1. March, 2010
Project: IEEE P Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: Integration lengths for long-range PHY
Date Submitted: 16th March 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 long-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 long-range PHY
March, 2010
Integration lengths for long-range PHY
Andy Ward
Ubisense
Andy Ward, Ubisense

3. doc.: IEEE 802.15-<doc#>
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doc.: IEEE <doc#>
March, 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. March, 2010
Long-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
March, 2010
Ideal coherent integrator performance
Andy Ward, Ubisense

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

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

10. How does integration gain vary against packet length?
March, 2010
How does integration gain vary against packet length?
Here, both systems are average-limited so graph flattens out
Here, 1-pulse-per-symbol system is peak-limited, so graph flattens out
NB: This graph is shows relative performance of the 2MHz 64-pulse-per-symbol and 1MHz 1-pulse-per-symbol modes. The increase in relative effective gain as packet length increases is due to the decrease in performance of the 1-pulse-per-symbol mode at packet lengths beyond 185 bits (up to 1000 bits). It does not indicate (nor is it intended to) an increase in absolute gain of the 64-pulse-per-symbol mode as packet length increases.
Andy Ward, Ubisense

11. What is a reasonable frequency tolerance to aim for?
March, 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. March, 2010
Proposal
Based on available TCXOs and integration performance, we propose a 2MHz 64-pulse-to-symbol mapping for the long-range UWB PHY mode, and a 2.3ppm system timing accuracy
This integration length and timing accuracy gives a relative gain over the 1MHz 1-pulse-per-symbol mapping of 4.5dB at 8.5GHz centre frequency and packet length of 185 bits
This number increases as:
Packet length increases (up to 1000 bits)
Centre frequency decreases
Timing accuracy increases
Andy Ward, Ubisense