Patent Publication Number: US-2022225255-A1

Title: Premium device-aided low-tier device group delay calibration for nr positioning

Description:
BACKGROUND 
     Determining the location of a mobile electronic device (also referred to herein as a User Equipment (UE)) using a cellular network may use signaling between the device and base stations of the cellular network. According to some techniques, Round-Trip-Time (RTT) measurements may be made to determine distances between the device and the base stations, from which the location of the device may be determined. But these measurements can suffer inaccuracy due to internal delays at the device. 
     BRIEF SUMMARY 
     Techniques described herein provide for calibrating group delay for a low-tier device by leveraging the relatively high accuracy of RTT positioning for a premium device. This can enable in-field group delay calibration of low-tier devices, allowing for low-tier devices to be calibrated when needed. Depending on desired functionality, techniques for calibration may include the use of RTT measurements with a base station, or an RTT measurement between the low-tier device and the premium device. 
     An example method of determining a group delay of a first mobile device, according to this description, comprises obtaining a first RTT measurement between the first mobile device and a base station, and identifying a second mobile device within a threshold distance of the first mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device. The method further includes obtaining a second RTT measurement between the second mobile device and the base station, and determining a group delay of the first mobile device based on a difference between the first RTT measurement and the second RTT measurement. 
     Another example method of determining a group delay of a first mobile device, according to this description, comprises obtaining information indicative of a known distance between the first mobile device and a second mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device and obtaining an RTT measurement between the first mobile device and the second mobile device. The method further includes determining a group delay of the first mobile device based on a difference between the known distance and a distance determined by the RTT measurement. 
     An example device, according to this description, comprises a transceiver, a memory, and one or more processing units communicatively coupled with the transceiver and the memory. The one or more processing units are configured to obtain a first RTT measurement between a first mobile device and a base station, and identify a second mobile device within a threshold distance of the first mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device. The one or more processing units are also configured to obtain a second RTT measurement between the second mobile device and the base station, and determine a group delay of the first mobile device based on a difference between the first RTT measurement and the second RTT measurement. 
     An example mobile device, according to this description, comprises a transceiver, a memory, and one or more processing units communicatively coupled with the transceiver and the memory. The one or more processing units are configured to obtain information indicative of a known distance between a first mobile device and a second mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device. The one or more processing units are also configured to obtain, using the transceiver, an RTT measurement between the first mobile device and the second mobile device, and determine a group delay of the first mobile device based on a difference between the known distance and a distance determined by the RTT measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a terrestrial positioning system, according to an embodiment. 
         FIG. 2  is a timing diagram illustrating the basic steps of an RTT measurement, according to an embodiment. 
         FIG. 3  is a timing diagram illustrating how group delay can impact the accuracy of RTT measurements. 
         FIG. 4  is a diagram of a first technique for group delay calibration of a low-tier device, according to an embodiment. 
         FIG. 5  is a diagram of a second technique for group delay calibration of a low-tier device, according to an embodiment. 
         FIG. 6  is a flow diagram of a method of determining the group delay of a first device (e.g., a low-tier device), according to an embodiment utilizing a base station. 
         FIG. 7  is a flow diagram of a method of determining the group delay of a first device (e.g., a low-tier device), according to an embodiment that uses direct communications with a second device (e.g., a premium device). 
         FIG. 8  is block diagram of an embodiment of a device. 
         FIG. 9  is block diagram of an embodiment of a base station. 
     
    
    
     Like reference symbols in the various drawings indicate like elements, in accordance with certain example implementations. In addition, multiple instances of an element may be indicated by following a first number for the element with a letter or a hyphen and a second number. For example, multiple instances of an element  110  may be indicated as  110 - 1 ,  110 - 2 ,  110 - 3 , etc. or as  110   a ,  110   b ,  110   c , etc. When referring to such an element using only the first number, any instance of the element is to be understood (e.g., element  110  in the previous example would refer to elements  110 - 1 ,  110 - 2 , and  110 - 3  or to elements  110   a ,  110   b , and  110   c ). 
     DETAILED DESCRIPTION 
     Several illustrative embodiments will now be described with respect to the accompanying drawings, which form a part hereof. While particular embodiments, in which one or more aspects of the disclosure may be implemented, are described below, other embodiments may be used and various modifications may be made without departing from the scope of the disclosure or the spirit of the appended claims. 
     Fifth-Generation (5G) New Radio (NR) is a wireless radio frequency (RF) interface undergoing standardization by the 3rd Generation Partnership Project (3GPP). 5G NR is poised to offer enhanced functionality over previous generation (Long-Term Evolution (LTE)) technologies, such as significantly faster and more responsive mobile broadband, enhanced conductivity through Internet of Things (IoT) devices, and more. Additionally, 5G NR enables new positioning techniques for UEs, including Angle of Arrival (AoA)/Angle of Departure (AoD) positioning, UE-based positioning, and multi-cell RTT positioning. With regard to RTT positioning, this involves taking RTT measurements between the UE and multiple base stations. 
       FIG. 1  is a diagram of a terrestrial positioning system  100 , according to an embodiment. Here, the terrestrial positioning system comprises multiple cellular transceivers, or base stations  110 - 1 ,  110 - 2 , and  110 - 3  (generically and collectively referred to herein as base stations  110 ), which are used to determine the location (e.g., in geographical coordinates) of a UE  120 . The base stations  110  and/or the UE  120  both may be communicatively coupled with a location server  130  via a Wide Area Network (WAN)  140 , which may comprise a network of the cellular carrier, as well as other data communication networks, as discussed in more detail below. (Solid arrows between components indicate communication links.) Although the UE  120  may be communicatively coupled with the WAN  140  via wireless communication with one or more of the base stations  110 , the UE  120  may have an additional or alternative communication link to the WAN  140 , as illustrated. 
     It should be noted that  FIG. 1  provides a generalized illustration of various components, any or all of which may be utilized as appropriate, and each of which may be duplicated or omitted as necessary. Specifically, although one UE  120  is illustrated, it will be understood that many UEs (e.g., hundreds, thousands, millions, etc.) may utilize the terrestrial positioning system  100 . Similarly, the terrestrial positioning system  100  may include a larger or smaller number of base stations  110 , location servers  130 , and/or other components. The illustrated communication links that communicatively connect the various components in the terrestrial positioning system  100  include data and signaling connections, which may include additional (intermediary) components, direct or indirect physical (wired) and/or wireless connections, and/or additional networks. Furthermore, components may be rearranged, combined, separated, substituted, and/or omitted, depending on desired functionality. 
     The UE  120 , as used herein, may be an electronic device and may be referred to as a device, a mobile device, a wireless device, a mobile terminal, a terminal, a wireless terminal, a mobile station (MS), a Secure User Plane Location (SUPL)-Enabled Terminal (SET), or as some other name. Moreover, UE  120  may correspond to a cellphone, smartphone, laptop, tablet, personal data assistant (PDA), wearable device (e.g., smart watch), tracking device, or some other portable or moveable device. In some cases, a UE  120  may be part of some other entity, for example, may be a chipset supporting a modem that is integrated into some larger mobile entity such as a vehicle, drone, package, shipment, or robotic device. Typically, though not necessarily, the UE  120  may support wireless communication using one or more Radio Access Technologies (RATs) (e.g., in addition to 5G NR), such as Global System for Mobile communication (GSM), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), LTE, High Rate Packet Data (HRPD), IEEE 802.11 Wi-Fi, Bluetooth® (BT), Worldwide Interoperability for Microwave Access (WiMAX), etc. The UE  120  may also support wireless communication using a Wireless Local Area Network (WLAN) which may connect to other networks (e.g. the Internet) using a Digital Subscriber Line (DSL) or packet cable, for example. The WAN  140  may comprise such wireless communication networks and/or technologies. 
     The UE  120  may include a single entity or may include multiple entities such as in a personal area network where a user may employ audio, video, and/or data input/output (I/O) devices and/or body sensors and a separate wireline or wireless modem. An estimate of a location of the UE  120  may be referred to as a location, location estimate, location fix, fix, position, position estimate, or position fix, and may be geodetic, thus providing location coordinates for the UE  120  (i.e., latitude and longitude) which may or may not include an altitude component (e.g., height above sea level, height above or depth below ground level, floor level or basement level). Alternatively, a location of the UE  120  may be expressed as a civic location (e.g., as a postal address or the designation of some point or small area in a building such as a particular room or floor). A location of the UE  120  may also be expressed as an area or volume (defined either geodetically or in civic form) within which the UE  120  is expected to be located with some probability or confidence level (e.g., 67%, 95%, etc.). A location of the UE  120  may further be a relative location comprising, for example, a distance and direction or relative X and Y (and, optionally, Z) coordinates defined relative to some origin at a known location which may be defined geodetically, in civic terms, or by reference to a point, area, or volume indicated on a map, floor plan, or building plan. In the description contained herein, the use of the term “location” may comprise any of these variants unless indicated otherwise. When computing the location of a UE, it is common to solve for local X, Y, and possibly Z coordinates and then, if needed, convert the local coordinates into absolute ones (e.g., for latitude, longitude, and altitude above or below mean sea level). 
     As noted, depending on desired functionality, the WAN  140  may comprise any of a variety of wireless and/or wireline communication networks. The WAN  140  can, for example, comprise any combination of public and/or private networks, local and/or wide-area networks, and the like. Furthermore, the WAN  140  may utilize one or more wired and/or wireless communication technologies. In some embodiments, the WAN  140  may comprise a cellular or other mobile network, a WLAN, a Wireless Wide-Area Network (WWAN), and/or the Internet, for example. Particular examples of a WAN  140  include a 5G NR network, an LTE network, a Wi-Fi WLAN, and the like. WAN  140  may also include more than one network and/or network type. 
     Base stations  110  may comprise nodes in a cellular network, which may allow the UE  120  to communicate wirelessly with other devices linked to the WAN  140 . The base stations  110  may have known locations, and may therefore be used for positioning as described herein. As described in further detail below, techniques are not necessarily limited to fixed base stations (i.e., base stations having a fixed position), but may also include mobile base stations and even other UEs  120 . For 5G NR, the base stations  110  may comprise a next-generation Node B (gNB). A WAN  140  comprising additional or alternative RATs may include base stations  110  comprising a node B, an Evolved Node B (eNodeB or eNB), a base transceiver station (BTS), a radio base station (RBS), a Next-Generation eNB (ng-eNB), a Wi-Fi access point (AP), and/or a BT AP. Thus, UE  120  can send and receive information with network-connected devices, such as location server  130 , by accessing the WAN  140 . And, as noted, the UE  120  may access the WAN  140  via a base station  110 . Base stations  110  and/or base station antennas may be referred to as Transmission Reception Points (TRPs). 
     The location server  130  may comprise a server and/or other computing device configured to determine an estimated location of UE  120  and/or provide data (e.g., assistance data) to UE  120  to facilitate the location determination. According to some embodiments, location server  130  may comprise an SUPL Location Platform (SLP), which may support the SUPL user plane (UP) location solution defined by the Open Mobile Alliance (OMA) and may support location services for UE  120  based on subscription information for UE  120  stored in location server  130 . The location server  130  may also comprise an Enhanced Serving Mobile Location Center (E-SMLC) that supports location of UE  120  using a control plane (CP) location solution for LTE radio access by UE  120 . The location server  130  may further comprise a Location Management Function (LMF) that supports location of UE  120  using a CP location solution for 5G NR radio access by UE  120 . In a CP location solution, signaling to control and manage the location of UE  120  may be exchanged between elements of WAN  140  and with UE  120  using existing network interfaces and protocols and as signaling from the perspective of WAN  140 . In a UP location solution, signaling to control and manage the location of UE  120  may be exchanged between location server  130  and UE  120  as data (e.g., data transported using the Internet Protocol (IP) and/or Transmission Control Protocol (TCP)) from the perspective of WAN  140 . 
     It can be further noted that, in some embodiments of a terrestrial positioning system  100 , the location server  130  may be executed by and/or incorporated into the UE  120  itself. That is, in the embodiments described herein, the functionality of the location server  130  may be performed by the UE  120 . In such instances, communication between the UE and location server may therefore occur between hardware and/or software components of the UE  120 . Similarly, the functions of the location server  130  described herein may be performed by a base station  110  or other device communicatively coupled to the terrestrial positioning system  100 . 
     Additionally, positioning of the UE  120  can be “UE-based” or “network-based.” UE-based positioning comprises the UE  120  determining its own location, which may be facilitated by information provided to the UE  120  by the network (e.g., the location server  130  and/or base stations  110 ). Network-based positioning comprises the network (e.g., the location server  130 ) determining the location of the UE, which may be facilitated by information provided to the network by the UE  120 . The techniques for RTT-based positioning provided herein may apply to either UE-based or network-based positioning. For example, for UE-based positioning, RTT measurements may be initiated by and/or communicated to the UE  120 , which, if provided the location of the base stations  110  from which RTT measurements were taken, can determine its own location. For network-based positioning, RTT measurements may be initiated by and/or communicated to one or more base stations  110 , which may send the measurements to the location server  130 , which can then determine the location of the UE  120 . 
     The terrestrial positioning system  100  can determine the location of the UE  120  by exploiting both downlink (DL) information transmitted by base stations  110  and uplink (UL) information transmitted by the UE  120 . As explained in more detail below, certain positioning methods can use RTT to determine the location of the UE  120  by determining one or more distances  150  from base stations  110 , then using multilateration or similar algorithms to determine the position of the UE  120 . In multilateration, for example, distances  150 - 1 ,  150 - 2 , and  150 - 3  trace respective circles  160 - 1 ,  160 - 2 , and  160 - 3  (only portions of which are shown in  FIG. 1 ), and the location of the UE  120  may be determined as the intersection of these circles  160 . Alternative positioning methods may use a combination of distance information from one or more RTT measurements with angle information (e.g., AoA, AoD). An illustration of how distance can be determined using RTT is shown in  FIG. 2 . 
       FIG. 2  is a timing diagram illustrating the basic steps of an RTT measurement, with which a position of a UE  120  can be determined, and which may be utilized in the embodiments provided herein, as described in more detail below. Here, an initiating device transmits a first Reference Signal (RS)  210  at a first time, T 1 , which propagates to a responding device. At a second time, T 2 , the first RS  210  arrives at the responding device. The Over-The-Air (OTA) delay (i.e., the propagation time it takes for the first RS  210  to travel from the initiating device to the responding device) is T prop . The responding device then transmits a second RS  220  at a third time, T 3 . Finally, the second RS  220  is received and measured by the initiating device at a fourth time, T 4 . As with the first RS  210 , the OTA delay of the second RS  220  is T prop . 
     Here, which devices comprise the initiating device and responding device may vary, depending on desired functionality. That is, in some instances, the UE  120  may be the initiating device, and a base station  110  may be the responding device. In other instances, the base station  110  may be the initiating device, and the UE  120  may be the responding device. Again, this may depend on whether the terrestrial positioning system  100  is performing UE-based positioning or network-based positioning. Additionally, as indicated in embodiments provided herein below, there may be instances in which RTT measurements are taken between two different UEs. Thus, the initiating device may be a first UE, and the responding device may be a second UE. 
     The RTT measurement shown in  FIG. 2  may be used to determine a distance, d, between the initiating and responding devices. This can be determined using the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       2 
                       ⁢ 
                       d 
                     
                     c 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             T 
                             4 
                           
                           - 
                           
                             T 
                             1 
                           
                         
                         ) 
                       
                       - 
                       
                         ( 
                         
                           
                             T 
                             3 
                           
                           - 
                           
                             T 
                             2 
                           
                         
                         ) 
                       
                     
                     = 
                     
                       
                         ( 
                         
                           
                             T 
                             4 
                           
                           - 
                           
                             T 
                             1 
                           
                         
                         ) 
                       
                       + 
                       
                         
                           ( 
                           
                             
                               T 
                               2 
                             
                             - 
                             
                               T 
                               3 
                             
                           
                           ) 
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     (As will be appreciated, distance, d, divided by the speed of RF propagation, c, equals the propagation delay, T prop .) Thus, a precise determination of the distance between the initiating device and responding device can be made. 
     The precision with which times T 1 , T 2 , T 3 , and T 4  are measured, however, can be a limiting factor to the precision of the distance determination made by the RTT measurement. The accuracy of the determination of the total time between T 2  and T 3 , known as the “Rx-Tx time offset,” can influence the precision of the distance determination. And with 5G NR&#39;s promise of increased capabilities, it is important that these times are measured accurately. This can be especially true for low-tier UEs, which have a reduced operating bandwidth compared with premium UEs. 
     As used herein, the term “low-tier UE” or “low-tier device” refers to a wireless device (UE) having a relatively low operating bandwidth, as compared with a “premium UE” or “premium device,” which has a relatively high operating bandwidth. Low-tier UEs may also be called “reduced-capability” UEs. Examples of low-tier UEs can include wearable devices (e.g., smart watches), relaxed/narrowband IoT devices, low-end mobile phones, and the like. The current operating bandwidth of these devices is roughly 5-20 megahertz (MHz), although some low-tier UEs may have a higher or lower operating bandwidth. Examples of premium UEs may comprise high-end mobile phones (e.g., smart phones), tablets, vehicles, and the like. Premium UEs currently operate at a bandwidth of 100 MHz or more. Generally speaking, low-tier UEs have a relatively lower bandwidth (e.g., less than 100 MHz), lower processing capabilities, and/or lower power budgets than premium UEs. Importantly, while the group delay of premium UEs is often accurately determined (e.g., using proprietary calibration techniques), it may be more difficult to determine the group delay of low-tier UEs. This can impact the accuracy of RTT measurements. 
       FIG. 3  is a timing diagram illustrating how group delay can impact the accuracy of RTT measurements. Group delay in this context refers to the time it takes an outgoing (TX) signal to travel from baseband processing circuitry (“BB” in  FIG. 3 ) to the antenna (“Ant”) of a device, or the time it takes an incoming (RX) signal to travel from the antenna to the baseband processing circuitry. (In a typical UE design, there may be one or more analog components between the baseband processing circuitry and the antenna.) As can be seen in  FIG. 3 , initiating device and responding device each have a respective total group delay of Δ RX +Δ TX  (which may be different for each device) that can impact the RTT measurement. For example, Rx-Tx time offset (the total time between the responding device&#39;s receipt of the first RS  210  and transmission of the second RS  220 ) is not simply T 3 −T 2 , but T 3 −T 2  plus the group delay (Δ RX +Δ TX ) of the responding device. 
     The impact of group delay can be significant. For example, delay of a single nanosecond can result in an error of two feet, resulting in limited precision of the determined location of the UE. This, in turn, can limit the number of applications for which location determination can be used. And, as noted, although the group delay for premium UEs is often determinable using proprietary means, the group delay for low-tier UEs is often not determinable using similar techniques. Moreover, because of the lower operating bandwidth of low-tier UEs, it may result in lower accuracy for calibration. (A 100 MHz premium UE would have a resolution of 10 nanoseconds (ns), whereas a 20 MHz low-tier UE would have a resolution of 50 ns.) Further complicating this issue is the fact that group delay can vary over time (e.g., it may vary across different operating temperatures), and therefore may not be determined by the manufacturer. 
     Embodiments provided herein solve these and other issues by providing techniques for calibrating low-tier UEs to accurately account for group delay by leveraging the relatively high accuracy of RTT positioning for premium UEs. This can enable online/in-field group delay calibration of low-tier UEs, allowing for low-tier UEs to be calibrated when needed. Depending on desired functionality, different techniques for calibration may be used. When properly calibrated, a low-tier UE can provide an accurate Rx-Tx time offset to account for group delay. 
       FIG. 4  is a diagram of a first technique for group delay calibration of a low-tier UE, according to an embodiment. Here, the technique involves using a base station  110 . In short, according to this technique, where a low-tier UE  410  is located near a premium UE  420 , a first RTT measurement (RTT_ 1 ) is taken between the base station  110  and the low-tier UE  410 , a second RTT measurement (RTT_ 2 ) is taken between the base station  110  and the premium UE  420 , and then the two RTT measurements (RTT_ 1  and RTT_ 2 ) are compared to determine the group delay of the low-tier UE  410 . (Because RTT_ 1  and RTT_ 2  should be approximately the same, the difference, therefore, can be attributed to the group delay of the low-tier UE  410 .) 
     The effectiveness of this technique can depend on the co-location of the low-tier UE  410  and premium UE  420 . That is, to accurately determine the group delay of the low-tier UE  410 , the low-tier UE  410  and premium UE  420  should be substantially the same distance from the base station  110 , such that the two RTT measurements should be substantially the same. 
     Any variety of techniques may be employed for choosing the premium UE  420  to use in this technique. In many instances, for example, the low-tier UE  410  may already be in communication with the premium UE  420  (e.g., via direct communications, such as “sidelink” in LTE and NR standards). A simple example of this would be the low-tier UE  410  comprising a smart watch worn by a user who was also carrying a premium UE  420  comprising a mobile phone. Some embodiments may enable a user to select a premium UE  420  to use for calibration. This can include enabling the user of the low-tier UE  410  to select from a list of premium UEs  420  in the approximate area of the low-tier UE  410 , as determined by the terrestrial positioning system  100 . Additionally or alternatively, the low-tier UE  410  may conduct a search for nearby premium UEs  420  (e.g., using RF signaling to conduct a scan of available premium UEs  420 ). Other embodiments may do this automatically (e.g., based on a premium UE  420  determined to be the closest to the low-tier UE  410  from among a plurality of premium UEs  420 , or a premium UE  420  being within a threshold distance from the low-tier UE  410 , as determined by the terrestrial positioning system  100 ). 
     Some embodiments may leverage AoA capabilities of the base station  110  to help determine a premium UE  420  to use for calibration. For example, a 5G NR base station  110  (e.g., a gNB) may be capable of performing AoA measurements to determine which premium UEs  420  are near the low-tier UE  410 . A premium UE  420  may then be selected if the AoA difference  430  (e.g., the difference between the AoA of the premium UE  420  and the AoA of the low-tier UE  410 , from the perspective of the base station  110 ) is within a certain threshold and/or the premium UE  420  has the smallest AoA difference  430  from a plurality of candidate premium UEs  420 . Additionally or alternatively, if the AoA difference  430  is still above a threshold minimum, the terrestrial positioning system  100  may conduct triangulation and/or another form of location determination of the premium UE  420 , to accommodate this offset in the location of the premium UE  420  and the low-tier UE  410 . This offset can then be accounted for when conducting the two RTT measurements, to help ensure the accuracy of the group delay determination for the low-tier UE  410 . 
     The initiation of the RTT measurements and/or the determination of the group delay for the low-tier UE  410  may be executed a variety of ways, depending on desired functionality. In some instances, for example (e.g., for network-based positioning), the base station  110  may initiate the RTT measurements and compare the RTT measurements to determine the group delay. In some embodiments, the base station  110  may further provide the determined group delay to the low-tier UE  410  for future use (e.g., during a window of time in which the determined group delay may be considered valid). In some instances, the low-tier UE  410  may initiate the first RTT measurement (RTT_ 1 ). In such instances, the second RTT measurement (RTT_ 2 ) may be initiated by the base station  110  (e.g., in response to taking the first RTT measurement) or premium UE  420  (e.g., in response to direct or indirect communications from the low-tier UE  410 ), then provided to the low-tier UE  410  by the base station  110  or premium UE  420  for determination of the group delay. (Once the low-tier UE  410  determines its group delay, it can, for example, then perform UE-based positioning.) According to some embodiments, the group delay may be reported back to the terrestrial positioning system  100 , which may then account for the group delay in subsequent network-based positioning of the low-tier UE  410 . (In some embodiments, the group delay of the low-tier UE  410  may be determined by or communicated to the premium UE  420 , which can then relay the group delay to the base station  110 . This way of communicating the group delay of the low-tier UE  410  may be preferable in certain instances, given that the premium UE  420  likely has a higher power budget than the low-tier UE  410 .) 
       FIG. 5  is a diagram of a second technique for group delay calibration of a low-tier UE, according to an embodiment, which may be used in addition or as an alternative to the first technique illustrated in  FIG. 4  and described above. Unlike the first technique, the technique illustrated in  FIG. 5  does not involve a base station  110 , but instead takes an RTT measurement at a known distance  510  to be able to determine the group delay of the low-tier UE  410 . That is, according to this technique, an RTT measurement is made by the premium UE  420  and low-tier UE  410  while the premium UE  420  and low-tier UE  410  are situated at a known distance  510  from each other. Because they are at a known distance  510  (and because the group delay of the premium UE  420  is known and accounted for), any difference between the known distance  510  and distance derived from the RTT measurement may be attributed to an inaccuracy in the determination of the group delay for the low-tier UE  410 . The group delay of the low-tier UE  410  may then be recalibrated to ensure accurate RTT measurements. According to some embodiments, the group delay may be reported back to the terrestrial positioning system  100 , which may then account for the group delay in subsequent network-based positioning of the low-tier UE  410 . 
     The technique illustrated in  FIG. 5  may be conducted in any of a variety of ways. According to some embodiments, a user of the low-tier UE  410  may be guided through a process for making this calibration using, for example, a user interface of the low-tier UE  410  and/or premium UE  420 . Some embodiments may allow the user to confirm that the premium UE  420  and low-tier UE  410  have been accurately placed via user input (e.g., the press of a button on a touchscreen display of the premium UE  420  or low-tier UE  410 ). In some embodiments, the user may be able to locate the premium UE  420  and low-tier UE  410  at a desired distance, then provide the distance (e.g., the known distance  510 ) via a user input. Additionally or alternatively, the user interface of the low-tier UE  410  and/or premium UE  420  may tell the user the distance at which to locate the premium UE  420  and low-tier UE  410 . (In some embodiments, the user may then confirm that the UEs have been placed at the appropriate distance.) 
     According to some embodiments, the RTT measurements between the premium UE  420  and low-tier UE  410  may be made, for example, using protocols for UE-based positioning based on communication with other UEs, as provided in 5G NR. In some embodiments, for example, this may involve utilizing a Channel State Information Reference Signal (CSI-RS), which can be transmitted for Channel Quality Information (CQI) purposes in sidelink for the purpose of positioning. In this case both UEs may transmit a CSI-RS inside a Physical Sidelink Shared Channel (PSSCH), and the receiving UE can measure the corresponding group delay. 
       FIG. 6  is a flow diagram of a method  600  of determining the group delay of a first mobile device (e.g., a low-tier UE), according to an embodiment utilizing a base station. The method  600 , therefore, may be seen as a method of performing the calibration previously described with regard to  FIG. 4 . As noted in the embodiments previously described, the initiation of RTT measurements and/or determination of delay for the first mobile device may be performed by one or more different devices. As such, the functionality shown in the blocks of  FIG. 6  may be performed by the first mobile device, a second mobile device (e.g., a premium UE), and/or the base station. Further, means for performing the functionality of method  600  may include hardware and/or software components of a mobile device (e.g., UE illustrated in  FIG. 8 ), and/or hardware and/or software components of the base station  110  illustrated in  FIG. 9 , both of which are described in more detail below. Additionally, it can be noted that, as with other figures appended hereto,  FIG. 6  is provided as a non-limiting example. Other embodiments may vary, depending on desired functionality. For example, the functional blocks illustrated in method  600  may be combined, separated, or rearranged to accommodate different embodiments. 
     At block  610 , the functionality comprises obtaining a first RTT measurement between the first mobile device and a base station. As noted, the RTT measurement itself may be initiated by the base station or first mobile device, depending on desired functionality. Moreover, in some embodiments, the measurement may be obtained by a device other than the device initiating the RTT measurement (e.g., the first mobile device may take the measurement and send it to the base station, or the base station may take the measurement and send it to the first mobile device). Means for performing the functionality at block  610  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. Additionally or alternatively, means for performing the functionality at block  610  may comprise software and/or hardware components of a base station, such as the bus  905 , processing unit(s)  910 , DSP  920 , wireless communication interface  930 , memory  960 , and/or other components of the base station  110  illustrated in  FIG. 9  and described in more detail below. 
     The functionality at block  620  comprises identifying a second mobile device within a threshold distance of the first mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device. In some embodiments, the first mobile device comprises a low-tier UE having a bandwidth of less than 100 MHz, and the second mobile device comprises a premium UE having a bandwidth of 100 MHz or more. 
     As indicated in the previously described embodiments, identifying the second mobile device may comprise any of a variety of techniques. In some embodiments, identifying the second mobile device comprises determining that a difference between a first AoA measurement by the base station of the first mobile device and a second AoA measurement by the base station of the second mobile device is within a threshold value. Additionally or alternatively, the first mobile device may perform a scan and allow a user to select a desired second mobile device with which to perform calibration. As such, according to some embodiments, identifying the second mobile device may comprise performing a scan by the first mobile device. Moreover, identifying may further comprise receiving a user selection of a premium device from a list of a plurality of devices detected from the scan. 
     Means for performing the functionality at block  620  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. Additionally or alternatively, means for performing the functionality at block  620  may comprise software and/or hardware components of a base station, such as the bus  905 , processing unit(s)  910 , DSP  920 , wireless communication interface  930 , memory  960 , and/or other components of the base station  110  illustrated in  FIG. 9  and described in more detail below. 
     At block  630 , the functionality comprises obtaining a second RTT measurement between the second mobile device and the base station. Again, the RTT measurement itself may be initiated by the base station or second mobile device, depending on desired functionality. Further, in some embodiments, the RTT measurement may be sent from one device to another (e.g., from UE to base station or vice versa). Means for performing the functionality at block  630  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. Additionally or alternatively, means for performing the functionality at block  630  may comprise software and/or hardware components of a base station, such as the bus  905 , processing unit(s)  910 , DSP  920 , wireless communication interface  930 , memory  960 , and/or other components of the base station  110  illustrated in  FIG. 9  and described in more detail below. 
     At block  640 , the functionality comprises determining a group delay of the first mobile device based on a difference between the first RTT measurement and the second RTT measurement. As previously noted, because the group delay of the second mobile device may be known and accounted for, this allows for determination of the group delay of the first mobile device. And again, the base station, first mobile device, or second mobile device may make this determination of the group delay using the obtained first and second RTT measurements. In instances in which the first mobile device determines the group delay, the first mobile device may further send information indicative of the determined group delay to the base station (e.g., for use in network-based positioning of the first mobile device). In some embodiments, the first mobile device can be calibrated to account for group delay. And thus, the information indicative of the determined group delay can include, for example, the Rx-Tx time offset, accounting for the determined group delay. To preserve power, the first mobile device may send the determined group delay to the second mobile device, and the second mobile device may send the information indicative of the determined group delay to the base station. In instances in which the second mobile device determines the group delay, the second mobile device may further send the information indicative of the determined group delay to the base station and/or first mobile device. In instances in which the base station determines the group delay, the base station may send the information indicative of the group delay to the first mobile device (e.g., for use in UE-based positioning of the first mobile device). 
     Means for performing the functionality at block  640  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. Additionally or alternatively, means for performing the functionality at block  640  may comprise software and/or hardware components of a base station, such as the bus  905 , processing unit(s)  910 , DSP  920 , wireless communication interface  930 , memory  960 , and/or other components of the base station  110  illustrated in  FIG. 9  and described in more detail below. 
       FIG. 7  is a flow diagram of a method  700  of determining the group delay of a first mobile device (e.g., a low-tier UE), according to an embodiment that uses direct communications with a second mobile device (e.g., a premium UE). The method  700 , therefore, may be seen as a method of performing the calibration previously described with regard to  FIG. 5 . As noted in the embodiments previously described, the initiation of RTT measurements and/or determination of delay for the first mobile device may be performed by either the first or second mobile device. As such, means for performing the functionality of method  700  may include hardware and/or software components of the UE illustrated in  FIG. 8 . Additionally, it can be noted that, as with figures appended hereto,  FIG. 7  is provided as a non-limiting example. Other embodiments may vary, depending on desired functionality. For example, the functional blocks illustrated in method  700  may be combined, separated, or rearranged to accommodate different embodiments. 
     The functionality at block  710  comprises obtaining information indicative of a known distance between the first mobile device and the second mobile device, wherein the second mobile device has a higher bandwidth than the first mobile device. In some embodiments, the first mobile device comprises a low-tier UE having a bandwidth of less than 100 MHz, and the second mobile device comprises a premium-tier UE having a bandwidth of 100 MHz or more. Again, according to embodiments, this information may be obtained using a guided process in which the first and/or second mobile device guides a user into positioning each mobile device such that there is a known distance between the first mobile device and the second mobile device. Thus, according to embodiments, obtaining information indicative of the known distance between the first mobile device and the second mobile device may comprise receiving user input of a distance between the first mobile device on the second mobile device, receiving user input verifying that the first mobile device and second mobile device have been placed at a requested distance, or the like. Means for performing the functionality at block  710  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , input device(s)  870 , output device(s)  815 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. 
     At block  720 , the functionality comprises obtaining an RTT measurement between the first mobile device and the second mobile device. As noted in the embodiments above, this may involve utilizing a sidelink channel (e.g., utilizing CSI-RS confined within a PSSCH transmission). Additionally or alternatively, the RTT measurement may be taken in response to user input. More specifically, the RTT measurement may be performed in response to receiving a user input comprising information confirming that the first mobile device and the second mobile device are located the known distance apart. (Depending on desired functionality, the first mobile device may be the initiating device and the second mobile device may be the responding mobile device, or vice versa. In either case, the RTT measurement may be provided to the device determining the group delay of the first mobile device.) Means for performing the functionality at block  720  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8  and described in more detail below. 
     The functionality at block  730  comprises determining a group delay of the first mobile device based on a difference between the known distance and a distance determined by the RTT measurement. A difference between the known distance and a distance determined using the RTT measurement may be indicative of the group delay (Δ RX +Δ TX ) of the first mobile device (or an error in a current group delay estimate for the first mobile device). The first mobile device can then be calibrated accordingly to account for the determined group delay. For example, if a distance derived from the RTT measurement is two feet longer than the known distance, the group delay is approximately 1 ns. Alternatively, if a current group delay estimate was accounted for the RTT measurement, this would mean the current group delay estimate is 1 ns shorter than it should be, and the group delay estimate can be adjusted accordingly. Again, this determination may be made by either the first mobile device or the second mobile device, and information indicative of this determination may be reported to the network (e.g., via a base station) for subsequent network-based positioning using RTT measurements of the first mobile device. 
     Means for performing the functionality at block  730  may comprise software and/or hardware components of a UE, such as the bus  805 , processing unit(s)  810 , DSP  820 , wireless communication interface  830 , memory  860 , and/or other components of the UE  120  illustrated in  FIG. 8 . 
       FIG. 8  is a block diagram of an embodiment of a UE  120 , which can be utilized as described in the embodiments described herein and in association with  FIGS. 1-7 . Specifically, the UE  120  of  FIG. 8  may correspond to any type of UE (e.g., low-tier and/or premium) discussed in the embodiments above, including the UE  120  of  FIG. 1  and/or either or both of the low-tier UE  410  and premium UE  420  of  FIGS. 4 and 5 . It should be noted that  FIG. 8  is meant only to provide a generalized illustration of various components of UE  120 , any or all of which may be utilized as appropriate. In other words, because UEs can vary widely in functionality, they may include only a portion of the components shown in  FIG. 8 . A premium UE, for example, may include more of the components shown in  FIG. 8  than does a low-tier UE. It can be noted that, in some instances, components illustrated by  FIG. 8  can be localized to a single physical device and/or distributed among various networked devices, which may be disposed at different physical locations. 
     The UE  120  is shown comprising hardware elements that can be electrically coupled via a bus  805  (or may otherwise be in communication as appropriate). The hardware elements may include a processing unit(s)  810  which may comprise, without limitation, one or more general-purpose processors, one or more special-purpose processors (such as digital signal processing (DSP) chips, graphics acceleration processors, application-specific integrated circuits (ASICs), and/or the like), and/or other processing structure or means, which can be configured to perform one or more of the methods described herein. As shown in  FIG. 8 , some embodiments may have a separate DSP  820 , depending on desired functionality. The UE  120  also may comprise one or more input devices  870 , which may comprise, without limitation, one or more touchscreens, touchpads, microphones, buttons, dials, switches, and/or the like; and one or more output devices  815 , which may comprise, without limitation, one or more displays, light-emitting diodes (LEDs), speakers, and/or the like. 
     The UE  120  might also include a wireless communication interface  830 , which may comprise, without limitation, a modem, a network card, an infrared communication device, a wireless communication device, and/or a chipset (such as a BT device, an IEEE 802.11 device, an IEEE 802.15.4 device, a Wi-Fi device, a WiMAX™ device, cellular communication facilities, etc.), and/or the like, which may enable the UE  120  to communicate via the networks (e.g., via a base station) described herein with regard to  FIG. 1 . The wireless communication interface  830  may permit data to be communicated with a network, base stations (e.g., eNBs, ng-eNBs, and/or gNBs), and/or other TRPs, network components, computer systems, and/or any other electronic devices described herein. The communication can be carried out via one or more wireless communication antenna(s)  832  that send and/or receive wireless signals  834 . 
     Depending on desired functionality, the wireless communication interface  830  may comprise separate base stations to communicate with base stations (e.g., eNBs, ng-eNBs, and/or gNBs) and other terrestrial base stations, such as wireless devices and APs. The UE  120  may communicate with different data networks that may comprise various network types. For example, a WWAN may be a CDMA network, a Time Division Multiple Access (TDMA) network, a Frequency Division Multiple Access (FDMA) network, an Orthogonal Frequency Division Multiple Access (OFDMA) network, a Single-Carrier Frequency Division Multiple Access (SC-FDMA) network, a WiMax (IEEE 802.16), and so on. A CDMA network may implement one or more RATs, such as cdma2000, WCDMA, and so on. Cdma2000 includes IS-95, IS-2000, and/or IS-856 standards. A TDMA network may implement GSM, Digital Advanced Mobile Phone System (D-AMPS), or some other RAT. An OFDMA network may employ LTE, LTE Advanced, NR, and so on. 5G, LTE, LTE Advanced, NR, GSM, and WCDMA are described in documents from 3GPP. Cdma2000 is described in documents from a consortium named “3rd Generation Partnership Project 2” (3GPP2). 3GPP and 3GPP2 documents are publicly available. A WLAN may also be an IEEE 802.11x network, and a wireless personal area network (WPAN) may be a BT network, an IEEE 802.15x, or some other type of network. The techniques described herein may also be used for any combination of WWAN, WLAN, and/or WPAN. 
     The UE  120  can further include sensor(s)  840 . Such sensors may comprise, without limitation, one or more inertial sensors (e.g., accelerometer(s), gyroscope(s), and or other Inertial Measurement Units (IMUs)), camera(s), magnetometer(s), compass, altimeter(s), microphone(s), proximity sensor(s), light sensor(s), barometer, and the like, some of which may be used to complement and/or facilitate the functionality described herein. 
     Embodiments of the UE  120  may also include a Global Navigation Satellite System (GNSS) receiver  880  capable of receiving signals  884  from one or more GNSS satellites using an GNSS antenna  882  (which may be combined in some implementations with antenna(s)  832 ). Such positioning can be utilized to complement and/or incorporate the techniques described herein. The GNSS receiver  880  can extract a position of the UE  120 , using conventional techniques, from GNSS satellites of a GNSS system, such as Global Positioning System (GPS), Galileo, GLObal NAvigation Satellite System (GLONASS), Compass, Quasi-Zenith Satellite System (QZSS) over Japan, Indian Regional Navigation Satellite System (IRNSS) over India, BeiDou Navigation Satellite System (BDS) over China, and/or the like. Moreover, the GNSS receiver  880  can use various augmentation systems (e.g., a Satellite-Based Augmentation System (SBAS)) that may be associated with or otherwise enabled for use with one or more global and/or regional navigation satellite systems. By way of example, but not limitation, an SBAS may include an augmentation system(s) that provides integrity information, differential corrections, and so on, such as Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Multi-functional Satellite Augmentation System (MSAS), GPS-Aided GEO-Augmented Navigation or GPS and GEO-Augmented Navigation system (GAGAN), and/or the like. Thus, as used herein, a GNSS may include any combination of one or more global and/or regional navigation satellite systems and/or augmentation systems, and GNSS signals may include GNSS, GNSS-like, and/or other signals associated with such one or more GNSS. 
     The UE  120  may further include and/or be in communication with a memory  860 . The memory  860  may comprise, without limitation, local and/or network-accessible storage, a disk drive, a drive array, an optical storage device, a solid-state storage device (such as a random access memory (RAM) and/or a read-only memory (ROM)), which can be programmable, flash-updateable, and/or the like. Such storage devices may be configured to implement any appropriate data stores, including, without limitation, various file systems, database structures, and/or the like. 
     The memory  860  of the UE  120  also can comprise software elements (not shown), including an operating system, device drivers, executable libraries, and/or other code, such as one or more application programs, which may comprise computer programs provided by various embodiments, and/or may be designed to implement methods and/or configure systems, provided by other embodiments, as described herein. Merely by way of example, one or more procedures described with respect to the functionality discussed above might be implemented as code and/or instructions executable by the UE  120  (e.g., using processing unit(s)  810 ). In an aspect, then, such code and/or instructions can be used to configure and/or adapt a general-purpose computer (or other device) to perform one or more operations in accordance with the described methods. 
       FIG. 9  illustrates an embodiment of a base station  110 , which can be utilized as described herein above (e.g., in association with  FIGS. 1-7 ). It should be noted that  FIG. 9  is meant only to provide a generalized illustration of various components, any or all of which may be utilized as appropriate. In some embodiments, the base station  110  may correspond to a gNB, an ng-eNB, and/or an eNB. 
     The base station  110  is shown comprising hardware elements that can be electrically coupled via a bus  905  (or may otherwise be in communication as appropriate). The hardware elements may include a processing unit(s)  910 , which can include, without limitation, one or more general-purpose processors, one or more special-purpose processors (such as DSP chips, graphics acceleration processors, ASICs, and/or the like), and/or other processing structure or means. As shown in  FIG. 9 , some embodiments may have a separate DSP  920 , depending on desired functionality. Location determination and/or other determinations based on wireless communication may be provided in the processing unit(s)  910  and/or wireless communication interface  930  (discussed below), according to some embodiments. The base station  110  also can include one or more input devices, which can include, without limitation, a keyboard, display, mouse, microphone, button(s), dial(s), switch(es), and/or the like; and one or more output devices, which can include, without limitation, a display, LED, speakers, and/or the like. 
     The base station  110  might also include a wireless communication interface  930 , which may comprise, without limitation, a modem, a network card, an infrared communication device, a wireless communication device, and/or a chipset (such as a BT device, an IEEE 802.11 device, an IEEE 802.15.4 device, a Wi-Fi device, a WiMAX device, cellular communication facilities), and/or the like, which may enable the base station  110  to communicate as described herein. The wireless communication interface  930  may permit data and signaling to be communicated (e.g., transmitted and received) with UEs, other base stations (e.g., eNBs, gNBs, and ng-eNBs), and/or other TRPs, or network components, computer systems, and/or any other electronic devices described herein. The communication can be carried out via one or more wireless communication antenna(s)  932  that send and/or receive wireless signals  934 . 
     The base station  110  may also include a network interface  980 , which can include support of wireline communication technologies. The network interface  980  may include a modem, network card, chipset, and/or the like. The network interface  980  may include one or more input and/or output communication interfaces to permit data to be exchanged with a network, communication network servers, computer systems, and/or any other electronic devices described herein. 
     In many embodiments, the base station  110  may further comprise a memory  960 . The memory  960  can include, without limitation, local and/or network-accessible storage, a disk drive, a drive array, an optical storage device, a solid-state storage device such as a RAM, and/or a ROM, which can be programmable, flash-updateable, and/or the like. Such storage devices may be configured to implement any appropriate data stores, including, without limitation, various file systems, database structures, and/or the like. 
     The memory  960  of the base station  110  also may comprise software elements (not shown in  FIG. 9 ), including an operating system, device drivers, executable libraries, and/or other code, such as one or more application programs, which may comprise computer programs provided by various embodiments, and/or may be designed to implement methods and/or configure systems, provided by other embodiments, as described herein. Merely by way of example, one or more procedures described with respect to the method(s) discussed above may be implemented as code and/or instructions in memory  960  that are executable by the base station  110  (and/or processing unit(s)  910  or DSP  920  within base station  110 ). In an aspect, then, such code and/or instructions can be used to configure and/or adapt a general-purpose computer (or other device) to perform one or more operations in accordance with the described methods. 
     It will be apparent to those skilled in the art that substantial variations may be made in accordance with specific requirements. For example, customized hardware might also be used, and/or particular elements might be implemented in hardware, software (including portable software, such as applets), or both. Further, connection to other computing devices, such as network I/O devices, may be employed. 
     With reference to the appended figures, components that can include memory can include non-transitory machine-readable media. The terms “machine-readable medium” and “computer-readable medium,” as used herein, refer to any storage medium that participates in providing data that causes a machine to operate in a specific fashion. In embodiments provided hereinabove, various machine-readable media might be involved in providing instructions/code to processing units and/or other device(s) for execution. Additionally or alternatively, the machine-readable media might be used to store and/or carry such instructions/code. In many implementations, a computer-readable medium is a physical and/or tangible storage medium. Such a medium may take many forms, including, but not limited to, non-volatile media, volatile media, and transmission media. Common forms of computer-readable media include, for example, magnetic and/or optical media, any other physical medium with patterns of holes, a RAM, a programmable ROM (PROM), an erasable PROM (EPROM), a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read instructions and/or code. 
     The methods, systems, and devices discussed herein are examples. Various embodiments may omit, substitute, or add various procedures or components as appropriate. For instance, features described with respect to certain embodiments may be combined in various other embodiments. Different aspects and elements of the embodiments may be combined in a similar manner. The various components of the figures provided herein can be embodied in hardware and/or software. Also, technology evolves, and thus, many of the elements are examples that do not limit the scope of the disclosure to those specific examples. 
     It has proven convenient at times, principally for reasons of common usage, to refer to such signals as “bits,” “information,” “values,” “elements,” “symbols,” “characters,” “variables,” “terms,” “numbers,” “numerals,” or the like. It should be understood, however, that all of these or similar terms are to be associated with appropriate physical quantities and are merely convenient labels. Unless specifically stated otherwise, as is apparent from the discussion above, it is appreciated that throughout this Specification, discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining,” “ascertaining,” “identifying,” “associating,” “measuring,” “performing,” or the like refer to actions or processes of a specific apparatus, such as a special-purpose computer or a similar special-purpose electronic computing device. In the context of this Specification, therefore, a special-purpose computer or a similar special-purpose electronic computing device is capable of manipulating or transforming signals, typically represented as physical, electronic, electrical, or magnetic quantities within memories, registers, or other information storage devices, transmission devices, or display devices of the special-purpose computer or similar special-purpose electronic computing device. 
     Terms “and” and “or,” as used herein, may include a variety of meanings which also are expected to depend at least in part upon the context in which such terms are used. Typically, “or” if used to associate a list, such as A, B, or C, is intended to mean A, B, and C, here used in the inclusive sense, as well as A, B, or C, here used in the exclusive sense. In addition, the term “one or more,” as used herein, may be used to describe any feature, structure, or characteristic in the singular or may be used to describe some combination of features, structures, or characteristics. However, it should be noted that this is merely an illustrative example and claimed subject matter is not limited to this example. Furthermore, the term “at least one of” if used to associate a list, such as A, B, or C, can be interpreted to mean any combination of A, B, and/or C, such as A, AB, AA, AAB, or AABBCCC. 
     Having described several embodiments, various modifications, alternative constructions, and equivalents may be used without departing from the spirit of the disclosure. For example, the above elements may merely be a component of a larger system, wherein other rules may take precedence over or otherwise modify the application of the various embodiments. Also, a number of steps may be undertaken before, during, or after the above elements are considered. Accordingly, the above description does not limit the scope of the disclosure.