PATENT DOCUMENT

Publication Number: US-11218876-B2
Application Number: US-201815883785-A
Country: US
Kind Code: B2

Title: Secure channel estimation architecture

Abstract:
Wireless communication between two electronic devices may be used to determine a distance between the two devices, even in the presence of an otherwise-disruptive attacker. A wireless receiver system of one device may receive a true wireless ranging signal from a first transmitting device and a false wireless ranging signal from an attacker. The wireless receiver system may correlate the wireless signals with a known preamble sequence and perform channel estimation using the result, obtaining a channel impulse response for the wireless signals. The wireless receiver system may filter the channel impulse response for the plurality of wireless signals by removing at least part of the channel impulse response due to the false wireless ranging signal while not removing at least part of the channel impulse response due to the true wireless ranging signal. The receiver system may perform a wireless ranging operation using the filtered channel impulse response.

Claims:
What is claimed is: 
     
       1. A method comprising:
 receiving, into a wireless receiver system, digital samples of a plurality of wireless signals, wherein the plurality of wireless signals comprises a true wireless ranging signal from a first transmitting device and a false wireless ranging signal from a second transmitting device; 
 correlating, in the wireless receiver system, the digital samples with a preamble sequence to generate preamble match signals corresponding to a correlation of the digital samples and the preamble sequence; 
 performing channel estimation, in the wireless receiver system, using the preamble match signals to obtain a channel impulse response for the plurality of wireless signals; 
 filtering, using the wireless receiver system, the channel impulse response, as a filtered channel impulse response, for the plurality of wireless signals, wherein the filtering removes at least part of the channel impulse response due to the false wireless ranging signal while not removing at least part of the channel impulse response due to the true wireless ranging signal; and 
 using the filtered channel impulse response to perform a wireless ranging operation. 
 
     
     
       2. The method of  claim 1 , wherein:
 the true wireless ranging signal includes a shared secret known to the wireless receiver system; 
 the false wireless ranging signal does not include the shared secret; 
 the method comprises correlating, in the wireless receiver system, the digital samples with the shared secret to generate shared secret match signals corresponding to a correlation of the digital samples and the shared secret; and 
 the method comprises estimating an attacker channel, as an estimated attacker channel, that conveys the false wireless ranging signal, wherein the estimated attacker channel enables the filtering of the channel impulse response. 
 
     
     
       3. The method of  claim 2 , wherein correlating the digital samples with the shared secret is performed using a first correlator block and correlating the digital samples with the preamble sequence is also performed using the first correlator block. 
     
     
       4. The method of  claim 2 , comprising frame-timing the digital samples as frame-timed digital samples, and subtracting the frame-timed digital samples from a first channel matched filter, wherein the first channel matched filter is matched based at least in part on the shared secret, before correlating the digital samples with the shared secret. 
     
     
       5. The method of  claim 1 , wherein the channel impulse response for the plurality of wireless signals is filtered based at least in part on a shared secret known to the wireless receiver system, wherein the true wireless ranging signal contains the shared secret but the false wireless ranging signal does not contain the true wireless ranging signal. 
     
     
       6. The method of  claim 5 , wherein the false wireless ranging signal contains a false shared secret, wherein the false wireless ranging signal is identifiable at least in part because an autocorrelation of the false shared secret with a local copy of the shared secret of the wireless receiver system is worse than an autocorrelation of the shared secret contained in the true wireless ranging signal with the local copy of the shared secret of the wireless receiver system. 
     
     
       7. The method of  claim 1 , wherein the wireless ranging operation comprises extracting a timestamp from the plurality of wireless signals, identifying from the filtered channel impulse response a timing of receipt of the true wireless ranging signal, and comparing the timestamp to the timing of receipt of the true wireless ranging signal to determine a time-of-flight of the true wireless ranging signal. 
     
     
       8. An electronic device comprising communication circuitry, wherein the communication circuitry comprises:
 an antenna configured to receive a plurality of wireless signals at least partly overlapping in time, wherein the plurality of wireless signals comprises:
 from a first transmitting device, a first wireless signal via a free-space path between the electronic device and the first transmitting device, wherein the first wireless signal comprises a preamble and a shared secret, wherein the shared secret is shared by the first transmitting device and the electronic device; and 
 from a second transmitting device, a second wireless signal comprising the preamble; and 
 
 a receiver system coupled to the antenna, wherein the receiver system is configured to:
 receive the plurality of wireless signals; 
 identify a free-space channel over which the first wireless signal is received; and 
 determine a time-of-flight of the first wireless signal after identifying the free-space channel. 
 
 
     
     
       9. The electronic device of  claim 8 , wherein the receiver system is configured to perform channel estimation based at least in part on the preamble and the shared secret to identify the free-space channel. 
     
     
       10. The electronic device of  claim 9 , wherein the receiver system is configured to use a same correlator block to correlate the preamble with the plurality of wireless signals to obtain preamble match signals and to correlate the shared secret with the plurality of wireless to obtain shared secret match signals, and wherein the receiver system is configured to use a same channel estimation block to perform channel estimation using the preamble match signals and to perform channel estimation using the shared secret match signals. 
     
     
       11. The electronic device of  claim 8 , wherein the receiver system is configured to:
 correlate digital samples of the first wireless signal and the second wireless signal with a preamble sequence to generate preamble match signals corresponding to a correlation of the digital samples and the preamble sequence; 
 performing channel estimation using the preamble match signals to obtain a channel impulse response for the first wireless signal and the second wireless signal; 
 filtering the channel impulse response, as a filtered channel impulse response, for the first wireless signal and the second wireless signal, wherein the filtering removes at least part of the channel impulse response due to a false wireless ranging signal associated with the preamble from the second transmitting device without the shared secret while not removing at least part of the channel impulse response due to a true wireless ranging signal associated with the preamble from the first transmitting device; and 
 using the filtered channel impulse response to perform a wireless ranging operation. 
 
     
     
       12. An electronic device, comprising:
 one or more antennas; and 
 a receiver system coupled to the one or more antennas, wherein the receiver system is configured to
 receive digital samples of a plurality of wireless signals, wherein the plurality of wireless signals comprises a true wireless ranging signal from a first transmitting device and a false wireless ranging signal from a second transmitting device, 
 correlate the digital samples with a preamble sequence to generate preamble match signals corresponding to a correlation of the digital samples and the preamble sequence, 
 perform channel estimation using the preamble match signals to obtain a channel impulse response for the plurality of wireless signals, 
 filter the channel impulse response, as a filtered channel impulse response, for the plurality of wireless signals, wherein the filtering removes at least part of the channel impulse response due to the false wireless ranging signal while not removing at least part of the channel impulse response due to the true wireless ranging signal, and 
 use the filtered channel impulse response to perform a wireless ranging operation. 
 
 
     
     
       13. The electronic device of  claim 12 , wherein:
 the true wireless ranging signal includes a shared secret known to the receiver system; 
 the false wireless ranging signal does not include the shared secret; and 
 the receiver system is configured to correlate the digital samples with the shared secret to generate shared secret match signals corresponding to a correlation of the digital samples and the shared secret. 
 
     
     
       14. The electronic device of  claim 13 , wherein the receiver system is configured to estimate an attacker channel as an estimated attacker channel that conveys the false wireless ranging signal, wherein the estimated attacker channel enables the filtering of the channel impulse response. 
     
     
       15. The electronic device of  claim 13 , wherein the receiver system is configured to correlate the digital samples with the shared secret by using a first correlator block and correlate the digital samples with the preamble sequence by using the first correlator block. 
     
     
       16. The electronic device of  claim 12 , wherein the receiver system is configured to frame-time the digital samples as frame-timed digital samples, and subtract the frame-timed digital samples from a first channel matched filter. 
     
     
       17. The electronic device of  claim 16 , wherein the first channel matched filter is matched based at least in part on the shared secret before correlating the digital samples with the shared secret. 
     
     
       18. The electronic device of  claim 12 , wherein the receiver system is configured to filter the channel impulse response for the plurality of wireless signals based at least in part on a shared secret known to the receiver system, the true wireless ranging signal containing the shared secret, and the false wireless ranging signal not containing the true wireless ranging signal. 
     
     
       19. The electronic device of  claim 18 , wherein the false wireless ranging signal contains a false shared secret, wherein the receiver system is configured to identify the false wireless ranging signal based at least in part on comparing an autocorrelation of the false shared secret with a local copy of the shared secret of the receiver system to an autocorrelation of the shared secret contained in the true wireless ranging signal with the local copy of the shared secret of the receiver system. 
     
     
       20. The electronic device of  claim 12 , wherein the receiver system is configured to perform the wireless ranging operation by extracting a timestamp from the plurality of wireless signals, identifying from the filtered channel impulse response a timing of receipt of the true wireless ranging signal, and comparing the timestamp to the timing of receipt of the true wireless ranging signal to determine a time-of-flight of the true wireless ranging signal.

Description:
This application claims priority to and benefit from U.S. Provisional Application No. 62/564,901, entitled “Secure Channel Estimation Architecture,” filed Sep. 28, 2017, the contents of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     This disclosure relates to systems and methods for securely identifying a wireless channel in a shortest free-space path between a transmitting device and a receiving device. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Many electronic devices, such as smartphones and computers, include antennas that are used for various forms of wireless communication. Some electronic devices may benefit from determining an accurate estimate of proximity to another electronic device using wireless signals. This may be accomplished by calculating a time-of-flight estimation for a wireless signal sent over a wireless channel of the shortest free-space path between a transmitting device and a receiving device. A simple way to determine the wireless channel of the shortest free-space path involves identifying the wireless channel between the receiving device and the transmitting device that provides the strongest signal. However, the strongest signal may not always represent the shortest free-space path between the receiving device and the transmitting device. In some cases, an obstruction—such as a person—may be positioned in the shortest free-space path between the receiving device and the transmitting device. This may lower the signal strength of the wireless channel of the shortest free-space path. Meanwhile, a wireless channel in a non-direct path, such as a reflection off a wall that goes around the obstruction, could provide a stronger signal. In cases like these, then, signal strength alone may not accurately identify the shortest free-space path between the receiving device and the transmitting device. 
     Since the signal strength does not always indicate the wireless channel of the shortest free-space path between the receiving device and the transmitting device, identifying the shortest free-space path may involve identifying the wireless channel with the signal having the earliest arrival time. For example, the signal from the transmitting device may include a defined preamble that can be used to determine which of the possible channels provides the earliest signal, even if the earliest signal is not the strongest signal. While this may allow the receiving device to accurately identify the proximity to the transmitting device in many cases, an attacker could provide a spoofed signal using the defined preamble. The spoofed signal could appear, from the perspective of the receiving device, to be earlier than the signal from the actual shortest free-space path. In this way, an attacker could cause the receiving device to misidentify the shortest free-space path between the receiving device and the transmitting device, which could thereby cause the receiving device to calculate a false proximity. 
     SUMMARY 
     A summary of certain embodiments disclosed herein is set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of these certain embodiments and that these aspects are not intended to limit the scope of this disclosure. Indeed, this disclosure may encompass a variety of aspects that may not be set forth below. 
     To protect against attacks that spoof a signal in a shortest free-space wireless channel, this disclosure provides several architectures that use both a preamble and a shared secret. Indeed, since a universally defined, plaintext preamble could be spoofed by an attacker, the systems and methods of this disclosure do not rely exclusively on a universally defined preamble to determine a shortest free-space path between a transmitting device and a receiving device. Instead, the transmitting device may send a signal that includes both a defined preamble and a cryptographically secure shared secret. Even if the attacker spoofs the preamble, the attacker may not be able to spoof the shared secret. As such, while the preamble may assist the receiving device in determining the earliest signal, and therefore the signal received over the wireless channel in the shortest free-space path, the receiving device may also rely on the shared secret. 
     The receiving device may use the shared secret differently according to different architectures. In one example, the preamble and the shared secret may be used together to perform channel estimation for identifying the wireless channel in the shortest free-space path to the transmitting device. However, while the preamble may be defined specifically to enable identifying the earliest signal, the shared secret may not be as effective as the preamble for this purpose. As such, in another architecture, the receiving device may use the preamble to identify the earliest signal, while using the shared secret to identify the attacker signal that lacks the correct shared secret present in the signals from the transmitting device. Having identified the attacker signal, the receiving device may filter away the attacker signal. Thus, the receiving device may estimate the wireless channel of the shortest free-space path using the preambles of the remaining non-attacker signals. In this way, a true wireless channel of the shortest free-space path between the transmitting device and the receiving device may be identified, even in the presence of an attacker. 
     Various refinements of the features noted above may be made in relation to various aspects of the present disclosure. Further features may also be incorporated in these various aspects as well. These refinements and additional features may be made individually or in any combination. For instance, various features discussed below in relation to one or more of the illustrated embodiments may be incorporated into any of the above-described aspects of the present disclosure alone or in any combination. The brief summary presented above is intended to familiarize the reader with certain aspects and contexts of embodiments of the present disclosure without limitation to the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of this disclosure may be better understood upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a schematic block diagram of an electronic device, in accordance with an embodiment; 
         FIG. 2  is a perspective view of a notebook computer representing an embodiment of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 3  is a front view of a hand-held device representing another embodiment of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 4  is a front view of another hand-held device representing another embodiment of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 5  is a front view of a desktop computer representing another embodiment of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 6  is a front view and side view of a wearable electronic device representing another embodiment of the electronic device of  FIG. 1 , in accordance with an embodiment; 
         FIG. 7  is a diagram of wireless signals between a transmitting device (initiator) and a receiving device (responder) in a room that includes a wireless channel in the free-space path and a wireless channel of a reflected path, in accordance with an embodiment; 
         FIG. 8  is a signal diagram illustrating a signal strength and timing of signals received via the wireless channel of the shortest free-space path and the wireless channel of the reflected path of  FIG. 7 , in accordance with an embodiment; 
         FIG. 9  is a diagram of wireless signals between the transmitting device (initiator) and the receiving device (responder) in the room of  FIG. 7  in which the wireless channel in the free-space path is obstructed, in accordance with an embodiment; 
         FIG. 10  is a signal diagram illustrating a signal strength and timing of signals received via the obstructed channel of the shortest free-space path and the wireless channel of the reflected path of  FIG. 9 , in accordance with an embodiment; 
         FIG. 11  is a block diagram of a system for identifying the wireless channel in the shortest free-space path to determine a time-of-flight through the shortest free-space path between the transmitting device (initiator) and the receiving device (responder), in accordance with an embodiment; 
         FIG. 12  is a timing diagram of signals transmitted and received between the transmitting device (initiator) and the receiving device (responder) using the system of  FIG. 11 , in accordance with an embodiment; 
         FIG. 13  is a diagram of wireless signals between the transmitting device (initiator) and the receiving device (responder) in the room of  FIG. 9  in which an attacker is sending a spoofed signal, in accordance with an embodiment; 
         FIG. 14  is a timing diagram of signals transmitted by the transmitting device (initiator) and the attacker and received by the receiving device (responder) using the system of  FIG. 11 , in accordance with an embodiment; 
         FIG. 15  is a block diagram of a system for securely identifying the wireless channel in the shortest free-space path to determine a time-of-flight through the shortest free-space path between the transmitting device (initiator) and the receiving device (responder) in the presence of an attacker, in accordance with an embodiment; 
         FIG. 16  is a block diagram of another system for securely identifying the wireless channel in the shortest free-space path to determine a time-of-flight through the shortest free-space path between the transmitting device (initiator) and the receiving device (responder) in the presence of an attacker, in accordance with an embodiment; and 
         FIG. 17  is a set of signal diagrams representing the operation of the system of  FIG. 16  to securely identify the wireless channel in the shortest free-space by filtering away a signal from the attacker, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but may nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Furthermore, the phrase A “based on” B is intended to mean that A is at least partially based on B. Moreover, unless expressly stated otherwise, the term “or” is intended to be inclusive (e.g., logical OR) and not exclusive (e.g., logical XOR). In other words, the phrase A “or” B is intended to mean A, B, or both A and B. 
     Many electronic devices, such as smartphones and computers, include antennas that are used for various forms of wireless communication. Wireless communication between two electronic devices may be used to determine a distance between the two devices. For example, a time-of-flight estimate for a signal may indicate the distance between two devices when the signal is received over a wireless channel of the shortest free-space path between the two devices. As noted above, a simple way to determine the wireless channel of the shortest free-space path involves identifying the wireless channel between a receiving device and a transmitting device that provides the strongest signal. However, the strongest signal may not always represent the shortest free-space path between the receiving device and the transmitting device. In some cases, an obstruction—such as a person—may be positioned in the shortest free-space path between the receiving device and the transmitting device. This may lower the signal strength of the wireless channel of the shortest free-space path. Meanwhile, a wireless channel in a non-direct path, such as a reflection off a wall that goes around the obstruction, could provide a stronger signal. In cases like these, then, signal strength alone may not accurately identify the shortest free-space path between the receiving device and the transmitting device. 
     Since the signal strength does not always indicate the wireless channel of the shortest free-space path between the receiving device and the transmitting device, identifying the shortest free-space path may involve identifying the wireless channel with the signal having the earliest arrival time. For example, the signal from the transmitting device may include a defined preamble that can be used to determine which of the possible channels provides the earliest signal, even if the earliest signal is not the strongest signal. While this may allow the receiving device to accurately identify the proximity to the transmitting device in many cases, an attacker could provide a spoofed signal using the defined preamble. The spoofed signal could appear, from the perspective of the receiving device, to be earlier than the actual earliest signal from the actual shortest free-space path. In this way, an attacker could cause the receiving device to misidentify the shortest free-space path between the receiving device and the transmitting device, which could thereby cause the receiving device to calculate a false proximity. 
     Several systems and methods may be used to defend against such an attack. Since a known, plaintext preamble could be spoofed by an attacker, the systems and methods of this disclosure do not rely exclusively on a defined preamble to determine a shortest free-space path between the transmitting device and the receiving device. Instead, the transmitting device may send a signal that includes both a defined preamble and a cryptographically secure shared secret. Even if the attacker spoofs the preamble, the attacker may not be able to spoof the shared secret. As such, while the preamble may assist the receiving device in determining the earliest signal, and therefore the signal received over the wireless channel in the shortest free-space path, the receiving device may use the shared secret to identify the true shortest free-space path. 
     The receiving device may use the shared secret differently according to different architectures. In one example, the preamble and the shared secret may be used together to perform channel estimation for identifying the wireless channel in the shortest free-space path to the transmitting device. The preamble may be defined specifically to enable identifying the earliest signal, whereas the shared secret may not be as effective for this purpose. As such, in another architecture, the receiving device may use the preamble to identify the earliest signal, while using the shared secret to identify the attacker signal that lacks the correct shared secret present in the signals from the transmitting device. Having identified the attacker signal, the receiving device may filter away the attacker signal. The receiving device may thus estimate the wireless channel of the shortest free-space path using the preambles of the remaining non-attacker signals. Thus, a true wireless channel of the shortest free-space path may be identified to determine the proximity between the transmitting device and the receiving device, even in the presence of an attacker. 
     With the foregoing in mind, a general description of suitable electronic devices that may use both a defined preamble and a shared secret to accurately and securely identify a shortest free-space path to another electronic device follows below. Turning first to  FIG. 1 , an electronic device  10  according to an embodiment of the present disclosure may include, among other things, one or more processor(s)  12 , memory  14 , nonvolatile storage  16 , a display  18 , input structures  22 , an input/output (I/O) interface  24 , a network interface  26 , a transceiver  28 , and a power source  29 . The various functional blocks shown in  FIG. 1  may include hardware elements (including circuitry), software elements (including computer code stored on a computer-readable medium) or a combination of both hardware and software elements. It should be noted that  FIG. 1  is merely one example of a particular implementation and is intended to illustrate the types of components that may be present in electronic device  10 . 
     By way of example, the electronic device  10  may represent a block diagram of the notebook computer depicted in  FIG. 2 , the handheld device depicted in  FIG. 3 , the handheld device depicted in  FIG. 4 , the desktop computer depicted in  FIG. 5 , the wearable electronic device depicted in  FIG. 6 , or similar devices. It should be noted that the processor(s)  12  and other related items in  FIG. 1  may be generally referred to herein as “data processing circuitry”. Such data processing circuitry may be embodied wholly or in part as software, firmware, hardware, or any combination thereof. Furthermore, the data processing circuitry may be a single contained processing module or may be incorporated wholly or partially within any of the other elements within the electronic device  10 . 
     In the electronic device  10  of  FIG. 1 , the processor(s)  12  may be operably coupled with the memory  14  and the nonvolatile storage  16  to perform various algorithms. Such programs or instructions executed by the processor(s)  12  may be stored in any suitable article of manufacture that includes one or more tangible, computer-readable media at least collectively storing the instructions or routines, such as the memory  14  and the nonvolatile storage  16 . The memory  14  and the nonvolatile storage  16  may include any suitable articles of manufacture for storing data and executable instructions, such as random-access memory, read-only memory, rewritable flash memory, hard drives, and optical discs. In addition, programs (e.g., an operating system) encoded on such a computer program product may also include instructions that may be executed by the processor(s)  12  to enable the electronic device  10  to provide various functionalities. 
     In certain embodiments, the display  18  may be a liquid crystal display (LCD), which may allow users to view images generated on the electronic device  10 . In some embodiments, the display  18  may include a touch screen, which may allow users to interact with a user interface of the electronic device  10 . Furthermore, it should be appreciated that, in some embodiments, the display  18  may include one or more organic light emitting diode (OLED) displays, or some combination of liquid crystal display (LCD) panels and OLED panels. The display  18  may receive images, data, or instructions from processor  12  or memory  14 , and provide an image in display  18  for interaction. More specifically, the display  18  includes pixels, and each of the pixels may be set to display a color at a brightness based on the images, data, or instructions from processor  12  or memory  14 . 
     The input structures  22  of the electronic device  10  may enable a user to interact with the electronic device  10  (e.g., pressing a button to increase or decrease a volume level). The I/O interface  24  may enable electronic device  10  to interface with various other electronic devices, as may the network interface  26 . The network interface  26  may include, for example, one or more interfaces for a personal area network (PAN), such as a Bluetooth network, for a local area network (LAN) or wireless local area network (WLAN), such as an 802.11x Wi-Fi network, and/or for a wide area network (WAN), such as a 3rd generation (3G) cellular network, 4th generation (4G) cellular network, long term evolution (LTE) cellular network, or long term evolution license assisted access (LTE-LAA) cellular network. The network interface  26  may also include one or more interfaces for, for example, broadband fixed wireless access networks (WiMAX), mobile broadband Wireless networks (mobile WiMAX), asynchronous digital subscriber lines (e.g., ADSL, VDSL), digital video broadcasting-terrestrial (DVB-T) and its extension DVB Handheld (DVB-H), ultra-Wideband (UWB), alternating current (AC) power lines, and so forth. 
     In certain embodiments, to allow the electronic device  10  to communicate over the aforementioned wireless networks (e.g., Wi-Fi, WiMAX, mobile WiMAX, 4G, LTE, and so forth), the electronic device  10  may include a transceiver  28 . The transceiver  28  may include any circuitry that may be useful in both wirelessly receiving and wirelessly transmitting signals (e.g., data signals). Indeed, in some embodiments, as will be further appreciated, the transceiver  28  may include a transmitter and a receiver combined into a single unit, or, in other embodiments, the transceiver  28  may include a transmitter separate from the receiver. Indeed, in some embodiments, the transceiver  28  may include several transmitters and receivers, some or none of which are combined into single units. The transceiver  28  may transmit and receive OFDM signals (e.g., OFDM data symbols) to support data communication in wireless applications such as, for example, PAN networks (e.g., Bluetooth), WLAN networks (e.g., 802.11x Wi-Fi), WAN networks (e.g., 3G, 4G, and LTE cellular networks), WiMAX networks, mobile WiMAX networks, ADSL and VDSL networks, DVB-T and DVB-H networks, UWB networks, and so forth. Further, in some embodiments, the transceiver  28  may be integrated as part of the network interfaces  26 . As further illustrated, the electronic device  10  may include a power source  29 . The power source  29  may include any suitable source of power, such as a rechargeable lithium polymer (Li-poly) battery and/or an alternating current (AC) power converter. 
     In certain embodiments, the electronic device  10  may take the form of a computer, a portable electronic device, a wearable electronic device, or other type of electronic device. Such computers may include computers that are generally portable (such as laptop, notebook, and tablet computers) as well as computers that are generally used in one place (such as conventional desktop computers, workstations, and/or servers). In certain embodiments, the electronic device  10  in the form of a computer may be a model of a MacBook®, MacBook® Pro, MacBook Air®, iMac®, Mac® mini, or Mac Pro® available from Apple Inc. By way of example, the electronic device  10 , taking the form of a notebook computer  10 A, is illustrated in  FIG. 2  in accordance with one embodiment of the present disclosure. The depicted computer  10 A may include a housing or enclosure  36 , a display  18 , input structures  22 , and ports of an I/O interface  24 . In one embodiment, the input structures  22  (such as a keyboard and/or touchpad) may be used to interact with the computer  10 A, such as to start, control, or operate a GUI or applications running on computer  10 A. For example, a keyboard and/or touchpad may allow a user to navigate a user interface or application interface displayed on display  18 . 
       FIG. 3  depicts a front view of a handheld device  10 B, which represents one embodiment of the electronic device  10 . The handheld device  10 B may represent, for example, a portable phone, a media player, a personal data organizer, a handheld game platform, or any combination of such devices. By way of example, the handheld device  10 B may be a model of an iPod® or iPhone® available from Apple Inc. of Cupertino, Calif. The handheld device  10 B may include an enclosure  36  to protect interior components from physical damage and to shield them from electromagnetic interference. The enclosure  36  may surround the display  18 . Enclosure  36  may also include sensing and processing circuitry that may be used to provide correction schemes described herein to provide smooth images in display  18 . The I/O interfaces  24  may open through the enclosure  36  and may include, for example, an I/O port for a hardwired connection for charging and/or content manipulation using a standard connector and protocol, such as the Lightning connector provided by Apple Inc., a universal service bus (USB), or other similar connector and protocol. 
     User input structures  22 , in combination with the display  18 , may allow a user to control the handheld device  10 B. For example, the input structures  22  may activate or deactivate the handheld device  10 B, navigate user interface to a home screen, a user-configurable application screen, and/or activate a voice-recognition feature of the handheld device  10 B. Other input structures  22  may provide volume control, or may toggle between vibrate and ring modes. The input structures  22  may also include a microphone may obtain a user&#39;s voice for various voice-related features, and a speaker may enable audio playback and/or certain phone capabilities. The input structures  22  may also include a headphone input to provide a connection to external speakers and/or headphones. 
       FIG. 4  depicts a front view of another handheld device  10 C, which represents another embodiment of the electronic device  10 . The handheld device  10 C may represent, for example, a tablet computer, or one of various portable computing devices. By way of example, the handheld device  10 C may be a tablet-sized embodiment of the electronic device  10 , which may be, for example, a model of an iPad® available from Apple Inc. of Cupertino, Calif. 
     Turning to  FIG. 5 , a computer  10 D may represent another embodiment of the electronic device  10  of  FIG. 1 . The computer  10 D may be any computer, such as a desktop computer, a server, or a notebook computer, but may also be a standalone media player or video gaming machine. By way of example, the computer  10 D may be an iMac®, a MacBook®, or other similar device by Apple Inc. It should be noted that the computer  10 D may also represent a personal computer (PC) by another manufacturer. A similar enclosure  36  may be provided to protect and enclose internal components of the computer  10 D such as the display  18 . In certain embodiments, a user of the computer  10 D may interact with the computer  10 D using various peripheral input devices, such as the keyboard  22 A or mouse  22 B (e.g., input structures  22 ), which may connect to the computer  10 D. 
     Similarly,  FIG. 6  depicts a wearable electronic device  10 E representing another embodiment of the electronic device  10  of  FIG. 1  that may be configured to operate using the techniques described herein. By way of example, the wearable electronic device  10 E, which may include a wristband  43 , may be an Apple Watch® by Apple, Inc. However, in other embodiments, the wearable electronic device  10 E may include any wearable electronic device such as, for example, a wearable exercise monitoring device (e.g., pedometer, accelerometer, heart rate monitor), or other device by another manufacturer. The display  18  of the wearable electronic device  10 E may include a touch screen display  18  (e.g., LCD, OLED display, active-matrix organic light emitting diode (AMOLED) display, and so forth), as well as input structures  22 , which may allow users to interact with a user interface of the wearable electronic device  10 E. 
     Wireless Ranging 
     Wireless communication to an electronic device  10  from a transmitting device may be used to determine a distance between the electronic device  10  and the transmitting device. This may be referred to as “wireless ranging.” For example, as shown in  FIG. 7 , an initiator  60  (e.g., a first electronic device  10 ) may communicate with a responder  62  (e.g., a second electronic device  10 ) in a room  64 . The room  64  may have walls  66 A,  66 B,  66 C, and  66 D. The initiator  60  may communicate wirelessly with the responder  62  by sending a wireless ranging signal in the form of a first wireless signal  68  that travels directly to the responder  62  via a free-space channel  69  through a shortest free-space path. Meanwhile, a second copy of the wireless ranging signal in the form of a second wireless signal  70  reaches the responder  62  via a reflected channel  71  that reflects off of the wall  66 A. A signal timing diagram  78  of  FIG. 8  shows that, as a consequence, the responder  62  may initially receive the free-space first wireless signal  68  in time  80  before receiving the reflected second wireless signal  70 . Because the reflected second wireless signal  70  loses energy when the second wireless signal  70  reflects against the wall  66 A, the free-space first wireless signal  68  has a greater signal strength than the reflected second wireless signal  70 . In a situation like this, the stronger signal strength correlates with the channel in the most direct path between the initiator  60  and the responder  62 . 
     But this is not always the case. Indeed, in some cases, such as the one shown by  FIG. 9 , an obstruction  90  may stand in the free-space path of the first wireless signal  68 . This could happen, for example, when a person or furniture is located directly between the initiator  60  and the responder  62 . Here, as shown by a signal timing diagram  98  of  FIG. 10 , the free-space first wireless signal  68  may still arrive earlier in time  80  than the reflected second wireless signal  70 . However, the free-space first wireless signal  68  is attenuated and may even have a lower signal strength than the reflected second wireless signal  70 . Accordingly, in some embodiments, the responder  62  may employ a receiver system  100  as shown in  FIG. 11 , which may aim to identify the shortest free-space channel  69  that conveys the first wireless signal  68 , even when the first wireless signal  68  has a lower signal strength than signals from other channels (such as the reflected second wireless signal  70  in the reflected channel  71 ). The receiver system  100  is described in block diagram form in  FIG. 11 . The various components of the receiver system  100  may be implemented in digital circuitry, software running on a processor (e.g., firmware), or some combination of these. 
     The receiver system  100  of  FIG. 11  may receive digitized analog-to-digital (ADC) samples  102  received from an antenna of the transceiver  28 . A correlator  104  may compare the received ADC samples  102  to a known preamble p. The preamble p may be a predefined set of values that is known at least to the initiator  60  and responder  62 . In some embodiments, the preamble p may be publicly known. As such, the preamble p may be sent via plaintext in at least some embodiments. Moreover, the preamble p may take any suitable signal structure that enables the correlator  104  to accurately and/or efficiently (e.g., with reduced or minimal signal sidelobes) produce a preamble correlation signal  105 . The correlator  104  may provide the preamble correlation signal  105  to a channel estimation block  106  and a start-of-frame delimiter (SFD) detector  108 . The channel estimation block  106  may identify characteristics of the various channels (e.g., free-space channel  69 , reflected channel  71 ), including which of the channels provides the earliest signal, by analyzing the preamble correlation signal  105  from the correlator  104 . The channel that provides the earliest signal may be referred to in this disclosure as the “earliest channel.” Having identified the earliest channel, a first path correction block  110  may identify when the signal from the earliest channel was received (e.g., when in time the first wireless signal  68  was received on the free-space channel  69 ) as a first path correction value. The first path correction value can be used in combination with other information to determine a proximity between the initiator  60  and the responder  62 . 
     The ADC samples  102  may also enter a channel-matched filter  112  that analyzes the ADC samples  102  for each channel based on the channel estimation from the channel estimation block  106 . The filtered results may be aligned in a frame timing block  114  according to the start-of-frame delimiter from the SFD detector  108  to extract data that can be demodulated in a demodulation block  116  and decoded in a decode block  118  to identify a timestamp  120 . The timestamp  120  represents the time provided by the initiator  60  that indicates when the initiator  60  transmitted the communication to the responder  62 . By comparing the result of the first path correction block  110  and the timestamp  120  in an adder  122 , a time-of-flight value  124  may be computed. The time-of-flight value  124  represents the time taken for the first wireless signal  68  to travel the shortest free-space path via the free-space channel  69  between the initiator  60  and the responder  62 . Using the time-of-flight value  124  and the physical parameter of the speed of electromagnetic radiation, the physical distance between the initiator  60  and the responder  62  can be estimated. 
     A timing diagram  130  shown in  FIG. 12  provides an example of the communication that may take place between the initiator  60  and the responder  62  using the system of  FIG. 11 . The timing diagram  130  shows that, at a time  132 , the initiator  60  begins to transmit a wireless signal  134 . The wireless signal  134  may contain several components, including an initial preamble p  136  and a start-of-frame delimiter (SFD)  138 , followed by data  140  (which may encode the timestamp  120 ). The wireless signal  134  may be received by the responder  62  as the free-space first wireless signal  68  and the reflected second wireless signal  70 . At a time  142 , the correlator  104  of the responder  62  may begin to analyze the received free-space first wireless signal  68  and the reflected second wireless signal  70  for a matching preamble p sequence (e.g., as the preamble correlation signal  105  shown in  FIG. 11 ). 
     In  FIG. 12 , a first preamble match is identified as signal  144  and occurs when a first preamble p sequence of the free-space first wireless signal  68  is received. Thereafter, a preamble match signal  146  occurs every time the preamble p sequence is found in a corresponding received preamble sequence  136 A of the free-space first wireless signal  68 . A preamble match signal  148  occurs every time the preamble p sequence is found in a corresponding received preamble sequence  136 B in the reflected second wireless signal  70 . In the example of  FIG. 12 , the preamble match signal  146  appears earlier than the preamble match signal  148 , but the preamble match signal  146  has a lower magnitude than the preamble match signal  148  because the signal strength of the free-space first wireless signal  68  is lower the reflected second wireless signal  70  (e.g., due to some obstruction along the free-space channel  69 ). The correlator  104  may also identify components of a received start-of-frame delimiter (SFD)  138 A of the free-space first wireless signal  68  and of a received start-of-frame delimiter (SFD)  138 B of the reflected second wireless signal  70  as SFD match signals  150 . For example, positive SFD match signal  152  relates to the received SFD  138 A, positive SFD match signal  154  relates to the received SFD  138 B, negative SFD match signal  156  relates to the received SFD  138 A, and negative SFD match signal  158  relates to the received SFD  138 B. The SFD match signals  150  allow the responder  62  to identify the start of received data  140 A or  140 B received via each wireless channel. Here, the correlator  104  is used to determine which of the signals  68  or  70  are in the earliest channel. 
     Yet the system of  FIGS. 11 and 12  could be vulnerable to certain attacks. Building on the previous examples of  FIGS. 7 and 9 , in  FIG. 13 , an attacker  170  may intercept the transmission from the initiator  60  (represented as intercepted wireless signal  172 ) and then delay and retransmit the intercepted wireless signal  172  as a false wireless ranging signal in the form of an attack signal  174  to the responder  62 . To distinguish from the true shortest free-space channel  69  and the reflected path channel  71 , the channel through which the attack signal  174  reaches the responder  62  will be referred to in this disclosure as an attack channel  176 . Note also that, while the attacker  170  is shown to be between the initiator  60  and the responder  62 , it is possible for the attacker  170  to be remote from the initiator  60  and the responder  62  and still mount an attack. In some cases, the attacker  170  could be very far (e.g., hundreds or even thousands of meters) from the initiator  60  and the responder  62 . 
     The effect of the attack signal  174  on the receiver system  100  is shown by a signal timing diagram  188  in  FIG. 14 , which builds on the example signal timing diagram  130  of  FIG. 12 . As such, a description of elements that appear in both  FIGS. 12 and 14  may be found in the previous discussion with reference to  FIG. 12 . In  FIG. 14 , the attacker  170  is shown to receive the intercepted wireless signal  172  quickly after it has been transmitted by the initiator  60 . The intercepted wireless signal  172  includes a preamble  136 C and a start-of-frame delimiter (SFD)  138 C that corresponds to the preamble  136  and the SFD  138  from the initiator  60 . The attacker  170  holds the signal for an attacker delay period  190  before transmitting the attack signal  174 , which includes a preamble  136 D and an SFD  138 D that corresponds to the preamble  136 C and the SFD  138 C. The attacker delay period  190  delays the attack signal  174  just enough to cause the attack signal  174 , when received by the responder  62  as a received attack signal  192 , to appear to be arriving earlier than either the free-space first wireless signal  68  or the reflected second wireless signal  70  due to the periodicity of the preambles  136 A,  136 B, and  136 D. Accordingly, when the correlator  104  generates a preamble match signal  194  corresponding to a match to the preamble  136 D, it recurs before the preamble match signals  144  and  146  in a repeating pattern  196 . As a consequence, the responder  62  may interpret the attack channel  176  that carries the attack signal  174  to be the earliest channel. This may prevent or complicate the efforts by the responder  62  to correctly identify the free-space channel  69 . 
     Secure Receiver Architecture Using Shared Secret for Channel Estimation 
     A secure receiver system  210 , shown in  FIG. 15 , may allow the responder  62  to thwart attacks like those discussed above, while still allowing the responder  62  to identify the shortest free-space channel  69  that conveys the first wireless signal  68 . The receiver system  210  is described in block diagram form in  FIG. 15 . The various components of the receiver system  210  may be implemented in digital circuitry, software running on a processor (e.g., firmware), or some combination of these. 
     The receiver system  210  of  FIG. 15  may receive digitized analog-to-digital (ADC) samples  102  received from an antenna of the transceiver  28 . A first correlator  104 A may compare the received ADC samples  102  to a known preamble p. The preamble p may be a predefined set of values that is known at least to the initiator  60  and responder  62 . In some embodiments, the preamble p may be publicly known. As such, the preamble p may be sent via plaintext in at least some embodiments. Moreover, the preamble p may take any suitable signal structure that enables the first correlator  104 A to accurately and/or efficiently (e.g., with reduced or minimal signal sidelobes) produce a preamble correlation signal  105 . The first correlator  104 A may provide the preamble correlation signal  105  to a first channel estimation block  106 A and a start-of-frame delimiter (SFD) detector  108 . The first channel estimation block  106 A may estimate the various channels (e.g., free-space channel  69 , reflected channel  71 , attack channel  176 ), by analyzing the preamble correlation signal  105  from the first correlator  104 A. Yet the first channel estimation block  106 A may not alone identify the earliest channel if the attack signal  174  is being sent through the attack channel  176  in a way that makes the attack signal  174  appear to be the earliest signal. Instead, the first channel estimation block  106 A may be used identify the various channels over which the responder  62  may be receiving signals, since it is possible that an attacker signal (e.g., the attack signal  174 ) could spoof the preamble. Instead, as will be discussed further below, the receiver system  210  may use a shared secrete b to identify the earliest channel. 
     As in the receiver system  100  discussed above, in the receiver system  210 , the ADC samples  102  may also enter a channel-matched filter  112  that analyzes the ADC samples  102  for each channel identified by the first channel estimation block  106 A. The filtered results may be aligned in a first frame timing block  114 A according to the start-of-frame delimiter from the SFD detector  108  to extract data that can be demodulated in a demodulation block  116  and decoded in a decode block  118  to identify a timestamp  120 . The timestamp  120  represents the time provided by the initiator  60  that indicates when the initiator  60  transmitted the communication to the responder  62 . 
     In addition, the receiver system  210  may identify the earliest channel using a shared secret b. Namely, the shared secret b may be any cryptographically secure value that is known by both the initiator  60  and the responder  62 , but which is not known by the attacker  170 . In one example, the shared secret b appears as a cryptographically secure pseudorandom number. Thus, if the attacker  170  attempts to retransmit the shared secret b, which does not have a known periodicity like the preamble p that the attacker  170  could exploit, the retransmitted shared secret b would arrive later and could be identified as late for that reason. On the other hand, if the attacker  170  attempts to use a false shared secret b′, it will not match the shared secret b that is known by the responder  62 . 
     Thus, the receiver system  210  may use a second frame timing block  114 B (which may reuse the same circuitry, software, or other processing logic as the first frame timing block  114 A) to align the ADC samples  102  to the start of the frame to begin receiving data that ostensibly contains the shared secret b. A second correlator  104 B (which may reuse the same circuitry, software, or other processing logic as the first correlator  104 A) may provide shared secret match signals  212  to a second channel estimation block  106 B (which may reuse the same circuitry, software, or other processing logic as the first channel estimation block  106 A). 
     Because the shared secret b has a sufficiently high entropy to be secure, and therefore lacks the predictable periodicity of the preamble p, the shared secret match signals  212  output by the second correlator  104 B may have a higher-order behavior in comparison to the preamble match signals  105  output by the first correlator  104 A. The higher-order behavior of the shared secret match signals  212  may manifest as sidelobes or other higher-order signal features. As such, when the shared secret match signals  212  enter the second channel estimation block  106 B, channel estimation may be more difficult when the signal strength is relatively low, which could happen if the earliest free-space channel is obstructed in some way (e.g., if there is an obstruction  90  that lowers the signal strength of the free-space first wireless signal  68  in the free-space channel  69 ). 
     Even so, because the attacker  170  does not know the shared secret b, any data from the attack signal  174  that purports to represent a false shared secret b′ will not reliably produce the shared secret match signals  212  that would be expected from the true shared secret b. As such, the second channel estimation block  106 B may not estimate the attack channel  176 . Consequently, when the results of the channel estimation from the second channel estimation block  106 B enter a first path correction block  110 , only the channels for the true signals may be estimated. Thus, provided the signal strength is sufficient to overcome the higher-order behavior of the shared secret match signals  212 , the first path correction block  110  may be able to determine the arrival time of the first wireless signal  68  on the free-space channel  69 . By comparing the result of the first path correction block  110  and the timestamp  120  in an adder  122 , a time-of-flight value  124  may be computed. The time-of-flight value  124  represents the time taken for the first wireless signal  68  to travel the shortest free-space path via the free-space channel  69  between the initiator  60  and the responder  62 . Using the time-of-flight value  124  and the physical parameter of the speed of electromagnetic radiation (e.g., the speed of light), the physical distance between the initiator  60  and the responder  62  can be estimated. 
     Secure Receiver Architecture to Filter Attack Signal Using Shared Secret 
     Another secure receiver system  240 , shown in  FIG. 16 , may allow the responder  62  to thwart attacks like those discussed above by filtering out the attack signal  174  using the shared secret b. The receiver system  240  is described in block diagram form. The various components of the receiver system  240  may be implemented in digital circuitry, software running on a processor (e.g., firmware), or some combination of these. 
     The receiver system  240  of  FIG. 16  may receive digitized analog-to-digital (ADC) samples  102  received from an antenna of the transceiver  28 . A first correlator  104 A may compare the received ADC samples  102  to a known preamble p. The preamble p may be a predefined set of values that is known at least to the initiator  60  and responder  62 . In some embodiments, the preamble p may be publicly known. As such, the preamble p may be sent via plaintext in at least some embodiments. Moreover, the preamble p may take any suitable signal structure that enables the first correlator  104 A to accurately and/or efficiently (e.g., with reduced or minimal signal sidelobes) produce a preamble correlation signal  105 . The first correlator  104 A may provide the preamble correlation signal  105  to a first channel estimation block  106 A and a start-of-frame delimiter (SFD) detector  108 . The first channel estimation block  106 A may estimate the various channels (e.g., free-space channel  69 , reflected channel  71 , attack channel  176 ) by analyzing the preamble correlation signal  105  from the first correlator  104 A. The result may include a channel impulse response (CIR) that includes the impulse response from the preambles of the various received signals (e.g., free-space first wireless signal  68 , reflected second wireless signal  70 , attack signal  174 ). This may be provided to a first path correction block  110 , but the first path correction block  110  may not alone rely on the CIR that includes all of the signals to identify the earliest signal arrival to perform first path correction. 
     Indeed, since the attack signal  174  could be sent through the attack channel  176  in a way that makes the attack signal  174  appear to be the earliest signal, the receiver system  210  may use a shared secrete b to identify the attack signal  174  so it can be filtered out of the CIR at the first path correction block  110 . This will be discussed further below. 
     Before doing so, it is noted that, as in the receiver systems  100  and  210  discussed above, in the receiver system  240 , the ADC samples  102  may also enter a channel-matched filter  112  that analyzes the ADC samples  102  for each channel identified by the first channel estimation block  106 A. The filtered results may be aligned in a first frame timing block  114 A according to the start-of-frame delimiter from the SFD detector  108  to extract data that can be demodulated in a demodulation block  116  and decoded in a decode block  118  to identify a timestamp  120 . The timestamp  120  represents the time provided by the initiator  60  that indicates when the initiator  60  transmitted the communication to the responder  62 . 
     In addition, the receiver system  240  may identify the attack signal  174  using a shared secret b. Namely, the shared secret b may be any cryptographically secure value that is known by both the initiator  60  and the responder  62 , but which is not known by the attacker  170 . In one example, the shared secret b appears as a cryptographically secure pseudorandom number. Thus, if the attacker  170  attempts to retransmit the shared secret b, which does not have a known periodicity like the preamble p that the attacker  170  could exploit, the retransmitted shared secret b would arrive later and could be identified as late for that reason. On the other hand, if the attacker  170  attempts to use a false shared secret b′, it will not match the shared secret b that is known by the responder  62 . 
     Thus, the receiver system  240  may use a second frame timing block  114 B (which may reuse the same circuitry, software, or other processing logic as the first frame timing block  114 A) to align the ADC samples  102  to the start of the frame to begin receiving data that ostensibly contains the shared secret b. A local copy  242  of the shared secret b may be provided to a second channel match filter  112 B (which may reuse the same circuitry, software, or other processing logic as the first channel matched filter  112 A) and the result subtracted in a subtraction operation  244  from the received data. Because the attacker  170  does not know the shared secret b, the attack signal  174  may use a false shared secret b′ that does not match the shared secret b. As a consequence, when the output of the subtraction operation  244  enters a second correlator  104 B (which may reuse the same circuitry, software, or other processing logic as the first correlator  104 A), any component related to a non-attacker signal (e.g., the free-space first wireless signal  68  or the reflected second wireless signal  70 ) may result in perfect correlation. 
     On the other hand, because the shared secret b has a sufficiently high entropy to be secure, and because the attacker  170  does not know the shared secret b, any data from the attack signal  174  that includes a false shared secret b′ will produce a noise signal when passed through the second correlator  104 B. Moreover, the noise signal will have a random pattern since the false shared secret b′ can be expected only to randomly correlate with the true shared secret b. Because the resulting noise signal caused by correlating the false shared secret b′ to the true shared secret b will have a predictable noise pattern, an attacker estimation block  246  may use this predictable noise pattern to identify the attack signal  174  on the attack channel  176 . An attack signal estimate  248  that corresponds to the attack signal  174  may be provided to the first path correction block  110 . 
     The first path correction block  110  may filter out the component of the CIR that corresponds to the attack signal estimate  248 , relying on the channel estimation from the channel estimation block  106  based on the preamble b to determine the first path correction. In this way, the first path correction block  110  of the receiver system  240  of  FIG. 16  may be able to determine the arrival time of the first wireless signal  68  on the free-space channel  69  based on the preamble b without performing channel estimation on a shared secret match signal that could have higher-order behavior (e.g., sidelobes), as in the receiver system  210  of  FIG. 15 . Accordingly, the receiver system  240  of  FIG. 16  may be more sensitive to a weaker signal through a true free-space path. By comparing the result of the first path correction block  110  and the timestamp  120  in an adder  122 , a time-of-flight value  124  may be computed. The time-of-flight value  124  represents the time taken for the first wireless signal  68  to travel the shortest free-space path via the free-space channel  69  between the initiator  60  and the responder  62 . Using the time-of-flight value  124  and the physical parameter of the speed of electromagnetic radiation (e.g., the speed of light), the physical distance between the initiator  60  and the responder  62  can be estimated. 
     In the receiver system  240  of  FIG. 16 , the operational results of processing the preamble may be represented as follows: 
                     p   RX     ⁢       =       p   *     h   true       +     p   *     h   attack                       h   ^     ⁢       =       p   RX     *   p                     ⁢     =       (       h   true     +     h   attack       )     ⁢     (     p   *   p     )                         ⁢     =       h   true     +     h   attack                     
where h true  represents the true free-space channel  69 , h attack  represents the attack channel  176 , p represents the preamble sequence known to both the initiator  60  and the attacker  170 , p RX  represents the correlation of the preambles from the various channels received by the responder  62 , and ĥ: represents the estimated earliest channel due to the combined true free-space channel  69  and attack channel  176 . Here, the known preamble p has perfect autocorrelation.
 
     On the other hand, since the shared secret b is not known to the attacker  170 , a false shared secret b′ sent in the attack signal  174  would be independent of the true shared secret b sent in the true free-space first wireless signal  68 . Thus, in the receiver system  240  of  FIG. 16 , the operational results of processing the true shared secret b in the true free-space first wireless signal  68  and the false shared secret b′ sent in the attack signal  174  may be represented as follows: 
                     b   RX     ⁢       =       b   *     h   true       +       b   ′     *     h   attack                         h   ^     ′     ⁢       =       (       b   RX     -     )     *   b                     ⁢     =       (     +       b   ′     *     h   attack       -   -     b   *     h   attack         )     *   b                       ⁢     =       h   attack     ⁡     (         -   b     *   b     +     b   *     b   ′         )                         ⁢     ≈     -     h   attack                     
where h true  represents the true free-space channel  69 , h attack  represents the attack channel  176 , b represents the true shared secret known to the initiator  60  but not the attacker  170 , b′ represents a false shared secret sent by the attacker  170 , b RX  represents the correlation of the false and true shared secrets from the various channels received by the responder  62 , and ĥ′ represents the estimated attack channel  176 . Here, the true shared secret b, but not the false shared secret b′, has at least partial autocorrelation. In other words, the use of the shared secret b, and the fact that it is not known to the attacker  170 , can be used to estimate the attacker channel  176  and reject the attack signal  174  on the attack channel  176 .
 
     An example is shown in  FIG. 17 . A plot  270  represents a channel impulse response (CIR) that includes the impulse response from the preambles of various received signals, including true signals from an initiator  60  (e.g., the free-space first wireless signal  68  from the free-space channel  69 , and the reflected second wireless signal  70  from the reflected channel  71 ), as well as a false signal from an attacker  170  (e.g., the attack signal  174  from the attack channel  176 ). A plot  272  represents a channel impulse response (CIR) from an attack channel estimate  248  as determined using the receiver system  240 , as discussed above. By rejecting the portions of the CIR signal due to the estimated attack channel  248  (e.g., as shown in plot  272 ) from the CIR signal due to all of the channels (e.g., as shown in plot  270 ), a corrected CIR may be obtained as shown in a plot  274 . The corrected CIR of plot  274  may include substantially only true signals from the initiator  60 . Indeed, this may allow even a faint CIR signal  276  to be detected, which may be due to the true free-space first wireless signal  68  of the free-space channel  69  because it is the earliest signal. Accordingly, an accurate first path correction may be determined even in the presence of an attacker that spoofs a preamble, and even when the rue free-space first wireless signal  68  of the free-space channel  69  is attenuated. With the accurate first path correction, an accurate and secure wireless ranging operation may be performed via the time-of-flight, to thereby determine a proximity between the initiator  60  and the responder  62 . 
     The specific embodiments described above have been shown by way of example, and it should be understood that these embodiments may be susceptible to various modifications and alternative forms. It should be further understood that the claims are not intended to be limited to the particular forms disclosed, but rather to cover all modifications, equivalents, and alternatives falling within the spirit and scope of this disclosure.

Metadata:
Filing Date: 20180130
Publication Date: 20220104
Grant Date: 20220104
Priority Date: 20170928
Inventors: YANG, SHANG-TE
CHEN, XU
MARQUEZ, Alejandro J.
NARANG, MOHIT
SEN, INDRANIL S.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01S5/0218", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01S5/0218", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04L63/1483", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L63/1483", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W12/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W12/08", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01S5/0273", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W12/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S11/06", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S11/02", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S5/0278", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W12/08", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01S5/0278", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W76/14", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04W12/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S5/0215", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01S5/0273", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L63/1483", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 65808165