Patent Publication Number: US-2022230162-A1

Title: Systems and methods for sensing locations of near field communication devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/218,216, entitled “SYSTEMS AND METHODS FOR SENSING LOCATIONS OF NEAR FIELD COMMUNICATION DEVICES” and filed on Dec. 12, 2018, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     Near field communication (NFC) devices are increasingly used in a variety of applications to communicate data. In NFC communication, a first NFC device is positioned sufficiently close (e.g., a few inches or less) to another NFC device, such as an NFC reader, so that the devices are inductively coupled. Load modulation is often used to communicate data. In this regard, the reader may transmit a wireless carrier signal, and the NFC device may change the impedance of its antenna circuit in order to modulate the carrier signal with data. The reader detects and demodulates the modulated signal in order to recover the data. 
     NFC standards require the wireless carrier signal emitted from a reader to exhibit at least a certain threshold power at any point within a specified three-dimensional (3D) test volume that extends a certain distance (e.g., about 4 centimeters) from the reader. Ensuring that the receive power of the carrier signal meets or exceeds the threshold at every point in the test volume, particularly at the boundaries of the test volume, can be problematic or result in inefficient power consumption. In this regard, one technique to ensure that the reader meets NFC specifications is to increase the reader&#39;s transmit power. However, such an increase undesirably increases the reader&#39;s power requirements, thereby reducing the useful life of its batteries. Further, reducing the reader&#39;s transmit power may affect the reader&#39;s ability to provide a suitable signal for satisfying NFC requirements at all points in the test volume. Improved techniques for enabling an NFC reader to comply with NFC standards while operating at reduced power levels are generally desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present disclosure, its nature and various advantages will be more apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  shows an illustrative block diagram of a payment system in accordance with some embodiments of the present disclosure; 
         FIG. 2  depicts an illustrative block diagram of a payment device and payment terminal in accordance with some embodiments of the present disclosure; 
         FIG. 3  depicts an illustrative payment reader and NFC device in accordance with some embodiments of the present disclosure; 
         FIG. 4  depicts an illustrative block diagram of a payment reader in accordance with some embodiments of the present disclosure; 
         FIG. 5  depicts an illustrative graph of impedance versus frequency for an antenna circuit of an NFC device in accordance with some embodiments of the present disclosure; 
         FIG. 6  depicts an illustrative graph of voltage versus frequency for signals received from an NFC device at multiple distances in accordance with some embodiments of the present disclosure; 
         FIG. 7  depicts a top view of an illustrative payment reader in accordance with some embodiments of the present disclosure; 
         FIG. 8  depicts a top view of an illustrative payment reader in accordance with some embodiments of the present disclosure; 
         FIG. 9  depicts an illustrative antenna for a payment reader in accordance with some embodiments of the present disclosure; and 
         FIG. 10  depicts a flow chart illustrating an exemplary process of determining a location of an NFC in accordance with some embodiments of the present disclosure; 
         FIG. 11  depicts an illustrative block diagram of a payment reader in accordance with some embodiments of the present disclosure; 
         FIG. 12  depicts an illustrative block diagram of a payment reader in accordance with some embodiments of the present disclosure; 
         FIG. 13  depicts a top view of an illustrative payment reader in accordance with some embodiments of the present disclosure; and 
         FIG. 14  depicts a side view of an illustrative payment reader in accordance with some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure generally pertains to systems and methods for determining a location of a near field communication (NFC) device. In some embodiments of the present disclosure, an NFC reader communicates with a remote NFC device using load modulation, as described above. In this regard, the NFC reader transmits a wireless carrier signal from an antenna, and the remote NFC device modulates such carrier signal by changing its internal impedance in order to transmit data to the NFC reader. The NFC reader has at least one antenna for receiving the signal that is load modulated by the remote NFC device. This antenna is coupled to circuitry that measures a spectrum of the received signal and identifies a resonant frequency of the NFC device based on the measured spectrum. The circuitry also determines the signal&#39;s amplitude at such resonant frequency and precisely estimates the distance of the remote NFC device from the antenna and, hence, reader based on such amplitude. By estimating the precise distance of the remote NFC device from the reader, the location of the remote NFC device relative to the reader can be precisely determined, and this information may be used to operate the NFC reader more efficiently. 
     As an example, the precise location of the remote NFC device from the reader may be used to tune the communication characteristics of the reader such that the power of the carrier signal received by the remote NFC device from the reader is higher. Thus, the NFC reader may operate at a lower transmit power relative to the transmit power that would otherwise be required in order to ensure compliance with applicable NFC standards for certification. 
     In some embodiments, the NFC reader has multiple antennas that are positioned at different locations on the reader. As described above, each antenna is coupled to circuitry that precisely measures a respective distance of the remote NFC device from the antenna based on the resonant frequency identified within the spectrum of a signal received by the antenna. The distances from multiple antennas may be used to determine the location of the remote NFC device relative to the reader in three dimensional (3D) space. In other embodiments, the distance measured by the circuitry for at least one of the antennas may be combined with data from other sensors to determine the location of the remote NFC device in 3D space. 
     In some embodiments, one of a plurality of antennas is selected for transmitting to the remote NFC device based on the determined location of the remote NFC device relative to the reader. As an example, the antenna that is better aligned with (e.g., closest to) the remote NFC device may be selected for transmitting the reader&#39;s wireless carrier signal. Thus, the NFC reader may operate at a lower transmit power relative to the transmit power that would otherwise be required in order to ensure compliance with applicable NFC standards for certification. 
     In some embodiments, the circuitry that is used to measure a spectrum of the signal received from the remote NFC device may also be used for other measurements with the reader, such as analyzing a signal from a virtual cage for sensing a tamper attempt. In addition, the spectrum measured by the circuitry for at least one antenna may be used detect a device type for the remote NFC device for use in processing information from the remote NFC device. As an example, if the remote NFC device is a payment device, such as a credit card, the reader may detect a type of payment device being used for use in processing a payment transaction. To identify device type, the NFC reader may identify physical characteristics of the remote NFC that are unique relative to other device types. Such physical characteristics may include the resonant frequency and amplitude of impedance loading from the device&#39;s antenna circuit. In some cases, based on the resonant frequency identified for the remote NFC device, the reader may detect a problem with the remote NFC device, such as damage to the antenna circuit of the remote NFC device. In yet other embodiments, the information about the received signal may be used for other purposes. 
     For illustrative purposes, various embodiments of an NFC communication system will be described in the context of payment systems that process payment transactions. However, it should be emphasized that the concepts described herein may be applied to other types of NFC communication systems as may be desired. 
       FIG. 1  depicts an illustrative block diagram of a payment system  1  that utilizes NFC communication in accordance with some embodiments of the present disclosure. In one embodiment, payment system  1  includes a payment device  10 , payment terminal  20 , network  30 , and payment server  40 . In an exemplary embodiment, payment server  40  may include a plurality of servers operated by different entities, such as a payment service system  50  and a bank server  60 . These components of payment system  1  facilitate electronic payment transactions between a merchant and a customer. 
     The electronic interactions between the merchant and the customer take place between the customer&#39;s payment device  10  and the merchant&#39;s payment terminal  20 . The customer has a payment device  10 , such as a credit card having a magnetic stripe, a credit card having an externally-driven processing device such as an EMV chip, or an NFC-enabled electronic device such as a smart phone running a payment application. The merchant has a payment terminal  20  such as a merchant device, payment reader, standalone terminal, combined customer/merchant terminals, electronic device (e.g., smart phone) running a point-of-sale application, or other electronic device that is capable of processing payment information (e.g., encrypted payment card data and user authentication data) and transaction information (e.g., purchase amount and point-of-purchase information). 
     In some embodiments (e.g., for low-value transactions or for payment transactions that are less than a payment limit indicated by an NFC or EMV payment device  10 ), the initial processing and approval of the payment transaction may be processed at payment terminal  20 . In other embodiments, payment terminal  20  may communicate with payment server  40  over network  30 . Although payment server  40  may be operated by a single entity, in one embodiment payment server  40  may include any suitable number of servers operated by any suitable entities, such as a payment service system  50  and one or more banks of the merchant and customer (e.g., a bank server  60 ). The payment terminal  20  and the payment server  40  communicate payment and transaction information to determine whether the transaction is authorized. For example, payment terminal  20  may provide encrypted payment data, user authentication data, purchase amount information, and point-of-purchase information to payment server  40  over network  30 . Payment server  40  may determine whether the transaction is authorized based on this received information as well as information relating to customer or merchant accounts, and respond to payment terminal  20  over network  30  to indicate whether or not the payment transaction is authorized. Payment server  40  may also transmit additional information such as transaction identifiers to payment terminal  20 . 
     Based on the information that is received at payment terminal  20  from payment server  40 , the merchant may indicate to the customer whether the transaction has been approved. In some embodiments such as a chip card payment device, approval may be indicated at the payment terminal, for example, at a display device of a payment terminal. In other embodiments such as a smart phone or watch operating as an NFC payment device, information about the approved transaction and additional information (e.g., receipts, special offers, coupons, or loyalty program information) may be provided to the NFC payment device for display at a screen of the smart phone or watch or storage in memory. 
     During transactions involving an EMV card, the EMV card may be inserted into a card slot of the payment terminal. The terminal may make a number of electrical connections with the EMV card including, inter alia, a power line, a ground line, a clock source line and a data line. The EMV card may have at least one processor that is powered by the power and ground lines, and that performs various functions in conjunction with the payment terminal, such as encryption and communication of card and transaction information, for example via an authorization request cryptogram (ARQC) and other transaction information. 
       FIG. 2  depicts an illustrative block diagram of payment device  10  and payment terminal  20  in accordance with some embodiments of the present disclosure. Although it will be understood that payment device  10  and payment terminal  20  of payment system  1  may be implemented in any suitable manner, in one embodiment the payment terminal  20  may comprise a payment reader  22  and a merchant device  29  (either or which may be an NFC device as will be described in more detail below). However, it will be understood that as used herein, the term payment terminal may refer to any suitable component of the payment terminal, such as payment reader  22 . In an embodiment, the payment reader  22  of payment terminal  20  may be a device that facilitates transactions between the payment device  10  and a merchant device  29  running a point-of-sale application. 
     In one embodiment, payment device  10  may be a device that is capable of communicating with payment terminal  20  (e.g., via payment reader  22 ), such as an NFC device  12  or an EMV chip card  14  (which also may be an NFC device capable of communicating with the payment reader  22  via NFC). Chip card  14  may include a secure integrated circuit that is capable of communicating with a payment terminal such as payment terminal  20 , generating encrypted payment information, and providing the encrypted payment information as well as other payment or transaction information (e.g., transaction limits for payments that are processed locally) in accordance with one or more electronic payment standards such as those promulgated by EMVCo. In some embodiments, chip card  14  may include an EMV chip that is an externally-driven processing device that receives signals necessary to operate the EMV chip (e.g., power, ground, and clock signals) from an external source. Chip card  14  may include contact pins for communicating with payment reader  22  (e.g., in accordance with ISO 7816) and in some embodiments, may be inductively coupled to payment reader  22  via a near field  15 . A chip card  14  that is inductively coupled to payment reader  22  may communicate with payment reader  22  using load modulation of a wireless carrier signal that is provided by payment reader  22  in accordance with a wireless communication standard such as ISO 14443. 
     NFC device  12  may be an electronic device such as a smart phone, tablet, or smart watch that is capable of engaging in secure transactions with payment terminal  20  (e.g., via communications with payment reader  22 ). NFC device  12  may have hardware (e.g., a secure element including hardware and executable code) and/or software (e.g., executable code operating on at least one processor in accordance with a host card emulation routine) for performing secure transaction functions. During a payment transaction, NFC device  12  may be inductively coupled to payment reader  22  via near field  15  and may communicate with payment terminal  20  by active or passive load modulation of a wireless carrier signal provided by payment reader  22  in accordance with one or more wireless communication standards such as ISO 14443 and ISO 18092. 
     Although payment terminal  20  may be implemented in any suitable manner, in one embodiment payment terminal  20  may include a payment reader  22  and a merchant device  29 . The merchant device  29  runs a point-of-sale application that provides a user interface for the merchant and facilitates communication with the payment reader  22  and the payment server  40 . Payment reader  22  may facilitate communications between payment device  10  and merchant device  29 . As described herein, a payment device  10  such as NFC device  12  or chip card  14  may communicate with payment reader  22  via inductive coupling. This is depicted in  FIG. 2  as near field  15 , which comprises a wireless carrier signal having a suitable frequency (e.g., 13.56 MHz) emitted from payment reader  22 . 
     In one embodiment, payment device  10  may be a contactless payment device such as NFC device  12  or chip card  14 , and payment reader  22  and the contactless payment device  10  may communicate by modulating the wireless carrier signal within near field  15 . In order to communicate information to payment device  10 , payment reader  22  changes the amplitude and/or phase of the wireless carrier signal based on data to be transmitted from payment reader  22 , resulting in a wireless data signal that is transmitted to the payment device. This signal is transmitted by an antenna of payment reader  22  that is tuned to transmit at 13.56 MHz, and if the payment device  10  also has a suitably tuned antenna within the range of the near field  15  (e.g., 0 to 10 cm), the payment device receives the wireless carrier signal or wireless data signal that is transmitted by payment reader  22 . In the case of a wireless data signal, processing circuitry of the payment device  10  is able to demodulate the received signal and process the data that is received from payment reader  22 . 
     When a contactless payment device such as payment device  10  is within the range of the near field  15 , it is inductively coupled to the payment reader  22 . Thus, the payment device  10  is also capable of modulating the wireless carrier signal via active or passive load modulation. By changing the tuning characteristics of the antenna of payment device  10  (e.g. by selectively switching a parallel load into the antenna circuit based on modulated data to be transmitted) the wireless carrier signal is modified at both the payment device  10  and payment reader  22 , resulting in a modulated wireless carrier signal. In this manner, the payment device is capable of sending modulated data to payment reader  22 . 
     In some embodiments, payment reader  22  also includes an EMV slot  21  that is capable of receiving chip card  14 . Chip card  14  may have contacts that engage with corresponding contacts of payment reader  22  when chip card  14  is inserted into EMV slot  21 . Payment reader  22  provides power and a clock signal to an EMV chip of chip card  14  through these contacts and payment reader  22  and chip card  14  communicate through a communication path established by the contacts. 
     Payment reader  22  may also include hardware for interfacing with a magnetic strip card (not depicted in  FIG. 2 ). In some embodiments, the hardware may include a slot that guides a customer to swipe or dip the magnetized strip of the magnetic strip card such that a magnetic strip reader can receive payment information from the magnetic strip card. The received payment information is then processed by the payment reader  22 . 
     Merchant device  29  may be any suitable device such as tablet payment device  24 , mobile payment device  26 , or payment terminal  28 . In the case of a computing device such as tablet payment device  24  or mobile payment device  26 , a point-of-sale application may provide for the entry of purchase and payment information, interaction with a customer, and communications with a payment server  40 . For example, a payment application may provide a menu of services that a merchant is able to select and a series of menus or screens for automating a transaction. A payment application may also facilitate the entry of customer authentication information such as signatures, PIN numbers, or biometric information. Similar functionality may also be provided on a dedicated payment terminal  28 . 
     Merchant device  29  may be in communication with payment reader  22  via a communication path  23 / 25 / 27 . Although communication path  23 / 25 / 27  may be implemented via a wired (e.g., Ethernet, USB, FireWire, Lightning) or wireless (e.g., Wi-Fi, Bluetooth, NFC, or ZigBee) connection, in one embodiment payment reader  22  may communicate with the merchant device  29  via a Bluetooth low energy interface. In some embodiments, processing of the payment transaction may occur locally on payment reader  22  and merchant device  29 , for example, when a transaction amount is small or there is no connectivity to the payment server  40 . In other embodiments, merchant device  29  or payment reader  22  may communicate with payment server  40  via a public or dedicated communication network  30 . Although communication network  30  may be any suitable communication network, in one embodiment communication network  30  may be the Internet and payment and transaction information may be communicated between payment terminal  20  and payment server  40  in an encrypted format such by a transport layer security (TLS) or secure sockets layer (SSL) protocol. 
       FIG. 3  depicts an exemplary payment reader  22 . The reader  22  may be configured to transmit wireless NFC signals to an NFC device  12  within close proximity to the reader  22 . As an example, when the reader  22  is sufficiently close to an NFC device  12  such that an antenna circuit  18  of the reader  22  is inductively coupled to an antenna of the NFC device  12 , the reader  22  may transmit to the NFC device  12  a wireless carrier signal that is load modulated by the NFC device  12  in order to transmit payment information for use in processing a payment transaction. The NFC device  12  may be referred to as “remote” from the reader in that it is not physically connected to the reader and uses wireless signals to communicate. 
     Applicable NFC standards may require the wireless signal transmitted by the reader  22  to exhibit an amplitude above a predefined threshold at each point within a test volume  33 . In some embodiments, the test volume  33  may extend about 4 cm from the reader  22  and may be about 5 cm wide, but other dimensions of the test volume  33  are possible. By adhering to the applicable NFC standards, the NFC device  12  should be able to communicate with the reader  22  with a certain minimum signal quality regardless of its location within the test volume  33 . In the context of a payment system, the NFC device  12  may be a payment device, such as a credit card or a mobile telephone storing a payment application, but other types of NFC devices  12  are possible. In some embodiments, the NFC device  12  may be a test probe that is used to confirm whether the reader  22  is in compliance with applicable NFC standards (e.g., to certify the reader  22 ). 
       FIG. 4  is a block diagram illustrating an exemplary embodiment of the reader  22 . The exemplary embodiment shown by  FIG. 4  has a transmitter  36  that is coupled to an antenna  39 , referred to hereafter for ease of illustration as “transmit antenna  39 .” The transmitter  36  is configured to transmit a wireless carrier signal from the transmit antenna  39 . As described above, an NFC device  12  ( FIG. 3 ) within the test volume  33  ( FIG. 3 ) may be configured to receive and modulate the wireless carrier signal in order to communicate information to the reader  22 . As shown by  FIG. 4 , the reader  22  has at least one antenna, referred to hereafter for ease of illustration as “receive antenna,” for receiving the modulated signal from the NFC device  12 . In the exemplary embodiment shown by  FIG. 4 , there are four receive antennas  41 - 44 , but there may be any number of receive antennas in other embodiments. 
     Each receive antenna  41 - 44  is coupled to a respective receiver  51 - 54 , as shown by  FIG. 4 , that receives and processes (e.g., filters and amplifies) the analog signal received from its corresponding antenna  41 - 44 . Each of the analog signals processed by the receivers  51 - 54  is received by an NFC decoder  56 , which is configured to demodulate the signal received by the antenna in order to recover and decode data transmitted by the NFC device  12 . That is, the NFC decoder  56  converts each analog signal from the receivers  51 - 54  into a respective digital data stream defining information from the NFC device  12 . A stream selector  57  may be configured to select one of the data streams for forwarding to other components of the reader  22  for further processing. As an example, the NFC device  12  may communicate payment information to be used for completing a payment transaction. In such an embodiment, the stream selector  57  may select one of the streams from a respective one of the receivers  51 - 54  for forwarding to payment processing circuitry  58 . Such data stream defines the payment processing information from the NFC device  12  and is used by the payment processing circuitry  58  in processing a payment transaction. In this regard, the payment processing circuitry  58  may be coupled to a communication interface  61 , such as a cellular radio, that is configured to wirelessly transmit payment information processed by the circuitry  58  to a payment server (not shown in  FIG. 4 ) for approval of the payment transaction. 
     Each receiver  51 - 54  is also configured to provide the processed analog signal to proximity detection circuitry  55  that then further processes the signal in order to detect a proximity of the NFC device  12  relative to the reader  22 , as will be described in more detail below. The proximity detection circuitry  55  may include one or more processors, field-programmable gate arrays (FPGAs), or other types of circuitry for performing the functions ascribed to such circuitry  55  herein. 
     The proximity detection circuitry  55  is configured to transmit to tuning circuitry  63  location data indicating the location of the payment device  12  relative to the reader  22 . The tuning circuitry  63  is configured to tune communication characteristics of the transmitter  36  based on the location of the NFC device  12  relative to the reader  22 . Specifically, the tuning circuitry  63  tunes the communication characteristics such that the received power of the wireless carrier signal at the location of the NFC device  12  is higher, thereby helping to ensure that applicable NFC standards are satisfied. As an example, the transmitter  36  may include variable capacitors and resistors that can be controlled by the tuning circuitry  63  in order to change the transmission profile of the wireless carrier signal, thereby changing the power of the carrier signal at various points within the test volume  33 . In addition, the tuning circuitry  63  may change the transmit power based on location, such as increasing transmit power when it is determined that the NFC device  12  is being moved to a dead zone or decreasing transmit power when it is determined that the NFC device  12  is being moved away from a dead zone. By optimizing the communication characteristics for the detected location of the NFC device  12 , it is possible for the reader  22  to use a lower transmit power while still ensuring that the receive power at the NFC device  12  is above a threshold specified by applicable NFC standards for the test volume  33 . That is, the communication characteristics may be tuned to ensure that the reader  22  remains compliant with the applicable specifications pertaining to receive power as the NFC device  12  is moved within the test volume  33 . 
     Note that there are various techniques that may be used to detect the location of the NFC device  12  and, specifically for example, the distance of the NFC device  12  from any of the receive antennas  41 - 44 . In one exemplary embodiment, the distance of the NFC device  12  from one or more receive antennas  41 - 44  is precisely determined based on the resonant frequency of the antenna circuit  18  within the NFC device  12 . 
     In this regard, the resonant frequency of the antenna circuit  18  of the NFC device  12  is the point where its impedance appears to be infinitely large or at open circuit. At other frequencies, the impedance of the antenna circuit  18  appears to be low.  FIG. 5  depicts an exemplary graph of impedance versus frequency for an antenna circuit  18  where resonant frequency is indicated by the peak of the curve. This high impedance at resonant frequency results in a substantial decrease in voltage for the signals received by the receive antennas  41 - 44  of the reader  22  due to the inductive coupling between the antennas of the reader  22  and the NFC device  12 . Further, the amount of the decrease is a function of distance. Specifically, the magnitude of the voltage drop increases as the NFC device  12  moves closer to the receive antennas  41 - 44  of the reader  22 . To detect a distance of the NFC device  12  from a receive antenna  41 - 44 , the reader  22  is configured to identify the resonant frequency of the antenna circuit  18  for the NFC device  12  and further to detect a change in voltage of the signal received by the receive antenna  41 - 44  resulting from the high impedance at the resonant frequency. 
     In one exemplary embodiment, in order to detect a distance of the NFC device  12  from a receive antenna  41 , the transmitter  36  is configured to transmit a continuous sine signal across a wide frequency band that includes the expected resonant frequency of the antenna circuit  18  for the NFC device  12 . The proximity detection circuitry  55  is configured to measure the voltage of the signal received by the antenna  41  from the NFC device  12  across the transmitted frequency band. As noted above, there will be a noticeable voltage drop at the resonant frequency of the antenna circuit  18  for the NFC device  12 .  FIG. 6  is a graph showing voltage versus frequencies measured by the proximity detection circuitry  55  when the NFC device is at different distances from the receive antenna  41 . Each curve of  FIG. 6  indicates the spectrum of the signal received by the antenna  41  when the NFC device  12  is located at a different distance from the antenna  41 . 
     Based on the measured spectrum of the signal received by the antenna  41 , the proximity detection circuitry  55  identifies the NFC device&#39;s resonant frequency within the spectrum, which is at the peak of the voltage drop that results from the increased impedance of the antenna circuit  18  for the NFC device  12 . The proximity detection circuitry  55  determines the magnitude of the voltage drop at resonant frequency from the curve&#39;s normal profile for other frequencies and estimates the NFC device&#39;s distance from the antenna  41  based on the voltage drop. Such estimation can be more precise relative to other techniques that may be used to estimate distance. However, in other embodiments, it is possible to use other techniques to estimate distance. 
     In some embodiments, such as the embodiment shown by  FIG. 4 , multiple receive antennas  41 - 44  may be used, and information from each of the multiple antennas  41 - 44  may be processed by the proximity detection circuitry  55  to precisely determine the NFC device&#39;s location in three-dimensional (3D) space. As an example, the proximity detection circuitry  55  may determine the x-coordinate, y-coordinate, and z-coordinate of the NFC device  12  in a coordinate system that is relative to the reader  22 . 
     In this regard, each receive antenna  41 - 44  may be at a different location on the reader  22  in order to facilitate the process of determining the location of the NFC device  12 .  FIG. 7  depicts an exemplary embodiment of the reader  22  for which the receive antennas  41 - 44  are arranged to completely surround the transmit antenna  39  in the x-direction and the y-direction. In this regard, antennas  41  and  43  are located on opposite sides of the transmit antenna  39 , and antennas  42  and  44  are located on opposite sides of the transmit antenna  39 . As shown by  FIG. 7 , it is possible for the receive antennas  41 - 44  to overlap each other, as well as the transmit antenna  39 . In some embodiments, the antennas may overlap in two dimensions (e.g., the x-direction and the y-direction, as shown by  FIG. 7 ) with at least some of the antennas being in a different plane (e.g., the plane in the x-direction and y-direction). Indeed, as will be described in more detail below, the transmit antenna  39  may be at a different z-coordinate than any of the receive antennas  41 - 44 . In other embodiments, other patterns and arrangements of the transmit antenna  39  and receive antennas  41 - 44  are possible. Note that for simplicity of illustration,  FIG. 7  shows antennas having a single turn, but each of the antennas may be a conventional NFC antenna having many turns. 
     The proximity detection circuitry  55  may be configured to determine the distance of the NFC device  12  from each receive antenna  41 - 44  using the distance estimation techniques described above. After determining the respective distances of the NFC device  12  from the receive antennas  41 - 44 , the proximity detection circuitry  55  may use triangulation, trilateration, or other techniques for determining the coordinates of the NFC device  12  in 3D space relative to the reader  22 . Such coordinates precisely indicate the location of the NFC device  12  within the test volume  33 , and the tuning circuitry  63  may use such coordinates to tune the communication characteristics of the transmitter  36 , as described above. 
     Note that the techniques for determining the location of the NFC device  12  may be simplified by assuming that the NFC  12  is within or close to the test volume  33 . For example, using such assumption in combination with conventional triangulation or trilateration techniques, it may be possible to determine the precise location of the NFC device  12  within the test volume  33  using just two receive antennas. 
     In some embodiments, the reader  22  may combine information from one or more receive antennas  41 - 44  with information from other sources in order to determine the location of the NFC device  12  in 3D space. As an example, the reader  22  may include one or more sensors  75 , as shown by  FIG. 4 , that are configured to detect information that may be used to determine one or more coordinates of the NFC device  12 . In one embodiment, the proximity detection circuitry  55  is configured to use information from the sensors  75  to determine the x-coordinate and y-coordinate of the NFC device  12  in a horizontal plane. In such an embodiment, the proximity detection circuitry  55  may use the distance determined from the signal received by one of the antennas  41 - 44  in order to determine the z-coordinate or depth of the NFC device  12  in the z-direction, which is perpendicular to the horizontal plane. Thus, a single receive antenna may be used in conjunction with the sensors  75  to determine the precise location of the NFC device relative to the reader  22  in 3D space. 
     Note that there are various types of sensors  75  that may be used to determine one or more coordinates, such as the x-coordinate and y-coordinate, of the NFC device  12 . As an example, it is possible for the sensors  75  to include one or more proximity sensors that measure distance or sense objects within a certain range. Such sensors may detect objects by transmitting a signal and measuring an amount of the signal that reflects from objects and returns to the sensor. As an example, an optical sensor may be used to transmit an optical signal, or a sonar sensor may be used to transmit an acoustic signal. In other embodiments, other types of sensors, such as capacitive sensors, may be used to sense a presence of the NFC device  12  or a distance of the NFC device from the respective sensor. 
       FIG. 8  shows an exemplary embodiment having a single receive antenna  41  used in conjunction with a horizontal row  81  of proximity sensors  82  for sensing the location of the NFC device  12  in the x-direction and a vertical row  83  of proximity sensors  82  for sensing the location of the NFC device  12  in the y-direction. In such an embodiment, each sensor  82  may sense when the NFC device  12  is directly above the sensor  82 . Thus, the proximity detection circuitry  55  may determine the x-coordinate of the NFC device  12  by determining which of the sensors  82  of row  81  are sensing a presence of the NFC device  12 , and the proximity detection circuitry  55  may determine the y-coordinate of the NFC device  12  by determining which of the sensors  82  of row  83  are sensing a presence of the NFC device  12 . In other embodiments, other types of sensors  75  and techniques may be used to determine the coordinates of the NFC device  12 . 
       FIG. 9  depicts an exemplary looped antenna  90  having multiple pins  92  that are electrically coupled to the proximity detection circuitry  55 . In the exemplary embodiment shown by  FIG. 9 , the proximity detection circuitry  55  is coupled to the pins  92  through a multiplexer (MUX)  94  so that the proximity detection circuitry  55  receives signals from different pins serially rather than in parallel, but the use of multiplexer  94  is unnecessary in other embodiments. 
     For each pin  92 , the proximity detection circuitry  55  is configured to measure the spectrum of the signal received from the pin  92  and to then estimate the distance of the NFC device  12  from the pin  92  using the same techniques described above for the receive antennas  41 - 44 . Note that the antenna  90  may have any number of pins  92  so that the proximity detection circuitry  55  may have any number of data points for determining the location of the NFC device  12 . 
     An exemplary operation and use of the system  1  will be described below with reference to  FIG. 10 . 
     In this regard, assume that the reader  22  is currently communicating with an NFC  12  device that is within the test volume  33 . In this regard, the communication characteristics of the transmitter  36  are such that the NFC device  12  receives a sufficient amount of power from the wireless carrier signal transmitted by the reader  22 . That is, the amount of power received by the NFC device  12  exceeds a predefined threshold so that the reader  22  is in compliance with applicable NFC standards. However, assume that the signal profile of the wireless carrier signal is such that there is a dead zone within the test volume  33  at or near a boundary of the test volume  33 . In this regard, if the NFC device  12  is moved to the dead zone without changing the communication characteristics of the transmitter  36 , then the amount of power received by the NFC device  12  would fall below the predefined threshold so that the reader  22  is no longer compliant with NFC standards. For illustrative purposes, assume that the NFC device  12  is moved into this dead zone. 
     When the NFC  12  is moved into the dead zone, the reader  22  detects the change in location of the NFC device  12  and then adjusts the communication characteristics of the transmitter  36  so that the NFC device  12  receives a suitable amount of power from the reader  22  in accordance with applicable NFC standards. In this regard, for at least one receive antenna  41 - 44 , the proximity detection circuitry  55  of the reader  22  measures the spectrum of the signal received by such antenna  41 - 44  across a range of frequencies, as shown by block  101  of  FIG. 10 . Based on this measured spectrum, the proximity detection circuitry  55  identifies the resonant frequency of the antenna circuit  18  for the NFC device  12 , as shown by block  103 . As described above, such resonant frequency is at the peak of the voltage drop resulting from the increased impedance of the NFC device&#39;s antenna circuit  18 . 
     As shown by block  106  and  109 , the proximity detection circuitry  55  of the reader  22  determines the amplitude of the received signal at resonant frequency and determines a distance of the NFC device  12  from the receive antenna based on this amplitude. As an example, the proximity detection circuitry  55  may determine a voltage drop of the signal at the resonant frequency and convert this voltage drop into a distance estimate. Specifically, the reader  22  may include one or more tables that can be used by the proximity detection circuitry  55  to look up or otherwise find the distance of the NFC device  12  from the reader using amplitude or voltage drop at the resonant frequency as a lookup key. In other embodiments, the proximity detection circuitry  55  may calculate the distance using a predefined formula that is based on the measured amplitude or voltage drop. 
     As shown by block  112 , the proximity detection circuitry  55  determines the precise location of the NFC device  12  relative to the reader  22  using at least the distance determined in block  109 . As an example, the proximity detection circuitry  55  may perform blocks  101 ,  103 ,  106 , and  109  for multiple receive antennas and then use the estimated distances to calculate the coordinates of the NFC device  12  in 3D space. In another example, the proximity detection circuitry  55  may use parameters sensed by sensors  75  to determine the x-coordinate and y-coordinate of the NFC device  12  and to use the distance estimated in block  109  to determine the NFC device&#39;s z-coordinate. In other examples, other techniques may be used to determine the location of the NFC device  12  in 3D space based on the distance estimated in block  109 . 
     Based on the location of the NFC device  12  determined in block  112 , the reader  22  is configured to perform at least one operation for enhancing the efficiency or operation of the reader  22 , as shown by block  115  of  FIG. 10 . As an example, the tuning circuitry  63  may change the communication characteristics of the transmitter  36 . In the instant example where the NFC device  12  has been moved to a dead zone, the tuning circuitry  63  tunes the communication characteristics of the transmitter  36  so that the NFC device  12  receives a greater amount of power from the reader  22 . Specifically, the receive power is increased by the tuning in the dead zone so that the NFC device  12  receives a sufficient amount of power for the reader  22  to remain compliant with applicable NFC standards. Thus, the reader  22  may operate at a relatively low power level that results in dead zones within the test volume  33  but nevertheless remain compliant with applicable NFC standards by adjusting the communication characteristics of the transmitter  36  as the NFC device  12  is moved to different locations in the test volume  33 . 
     It should be emphasized that the above configurations and processes are exemplary, and various modifications may be made to the aforementioned embodiments. As an example, it is possible for components of the proximity detection circuitry  55  used to analyze the spectra of signals received by the antennas  41 - 44  to also be used for measuring the spectra of other signals within the reader  22 , as may be desired. In this regard, as shown by  FIG. 11 , the proximity detection circuitry  55  may include a spectrum analyzer  133  for measuring the spectra of signals, such as the signals received by the antennas  41 - 44 . Depending on its design, the spectrum analyzer  133  has certain electrical components for measuring spectra. As an example, the spectrum analyzer  133  may have a narrowband filter with an adjustable center frequency that can be swept through a range of frequencies so that a voltage sensor can measure the voltage of the received signal at multiple frequencies. In another embodiment, the spectrum analyzer  133  may have a Fourier transform device that transforms the received signal from the time domain to the frequency domain to allow an analysis of the signal in the frequency domain. The spectrum analyzer  133  may also have a signal generator that varies a frequency of its output so that it can sweep across a range of frequencies. These same components, such as filters, voltage sensors, Fourier transform devices, signal generators, and other devices that may be used to measure a spectrum of a signal from at least one of the receive antennas  41 - 44  may also be used to measure the spectrum of at least one other signal in the reader  22 . 
     As an example, the payment processing circuitry  58  may define a secure area for processing sensitive information such as personal identification numbers (PINs) used in payment transactions. As shown by  FIG. 11 , one or more components, such as a secure processor  138  for processing sensitive information of a payment transaction, may be located in the secure area, which is protected from tampering attempts by a virtual cage  141 . As known in the art, the virtual cage  141  may comprise a plurality of conductive traces that pass over the components within the virtual cage  141  such that a hacker would need to touch or modify (e.g., cut) at least one of the conductive traces in order to physically access the components in the secure area. 
     The spectrum analyzer  133  of the proximity detection circuitry  55  may be electrically coupled to the traces of the virtual cage  141  and transmit a probe signal through the traces of the virtual cage  141 . The spectrum analyzer  133  may also analyze the response (i.e., the probe signal after passing through the virtual cage) to determine the spectrum of the received signal. In particular, the same components of the spectrum analyzer  133  used to measure the spectrum of a signal from at least one receive antenna  41 - 44 , such as the filter, voltage sensor, or Fourier transform device described above, may also be used to measure the spectrum of the signal received from the virtual cage  141 . If the measured spectrum from the virtual cage  141  materially changes (e.g., the voltage at one or more frequencies changes by at least a threshold amount), the proximity detection circuitry  133  may be configured to detect a tamper attempt. In response to such detection of a tamper event, the proximity circuitry  133  may take one or more actions, such as transmitting a warning message or disabling one or more components or functions of the reader  22 . Moreover, using the same components of the spectrum analyzer  133  to measure spectra of different signals within the reader  22  helps to reduce the circuitry within and the overall cost of the reader  22 . 
     In some embodiments, the proximity detection circuitry  55  may be configured to detect a problem with the NFC device  12  based on the spectrum of the signal received from it. In this regard, certain types of NFC devices  12  are expected to have resonant frequencies within a certain frequency range. As an example, a design of an NFC device  12  from a first issuing bank may be expected to have a resonant frequency in one range, and an NFC device  12  from another issuing bank may be expected to have a resonant frequency in another range. In the context of test probes, there may be multiple types of test probes that could be used to certify the reader  22 . The proximity detection circuitry  55  may include memory for storing predefined data indicating the expected resonant frequency range for the NFC device  12 . If there are multiple types of NFC devices  12  with different frequency ranges, the proximity detection circuitry  55  may store the expected resonant frequency range for each device type. 
     When the proximity detection circuitry  55  identifies the resonant frequency for the antenna circuit  18  of the NFC device  12 , it is configured to compare the measured resonant frequency to the expected resonant frequency for the NFC device  12  stored in memory. If the measured resonant frequency is outside of this range, the proximity detection circuitry  55  may be configured to detect a problem with the NFC device and report the problem to a user. As an example, the resonant frequency of the NFC device  12  may be outside of the expected range when the antenna circuit  18  has been damaged, which may impair the device&#39;s ability to reliably communicate with the reader  22 . In such case, the proximity detection circuitry  55  may be configured to transmit a warning message from the reader  22  using the communication interface  61  or otherwise to a server or other location so that corrective action may be taken. As an example, the reader  22  may transmit the warning message to the payment server to be used for approving the payment transaction so that the issuing bank may take corrective action, such as issuing a new NFC device  12  to the consumer making the purchase. In other embodiments, other types of corrective actions may be taken. 
     The reader  22  may also be configured to tailor its communication characteristics based on the determined device type for the NFC device  12 . In this regard, the tuning circuitry  63  may adjust the communication characteristics of the transmitter  36  based on the device type determined by the proximity detection circuitry  55 . In some cases, the tuning circuitry  63  may tune the communication characteristics based on both device type and the location of the NFC device  12 . In some embodiments, device type may be determined based on data communicated from the NFC device  12 , such as an account number or other payment information, to be used in a payment transaction. In other embodiments, device type may be determined based on the characteristics of a signal received from the NFC device  12 . As an example, the proximity detection circuitry  55  may identify device type based on the device&#39;s resonant frequency, as determined by the circuitry  55  according to the techniques described above. Other physical characteristics indicated by the spectrum of the received signal, such as the amplitude of impedance loading may also be used to identify device type. In other embodiments, other techniques for determining device type are possible. 
     In some embodiments, the location of the NFC device  12  may be used to perform other functions in the reader  22  in addition to or in lieu of tuning the transmitter  36 . As an example, the location of the NFC device  12  may be used to select one or more transmit antennas for communicating with the NFC device  12 .  FIG. 12  depicts an exemplary embodiment of a reader  22  having a plurality of transmit antennas  161 - 164 . In the embodiment shown by  FIG. 12 , each transmit antenna  161 - 164  is coupled to the transmitter  36  through a multiplexer  172 . However, other configurations are possible in other embodiments. As an example, it is possible for each transmit antenna  161 - 164  to be coupled to a respective transmitter without the use of a multiplexer. 
     Each transmit antenna  161 - 164  is preferably positioned at a different location on the reader  22 . As an example,  FIG. 13  shows an exemplary embodiment for which the transmit antennas  161 - 164  are positioned at different locations and are overlapping. Other patterns and locations of the antennas  161 - 164  are possible in other embodiments. 
     Once the proximity detection circuitry  55  has determined the location of the NFC device  12 , as described above, the circuitry  55  may be configured to selectively enable the transmit antennas  161 - 164  based on the determined location of the NFC device  12 . As an example, the proximity detection circuitry  55  may enable the transmit antenna that is better aligned with (e.g., closest to) the NFC device  12  and to disable the other transmit antennas. Thus, the signal strength of the wireless carrier signal received from the enabled transmit antenna should be stronger relative the signal strength of a wireless carrier that would be transmitted from one of the other transmit antennas. If the NFC device  12  is moved to a different location so that it is better aligned with another transmit antenna, then this other transmit antenna may be selected for communication with the NFC device. Thus, as the NFC device  12  moves within the test volume  33 , the antenna that is likely to provide the strongest wireless carrier signal to the NFC device  12  is selected for communication and enabled while the other transmit antennas are disabled. Selective use of multiple transmit antennas  161 - 164  in this way may help to eliminate dead zones within the test volume  33  and ensure that the NFC device  12  receives a strong signal. That is, selection of the transmit antenna  161 - 164  to communicate with the NFC device  12  is optimized based on the NFC device&#39;s current location in order to ensure that the NFC device  12  receives a strong signal from the reader  22  regardless of its location within the test volume  33 . 
     Note that there are various techniques that can be used to enable and disable the transmit antennas  161 - 164 . In the embodiment depicted by  FIG. 12 , the proximity detection circuitry  55  controls the multiplexer  172  in order to control selection of the transmit antenna  161 - 164  to be used for communication. In this regard, the proximity detection circuitry  55  controls the multiplexer  172  such that the transmit antenna  161 - 164  selected for communication receives the carrier signal from the transmitter  36  while the remaining transmit antennas do not. In other embodiments, other techniques for enabling and disabling the transmit antennas  161 - 164  are possible. As an example, if multiple transmitters  36  are used, the proximity detection circuitry  55  may control the transmitters such that only the one that is coupled to the transmit antenna  161 - 164  to be enabled actually transmits the carrier signal. 
     In other embodiments, it is possible for multiple transmit antennas  161 - 164  to be enabled for communicating with the NFC device  12 . However, enabling more than one transmit antenna  161 - 164  for communication has various disadvantages that can be avoided by enabling only one transmit antenna  161 - 164  at a time. For example, enabling multiple transmit antennas  161 - 164  increases power requirements and can also create interference between the multiple carrier signals that are being transmitted. Further, when multiple transmit antennas  161 - 164  are turned on, it is possible for the antennas  161 - 164  to detune one another. Selecting only one transmit antenna  161 - 164  for communication helps to provide a more efficient solution and, in particular, helps to reduce power and increase reliability of the data communications. 
     As described above, it is possible to use one or more receive antennas  41 - 44  to receive signals that are used for both data communication and proximity detection of the NFC device  12 . In addition, the stream selector  57  may be configured to select a data stream from one of the receive antennas  41 - 44  for processing a payment transaction or other type of transaction by the circuitry  58 . As an example, cyclic redundancy check (CRC) data or other types of data used for detecting errors may be included in the information transmitted by the NFC device  12 , and the stream selector  57  may be configured to use such information to detect errors in the received data streams. The stream selector  57  may be further configured to count the errors from each receive antenna  41 - 44  over a given time window, and select the data stream from the receive antenna  41 - 44  associated with the lowest error count. The stream selector  57  may then send the information from the selected data stream to the payment processing circuitry  58  for use in processing the payment transaction. In other embodiments, other techniques for selecting the data stream to be used for the payment transaction are possible. 
     As an example, it is possible for the proximity detection circuitry  55  to inform the stream selector  57  of the location of the NFC device  12  and for the stream selector  57  to then select the data stream from the receive antenna  41 - 44  that is better aligned with (e.g., closest to) the NFC device  12 . In yet other embodiments, other techniques for selecting the desired data stream to use for further processing are possible. In addition, in other embodiments, the information from the stream selector  57  may be used for other purposes or types of transactions. That is, the use of the reader  22  in performing payment transactions is unnecessary, and the reader  22  may be used in the same way as described above in order to process other types of transactions. 
     Note that simultaneous use of multiple receive antennas  41 - 44 , as described above, does not have the same disadvantages that are associated with simultaneous use of multiple transmit antennas  161 - 164 . In this regard, the receive antennas  41 - 44  can be passive such that they do not consume any power from the battery or other power resources of the reader  22 . That is, the receive antennas  41 - 44  do not increase the power requirements of the reader  22 . Further, since the receive antennas  41 - 44  are passive, they do not have a tuning circuit, and their impedance does not load the tuning circuitry  63 . 
     In addition, there is not a stringent impedance requirement for passive receive antennas  41 - 44 . In some embodiments, transparent materials, such as Indium Thin Oxide (ITO), may be used for the receive antennas  41 - 44  instead of opaque materials having a higher conductance, such as copper. By using transparent materials, the receive antennas  41 - 44  can be placed on a surface of the reader  22  without adversely affecting the aesthetic appearance of the reader to users. As an example, if the reader  22  has a display device, such as a liquid crystal display (LCD), the receive antennas  41 - 44  could be placed on or near the surface of the display device in view of the user without being significantly noticeable. Such placement of the receive antennas  41 - 44  may provide a larger receive area, thereby helping to improve receive sensitivity. Further, placement of the antennas  41 - 44  on or near the surface of the display device or other surface of the reader  22  may prevent the display device or other components of the reader  22  from significantly attenuating the signal received by the antennas  41 - 44 . That is, such signal does not need to pass through the display device or other components before being received by the antennas  41 - 44 . 
       FIG. 14  depicts an exemplary reader  22  having a display device  205 , such as an LCD, embedded in or otherwise coupled to the reader  22 . As shown by  FIG. 14 , the receive antennas  41 - 44  may be composed of a transparent material and positioned on or near a surface of the display device  205 . The transmit antennas  161 - 164 , which may be composed of an opaque material, such as copper, may positioned on an opposite side of the display device  205  such that the wireless carrier signal transmitted by one of the transmit antennas  161 - 164  passes through the display device  205  before reaching the NFC device  12 . Such placement of the transmit antennas  161 - 164  undesirably increases signal attenuation since the wireless carrier signal passes through the display device  205 , but the transmit antennas  161 - 164  are hidden by the display device  205  so that they do not adversely affect the aesthetic appearance of the reader  22 . In other embodiments, other configurations and placement of the antennas  41 - 44  and  164 - 164  are possible. 
     Note that, in several embodiments described above, the circuitry of the reader  22  is shown as disparate blocks for illustrative purposes. It is unnecessary for the circuitry to be separated or segmented in any manner, and it is possible for the same set of circuitry to be used for multiple blocks. As an example, the term “circuitry” may be used to refer to any block of circuitry shown by the figures or to refer collectively to multiple blocks. As an example, circuitry may include a processor that is programmed with instructions for performing functions of the tuning circuitry  63 , functions of the proximity detection circuitry  55 , and/or functions of the payment processing circuitry  58 . Moreover, the same hardware resources, such as one or more processors or other types of circuitry, may be used to implement the functionality of multiple blocks. 
     The foregoing is merely illustrative of the principles of this disclosure and various modifications may be made by those skilled in the art without departing from the scope of this disclosure. The above described embodiments are presented for purposes of illustration and not of limitation. The present disclosure also can take many forms other than those explicitly described herein. Accordingly, it is emphasized that this disclosure is not limited to the explicitly disclosed methods, systems, and apparatuses, but is intended to include variations to and modifications thereof, which are within the spirit of the following claims. 
     As a further example, variations of apparatus or process parameters (e.g., dimensions, configurations, components, process step order, etc.) may be made to further optimize the provided structures, devices and methods, as shown and described herein. In any event, the structures and devices, as well as the associated methods, described herein have many applications. Therefore, the disclosed subject matter should not be limited to any single embodiment described herein, but rather should be construed in breadth and scope in accordance with the appended claims.