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N-Way Time Transfer (NWTT) Method for Cooperative Ranging

Discusses the NWTT method for cooperative ranging between multiple devices in wireless networks, addressing MAC implications and scalability challenges for positioning applications.

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N-Way Time Transfer (NWTT) Method for Cooperative Ranging

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  1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [N-Way Time Transfer (“NWTT”) Method for Cooperative Ranging] Date Submitted: [12 August, 2005] Source: [Marilynn P. Green] Company [Nokia Research Center] Address [6000 Connection Drive; Irving, Texas 75039; USA] Voice:[(972) 894-5000], E-Mail: [marilynn.green@nokia.com] Re: [802.15.4a.] Abstract: [This material discusses the “NWTT” method for cooperative ranging between three devices, its implication to the MAC and addresses the scalability of the 3WTT method (482r0).] Purpose: [To promote discussion in 802.15.4a.] 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. Marilynn P. Green, PhD, Nokia Research Center

  2. 4 c·D24 2 c·D45 c·D34 c·D12 c·D23 c·D25 c·D14 1 5 c·D15 Initiator 3 c·D35 c·D13 Cooperative Ranging: “NWTT” Method • Future location-based services may require cooperation between multiple devices in order to map their relative positions within a local coordinate system. • Devices can coordinate in order to determine the range between every pair of devices in the positioning network. • The 3WTTmethod is a 2-phase approach that allows three devices to determine their relative distances (presented to the ranging subcommittee on 7/25/05 from #0482r0). • By combining Phase I and II of the 3WTT method, we can scale it to become an N-Way Time Transfer “NWTT” method which allows N devices within communication range to determine their pair-wise distances. • This presentation aims to discuss the MAC implications as well as the issues involved with scaling the 3WTT method to accommodate more devices. c: speed of light Dmn: prop. delay between Dev-m and Dev-n Marilynn P. Green, PhD, Nokia Research Center

  3. 3WTT Method (482r0) • The 3WTT method required two phases to determine the pair-wise distances between three cooperating devices. • During Phase I, Dev1 and Dev2 actively exchanged packets while Dev3 passively measured. • At the end of this phase, the distance between Dev1 and Dev2 (D12)could be calculated • Dev 3 took made passive time difference measurements • During Phase II, Dev1, Dev2 and Dev3 sequentially transmitted. • The result was that by combining measurements taken from Phase I and Phase II, there was sufficient information to determine D12, D23 and D13 • We expanded on the core ideas from the 3WTT method to accommodate N-way ranging by combining the operations from Phase I and Phase II into a single phase. Marilynn P. Green, PhD, Nokia Research Center

  4. Required Measurements for the NWTT Method • The NWTT method requires each device to transmit 2 + P packets, where P denotes the number of consecutive measurement periods. • Initially, 1 RRQ or ACK • P range measurement (“RM”) packets (one per measurement period) • 1 range measurement report (“RMR”) RRQ and ACKs Measurement Reporting (1) Measurement Period RMR_1 RRQ RM_1 RMR_2 ACK RM_2 RMR_3 ACK RM_3 . . . . . . . . . RMR_N ACK RM_N Dev1, Initiator Dev1, Initiator Dev1, Initiator . . . . . . . . . Dev2 Dev3 DevN Dev2 Dev3 DevN Dev2 Dev3 DevN Marilynn P. Green, PhD, Nokia Research Center

  5. NWTT Method • As it turns out, Phase I and II of the 3WTT method (482r0) can be combined to provide an N-way mechanism for all of the devices to determine their pair-wise distances. • A total of N packets (1 per device) is transmitted during each measurement period. TWTT’s D12 D14 D13 D1N RM2 RM4 RM1 RM3 RM_N T1c R(1)2c=T2c+D12 =T1c+2D12+∆1 R(1)4c=T4c+D14 =T1c+D12+D23+D34+D14 +∆1+∆2+∆3 R(1)Nc=TNc+D1N =T1c+D12+D23+…+DN-1,N+D1N +∆1+∆2+…+∆N-1 R(1)3c=T3c+D13 =T1c+D12+D23+D13+∆1+∆2 ∆1 D24 D23 D2N D12 RM4 RM2 RM3 RM1 RM_N T2c=R(2)1c+ ∆1=T1c+D12+ ∆1 R(2)4c=T4c+D24 =T1c+D12+D23+D34+D24 +∆1+∆2+∆3 R(2)1c=T1c+D12 R(2)3c=T3c+D23 =T1c+D12+2D23+∆1+∆2 R(2)Nc=TNc+D2N =T1c+D12+D23+…+DN-1,N+D2N +∆1+∆2+…+∆N-1 ∆2 D23 D34 D3N D13 RM3 RM1 RM2 RM4 RM_N R(3)2c=T2c+D23 =T1c+D12+D23+∆1 R(3)1c=T1c+D13 R(3)4c=T4c+D34 =T1c+D12+D23+2D34 +∆1+∆2+∆3 T3c=R(3)2c+ ∆1+ ∆2 =T1c+D12+D23+∆1+ ∆2 R(3)Nc=TNc+D3N =T1c+D12+D23+…+DN-1,N+D3N +∆1+∆2+…+∆N-1 ∆3 D14 D24 D34 RM_N RM2 RM4 RM1 RM3 R(4)Nc=TNc+D4N =T1c+D12+D23+D34 ….+DN-1,N+D4,N + ∆1+∆2+…+∆N-1 R(4)2c=T2c+D24 =T1c+D12+D24+∆1 T4c=R(4)3c+∆3 =T1c+D12+D23+D34 +∆1+∆2+∆3 . . . R(4)3c=T3c+D34 =T1c+D12+D23+D34+∆1+∆2 R(4)1c=T1c+D14 ∆N-1 . . . . . . D2N D4N D3N D1N RM1 RM2 RM4 RM_N RM3 TNc=R(N)N-1,c+∆N =T1c+D12+D23+D34+…+ DN-1,N +∆1 + ∆2 +…+ ∆N-1 R(N)3c=T3c+D3N =T1c+D12+D23+D3N+∆1+∆2 R(N)1c=T1c+D1N R(N)2c=T2c+D2N =T1c+D12+D2N+∆1 R(N)4c=T4c+D4N =T1c+D12+D23+D34+D4N +∆1+∆2+∆3 One measurement period Yellow: transmitted packet White: received (measured) packet Marilynn P. Green, PhD, Nokia Research Center

  6. Resulting Measurements • During the active period, we obtain three types of measurements: • Receive-to-transmit delays, {∆i} • Two-way distance measurements • Time difference measurements • These measurements are sufficient to determine the pair-wise distances between all of the devices… Marilynn P. Green, PhD, Nokia Research Center

  7. Resulting Measurements (1/3): Receive-to-Transmit Delays • The m-th device is responsible for measuring the delay between reception of a packet from the (m-1)-th device and its time of transmission during the next time slot. • The delays, {∆m} , might be related to a fixed delay but in practice will differ due to clock uncertainties. Marilynn P. Green, PhD, Nokia Research Center

  8. Resulting Measurements (2/3):Two-Way Distance Measurements • There are N-1 TWTT (Two-Way Time Transfer) measurements between the devices that transmit in adjacent slots. • From these measurements, we can determine the range between sequentially transmitting devices: Marilynn P. Green, PhD, Nokia Research Center

  9. Resulting Measurements (3/3):Time Difference Measurements • Each device can also make TDOA measurements by noting the difference between the time of reception from other devices and the time of its own transmission (excluding devices that transmit in adjacent time slots). • For example: Dev1 can make the following measurements to determine its range to Dev3…DevN. • And the n-th device can make the following measurements to solve for all pair-wise differences between itself and the other devices. Marilynn P. Green, PhD, Nokia Research Center

  10. Measurement Reporting • In order for each device to be able to calculate the distances between devices, we require measurement reports to be sent. • The measurement report sent by the m-th mobile should obtain • Its receive-to-transmit delay, ∆m-1 • Its measured time differences, Mnk • Thus, there should be a total of N range measurement reports packets sent at the end of the exchange Marilynn P. Green, PhD, Nokia Research Center

  11. Summary • The NWTT method allows N devices to cooperate in order to determine their inter-device ranges • The two-way range is determined between sequentially transmitting devices using the TWTT method • For those devices that are not transmitting in adjacent time slots…the range between Dev-n and Dev-k is determined by observing the difference between the time of arrival of a packet from Dev-k and the time of transmission from Dev-n • This method minimally requires a total of 2+P packets to be sent by each device, where P denotes the number of measurement periods • By allowing all of the devices to send measurement reports, we provide each device with enough data to determine the network geometry • Equal burden on all of the devices • A device can also choose to calculate ranges between a subset of devices • This method is a straightforward extension of the use of the TWTT and 3WTT methods, but it exploits both active and passive measurements so that N devices can cooperatively determine their range Marilynn P. Green, PhD, Nokia Research Center

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