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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [ Sample MAC Requirements for Angle of Arrival Based Ranging ] Date Submitted: [ 29 Sept, 2004 ] Source: [ Marilynn P. Green ] Company [ NOKIA ] Address []

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Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Sample MAC Requirements for Angle of Arrival Based Ranging] Date Submitted: [ 29 Sept, 2004] Source: [Marilynn P. Green] Company [NOKIA] Address [] Voice:[], FAX: [], E-Mail:[Marilynn.Green@nokia.com] Re: [] Abstract: [] Purpose: [Presented as a basis for discussion to the IEEE 802.15 TG4a on September 30, 2004.] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15. Dr. Marilynn P. Green, Nokia Research Center

  2. Sample MAC Requirements for Angle of Arrival Based RangingPresented as a basis for discussion to the IEEE 802.15 TG4aonSeptember 30, 2004 by: Dr. Marilynn P. Green Nokia Research Center Dr. Marilynn P. Green, Nokia Research Center

  3. Outline of Presentation • AOA Basics: • AOA Modeling • AOA Determination • Error Sources • Basic Assumptions: • Device Capability • MAC Requirements • AOA Ranging and Sample MAC Scenarios: • AOA-Based Positioning with Passive Anchors • AOA-Based Triangulation with Active Anchors • Cooperative AOA-Based Positioning • Summary of MAC Resources and Other Considerations Dr. Marilynn P. Green, Nokia Research Center

  4. Z Y  Azimuth Array Elements R Range Source (x,y,z) Source Elevation R (x,y)  Range X Y  Array Elements Azimuth 2D model 3D model X AOA Basics • Angle of Arrival (AOA) is the direction to the source of an incoming wave field as measured by an array of antenna elements. • While the 3D model is exact, we often use the simpler 2D model when the source and antenna elements are co-planar ( = /2)  for this presentation, we assume 2D. • The local coordinate system of the receiving array may be arbitrarily oriented. True North Dr. Marilynn P. Green, Nokia Research Center

  5. AOA Modeling • The planar wave front models the incoming wave field in the far field. • The AOA may be determined by measuring the phase (time) difference of the wave front at different array elements. Wave front Array Response Vector of a Linear Equi-Spaced Array (“M” Antenna Elements)  2d Plane Wave Model To Source  Y  d 2dsin Reference Element X Parallel ray approximation y(t): M x 1 vector of reception s(t): Scalar source signal n(t): M x 1 noise vector c: Speed of Light Dr. Marilynn P. Green, Nokia Research Center

  6. AOA Determination – Basic Principle • A phased array antenna system consists of any number of antenna elements distributed in a particular geometrical pattern: • Antenna elements are typically spaced at regular intervals, such as a linear array, a planar array or a circular array. • Phased array receive antenna systems connect the antenna elements by an adder network: • Output of all the antenna elements is phase shifted using pre-determined phase shifts and added to locally maximize the receiver antenna pattern in the direction of the incoming source field(s). • Other optimization methods exist (MUSIC, ESPRIT, MLE,…) which give better resolution of closely spaced sources at the expense of computational complexity. Array Elements W2*() W1*() WM*() Variable phase shifters. …  Maximum at  = . Beamformer Dr. Marilynn P. Green, Nokia Research Center b()

  7. DEV-A DEV-B DEV-C Measurement error True line of position Line of sight measurements corrupted by measurement errors. Errors in AOA-Based Positioning • 2D positioning requires measurement of AOA by at least two antenna array systems. • If two devices (DEV-A and DEV-B) are each equipped with an antenna array, they can each determine the line of position along a third device (Dev-C) lies. • Dev-C ideally lies at the point of intersection of these two lines. • In practice, measurement errors due to imperfect array phase and gain calibration, mis-modeling of the mutual coupling between elements, and the error due to the presence of a strong indirect path, etc. may all corrupt the AOA measurements. • It is usually desirable to obtain multiple (> 2) lines of position to reduce the final positioning error. Dr. Marilynn P. Green, Nokia Research Center

  8. Outline of Presentation • AOA Basics • AOA Modeling • AOA Determination • Error Sources • Basic Assumptions • Device Capability • Basic MAC Requirements • AOA Ranging and Sample MAC Scenarios • AOA-Based Positioning with Passive Anchors • AOA-Based Triangulation with Active Anchors • Cooperative AOA-Based Positioning • Summary of MAC Resources and Other Considerations Dr. Marilynn P. Green, Nokia Research Center

  9. Basic Assumptions for Devices • Each device is equipped with an antenna array to measure AOA to neighbor nodes. • Each device has a main axis against which all angles are reported. • The axis of each node has an arbitrary but unknown orientation (heading) with respect to True North. • Some devices may have self positioning (eg. GPS capability) and compass capability. : Heading a Axis aligned with True North Dev-A Orientation with respect to True North ac ab DEV-B True North AOA: ab and ac Heading: a DEV-C Dr. Marilynn P. Green, Nokia Research Center

  10. Basic MAC Requirements (1/2) • MAC will have to adapt to different capabilities of the local and remote devices. • For example: One type of device may be fully equipped with GPS, a compass, and an antenna array to measure AOA…but another device may only be able to measure AOA. • In the simplest case, the PHY passes AOA results to the MAC and the MAC leaves more complex decisions to higher layers, like… • Need for repeated measurements. • Calculation of position. • MAC can reserve the time needed to make the AOA measurements. • Guaranteed time slots may be required for: • ARQ: Initiator’s AOA request. • ACK: Responder’s acknowledgement. • AM: Responder’s AOA measurement frame. • AMR: Responder’s AOA measurement report. Dr. Marilynn P. Green, Nokia Research Center

  11. Basic MAC Requirements (2/2) • MAC may need to have pre-programmed constants to correct for device-specific measurement errors (ex.: drift in the phase and gain of the antenna elements). • Power efficiency: • Power control to conserve battery power during device idle periods. • Reply frame with measurement results can be sent in the same frame as the request so that the device does not have to store measurement reports for a long time and can more quickly return to idle mode. • In some cases, ACK frame may be used to obtain the AOA measurements. • MAC measurement report to higher layers may contain: • AOA. • Success or failure. • Quality of measurement. • Number of measurement periods required to satisfy accuracy requirements can be decided by higher layers. Dr. Marilynn P. Green, Nokia Research Center

  12. Outline of Presentation • AOA Basics • AOA Modeling • AOA Determination • Error Sources • Basic Assumptions • Device capability • MAC requirements • AOA Ranging and Sample MAC Scenarios • AOA-Based Positioning with Passive Anchors • AOA-Based Triangulation with Active Anchors • Cooperative AOA-Based Positioning • Summary of MAC Resources and Other Considerations Dr. Marilynn P. Green, Nokia Research Center

  13. Y Y X X AOA-Based Positioning with Passive Anchors • Basic Idea: If two anchors in known positions and with known heading each measure the direction to Dev-C, then we can determine the position of Dev-C as the intersection of two lines of position. • Certain pathological geometries must be avoided  Dev-A Dev-C Dev-B Anchor B Known: (xB,yB, Heading) Anchor A Known: (xA,yA,Heading) BC AC Line of Position Line of Position DEV-C (xC,yC) Transmission by Dev-C. Dr. Marilynn P. Green, Nokia Research Center

  14. DEV-B (Anchor) DEV-A (Anchor) DEV-C Sample MAC Resources: AOA-Based Positioning with Passive Anchors • 4 frame AOA exchange transaction between DEV-C and each anchor device: • ARQ1: Initiator’s AOA request: • DEV-C transmits AOA measurement request to DEV-X (X = A, B). • ACK1: Responder’s acknowledgement. • AM1: Responder’s measurement frame. • DEV-C transmits to DEV-X. • AMR1: Responder’s measurement report: • DEV-X transmits report to DEV-C. • Two anchor devices  8 exchange transactions (minimum). • Initiator (DEV-C) calculates AOA from AMR1 and AMR2. • 3 frame AOA exchange between DEV-C and each anchor device: • ARQ1: Initiator’s AOA request: • DEV-C transmits AOA measurement request to DEV-X (X = A, B). • Measurement request itself can be used by anchor to measure AOA. • ACK1: Responder’s acknowledgement. • AMR1: Responder’s measurement report: • DEV-X transmits report to DEV-C. • Two anchor devices  6 exchange transactions (minimum). • Initiator (DEV-C) calculates AOA from AMR1 and AMR2. OR… OR…? Dr. Marilynn P. Green, Nokia Research Center

  15. Anchor A (xA,yA) Anchor A (xA,yA) Anchor B (xB,yB) Anchor B (xB,yB) Anchor A (xA,yA) Dev-D Dev-D Anchor C (xC,yC) Anchor C (xC,yC) AOA-Based Triangulation with Active Anchors • Basic Idea: If we know the positions of the vertices of a triangle and the angles at which an interior point sees those vertices, we can determine the position of the interior point (i.e. measure BDA, ADC and CDB to estimate (xD,yD)). Dev-D Anchor C (xC,yC) Anchor B (xB,yB) T0: Anchor A transmits. Dev-D measures DA. T1: Anchor B transmits. Dev-D measures DB. T2: Anchor C transmits. Dev-D measures DC. Dr. Marilynn P. Green, Nokia Research Center

  16. Anchor A (xA,yA) Dev-D Anchor C (xC,yC) Anchor B (xB,yB) Sample MAC Resources: AOA-Based Triangulation with Active Anchors • 3 frame AOA exchange transactions between DEV-A and each anchor device: • ARQ1: Initiator’s AOA request: • DEV-D transmits AOA measurement request to DEV-X. (X = A, B, C). • ACK1: Responder’s acknowledgement. • AM1: Initiator’s measurement frame : • DEV-X transmits to DEV-D and sends its coordinates + other info. • DEV-D makes its AOA measurement. • Three anchor devices  9 exchange transactions (minimum). • Initiator (DEV-D) calculates AOA. • 2 frame AOA exchange transactions between DEV-A and each anchor device: • ARQ1: Initiator’s AOA request: • DEV-D transmits AOA measurement request to DEV-X. (X = A, B, C). • ACK1: Responder’s acknowledgement. • DEV-X ACKs and also sends its coordinates + other info. • DEV-D uses this ACK as a measurement frame. • Three anchor devices  6 exchange transactions (minimum). • Initiator (DEV-D) calculates AOA. • Anchors are pre-assigned beacon slots during which they regularly transmit. • DEV-D wakes up to listen during the beacon periods to obtain three (or more) measurements. • This option minimizes the number of transactions required per location request BUT in the long run may waste some MAC resources. OR… OR… OR…? Dr. Marilynn P. Green, Nokia Research Center

  17. a b bc Dev-A ba ac ab DEV-B c cb ca True North DEV-C Cooperative AOA-Based Positioning • Basic Idea: If we can measure the interior angles of a triangle and we know its orientation, then we can determine the positions of its vertices. • A cooperative AOA ranging scheme can be used to measure the interior angles (BAC, ACB, CBA) of a triangle that is formed by Dev-A, Dev-B and Dev-C. Heading Known Export compass ability from Dev-B to Dev-A. Dr. Marilynn P. Green, Nokia Research Center

  18. Dev-A DEV-B True North DEV-C Sample MAC Resources: Cooperative AOA-Based Positioning • DEV-A initiates a cooperative positioning session between itself, DEV-B and DEV-C. • 3 frame AOA exchange transaction between DEV-A and each cooperating device. • ARQ1: Initiator’s AOA request • DEV-A transmits AOA measurement request to DEV-X. (X = B, C). • ACK1: Responder’s acknowledgement. • AM1: Initiator’s measurement frame • DEV-X transmits to DEV-A. • 3 frame AOA exchange between DEV-B and each cooperating device + 1 Measurement report: • ARQ1 • ACK1 • AM1 • AMR1 • DEV-B sends all measurement results to DEV-A. • 3 frame AOA exchange between DEV-C and each cooperating device + 1 Measurement report: • ARQ1 • ACK1 • AM1 • AMR1 • DEV-C sends all measurement results to DEV-A. • Three cooperating devices  Minimum of 20 frames. • If ACK can be used as for measurements  minimum of 14 frames required. • Initiator (DEV-C) calculates position based on the measurement reports. • Initiator sends final measurement report (with all coordinates calculated) to DEV-B and DEV-C. • Total of 21 frames used (or 15 if ACKs can be used for measurements). Alternative: ACK may be used as measurement frame as well.  2 frame AOA exchange transaction per cooperating device. Dr. Marilynn P. Green, Nokia Research Center

  19. Outline of Presentation • AOA Basics • AOA Modeling • AOA Determination • Error Sources • Basic Assumptions • Device Capabilities • MAC Requirements • AOA Ranging and Sample MAC Scenarios • AOA-Based Positioning with Passive Anchors • AOA-Based Triangulation with Active Anchors • Cooperative AOA-Based Positioning • Summary of MAC Resources and Other Considerations Dr. Marilynn P. Green, Nokia Research Center

  20. Summary of MAC Resources • PHY notifies the MAC protocol of the signal reception information (for example, AOA and reception power) and lets the higher layers make complex decisions. • MAC can assign time slots to be used for AOA measurements. • The total number of measurement periods required to make one position estimate depends on the accuracy requirement of the application. • At MAC sub-layer each device could maintain a cache table to keep the AOA, reception time, reception power, etc. of the last signal from each neighboring device. • In practice, each device may update the AOA and reception time that corresponds to a neighboring device even when overhearing any signal, regardless of whether the signal is sent to that device. • ACK frames may be used as measurement frames to conserve power and MAC resources. • Beacon frames may be used for broadcasts by anchor nodes: • May not be the most resourceful use of MAC resources if positioning is not in high demand. • Power conservative approach from the point of view of the device to be located. Dr. Marilynn P. Green, Nokia Research Center

  21. Other Considerations • AOA does not require the precise synchronization needed for TOA and TDOA methods. • Maturity of UWB antenna array technology must be taken into consideration. • The types of algorithms that might be required to give the desired accuracy might be too power consumptive. • There is a spatial sampling requirement that limits the inter-element spacing of antenna elements to be  ½ minimum source wavelength. • As long as the spatial sampling requirement is met, larger arrays generally provide better resolution of the source field  size and cost issues. • Special consideration is needed for multipath environments and for multi-source cases since sources can be closely spaced. • In non-line-of-sight environments, the measured AOA might not correspond to the direct path component of the incoming wave field  can lead to large positioning errors. • Array phase and gain calibration is important. • Whether or not the far-field planar wave approximation holds well will depend on the array aperture and the minimum wavelength of the source signal. • Near field: The phase (time) difference at different array elements becomes a non-linear function of the source’s position. Array aperture. Standard calculation for the far field distance  Minimum wavelength in source signal. Dr. Marilynn P. Green, Nokia Research Center

  22. Thank You! Dr. Marilynn P. Green, Nokia Research Center

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