WO2019152034A1

WO2019152034A1 - Systems and methods for wireless monitoring

Info

Publication number: WO2019152034A1
Application number: PCT/US2018/016322
Authority: WO
Inventors: Mohammad Nekoui; Shinhaeng Lee; Tao Zhang; Yuxuan ZHANG; Lichung Chu
Original assignee: Olympus Communication Technology Of America, Inc.
Priority date: 2018-01-31
Filing date: 2018-01-31
Publication date: 2019-08-08
Also published as: WO2019152031A1

Abstract

A method for scheduling simultaneous communications among a plurality of transceivers includes: identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another; selecting a first set of transceivers from among the one or more groups of transceivers identified as not interfering with one another, scheduling transceivers of the first set to initiate communication at the same time; determining a first transceiver of the first set of transceivers that will require the greatest amount of time to complete its communication relative to the other transceiver or transceivers in the first set; selecting additional transceivers from the one or more groups of transceivers identified as not interfering with one another, that can complete their communication operations before the first transceiver will finish its communication operations; and scheduling one or more of the additional transceivers to begin communicating before the first transceiver finishes its communication operations.

Description

SYSTEMS AND METHODS FOR WIRELESS MONITORING

Technical Field

[0001] The disclosed technology relates generally to communications systems, and more particularly, some embodiments relate to a multi-tier wireless network.

Description of the Related Art

[0002] Two-tier systems for Structural Health Monitoring (SHM) and other applications have been proposed in various forms. For example, a hybrid system using a combination of a Wireless Sensor Network (WSN) and Radio Frequency Identification (RFID) technology has been proposed for smart healthcare systems. The WSN is a low-power personal area network based on IPv6 over Low-Power Wireless Personal Area Networks (6L0WPAN). The goal of such hybrid systems is mainly focused on ultra-low power consumption.

[0003] However, issues often arise with multiple RFID devices and multiple RFID readers in close proximity. In an environment with multiple RFID readers in close proximity, simultaneous operation of these readers might cause interference among the RFID tags sought to be interrogated. Likewise, simultaneous operation of these readers can lead to one reader causing interference on a neighboring reader. Accordingly, there are two kinds of interference that can arise in a multi-RFID reader environment. Those are reader-tag collisions and reader-reader collisions. Figure 1 illustrates a simple example of reader-tag and reader-reader collisions. These types of collisions are illustrated using an example scenario of a system having two readers 122, 124 and an RFID tag 126. Each reader is shown with an example interrogation region 110, which is essentially the useful operating range of the reader. Reader 122 on the left-hand side is also shown with an example interference region 112. Although only one reader is illustrated as having an interference region 112, it is generally understood that each reader may also have its corresponding interference region.

[0004] In the example of reader-tag collisions 105, the signal from tag reader 122 (e.g., due to its interference region 112) interferes with the desired RFID signal from RFID tag 126. Accordingly, interference due to transmissions from tag reader 122 can hinder the ability of tag reader 124 to properly interrogate RFID tag 126. In the example of a reader-reader condition 106 as illustrated in Figure 1, two readers 122, 124, are attempting to interrogate RFID tag 126 at the same time. According, readers 122, 124, are interfering with each other's operations. Each of these examples illustrates how interference from nearby devices can interrupt operation of a reader system.

[0005] To combat reader-tag collisions, some conventional systems have exploited various techniques to ensure that multiple readers are not active on the same channel simultaneously or that they are not operating at the same time, or a combination of the two. On the other hand, to combat reader-reader collisions, the readers should operate at different times because the tag cannot selectively receive only one channel frequency but not the other. However, industrial, scientific and medical devices operating in the ISM frequency bands may have additional restrictions placed on them. In the United States, for example, unlicensed devices operating in the ISM frequency bands are governed by Title 47, Part 15 of the Code of Federal Regulations (47 CFR 15). While the FCC regulations require transmitters to implement a pseudorandom hopping sequence, the FCC does not allow multiple readers to coordinate their frequency hopping patterns. Accordingly, this places limits on how frequency hopping can be used to minimize or reduce reader-tag collisions.

Brief Summary of Embodiments

[0006] Embodiments of the technology disclosed herein are directed toward devices and methods for providing wireless sensor systems for applications such as, for example, structural health monitoring, or SHM, medical monitoring, or other monitoring applications. More particularly, various embodiments of the technology disclosed herein provide a method for scheduling simultaneous communications among a plurality of transceivers, including: identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another; selecting a first set of transceivers from among the one or more groups of transceivers identified as not interfering with one another, and scheduling all transceivers of the first set to initiate communication at the same time; determining a first transceiver of the first set of transceivers that will require the greatest amount of time to complete its communication relative to the other transceiver or transceivers in the first set of transceivers; selecting additional one or more transceivers from the one or more sets of transceivers identified as not interfering with one another, that can complete their communication operations before the first transceiver will finish its communication operations; and scheduling one or more of the additional transceivers to begin communicating before the first transceiver finishes its communication operations.

[0007] Identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another may include constructing an interference graph that may include, a plurality of vertices, wherein a vertex represents a respective transceiver; edges between vertices, wherein an edge between two vertices indicates that the transceivers represented by those two vertices are within an interference region with respect to each other; and weights for the vertices, wherein a weight for a vertex represents a required communication time for the transceiver represented by that vertex. Also, selecting a first set of transceivers from among the one or more groups of transceivers may include identifying a priority-aware maximum weighted independent set in the interference graph by selecting a root transceiver based on weight and degree values of the vertices in the interference graph. The method may further include removing from the interference graph the selected vertex and each vertex connected directly to the selected vertex by an edge. [0008] A vertex in the interference graph may include a degree D_v and a weight W_v, and wherein identifying a priority-aware maximum weighted independent set may include determining a root transceiver by determining a vertex in the interference graph maximizing

, and determining a set of one or more

non-interfering transceivers that can communicate simultaneously with the root transceiver. In some embodiments, a priority-aware maximum weighted independent set may also include determining a maximum total weight of the set of noninterfering transceivers. In further embodiments, determining a maximum total weight of the set of noninterfering transceivers may include comparing the communication durations of all noninterfering transceivers to the communication duration for the root transceiver and selecting all noninterfering transceivers whose read times are less than the read time of the root transceiver.

[0009] Scheduling one or more of the additional transceivers may include constructing a tree graph including a plurality of leaf nodes; iteratively assigning second weights to the leaf nodes; adding new children to the tree graph until all leaf nodes are assigned a second weight. In some embodiments, constructing a tree graph may include, adding a vertex as a leaf node of the tree graph if the transceiver associated with that vertex is not already scheduled and is not a neighbor of a currently active transceiver, wherein the added vertex is added as a child leaf node of a leaf node representing a vertex of a previously active transceiver with which the added vertex is directly connected by an edge on the interference graph. In further embodiments, the method may further include determining a path in the tree graph with a largest sum of second weights and selecting children of all leaf nodes in the path.

[0010] In some embodiments, a vertex in the interference graph includes a degree D_v and a weight W_v, and identifying a priority-aware maximum weighted independent set may include determining a root transceiver by determining a vertex in the interference graph maximizing W_V(D_V + 1), and determining a set of one or more non-interfering transceivers that can communicate simultaneously with the root transceiver.

[0011] In other embodiments, a vertex in the interference graph includes a degree D_v and a weight W_v, and identifying a priority-aware maximum weighted independent set may include determining a root transceiver by determining a vertex in the interference graph maximizing W_v ^a(D_v + \)^b , and determining a set of one or more non-interfering transceivers that can communicate simultaneously with the root transceiver, where a and b are weighting factors between zero and one.

[0012] In various embodiments, two transceivers may be said to be within an interference region with respect to each other if the distance between the two transceivers is less than the sum of an interference range of one transceiver of the two transceivers and a read range of the other transceiver of the two transceivers.

[0013] Selecting a first set of transceivers from among the one or more groups of transceivers may include determining a priority-aware maximum weighted set of transceivers using weights and degrees assigned to the transmitters, wherein a weight represents a total communication time for the transceiver to which that weight is assigned and a degree represents a quantity of other transceivers within an interference region of the transceiver to which that degree is assigned. Determining a priority-aware maximum weighted set of transceivers using weights assigned to the transmitters may include determining a first transceiver by maximizing at least l), and W_v ^a (D_v +\)^b , and determining a set of one or more

non-interfering transceivers that can communicate simultaneously with the first transceiver. In some embodiments, the interference region defines a region surrounding a transceiver within which transmissions by the transceiver will generate interference above a determined threshold to another transceiver within the defined region.

[0014] Identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another may include for each transceiver in the plurality of transceivers, determining which other transceivers in the plurality of transceivers is within its interference region.

[0015] Selecting a first set of transceivers from among the one or more groups of transceivers identified as not interfering with one another may include: selecting a transceiver represented by a vertex with a maximum weight v = argmax^_g^C,,); adding the selected transceiver to a set of active nodes; removing the selected transceiver and all transceivers within an interference region of the selected transceiver from further selection; and iteratively repeating the steps of selecting and adding transceivers for transceivers that have not been removed from further selection until all transceivers have been removed from further selection. [0016] Scheduling the one or more of the additional transceivers to begin communicating before the first transceiver finishes its communication operations, may include identifying a second transceiver represented by a vertex with a minimum weight vertex: v = argmin,,^ (t_eTld), identifying all transceivers within an interference region of the second transceiver, and removing the second transceiver from consideration if the second transceiver is within an interference region of any transceiver in the set of active nodes.

[0017] In another embodiments method for scheduling simultaneous communications among a plurality of RFID tag readers includes: constructing an interference graph including a plurality of vertices selectively connected by edges, wherein the vertices each represent a corresponding RFID reader and an edge connecting two vertices represents to RFID readers within an interference range of each other; determining a priority-aware maximum weighted independent set of RFID readers by identifying a first RFID reader based on weight and degree values for the vertices in the interference graph, wherein a weight for a vertex represents a required read time for the RFID reader represented by that vertex and a degree for a vertex represents a number of other RFID readers within the interference region with respect to the RFID reader represented by that vertex; removing the vertex representing the identified RFID reader and all of its neighbor vertices from the interference graph and repeating the determining step until no further vertices can be chosen. [0018] Other features and aspects of the disclosed technology will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features in accordance with embodiments of the disclosed technology. The summary is not intended to limit the scope of any inventions described herein, which are defined solely by the claims attached hereto.

Brief Description of the Drawings

[0019] The technology disclosed herein, in accordance with one or more various embodiments, is described in detail with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict typical or example embodiments of the disclosed technology. These drawings are provided to facilitate the reader's understanding of the disclosed technology and shall not be considered limiting of the breadth, scope, or applicability thereof. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

[0020] Figure 1 illustrates a simple example of reader-tag and reader-reader collisions.

[0021] Figure 2 illustrates an example of a structural health monitoring environment with which embodiments of the systems and methods disclosed herein may be implemented. [0022] Figure 3 is a diagram illustrating an example 2-tier wireless network in accordance with one embodiment of the systems and methods described herein.

[0023] Figure 4 is a diagram illustrating an example implementation of an interrogation and communication circuit in accordance with one embodiment of the systems and methods described herein.

[0024] Figure 5 is a diagram illustrating an example of an RFID sensor tag 344 in accordance with one embodiment of the systems and methods disclosed herein.

[0025] Figure 6 is a diagram illustrating an example operation of a two-tier network in accordance with one embodiment of the systems and methods disclosed herein.

[0026] Figure 7 is a diagram illustrating an example timeline of subsystem performance in an example 2-tier wireless network in accordance with various embodiments of the systems and methods disclosed herein.

[0027] Figure 8A is a diagram illustrating an example of a plurality of antennas configured to operate with a single field device in accordance with one embodiment of the systems and methods described herein.

[0028] Figure 8B is a diagram illustrating another example of a plurality of antennas configured to operate with a single field device in accordance with one embodiment of the systems and methods described herein.

[0029] Figure 9 is a diagram illustrating a simple example of reader-reader collisions that can occur when the distance between two readers is less than the sum of the interference range of one reader and the read range of the other reader. [0030] Figure 10 illustrates an example interference graph in accordance with one embodiment of the systems and methods described herein.

[0031] Figure 11 illustrates an example scheduling solution in accordance with one embodiment of the systems and methods described herein.

[0032] Figure 12 illustrates another example process for scheduling in accordance with one embodiment of the systems and methods described herein.

[0033] Figure 13 illustrates an example of the scheduling algorithm in accordance with one embodiment of the systems and methods described herein.

[0034] Figure 14 illustrates this embodiment of the scheduling process in terms of the example network of Figure 10.

[0035] Figure 15 illustrates an example process for scheduling in accordance with one embodiment of the systems and methods described herein.

[0036] Figures 16 and 17 illustrate additional examples of the sample network shown in figure 10 with an illustration of vertices that are not already scheduled and that are not a neighbor of a currently active node, and example timelines according to the scheduling tree.

[0037] Figure 18 illustrates an example of a partial scheduling tree.

[0038] Figure 19 illustrates a completed scheduling tree in accordance with the example described above with reference to Figures 16, 17 and 18.

[0039] Figure 20 illustrates an example of a final scheduling timeline of all the readers in the foregoing example along with the corresponding scheduling diagram. [0040] Figure 21 illustrates an example scheduling solution in accordance with one embodiment of the systems and methods described herein.

[0041] Figure 22 illustrates the scheduling of the three nodes according to this example and their respective read times.

[0042] Figure 23 illustrates the scheduling of the next two nodes according to this example.

[0043] Figure 24 illustrates the scheduling of the next two nodes according to this example.

[0044] Figure 25 illustrates an example resultant scheduling algorithm in accordance with one embodiment of the systems and methods described herein.

[0045] Figure 26 illustrates an example circuit that may be used in implementing various features of embodiments of the disclosed technology.

[0046] The figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration, and that the disclosed technology be limited only by the claims and the equivalents thereof.

Detailed Description of the Embodiments

[0047] Embodiments of the technology disclosed herein are directed toward devices and methods for providing wireless sensor systems for applications such as, for example, structural health monitoring, or SFI M. More particularly, various embodiments of the technology disclosed herein provide a 2-tier wireless sensor network that includes a plurality of RFID tag sensors that can include a sensor circuit and an RFID communication circuit. The RFID sensors can be distributed and deployed at various locations to sense physical, environmental or other conditions. Field devices that include interrogation and communication circuits can be deployed to communicate with a group of one or more RFID tag sensors within their range. The interrogation and communication circuits may interrogate the nearby RFID tag sensors, gather sensor data from the RFID tag sensors and communicate that sensor data to a monitoring and analysis system. In some embodiments, the interrogation and communication circuits communicate with the monitoring and analysis system via a wireless mesh network, although other communication infrastructure can be used.

[0048] Before describing the disclosed technology in detail, it is useful to describe an example application with which the technology can be implemented. One such example application is in the field of Structural Hea Ith Monitoring (SFI M). Structural health monitoring systems can be used to provide early detection of damage or out of specification stress or strain on the structure and can be used for early identification of issues that may need to be corrected. Structural health monitoring systems typically employ a plurality of sensors deployed on or near the components of a monitored structure to sense various conditions that may indicate the health of the monitored structure. For example, sensors may measure parameters relevant to the structure such as vibration, stress, strain, humidity, corrosion, deformation, panel thickness, position and so on. Data gathered by the sensors is collected and communicated to a monitoring facility that can evaluate the data to determine the health of the monitored structure.

[0049] Figure 2 illustrates an example of a structural health monitoring environment with which embodiments of the systems and methods disclosed herein may be implemented. In the example of Figure 2, there are a plurality of monitored structures 222. Monitored structures 222 might include, for example, various structural elements or other components of the system being monitored. Systems to be monitored can include, for example, aircraft, automobiles, trains or other vehicles, bridges, buildings, roadways, industrial machinery, and so on. Multiple sensors 232 can be deployed throughout various structures of the system to monitor the health and status of the system.

[0050] Sensors 232 communicate the collected sensor information via a communication link 225 to one or more monitoring and analysis systems 226. For example, wireless sensor networks or other communications infrastructure can be used to collect sensor data and provide that data to the monitoring systems. Monitoring and analysis systems 226 can be centralized or distributed and typically include a plurality of computing systems used to analyze the data received to detect damage, predict failure, assess the need for maintenance, or otherwise monitor and analyze the structural health of the system. In addition to performing analytics, monitoring and analysis systems 226 can also send commands back to the sensors if needed. [0051] Although embodiments of the systems and methods disclosed herein are at times described in terms of the example environment of a structural health monitoring system, one of ordinary skill in the art reading this description will understand how the technology disclosed herein can be implemented in other applications and environments. For example, other applications with which the disclosed technology may be used can include, for example, medical device, patient, and health-care facility applications; environmental monitoring applications; commercial and industrial systems monitoring, feedback and control systems; and others.

[0052] Embodiments of the systems and methods disclosed herein provide a communication infrastructure to collect data from a plurality of sensors and to communicate the collected sensor data to a collection point such as, for example, monitoring and analysis systems 226. Some embodiments may be implemented as a hybrid 2-tier wireless network system to gather data from one or more sensor nodes and transport the data to a monitoring circuit such as in a backend server system. Such embodiments may include a plurality of RFID tags, such as passive tags embedded with sensors. The RFID tags can be used to send sensed data to a corresponding RFID reader. These RFID tags can be implemented as passive tags that don't require batteries, but instead harvest energy from the tag reader's RF signal that is used to interrogate the sensor. Once the RFID reader gathers sensor readings from its corresponding tags, it routes this information over a network to the backend servers. In some applications, the readers can be implemented to packetize the sensor data from the plurality of RFID tags, and the network is a mesh network that can be implemented as a WirelessHART (WiHART) network. Embodiments may be further implemented in which each reader, which also serves as a node in the mesh network, and services multiple RFID tags such that there are a limited number of network components that require power. Accordingly, embodiments may be implemented in which there are a limited number of components requiring battery replacement.

[0053] Figure 3 is a diagram illustrating an example 2-tier wireless network in accordance with one embodiment of the systems and methods described herein. Referring now to Figure 3, this example includes a plurality of field devices 342, each serving a plurality of RFID tags 344 within range of its interrogation signal. Field devices 342 may be implemented, for example, to include an interrogation and communication circuit (an example of which is shown in Figure 4 at 400) that may be used to interrogate the RFID tag sensors and to communicate information gathered from the RFID tag sensors to one or more access points 346 directly or via other field devices 342. The interrogation and communication circuit can also be implemented in various embodiments to provide a return communication path. That is, they can receive information from the wireless access point 346 and transmit information to the RFID tag sensors.

[0054] Figure 4 is a diagram illustrating an example implementation of an interrogation and communication circuit in accordance with one embodiment of the systems and methods described herein. With reference now to Figure 4, in this example, interrogation and communication circuit 400 includes an RFID reader 410, a control circuit 411, a WiHART mote 412, memory 413 and power source 414.

[0055] RFID reader 410 can be implemented utilizing commercially available RFID reader technology to interrogate one or more RFID tag sensors 344. As noted above, in various embodiments RFID tag sensors 344 are passive devices and do not require a battery or other 'internal' source of energy. Instead of relying on a battery, the radio frequency (RF) interrogation signal used by RFID reader 410 to interrogate RFID tag sensors 344 also provides energy to the RFID tag sensors 344. The RFID sensors can include a storage device (e.g., a capacitive or other storage device) to store energy harvested from the RF signal. Although 3 RFID tag sensors 344 are illustrated in the example of figure 4, RFID reader 410 can be implemented to interact with a different quantity of one or more RFID tag sensors 344.

[0056] Control circuit 411 can be provided to control the operation of the device, including the operation of RFID reader 410 and WiFlart mote 412. Control circuit 411 can be implemented using hardware circuitry, software, or combination of the foregoing to implement the desired functions. Where control circuit 411 is implemented using a processor or other computing device, memory 413 can be used to store variables and program instructions as may be needed for the computing operations. In some examples, memory four thirteen may also provide a memory buffer for communications or other values. Memory 413 can be implemented using any of a number of different memory or storage technologies, examples of which are set forth in more detail below. [0057] Power supply 414 can be implemented using any of a number of different power technologies to provide energy that can be used to power the various components of interrogation and communication circuit 400. Power supply 414 can be implemented as a battery (e.g., rechargeable battery), capacitor bank or other energy storage device. In some embodiments, power supply 414 is configured to be charged, for example, via a wired interface, by a wireless inductive charging system, through an array of photovoltaic cells included with interrogation and communication circuit 400, or other charging systems. Alternatively, photovoltaic cells can be used to provide power for interrogation and communication circuit 400 directly. Power supply 414 can also be implemented using a power supply connected to AC mains (which may include an AC to DC power converter, as needed) or to another suitable power source.

[0058] WiHART mote 412 provides the communication function for the wireless mesh network. WiHART mote 412 includes a wireless communications transceiver and can be implemented to receive information from RFID reader 410, directly or indirectly, and to communicate this information to the wireless access point 346 via the mesh network. WiHART mote 412 in some embodiments implements a MAC layer using Time Division Multiple Access (TDMA) and Channel Hopping for collision avoidance, and deterministic communication between two devices.

[0059] Figure 5 is a diagram illustrating an example of an RFID sensor tag 344 in accordance with one embodiment of the systems and methods disclosed herein. With reference now to Figure 5, the example RFID sensor tag 344 includes an energy harvesting circuit 512, an RFID tag 535, and a communication interface between sensing circuit 540 and RFID tag 535. Accordingly, RFID sensor tag 344 includes circuitry for sensor functionality and circuitry for RFID tag operations. Because RFID sensor tag 344 is a passive sensor in this example embodiment, this example also includes energy harvesting circuit 512 which is used to capture some of the energy from the radiofrequency (RF) interrogation signal and use that energy to power circuitry in the RFID sensor tag 344. Energy harvesting circuit 512 can include a capacitor or other energy storage device that can be charged by the incoming RF signal from the corresponding RFID reader. Accordingly, energy harvesting circuit 512 can be used to provide power for sensing circuit 540, RFID tag 535 and other components of RFID sensor tag 344.

[0060] RFID sensor tags 344 can be designed to work in the particular environment in which the sensors are intended to be used. In some embodiments, RFID sensor tags 344 can use the EPC Radio-Frequency Identity Protocols Generation-2 U H F RFID standard, which outlines the communications between passive tags and their corresponding reader via backscatter in the 860-960 MFIz frequency range. As noted RFID sensor tags 344 receive both information and operating energy from the reader's RF signal. For example, the reader and RFID sensor tags 344 can communicate by the reader transmitting a continuous-wave

(CW) RF signal to the RFID sensor tag 344 and the Tag responding by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the Interrogator.

[0061] In various embodiments, the reader (e.g., RFID reader 410) can manage its corresponding RFID sensor tags 344 using any of a number of different operations. The first operation may be referred to as a Select operation. The Select operation allows RFID reader 410 to select a RFID sensor tag population with which it is going to interact. Accordingly, RFID reader 410 may use a first command to select one or more RFID sensor tags 344 using the value or values stored in tag memory. RFID reader 410 may also inventory and access the selected RFID sensor tags 344.

[0062] Another operation is the Inventory operation. In this operation, RFID reader 410 identifies an individual RFID sensor tag 344 with which it is going to interact. In various embodiments, RFID reader 410 can begin an inventory round by transmitting a query command in a session. One or more of the RFID sensor tags 344 may reply to the query command sent by RFID reader 410. RFID reader 410 detects a reply from an RFID sensor tag 344 and requests that tag's EPC.

[0063] The read, or access, operation is the operation of communicating with an identified RFID sensor tag 344. In this operation, RFID reader 410 may perform a core operation such as, for example, reading, writing, locking, or disabling the identified RFID sensor tag 344; a security-related operation such as authenticating the identified RFID sensor tag 344; or a file-related operation such as opening a particular file in the identified RFID sensor tag's 344 user memory. [0064] Figure 6 is a diagram illustrating an example operation of a two-tier network in accordance with one embodiment of the systems and methods disclosed herein. Referring now to figure 6, at operation 622 the system controller (e.g. control circuit 411) commands the RFID circuit (e.g. RFID reader 410) to obtain necessary sensor measurement. At operation 624, RFID reader 410 interrogates the identified RFID sensor tags 344 to obtain the sensor information. In various embodiments, control circuit 411 or RFID reader 410 can determine the appropriate RFID sensor tag or tags 344 to interrogate to obtain a desired sensor information.

[0065] At operation 626, an identified RFID sensor tag 344 receives the interrogation signal, powers up its sensor circuit 540 using energy from the interrogation signal or energy ha rvesting circuit 512, and sensor circuit 540 takes a necessary sensor measurements. Sensor circuit 540 provides a sensor data to RFID tag 550, and RFID tag 550 returns the measurement data to the RFID reader 410 at interrogation and communication circuit 400.

[0066] At operation 628, RFID reader 410 receives the sensor data from the identified RFID sensor tag 344 and provides the sensor data to the control circuit 411. Control circuit 411 converts the RFID measurements into packets and provides the packets to WiFIART Mote 412. This is illustrated at operation 630. WiFIART Mote 412 release this information to the backend server via the access point (e.g. wireless access point 346) over the wireless mesh network via a plurality of other network nodes. [0067] Using a mesh topology such as that illustrated in Figure 3 can enhance the reliability of the network as compared to other topologies because it helps to avoid single points of failure. Using time division multiple access and frequency hopping functionality in WiHART mote 412 helps to provide stability against impairments of the environment such as multipath fading, jamming and other external interferers.

[0068] As this example serves to illustrate, functionality of the wireless sensing system can be distributed across a plurality of RFID sensor tags 344 and interrogation and communication circuits 400. Where RFID sensor tags 344 are implemented using passive sensors, battery replacement concerns can be virtually eliminated and the use of batteries can be limited only to interrogation and communication circuits 400. Because interrogation communication circuit 400 can service a plurality of RFID sensor tags 344, in various embodiments, the system can be implemented with fewer interrogation and communication circuits 400 than RFID sensor tags 344. Accordingly, this configuration can reduce the number of network elements that require battery replacement, which may lead to more reliable communications across the system. Depending on the number of sensors deployed for a given application, the reduction in the amount of batteries that would otherwise be required can be considerable. Consider an example scenario wherein there are 1000 sensors distributed to monitor the health of a structure and there are 30 interrogation and communication circuits that are each capable of interrogating approximately 33 readers. In this scenario, because the RFID tag sensors are passive, there are only 33 network components requiring batteries (the interrogation and communication circuit nodes) as opposed to 1033 components requiring batteries.

[0069] In some embodiments, the TDMA communication scheme used by the network nodes can implement a MAC layer using TDMA super frames. A TDMA superframe includes a series of time slots, and communication between two devices may occur during one time slot of fixed size 10ms. Superframes may be repeated continuously with a fixed repetition rate (link cycle). Devices can support multiple superframes corresponding to various sensor reporting frequencies, each with its own routing graph. Between consecutive transmission opportunities, channel hopping may be performed to provide frequency diversity. In various embodiments, the nodes can be implemented using WiHART motes compliant with the WirelessHART industrial wireless standard (IEC 62591), which is built on top of the IEEE 802.15.4 standard.

[0070] As noted above, there are various interference issues that can arise in an RFID environment. Embodiments of the systems and methods disclosed herein can be implemented to avoid one or more of the interference issues discussed above. As already noted, embodiments can be implemented using WiHART scheduling and routing, or other like scheduling and routing techniques. Accordingly, interference between field devices 342 can be avoided using WiHART or similar TDMA and frequency hopping schemes.

[0071] In further embodiments, interference between the WiHART and RFID subsystems can also be avoided. Figure 7 is a diagram illustrating an example timeline of subsystem performance in an example 2-tier wireless network in accordance with various embodiments of the systems and methods disclosed herein. As this example illustrates, the system can be designed such that WiHART network operations 710 occur in a different frequency range (2.45 GHz in this example) as compared to RFID operations 712 (860-960 MHz in this example). Operation at different frequency bands helps to reduce or eliminate interference between the WiHART and RFID subsystems.

[0072] Operation at different frequency bands may allow the WiHART and RFID subsystems to operate simultaneously. Therefore, in some embodiments the WiHART transmitters may transmit during RFID operations 712 as well as during WiHART operations 710 as illustrated by the example WiHART transmissions 740 in Figure 7. Other embodiments, however, may separate the operation of the WiHART and RFID subsystems temporally. In such embodiments, WiHART network operations 710 occur at a different time from RFID operation 712, and the illustrated example WiHART transmissions 740 during RFID operations would not occur.

[0073] The example of Figure 7 shows that RFID operations 712 can include interrogation of a plurality of RFID sensor tags in multiple clusters. Illustrated in this example are charge operation 722 in which the RFID sensor tag is charged, and selection, and inventory and access operation 724 during which the RFID sensor tag is selected, sensor operations are performed, and information is returned to the interrogating reader. Embodiments discussed in detail below can implement a novel time division multiplexing scheme that reduces reader-reader collisions. [0074] In a WirelessHART network, the total network throughput has an upper limit dictated by the available uplink receive links of the Access Point (AP). A link is a transmission opportunity (in a given time slot at a specific frequency) between neighboring devices. According to the IEC standard, the timeslot duration is fixed at 10ms. Therefore, there are a total of 100 available timeslots per second on the uplink to the AP. Because the AP can only listen to one of its neighbors at any given time, and because the standard allows for only 1 packet transaction within one timeslot, the total throughput ( W ) in the number of packets is:

W = 100 packet/sec

[0075] In some embodiments, only about 70% of the total links are dedicated for uplink transmissions. Therefore, the total throughput reduces to:

W = 70 packet/sec

[0076] In order to attain high reliability, the WirelessHART standard foresees at least a 3x provisioning (retransmission opportunities) in the allocated links i.e. for each packet transmission two more links are also allocated in case the first one is not successful. Considering this and also reserving some links for management, the total network throughput that can be handled by a single AP is:

W = 24 packet/sec [0077] This throughput can be shared among all devices in the network. For- example if there are 24 devices in the network they can all transmit at 1 packet/sec. Note that the transmission rate of all devices need not be the same.

[0078] Figure 8A is a diagram illustrating an example of a plurality of antennas configured to operate with a single field device in accordance with one embodiment of the systems and methods described herein. This example illustrates a field device 342 (e.g., communication and interrogation circuit 400) including 3 antennas for RFID interrogation operations. As this illustrates, each antenna is directed to interrogate a group of RFID sensor tags 344 (only two numbered for clarity of the illustration) monitoring the condition of a pipeline 810. As this example illustrates, multiple antennas pointed in different directions can be used to extend the reader's field of view. Multiple antennas can allow a system configuration in which the beam width of each antenna can be tightened to reach tags at a greater distance or non-line-of-site points. Reducing the beam width also saves power by having fewer tags contending for channel access. Flowever, this might not be sufficient to compensate for the increased power consumption due to the employment of multiple antennas.

[0079] Figure 8B is a diagram illustrating another example of a plurality of antennas configured to operate with a single field device in accordance with one embodiment of the systems and methods described herein. This example illustrates how multiple antennas might be used in different locations to handle non-line-of-site sensing operations. In this example, a separate antenna 850 is included to communicate with RFID sensor tags 344B that may be located in a position such that these RFID sensor tags 344B cannot be interrogated by the other antennas of field device 342. For example, these RFID sensor tags 344B might be located on the back side of the metallic pipeline (in the example of Figure 8B) or otherwise located out of range or in a blocked location.

[0080] As described above, embodiments can be implemented in which the number of battery-operated components can be reduced by using passive RFID sensor tags in combination with RFID readers configured to read multiple sensor tags. In some embodiments, sensor tags can be of a sufficiently small form factor such that they can be deployed in out of reach places with tight spaces and cause minimal mechanical interference. Because distance between RFID readers and sensor tags can be as much as a few meters, there can be flexibility on deployment of the readers to provide benefit to exploiting energy harvestable locations. Also, the wireless interface between the sensor tags and the RFID readers can make deployment of the sensors more convenient.

[0081] As noted above, various embodiments can include a Time Division Multiplexing (TDM) scheduling scheme to reduce or eliminate reader-reader collisions. More particularly, embodiments can be implemented such that the interfering readers do not operate at the same time. Some embodiments can be implemented to reduce or minimize the total tag read time to allow more flexibility in the TDM scheme. [0082] Some applications may require consideration of various constraints that can affect the tag read time. These can include the requirement for low power consumption and the relative difference in priority between multiple readers. To address the low power consumption constraint, embodiments can be implemented to allow the readers to be inactive for as long as possible. For example, the scheduling can be implemented such that each reader inventories and reads all tags within its read range in a single session. This can be used to avoid operations in which a reader reads part of its population of tags, sleeps, reactivates later and reads the rest of its tags. This can be beneficial in embodiments that require a charging face during which the reader charges the tag followed by an inventory and access phase. Where the reader is required to use multiple sessions to read all of the designated tags, it may be required to perform the charging task multiple times in the event the tag does not retain sufficient charge between sessions. Additionally, transitioning in and out of the active state multiple times requires the circuitry to be used to be turned on and off each time, and may also require additional overhead to reestablish the communications channel with the tags. For one or more of these reasons, some embodiments can be implemented to avoid reading the identified tags across multiple sessions.

[0083] The other constraint, as mentioned, relates to the difference in priority between the readers. A number of different priority schemes could be established. One example priority scheme is a location-based scheme in which different monitored locations have different priorities. For example, readers interrogating sensors monitoring critical systems might have a higher priority than those interrogating sensors monitoring noncritical systems or systems with built-in redundancies. Another example priority scheme might be an emergency-based priority scheme. In some embodiments, readers can take on increase priority depending on their readings. For example, where a fault or malfunction in the structure occurs, the readers can be assigned a higher priority.

[0084] Table 1 describes reader parameter notation that is used in this disclosure. The systems and methods disclosed herein are not dependent upon the particular nomenclature adopted. Indeed, in other embodiments and applications, other nomenclature can be adopted and used to denote the noted parameters.

Table 1: RFID Reader Parameters

[0085] Figure 9 is a diagram illustrating a simple example of reader-reader collisions that can occur when the distance between two readers is less than the sum of the interference range of one reader and the read range of the other reader. This example illustrates the interference range interference range (d_int) of a first reader

910 and the read range (d_R) of a second reader 912. The interference may generally be greater than the read range, and the interference range typically overlaps with the maximum read range, defined by a circle of radius d_Rmax in the shaded area. It can be assumed that in many systems > 1 due to the signal-to-interference d_R

ratio of a receiver typically being required to be larger than 1. However, the actual relationship between the interference range in the read range may differ based on any of a number of factors, and these ranges may not be circular as illustrated in this simplified example.

[0086] To determine reader coordination, and interference graph can be created based on the anticipated interference between readers in the system. In some embodiments, the assumption that d;_nt/d_R=C >l, where Cm is a constant, may be used when creating the interference graph. In other embodiments, other assumptions can be used or actual interference measurements can be used to create the graph.

[0087] Figure 10 illustrates an example interference graph in accordance with one embodiment of the systems and methods described herein. In this graph, G(V, E), the set of vertices V = {v , ... , v_n] denotes the RFID readers 1010. This example includes ten readers 1010, within an interference range of one or more of the other readers 1010 in as illustrated by the depicted edges. Furthermore, (Vi, V_j) E E only if d_V(Vj < d_{int max} + d_{R 7nax}. Also, associated with each vertex (i.e., with each reader 1010) is a weight W_a = T(J_a, R_a). This weight denotes the expected time it takes for the reader 1010 to complete reading all tags in its read range. [0088] As noted above, in some embodiments the TDM scheduling scheme is crafted to minimize the total time required to read the tags of all the readers. In further embodiments, this may be subject to various constraints. These constraints can include requiring each reader to read all tags in a single session, and scheduling higher priority readers before lower priority readers. In some applications, a scheduling solution is provided that determines a locally optimal choice at each stage in the process, and uses this locally optimal choice.

[0089] Figure 11 illustrates an example scheduling solution in accordance with one embodiment of the systems and methods described herein. This example scheduling solution is described in terms of the network example in Figure 10. With reference now to Figures 10 and 11, at operation 1142 a scheduling circuit prioritizes the readers (e.g. RFID readers 410 within interrogation and communication circuits 400) into groups of one or more readers at a plurality of priority levels. For example, consider a scenario in which the nodes 1010 belong to the priority classes: V_P={1,8,9} and {2,7,3} and {6,4,5,10}, based on decreasing order of priority. The scheduling circuit can be implemented in some examples as part of the controller circuit 411 in the nodes 1010, or it may include other circuitry within the nodes 1010. In other examples, the scheduling circuit can be included in a master node that is responsible for scheduling a plurality of network nodes 1010. With the priorities determined, the scheduling circuit selects the highest priority readers set as the first set for scheduling, and schedules the reader set as illustrated at operations 1144, 1146 and

1148 [0090] At operation 1144, the scheduling circuit identifies a reader within the first reader set that has the longest read time as compared to the other readers. That is, the scheduling circuit identifies the reader that will take the longest time to read all of the RFID sensor tags that it is responsible for reading. In some embodiments the scheduling circuit considers not only the respective read times (which may be denoted by the weight of each vertex, W_v), but also considers the number of neighboring RFID readers, which may be indicated by the degree of the vertex, D_v. As a further example, the vertex maximizing the ratio

may be

chosen. At operation 1146, the scheduling circuit schedules determines whether any other readers in the set can complete the read operation before the reader in the set identified at operation 1144. If so, these additional readers are scheduled. Once these readers are scheduled, they are removed from the scheduling algorithm. This is illustrated at operation 1148.

[0091] At operation 1150, the scheduling circuit determines whether all network nodes have been scheduled. If so, the scheduling operation is completed as illustrated at block 1153. If there are remaining network nodes that have not been scheduled, the scheduling circuit schedules the next reader set based on the determined priorities. In so doing, the scheduling circuit schedules this reader set following the same process used to schedule the highest priority reader set from the remaining sets as illustrated by flow line 1160.

[0092] Accordingly, the scheduling circuit implements a process that forms a maximal independent set by choosing the minimum degree vertex in the graph and removing its neighbors. This can be implemented to achieve a solution with an approximation ratio of (D+2)/3 against an optimal solution on graphs with maximum degree D. This outputs an independent set of size at least , where D_v is the

degree of vertex v. Likewise, the size of the maximum independent set in G is given by a(G) , where a(G ) is the size of a maximum independent set in G. A

maximum Weighted MIS is an independent set with maximum total weight. In some embodiments, the vertex maximizing

may be chosen to be in the Weighted M IS

(WMIS), and all its neighbors removed, and the process continued until no more vertex can be added to the WMIS. This scheme achieves WMIS of weight at least v^e _V ^~D ^~+i Furthermore this scheme may achieve performance with an approximation ratio of l/D as compared to an optimal solution.

[0093] Note that the weight of a vertex in the interference graph represents the time required to read all of the tags in the read range of the corresponding reader i.e. W_v = T(J_V, R_V). Embodiments may be implemented to extend the WMIS problem into the Priority-Aware WM IS (PAWM IS) which is used in further algorithms devised to solve the sum read time minimization problem.

[0094] An example scheduling process that the scheduling circuit may be implement to carry out is illustrated in Table 2. Figure 12 il lustrates an example process for scheduling in accordance with another embodiment of the systems and methods described herein. Table 2: Master RFID Scheduling Algorithm

[0095] Consistent with the above description, this example defines an interference graph, G(V,E), with a set of vertices V={vi,...,v_n} (e.g., denoting RFID readers 1010). In this construct, each v E V has a degree D_v and a weight W_v. In accordance with the example operation described with reference to Figure 11, the vertices can be grouped based on priority in which V_p is the subset of V including the highest priority nodes.

[0096] To begin, at operation 1212 the procedure is initialized such that there are no scheduled readers in the set. Then, at operation 1214 the vertex maximizing

may be chosen to be in the Weighted MIS (WMIS) as the root node.

An example algorithm to select the next Priority-Aware Maximum Weighted node is expressed as r = argmax-^-, and outlined in Table 3 as Priority-Aware Maximum

VEV_p ^{Dv + 1}

Weighted Node (PAMWN) function. This example determines and outputs the vertex, r, with the highest priority level and the maximum weighted value among the nodes with the same priority level. As illustrated in Table 3, the function compares the values for nodes at a given priority level and selects the node with the largest at the highest priority level.

Table 3. Function to select next PAMWN

[0097] At operation 1216, a Priority-Aware Maximum Weighted Independent

Set (PAMWIS) scheduler may be called to determine the maximum total weight of a set of noninterfering readers that can be active simultaneously with the chosen root node. Table 4 outlines an example of a PAMWIS scheduler according to one embodiment.

Table 4: PAMWIS Scheduling Algorithm

[0098] Figure 13 illustrates an example of the scheduling algorithm in accordance with one embodiment of the systems and methods described herein. This process is one way of carrying out operation 1216 in which the noninterfering- node weights are evaluated to determine readers that can be active with the root node simultaneously. With reference now to Figure 13, at operation 1312, the example PAMWIS Scheduling Algorithm compares the read times of all non interfering nodes with the root node. At operation 1314 it eliminates all nodes that have a read time larger than the root node. Then, at operation 1316 it returns the set of nodes whose read times are less than the read time of the root node.

[0099] Figure 14 illustrates this embodiment of the scheduling process in terms of the example network of Figure 10. Part 1410 illustrates the network, with vertex "1” as root, and S' = {2,3} as output of the PAMWIS algorithm. These are illustrates as 'white' nodes in the network 1410. The weights of the vertices show the read duration of the corrsponding reader.

[00100] 1420 illustrates an example timeline of nodes in S'. As this example illustrates, nodes {2,3} finish reading before node 1 does. Accordingly, the system may be able to schedule more readers while node 1 is still in operation, as these are noninterfering nodes. In some embodiments, nodes can be scheduled immediately after reader 2 finishes, while in other embodiments, the scheduler waits until reader 3 finishes before scheduling more nodes. In further embodiments, both options may be viable.

Table 5: Tree Scheduler Algorithm

_ _

[00101] Returning now to figure 12, at operation 1218, a tree scheduler algorithm may be implemented to make the determination as to how the nodes may be scheduled. An example of a scheduling tree in accordance with various embodiments is set forth in Table 5. Figure 15 illustrates an example process for scheduling in accordance with one embodiment of the systems and methods described herein. Figures 16 and 17 illustrate additional examples of the sample network shown in figure 10 with an illustration of vertices that are not already scheduled and that are not a neighbor of a currently active node, and example timelines according to the scheduling tree. Figure 18 illustrates an example of a partial scheduling tree.

[00102] As these examples illustrate, in a scheduling tree T(H, L ) each vertex i G H, denotes the corresponding reader completing a read session. The weight of a vertex, however, has a different meaning from the meaning used in an interference graph. The weight rU_j is the sum weight of the vertices in the achievable PAMWIS after node i has finished reading. In fact, this is the maximum total read time that can be added to the schedule after node i is done reading. Further, each parent node may have a number of child nodes equal to the size of the PAMWIS that can be scheduled after it is done reading plus any other active node other than the root node.

[00103] With reference now to figures 15, 16, 17 and 18, at operation 1512, the root of the scheduling tree, T, r is initialized with w_Gi = 0. Then, at operation 1514, ||S' || vertices are added as children of r_t. In the illustrated example, two nodes are added as its children. Each of the added children point to the read end time of the respective RFID reader. This operation can be seen in the example algorithm at lines 1 and 2 of Table 5.

[00104] Note that the tree vertices in Figure 18 are marked with their weights, w , where the subscript i, denotes the reader index. At operation 1516, the "while" loop in the scheduler iteratively assigns weights to leaf nodes. This operation can be seen in the example algorithm at line 3 of Table 5. At operations 1518 and 1520, the path with the largest sum weight of the vertices in tree, T, is computed and the path is traversed to return children of all vertices in the path along with their start time.

[00105] With continued reference to Figures 16, 17 and 18, an example of the weight assignment is now described in terms of the prior example. Because node 2 is not yet assigned a weight, graph Q(M, N ) is constructed. In the example embodiment, the graph is constructed in accordance with the process set forth in line 3a of Table 5. The resulting graph is shown as 1620. For this case l₂ =

W₂, and hence the PAMWIS function call in line 3c returns nodes 4, 5 which are added as child nodes of node 2. Node 3 is also added because it is still active when node 2 is done reading. Also, w₂ is set to be the total weight of the nodes in the PAMWIS which is W₄ + W₅ in this case. With reference to Figure 17, the same process may be carried out to add child nodes 4, 6, and 7 as child nodes of node 3. An example of this is illustrated at 1720.

[00106] In the illustrated example, the scheduling tree in figure 18 has two distinct vertices with weights w₃ and w₃. Both vertices denote reader 3 completing its read tasks, yet the total weight of readers that can be scheduled in this example after reader 3 is finished, is different (denoted by different values w₃ and w₃).

[00107] Figure 19 illustrates a completed scheduling tree in accordance with the example described above with reference to Figures 16, 17 and 18. As illustrated by 1910, the path with the largest sum weight of vertices is denoted by the added arrow. The timeline of readers corresponding to this path is shown at 1915. In some embodiments, the timeline may be derived from the tree path by traversing the path from the root to the leaf, and upon encountering a node, for example W₃, add all of its child nodes, for example W₄ 14 1 4 and W₁₀, that are not already scheduled to the timeline right after the end of the W₃ read period.

[00108] Returning again to figure 12, at operation 1220, with the scheduling algorithm complete, the master scheduler adds the scheduled nodes to the set of schedule nodes, and remove seeds from the set of vertices of the original interference graph G. The process iterates further until all nodes are scheduled. Figure 20 illustrates an example of a final scheduling timeline of all the readers in the foregoing example along with the corresponding scheduling diagram.

[00109] As described above, a criterion used for picking the valid nodes is shown in Table 2 and Table 4 as:

[00110] This criterion means that for two nodes with the same read time W_v, the node with smaller degree D_v , which means fewer neighboring nodes, should be selected. In another embodiment, an alternative criterion may be defined as:

[00111] Here, the alternative criterion means that for two nodes with the same read time W_v, the node with higher degree D_v , which means a greater number of neighboring nodes, should be selected.

[00112] Moreover, a more general form of the criterion could be used: r = argmax, ,..- [W“ (D_v + 1 ] (3)

[00113] where a and 6 are two weighting factors for the read time W_v and degree D_v, respectively. If a = 1 and 6 = -1, then the criterion (3) is the same as criterion (1); If a = 1 and 6 = 1, then the criterion (3) is the same as criterion (2). In further embodiments, to omit the effect from either W_v or D_v, the corresponding a or 6 can be set = 0. To reduce (rather than completely eliminate) the effect from the factors W_v or D_v, weighting factors of a and b between 0 to 1 can be selected.

[00114] The algorithms described in various embodiments provide a combination of the local optimization and greedy searching. In other embodiments, another approach to solve the PPAS problem uses a global greedy algorithm as an alternative solution. Table 6 illustrates an example implementation of a global greedy algorithm in accordance with various embodiments.

Table 6. Algorithm for Master RFID Scheduling

[00115] Where the C_v in line 1 is the criterion for choosing the next node. The detailed form of the C_v could be any of the form expressed in Equations (D-(3).

[00116] This algorithm is now described using the same example interference graph as used for the above-described examples as shown in Figure 10. Figure 21 illustrates an example process for scheduling in accordance with one embodiment of the systems and methods described herein. At operation 2112, the system is initialized to V = V, A = 0.

[00117] At operation 2114, the system selects the vertex v from V with the maximum weight. In the example embodiment of Table 6, this is: v = argmax^_g^C,,). At operation 2116, the system adds the selected v to the set S_act that consists of all the active nodes. Then, the system removes v and all its neighbors from V . This is repeated until there are no more vertices in V as illustrated at operation 2118. This yields the same results as shown in Figure 14, with nodes 1, 2 and 3 as the starting nodes. As illustrated by operation 2120, the system repeats operations 2114, 2116 and 2118 until no more vertices remain in V.

[00118] At operation 2122, the system searches from the set of active nodes to find the next ending vertex. In the example of Table 6, this is done by identifying the vertex that satisfies v = argmin_t,_e5_act(t_end) . This vertex, v, is removed from the set of active nodes, S_act and added into the scheduled vertices A. All neighboring vertices of v are added into V' and the removed from V' if it is the neighbor of any vertex in the set of active nodes, S_act . This is shown at operations 2124 and 2126. The process for master RFID scheduling continues until all vertices are scheduled as illustrated at operations 2128 and 2130.

[00119] Figure 22 illustrates the scheduling of the three nodes according to this example and their respective read times. Because node 2 has the shortest reader time among these three nodes, it will be the first node that finishes among all three nodes among the active nodes set. Therefore, at the end of W₂ the Node 2 is removed from the active set and added into the scheduled Set, , *, = {1, 3} ,A = { 2} .

[00120] Next, the system selects additional node(s) that could be scheduled. Figure 23 illustrates the scheduling of the next two nodes according to this example. Based on the interference graph illustrated in Figure 22, nodes 4 and 5 are chosen and may be added into the active node Set as S_act = {1, 3, 4, 5} shown in Figure 23. These may be selected because these nodes don't interfere with the other two active nodes, 1 and 3. In some embodiments, the selection of any active nodes is final and won't be changed.

[00121] In this example, The next node that will finish first among all the active nodes is Node 4. Flowever, after Node 4, no free node can be scheduled because nodes 1 and 3 have not finished and the remaining unscheduled nodes will interfere. The next node that will finish after Node 4 is Node 3, and only Node 10 can be activated after Node 3. Figure 24 illustrates the scheduling of the next two nodes according to this example. At this moment, the active nodes set is ^ = {1, 5, 10} and the scheduled node set is A = {2, 4, 3} .

[00122] As noted, the scheduling process will be continued following the procedure described in Table 6 for all the nodes until all nodes are activated and scheduled. The final scheduling result with the global greedy algorithm in this example is shown in Figure 25.

[00123] The total reading time using the global greedy algorithm is T_greedy = W₂ + W₅ + W₆ + W_{9 )} while the total time using the combined local optimization and greedy searching in the example culminating in Figure 20 is Ί_combine = W_X + W₉. Because the system in this example did not specify the exact value of the reading time for each node, the inequality relation between T _d and T_combme is unknown, which means that T_greedy could be either smaller or greater than T_combme.

In other words, either of the algorithms may yield better result than the other in certain cases depending on the input interference graph and reader time of each nodes.

[00124] In some embodiments, a central scheduling unit may be used that has access to the expected read time of all readers, performs the task of scheduling (according to the algorithms provided) beforehand and sends the schedules to all the readers through a wired or wireless backbone connection. The readers may then each start reading their tags based on the received schedules. As mentioned before, a safety margin may be incorporated into the allocated scheduling intervals, in case readers are not able to finish reading before the expected deadline. The amount of safety margin can be determined based on a target error rate. Note that an added safety margin generally leads to some bandwidth inefficiency in the system.

[00125] To combat this bandwidth inefficiency problem, some embodiments may use an "online" scheduling process. Note that, as mentioned before, the actual read time of readers could be shorter or longer than the expected value. With this in mind, the centralized scheduler can Initially, compute the "scheduling forest" of all readers according to the master scheduling algorithm and dispatch the computed schedule to all readers.

[00126] For the chosen schedule, each time a reader completes its read session (which could happen before or after its predetermined expected completion time), the system may re-compute the "scheduling forest" and find the new solution based on the realistic completion time. Then dispatch the new schedule to all readers. In some embodiments, this rescheduling may be performed every time a reader completes its task.

[00127] Note that in this scenario, embodiments may be implemented such that readers typically will not commence their read operation before being instructed by the scheduler (whether by telling them to stick to the old schedule or giving them a new start time).

[00128] Also, note that a low latency network connection is preferably available between the readers and the scheduler to allow reporting and scheduling information to be propagated fast between the readers and the scheduler and not be a cause of bandwidth inefficiency itself.

[00129] In various embodiments, during each read session, no frequency coordination is performed across readers. Instead the system focuses on reducing the read time of tags by concurrently scheduling those readers that don't cause reader-reader collisions. Accordingly, the system can be configured to conform to the built-in pseudo random hopping patterns of readers even where such patterns are not coordinated among readers.

[00130] As used herein, a circuit might be implemented utilizing any form of hardware, software, or a combination thereof. For example, one or more processors, controllers, ASICs, PLAs, PALs, CPLDs, FPGAs, logical components, software routines or other mechanisms might be implemented to make up a circuit. In implementation, the various circuits described herein might be implemented as discrete circuits or the functions and features described can be shared in part or in total among one or more circuits. In other words, as would be apparent to one of ordinary skill in the art after reading this description, the various features and functionality described herein may be implemented in any given application and can be implemented in one or more separate or shared circuits in various combinations and permutations. Even though various features or elements of functionality may be individually described or claimed as separate circuits, one of ordinary skill in the art will understand that these features and functionality can be shared among one or more common circuits, and such description shall not require or imply that separate circuits are required to implement such features or functionality.

[00131] Where circuits are implemented in whole or in part using software, in one embodiment, these software elements can be implemented to operate with a computing or processing system capable of carrying out the functionality described with respect thereto. One such example computing system is shown in Figure 26. Various embodiments are described in terms of this example computing system 2600. After reading this description, it will become apparent to a person skilled in the relevant art how to implement the technology using other computing systems or architectures.

[00132] Referring now to Figure 26, computing system 2600 may represent, for example, computing or processing capabilities found within desktop, laptop and notebook computers; hand-held computing devices (smart phones, cell phones, palmtops, tablets, etc.); mainframes, supercomputers, workstations or servers; or any other type of special-purpose or general-purpose computing devices as may be desirable or appropriate for a given application or environment. Computing system 2600 might also represent computing capabilities embedded within or otherwise available to a given device. For example, a computing system might be found in other electronic devices such as, for example, digital cameras, navigation systems, cellular telephones, portable computing devices, modems, routers, WAPs, terminals and other electronic devices that might include some form of processing capability. [00133] Computing system 2600 might include, for example, one or more processors, controllers, control modules, or other processing devices, such as a processor 2604. Processor 2604 might be implemented using a general-purpose or special-purpose processing engine such as, for example, a microprocessor (whether single-, dual- or multi-core processor), signal processor, graphics processor (e.g., GPU) controller, or other control logic. In the illustrated example, processor 2604 is connected to a bus 2602, although any communication medium can be used to facilitate interaction with other components of computing system 2600 or to communicate externally.

[00134] Computing system 2600 might also include one or more memory modules, simply referred to herein as main memory 2608. For example, in some embodiments random access memory (RAM) or other dynamic memory, might be used for storing information and instructions to be executed by processor 2604. Main memory 2608 might also be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 2604. Computing system 2600 might likewise include a read only memory ("ROM") or other static storage device coupled to bus 2602 for storing static information and instructions for processor 2604.

[00135] The computing system 2600 might also include one or more various forms of information storage mechanism 2610, which might include, for example, a media drive 2612 and a storage unit interface 2620. The media drive

2612 might include a drive or other mechanism to support fixed or removable storage media 2614. For example, a hard disk drive, a floppy disk drive, a magnetic tape drive, an optical disk drive, a CD or DVD drive (R or RW), a flash drive, or other removable or fixed media drive might be provided. Accordingly, storage media 2614 might include, for example, a hard disk, a floppy disk, magnetic tape, cartridge, optical disk, a CD or DVD, or other fixed or removable medium that is read by, written to or accessed by media drive 2612. As these examples illustrate, the storage media 2614 can include a computer usable storage medium having stored therein computer software or data.

[00136] In alternative embodiments, information storage mechanism 2610 might include other similar instrumentalities for allowing computer programs or other instructions or data to be loaded into computing system 2600. Such instrumentalities might include, for example, a fixed or removable storage unit 2622 and an interface 2620. Examples of such storage units 2622 and interfaces 2620 can include a program cartridge and cartridge interface, a removable memory (for example, a flash memory or other removable memory module) and memory slot, a flash drive and associated slot (for example, a USB drive), a PCMCIA slot and card, and other fixed or removable storage units 2622 and interfaces 2620 that allow software and data to be transferred from the storage unit 2622 to computing system 2600.

[00137] Computing system 2600 might also include a communications interface 2624. Communications interface 2624 might be used to allow software and data to be transferred between computing system 2600 and external devices. Examples of communications interface 2624 might include a modem or softmodem, a network interface (such as an Ethernet, network interface card, WiMedia, IEEE 802.XX, Bluetooth^® or other interface), a communications port (such as for example, a USB port, IR port, RS232 port, or other port), or other communications interface. Software and data transferred via communications interface 2624 might typically be carried on signals, which can be electronic, electromagnetic (which includes optical) or other signals capable of being exchanged by a given communications interface 2624. These signals might be provided to communications interface 2624 via a channel 2628. This channel 2628 might carry signals and might be implemented using a wired or wireless communication medium. Some examples of a channel might include a phone line, a cellular link, an RF link, an optical link, a network interface, a local or wide area network, and other wired or wireless communications channels.

[00138] In this document, the terms "computer program medium" and "computer usable medium" are used to generally refer to media such as, for example, memory 2608, storage unit 2620, media 2614, and channel 2628. These and other various forms of computer program media or computer usable media may be involved in carrying one or more sequences of one or more instructions to a processing device for execution. Such instructions embodied on the medium, are generally referred to as "computer program code" or a "computer program product"

(which may be grouped in the form of computer programs or other groupings). When executed, such instructions might enable the computing system 2600 to perform features or functions of the disclosed technology as discussed herein.

[00139] While various embodiments of the disclosed technology have been described above, it should be understood that they have been presented by way of example only, and not of limitation. Likewise, the various diagrams may depict an example architectural or other configuration for the disclosed technology, which is done to aid in understanding the features and functionality that can be included in the disclosed technology. The disclosed technology is not restricted to the illustrated example architectures or configurations, but the desired features can be implemented using a variety of alternative architectures and configurations. Indeed, it will be apparent to one of skill in the art how alternative functional, logical or physical partitioning and configurations can be implemented to implement the desired features of the technology disclosed herein. Also, a multitude of different constituent module names other than those depicted herein can be applied to the various partitions. Additionally, with regard to flow diagrams, operational descriptions and method claims, the order in which the steps are presented herein shall not mandate that various embodiments be implemented to perform the recited functionality in the same order unless the context dictates otherwise.

[00140] Although the disclosed technology is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations, to one or more of the other embodiments of the disclosed technology, whether or not such embodiments are described and whether or not such features are presented as being a part of a described embodiment. Thus, the breadth and scope of the technology disclosed herein should not be limited by any of the above-described exemplary embodiments.

[00141] Terms and phrases used in this document, and variations thereof, unless otherwise expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term "including" should be read as meaning "including, without limitation" or the like; the term "example" is used to provide exemplary instances of the item in discussion, not an exhaustive or limiting list thereof; the terms "a" or "an" should be read as meaning "at least one," "one or more" or the like; and adjectives such as "conventional," "traditional," "normal," "standard," "known" and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future. [00142] The presence of broadening words and phrases such as "one or more/' "at least/' "but not limited to" or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term "module" does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations.

[00143] Additionally, the various embodiments set forth herein are described in terms of exemplary block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.

Claims

Claims What is claimed is:

1. A method for scheduling simultaneous communications among a plurality of transceivers, comprising:

identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another;

selecting a first set of transceivers from among the one or more groups of transceivers identified as not interfering with one another, and scheduling all transceivers of the first set to initiate communication at the same time;

determining a first transceiver of the first set of transceivers that will require the greatest amount of time to complete its communication relative to the other transceiver or transceivers in the first set of transceivers;

selecting additional one or more transceivers from the one or more sets of transceivers identified as not interfering with one another, that can complete their communication operations before the first transceiver will finish its communication operations; and

scheduling one or more of the additional transceivers to begin

communicating before the first transceiver finishes its communication operations.

2. The method of Claim 1, wherein identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another comprises constructing an interference graph that comprises, a plurality of vertices, wherein a vertex represents a respective transceiver; edges between vertices, wherein an edge between two vertices indicates that the transceivers represented by those two vertices are within an interference region with respect to each other; and weights for the vertices, wherein a weight for a vertex represents a required communication time for the transceiver represented by that vertex.

3. The method of Claim 2, wherein selecting a first set of transceivers from among the one or more groups of transceivers comprises identifying a priority-aware maximum weighted independent set in the interference graph by selecting a root transceiver based on weight and degree values of the vertices in the interference graph.

4. The method of claim 3, wherein a vertex in the interference graph includes a degree D_v and a weight W_v, and wherein identifying a priority-aware maximum weighted independent set comprises determining a root transceiver by determining a vertex in the interference graph maximizing

, and determining a set of one or

more non-interfering transceivers that can communicate simultaneously with the root transceiver.

5. The method of claim 4, wherein identifying a priority-aware maximum weighted independent set further comprises determining a maximum total weight of the set of noninterfering transceivers.

6. The method of claim 5, wherein determining a maximum total weight of the set of noninterfering transceivers comprises comparing the communication durations of all noninterfering transceivers to the communication duration for the root transceiver and selecting all noninterfering transceivers whose read times are less than the read time of the root transceiver.

7. The method of claim 3, wherein scheduling one or more of the additional transceivers comprises constructing a tree graph comprising a plurality of leaf nodes; iteratively assigning second weights to the leaf nodes; adding new children to the tree graph until all leaf nodes are assigned a second weight.

8. The method of claim 7 , wherein constructing a tree graph comprises, adding a vertex as a leaf node of the tree graph if the transceiver associated with that vertex is not already scheduled and is not a neighbor of a currently active transceiver, wherein the added vertex is added as a child leaf node of a leaf node representing a vertex of a previously active transceiver with which the added vertex is directly connected by an edge on the interference graph.

9. The method of claim 7, further comprising determining a path in the tree graph with a largest sum of second weights and selecting children of all leaf nodes in the path.

10. The method of claim 3, wherein a vertex in the interference graph includes a degree D_v and a weight W_v, and wherein identifying a priority-aware maximum weighted independent set comprises determining a root transceiver by determining a vertex in the interference graph maximizing W_V(D_V + 1), and determining a set of one or more non-interfering transceivers that can communicate simultaneously with the root transceiver.

11. The method of claim 3, wherein a vertex in the interference graph includes a degree D_v and a weight W_v, and wherein identifying a priority-aware maximum weighted independent set comprises determining a root transceiver by determining a vertex in the interference graph maximizing W_v ^a (D_v + 1)^ , and determining a set of one or more non-interfering transceivers that can communicate simultaneously with the root transceiver, where a and b are weighting factors between zero and one.

12. The method of claim 3, further comprising removing from the interference graph the selected vertex and each vertex connected directly to the selected vertex by an edge.

13. The method of Claim 2, wherein two transceivers are within an interference region with respect to each other if the distance between the two transceivers is less than the sum of an interference range of the first transceiver of the two transceivers and a read range of the second transceiver of the two transceivers.

14. The method of Claim 1, wherein selecting a first set of transceivers from among the one or more groups of transceivers comprises determining a priority- aware maximum weighted set of transceivers using weights and degrees assigned to the transmitters, wherein a weight represents a total communication time for the transceiver to which that weight is assigned and a degree represents a quantity of other transceivers within an interference region of the transceiver to which that degree is assigned.

15. The method of Claim 14, wherein determining a priority-aware maximum weighted set of transceivers using weights assigned to the transmitters comprises determining a first transceiver by maximizing at least one of

, W_V(D_V + 1), and

D_v+ 1

W (D_v + \)^b , and determining a set of one or more non-interfering transceivers that can communicate simultaneously with the first transceiver.

16. The method of Claim 1, wherein the interference region defines a region surrounding a transceiver within which transmissions by the transceiver will generate interference above a determined threshold to another transceiver within the defined region.

17. The method of Claim 1, wherein identifying one or more groups of transceivers of the plurality of transceivers that do not interfere with one another comprises for each transceiver in the plurality of transceivers, determining which other transceivers in the plurality of transceivers is within its interference region.

18. The method of Claim 2, wherein, selecting a first set of transceivers from among the one or more groups of transceivers identified as not interfering with one another comprises: selecting a transceiver represented by a vertex with a maximum weight v = argmax^_g^C,,); adding the selected transceiver to a set of active nodes; removing the selected transceiver and all transceivers within an interference region of the selected transceiver from further selection; and iteratively repeating the steps of selecting and adding transceivers for transceivers that have not been removed from further selection until all transceivers have been removed from further selection.

19. The method of Claim 18, wherein scheduling the one or more of the additional transceivers to begin communicating before the first transceiver finishes its communication operations, comprises identifying a second transceiver represented by a vertex with a minimum weight vertex: v = argmin_t;gSact(t_end), identifying all transceivers within an interference region of the second transceiver, and removing the second transceiver from consideration if the second transceiver is within an interference region of any transceiver in the set of active nodes.

20. A method for scheduling simultaneous communications among a plurality of RFID tag readers, the method comprising:

constructing an interference graph comprising a plurality of vertices selectively connected by edges, wherein the vertices each represent a corresponding RFID reader and an edge connecting two vertices represents to RFID readers within an interference range of each other; determining a priority-aware maximum weighted independent set of RFID readers by identifying a first RFID reader based on weight and degree values for the vertices in the interference graph, wherein a weight for a vertex represents a required read time for the RFID reader represented by that vertex and a degree for a vertex represents a number of other RFID readers within the interference region with respect to the RFID reader represented by that vertex;

removing the vertex representing the identified RFID reader and all of its neighbor vertices from the interference graph and repeating the determining step until no further vertices can be chosen.