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: [Header Length Comments] Date Submitted: [8 May, 2005] Source: [Vern Brethour] Company [Time Domain Corp.] Address [7057 Old Madison Pike; Suite 250; Huntsville, Alabama 35806; USA] Voice:[(256) 428-6331], FAX: [(256) 922-0387], E-Mail: [vern.brethour@timedomain.com] Re: [802.15.4a.] Abstract: [Companion discussion for a spreadsheet contributed as IEEE802.15-05-0245.] Purpose: [To provoke a discussion of header lengths 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. Brethour, Time Domain

2. One of the most important decisions we will make is picking the length of the packet header. How much time for the header? Data (to include the time stamp of when the delimiter was at the antenna of the transmitter. Channel sounding Acquisition A delimiter signaling event separates the header from the rest of the packet. Brethour, Time Domain

3. The length of the packet header plays a huge roll in determining our long range positioning performance. • Our Standard is about the signal on the air. • The Signal on the air must support our performance targets. • Yet our performance is also largely determined by the receiver. Brethour, Time Domain

4. Simulations are best for predicting performance • Even simulators are costly, so we need something quick and simple to pick an initial direction. • This is a companion document to 0245r0, which is a spreadsheet to quickly evaluate the impact of architectural trade-offs. Brethour, Time Domain

5. The 0245r0 spreadsheet is full of assumptions about the receiver architecture. • The receiver is NOT part of the standard. • I would love to ignore the receiver, but: {The receiver does exist and it has performance determining properties.} • So this discussion will include a reference receiver. Brethour, Time Domain

6. Include a reference receiver …..okay, but first…….. disclaimers! • This is not and will not be part of the standard. • There are lots of ways to build a radio. • There is absolutely no claim here that this reference receiver is the best way to build a 15.4a radio. • This is simply a structure to put the performance spreadsheet into some context. Brethour, Time Domain

7. Reference Receiver for 0245r0 (in phase data stream) RF Front End: LNA, Band definition filters, etc. A2D (1024 M Sa/s) Low pass filter I Rectangular to polar transform Magnitude (to Acquisition & Ranging) Local Oscillator @ Tx center frequency Phase (to tracking) 90 degree phase shift A2D (1024 M Sa/s) Low pass filter Q (quadrature data stream) Brethour, Time Domain

8. This is the spreadsheet contributed as 05-0245r0 The cover sheet is not interesting. Go to the sheet named “Computation” Brethour, Time Domain

9. How do we use the spreadsheet? We make decisions and trade off numbers in this part of the spreadsheet While we keep our eye on these two answers: These are the projected preamble lengths needed to satisfy the conditions Brethour, Time Domain

10. So, where do the numbers come from? Band sketch from Welborn 0240r0 -40 -45 -50 dBm/MHz -55 -60 Center Frequency matters: the performance can be different in each band -65 -70 2 2.5 3 3.5 4 4.5 5 5.5 6 Frequency 9 x 10 Brethour, Time Domain

11. More numbers Band sketch from Welborn 0240r0 -40 -45 -50 dBm/MHz -55 Depending on how much pulse shaping we do, the 3dB Tx bandwidth might only be 63% of the 10 dB bandwidth. We must convert, because most of the calculations are with respect to a 3 dB bandwidth. -60 -65 -70 2 2.5 3 3.5 4 4.5 5 5.5 6 Frequency 9 x 10 Brethour, Time Domain

12. I low pass A2D LNA Mag Rectangle to polar osc Phase Q 90 low pass A2D More numbers The pass band of the low pass filter is often wider than the incoming signal envelope bandwidth. That allows in more noise, which we account for with this cell. Brethour, Time Domain

13. More numbers: This one is the Biggie! For performance in long links, in simulations, in spreadsheets, and in real life, this number is dominant. A free space channel has a path loss exponent of “2”. A moderately nasty indoor channel has a path loss exponent of “3”. A really nasty channel can have a path loss exponent much higher. I would feel better if this cell contained the value “3”. But if it did, the spreadsheet would say that the 50 meter link does not work in 4 ms. The value 2.6 represents a benign channel. This needs discussion. Brethour, Time Domain

14. More numbers: Acquisition S/N For the purposes of Mr. Boltzmann, this number really is just the system ambient temperature, not the junction temperature of the devices in the LNA. (Thank goodness! We capture that other nasty stuff separately in the Noise Figure two cells below. The Noise Figure is not a simple function of temperature, so we just make it a number and don’t even try to model thermal effects in the NF.) Brethour, Time Domain

15. More numbers : By the time we get done building the Tx and take it to the compliance test lab, the output spectrum will never be as smooth as the blue curves. We must then back off the Tx power across the entire band to keep the worst little spike below the FCC emissions mask. Band sketch from Welborn 0240r0 -40 -45 -50 dBm/MHz -55 -60 -65 -70 2 2.5 3 3.5 4 4.5 5 5.5 6 Frequency 9 x 10 Brethour, Time Domain

16. I low pass A2D LNA Mag Rectangle to polar osc Phase Q 90 low pass A2D More numbers A 7dB noise figure will sound high to people used to narrow band radios. This is UWB , and we’re targeting a system we can build in CMOS Brethour, Time Domain

17. More numbers: Acquisition S/N This number is an estimate of the post-integration S/N needed to acquire with a high probability of detect as well as a low probability of false alarm. Even after extensive simulations, it is often hard to get consensus on this number. It’s certainly more than 6 dB. 9 dB is a reasonable guess. Others are free to make their own guesses. Brethour, Time Domain

18. More numbers: Acquisition S/N This number is hard for me to distinguish from the S/N in the cell directly above. Some people like to manage issues like degradation due to oscillator drift during the integration period with a separate number. This spreadsheet is organized to please those people. In this spreadsheet, this number and the one in the cell above are never used separately but rather always used as a summed pair. Brethour, Time Domain

19. I low pass A2D LNA Mag Rectangle to polar osc Phase Q 90 low pass A2D Another key number: S/N for leading edge. The channel sounding is characterized by looking at magnitude information. But what algroithm is used for this is another issue with the reference receiver. What algorithm? Brethour, Time Domain

20. Algorithm for characterization of LOS. We’re trying to find this leading edge energy in the channel sounding. An indoor channel sounding. Brethour, Time Domain

21. Let’s think about the problem in free space: Artists’ concept of a raised cosine envelope Base band envelope (500 MHz) mixed to DC. About 5 ns for 500 MHz Brethour, Time Domain

22. Consider finding the leading edge in free space: only one arriving pulse envelope. Base band envelope (500 MHz) mixed to DC. Sample times (1 GHz) Actual Samples Correct answer for position of leading edge Brethour, Time Domain

23. How do we find the green arrow? 500 MHz base band envelope mixed to DC. and sampled at 1 GHz Correct answer for position of leading edge (The elusive green arrow) One popular algorithm simply finds the first non-zero (in practice, above some threshold) value and calls that sample position the location of the leading edge. In this example, that algorithm would say the leading edge is here. Brethour, Time Domain

24. Alternative algorithm: Find the green arrow! Do some math & calculate this position. Correct answer for position of leading edge Another algorithm uses the first two non-zero (in practice, above some threshold) values and does trig computations knowing that they are samples of a known length cosine to calculate the location of the leading edge. Brethour, Time Domain

25. Find the LOS path: we have choices! Algorithm #1: Pick the first value above a threshold and call the leading edge position here. Brethour, Time Domain

26. Find the LOS path: we have choices! Algorithm #2: Do some math & calculate this position. Brethour, Time Domain

27. Leading edge algorithms and ranging performance. This is a receiver design issue. This is NOT a recommendation about which algorithm to pick. Pick 1st big one Pick an algorithm: 2 choices are shown here. There are other choices as well. Trig. look up table Brethour, Time Domain

28. Algorithm selection determines the S/N value. The modeling of this algorithm is where this particular number comes from. trigonometry Brethour, Time Domain

29. Allowance for attenuation of the leading edge. How much attenuation of the Line of Sight energy will our algorithm tolerate? We make allowance for that here. Brethour, Time Domain

30. LOS algorithm implementation loss. This number captures stray effects like imperfect tracking of oscillator drift during the channel sounding and round-off errors in trig tables and such distractions. I find it useful to characterize the S/N needed for the algorithm (two cells above) as if everything about the implementation of the algorithm were perfect and then account for imperfections separately here. Brethour, Time Domain

31. Chip time is an element of the computation. Brethour, Time Domain

32. Symbol time is an element of the computation. The Barker 13 sequence is chosen as an example 10 13 12 9 1 2 3 4 5 6 7 8 11 Brethour, Time Domain

33. The signaling scheme from Zafer 0223r0 One Bit Always Empty Always Empty Always Empty Time Hop freedom This is our channel multipath tolerance. 13 chip times The Other Bit Always Empty Always Empty Always Empty Time Hop freedom This is our channel multipath tolerance. 10 13 12 9 1 2 3 4 5 6 7 8 11 Brethour, Time Domain

34. Time hopping and multipath tollerance. The “Time Hop Freedom” cell is to optionally support the time hopping proposed by Zafer. I set it to zero, to keep it out of the way in this analysis. Signaling from Zafer 0223r0 Brethour, Time Domain

35. The Symbol repetition rate computation. These numbers make up the inputs for the computation of the symbol rep rate over here. This gets multiplied by the integration count to get the needed header times for acquisition and channel sounding. The final “green cell” number is the total of the acquisition time and the channel sounding time. Brethour, Time Domain

36. The Path loss computation. The path loss is determined as if the first meter is free space. The antenna capture crossection with different center frequencies comes in to play here. For all ranges beyond the first meter, the Path loss exponent is in effect. Brethour, Time Domain

37. The integration count computations. These are the areas where a link budget style computation is done to find the link margin (negative at long ranges) that must be “bought back” with integration gain for acquisition and Line of Sight path arrival characterization. Brethour, Time Domain

38. What does the spreadsheet say?Bad news, mostly! This number is small for conservative design practice. (Would rather it be 3.) These numbers are crowding our target sounding times. Brethour, Time Domain

39. Conclusion: We should stick with our ranging performance targets, for now. • 50 meter positioning to 1 meter accuracy in under 10 ms (per round trip, with a small allowance for overhead) will be hard. • 20 meter positioning to 1 meter accuracy in under 2 ms (per round trip, with a small allowance for overhead) will be hard. • These performance targets are only hard, not impossible. • There are other positioning solutions in the marketplace, but if we hit these targets (or get close) we will bring unique value to our customers. Brethour, Time Domain