Integration Lengths for Extended-Range PHY

1. Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: Integration lengths for extended-range PHY Date Submitted: 18th May 2010 Source: Andy Ward, Ubisense Address: St Andrew’s House, St Andrew’s Road, Chesterton, Cambridge, CB4 1DL, ENGLAND Voice: +44 1223 535170, FAX: +44 1223 535167, E-Mail: andy.ward@ubisense.net Re: TG4f Call for Preliminary Proposals and Final Proposals, IEEE P802.15-09-0419-01-004f Abstract: Integration lengths for extended-range PHY Purpose: To be considered by 802.15TG4f 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. Andy Ward, Ubisense

2. Integration lengths for extended-range PHY Andy Ward Ubisense Andy Ward, Ubisense

3. Overview • Discuss trade-offs involved in selecting the pulse-to-symbol ratio • Show how system frequency accuracy is important • Discuss ‘state-of-the-art’ in cheap crystal technology Andy Ward, Ubisense

4. Extended-range UWB PHY • Rather than using one pulse-per-symbol mapping, we use m pulses-per-symbol • Still very simple for transmitter to generate • Still relatively simple for receivers as well! • Use a different PRF (2MHz) so that receivers can do tone detection to figure out what type of packet is coming • Integrate the pulses at the receiver to increase signal to noise • For example, integration of four pulses increases SNR by 6dB (ideally) • System will be average-power limited rather than peak-power limited • Long pulse trains will mean that ‘average in 1ms’ trick can’t be used • However, once you are average-power limited, there is no (regulatory) limit to the length of packet you can transmit at the same power Andy Ward, Ubisense

5. Ideal coherent integrator performance Andy Ward, Ubisense

6. System frequency accuracy Nominal pulse centre time • The previous graph assumes perfect frequency sources throughout the system • When the frequency sources differ, the pulses won’t line up perfectly in the coherent receiver • So the achievable gain is less • As the number of pulses-per-symbol increases, frequency accuracy gets more important • You’re integrating over a longer time… • …so the amount of drift between the transmitter and receiver increases… • …so the pulses at the start and end of the integration period don’t line up so well Some time later…. Nominal pulse centre time Andy Ward, Ubisense

7. Effect of system frequency accuracy on integration gain Andy Ward, Ubisense

8. System frequency accuracy vs centre frequency Nominal pulse centre time • There is some dependence of integration performance (with non-ideal frequency sources) on system centre frequency • As CF increases, it takes less drift to make the pulses align less well in the coherent integrator • So you need a higher frequency accuracy to maintain integration gain at high pulse-to-symbol numbers NB: What is shown here is a half-cycle of the CF of the UWB pulse (which will typically consist of a few complete cycles in a short-duration envelope) Some time later…. Nominal pulse centre time Extra energy for red (low frequency) signal Extra energy for blue (high frequency) signal Andy Ward, Ubisense

9. System frequency accuracy vs CF Andy Ward, Ubisense

10. How does integration gain vary against packet length? NB: 32-pulse-per-symbol mapping is 3dB below this line Here, both systems are average-limited so graph flattens out Here, 1-pulse-per-symbol system is peak-limited, so graph flattens out Andy Ward, Ubisense

11. What is a reasonable frequency tolerance to aim for? • Remember – this is ONLY relevant to systems that want to do coherent integration. You don’t need a super crystal if you don’t care about this! • Plain crystal – maybe 10ppm • TCXO – maybe 0.3ppm • 0.5ppm TCXOs over -40C to 85C are now readily available • Used for GPS/GSM • Very cheap • Need to cope with other effects too: • Initial frequency accuracy (tune out at manufacture) • Crystal ageing (either tune out during use, or pre-age) • As a data point (nothing more!), system frequency accuracy budget in current Ubisense systems is better than 2ppm • Better than 1ppm would be achievable at same cost today Andy Ward, Ubisense

12. Synchronisation for integration • Need to be able to frequently update receiver’s integration point • Otherwise will ‘drift off’ bits • Easiest way to do this is to Manchester-code the symbols • Has additional advantage that as long as we don’t choose a huge integration period, we can do without the previous proposal to transmit single pulses within long-range ‘0’ symbols • If we choose an integration length of 32, and then Manchester-code the symbols, then each data bit is 64 pulses (32 on, 32 off) and the maximum period between 1s, as seen by a non-coherent receiver is 64 pulses, which is less than the ~156 pulse limit in base mode. Andy Ward, Ubisense

13. Proposal Based on available TCXOs and integration performance, we propose a 32-pulse-to-symbol mapping with Manchester encoding for the long-range UWB PHY mode Andy Ward, Ubisense