1. May, 2010
Project: IEEE P 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: , FAX: ,
Re: TG4f Call for Preliminary Proposals and Final Proposals, IEEE P f
Abstract: Integration lengths for extended-range PHY
Purpose: To be considered by TG4f
Notice: This document has been prepared to assist the IEEE P 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 P
Andy Ward, Ubisense

2. Integration lengths for extended-range PHY
May, 2010
Integration lengths for extended-range PHY
Andy Ward
Ubisense
Andy Ward, Ubisense

3. doc.: IEEE 802.15-<doc#>
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doc.: IEEE <doc#>
May, 2010
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
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4. Extended-range UWB PHY
May, 2010
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
May, 2010
Ideal coherent integrator performance
Andy Ward, Ubisense

6. System frequency accuracy
May, 2010
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
May, 2010
Effect of system frequency accuracy on integration gain
Andy Ward, Ubisense

8. System frequency accuracy vs centre frequency
May, 2010
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
May, 2010
System frequency accuracy vs CF
Andy Ward, Ubisense

10. How does integration gain vary against packet length?
May, 2010
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?
May, 2010
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
May, 2010
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. May, 2010
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