# Patent

## Title
Method for determining location of wireless devices

## Abstract

Methods for determining a location of a client device in a wireless network having at least a first other network device with a known location are disclosed. Transmission-to-self calibrations are performed at the first other network device and/or the client device to determine RF group delay of the first other network device and/or the client device. The client device receives messages and acknowledgements from the other network devices and determines its location based on information received including the time at which it received the messages, the acknowledgements and the RF groupdelay of the first other network device. As such, the location calculation can be refined by accounting for the RF group delays of the first other network device and/or the client device.

## Background
FIELD OF THE INVENTION
The present invention is related to location detection of wireless devices, and more particularly, to methods and apparatus of measuring differential distance of a wireless device in a wireless network with respect to other network devices with known locations.
BACKGROUND OF THE INVENTION
Determining the location of a wireless device, such as a wireless telephone typically uses GPS or trilateration of mobile telephone signals broadcast from cell towers at known positions. These methods, however do not work well indoors due to the limited availability of GPS satellite signals and mobile telephone signals. Indoor positioning is important for determining the location of a portable device in buildings such as malls, hotels, offices, train stations etc.
SUMMARY
Increasingly, indoor locations such as malls, hotels, offices and train stations, provide multiple WiFi access points that, as long they operate according to an appropriate protocol, can be used to determine the location of a WiFi capable device with high accuracy.
Various aspects of the present invention for performing WiFi location include Round Trip Time (RTT) measurements and other methods using Time of Arrival (ToA) and Time of Departure (ToD) measurements. The materials that follow disclose: 1) a method to compute location using RTT which reduces the burden on the client, 2) a first receive-only location method which reduces communications overhead and may reduce power consumption by the client device. Although the invention is described in terms of a WiFi network, it is contemplated that it may be used for other wireless technologies including, without limitation, LTE, 3GPP, Bluetooth®, Zigbee® and WiGig.
Time of flight (ToF) methods to compute location include RTT measurements as described in IEEE 802.11v. These methods determine location by measuring the RTT of signals between a client device and one or more access points.
A method is disclosed for determining a location of a client device in a wireless network having at least one other network device having a known location. The method includes determining the RF delay of the client device by performing a transmission-to-self calibration at the client device and determining the RF delay of the other network device by performing a transmission-to-self calibration at the other network device. The method further includes the client device receiving a first message and a second message transmitted by the other network device to the client device, and calculating the location of the client device according to the first message, the second message, the determined RF delays and the known location of the other network devices.
According to one aspect of the method, the method further comprises transmitting from the client device an acknowledgement in response to the first message to the other network device, wherein the second message includes actual transmission time of the first message at the client device and actual reception time of the acknowledgement corresponding to the first message at the other network device.
A method is also disclosed for determining the location of a client device in a wireless network including the client device and at least a first and a second other network devices each having a known location. The method comprises determining the RF delay of the first other network device by performing a transmission-to-self calibration at the first other network device; transmitting by the first other network device, a message to the second other network device; and transmitting, by the second other network device, an acknowledgement in response to receiving the message. The method further comprises receiving, by the client device, the message, the acknowledgement and information regarding the RF delay; and calculating the location of the client device based on information including the time when the client device receives the message, the time when the client device receives the acknowledgement, the RF delay and the known locations of the first and the second other network devices.



BRIEF DESCRIPTION OF THE DRAWINGS
Aspects and features of the invention will become more apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein:
FIG. 1 (prior art) is a network communications diagram that is useful for describing a prior-art method for determining location of a client device based on round-trip time.
FIGS. 2, 3, 5A, 5B, 6A, 6B, 7-9 and 11 are network communications diagrams that are useful for describing example embodiments of the invention.
FIG. 4 is a graph of displacement along the x axis versus displacement along the y axis that is useful for describing the embodiment of the invention described with reference to FIGS. 2, 3, 5A, 5B, 6A, 6B, 7-9 and 11.
FIG. 10 is a block diagram illustrating the Tx and Rx RF delays between the ADC/DAC and the RF antenna ports of a transceiver in a communication network for example embodiments of the invention.
FIG. 12 is a proposed change to the protocol implemented in the current standard of IEEE 802.11v, according to one aspect of the invention.



DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. Embodiments described as being implemented in software should not be limited thereto, but can include embodiments implemented in hardware, or combinations of software and hardware, and vice-versa, as will be apparent to those skilled in the art, unless otherwise specified herein.
In the present specification, an embodiment showing a singular component should not be considered limiting, rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the invention encompasses present and future known equivalents to the known components referred to herein by way of illustration.
With reference to FIG. 1, the ToF between two stations, STA A and STA B, may be computed according to equation (1).

ToF=[(t2−t1)+(t4−t3)]/2  (1)

Note that the times t1 to t4 are all assumed to be measured as the signal leaves or is received at the antenna port. It is desirable to carefully calibrate the transmit (Tx) and receive (Rx) radio-frequency (RF) delays through the analogue elements of the transceivers before making these measurements. The calibration of these delays may be relatively easy for an access point device but may not be cost effective for a less expensive client device.
In FIG. 1, it is noted that six packets are transmitted over the medium for every measurement. In an indoor location such as in a train station or in a shopping mall, where 400 users are maintaining their locations by sending an automated location request every five seconds, each pair of transactions would use (100+16+44)μs where 100 μs is the typical time to transmit the location request message M1, 16 μs is the SIFS duration and 44 μs is the ACK duration. As 3 pairs of these measurements are used with each AP and this measurement is done using 3 APs, the total medium occupancy time would be 160 μs*3*3*400/5s=11.52% of the total available time. This calculation does not include retransmissions caused by erroneous or overlapping packets. Therefore it is reasonable to assume that the use of this location technique would reduce the wireless throughput substantially. There would also be additional power consumption incurred by the client device to transmit and receive the packets.
One embodiment of the subject invention concerns a method for performing location measurements and calculations that relaxes the requirement on the knowledge and calibration of the RF delays of the client device.
In this method the client device essentially measures the time-of-arrival (ToA) and the time-of-departure (ToD) at the analogue-to-digital (ADC) and digital to analogue (DAC) interfaces rather than at the antenna port. The measurements may be done in other parts of the transmit and receive chain as long as the delay between the measurement point and the packet being transmitted over the air is fixed and the delay between the packet receive at the antenna and the measurement point is fixed. Using these measurements relaxes the requirements on the knowledge of Rx and Tx RF delays of the client device.
FIG. 2 illustrates a solution using parabolic equations. In FIG. 2 the RTT transmission and the measured times is depicted for the case when the RTT transmission starts and ends at AP 210. Note that according to the protocol shown in FIG. 2 the client device 212 nominally is the device that requests this transmission exchange, according to the 802.11 standard. It is preferable for the client device 212 to request the RTT transmission as it may change channels to address various APs with relative ease in order to initiate the RTT measurement with the different APs. While this embodiment of the invention is described as using APs, it is contemplated that it may use any station (STA) in the wireless network that has a known location and operates according to a protocol similar to that described below.
When the client device 212 does not have good knowledge of its Tx and Rx RF delays, it may be desirable to devise a methodology that minimally relies on this knowledge. This can be achieved by considering differential RTT times from a client device to multiple APs.
Consider the two-dimensional example below where the client device requests measurements relative to 3 APs (not shown in FIG. 2), AP 1, AP 2 and AP 3. A two-dimensional embodiment may be useful, for example, in a region having APs with a single elevation above the floor that is sufficiently close to the elevation of the client device that any error caused by the difference in elevation may be ignored. This may be the case, for example, when STAs having known positions are used in place of APs. The measurements for AP 1, AP 2, and AP 3 are shown in Tables 1, 2 and 3, below.







TABLE 1





Measurements with AP 1:

















ToD at AP 1 at Antenna port = t11



ToA at client device antenna port = t12



ToA at client device ADC = t12′



ToD at client device DAC = t13′



ToD at client device antenna port = t13



ToA at antenna port of AP 1 = t14
















TABLE 2





Measurements with AP 2:

















ToD at AP 2 at antenna port = t21



ToA at client device antenna port = t22



ToA at client device ADC = t22′



ToD at client device DAC = t23′



ToD at client device antenna port = t23



ToA at antenna port of AP 2 = t24
















TABLE 3





Measurements with AP 3

















ToD at AP 3 at antenna port = t31



ToA at client device antenna port = t32



ToA at client device ADC = t32′



ToD at client device DAC = t33′



ToD at client device antenna port = t33



ToA at antenna port of AP 3 = t34









The following definitions apply to the equations set forth below for this example embodiment:
D1=Distance between AP1 and the client device
D2=Distance between AP2 and the client device
D3=Distance between AP3 and the client device
TTX=Delay in client device Tx chain
TRX=Delay in client device Rx chain
The roundtrip time from the antenna ports of the APs to the ADC of the client device 212 and from the DAC of the client device 212 to the antenna ports of the APs for AP 1, AP 2 and AP 3 may be described by respective equations (2), (3) and (4):




  
    
      
        
          
            2
            ⁢
            
              
                D
                1
              
              c
            
          
          +
          
            T
            Rx
          
          +
          
            T
            Tx
          
        
        =
        
          
            (
            
              
                t
                12
                ′
              
              -
              
                t
                11
              
            
            )
          
          +
          
            (
            
              
                t
                14
              
              -
              
                t
                13
                ′
              
            
            )
          
        
      
    
    
      
        (
        2
        )
      
    
  
  
    
      
        
          
            2
            ⁢
            
              
                D
                2
              
              c
            
          
          +
          
            T
            Rx
          
          +
          
            T
            Tx
          
        
        =
        
          
            (
            
              
                t
                22
                ′
              
              -
              
                t
                21
              
            
            )
          
          +
          
            (
            
              
                t
                24
              
              -
              
                t
                23
                ′
              
            
            )
          
        
      
    
    
      
        (
        3
        )
      
    
  
  
    
      
        
          
            2
            ⁢
            
              
                D
                3
              
              c
            
          
          +
          
            T
            RX
          
          +
          
            T
            TX
          
        
        =
        
          
            (
            
              
                t
                32
                ′
              
              -
              
                t
                31
              
            
            )
          
          +
          
            (
            
              
                t
                34
              
              -
              
                t
                33
                ′
              
            
            )
          
        
      
    
    
      
        (
        4
        )
      
    
  




For simplicity and without any loss of generality, the STA may be assumed to be at position (x, y) and the three APs may be assumed to be located at coordinates (0, 0), (x2, 0) and (x3, y3), respectively. This results in basic differential distance equations (5) and (6):




  
    
      
        
          D
          12
        
        =
        
          
            
              
                x
                2
              
              +
              
                y
                2
              
            
          
          -
          
            
              
                
                  (
                  
                    x
                    -
                    
                      x
                      2
                    
                  
                  )
                
                2
              
              +
              
                y
                2
              
            
          
        
      
    
    
      
        (
        5
        )
      
    
  
  
    
      
        
          D
          13
        
        =
        
          
            
              
                x
                2
              
              +
              
                y
                2
              
            
          
          -
          
            
              
                
                  (
                  
                    x
                    -
                    
                      x
                      3
                    
                  
                  )
                
                2
              
              +
              
                
                  (
                  
                    x
                    -
                    
                      y
                      3
                    
                  
                  )
                
                2
              
            
          
        
      
    
    
      
        (
        6
        )
      
    
  




where D12 and D13 are calculated according to equations (6a) and (6b) and





  
    
      
        
          D
          12
        
        =
        
          
            
              D
              1
            
            -
            
              D
              2
            
          
          =
          
            
              1
              2
            
            ⁢
            
              c
              ⁡
              
                (
                
                  
                    (
                    
                      
                        t
                        12
                        ′
                      
                      -
                      
                        t
                        11
                      
                    
                    )
                  
                  +
                  
                    (
                    
                      
                        t
                        14
                      
                      -
                      
                        t
                        13
                        ′
                      
                    
                    )
                  
                  -
                  
                    (
                    
                      
                        t
                        22
                        ′
                      
                      -
                      
                        t
                        21
                      
                    
                    )
                  
                  -
                  
                    (
                    
                      
                        t
                        24
                      
                      -
                      
                        t
                        23
                        ′
                      
                    
                    )
                  
                
                )
              
            
          
        
      
    
    
      
        (
        
          6
          ⁢
          a
        
        )
      
    
  
  
    
      
        
          D
          13
        
        =
        
          
            
              D
              1
            
            -
            
              D
              3
            
          
          =
          
            
              1
              2
            
            ⁢
            
              c
              ⁡
              
                (
                
                  
                    (
                    
                      
                        t
                        12
                        ′
                      
                      -
                      
                        t
                        11
                      
                    
                    )
                  
                  +
                  
                    (
                    
                      
                        t
                        14
                      
                      -
                      
                        t
                        13
                        ′
                      
                    
                    )
                  
                  -
                  
                    (
                    
                      
                        t
                        32
                        ′
                      
                      -
                      
                        t
                        31
                      
                    
                    )
                  
                  -
                  
                    (
                    
                      
                        t
                        34
                      
                      -
                      
                        t
                        33
                        ′
                      
                    
                    )
                  
                
                )
              
            
          
        
      
    
    
      
        (
        
          6
          ⁢
          b
        
        )
      
    
  




While the differential distances as described in equations (6a) and (6b) are described in a two-dimensional setting, the same equations for additional or fewer APs also apply in a three-dimensional or one-dimensional setting.
These two basic differential distance equations give rise to the two curves according to equations (7) and (8):



  
    
      
        
          {
          
            
              
                
                  
                    y
                    =
                  
                
                
                  
                    gx
                    +
                    h
                  
                
              
              
                
                  
                    y
                    =
                  
                
                
                  
                    
                      
                        
                          (
                          
                            
                              e
                              2
                            
                            -
                            1
                          
                          )
                        
                        ⁢
                        
                          x
                          2
                        
                      
                      +
                      
                        2
                        ⁢
                        
                          
                        
                        ⁢
                        edx
                      
                      +
                      
                        d
                        2
                      
                    
                  
                
              
            
            ⁢
            
              
            
            ⁢
            where
          
          ⁢
          
            
          
        
      
      
        
          
            
              
                
                  
                
                ⁢
                
                  (
                  7
                  )
                
              
            
          
          
            
              
                
                  
                
                ⁢
                
                  (
                  8
                  )
                
              
            
          
        
      
    
  




  
    d
    =
    
      
        
          
            D
            12
          
          2
        
        -
        
          
            
              x
              2
              2
            
            
              D
              12
            
          
          ⁢
          
            

          
          ⁢
          e
        
      
      =
      
        
          
            
              x
              2
            
            
              D
              12
            
          
          ⁢
          
            

          
          ⁢
          g
        
        =
        
          
            
              
                
                  D
                  13
                
                ⁢
                
                  x
                  2
                
              
              
                
                  D
                  12
                
                ⁢
                
                  y
                  3
                
              
            
            -
            
              
                
                  x
                  3
                
                
                  y
                  3
                
              
              ⁢
              
                
              
              ⁢
              and
              ⁢
              
                

              
              ⁢
              h
            
          
          =
          
            
              
                x
                3
                2
              
              +
              
                y
                3
                2
              
              -
              
                D
                13
                2
              
              +
              
                
                  D
                  13
                
                ⁢
                
                  
                    D
                    12
                  
                  ⁡
                  
                    (
                    
                      1
                      -
                      
                        
                          (
                          
                            
                              
                                
                                  x
                                  2
                                
                              
                            
                            
                              
                                
                                  D
                                  12
                                
                              
                            
                          
                          )
                        
                        2
                      
                    
                    )
                  
                
              
            
            
              2
              ⁢
              
                
              
              ⁢
              
                x
                3
              
            
          
        
      
    
  



The location of the client device 212 in two dimensions lies at the intersection of these two curves, as shown in FIG. 4. This methodology can be applied and generalized to more than two dimensions as well as to systems using more than 3 APs or STAs. One problem is that, in general, there may be two or more valid intersection points. An additional differential distance equation (i.e. to an additional AP or STA) can be used to resolve this ambiguity. The use of an additional AP or STA results in additional parabolic equation(s). Methods for resolving parabolic equations are similar to the methods used for hyperbolic navigation equations disclosed in a thesis by H. Senturk entitled “Performance Evaluation of Hyperbolic Position Location Technique in Cellular Wireless Networks,” Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio, which is available from OTIC Online. The two-dimensional solution may also be useful where auxiliary information, such as a map is available to resolve a valid location from among multiple intersecting points of the curves resulting from the equations.
The method described with reference to FIG. 2 allows a client device to measure the arrival time and the departure time at the DACs and ADCs or at any fixed point in the transmit and receive circuitry. The RF Rx and Tx delays can then largely be ignored. This relaxes the calibration requirements on the client device. It is noted, however, that if the Rx RF delay of the client device depends on the Rx gain, then it may be desirable for the client device to compensate for any discrepancy in these delays as transmissions received from two different APs can result in different receive gain settings in the client device. Similarly, if the Rx and/or Tx RF delays depend on the channel and/or bandwidth used for the RTT measurement to the pair of APs, then the client device may need to compensate for those factors as well. Nevertheless, this methodology opens up the possibility to cancel all or some of the dependency on the client device's RF delays.
The RTT method for determining location is simple but may suffer from high overhead if many clients try to determine their location concurrently. This may be compounded if the client devices continually try to determine their location at a high rate. Moreover, in order for a client to determine its location using the RTT method, it not only receives signals but transmit signals. This can cause unnecessarily high power consumption in the client device which is typically powered by a battery having a relatively small capacity. The following embodiments of the invention disclose a method that enables a device to determine its location by only receiving signals from WiFi access points or other stations (STAs) in the wireless network or by transmitting only one message.
Because the embodiments of the invention described use one transmission or no transmissions by the client device they may significantly reduce overhead in the system as well as power consumption of the client devices.
In a first example embodiment, shown in FIG. 3 the STA may determine its position without any transmissions. In this embodiment, each AP or STA (e.g. AP310) scans its neighborhood and finds other APs or STAs (e.g. AP 320) that support the client device receive only location scheme. Alternatively, the AP or STA may use the existing Fine Timing Measurement (FTM) mechanism from the IEEE 802.11v protocol or a variant of the FTM mechanism to find other suitable APs. It is contemplated that an AP may also learn about the support of neighboring APs by a Neighbor Report Element that the AP receives from the non-AP STAs in its basic service set (BSS) (e.g. its network). For the sake of clarity, the embodiments below are described in terms of actions performed by APs. It is contemplated, however, that any operation performed by an AP may also be performed by an STA having a known location.
The identified APs exchange information about their respective locations. Each AP (e.g. AP1 310 in FIG. 3) may periodically initiate a location measurement exchange by sending a special location measurement message as an FTM message. The other AP (e.g. AP2 320 in FIG. 3), which is still in its normal channel, receives this message and records the time at which the message was received at its antenna port. AP 320 then replies with an ACK and records when the ACK leaves its antenna port. Next AP 320 follows up with an FTM message containing the time stamps of when it received the location measurement message from AP 310 and when it transmitted the ACK. (AP 310 may or may not actually receive this message, it does not really matter. This message is intended only for the client device 312 to use for its location calculation). The above message exchange may be repeated multiple times to refine the timing measurement.
The APs 310 and 320 may repeat the whole procedure from the start of initial message exchange in an approximate period but in a non-asynchronous fashion, possibly somewhat randomized to avoid lengthy collision conditions. It is contemplated that the special location measurement message may be preceded by a Request and ACK message (as shown in FIG. 2) to determine if the other AP (e.g. AP2 320 in FIG. 3, if the Request is sent by AP1 310, or AP1 310, if the Request is sent by AP2 320) is ready for the Location Message Exchange sequence. In addition, the exchange of the Request-ACK frames may start the time stamping processes in the respective devices.
In a variant of this method, the time stamps of when AP2 320 received the location measurement message from the AP1 310 and when AP2 320 transmitted the ACK, may be contained in the ACK itself. This would eliminate one message, the second message transmitted by AP2 320, from the procedure.
In another embodiment, the special location measurement message from AP1 310 that is received by AP2 320 might be used by AP1 310 to initiate the time stamping process. To facilitate this flexibility, when AP1 310 does not receive the time stamps from AP2 320, AP1 310 may initiate the message exchange again. Additionally, AP2 320, after transmitting the ACK message, may follow up by sending the special location measurement message to AP1 310.
Each AP may also broadcast (e.g. during the Beacon transmission time) its location as well as the location of its neighboring APs, the capability of the neighboring APs to support the Location Measurement procedure, and the next possible time when the Location Measurement procedure may be initiated. This broadcast may take place at any instant and, preferably, the information is broadcast with regularity such that new client devices entering the AP's channel get this information in a timely fashion. Note that the AP may encode the location of its neighbor APs in relation to its own position and thus reduce the volume of data it transmits. Alternatively, a message may be sent periodically by some or all of the APs indicating the positions of all APs in the wireless network.
In this example, the client device 312 listens to these communications and may determine the wireless channel and the time at which the client device is to tune to the channel to receive the location measurement messages and the subsequent ACKs. It also listens to the follow up message from AP2 320 to get the time the location message was received by the AP2 320 on the channel on which it is camped, and the time that AP2 320 transmitted the ACK. With this timing information and the location of the APs 310 and 320 that the client 312 device extracts from the AP messages, the client device 312 may now estimate its location.
From the reception timing of each location measurement message and following ACK, the client device 312 computes the differential distance to the two Aps 310 and 320. The method for doing this is as follows:
The client device computes the differential distance between the two APs as shown in equation (9):

D12=C(tCD,1−(tCD,2−TAP12−(TAP2,Tx−TAP2,Rx)))  (9)

Where, as shown in FIG. 3,

    
    
        D12=The differential distance between AP1 and AP2,
        tCD,1=Time of arrival of the location measurement message from AP1 at the client device, tCD,2=Time of arrival of the ACK from AP2 at the client device,
        TAP12=Time of flight between AP1 and AP2,
        tAP2,Rx=Time of arrival of the location measurement message from AP1 at AP2,
        tAP2,Tx=Time of transmission of the ACK from AP2, and
        c=The speed of light.
    
    


Assuming a clear line of sight between AP1 310 and AP2 320, the time of flight between AP 310 and AP 320, TAP12, may be calculated as shown in equation (10):




  
    
      
        
          T
          
            A
            ⁢
            
              
            
            ⁢
            P
            ⁢
            
              
            
            ⁢
            12
          
        
        =
        
          
            
              D
              
                A
                ⁢
                
                  
                
                ⁢
                P
                ⁢
                
                  
                
                ⁢
                1
                ⁢
                
                  
                
                ⁢
                2
              
            
            c
          
          .
        
      
    
    
      
        (
        10
        )
      
    
  




Where DAP12 is the distance between AP1 310 and AP2 320. If there is not a clear line of sight between AP1 310 and AP2 320, further processing of the signals may be desirable to ensure that the calculation is based on the most direct signal (e.g. the first to arrive) and later multipath signals are ignored.
It is noted that very precise synchronization of the clocks of the three devices involved (AP1 310 and AP2 320 and the client device 312) is not required. This is because only the time differences measured close in time on the same device are relevant to the equation. Thus small discrepancies in the clocks among the devices 310, 320 and 312 tend to cancel out. Furthermore, the client device 312 does not need to make the measurements at the antenna port. It may, instead, make measurements at the ADCs or at any point in the receive circuitry that has a fixed delay from the antenna.
In a variant of this scheme the timing of the ACK transmission of AP 320 that receives the location measurement message from AP 310 in relation to the reception of that measurement message is predetermined with high accuracy and, in this case, no time-stamps may be transmitted. The client device, knowing the timing of the ACK relative to the original message, simply calculates the differential time distance as the time difference between the reception of the location measurement message and the ACK minus the ToF between the access points and the predetermined delay between the reception of the location measurement message and the ACK.
The STA measures such differential distances to multiple pairs of APs and uses them to estimates its location. The thesis by Senturk, referenced above, describes methods for calculating a location in one, two or three dimensions. To illustrate how this can be done, consider the following simple two-dimensional example with three APs, AP 1, AP 2 and AP 3. It is noted that this example may produce multiple intersecting points. A differential distance from at least one other AP or STA may be used to resolve to a single intersecting point.
For the sake of simplicity and without any loss of generality, the client device (STA) is located at coordinates (x,y) and the three APs are located at the coordinates (0,0), (x2,0) and (x3,y3) respectively. We then have the basic differential distance equations (11) and (12):




  
    
      
        
          D
          12
        
        =
        
          
            
              
                x
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where D12 and D13 are known from the measurement procedure described above.
These two basic differential distance equations give rise to the two curves described by equations (7) and (8):



  
    
      
        
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The location of the STA lies at the intersection of these two curves. Example curves are shown in FIG. 4. There may be more than one valid intersection (not counting the invalid solution shown in FIG. 4). As described above, to resolve the location measurement to a single point, it may be desirable to use additional AP differential distances to obtain additional hyperbolic equations and solve these equations using techniques described in the above-referenced thesis by Senturk.
An example of a three-dimensional scheme uses a pair of participating APs or STAs, A and B, and a mobile device C. In this example, APs A and B correspond to the respective APs 310 and 320 shown in FIG. 3 and the client device C corresponds to the client device (STA) 312.
AP A broadcasts a signal, received by AP Band client device C. Upon receiving the signal, AP B broadcasts a signal that is received by client C. The signal transmitted by AP B, (e.g. the ACK shown in FIG. 3 or the subsequent signal, described above, sent by AP B) contains the difference, 8, in time between the signal arriving at AP B from AP A and the signal departing from AP B.
At STA C, the difference in the arrival of the two signals is given by equation (13)

tB−tA=∥A−B∥+δ+∥B−C∥−∥A−C∥,  (13)

which is equivalent to equation (14):

∥B−C∥−∥A−C∥=tE−tA∥A−B∥−δ.  (14)

Where ∥A−B∥=((XA−XB)2+(YA−YB)2+(ZA−ZB)2)1/2 

and the Xs, Ys and Zs are the x, y and z coordinates of the APs A and B.

Because all of the quantities on the right hand side of equation (14) are known or measured, this is simply the equation of a hyperbola, symmetric about the line, AB, between the access points A and B.
If the signaling procedure is repeated but with an additional access point D (not shown in FIG. 3) replacing B, another hyperbola is obtained on which C should lie. In general, two hyperbolas intersect at two distinct points, so the procedure is repeated with another access point, E replacing D. The intersection of these three hyperbolas uniquely provides a position estimate of C. This analysis assumes that none of the APs A, B, D or E is collocated. Additionally, the pair of APs that the client device may use to determine its location need not have a common AP (e.g. AP A in the above example).
Methods for solving the three hyperbolic equations, centered on the lines AB, AD and AE, to obtain the estimated position of the client Care described in sections 2.5.2, of the above-referenced Senturk thesis at pages 33-36. Times are resolved into distances by multiplying the times by the speed of light, c.
The methods described above considers only the case when the client device listens to the traffic on one channel. It is contemplated, however, that the client device may listen to traffic and location messages on multiple channels, thereby possibly increasing the number of location measurements it processes.
The information that is broadcast by the AP may include an indication of the channel on which both the transmitting AP and the neighbor APs will next broadcast the information. In addition, this information may include the time stamp of the transmission time of the message by the broadcasting AP as the transmission timestamp of the neighboring APs. Alternatively, the broadcast times of the neighbor APs may be predetermined relative to the broadcast time of the current AP. Knowing these times allows a client device to change the channel if it chooses to listen to the information transmitted by the neighbor APs in order to speed up its location determination. For example, it may measure Received Carrier Power and the location of the AP. As described below, knowing the approximate timing of the message may also allow the client device to sleep until the next transmission to preserve battery life.
As shown in FIG. 5A, AP1 310 transmits a Location Message M1 to AP2 320. AP2 320 responds to the message M1 by sending an ACK to AP1 310. AP2 then sends a second Location Message M2 containing the actual transmission time (t11) of the first Location Message M1 and the reception time (t14) of the ACK message transmitted by AP2 320 in response to the first location message. Using the information contained in M2 or the actual time of reception, by the client device 312, of the same location measurement message M1 and the following ACK message, the client device can compute its differential distance to the two APs involved.
The client device may compute its differential distance with respect to the two APs 310 and 320 as shown in equation (15):

D12=c(t15−(t16−(t14−t11−TAP12)))  (15)

where

t15=Time of arrival of the location measurement message from AP1 at the client device,

t16=Time of arrival of the ACK from AP2 at the client device,

TAP12=Time of flight between AP1 and AP2,

T11=Time of transmission of the location measurement message from AP1,

t14=Time of arrival of the ACK from AP2 at AP1, and

c=The speed of light.

The rest of the scheme is the same as the method described above with reference to FIG. 3, in one, two or three dimensions.

Furthermore, the message exchange may include multiple transmissions of Location Message M2 (that includes the transmission time of the last M1 message and the reception time of its ACK), the client device can use the content of the messages (timing information) along with the actual reception time of the messages and their ACKs at the client device 312 as described above with reference to equation (13) to refine the measurement of its differential distance with the two APs 310 and 320.
In addition, AP2 320 may compute the distance DAP12, which is equivalent to D12, using equation (1). AP2 may then use a triangulation method to compute its location if the location of some APs is well known (possibly through GPS). This mechanism allows an easy deployment of location services where the location of some APs is known and other APs compute their location using RTT and triangulation methods.
A possible problem with the above-described method for receive-only client device location is that the ToF between the two APs may not be known. For example, TAP12 in equation (15) above may be unknown when there is no line of sight between the two APs (e.g., AP1 and AP2 in FIG. 5A). To solve this problem, an embodiment of the invention is described below with reference to FIG. 5B in addition to FIG. 5A described above.
In FIG. 5B, AP2 320 transmits a Location Message M1′ to AP1 310. AP1 310 responds to the message M1′ by sending an ACK′ to AP2 320. AP2 320 then sends a second Location Message M2′ containing the actual transmission time (t21) of the first Location Message M1′ and the reception time (t24) of the ACK′ message transmitted by AP1 310 in response to the first location message M1′. Using the information contained in M2′ or the actual time of reception, by the client device 312, of the same location measurement message M1′ and the following ACK′ message, the client device can compute its differential distance to the two APs involved.
Similarly, the client device 312 may compute its differential distance with respect to the two APs 310 and 320 as shown in equation (16):

D21−c(t25−(t26−(t24−t21−TAP21)))  (16)

where

t25=Time of arrival of the location measurement message M1′ from AP2 at the client device,

t26=Time of arrival of the ACK′ from AP1 at the client device,

TAP21=Time of flight between AP1 and AP2,

t21=Time of transmission of the location measurement message M1′ from AP2,

t24=Time of arrival of the ACK′ from AP1 at AP2, and

c=the speed of light.

It is assumed that the ToF between AP1 and AP2 is the same in one direction as in the other, i.e., TAP21=TAP12. Also, it is noted that D21=−D12. Therefore, plugging in equations (15) and (16), we have

D12=(D12−D21)/2=c[(t15−t25)−(t16−t26)+(t14−t24)−(t11−t21)]/2  (17)

According to equation (17), the ToF between the two APs is no longer required in computing the differential distance of the client device with respect to the two APs. Instead, the client device only needs to receive two sets of messages that the two APs transmit to each other according to FIGS. 5A and 5B.
Further, because this scheme is immune to errors in the ToF between the two APs, this scheme can also be used for a client device to calculate its differential distance with respect to two APs in situations where there is a clear line of sight between the two but accurate ToF between the two APs is not available due to other reasons (e.g., communication between the two is somehow impaired).
Another possible problem with the above-described asynchronous method for receive-only client device location is that the client device may need to be continually receiving when it wants to be able to know its current position. That is, it may not be able to enter a sleep mode if it does not want to miss some location measurement opportunities. In one contemplated variant of the above scheme, the AP pairs agree on certain periodic times when they will communicate on a channel to exchange location measurement messages. Alternatively, one AP in an AP pair may signal when it is planning to initiate the next location measurement exchange. Either of these variants may be part of a detailed protocol specifying how the APs agree and communicate at times and how they convey this information to the client device.
The following is a high-level description of the portion of the protocol. Once an AP has formed a pair with another AP it sets up a periodic communication to send location messages to the other AP. The APs negotiate on what channel(s) and with what bandwidth(s) these communications will take place. The APs also agree on the nominal timing and periodicity of the communications. It is contemplated that the timing and periodicity may be set by both APs or by one of the APs in the AP pair (e.g. AP1 310 in FIG. 5A). This is because the message sequence shown in FIG. 5A does not require the other AP to transmit any special message (e.g. AP2 320 in FIG. 5A). This is the case even if the message sequence in FIG. 5A is preceded by an exchange of Request-ACK frames (as shown in FIG. 2) that is initiated by AP1 310. The schedule may be conveyed by a Schedule Element that may be included in the message M1 and also may be included in the Neighbor Element that may be broadcast by the AP, or in any other Broadcast message.
The Schedule Element along with any additional information may include the information related to the start of next Location message Exchange, the periodicity of the message exchange and the identities of one or both the devices involved in the message exchange (e.g. MAC address, BSSID, etc.). The parameters that may be included in the Schedule Element may be conveyed by any additional frame(s) and might not be limited to the Schedule Element. To the extent they can, the APs manage the traffic in their respective basic service sets (BSSs) (i.e. the AP and all associated STAs) so that each AP is available to communicate with its paired AP at the time for the synchronization communication. In this example embodiment, for a short period of time that one AP (AP2 320), is to receive the initiating synchronization message from the other AP (AP1 310). AP2 320 ceases to transmit and listens for that synchronization transmission. Likewise AP1 310, which sends that initiating synchronization message, ceases other transmissions and instead transmits the synchronization message.
After the initiating location message has been sent, AP2 320 responds with an ACK as in the asynchronous method. Similarly, AP2 320 may follow up with a message containing the time stamps of the reception time of the location message and the transmission time of the ACK or this information may be contained in the ACK.
AP1 310 may then send a message containing the time stamp of the location measurement message it sent earlier as well as the time it received the ACK from AP2 320. Similarly, the AP2 320 may respond with a message containing the time-stamp of its location measurement transmission. In this instance, AP2 320 may also include information about the scheduling of its next beacon, including the time and periodicity of the beacon. Likewise, the AP1 310 may send a follow up message informing the AP2 320 as to the scheduling of its beacon transmissions. This beacon timing information may be passed on by the AP to the client devices in its BSS so that they know the timing of the beacons of the AP and its neighboring APs.
Another alternative is shown in FIG. 7. After the initiating location message M1 has been sent by AP1 310, AP2 320 responds with an ACK as in the asynchronous method. AP2 320 may follow up with a message M2 containing the time stamps of the reception time of the location message and the transmission time of the ACK. Alternatively, this information may be contained in the ACK.
AP1 310 may then send an ACK followed by a message, M3, containing the time stamp of the location measurement message that it sent earlier as well as the time it received the ACK from AP2 320. Similarly, the AP 320 may respond with an ACK. The messages M2 and/or M3 may include information about the scheduling of its next beacon, including its next beacon time and periodicity. In this embodiment, the client device 312 may listen to all of the messages and calculate its position using the timing data in from messages M2 and/or M3.
In particular, the client device may compute the differential distance between the two APs as shown in equation (20):

D12=c(tCD,1−tCD,2+1⁄2(tAP1,RX+tAP2,TX−tAP1,TX−tAP2,RX))  (18)

Where

tCD,1=Time of arrival of the location measurement message, M1, from AP1 at the client device,

tCD,2=Time of arrival of the ACK from AP2 at the client device,

tAP1,Tx=Time of transmission of the location measurement message M1 from AP1,

tAP1,Rx=Time of arrival of the ACK from AP2 at AP1

tAP2,Rx=Time of arrival of the location measurement, M1, message from AP1 at AP2

tAP2,Tx=Time of transmission of the ACK from AP2, and

c=The speed of light.

As with the embodiment described with reference to FIGS. 5A and 5B, the beacon timing information may be passed on by the AP to the client devices in its BSS so that they know the timing of the beacons of neighboring APs.
The APs broadcast certain information to the client devices so that each client can compute its location. This information includes:

    
    
        The location of the AP
        For each AP with which the AP is paired:
        
            The (approximate) time to the next location measurement transmission with this AP.
            The nominal periodicity of the location measurement transmissions with this AP.
            The channel and bandwidth choice for the location measurement transmission with this AP.
            The differential location of the paired AP.
        
        
    
    


Assuming a maximum relative distance of 1 km. With 17 bits per x and y dimension, the differential x-y distance may be specified with an accuracy of less than 1 cm. Assuming a maximum relative height of 250 m, with 14 bits, the relative height may be specified with an accuracy of less than 1 cm.
It is noted that this information does not need to be included in every beacon but only in beacons transmitted at times so that new client devices will be able to compute their locations in a timely fashion and so that if any client device loses track of the timing and periodicity of the location measurements it can obtain this information from the beacon broadcasts of its AP.
In another alternative embodiment of the invention, the AP may not include the location of the neighboring APs. The client device may, instead, obtain this information from the beacon transmissions of the neighboring APs. In this case, however, in order to avoid the client device having to scan for the neighboring APs, the transmitted message may also include information about when and on what channel the neighboring APs beacon transmissions will occur. If the maximum spacing of the beacons is, for example, 10 seconds and the precision of the next transmission time is, for example 10 μs, then 19 bits may be used express the timing of the next beacon and possibly another 19 bits may be used to express the periodicity of the beacon. The alternative, as described above, is to include the relative location of the neighboring AP(s). If the maximum distance between APs may be 1 km (in x, y and z dimensions) and their location is to be estimated with the precision of one cm, the transmitted relative distance would use 51 bits. Thus there could be some bandwidth saving made by including the timing of the neighboring APs beacons instead of providing their relative locations. The client, however, may need to switch between channels in order to pick up all the information used to compute its location. (In addition to the switching it would do to listen to the location measurement transmissions).
The client device receives the above broadcast information from an AP it is listing to. Based on this information it determines when and on what channel it will receive the location measurement transmissions. Once it has this information, the client device may turn off its receiver part of the time and therefore save battery power.
The location calculation for the client device is otherwise the same here as in the asynchronous method.
Other variants of this scheme are shown in FIGS. 6A and 68. The variant shown in FIG. 6A, employs access points 310 and 320 and the client device 312 and uses hyperbolic equations. The embodiment shown in FIG. 68 employs stations STAs 620 and 630 and uses parabolic equations.
In the embodiment shown in FIG. 6A, the client device 312 may be coupled to a location server 610 or to one of the APs 310 or 320. All connections to the client device 312 may be via a network, for example, a wired link or a wireless link. Some of the information, described above, that otherwise would have been transmitted over-the-air on the WiFi channel may be communicated to (and from) the location server 610 or one of the APs 310 or 320 on those other wired and/or wireless links. Examples of the information that may be communicated to the location server 610 or one of the APs include the measured time stamps and the locations of the access points 310 and 320. The location server 610, AP1 310 or AP2 320 may also perform the majority of the location calculations. This may be advantageous where it is preferred to off-load the calculations from the client device 312.
In the example shown in FIG. 6A, at time t1, AP1 310 sends the Location Message, M1, to AP2 320. At time t2, AP2 320 receives the Location Message. At time t3, AP2 320 responds with an ACK that may include values for the times t2 and t3 of the current message exchange or of a previous message exchange. At time t4, AP1 310 receives the ACK and at time t5, it responds by sending another Location Message, M2, with the times t1 and t4. As described above, using the values TCD1 and TCD2 and either the values t2 and t3 or the values t1 and t4, the client device 312 may calculate its position.
The calculations described above, however, may be complex for the client device. To reduce this burden, the client device may provide this information to the location server 610, to AP1 310 or to AP2 320. Any of these devices may perform the calculations described above to determine the location of the client device 312. Whichever device performs the calculation may then send the location information to the client device via the wireless network or by a wired network. Additionally, the client device may also provide the location information or the identification of the devices (Ex: MAC address) of the network device that made the timing measurements to allow for the location computation device to have a reference of the device. For example, if t1, t4 are sent, then the location or identification of AP1 is sent, along with TCD1 and TCD2 and the identification of client device. The accuracy of the measurement may be further improved by including an estimate of the errors in the time measurements in the information provided to compute the location.
If the data is sent by the client device 312 to AP1 310 or AP2 320, it is desirable that it not be sent during the fine timing message (FTM) exchange. As shown in FIG. 6A, the exchange begins with message M1 and ends with message Mn. The last FTM has a dialogue token of 0 to indicate that it is the last message. In one implementation, the client device 312 may wait to send the location data to AP1 310, AP2 320 or location server 610 until after this last message has been transmitted. This timing prevents interference with the FTM exchange. Alternatively, the client device 312 may send this message before the last message if the client device 312 knows the beacon scheduling of the APs 310 and 320. A Client device may choose to send the timing information to the device to do the computation based upon the signaling in the FTM Message which indicates that the device sending the FTM message can take data from the client to compute the location from the client or the client may be pre-configured to make such a transfer of the timing information.
As described above, with reference to FIG. 2, the location determination may be made via a RTT calculation, using parabolic equations. This example is shown in FIG. 6B. This implementation includes stations STA1 620 and STA2 630 as well as location server 610. Desirably, one of STA1 620 and STA2 630 is a stationary device with a known position. STA1 620 may be an AP and STA2 630 may be a mobile device. Alternatively, STA2 630 may be an AP and STA1 620 may be a mobile device. As described above with reference to FIG. 2, for a mobile device to accurately calculate its position using the parabolic technique, it desirably calculates its position with respect to multiple fixed STAs having known locations. FIG. 6B illustrates the message exchange between one fixed STA and one mobile STA. To resolve ambiguity in the position, the messages of this exchange may be interleaved with or followed by messages transmitted between the mobile STA and one or more other STAs neighboring the fixed STA.
In the embodiment shown in FIG. 6B, STA1 620 is fixed and STA2 630 is a mobile device. The FTM message exchange in FIG. 6B is essentially the same as in FIG. 6A. In this embodiment, however, the position of STA2 630 is calculated based on the position of STA1 620. Once it has obtained the RTT information, as described above with reference to FIG. 2, STA2 630 transmits the information to the location server 610 (which may be any STA) or to STA1 along with the Location or identification (Ex: MAC address) of STA1. As described above with reference to FIG. 6A, this transmission may occur after the last message in the FTM exchange or, at any time even during the FTM exchange. The transmission is desirably timed not to interfere with the FTM exchange. Additionally, the STA2 may be signaled by STA1 in the FTM Message to send its measured time stamps to the Location Server or STA1 or to any other STA that is reachable to STA2.
It is contemplated that the messages used in the message exchange may be configured to accommodate either hyperbolic or parabolic location determinations. The example message may have a flag bit indicating whether the parabolic or hyperbolic technique is being used. Each message may also transmit the various transmit and receive time values, as described above, together with an indication of the maximum error in each of the time values. The messages may also provide the MAC addresses of the STAs in the message and the media access control (MAC) address of STA originating the message. The content including the time stamps, identification (or location) details of the STAs, parabolic or hyperbolic computation flag, and errors in the time stamps, can be carried in an “Information Element”, and a single message can have one or more of these “Information Elements”.
In any of the embodiments, described above with reference to FIGS. 3-6A and 7, AP 310, when it receives the signal broadcast from AP 320, may estimate the RTT and, knowing the true distance from AP 320 or the processing time of the time stamps in the device, may estimate an error in the RTT calculation. In response to this estimated error, AP 310 may 1) issue a warning that multipath errors may be present, 2) provide an estimate of the magnitude of the multipath error or 3) provide a confidence measurement of the estimated RTT which includes the multipath error. This information may be provided by AP 310 to the client 312 for use in the position calculation. If multipath error is detected, the signaling procedure may be repeated a number of times, possibly using different frequencies and possibly in different frequency bands (e.g. 2.4 GHz and 5 GHz for WiFi) to ameliorate the multipath error.
An alternative method for augmenting a location determination is shown in FIG. 8. This method utilizes sounding signals sent as a part of a beamforming operation used by certain wireless technologies (e.g. 802.11v and 802.11ac) to implement steered signaling between a base station and a mobile device.
The example method assumes that a mobile STA can receive a signal from one or more APs that have multiple antennas. An example system in which the method may be implemented is shown in FIG. 8. The system includes a single AP 810 with two antennas 812 and 814 and non AP-STA 820 with one antenna 822.
According to this method, the transmitting AP has a known orientation and location, and a known antenna geometry. If a transmitting AP has multiple antennas, separate sounding signals may be transmitted identifiably from each antenna so that the receiving STA can determine the channel characteristics of the transmitted signal through the associated antennas.
The sounding signals may be sent periodically to determine the characteristics of the channel between the AP 810 and the STA 820. Upon receiving the sounding signal for the antennas, the STA 820 sends a response to the AP 810 with a measure of the channel quality and a matrix that addresses any phase difference between the sounding signals received from the antennas 812 and 814. The AP 810 uses the quality measure and the matrix to form a steered beam transmission channel to the STA 820.
An example embodiment may use these sounding signals to at least partially determine the location of the STA 820 relative to one or more APs 810. In the example embodiment, the received sounding measurements are used to determine the phase difference between the signals that are transmitted through the available transmit antennas 812 and 814. This angle defines a set of lines of constant phase difference that identify possible signal paths between the transmitting AP and receiving STA. In the example shown in FIG. 8, with two antennas, the non-AP STA receives sounding signals from the AP sent concurrently via the antennas 812 and 814.
By analysis of the respective times at which the signals are received (and the relationship between them), the STA 820 may determine, for example, that the signal from antenna 812 (path A) is delayed with respect to the signal from antenna 814 (path B) by an amount approximated by d·sin(Φ), where d is the distance between the antennas 812 and 814 of the AP 810. From this measurement, the STA 820 may deduce that the angle of the signal leaving the AP is Φ radians (or also possibly Φ+π radians). This angle is the angle of a straight-line path from a perpendicular line drawn through a central point between the two antennas, as shown in FIG. 8.
Similarly, the timings of reflected (multipath) signals can also be extracted, to determine their angles of departure. In general, the direct signal transmitted by an antenna is the first one to arrive at the antenna. Any later-arriving signals from that antenna represent multipath distortion. If the geometry of the area in which the signals are received is known, the angles at which the multipath signals arrive may be used to augment the location determination made by the STA 820.
The phase angle calculation may be ambiguous across complete cycles of the measured signal. However, the number of possible locations that satisfy the a positioning solution may still be smaller than would apply without the use of the measurement. For example, as described above, the angle of a pair of sounding signals received by STA 820 may be Φ radians or Φ+π radians. These angles, however, provide additional constraints which can be used with other angle measurements or with the parabolic or hyperbolic methods described above, to further refine the position of the STA 820 relative to one or more APs.
Where a larger number of APs is used, angles from multiple sets of antennas, each associated with a respective AP, can be calculated. Alternatively, where a single AP includes more than two antennas, the phase angle to the single AP may be measured more accurately. In this embodiment, the angle Φ may be an angle relative to a centroid of the multiple antennas. When geometry permits, these measurements may further limit the number of possible locations of the STA 820 in relation to reference AP 810.
When the STA 820 processes signals from multiple APs 810 (not shown), the possible locations of the STA may be the further reduced by applying triangulation based on the respective determined angles between the STA 820 and each of the APs 810.
If signals from a sufficient number APs are measured, it may be possible to determine the location of the STA uniquely, but even if the location cannot be determined uniquely, the bearing estimates made by this method may be used to augment a position estimate made using another technique, such as the parabolic and hyperbolic techniques described above, by reducing the number of possible solutions that fit both sets of data.
Where measurements are made using other techniques, information about the bearing of the received signal may also be used to evaluate whether multipath error is present. An estimate of the amount of multipath, or a confidence relating to that measurement, and can be relayed to the STA 820 for use in the position calculation employing one of the other methods.
The measurement procedure can be effected multiple times using the same APs, with the mean values of the angles used in the position calculation. The procedure can be repeated on different frequencies and possibly on a different band (e.g. 2.4 GHz and 5 GHz for Wi-Fi) to ameliorate any problems in the channel between the APs 810 and the STA 820, such as multipath.
It is also contemplated that the STA 820 may be fitted with multiple antennas (not shown). In this case further information about the radio propagation paths between the AP 810 and STA 820 may be exploited. In particular the STA 820 may be able to obtain more accurate information about the angle of arrival of the received signals by processing each of the arrived signals by each of its antennas.
In some embodiments of the invention, e.g., as discussed with respect FIG. 2 above, a client device measures the arrival time and the departure time at the DACs and ADCs or at any fixed point in the transmit and receive circuitry. As such, the RF Rx and Tx delays can then largely be ignored. This relaxes the calibration requirements on the client device.
In some embodiments of the invention, however, if the Rx RF delay of the client device depends on the Rx gain, then it may be desirable for the client device to compensate for any discrepancy in these delays as transmissions received from two different APs can result in different receive gain settings in the client device. In some implementations, the compensation is done by deduct from the TOF the RX RF delay before applying the various embodiments of RTT measurement method and receive-only method discussed above.
In yet some other embodiments of the invention, the following scheme allows a client device or an AP to compensate for any Tx and Rx group RF delays thus further improve the accuracy of the differential distance measurement discussed above.
FIG. 9 is a revised FIG. 2 that also shows the Rx and Tx RF delays of the client device and an AP. FIG. 10 is a simplified block diagram illustrating the RF Rx and Tx delays of a client device or an AP relative to the times actually reported (e.g., t1—reported) and the more accurate time (e.g., t1) that should be used in the calculation of the location of the client device as discussed above with respect to FIG. 2. Specifically,

t1=t1—reported+tAP—TX—delay 

t2=t2—reported−tclient—RX—delay 

t3=t3—reported+tclient—TX—delay 

t4=t4—reported−tAP—RX—delay 

With reference to FIGS. 9 and 10, Plugging in the above expressions, Equation (1) thus becomes equation (18):

TOF=RTT/2=(t2—reported−tAP—RX—delay)−(t1—reported+tclient—TX—delay)+(t4—reported−tclient—RX —delay)−(t3—reported+tAP—TX—delay)=((t4—reported−t1—reported)−(t3—reported−t2—reported)−Tb−Ta)/2  (18)

Where Ta=tAP—TX—delay+tAP—RX—delay, Tb−tclient—TX—delay+tclient—RX —delay.
According to some implementations, instead of reporting t4—reported, let the AP report t4—reported−Ta, and subtract Tb when computing TOF, i.e.,

TOF=((t′4—reported−t1—reported)−(t3—reported−t2—reported)−Tb))/2

Where t′4—reported=t4—reported−Ta.

With this scheme, the Tx and Rx group delays on both the AP and the client device are compensated for. By adjusting what the AP reports with the sum of the tAP—TX—delay and the tAP—RX—delay, the client is provided with the data it needs to compute the correct RTT. It should also be noted that the tAP—TX—delay and tAP—RX—delay do not need to be reported separately. The client device just needs the sum of the Rx delay and the Tx delay, i.e., the Tx and Rx group delay. The same is true for the tclient—TX—delay and tclient—RX—delay on the client device side: only the Tx and Rx group delay is necessary. A person skilled in the art will understand that these Rx and Tx RF delays can be obtained through Transmit-to-self calibration by a client device or an AP.
Generally, Transmit-to-self calibration as understood by a person skilled in the art is a process by which a modem transceiver calibrates properties of its transceiver by transmitting a signal, receiving the signal and measuring and calibrating during this process the various properties of the transceiver. The various properties measured and calibrated vary depending on the purpose of the calibration and may include the analog part of the transmitter and/or receiver, for example the transmit power, transmit and/or receiver IQ and DC imbalance. In the context of this invention, the sum of the transmitter and receivers group delays are measured and calibrated.
Similarly, FIG. 11 is a revised version of FIGS. 5A and 5B that also account for the RF delays. Equations (15) and (16) can be revised and simplified as:

D12=c(t5−(t6−(t4—reported−t1−T)))  (19)

Where T4—reported=t4−Ta, and Ta=tAP—RX—delay+tAP—TX—delay.

Therefore, the errors in equations (1), (15) and (16) are the sum of the respective Tx and Rx group delays, e.g., Ta and Tb respectively in FIG. 9, where Ta=tAP—RX—delay+tAP—TX—delay and Tb=tclient—TX—delay+tclient—RX—delay. Consequently, what is needed is accuracy in the differences of t4−t1 and t3−t2. Thus, the current standard of IEEE 802.11v which implements equation (1) can be modified to compensate for the Tx and Rx group RF delays by adding an accuracy reporting on the difference t4−t1. Similarly, a STA that can measure its sum Tx+Rx group RF delay can then report a low error in the difference t4−t1. FIG. 12 depicts an embodiment of this scheme.
All embodiments described above concern determination of a location of a client device relative to one or two fixed APs or STAs. While these locations may be used to determine an absolute location of the client device, more accurate positioning information may be obtained by obtaining and combining additional location determinations.
In the various embodiments discussed above, the client device (e.g., 212 and 312) is any type of mobile device with built-in Wi-Fi or similar wireless communications functionality. The client device is typically any cellular or similar type of telephone (e.g., iPhone, Blackberry, Android smartphone, etc.). However, many alternatives are possible such as portable or tablet computers (e.g., iPad, Galaxy), etc. WiFi transceiver functionality in a client device can be implemented using any combination of hardware and/or software, including WiFi chipsets and associated firmware/software from CSR Ltd., Marvell, or Qualcomm, possibly as adapted and/or supplemented with functionality in accordance with the present invention, as described in more detail herein. More particularly, those skilled in the art will be able to understand how to implement software or firmware with the location techniques of the present invention after being taught by the present specification.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

## Claims

1. A method for determining the location of a client device in a wireless network including the client device and at least one other network device having a known location, the method comprising:
determining a RF delay of the client device by performing, at the client device, a transmission-to-self calibration;
receiving, at the client device, a first message transmitted by the other network device to the client device;
transmitting, from the client device, an acknowledgement message to the other network device in response to receiving the first message;
receiving, at the client device, a second message transmitted by the other network device to the client device, the second message including an indication of a difference between a receipt time of the acknowledgement message at the other network device adjusted for an RF delay of the other network device determined at the other network device by a transmission-to-self calibration and a transmission time of the first message from the other network device; and
calculating, at the client device, the location of the client device based on a receipt time of the first message at the client device, a transmission time of the acknowledgment message from the client device, the indication of the difference included in the second message, and the known location of the other network device.

2. The method according to claim 1, wherein the client device includes an RF section and a digital section where receive circuitry of the RF section is coupled to the digital section by analogue-to-digital conversion (ADC) circuitry and transmit circuitry of the RF section is coupled to the digital section by digital-to-analogue conversion (DAC) circuitry.

3. The method according to claim 2, wherein the receipt time of the first message at the client device is a time at which the first message is processed by the ADC circuitry of the client device, and the transmission time of the acknowledgment message from the client device is a time at which the acknowledgement message is processed by the DAC circuitry of the client device.

4. The method according to claim 2, wherein determining the RF delay of the client device comprises determining a RF transmission delay and a RF reception delay.

5. The method according to claim 4, wherein the RF transmission delay is the time for a signal to travel from the DAC circuitry to the transmit circuitry of the RF section.

6. The method according to claim 5, wherein the RF reception delay is the time for a signal to travel from the receive circuitry of the RF section to the ADC circuitry.

7. The method according to claim 2, wherein the receipt time of the first message at the client device is a time at which the first message is received at the RF section or the receive circuitry of the RF section, and the transmission time of the acknowledgment message from the client device is a time at which the acknowledgment message is transmitted at the RF section of the transmit circuitry of the RF section.

8. A method for determining a location of a client device in a wireless network including the client device and at least a first other network device and a second other network device both having known locations, the method comprising:
receiving, at the client device, a first message transmitted by the first other network device to second other network device;
receiving, at the client device, an acknowledgement message transmitted by the second other network device to the first other network device in response to the second other network device receiving the first message;
receiving, at the client device, a second message transmitted by the first other network device, the second message including an indication of a difference between a receipt time of the acknowledgement message at the first other network device adjusted for an RF delay of the first other network device determined at the first other network device by a transmission-to-self calibration and a transmission time of the first message from the first other network device; and
calculating the location of the client device based on based on a receipt time of the first message at the client device, a receipt time of the acknowledgment message at the client device, the indication of the difference included in the second message, and the known locations of the first other network device and the second other network device.