Three-Way Time Transfer (3WTT) Method for Cooperative Ranging

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Three-Way Time Transfer (“3WTT”) Method for Cooperative Ranging] Date Submitted: [22 July, 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 3WTT method for cooperative ranging between three devices.] 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. Cooperative Ranging • 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: c·D12, c·D13andc·D23. c·D24 4 2 c·D45 c·D12 c·D23 c·D34 1 5 3 c·D35 c·D13 Marilynn P. Green, PhD, Nokia Research Center

3. Sample Timing and Notation • In this sample ranging system, the devices exchange ranging packets • Dev1 is the Initiator and Dev2 is the Responder. • T1c(T2c) is the corrected time of transmission from Dev1 (Dev2). • R1c(R2c) is the corrected time of reception of the signal transmitted by Dev2 (Dev1). • SIFS denotes the fixed (or known) unit of time between the end of a received packet and the beginning of the next transmission. Packet Transmission (yellow) T1c=T1+Range_transmit_delay_Dev1 (T1 = Transmit time from Dev1) R1c = R1 – Range_transmit_delay_Dev1 (R1 = Time of reception at Dev2) T2c=T2+Range_transmit_delay_Dev2 (T2 = Transmit time from Dev2) R2c = R2 – Range_transmit_delay_Dev2 (R2 = Time of reception at Dev2) PAYLOAD PAYLOAD PAYLOAD PAYLOAD Packet Reception (white) Marilynn P. Green, PhD, Nokia Research Center

4. Range Calculation based on Two-Way Time Transfer (TWTT) method • The TWTT method calculates the range between the two devices, c·D12, using the following relation: c: speed of light PAYLOAD PAYLOAD PAYLOAD PAYLOAD Marilynn P. Green, PhD, Nokia Research Center

5. Fundamentals of the 3WTT Method • The 3WTT method builds upon the TWTT method to allow three devices to mutually determine their range in a two-phased approach. • Phase 1: • Dev1 and Dev2 cooperate to determine their range, c·D12 • Dev3 measures the inter-flight time between packets received from Dev1 and Dev2 • Phase 2: • Dev1, Dev2 and Dev3 all exchange ranging packets • Dev3 measures the inter-flight time between packets • At the end of the session, there is enough information to calculate all of the inter-device ranges: c·D12 , c·D12 and c·D12 Marilynn P. Green, PhD, Nokia Research Center

6. Phase I of the 3WTT Method (1/2) • During Phase I, Dev1 and Dev2 are active • Dev1 and Dev2 perform a TWTT-like exchange to determine their range, c·D12 • RMR1 contains T1c and R2c • RMR2 contain T2c and R1c • By exchanging RMR1 and RMR2, all three devices can then calculate c·D12 • Dev3 passively measures the inter-flight time, R’2c and also captures RMR1 and RMR2 SIFS RMR2 RMR1 RMR2 RMR1 RMR1 RM1, RM2: Range Measurement packet payload RMR1, RMR2: Range Measurement report from Dev1, Dev2 Marilynn P. Green, PhD, Nokia Research Center

7. Phase I of the 3WTT Method (2/2) • At the end of Phase I, there are several measurements available: • T1c, R1c, T2c, R2c measured by Dev1, Dev2 can be used to determine inter-device range, c·D12: • Inter-flight time measured by Dev3 can be used to make a differential distance measurement: • Phase II is necessary in order to separately determine D13 and D23 Marilynn P. Green, PhD, Nokia Research Center

8. SIFS RMR3 RMR3 RMR3 Phase II of the 3WTT Method (1/2) • During Phase II, Dev1, Dev2 and Dev3 are all active • Packets are exchanged in a round-robin fashion with Dev3 being the initiator • Dev3 measures the inter-flight time, R1c – T3c and sends its range measurement report, RMR3 Marilynn P. Green, PhD, Nokia Research Center

9. Phase II of the 3WTT Method (2/2) • RMR3 contains T3c, R1c (from Phase II) and R’2c, R’1c (from Phase I). • At the end of Phase II, Dev3 has an additional inter-flight measurement: • …and from Phase I we have: • Now we have two equations in two unknowns: D23 and D13 which are easily solved. • Since D12 is also known from Phase I, we now know the inter-device range between all three devices. Marilynn P. Green, PhD, Nokia Research Center

10. 3WTT Summary • Phase I: • Phase II: Marilynn P. Green, PhD, Nokia Research Center

11. Summary • The 3WTT method allows three devices to cooperate in order to determine their inter-device ranges • This approach may be extended so that N (N > 3) devices may coordinate in order to determine their relative positions in a local coordinate system • 3WTT is a straightforward method to implement which can be extended to allow devices to make multiple measurements for improved accuracy • The details of ACKs and Ranging Requests were not discussed but should be straightforward to incorporate into this scheme Marilynn P. Green, PhD, Nokia Research Center