May, 2005
Brethour, Time DomainSlide 1
doc.: IEEE 802.15-05-0246-00-004a
Submission
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: [[email protected]]
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.
May, 2005
Brethour, Time DomainSlide 2
doc.: IEEE 802.15-05-0246-00-004a
Submission
One of the most important decisions we will make is picking the length of the packet header.
Acquisition Channel sounding Data (to include the time stamp of when the delimiter was at the antenna of the transmitter.
A delimiter signaling event separates the header from the rest of the packet.
How much time for the header?
May, 2005
Brethour, Time DomainSlide 3
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 4
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 5
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 6
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 7
doc.: IEEE 802.15-05-0246-00-004a
Submission
Reference Receiver for 0245r0
RF FrontEnd: LNA, Band
definition filters, etc.
A2D (1024 M
Sa/s)
Low pass filter
90 degreephase shift
Low pass filter
(in phase data stream)
(quadrature data stream)
A2D (1024 M
Sa/s)
Rectangular to polar
transform
I
Q
Magnitude(toAcquisition& Ranging)
Phase(totracking)
Local Oscillator @ Tx center frequency
May, 2005
Brethour, Time DomainSlide 8
doc.: IEEE 802.15-05-0246-00-004a
Submission
This is the spreadsheet contributed as 05-0245r0
The cover sheet is not interesting. Go to the sheet named “Computation”
May, 2005
Brethour, Time DomainSlide 9
doc.: IEEE 802.15-05-0246-00-004a
Submission
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
May, 2005
Brethour, Time DomainSlide 10
doc.: IEEE 802.15-05-0246-00-004a
Submission
So, where do the numbers come from?
2 2.5 3 3.5 4 4.5 5 5.5 6x 10
9
-70
-65
-60
-55
-50
-45
-40
Frequency
dBm
/MH
z
Band sketch from Welborn 0240r0
Center Frequency matters: the performance can be different in each band
May, 2005
Brethour, Time DomainSlide 11
doc.: IEEE 802.15-05-0246-00-004a
Submission
More numbers
2 2.5 3 3.5 4 4.5 5 5.5 6x 10
9
-70
-65
-60
-55
-50
-45
-40
Frequency
dBm
/MH
z
Band sketch from Welborn 0240r0
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.
May, 2005
Brethour, Time DomainSlide 12
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
I
Q
osc
90
lowpass
lowpassLNA A2D
A2D
Rectangle to
polar
Pha
seMag
May, 2005
Brethour, Time DomainSlide 13
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 14
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.)
May, 2005
Brethour, Time DomainSlide 15
doc.: IEEE 802.15-05-0246-00-004a
Submission
More numbers :
2 2.5 3 3.5 4 4.5 5 5.5 6x 10
9
-70
-65
-60
-55
-50
-45
-40
Frequency
dBm
/MH
z
Band sketch from Welborn 0240r0
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.
May, 2005
Brethour, Time DomainSlide 16
doc.: IEEE 802.15-05-0246-00-004a
Submission
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
I
Q
osc
90
lowpass
lowpassLNA A2D
A2D
Rectangle to
polar
Pha
seMag
May, 2005
Brethour, Time DomainSlide 17
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 18
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 19
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
I
Q
osc
90
lowpass
lowpassLNA A2D
A2D
Rectangle to
polar
Pha
seMag
What algorithm?
May, 2005
Brethour, Time DomainSlide 20
doc.: IEEE 802.15-05-0246-00-004a
Submission
Algorithm for characterization of LOS.
An indoor channel sounding.
We’re trying to find this leading edge energy in the channel sounding.
May, 2005
Brethour, Time DomainSlide 21
doc.: IEEE 802.15-05-0246-00-004a
Submission
Base band envelope (500 MHz) mixed to DC.
Artists’ concept of a raised cosine envelope
About 5 ns for 500 MHz
Let’s think about the problem in free space:
May, 2005
Brethour, Time DomainSlide 22
doc.: IEEE 802.15-05-0246-00-004a
Submission
Base band envelope (500 MHz) mixed to DC.
Sample times (1 GHz)
Actual Samples
Correct answer for position of leading edge
Consider finding the leading edge in free space: only one arriving pulse envelope.
May, 2005
Brethour, Time DomainSlide 23
doc.: IEEE 802.15-05-0246-00-004a
Submission
500 MHz base band envelope mixed to DC. and sampled at 1 GHz
Correct answer for position of leading edge (The elusive green arrow)
How do we find the 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.
May, 2005
Brethour, Time DomainSlide 24
doc.: IEEE 802.15-05-0246-00-004a
Submission
Correct answer for position of leading edge
Alternative algorithm: Find the green arrow!
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.
Do some math & calculate this position.
May, 2005
Brethour, Time DomainSlide 25
doc.: IEEE 802.15-05-0246-00-004a
Submission
Pick the first value above a threshold and call the leading edge position here.
Find the LOS path: we have choices!
Algorithm #1:
May, 2005
Brethour, Time DomainSlide 26
doc.: IEEE 802.15-05-0246-00-004a
Submission
Find the LOS path: we have choices!
Algorithm #2:
Do some math & calculate this position.
May, 2005
Brethour, Time DomainSlide 27
doc.: IEEE 802.15-05-0246-00-004a
Submission
Trig. look up table
Pick 1st big one
Pick an algorithm: 2 choices are shown here. There are other choices as well.
This is a receiver design issue. This is NOT a recommendation about which algorithm to pick.
Leading edge algorithms and ranging performance.
May, 2005
Brethour, Time DomainSlide 28
doc.: IEEE 802.15-05-0246-00-004a
Submission
Algorithm selection determines the S/N value.
The modeling of this algorithm is where this particular number comes from.
trigonometry
May, 2005
Brethour, Time DomainSlide 29
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 30
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 31
doc.: IEEE 802.15-05-0246-00-004a
Submission
Chip time is an element of the computation.
May, 2005
Brethour, Time DomainSlide 32
doc.: IEEE 802.15-05-0246-00-004a
Submission
1 2 3 4 5 6 7 8 9 10 11 12 13
The Barker 13 sequence is chosen as an example
Symbol time is an element of the computation.
May, 2005
Brethour, Time DomainSlide 33
doc.: IEEE 802.15-05-0246-00-004a
Submission
13 chip times
One Bit
The Other Bit
Time Hopfreedom
Always EmptyAlways Empty
Always EmptyAlways Empty Always Empty
Always Empty
The signaling scheme from Zafer 0223r0
Time Hopfreedom
This is our channel multipath tolerance.
This is our channel multipath tolerance.
1 2 3 4 5 6 7 8 9 10 11 12 13
May, 2005
Brethour, Time DomainSlide 34
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
Time hopping and multipath tollerance.
Signaling from Zafer 0223r0
May, 2005
Brethour, Time DomainSlide 35
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 36
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 37
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
The integration count computations.
May, 2005
Brethour, Time DomainSlide 38
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
May, 2005
Brethour, Time DomainSlide 39
doc.: IEEE 802.15-05-0246-00-004a
Submission
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.
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