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

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [Ranging Accuracy of SDS-TWR Subject to Unequal Reply Times] Date Submitted: [July 2005] Source: [Shahriar Emami and Celestino A. Corral] Company [Freescale Semiconductor, Inc] Address [Freescale Semiconductor, Inc. 6100 Broken Sound Pkwy., N.W. Suite 1 Boca Raton, FL  33487-2790] Voice:[(561) 544-4064] Re: [802.154a] Abstract: [This document addresses the performance of SDS-TWR technique] Purpose: [To help evaluate ways to manage crystal drift] 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. Shahriar Emami, Freescale

2. Symmetric Double Sided-Two Way Ranging (SDS -TWR)Proposed in document IEEE-15-05-0334-00-004 Device B Device A unknown propagation delay reply time Shahriar Emami, Freescale

3. Sensitivity of 4 message TWR • In 4 message two way ranging (SDS-TWR) it is required that treplyA ~treplyB • Is ranging error sensitive to variation of treplyA in reference to treplyB? And to what extent? • Document IEEE-15-05-0002-01-004a considers differential reply times in between 2 and 20 us. • We will compute the range and ranging error due to variations of reply time at various ranges when absolute reply time is in 0.1-1 ms range. Shahriar Emami, Freescale

4. Calculated Ranges+ + 10 PPM crystals are deployed and reply time is assumed to be 1 ms. * Excess reply time of B in reference to A or 100 x (treplyB-treplyA)/treplyA . Shahriar Emami, Freescale

5. Actual and Calculated Ranges Shahriar Emami, Freescale

6. Ranging Error Ranging error at 10m and 50m are 15% and 3%, respectively (10 PPM) subject to 100% variation in reply time (1ms reply time). Shahriar Emami, Freescale

7. Actual and Calculated Ranges Shahriar Emami, Freescale

8. Ranging Error Ranging error at 10m and 50m are 60% and 12%, respectively (40 PPM) subject to 100% variation in reply time (1ms reply time Shahriar Emami, Freescale

9. What If Reply Time Is Decreased? Shahriar Emami, Freescale

10. What If Reply Time Is Decreased? Shahriar Emami, Freescale

11. Summary (1) • For a given tolerable percentage error, shorter ranges determine the maximum tolerable excess reply time. • If low PPM crystals are used, the ranging error is not significant particularly at larger distances and especially when reply time variations are held under 40%. • On the other hand, the ranging errors for high PPM crystal devices if reply time high could are unacceptable. Other methods would have to be deployed to compensate for or correct crystal drifts • When reply time was reduced up to an order of magnitude (1ms-0.1 ms), ranging error and percentage ranging error also went down by up to an order of magnitude. Shahriar Emami, Freescale

12. Summary (2) % Ranging Error Acceptable Error Minimum Range Maximum Range Shahriar Emami, Freescale

13. Bottom Line For a given set of parameters: • PPM ( ) • Reply Time • Excess Reply Time( ) • Minimum Desired Range Calculate % ranging error If percent ranging error acceptable, you are done. • Otherwise one needs to reduce PPM, reply time, excess reply time or increase minimumrange. Shahriar Emami, Freescale