Patent Publication Number: US-9408172-B2

Title: High precision network synchronization in an indoor position location system

Description:
BACKGROUND 
     In some settings, such as in indoor and enterprise environments, it may be important to easily locate various types of assets or people, or both. Examples of such settings include hospitals, retail stores, warehouses, etc. The accuracy and speed with which the location of assets or people is monitored in an indoor setting may be an important factor in determining the usefulness of the tracking system. In addition, having a location system that is cost effective, scalable, and that can provide continuous, accurate, and precise location monitoring is also desirable. 
     Different systems and devices may be used to locate assets and/or people in a particular indoor environment. An ultra-wideband (UWB) network, or some other radio frequency network deployed throughout at least a portion of the indoor environment, may be configured to perform indoor tracking. Systems may employ multiple access points (APs) placed at specific locations in the indoor environment. A location tracking tag also may be attached to each mobile asset and/or to each person to be tracked. The tag may send waveforms (e.g., beacon signals) that are received by the APs for ranging measurements to determine the distance between the tag and the APs that receive the waveforms. Once the distances between the tag and at least three different APs are obtained, triangulation or trilateration may be used to estimate the location of the asset or person to which the tag is attached. 
     A position location network may be calibrated to provide accurate location measurements. Calibration may include synchronizing each AP to a master AP within the network. It is desirable to obtain the highest precision synchronization possible, which may require network wide synchronization, not just local synchronization. 
     SUMMARY 
     Described below are methods, systems, and/or devices that provide for high precision network wide synchronization in a position location network. The methods, systems, and/or devices may include tools and techniques that provide for incrementally synchronizing timers and oscillators located at respective APs to the timers and oscillator of a master AP within a tracking area of a position location network. The synchronization may include a coarse frequency and timing acquisition and a fine frequency and timing acquisition. These techniques may be utilized over a multi-hop wireless network. 
     A first, or master, AP with one or more oscillators and timers may be used to synchronize neighboring APs. For example, the master AP may broadcast a synchronization signal, which may include information related to the oscillator frequency and/or timer time (as referred to as timer count). APs within the vicinity may receive the synchronization signal and make estimates and/or corrections to their oscillators and/or timers. Once the other APs synchronize to the master AP, they may send an acknowledgment message. In some cases, APs that have synchronized to the master AP and sent an acknowledgment message may be designated as acting master APs. Acting master APs may operate similarly to master APs and may broadcast a synchronization message to neighboring APs, so that the neighboring APs may synchronize to the system. The master AP and/or a tracking management server may keep, or update, a list of APs within the network that have been synchronized to the system time broadcast from the master AP. 
     A method of synchronizing a position location network is described. A master access point (AP) may be selected from among a plurality of APs. Each of the APs may have a reference oscillator and a timer. A first synchronization message may be broadcast from the master AP. A first acknowledgment message may be received at the master AP from each AP that synchronized to the master AP based on the first synchronization message. One or more APs from which an acknowledgment message is received may be designated as an acting master AP. 
     In one embodiment, each AP may be configured with a narrowband transceiver and an ultra-wideband (UWB) transceiver. Broadcasting the first synchronization message may include broadcasting a narrowband signal. The first synchronization message may include information related to an oscillator frequency and a timer count. An AP that synchronized to the master AP may be an AP that estimated a frequency offset and a time offset with respect to the master AP. Broadcasting the first synchronization message may include broadcasting an ultra-wideband (UWB) signal. 
     In one configuration, an AP that synchronized to the master AP may include an AP that adjusted an oscillator frequency and a timer count based on the first synchronization message. A second synchronization message may be broadcast from a designated acting master AP. A second acknowledgment message may be received at the designated acting master AP from each AP that synchronized to the designated acting master AP based on the second synchronization message. Broadcasting the second synchronization message may include broadcasting at least one of a narrowband signal or a UWB signal. The second synchronization message may include information related to an oscillator frequency and a timer count. 
     In one configuration, one or more lists of neighboring APs synchronized to the one or more designated acting master APs may be received from the one or more designated acting master APs. Receiving the one or more lists may include receiving, from the one or more designated acting master APs at the master AP, one or more lists of neighboring APs synchronized to the one or more designated acting master APs. Receiving the one or more lists may further include receiving, from the master AP at a tracking management server, one or more lists of neighboring APs synchronized to at least one of the master AP and the one or more designated acting master APs. 
     A system of synchronizing a position location network is also described. The system may include means for selecting a master access point (AP) from among a plurality of APs. Each of the APs having a reference oscillator and a timer. The system may further include means for broadcasting a first synchronization message from the master AP, means for receiving, at the master AP, a first acknowledgment message from each AP that synchronized to the master AP based on the first synchronization message, and means for designating as acting master APs one or more APs from which an acknowledgment message is received. 
     An apparatus for synchronizing a position location network is also described. The apparatus may include a processor and memory in electronic communication with the processor. Instructions may be stored in the memory. The instructions may be executable by the processor to broadcast a first synchronization message from the master AP, receive, at the master AP, a first acknowledgment message from each AP that synchronized to the master AP based on the first synchronization message, and designate as acting master APs one or more APs from which an acknowledgment message is received. 
     A computer-program product for synchronizing a position location network is also described. The computer-program product may include a non-transitory computer-readable medium storing instructions executable by a processor to broadcast a first synchronization message from the master AP, receive, at the master AP, a first acknowledgment message from each AP that synchronized to the master AP based on the first synchronization message, and designate as acting master APs one or more APs from which an acknowledgment message is received. 
     Further scope of the applicability of the described methods and apparatuses will become apparent from the following detailed description, claims, and drawings. The detailed description and specific examples are given by way of illustration only, since various changes and modifications within the spirit and scope of the description will become apparent to those skilled in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the nature and advantages of the present invention may be realized by reference to the following drawings. In the appended figures, similar components or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
         FIGS. 1A and 1B  show an example(s) of a position location network in accordance with various embodiments; 
         FIGS. 2A and 2B  show block diagrams of an example device(s) that may be employed in position location networks in accordance with various embodiments; 
         FIG. 3  shows a block diagram of an example of a position location network in accordance with various embodiments; 
         FIG. 4  shows a block diagram of an example of a position location network in accordance with various embodiments; 
         FIG. 5  shows a block diagram of an example of a position location network in accordance with various embodiments; 
         FIG. 6  shows an example of a position location network in accordance with various embodiments; 
         FIG. 7  shows a call flow diagram that illustrates an example of access point synchronization within a position location network in accordance with various embodiments; and 
         FIGS. 8, 9, and 10  are flow diagrams that depict a method or methods of access point synchronization within a position location network in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Methods, systems, and devices are described that address issues pertaining to synchronizing APs within a network of APs in a position location network (as referred to as location tracking and/or position location systems). The methods, systems, and/or devices may include tools and techniques that provide for incrementally synchronizing timers and oscillators located at respective APs. The synchronization may include a master AP with a wired or wireless connection to a central server, or tracking management server. The master AP may have a stable oscillator and stable timer(s). 
     The master AP may broadcast a synchronization signal, which may include information related to oscillator frequency and/or timer time. APs within the vicinity may receive these signals and make estimates and/or corrections to their oscillators and timers. In this way, the system frequency and/or time broadcast from the master AP may be relayed to other APs. Once the other APs synchronize to the master AP, they may send an acknowledgment message. 
     In some cases, APs that have synchronized to the master AP and sent an acknowledgment message may assume the role of acting master AP. The master AP and/or a tracking management server may designate synchronized APs as acting master APs. Acting master APs may perform a role similar to the master AP. For example, APs in the vicinity of an acting master AP may synchronize with the acting master AP and acquire the system information, just as if it were synching with the master AP. As the number of acting master APs within the network increases, synchronization rate may also increase. That is, the number of APs exposed to synchronization broadcasts may increase as the number of acting master APs increases, creating a “synchronization wave” that propagates out from the master AP. This process may continue until each AP within the network is synchronized. 
     Each acting master AP may transmit to the master AP a list of synchronized APs. The master AP may keep track of each synchronized AP within the network. Additionally or alternatively, the master AP may forward the list(s) of synchronized APs to the tracking management server. 
     The following description provides examples, and is not limiting of the scope, applicability, or configuration set forth in the claims. Changes may be made in the function and arrangement of elements discussed without departing from the spirit and scope of the disclosure. Various embodiments may omit, substitute, or add various procedures or components as appropriate. For instance, the methods described may be performed in an order different from that described, and various steps may be added, omitted, or combined. Also, features described with respect to certain embodiments may be combined in other embodiments. 
     First,  FIG. 1A  depicts an example of a position location network  100  in accordance with various embodiments. The system  100  provides location tracking of assets (e.g., objects) or people, or both, throughout the coverage area  110  associated with an indoor and/or enterprise environment. In some embodiments, the coverage area  110  represents an area of coverage inside a building, such as a hospital, a retail store, or a warehouse. Within the coverage area  110 , multiple APs  105  may be deployed at specific locations, as may multiple tags  115  (as referred to as tag units and/or location tracking tags), which may be tracked within the coverage area  110 . Because of their stationary nature, the exact distance between any two APs  105  is typically known, or may be determined, throughout the operation of the system  100 . Any two APs  105  may ascertain the distance between themselves through a ranging operation, which may be a two-way ranging operation. The ranging operation may be performed via communication links  125 . 
     The arrangement of APs  105  shown in  FIG. 1A  is intended as a non-limiting example. The APs  105  may be deployed or distributed within the coverage area  110  in a manner or pattern different from that depicted in  FIG. 1A . For example, the APs  105  may be arranged at different distances form one another. In some cases, the coverage area  110  may represent a two-dimensional deployment, such as a single floor within a building. But in some embodiments, the APs  105  are deployed in a three-dimensional manner by placing some of the APs  105  on different floors or levels of a building within the coverage area  110 . 
     Each of the APs  105  may be equipped with a narrowband transceiver or a UWB transceiver, or both. Additionally or alternatively, the APs  105  may include one or more oscillators or timers, or both. The oscillators may each produce a repetitive, oscillating electronic signal, which may be adjustable and/or variable. The oscillators may be RF oscillators. The oscillators may be linear- or relaxation-type. In some embodiments, the oscillators are voltage controlled, temperature compensated crystal oscillators (VCTCXO). The timers may include quartz clock(s), they may be digital, and/or they may be implemented in software. 
     The APs  105  may need to undergo a calibration procedure in order to increase the precision and/or accuracy of the tracking system  100 . Calibration may include synchronizing the APs  105  to one another, to a network  140 , and/or to a tracking management server  150 . Additionally or alternatively, calibration may include determining coordinates of each AP  105 . 
     In some cases, one or more APs  105  are designated or selected as master APs or acting master APs that facilitate synchronization. Network-wide synchronization of APs  105  may involve designating or selecting a master AP  105  with a stable oscillator and stable timer. Each of the other APs  105  may synchronize their respective oscillators and timers to the master AP or to an acting master AP. This synchronization may include coarse and fine synchronization steps, which, in some embodiments, involves receiving and transmitting both narrowband and UWB signals. 
     Calibration may also include determining the coordinates of each of the APs  105  within the coverage area  110 . Coordinates of each of the APs  105  may be determined incrementally, based on known coordinates of one of the APs  105  and known or determined distances between APs  105 . 
     Each of the tag units  115  may be attached to an asset or person being tracked within the coverage area  110 . The tag units  115  may be equipped with a narrowband transceiver or a UWB transceiver, or both. The tag units  115  may also have one or more oscillators or timers, or both. The oscillators may each produce a repetitive, oscillating electronic signal, which may be adjustable and/or variable. The oscillators may be RF oscillators. The oscillators may be linear- or relaxation-type. By way of example, the oscillators are VCTCXO. The timers may include quartz clock(s), they may be digital, and/or they may be implemented in software. 
       FIG. 1A  depicts an example position location network  100  with six tag units at locations A, B, C, D, E, and F. Over time, these locations may change as the assets or people to which the tags  115  are attached move or are moved within the coverage area  110 . The system  100 , shown with six tags  115 , is intended as a non-limiting example of a position location network. Those skilled in the art will recognize that the system  100  is scalable, and it may be capable of tracking more or fewer 0  assets or people. 
     The system  100  includes a tracking management server  150 , which also may be referred to as a tag tracking management server. In some embodiments, the tracking management server  150  is connected to the APs  105  through a network  140 . The connection may be by way of a radio network associated with the APs  105 . The tracking management server  150  may receive information from the APs  105  to perform various types of calculations, including: determining one or more sets of receive filters for the APs  105 ; detecting whether a tag  115  is mobile or stationary and adjusting update rates accordingly; estimating characteristics of communication channels; and/or estimating a location of an asset or person being tracked within the coverage area  110 . The tracking management server  150  may also schedule or coordinate various operations associated with the APs  105 , including when to have an AP  105  wirelessly communicate (e.g., when to transmit UWB and/or narrowband signals) with other APs  105  or with tags  115 . In some embodiments, the tracking management server  150  stores information about different APs  105  and subsets of APs  105 ; and it may use stored information to schedule or coordinate various operations between individual APs  105  and/or subsets of APs  105 . 
     The APs  105  may communicate with one another by sending and/or receiving UWB signals and/or narrowband signals. The channels between APs  105 , which are associated with communication links  125 , are often characterized by noise and signal-degrading impedances. It may therefore be beneficial to maximize the signal transmit power. 
       FIG. 1B  illustrates transmissions or broadcasts between APs  105  and tags  115  via communication links  135 . In some embodiments, the tags  115  communicate with APs  105  via the communication links  135  using either or both UWB and narrowband signals. Whether a tag  115  communicates primarily with narrowband or UWB may be a function of whether the tag  115  is mobile or stationary. 
     An AP  105  may communicate with other APs  105  using either or both UWB and narrowband signals. During this communication a second AP may synchronize with a first AP. Upon receipt of an acknowledgment message from the second AP, the first AP may designate the second AP as an acting master AP. Once the second AP is designated as an acting master AP, the second AP may be used by neighboring APs to synchronize their oscillator and/or timer(s). The neighboring APs may send an acknowledgment message to the second AP to communicate that they have been properly synchronized. The second AP may send a synchronization list to the first AP including a list of APs that have been synchronized. The first AP may keep track of which APs within the network have been synchronized. In some cases, the first AP may transmit the synchronization list to a tracking management server  150 . 
     Next, turning to  FIG. 2A , a block diagram illustrates a device  200  configured for synchronizing APs in a position location network in accordance with various embodiments. The device  200  may be an AP  105 - a , which may be an example of an AP  105  of  FIG. 1A  or  FIG. 1B , or both. In some embodiments, the device  200  is a processor. The device  200  may include a receiver module  205 , a synchronization module  210 , an acting master designation module  215 , and/or a transmitter module  220 . In some cases, the receiver module  205  and the transmitter module  220  are a single, or multiple, transceiver module(s). The receiver module  205  and/or the transmitter module  220  may include an integrated processor. They may also include an oscillator and/or a timer. In some embodiments, the receiver module  205  and the transmitter module  220  are a part of a UWB transceiver module, a narrowband transceiver module, or both. 
     The device  200  may include a synchronization module  210 . The synchronization module  210  may include an oscillator and/or timer(s). It may also include an integrated processor. The synchronization module  210  may prepare a synchronization message to be transmitted to another AP which includes information based on an oscillator and/or a timer(s). The synchronization module  210  may analyze a received message to determine a frequency offset and/or timer offset, or an adjustment that needs to be made in order to synchronize an oscillator and/or timer. In some cases, the synchronization module  210  may adjust or change an oscillator and/or timer(s) in order to synchronize with another AP. 
     The device may include an acting master designation module  215 . The acting master designation module  215  may include an integrated processor. The acting master designation module  215  may include a database. In some cases, the acting master designation module  215  prepares a communication to let another AP know that it may act as a master AP. A master AP, or an acting-master AP, may have a synchronized oscillator and/or timer(s) and may be used for synchronization by neighboring APs. 
     In some embodiments, the components of the device  200  are, individually or collectively, implemented with one or more application-specific integrated circuits (ASICs) adapted to perform some or all of the applicable functions in hardware. Alternatively, the functions may be performed by one or more processing units (or cores), on one or more integrated circuits. In other embodiments, other types of integrated circuits are used (e.g., Structured/Platform ASICs, field-programmable gate arrays (FPGAs), and other Semi-Custom integrated circuits (ICs)), which may be programmed in any manner known in the art. The functions of each unit also may be wholly or partially implemented with instructions embodied in a memory, formatted to be executed by one or more general or application-specific processors. 
     By way of illustration, the device  200 , through the receiver module  205 , the synchronization module  210 , the acting master designation module  215 , and the transmitter module  220 , may prepare a signal at the synchronization module  210  including information regarding an oscillator and/or timer(s). The transmitter module may then transmit the signal to another AP. The receiver module  205  may receive an acknowledgment message letting the device  200  know that synchronization with the other AP has been successful. The acting master designation module  215  may prepare a signal, to be transmitted by the transmitter module  220 , including instructions designating the other AP as an acting master AP. 
     Next,  FIG. 2B  shows a block diagram of a device  200 - a  configured for synchronizing APs in a position location network in accordance with various embodiments. The device  200 - a  may be an example of the device  200  of  FIG. 2A ; and the device  200 - a  may perform the same or similar functions as described above for device  200 . In some embodiments, the device  200 - a  is an AP  105 - b , which may include one or more aspects of the APs  105  described above with reference to any or all of  FIGS. 1A, 1B, and 2A . The device  200 - a  may also be a processor. In some cases, the device  200 - a  includes a receiver module  205 - a , which may be an example of the receiver module  205  of  FIG. 2A ; and the receiver module  205 - a  may perform the same or similar functions as described above for receiver module  205 . In some cases, the device  200 - a  includes a transmitter module  220 - a , which may be an example of the transmitter module  220  of  FIG. 2A ; and the transmitter module  220 - a  may perform the same or similar functions as described above for transmitter module  220 . 
     In some embodiments, the device  200 - a  includes a synchronization module  210 - a , which may be an example of the synchronization module  210  of  FIG. 2A . The synchronization module  210 - a  may estimate an offset, such as a frequency offset and/or a time offset. Further, the synchronization module  210 - a  may adjust an estimated offset. In some cases, the synchronization module  210 - a  corrects a frequency of an oscillator and/or a time (or count) of a timer. In some cases, the synchronization module  210 - a  reports a frequency and/or time so other devices may synchronize with the device  200 - a    
     In some embodiments, the device  200 - a  includes an acting master designation module  215 - a , which may be an example of the acting master designation module  210  of  FIG. 2A . The acting master designation module  210 - a  may prepare a communication to let another AP know that it may act as a master AP. A master AP, or an acting master AP, may have a synchronized oscillator and/or timer(s) and may be used for synchronization by neighboring APs. Further, the acting master designation module  210 - a  may prepare a communication including an acknowledgment message informing another device that the device  200 - a  has been successfully and/or unsuccessfully synchronized. 
     In some cases, the device  200 - a  includes an acknowledgment module  230 . The acknowledgment module  230  may prepare communications including synchronization information, such as an indication that synchronization was successful. Further the acknowledgment module  230  may analyze communications from other devices to determine a device to designate as an acting master. 
     In some cases, the device  200 - a  includes a synchronized list module  240 . The synchronized list module  240  may include an integrated processor. In an embodiment the synchronized list module  240  includes a database. The synchronized list module  240  may keep a list of devices, such as APs, that have been successfully synchronized. In some cases, the synchronized list module  240  keeps a list of devices that have not been successfully synchronized. The synchronized list module  240  may collect synchronization information to be transmitted to a tracking management server. 
     According to some embodiments, the components of the device  200 - a  are, individually or collectively, implemented with one or more ASICs adapted to perform some or all of the applicable functions in hardware. In other embodiments, the functions of device  200 - a  are performed by one or more processing units (or cores), on one or more integrated circuits. In other embodiments, other types of integrated circuits are used (e.g., Structured/Platform ASICs, FPGAs, and other Semi-Custom ICs), which may be programmed in any manner known in the art. The functions of each unit may also be implemented, in whole or in part, with instructions embodied in a memory, formatted to be executed by one or more general or application-specific processors. 
     Turning now to  FIG. 3 , which depicts a block diagram of a system  300  configured for synchronizing APs in a position location network in accordance with various embodiments. The system  300  may include a APs  105 - c ,  105 - d , and  105 - e , which may be examples of the APs  105  described with reference to one or more of  FIGS. 1A, 1B, 2A, and 2B . The AP  105 - c  may include a memory module  310 , which, in some embodiments, includes a software module  315 . The AP  105 - c  may include a processor and scheduler module  320 , an oscillator module  330 , antenna(s) module  335 , a network communications module  340 , timer(s) module  350 , a transceiver module  360 , a synchronization module  210 - b , and/or an acting master designation module  215 - b . In some cases, the timer(s) module  350  and/or the oscillator module  330  operate based on a 32 MHz reference timer and/or oscillator. In some embodiments, the AP  105 - c  and/or the transceiver module  360  also includes an UWB module  370  and/or a narrowband module  380 . Each of the components of the AP  105 - c  may be in communication with each other. The network communications module  340  may be in communication with the network  140 - a , which may be an example of the network  140  of  FIGS. 1A and 1B . 
     The memory module  310  may include random access memory (RAM) and read-only memory (ROM). In some embodiments, the memory module  310  also stores computer-readable, computer executable software (SW) code  315  containing instructions configured to, when executed, cause the processor and scheduler module  320  to perform various functions described herein related to synchronizing APs in a position location network. In other embodiments, the software (SW) code  315  may not be directly executable by the processor and scheduler module  320 ; but it may be configured to cause a computer, e.g., when compiled and executed, to perform the functions described herein. 
     The processor and scheduler module  320  may include an intelligent hardware device, such as a central processing unit (CPU). The processor and scheduler module  320  may perform various operations associated with synchronizing APs in a position location network. The processor and scheduler module  320  may use scheduling information received from, for example, the tracking management server  150 , by way of the network  140 - a , to determine when it is desirable to synchronize APs  105 . The processor and scheduler module  320  may perform various operations associated with synchronizing APs in a position location network, including determining when to update a synchronization of APs  105 , and/or when to update a synchronization list of APs  105 . 
     The transceiver module  360  may include an ultra wideband (UWB) transceiver  370  and/or a narrowband transceiver  380 . Either or both of the UWB transceiver  370  and narrowband transceiver  380  may include a modem configured to modulate data (e.g., packets) and provide the modulated data to the antenna(s) module  335  for transmission, and to demodulate data received from the antenna(s) module  335 . Some embodiments of the AP  105 - c  include a single antenna; other embodiments include multiple antennas. As shown in  FIG. 3 , signals transmitted from a tag  115 - a  may be transmitted to and/or received by the AP  105 - c  via the antenna(s) in or connected to the antenna(s) module  335 . The AP  105 - c  may also wirelessly communicate with other APs, such as APs  105 - d  through  105 - e . In some embodiments, the AP  105 - c  may receive signals, including UWB, narrowband, and reference signals from other APs  105 ; and the AP  105 - c  may use the received signals for calibrating and/or synchronizing components of the AP  105 - c ; and/or the AP  105 - c  may use the received signals for determining a location (e.g., a position) of a tag unit  115 . In some cases, the AP  105 - c  may transmit received signals to the tracking management server  150  via the network communications module  340  and the network  140 - a.    
     The oscillator module  330  may be connected to the UWB transceiver  370 . The UWB transceiver  370  may include an UWB modulator and a radio frequency (RF) transceiver. In some embodiments, the UWB transceiver  370  includes, or is in communication with, a timer, such as the timer(s) module  350 . The UWB transceiver  370  may include an integrated processor. The UWB transceiver may work with, or for, the synchronization module  210 - b  to synchronize, for example, APs  105 . 
     The transceiver module  360  may further include the narrowband transceiver  380 . The narrowband transceiver  380  may include an integrated processor. It may also include a timer and/or oscillator. In some cases, it is in communication with the oscillator module  330  and/or the timer(s) module  350 . The narrowband transceiver  380  may be capable of communicating with wireless local area network (WLAN) products that are based on the IEEE 802.11 family of standards (WiFi). In some embodiments, the narrowband transceiver is a two-way digital radio based on the IEEE 802.15 family of standards (ZigBee). In another embodiment, the narrowband transceiver is a two-way digital radio based on the IEEE 802.15.1 family of standards (Bluetooth). 
     Next,  FIG. 4  shows a block diagram illustrating a system  400  configured for synchronizing APs in a position location network, which may include a tag unit  115 - b . In some embodiments, the tag unit  115 - b  includes one or more aspects of the tag units  115  of any or all of  FIGS. 1A, 1B, 2A, 2B, and 3 . The tag unit  115 - b  may include a controller and scheduler module  410 , a memory module  420 , a UWB transceiver module  450 , a narrowband transceiver module  460 , and antenna(s) module  440 . In some embodiments, the tag unit  115  includes an oscillator module  430  or a timer module  435 , or both. The oscillator module  450  and the timer module  435  may each include several oscillators and timers, respectively. 
     By way of illustration, the controller and scheduler module  410  includes logic or code, or both, that enables it to control the operations of the tag unit  115 - b . In some cases, the controller and scheduler module  410  includes a microcontroller or a state machine to control the UWB transceiver module  450  and the narrowband transceiver module  460 . 
     The memory module  420  may include random access memory (RAM) or read-only memory (ROM), or both. In some embodiments, the memory module  420  stores computer-readable, computer-executable software (SW) code  425  containing instructions that are configurable to, when executed, cause the controller and scheduler module  410  to perform various functions described herein for controlling the tag unit  115 - b . In other embodiments, the software code  425  is not directly executable by the controller and scheduler module  410 , but it may be configured to cause a computer, for example, when compiled and executed, to perform functions described herein. 
     The UWB transceiver module  450  may support radio frequency (RF) communication technology to broadcast UWB signals through the antenna(s) module  440 . Likewise, the narrowband transceiver module  460  may support RF communication technology to broadcast narrowband signals through the antenna(s) module  440 . In some embodiments, the UWB transceiver module  450  or the narrowband transceiver module  460 , or both, include a modulator (not shown) to modulate location tracking information and provide the modulated information to the antenna(s) module  440  for transmission of signals.  FIG. 4  shows broadcast and reception of signals between the tag unit  115 - b  and several APs  105 . In the system  400 , at least two APs  105 - f  and  105 - g  are shown communicating with the tag unit  115 - b ; but the tag unit  115 - b  may communicate with more or fewer APs  105 . 
     Referring next to  FIG. 5 , a system  500  is illustrated with a block diagram. The system  500  may be configured for synchronizing APs in a position location network in accordance with various embodiments. In some embodiments, the system  500  includes a tracking management server  150 - a , which may be the tracking management server  150  of  FIGS. 1A and/or 1B . The tacking management server  150 - a  may include a processor module  510 , a memory module  520 , a network communications module  530 , a synchronized list module  540 , and/or a management module  550 . The management module  550  may be configured to perform calibration, synchronization, coordinate determination, filter determination, channel estimation, and/or tag update mode adjustments. In some embodiments, the management module  550  determines or selects a master AP. 
     The processor module  510  may also perform various operations and may include an intelligent hardware device, e.g., a CPU. In some embodiments, the processor module  510  performs various operations associated with synchronizing APs in a position location network. For example, the processor module  510  may determine what APs need to be synchronized. The tracking management server  150 - a  also may communicate with a network  140 - b  through the network communications module  530  to receive information from the APs  105  and/or to send information to the APs  105 . The network  140 - b  may be an example of the networks  140  of any or all of  FIGS. 1A, 1B, and 3 . 
     The memory module  520  may include RAM and/or ROM. In some embodiments, the memory module  520  stores computer-readable, computer-executable software code  525  containing instructions that are configured to, when executed, cause the processor module  510  to perform various functions described herein. In other embodiments, the software code  525  may not be directly executable by the processor module  510 ; but the software code module may be configured to cause a computer, e.g., when compiled and executed, to perform functions described herein. The memory module  520  may include a database to store a list of synchronized APs  105 , and/or determined coordinate sets comprising coordinates of various APs  105  and/or tag units  115 . 
     The synchronized list module  540 , may perform various operations and may include an intelligent hardware device, e.g., a CPU. The synchronized list module  540  may include a database. The synchronized list module  540  may store a list of synchronized APs  105 . In some cases, the synchronized list module  540  may update a stored list of synchronized APs  105 . 
       FIG. 6  shows an example of a position location network  100 - a  in accordance with various embodiments. The system  100 - a  may be an example of a system  100  of  FIG. 1A  or  FIG. 1B , or both. The system  100 - a  may use the communications  125  between APs  105  to synchronize the network within the coverage area  110 . A master AP  105 - h  may use communications  125  to determine which APs need to be synchronized, such as APs  105 - j ,  105 - i ,  105 - l , and  105 - k . The APs may exchange messages  125  including oscillator and/or timer information. In some cases, a second AP  105 - l  transmits an acknowledgment message to the master AP  105 - h  confirming that synchronization was successful. Upon receipt of the acknowledgment message, the master AP  105 - h  may designate the second AP  105 - l  as an acting master AP. 
     In an embodiment, the acting master AP  105 - l  contains an oscillator and/or timer(s) that are synchronized with the master AP  105 - h . The acting master AP  105 - l  may then be used to synchronize neighboring APs  105 - m  and  105 - n . In some cases, the acting master AP  105 - l  transmits information relating to the APs  105 - n  and  105 - m  that have been synchronized with the acting master AP  105 - l , to the master AP  105 - h . In an embodiment, a neighboring AP  105 - m  may receive synchronizing information from more than one acting master APs  105 - k  and  105 - l . The neighboring AP  105 - m  may decide with which acting master AP—e.g., AP  105 - k  or  105 - l —to synchronize. For example, the neighboring AP  105 - m  may decide which acting master AP  105 - k  or  105 - l  to synchronize with based on the acting master AP from which it received the synchronization information from first and/or the acting master AP from which the received synchronization information has the strongest signal strength. 
     Next,  FIG. 7  shows a call flow diagram, which illustrates a system  700  configured for synchronizing APs in a position location network within a position location network, according to some embodiments.  FIG. 7  shows synchronization between three APs  105 . In the system  700 , three APs  105 - o ,  105 - p , and  105 - q  are shown synchronizing, but synchronization may occur with more APs  105  as well. In some cases, a first, or master, AP  105 - o  is used to synchronize secondary APs, such as a second AP  105 - p . A first synchronization communication  710  between the master AP  105 - o  and the second AP  105 - p  provides the second AP  105 - p  with information necessary to synchronize an oscillator and/or timer(s). The second AP  105 - p  may use the information to synchronize an oscillator and/or timer(s)  720 . In some cases, the second AP  105 - p  transmits an acknowledgment message  730  to the master AP  105 - o , to inform the master AP  105 - o  that synchronization was successful. 
     In some cases, the master AP  105 - o  adds the second AP  105 - p  to, or updates an existing value of, a stored synchronized list. The master AP  105 - o  may designate  740  the second AP  105 - p  as an acting master AP, and AP  105 - p  may assist in synchronizing APs that neighbor the second AP  105 - p , such as a third AP  105 - q . The master AP  105 - o  may send a message  750  designating the second AP  105 - p  as an acting master AP. An acting master AP may be an AP with an oscillator and/or timer(s) that is synchronized with the master AP. 
     In some cases, the acting master AP  105 - p  broadcasts a synchronization message  760  which provides the third AP  105 - q  with information necessary to synchronize an oscillator and/or timer(s). The third AP  105 - q  may use the information to synchronize an oscillator and/or timer(s)  770 . In some cases, the third AP  105 - q  transmits an acknowledgment message  780  to the acting master AP  105 - p , to inform the acting master AP  105 - p  that synchronization with the third AP  105 - q  was successful. The acting master AP  105 - p  may then send an acknowledgment message  790  to the master AP  105 - o  informing the master AP  105 - p  that synchronization with the third AP  105 - q  was successful. 
     In an embodiment, the third AP  105 - q  sends an acknowledgment message directly to the master AP  105 - o . In some cases, the master AP  105 - o  adds the third AP  105 - q  to, or updates an existing value of, a stored synchronized list. In some cases, the acting master AP  105 - p  may designate the third AP  105 - q  as another acting master AP. In an embodiment, the acknowledgment messages  730 ,  780 , and  790  are transmitted to a tracking management server. In some cases, the synchronized list is stored and/or managed by the tracking management server. In still further embodiments, an acting master AP transmits a list (or an acknowledgment message representative of a list) of all APs  105  which have synchronized to that acting master AP. For example, the acting master AP  105 - p  may transmit a list of all APs  105 , which may include AP 105 - q , that have synchronized to AP  105 - p.    
     Those skilled in the art will recognize that the system  700  is but one implementation of the tools and techniques discussed herein. The operations of the system  700  may be rearranged or otherwise modified such that other implementations are possible. 
       FIG. 8  shows a flow diagram that illustrates a method  800  for synchronizing APs in a position location network within a position location network, according to various embodiments. The method  800  may be implemented using, for example, the devices and systems  100 ,  200 ,  200 - a ,  300 ,  400 ,  500 ,  100 - a , and  700  of  FIGS. 1A, 1B, 2A, 2B, 3, 4, 5, 6 , and  7 . 
     At block  805 , an AP  105 , tracking management server  150 , and/or some other network component may select a master AP from among a plurality of APs  105 , each of the APs  105  having a reference oscillator and a timer. In some embodiments, the operations at block  805  are performed by the management module  550  of  FIG. 5 . 
     At block  810 , a master AP may broadcast a first synchronization message. For example, the operations at block  810  may be performed by: the device  200  of  FIG. 2A ; the device  200 - a  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  815 , the master AP may receive a first acknowledgment message from each AP  105  that synchronized to the master AP based on the first synchronization message. For example, the operations at block  815  may be performed by: the device  200  of  FIG. 2A ; the acknowledgment module  230  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  820 , the master AP and/or the tracking management server  150 , may designate as acting master APs one or more APs  105  from which an acknowledgment message is received. In some cases, the operations at block  820  are performed by: the tracking management server  150  of  FIGS. 1A and/or 1B ; the acting master designation module  215  of  FIG. 2A ; the action master designation module  215 - a  of  FIG. 2B ; the acting master designation module  215 - b  of  FIG. 3 ; and/or the management module  550  of  FIG. 5 . 
       FIG. 9  shows a flow diagram that illustrates a method  900  for synchronizing APs in a position location network within a position location network, according to some embodiments. In some cases, the method  900  may be implemented using some or all of the devices and systems  100 ,  200 ,  200 - a ,  300 ,  400 ,  500 ,  100 - a , and  700  of  FIGS. 1A, 1B, 2A, 2B, 3, 4, 5, 6, and 7 . 
     At block  905 , an AP  105 , tracking management server  150 , and/or some other network component may select a master AP from among a plurality of APs  105 , each of the APs  105  having a reference oscillator and a timer. In some embodiments, the operations at block  905  are performed by the management module  550  of  FIG. 5 . 
     At block  910 , a master AP may broadcast a first synchronization message. For example, the operations at block  910  may be performed by: the device  200  of  FIG. 2A ; the device  200 - a  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  915 , the master AP may receive a first acknowledgment message from each AP  105  that synchronized to the master AP based on the first synchronization message. For example, the operations at block  915  may be performed by: the device  200  of  FIG. 2A ; the acknowledgment module  230  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  920 , the master AP and/or the tracking management server  150 , may designate as acting master APs one or more APs  105  from which an acknowledgment message is received. In some cases, the operations at block  920  are performed by: the tracking management server  150  of  FIGS. 1A and/or 1B ; the acting master designation module  215  of  FIG. 2A ; the action master designation module  215 - a  of  FIG. 2B ; the acting master designation module  215 - b  of  FIG. 3 ; and/or the management module  550  of  FIG. 5 . 
     At block  925 , a designated acting master AP may broadcast a second synchronization message. For example, the operations at block  925  may be performed by the device  200  of  FIG. 2A ; the device  200 - a  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - l  of  FIG. 6 . 
     At block  930 , the designated acting master AP may receive a second acknowledgment message from each AP  105  that synchronized to the designated acting master AP based on the second synchronization message. In some embodiments, the operations at block  930  may be performed by the device  200  of  FIG. 2A ; the acknowledgment module  230  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     Turning to  FIG. 10 , which shows a flow diagram that illustrates a method  1000  for synchronizing APs in a position location network within a position location network, according to various embodiments. By way of example, the method  1000  is implemented using some or all of the devices and systems  100 ,  200 ,  200 - a ,  300 ,  400 ,  500 ,  100 - a , and  700  of  FIGS. 1A, 1B, 2A, 2B, 3, 4, 5, 6, and 7 . 
     At block  1005 , an AP  105 , tracking management server  150 , and/or some other network component may select a master AP from among a plurality of APs  105 , each of the APs  105  having a reference oscillator and a timer. In some embodiments, the operations at block  1005  are performed by the management module  550  of  FIG. 5 . 
     At block  1010 , a master AP may broadcast a first synchronization message. At block  1015 , the master AP may receive a first acknowledgment message from each AP  105  that synchronized to the master AP based on the first synchronization message. For example, the operations at block  1010  may be performed by: the device  200  of  FIG. 2A ; the device  200 - a  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  1020 , the master AP and/or the tracking management server  150 , may designate as acting master APs one or more APs  105  from which an acknowledgment message is received. In some cases, the operations at block  1020  are performed by: the tracking management server  150  of  FIGS. 1A and/or 1B ; the acting master designation module  215  of  FIG. 2A ; the action master designation module  215 - a  of  FIG. 2B ; the acting master designation module  215 - b  of  FIG. 3 ; and/or the management module  550  of  FIG. 5 . 
     At block  1025 , a designated acting master AP may broadcast a second synchronization message. For example, the operations at block  1025  may be performed by the device  200  of  FIG. 2A ; the device  200 - a  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - l  of  FIG. 6 . 
     At block  1030 , the designated acting master AP may receive a second acknowledgment message from each AP  105  that synchronized to the designated acting master AP based on the second synchronization message. In some embodiments, the operations at block  1030  may be performed by the device  200  of  FIG. 2A ; the acknowledgment module  230  of  FIG. 2B ; the AP  105 - c  of  FIG. 3 ; and/or the AP  105 - h  of  FIG. 6 . 
     At block  1035 , the master AP and/or the tracking management server  150  may receive from the one or more designated acting master APs one or more lists of neighboring APs  105  synchronized to the one or more designated acting master APs. In some cases, the operations at block  1035  are performed by: the tracking management server  150  of  FIGS. 1A and/or 1B ; the acting master designation module  215  of  FIG. 2A ; the action master designation module  215 - a  of  FIG. 2B ; the acting master designation module  215 - b  of  FIG. 3 ; and/or the synchronized list module  540  of  FIG. 5 . 
     It will be apparent to those skilled in the art that the methods  800 ,  900 , and  1000  are but example implementations of the tools and techniques described herein. The methods  800 ,  900 , and  1000  may be rearranged or otherwise modified such that other implementations are possible. 
     The detailed description set forth above in connection with the appended drawings describes exemplary embodiments and does not represent the only embodiments that may be implemented or that are within the scope of the claims. The term “exemplary” used throughout this description means “serving as an example, instance, or illustration,” and not “preferred” or “advantageous over other embodiments.” The detailed description includes specific details for the purpose of providing an understanding of the described techniques. These techniques, however, may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the concepts of the described embodiments. 
     Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals, bits, symbols, and chips that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof. 
     The various illustrative blocks and modules described in connection with the disclosure herein may be implemented or performed with a general-purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     The functions described herein may be implemented in hardware, software executed by a processor, firmware, or any combination thereof. If implemented in software executed by a processor, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium. Other examples and implementations are within the scope and spirit of the disclosure and appended claims. For example, due to the nature of software, functions described above can be implemented using software executed by a processor, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions may also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. Also, as used herein, including in the claims, “or” as used in a list of items prefaced by “at least one of” indicates a disjunctive list such that, for example, a list of “at least one of A, B, or C” means A or B or C or AB or AC or BC or ABC (i.e., A and B and C). 
     Computer-readable media includes both computer storage media and communication media including any medium that facilitates transfer of a computer program from one place to another. A storage medium may be any available medium that can be accessed by a general purpose or special purpose computer. By way of example, and not limitation, computer-readable media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that can be used to carry or store desired program code means in the form of instructions or data structures and that can be accessed by a general-purpose or special-purpose computer, or a general-purpose or special-purpose processor. Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. Disk and disc, as used herein, include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above are also included within the scope of computer-readable media. 
     The previous description of the disclosure is provided to enable a person skilled in the art to make or use the disclosure. Various modifications to the disclosure will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other variations without departing from the spirit or scope of the disclosure. Thus, the disclosure is not to be limited to the examples and designs described herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.