Abstract:
Apparatus having corresponding methods and computer-readable media comprise: a receiver configured to receive a wireless signal transmitted by an access point, wherein the wireless signal includes one or more transmit beamforming parameters used by the access point to transmit the wireless signal; wherein a location of the apparatus is determinable based, at least in part, on i) the one or more transmit beamforming parameters included in the wireless signal, and ii) a location of the access point.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 61/491,691, filed May 31, 2011, entitled “Indoor Positioning Using Transmit Beam Steering,” the disclosure thereof incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The present disclosure relates to the field of position determination. 
     BACKGROUND 
     Many modern mobile devices have positioning capabilities. These capabilities have many uses, for example, to provide the location to a user, to locate a lost mobile device, to provide location-based services to a user, and the like. One of the most popular positioning technologies is provided by a global positioning system (GPS). With GPS, a GPS receiver in a mobile device measures a timing of GPS signals transmitted by multiple GPS satellites, and determines a position of the GPS receiver through trilateration. The principal disadvantage of GPS technology is that the GPS signals typically experience high path loss in some environments, for example indoors, in urban canyons, and the like, thereby potentially rendering the technology ineffective in such environments. 
     SUMMARY 
     In general, in one aspect, an embodiment features an apparatus comprising: a receiver configured to receive a wireless signal transmitted by an access point, wherein the wireless signal includes one or more transmit beamforming parameters used by the access point to transmit the wireless signal; wherein a location of the apparatus is determinable based, at least in part, on i) the one or more transmit beamforming parameters included in the wireless signal, and ii) a location of the access point. 
     In general, in one aspect, an embodiment features a method for a wireless device, the method comprising: receiving a wireless signal transmitted by an access point, wherein the wireless signal includes one or more transmit beamforming parameters used by the access point to transmit the wireless signal; wherein a location of the wireless device is determinable based on i) the one or more transmit beamforming parameters, and ii) a location of the access point. 
     In general, in one aspect, an embodiment features computer-readable media embodying instructions executable by a computer to perform functions comprising: determining a location of a wireless device based on a location of an access point and one or more transmit beamforming parameters used by the access point to transmit a wireless signal to the wireless device, wherein the wireless signal includes the one or more transmit beamforming parameters. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows elements of a position determination system according to one embodiment. 
         FIG. 2  shows elements of an access point that is capable of transmit beamforming according to one embodiment. 
         FIGS. 3-10  show antenna patterns that can be produced by one radio and two antennas. 
         FIG. 11  shows elements of the device of  FIG. 1  according to one embodiment. 
         FIG. 12  shows a process for the position determination system of  FIG. 1  according to an embodiment where the device measures signals transmitted by the access point, and the location server determines the location of the device using those measurements. 
         FIG. 13  shows the format of the IEEE 802.11v Location Configuration Request Frame. 
         FIG. 14  shows the format of the IEEE 802.11v Location Indication Parameters subelement. 
         FIG. 15  shows the format of the IEEE 802.11v Location Track Notification Frame. 
         FIG. 16  shows the format of the IEEE 802.11v Radio Information subelement. 
         FIG. 17  shows the format of the IEEE 802.11v Time Of Departure subelement. 
         FIG. 18  shows a process for the position determination system of  FIG. 1  according to an embodiment where the device measures the signals transmitted by the access point, and determines the location of the device using those measurements. 
         FIG. 19  shows a block diagram of a transmitter drawn from the IEEE 802.11ac draft standard where transmit beamforming is applied in the frequency domain. 
     
    
    
     The leading digit(s) of each reference numeral used in this specification indicates the number of the drawing in which the reference numeral first appears. 
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure provide position determination using transmit beamforming. According to these embodiments, one or more access points transmit wireless signals using transmit beamforming. The wireless signals include the beamforming parameters used to transmit the wireless signals. For example, the wireless signals can indicate the antenna pattern used to transmit the wireless signals. A wireless device receives and processes the wireless signals. In particular, the wireless device obtains the beamforming parameters used to transmit the wireless signals. The wireless device uses the beamforming parameters to determine an angle for each access point. The location of the wireless device is determined based on one or more of the angles. The location determination can also include distances determined between the wireless device and the access points. The distances can be determined by time-of-flight of the wireless signals, signal strength measurements of the wireless signals by the wireless device, and the like. The location determination can be performed by the wireless device or by a location server remote from the wireless device. 
     As used herein, the term “server” generally refer to an electronic device or mechanism. As used herein, the term “mechanism” refers to hardware, software, or any combination thereof. These terms are used to simplify the description that follows. The servers and mechanisms described herein can be implemented on any standard general-purpose computer, or can be implemented as specialized devices. Furthermore, while some embodiments are described with reference to a client-server paradigm, other embodiments employ other paradigms, such as peer-to-peer paradigms and the like. 
       FIG. 1  shows elements of a position determination system  100  according to one embodiment. Although in the described embodiments the elements of position determination system  100  are presented in one arrangement, other embodiments may feature other arrangements. For example, various elements of position determination system  100  can be implemented in hardware, software, or combinations thereof. Referring to  FIG. 1 , position determination system  100  includes device  102 , three access points  104 - 1 ,  104 - 2 , and  104 - 3 , and a location server  106 . Access points  104 - 1 ,  104 - 2 , and  104 - 3  transmit wireless signals  108 - 1 ,  108 - 2 , and  108 - 3 , respectively. Device  102  transmits wireless signals  110 . Location server  106  can communicate with access points  104  wirelessly, by wired link, or the like. 
     In some embodiments, location server  106  determines the position of device  102 . In other embodiments, device  102  determines its own position, so location server  106  is not required. Device  102  can be any sort of device capable of performing the functions described herein for device  102 . In most cases, device  102  is a mobile device. However, in some cases device  102  is a fixed device. Device  102  can be implemented in an electronic device such as a smartphone, tablet or other computer, or the like. 
     In the described embodiment, access points  104  provide one or more IEEE 802.11 wireless local-area networks (WLANs). However, other embodiments are not limited to IEEE 802.11 WLANs or to network communications. Furthermore, other numbers of access points can be used. For example, if only one access point  104  is available, location information obtained from that access point  104  can be combined with location information from other sources to determine the position of device  102 . 
     Access points  104 - 1 ,  104 - 2 , and  104 - 3  have known locations (x1, y1), (x2, y2) and (x3, y3), respectively. Device  102  has an unknown location (x, y). In some embodiments, distances are computed between device  102  and one or more of the access points  104 . The respective distances d1, d2, and d3 for access points  104 - 1 ,  104 - 2 , and  104 - 3  are given by equations (1), (2), and (3), respectively.
 
 d 1=√{square root over (( x−x 1) 2 +( y−y 1) 2 )}{square root over (( x−x 1) 2 +( y−y 1) 2 )}  (1)
 
 d 2=√{square root over (( x−x 2) 2 +( y−y 2) 2 )}{square root over (( x−x 2) 2 +( y−y 2) 2 )}  (2)
 
 d 3=√{square root over (( x−x 3) 2 +( y−y 3) 2 )}{square root over (( x−x 3) 2 +( y−y 3) 2 )}  (3)
 
     In some embodiments, these distances are obtained by time-of-flight measurements. For example, the signal  108  transmitted by an access point  104  includes a timestamp indicating a time of departure of the signal  108  from access point  104 . Device  102  measures the time of arrival of the signal  108  at device  102 , and computes the time of flight as the difference between the time of departure and the time of arrival. In some embodiments, wireless device  102  measures the received signal strengths of signals  108 , and determines the distances based on those measurements. 
     In some embodiments, angles are computed between device  102  and one or more of the access points  104  with reference to a chosen reference angle, for example, magnetic North. The respective angles a1, a2, and a3 for access points  104 - 1 ,  104 - 2 , and  104 - 3  are given by equations (4), (5), and (6), respectively. 
     
       
         
           
             
               
                 
                   
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     In some embodiments, the angles represent transmit beamforming parameters employed by access points  104  when transmitting the signals  108 . Many modern access points  104  have multiple antennas, and are capable of transmit beamforming.  FIG. 2  shows elements of an access point  104  that is capable of transmit beamforming according to one embodiment. Although in the described embodiments the elements of access points  104  are presented in one arrangement, other embodiments may feature other arrangements. For example, various elements of access points  104  can be implemented in hardware, software, or combinations thereof. In addition, while access point  104  is described as having two radios, this is not required. Various embodiments apply to access points  104  having three or more radios as well. Access points  104  can be compliant with all or part of IEEE standard 802.11, including draft and approved amendments such as 802.11a, 802.11b, 802.11e, 802.11g, 802.11i, 802.11k, 802.11n, 802.11v, and 802.11w. 
     Referring to  FIG. 2 , access point  104  includes two radios  202 - 1  and  202 - 2 , a beamformer  204 , a beamformer controller  206 , and N antennas  208 - 1  through  208 -N that transmit wireless signals  108 . Each radio  202  generates a respective signal  210 - 1 ,  210 - 2  to be transmitted. Beamformer  204  performs transmit beamforming on the signals  210  in accordance with a control signal  212  provided by beamformer controller  206 . Antennas  208  transmit the beamformed wireless signals  108 . 
     Beamformer  204  can be implemented in any conventional manner. For example, beamformer  204  can employ predetermined phase delays for each antenna  208 . The phase delays can be implemented in the frequency domain or in the time domain. For example, in some embodiments, a time domain delay is applied as a constant phase ramp in the frequency domain. In other embodiments the phase delays are applied in the frequency domain, as described below. 
     Referring again to  FIGS. 1 and 2 , using transmit beamforming, an access point  104  can transmit signals  108  with more energy focused in a predetermined direction. For example, an access point with four antennas  208  can transmit 10 log 4=6 dB more power in one sector that another. According to various embodiments, knowledge of the transmit beamforming parameters used by an access point  104  to transmit a signal  108 , along with measurements taken by device  102  of that signal  108 , can be used to determine a relative angle between the device  102  and the access point  104 . In some embodiments, the transmit beamforming parameters include the identity of the antenna pattern used by access point  104  to transmit signals  108 . 
       FIGS. 3-10  show antenna patterns that can be produced by one radio  202  and two antennas  208 .  FIGS. 3-6  show antenna patterns where antennas  208  are separated by half a wavelength (λ/2).  FIG. 3  shows an antenna pattern with no relative delay imposed between antennas  208 .  FIG. 4  shows an antenna pattern with a delay of 0.32λ imposed on one antenna  208 .  FIG. 5  shows an antenna pattern with a delay of 0.64λ imposed on one antenna  208 .  FIG. 6  shows an antenna pattern with a delay of λ imposed on one antenna  208 . 
       FIGS. 7-10  show antenna patterns where antennas  208  are separated by a quarter of a wavelength (λ/4).  FIG. 7  shows an antenna pattern with no relative delay imposed between antennas  208 .  FIG. 8  shows an antenna pattern with a delay of 0.32λ imposed on one antenna  208 .  FIG. 9  shows an antenna pattern with a delay of 0.64λ imposed on one antenna  208 .  FIG. 10  shows an antenna pattern with a delay of λ imposed on one antenna  208 . 
     From the examples of  FIGS. 3-10  it is clear that a device  102  at a particular relative angle to an access point  104  will measure a larger RSSI with some antenna patterns than with others. Such measurements, combined with knowledge of the antenna pattern measured, allow accurate determination of that angle. The antenna patterns can be chosen based on directionality. For example, the antenna pattern of  FIG. 8  provides better directionality than the antenna pattern of  FIG. 7 . The calibration of the antennas  208  of the access points  104  can be done during production time. In one embodiment, the antennas transmit a signal with an increasing frequency ramp, which corresponds to a time delay. The effective antenna pattern can be measured using a turntable. The frequency ramps that correspond to directionality in specific directions are selected. Predetermined antenna IDs and antenna gains are assigned to the selected frequency ramps. The antenna ID can be transmitted in the physical layer convergence procedure (PLPC) protocol data unit (PPDU). 
       FIG. 11  shows elements of device  102  according to one embodiment. Although in the described embodiments the elements of device  102  are presented in one arrangement, other embodiments may feature other arrangements. For example, various elements of device  102  can be implemented in hardware, software, or combinations thereof. 
     Referring to  FIG. 11 , device  102  includes a radio  1102  and an antenna  1104 . Radio  1102  includes a receiver  1106 , a transmitter  1108 , a timing circuit  1110 , a signal meter  1112 , a location circuit  1114 , and a controller  1116 . In some embodiments, location circuit  1114  determines the position of device  102 . In other embodiments, location server  106  determines the position of device  102 , so location circuit  1114  is not required. In some embodiments, time-of-flight measurements are not used. In such embodiments, timing circuit  1110  is not required. Location circuit  1114  and controller  1116  can be implemented as one or more processors. Processors according to various embodiments can be fabricated as one or more integrated circuits. Device  102  can be compliant with all or part of IEEE standard 802.11, including draft and approved amendments such as 802.11a, 802.11b, 802.11e, 802.11g, 802.11i, 802.11k, 802.11n, 802.11v, and 802.11w. 
     In some embodiments, device  102  measures signals  108  transmitted by access point  104 , and location server  106  determines the location of device  102  using those measurements.  FIG. 12  shows a process  1200  for position determination system  100  of  FIG. 1  according to one such embodiment. Although in the described embodiments the elements of process  1200  are presented in one arrangement, other embodiments may feature other arrangements. For example, in various embodiments, some or all of the elements of process  1200  can be executed in a different order, concurrently, and the like. Also some elements of process  1200  may not be performed, and may not be executed immediately after each other. 
     Referring to  FIG. 12 , at  1202  device  102  transmits a wireless signal  110  that represents an IEEE 802.11v Location Configuration Request Frame.  FIG. 13  shows the format of the IEEE 802.11v Location Configuration Request Frame. The Location Configuration Request Frame includes a one-octet Category field, a one-octet Action field, a one-octet Dialog Token field, and a variable-length Location Parameters Element. The Category field is the value indicating the Wireless Network Management category. The Action field is the value indicating Location Configuration Request. The Dialog Token field is a nonzero value that is unique among the Location Configuration Request frames sent to each destination MAC address for which a corresponding Location Configuration Response frame has not been received. 
     One of the Location Parameters subelements is the Location Indication Parameters subelement.  FIG. 14  shows the format of the IEEE 802.11v Location Indication Parameters subelement. The Location Indication Parameters subelement includes a one-octet Subelement ID field, a one-octet Length field, a six-octet Indication Multicast Address field, a one-octet Report Interval Units field, a two-octet Normal Report Interval field, a one-octet Normal Number of Frames per Channel field, a two-octet In-Motion Report Interval field, a one-octet In-Motion Number of Frames per Channel field, a one-octet Burst Interframe Interval field, a one-octet Tracking Duration field, and a one-octet ESS Detection Interval field. 
     The Location Indication Parameters subelement contains location reporting characteristics for device  102 . The Subelement ID field contains the value 1, indicating that the subelement is the Location Indication Parameters subelement. The Length field contains the value 16. The Indication Multicast Address field specifies the destination address to which the Location Track Notification frames are sent in a non-IBSS network. The value of this field is a locally administered multicast address. The field is reserved when Location Track Notifications are transmitted in an IBSS. 
     The Report Interval Units field contains the units used for the Normal Report Interval field and In-Motion Report Interval field. The Normal Report Interval is the time interval, expressed in the units indicated in the Report Interval Units field, at which access point  104  is expected to transmit one or more Location Track Notification frames. The access point  104  will not transmit Location Track Notification frames when the Normal Report Interval is 0. The Normal Number of Frames per Channel is the number of Location Track Notification frames per channel sent or expected to be sent by access point  104  at each Normal Report Interval. 
     The In-Motion Report Interval is the time interval, expressed in the units indicated in the Report Interval Units field, at which access point  104  reports its location by sending a Location Track Notification frame when access point  104  is in motion. The In-Motion Number of Frames per Channel is the number of Location Track Notification frames per channel sent or expected to be sent by access point  104  at each In-Motion Report Interval. The Burst Inter-frame Interval is the target time interval, expressed in milliseconds, between the transmissions of each of the Normal or In-Motion frames on the same channel. The Burst Inter-frame interval value is 0 to indicate that frames will be transmitted with no target inter-frame delay. 
     The Tracking Duration is the amount of time, in minutes, that access point  104  sends the Location Track Notification frames. The duration starts as soon as access point  104  sends a Location Configuration Response frame with a Location Status value of Success. If the Tracking Duration value is a non-zero value access point  104  will send Location Track Notification Frames, based on the Normal and In-Motion Report Interval field values, until the duration ends. If the Tracking Duration is 0 access point  104  will continuously send Location Track Notification frames as defined by Normal and In-Motion Report Interval field values until transmission is terminated. 
     The ESS Detection Interval is the periodicity, in minutes, that a STA checks for beacons transmitted by one or more access points belonging to the same Extended Service Set (ESS) that configured access point  104 . If no beacons from the ESS are received for this period, the STA terminates transmission of Location Track Notification frames. The ESS Detection Interval field is not used when the ESS Detection Interval field value is 0. 
     At  1204 , responsive to the IEEE 802.11v Location Configuration Request Frame transmitted by device  102 , access point  104  transmits a wireless signal that represents an IEEE 802.11v Location Track Notification Frame.  FIG. 15  shows the format of the IEEE 802.11v Location Track Notification Frame. The Location Track Notification Frame includes a one-octet Category field, a one-octet Action field, a variable-length Location Parameters Element, and an optional variable-length Measurement Report Element. The Category field is the value indicating the WNM category. The Action field is the value indicating Location Track Notification. The Parameters Element field contains the Location Parameters subelements. The Measurement Report Element is optional. 
     One of the Location Parameters subelements is the Radio Information subelement.  FIG. 16  shows the format of the IEEE 802.11v Radio Information subelement. The Radio Information subelement includes a Subelement ID field, a Length field, a Transmit Power field, an Antenna ID field, an Antenna Gain field, a RSNI field, and a RCPI field. Each field is one octet in length. 
     The Subelement ID field contains the value 4, indicating that the subelement is the Radio Information subelement. The Length field contains the value 5. The Transmit Power field is the transmit power used to transmit the current Location Track Notification frame containing the Location Parameters element with the Radio Information subelement and is a signed integer, reported in dBm. A value of −128 indicates that the transmit power is unknown. 
     The Antenna ID field is described below. 
     The Antenna Gain field is the antenna gain of the antenna (or group of antennas) over which the Location Track Notification frame is transmitted and is a signed integer, reported in dB. A value of −128 indicates that the antenna gain is unknown. The RSNI field contains the received signal to noise indication (RSNI) value (dB) measured against the most recently received Location Configuration Request frame requesting that a Radio Information subelement be included in the Location Track Notification frame. A value of 255 indicates that the RSNI value is unknown or is not used. The RCPI field contains the received channel power indication (RCPI) value (dBm) measured against the most recently received Location Configuration Request frame requesting that a Radio Information subelement be included in the Location Track Notification frame. A value of 255 indicates that the RCPI value is unknown or is not used. 
     According to IEEE 802.11v, the Antenna ID field identifies the antenna(s) used to transmit the Location Track Notification frame. However, in the described embodiments, the Antenna ID filed is not used in this manner. Instead, in the described embodiments, the Antenna ID field is used to indicate the antenna pattern used to transmit the Location Track Notification frame. For example, each of the antenna patterns in  FIGS. 3-10  can be assigned a different integer in the range 0-255. When an antenna pattern is used to transmit a Location Track Notification frame, the corresponding integer is placed in the Antenna ID field of that frame. 
     In embodiments that employ time-of-flight measurements, the Location Track Notification frame includes the Time Of Departure subelement.  FIG. 17  shows the format of the IEEE 802.11v Time Of Departure subelement. The Time Of Departure subelement includes a one-octet Subelement ID field, a one-octet Length field, a four-octet TOD Timestamp field, a two-octet TOD RMS field, and a two-octet TOD Clock Rate field. 
     The Subelement ID field contains the value 7, indicating that the subelement is the Time of Departure subelement. The Length field contains the value 8. The TOD Timestamp field carried within the Location Track Notification frame specifies when the first frame energy is sent by the transmitting port in units equal to 1/TOD Clock Rate, where the TOD Clock Rate is specified in the TOD Clock Rate field. The TOD RMS field specifies the RMS time of departure error in units equal to 1/TOD Clock Rate, where the TOD Clock Rate is specified in the TOD Clock Rate field. The TOD Clock Rate field contains the clock rate used to generate the TOD timestamp value reported in the TOD Timestamp field, and it is specified in units of MHz. 
     The Location Track Notification frame includes the Location Indication Parameters used by access point  104 . If in the Location Configuration Request Frame device  102  requested changes to the Location Indication Parameters, other devices can respond with an IEEE 802.11v Location Configuration Response Frame. 
     Referring again to  FIG. 12 , at  1206  device  102  processes the Location Track Notification frame received from access point  104 . In particular, antenna  1104  receives the wireless signal  108  representing the Location Track Notification frame. Receiver  1106  receives the Location Track Notification frame into radio  1102 . Signal meter  1112  measures the received signal strength of wireless signal  108 . Device  102  may receive signals  108  from multiple access points  104 . Controller  1116  notes the signal  108  with the highest signal strength, the access point  104  that transmitted that signal  108 , and the antenna ID in the Location Track Notification frame conveyed by that signal  108 . 
     In embodiments that employ time-of-flight measurements, timing circuit  1110  measures the time of arrival of signal  108 , and notes the time of departure of the frame from access point  104  as presented in the Time Of Departure field of the Time Of Departure subelement of the Location Track Notification frame. 
     At  1208 , device  102  reports the measurements to access point  104 . In particular, transmitter  1108  of radio  1102  transmits a wireless signal  110  from antenna  1104 . Wireless signal  110  represents a Location Track Notification frame. The Location Track Notification frame includes the antenna ID reported in the strongest signal  108  measured by signal meter  1112 . Device  102  may transmit multiple Location Track Notification frames, including the antenna ID and the noise indication (RSNI) value and/or received channel power indication (RCPI) values of multiple received measurement frames, which can be received from multiple access points  104 . These Location Track Notification frames can be transmitted to a single access point  104  as well. 
     In some embodiments, signal meter  1112  of radio  1102  also measures the signal  108  received from access point  104  to obtain a RSNI value and/or a RCPI value. In these embodiments, device  102  includes the RSNI and/or RSPI value(s) in the optional Measurement Report Element of the Location Track Notification frame sent to access point  104  at  1208 . 
     In embodiments that employ time-of-flight measurements, device  102  includes the time of arrival measurement and the reported time of departure, or the difference between the two, in the Location Track Notification frame sent to access point  104  at  1208 . 
     At  1210 , access point sends the reported device measurements to location server  106 . At  1212 , location server  106  determines the location of device  102  using at least the angle information, augmented in some embodiments by distance information such as time-of-flight information, signal strength information, and the like. 
     In some embodiments, device  102  measures signals  108  transmitted by access point  104 , and determines the location of device  102  using those measurements.  FIG. 18  shows a process  1800  for position determination system  100  of  FIG. 1  according to one such embodiment. Although in the described embodiments the elements of process  1800  are presented in one arrangement, other embodiments may feature other arrangements. For example, in various embodiments, some or all of the elements of process  1800  can be executed in a different order, concurrently, and the like. Also some elements of process  1800  may not be performed, and may not be executed immediately after each other. 
     Referring to  FIG. 18 , at  1802  device  102  transmits a wireless signal that represents an IEEE 802.11v Location Configuration Request Frame.  FIG. 13  shows the format of the IEEE 802.11v Location Configuration Request Frame. At  1204 , responsive to the IEEE 802.11v Location Configuration Request Frame transmitted by device  102 , access point  104  transmits a wireless signal  108  that represents an IEEE 802.11v Location Track Notification Frame.  FIG. 15  shows the format of the IEEE 802.11v Location Track Notification Frame. 
     The Location Track Notification Frame includes the variable-length Location Parameters Element. One of the Location Parameters subelements is the Radio Information subelement.  FIG. 16  shows the format of the IEEE 802.11v Radio Information subelement. The Radio Information subelement includes the Antenna ID field. According to IEEE 802.11v, the Antenna ID field identifies the antenna(s) used to transmit the Location Track Notification frame. However, in the described embodiments, the Antenna ID filed is not used in this manner. Instead, in the described embodiments, the Antenna ID field is used to indicate the antenna pattern used to transmit the Location Track Notification frame. For example, each of the antenna patterns in  FIGS. 3-10  can be assigned a different integer in the range 0-255. When an antenna pattern is used to transmit a Location Track Notification frame, the corresponding integer is placed in the Antenna ID field of that frame. 
     In embodiments that employ time-of-flight measurements, the Location Track Notification frame includes the Time Of Departure subelement.  FIG. 17  shows the format of the IEEE 802.11v Time Of Departure subelement. The Time Of Departure subelement includes the TOD Timestamp field. 
     The Location Track Notification frame includes the Location Indication Parameters used by access point  104 . If in the Location Configuration Request Frame device  102  requested changes to the Location Indication Parameters, other devices can respond with an IEEE 802.11v Location Configuration Response Frame. 
     Referring again to  FIG. 18 , at  1806  device  102  processes the Location Track Notification frame received from access point  104 . In particular, antenna  1104  receives the wireless signal  108  representing the Location Track Notification frame. Receiver  1106  receives the Location Track Notification frame into radio  1102 . 
     Location circuit  1114  determines an angle for access point  104 . In particular, signal meter  1112  measures the received signal strength of wireless signal  108 . Device  102  may receive signals  108  from multiple access points  104 . Controller  1116  notes the signal  108  with the highest signal strength, the access point  104  that transmitted that signal  108 , and the antenna ID in the Location Track Notification frame represented by that signal  108 . Based on the antenna ID and knowledge of the antenna pattern represented by that antenna ID, location circuit  1114  determines an angle for access point  104 . 
     In embodiments that employ time-of-flight measurements, timing circuit  1110  measures the time of arrival of signal  108 , and notes the time of departure of the frame from access point  104  as presented in the Time Of Departure field of the Time Of Departure subelement of the Location Track Notification frame. Location circuit  1114  uses the difference between the time of arrival and the time of departure to compute the distance to access point  104 . 
     Device  102  can also employ signal strength measurements obtained by signal meter  1112  for location determination. Signal meter  1112  measures the received signal strength of the wireless signal  108  representing the Location Track Notification frame transmitted by access point  104  at  1804 . Controller  1116  notes the transmit power reported in the Radio Information subelement of that Location Track Notification frame. Based on the received signal strength and transmit power, controller  1116  estimates the signal loss and the corresponding path loss, and determines the distance of device  102  from access point  104 . 
     At  1808 , device  102  determines the location of device  102  using at least the angle information, augmented in some embodiments by the computed distance(s) and other information. Device  102  can use information derived from multiple access points  104  to determine the location of device  102 . In some embodiments, device  102  also uses location information from other sources to determine the position of device  102 . 
     In some embodiments the phase delays for transmit beamforming are applied in the frequency domain.  FIG. 19  shows a block diagram of one such transmitter  1900  drawn from the IEEE 802.11ac draft standard. Transmitter  1900  includes a physical-layer (PHY) padding block  1902 , a scrambler  1904 , an encoder parser  1906 , forward error correction (FEC) encoders  1908 , stream parser  1910 , braided convolutional code (BCC) interleavers  1912 , constellation mappers  1914 , low-density parity-check (LDPC) tone mappers  1916 , space-time block coder (STBC)  1918 , cyclic shift diversity per space time shift (CSD per STS) blocks  1920 , spatial mapper  1922 , inverse discrete Fourier transform (IDFT) blocks  1924 , insert guard interval (GI) and window blocks  1926 , and analog and radio frequency (RF) blocks  1928 . There can be 1 to 12 FEC encoders  1908  when BCC encoding is used. Stream parser  1910  can have 1-8 outputs. For streams encoded using LDPC, BCC interleavers  1912  are not used. For streams encoded using BCC, LDPC tone mappers  1916  are not used. When STBC is used, STBC block  1918  has twice as many outputs than inputs. When spatial mapping is used, there can be more transmit chains than space time streams. The number of inputs to spatial mapper  1922  can be 1-8. Note that, in transmitter  1900 , the spatial mapping occurs before the IDFT transform. 
     Various embodiments of the present disclosure can be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations thereof. Embodiments of the present disclosure can be implemented in a computer program product tangibly embodied in a computer-readable storage device for execution by a programmable processor. The described processes can be performed by a programmable processor executing a program of instructions to perform functions by operating on input data and generating output. Embodiments of the present disclosure can be implemented in one or more computer programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Each computer program can be implemented in a high-level procedural or object-oriented programming language, or in assembly or machine language if desired; and in any case, the language can be a compiled or interpreted language. Suitable processors include, by way of example, both general and special purpose microprocessors. Generally, processors receive instructions and data from a read-only memory and/or a random access memory. Generally, a computer includes one or more mass storage devices for storing data files. Such devices include magnetic disks, such as internal hard disks and removable disks, magneto-optical disks; optical disks, and solid-state disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     A number of implementations have been described. Nevertheless, various modifications may be made without departing from the scope of the disclosure. Accordingly, other implementations are within the scope of the following claims.