example, trilateration techniques;   an AP timing synchronization software module  438  to facilitate the timing synchronization of multiple APs associated with the WLAN  120  (e.g., as described for operations  608 ,  610 , and  612  of  FIG. 6A );   a power save software module  440  to instruct one or more STAs to selectively enter power save mode, to selectively exit power save mode, and/or to stay awake for longer periods of time upon waking up from power save mode (e.g., as described for operations  604  and  622  of  FIG. 6A , for operation  662  of  FIG. 6C , and/or for operation  672  of  FIG. 6D ).
 
Each software module includes instructions that, when executed by processor  420 , cause the AP  400  to perform the corresponding functions. The non-transitory computer-readable medium of memory  430  thus includes instructions for performing all or a portion of the AP-side operations of methods  600 ,  650 ,  660 , and  670  of  FIGS. 6A ,  6 B,  6 C, and  6 D, respectively.
       

     Processor  420 , which is coupled to network interface  410  and memory  430 , can be any suitable processor capable of executing scripts or instructions of one or more software programs stored in the AP  400  (e.g., within memory  430 ). For example, processor  420  can execute ranging software module  434 , positioning software module  436 , AP timing synchronization software module  438 , and/or power save software module  440 . 
     As discussed above with respect to  FIG. 1 , the NBP system  100  allows any of the APs associated with WLAN  120  to initiate ranging operations with the STA, even if the STA avails itself of power save mode and/or if the ranging AP does not have an established wireless connection with the STA (e.g., even if the ranging AP is not currently associated with the STA). In accordance with the present embodiments, the WLAN server  130  can synchronize the timing of the non-associated APs of WLAN  120  with the timing of the associated AP of the WLAN  120  to ensure that probes intended to initiate ranging operations with the STA are sent from the non-associated APs to the STA precisely during the time periods that the STA is awake from power save mode (e.g., during STA wake-up periods intended to allow the STA to listen for beacon frames transmitted by the associated AP). In this manner, probes sent from the non-associated AP(s) will arrive at the STA during the STA&#39;s wake-up periods, thereby ensuring that the STA is able to (1) receive such probes and (2) respond with corresponding ACK frames sent back to the ranging AP(s). Thereafter, each of the ranging APs can use the TOD of the probe and the TOA of the ACK frame to calculate an RTT value between the ranging AP and the STA. For other embodiments, the ranging AP(s) can send the TOD/TOA information to the WLAN server  130 , which in turn can calculate the RTT values, correlate the RTT values to distances, and thereafter determine the location of the STA using, for example, trilateration techniques. 
     Further, for some embodiments, the NBP system  100  can instruct the STA to stay awake for longer periods of time when waking-up from the power save mode, thereby increasing the time period that the STA is able to receive and respond to the NULL frames sent by the non-associated AP(s). For one embodiment, the associated AP can cause the STA to stay awake longer by informing the STA that the associated AP has additional data waiting to be sent to the STA (e.g., even if there really isn&#39;t any additional data to be sent). For another embodiment, the associated AP can send “keep-awake” frames to the STA that cause the STA to remain awake for additional periods of time. These and other mechanisms for keeping the STA awake for longer than the STA&#39;s normal or scheduled wake-up periods are described in more detail below. 
     An exemplary ranging operation performed by the NBP system  100  of  FIG. 1  is depicted in the sequence diagram  500  of  FIG. 5A  and the timing diagram  520  of  FIG. 5B . As shown in  FIG. 5A , WLAN server  130  is connected to and controls the operation of access points AP 1 -AP 3 , which together form the WLAN  120  and, along with WLAN server  130 , implement the NBP system  100  of  FIG. 1 . For the sequence diagram  500  of  FIG. 5A , the STA is currently associated with AP 1  (e.g., the STA has an established wireless connection with AP 1 ), and is not currently associated with AP 2  or AP 3 . Thus, AP 1  is denoted as an “associated” access point, while AP 2  and AP 3  are denoted as “non-associated” access points. 
     In accordance with the present embodiments, the WLAN server  130  can synchronize the timer and/or clocks of the non-associated access points AP 2  and AP 3  with the timer and/or clocks of the associated access point AP 1 , thereby allowing the non-associated APs to synchronize their initiation of STA ranging operations (e.g., their transmission of probes to the STA) with the beacon transmission schedule of the associated AP. In this manner, the present embodiments ensure that probe frames sent from non-associated access points AP 2  and/or AP 3  arrive at the STA during its wake-up periods from power save mode, thereby ensuring that the STA is awake to participate in ranging operations initiated from one or more of the non-associated APs. 
     It is noted that because AP 1  is associated with the STA, the STA knows when AP 1  is scheduled to broadcast beacon frames to the STA, and AP 1  knows when the STA is to enter the power save mode. More specifically, in accordance with the IEEE 802.11 standards, once AP 1  and the STA establish a wireless connection such that AP 1  is associated with the STA, AP 1  periodically broadcasts beacon frames to the STA, and the STA periodically wakes up from power save mode to listen for such beacon frames. The beacon frames broadcast from AP 1  include AP 1 &#39;s MAC address, AP 1 &#39;s timing synchronization function (TSF) timer, “more data” bits indicating whether AP 1  has more data to send to the STA, and other information. For the exemplary embodiments described herein, the TSF timer is a modulus 2 64  counter that increments in micro-seconds, and thus has a maximum count value of 2 64 =102,400 micro-seconds (although other suitable timers, clocks, and/or counters may be used). Thus, for such embodiments, the TSF timer of associated AP 1  provides a beacon interval of 102,400 micro-seconds. 
     The STA uses AP 1 &#39;s TSF timer to synchronize its own local TSF timer, thereby allowing AP 1  and the STA to establish a series of target beacon transmission times that are spaced apart by the beacon interval. Thus, once the beacon transmission schedule is synchronized between the STA and its associated access point AP 1 , AP 1  broadcasts beacon frames to the STA at the scheduled beacon transmission times using its TSF timer, and the STA wakes up to listen for such beacon frames at scheduled wake-up times. Once awake, the STA typically remains awake for a predetermined period of time (which is referred to herein as the “STA wake-up period”). For some embodiments, the STA wakes up a few milliseconds prior to AP 1 &#39;s scheduled broadcast of each beacon frame, and stays awake a few hundred microseconds after the end of AP 1 &#39;s scheduled broadcast of each beacon frame. Thus, the STA&#39;s wake-up period typically begins before and ends after the scheduled beacon transmission time period. 
     Upon receipt of each beacon frame, the STA sends a beacon response frame to AP 1 . Further, in accordance with the IEEE 802.11 standards, the STA informs its associated access point AP 1  when it is going to enter the power save mode, for example, by STA setting the power management (PM) bit=1 in the beacon response frames sent to AP 1 . If the STA is not going to enter the power save mode during the beacon interval period, then the STA can set the PM bit=0 to indicate that it is not going to enter the power save mode. 
     For example,  FIG. 5B  is a timing diagram depicting the temporal relationship between the beacon transmission times, the beacon intervals, the STA wake-up periods, and the initiation of STA ranging operations from non-associated APs in accordance with the present embodiments. More specifically,  FIG. 5B  shows the associated AP transmitting beacon frames BCN 0 , BCN 1 , and BCN 2  at scheduled transmission times t 0 , t 1 , and t 2 , respectively. The beacon transmission times are separated by a beacon interval of 2 64 =102,400 micro-seconds provided by the TSF timer of the associated AP. The STA has scheduled wake-up periods that are substantially centered around the beacon transmission times, where each of the STA&#39;s scheduled wake-up periods begins between approximately 2500-3500 micro-seconds before the corresponding scheduled beacon transmission time and ends between approximately 400-500 micro-seconds after the corresponding scheduled beacon transmission time. As mentioned earlier, the STA can be kept awake for longer periods of time, for example, as depicted by the “additional wake-up periods” in  FIG. 5B , in response to receiving “stay awake” instructions from its associated AP. Thereafter, the non-associated AP(s) can be instructed to transmit probes PR 0 -PR 2  to the STA at similar times as corresponding beacons BCN 0 -BCN 2  are transmitted from the associated AP to the STA to ensure that the probes are sent to and received by the STA during corresponding STA wake-up periods. 
     Referring again to  FIG. 5A , once the communication link between AP 1  and the STA has been established and the STA&#39;s TSF timer has been synchronized with AP 1 &#39;s TSF timer, the WLAN server  130  issues a ranging instruction that instructs the non-associated access point AP 2  to obtain AP 1 &#39;s beacon transmission schedule and to thereafter initiate ranging operations with the STA. In response thereto, AP 2  sends a probe request to AP 1 , which in turn sends to AP 2  a response frame that includes AP 1 &#39;s TSF timer. AP 2  can determine AP 1 &#39;s target beacon transmission times from AP 1 &#39;s TSF timer, and can synchronize its own TSF timer with AP 1 &#39;s TSF timer. 
     Thereafter, the non-associated AP 2  can initiate ranging operations with the STA according to AP 1 &#39;s scheduled beacon transmission times so that probes sent from the non-associated AP 2  arrive at the STA during the STA wake-up times, for example, as depicted in  FIG. 5B . In this manner, the present embodiments ensure that the STA can receive probes sent from non-associated APs and respond with corresponding acknowledgement (ACK) frames. For some embodiments, the non-associated APs can spoof the MAC address of the associated AP 1  when sending probes to the STA. 
     For exemplary embodiments described herein, the probes sent by the non-associated APs (e.g., AP 2  and/or AP 3 ) can be either NULL frames or QoS NULL frames. For other embodiments, any frames that elicit a response from the STA can be used as probe frames (e.g., RTS/CTS frame exchanges). 
     Thereafter, AP 2  can use the TOD of the NULL frame and the TOA of the ACK frame to determine an RTT value between AP 2  and STA, which in turn can be correlated to a distance between AP 2  and STA. For some embodiments, AP 2  can send the RTT measurements back to WLAN server  130 , which in response thereto can calculate the RTT value and thereafter correlate the RTT value to a distance. For other embodiments, AP 2  can send the ranging measurements (e.g., TOD/TOA information the NULL/ACK frame exchange) to the WLAN server  130 , which in can calculate the RTT value and then correlate the RTT value to a distance. 
     This process can be repeated with the non-associated AP 3  to initiate ranging operations between AP 3  and the STA, which in turn can be used to calculate the distance between AP 3  and the STA. Subsequently, when the WLAN server  130  has obtained or calculated the distances between three access points (e.g., AP 1 -AP 3 ) and the STA, the WLAN server  130  can determine the location of the STA using, for example, trilateration techniques. After the ranging operations are complete, the associated AP 1  can instruct the STA to re-enter the power save mode (e.g., to go to sleep). 
     As mentioned above, for some embodiments, the WLAN server  130  can cause the STA to stay awake for longer periods of times than the STA&#39;s predetermined wake-up period, thereby providing a larger time window for the STA to receive and respond to probes transmitted by the non-associated access points AP 2  and/or AP 3 , for example, as depicted in  FIG. 5B . In this manner, the WLAN server  130  not only coordinates the ranging operations of multiple access points with the STA but also keeps the STA awake for longer periods of time to ensure that the STA is able to receive and respond to multiple probes sent from a number of different APs. For one embodiment, the WLAN server  130  can instruct the associated AP 1  to set the traffic indication message (TIM) bits provided within its beacon frames to a state that indicates AP 1  has additional data frames to be delivered to the STA (e.g., even if AP 1  does not have any additional data frames), which in turn causes the STA to remain its in awake state longer periods of time. For another embodiment, the WLAN server  130  can instruct the associated AP 1  to set the “more data” bit provided within one or more MAC frames sent to the STA, thereby turning such frames into “keep-awake” instructions to the STA to keep the STA awake for longer periods of time. Note that the length of time that AP 1  can keep the STA awake beyond the scheduled wake-up period varies depending upon the make-and-model of the STA. 
     Further, note that the associated AP 1  can set the delay traffic indication message (DTIM) bit in its beacon frames to a value of 1 so that the STA wakes up on every beacon transmission period. Also, note that if the non-associated AP 2  is on a different channel than the STA, then AP 2  can change channels to perform the ranging operation and then return to its home channel. 
       FIG. 6A  is a flowchart illustrating a method  600  for performing ranging operations in accordance with the present embodiments. As described above, the present embodiments allow one or more APs of a WLAN system to initiate ranging operations with a STA, regardless of whether a particular AP is associated with the STA and regardless of whether the STA avails itself of power save mode. Referring also to  FIGS. 1 ,  5 A, and  5 B, in the method  600 , the WLAN server  130  issues a ranging instruction that may request the STA to stay awake longer than its predetermined or scheduled wake-up periods ( 602 ). The ranging instruction also requests the non-associated AP to obtain the synchronization timer of the associated AP and thereafter initiate ranging operations with the STA ( 606 ). In response to the request to stay awake, the associated AP can embed a stay-awake instruction in frames sent to the STA device ( 604 ). In response to the request to obtain the synchronization timer of the associated AP, the non-associated AP sends a probe request to the associated AP ( 608 ). The associated AP responds by sending, to the non-associated AP, a probe response that includes its synchronization timer ( 610 ). Once the associated AP&#39;s synchronization timer is obtained, the non-associated AP can synchronize itself with the associated AP using the obtained synchronization timer, and can then determine the associated AP&#39;s beacon transmission schedule and/or determine when the STA wake-up periods occur ( 612 ). After the non-associated AP is synchronized with the associated AP, the non-associated AP can range the STA during the STA&#39;s wake-up periods ( 614 ), and then send the ranging measurements (e.g., TOD/TOA information) to the WLAN server  130  ( 616 ). 
     Next, the WLAN server  130  can use the ranging measurements provided by the non-associated AP to calculate an RTT value, and then correlate the RTT value to a distance between the non-associated AP and the STA ( 618 ). After the ranging operation is complete, the WLAN server  130  may issue an instruction requesting the STA to return to power save mode ( 620 ). In response thereto, the associated AP may embed a sleep instruction into one or more frames sent to the STA, thereby causing the STA to enter power save mode ( 622 ). 
       FIG. 6B  is a flowchart illustrating a method  650  that is one embodiment of step  614  of  FIG. 6A . In the method  650 , to range the STA, the non-associated AP sends a probe frame to the STA during the STA&#39;s wake-up period ( 652 ). As described above, for some embodiments, the probe frame can be either a NULL frame or a QoS NULL frame; for other embodiments, the probe frame can be a RTS frame or any data frame that elicits a response from the STA. Further, if the STA responds only to frames sent from its associated AP, then the non-associated AP can spoof the MAC address of the associated AP and send a spoofed frame to the STA. After receiving the probe frame, the STA sends an acknowledgement (ACK) frame back to the non-associated AP ( 654 ). Then, the non-associated AP sends the ranging measurements to the WLAN server ( 656 ). 
       FIG. 6C  is a flowchart illustrating a method  660  that is one embodiment of step  604  of  FIG. 6A . In the method  660 , to keep the STA awake for longer periods of time, the associated AP can assert the TIM bit in beacon frames sent to the STA ( 662 ). Assertion of the TIM bit indicates that the associated AP has additional data buffered for the STA. Thus, in response to receiving the asserted TIM in the beacon frame, the STA stays awake longer than its scheduled wake-up period to await transmission of additional data from the associated AP ( 664 ). This additional wake-up period, as depicted in  FIG. 5B , allows the STA more time to receive and respond to probes sent from one or more non-associated APs. 
       FIG. 6D  is a flowchart illustrating a method  670  that is another embodiment of step  604  of  FIG. 6A . In the method  670 , to keep the STA awake for longer periods of time, the associated AP can set the “more data” bit provided within any data frames sent to the STA ( 672 ). Thus, in response to receiving the asserted “more data” bit, the STA stays awake longer than its scheduled wake-up period to await transmission of additional data from the associated AP ( 674 ). This additional wake-up period, as depicted in  FIG. 5B , allows the STA more time to receive and respond to probes sent from one or more non-associated APs. 
     In the foregoing specification, the present embodiments have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the disclosure as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense. For example, method steps depicted in the flow charts of  FIG. 6A-6D  can be performed in other suitable orders and/or one or more methods steps may be omitted. In addition, although described above in the context of RTT ranging operations, the present embodiments are equally applicable to other types of ranging operations (e.g., RSSI and/or TDOA ranging techniques).