May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)
Submission Title: [Header Length Comments]
Date Submitted: [10 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 corrected spreadsheet contributed as IEEE802.15-05-0245r1.]
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.
Submission
Slide 1
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
One of the most important decisions we will
make is picking the length of the packet header.
How much time for the 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.
Submission
Slide 2
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 3
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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 0245r1,
which is a spreadsheet to quickly evaluate
the impact of architectural trade-offs.
Submission
Slide 4
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
The 0245r1 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.
Submission
Slide 5
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 6
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Reference Receiver for 0245r1
(in phase
data stream)
Low
pass
filter
A2D
(1024 M
Sa/s)
I
Local Oscillator
@ Tx center
frequency
90 degree
phase shift
Low
pass
filter
Submission
A2D
(1024 M
Sa/s)
Slide 7
Q
Rectangular to polar transform
RF Front
End:
LNA,
Band
definition
filters, etc.
Magnitude
(to
Acquisition
& Ranging)
Phase
(to
tracking)
(quadrature
data stream)
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
This is the spreadsheet contributed as 05-0245r1
The cover sheet is not interesting. Go to the sheet named “Computation”
Submission
Slide 8
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
How do we use the spreadsheet?
We make
decisions
and trade
off
numbers in
this part of
the
spreadsheet
Submission
While we
keep our
eye on
these two
answers:
These are
the
projected
preamble
lengths
needed to
satisfy the
conditions
Slide 9
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
So, where do the numbers come from?
Band sketch from
Welborn 0240r0
-40
-45
dBm/MHz
-50
-55
-60
Center Frequency matters: the performance
can be different in each band
-65
-70
2
Submission
Slide 10
2.5
3
3.5
4
Frequency
4.5
5
5.5
6
9
x 10
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
More numbers
Band sketch from
Welborn 0240r0
-40
-45
dBm/MHz
-50
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.
Submission
-55
-60
-65
-70
2
Slide 11
2.5
3
3.5
4
Frequency
4.5
5
5.5
6
9
x 10
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
A2D
osc
low
pass
Submission
Slide 12
A2D
Phase
90
Mag
I
Rectangle to polar
low
pass
LNA
Q
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 13
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.)
Submission
Slide 14
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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
dBm/MHz
-50
-55
-60
-65
-70
2
Submission
Slide 15
2.5
3
3.5
4
Frequency
4.5
5
5.5
6
9
x 10
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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
A2D
osc
low
pass
Submission
Slide 16
A2D
Phase
90
Mag
I
Rectangle to polar
low
pass
LNA
Q
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
More numbers: Acquisition S/N
This number is an estimate of the postintegration 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.
Submission
Slide 17
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 18
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Another key number: S/N for leading edge.
The channel sounding is characterized by
looking at magnitude information. But
what algorithm is used for this is another
issue with the reference receiver.
What algorithm?
A2D
osc
low
pass
Submission
Slide 19
A2D
Phase
90
Mag
I
Rectangle to polar
low
pass
LNA
Q
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Algorithm for characterization of LOS.
We’re
trying to
find this
leading
edge
energy in
the channel
sounding.
An indoor channel sounding.
Submission
Slide 20
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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
Submission
Slide 21
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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
Submission
Slide 22
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 23
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 24
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Find the LOS path: we have choices!
Algorithm #1:
Pick the first value
above a threshold and
call the leading edge
position here.
Submission
Slide 25
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Find the LOS path: we have choices!
Algorithm #2:
Do some math
& calculate
this position.
Submission
Slide 26
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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
Trig. look
up table
Submission
Pick an algorithm: 2 choices
are shown here. There are
other choices as well.
Slide 27
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Algorithm selection determines the S/N value.
The modeling of this
algorithm is where
this particular
number comes from.
trigonometry
Submission
Slide 28
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 29
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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.
Submission
Slide 30
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Chip time is an element of the computation.
Submission
Slide 31
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Symbol time is an element of the computation.
The Barker 13
sequence is chosen
as an example
1
Submission
2
3
4
5
6
Slide 32
7
8
9 10
11
12 13
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
The signaling scheme from Zafer 0223r0
One Bit
Always Empty
13 chip times
Always Empty
Always Empty
Time Hop
freedom
This is our channel
multipath tolerance.
The Other Bit
Always Empty
Always Empty Always Empty
Time Hop
freedom
This is our channel
multipath tolerance.
1
Submission
2
3
4
5
6
7
Slide 33
8
9 10
11
12 13
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Time hopping and multipath tolerance.
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
Submission
Slide 34
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
The 1’st meter of the path loss computation.
This is where the Frequency dependence
of the performance happens: Higher
frequencies have smaller Antenna
Capture Areas.
Transmit energy density per square meter
after it’s spread over the surface of a 1
meter radius sphere. The path loss
exponent for this first meter is 2.
Transmit energy captured by a receive
antenna (at our center frequency) at a
range of 1 meter.
Submission
Slide 35
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
The thermal noise floor computation.
This is system ambient, not LNA junction
temperature.
Invoking Mr. Boltzmann to compute the
absolute quietest that the noise could
possibly ever be.
Converting the units on the quietest noise
that could ever be.
Adding a touch of LNA receiver noise
reality to the quietest noise that could
ever be.
A2D
osc
low
pass
Submission
A2D
Phase
90
Mag
I
Rectangle to polar
low
pass
LNA
Determining how much of Mr. Boltzmann’s
noise that we are going to let get through
these low pass filters.
Q
Slide 36
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Transmit duty cycle computation.
On time = chip time * symbol length
The ratio of {total time} to {on time}
The ratio of {total time} to {on time}
expressed as a dB increase.
Total symbol rep rate is the sum
of all the different allowances
Always Empty
Always Empty Always Empty
Time Hop
freedom
Submission
Slide 37
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Transmit power computation.
The 3dB bandwidth (as opposed to the
regulatory 10 dB bandwidth) that is
available for the transmitter to radiate
power into.
The amount of long term average
power that we can radiate into the
3dB bandwidth.
Now back that average power off
to allow for an unlucky unintended
spectral peak in the test lab.
Increase the transmit power during the
“on time” to make the average right
given that we’re not transmitting
during all the rest of this time.
The number of chips in a symbol expressed as dB
because later, I will coherently add all these
pulses up and do the processing on the
“compressed pulse” result.
Always Empty
Always Empty Always Empty
Submission
Slide 38
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
The path loss computations for each target.
The target distance in meters that we’re going to
evaluate. This number is user changeable if we
want to try another distance. There is an
assumption that the distance will be long. If it is
short, the assumptions about the signs of the
numbers breaks down and the answers are garbage.
Path loss calculated using the deadly Path Loss
Exponent.
Total path loss which now includes the first
meter, the rest of the meters, as well as the
capture area of the receive antenna.
Rx power is the Tx power plus the path loss.
Submission
Slide 39
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Acquisition computations for each target.
Link margin is the Rx signal minus the noise,
minus the required S/N for reliable acquisition,
minus the acquisition implementation loss.
Now we get credit for our symbol compression
gain. I held off on adding it in to the receive power
number before because I wanted to show the
required receiver sensitivity to pick up the itty bitty
chip signals before compression. In this case, it’s
minus 92.95 dB minus another 21.77 dB for a scary
total of minus 114.7 dB and that gets worse for
longer links. I took that cell out of the r1 version
because Receiver sensitivity means other things in
the regular vocabulary of communications theory.
Submission
Slide 40
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
Acquisition header computations.
The negative link margin tells how much gain is
needed to successfully do the job. I get this gain by
doubling by integration as often as required.
Dividing the link margin by 3 dB per integration
doubling, yields how many doublings are required.
To keep the implementation easy, we only want to
have integration counts which are powers of 2. So
here we round the number of doublings up to the
next biggest integer.
2 raised to the number of doublings gives the
needed integration count
The integration count times the symbol repetition
period equals the number of ms of header time
needed for acquisition.
Submission
Slide 41
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
LOS computations for each target.
Link margin is the Rx signal minus the noise,
minus the required S/N for reliable leading edge
characterization minus the leading edge
characterization implementation loss.
Now we get credit for our symbol compression
gain. Again I held off on adding it in to the receive
power number before because I wanted to show the
required receiver sensitivity to pick up the itty bitty
chip signals before compression. And now, it’s
even worse than it was for acquisition.
Submission
Slide 42
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
LOS header computations.
The negative link margin tells how much gain is
needed to successfully do the job. I get this gain by
doubling by integration as often as required.
Dividing the link margin by 3 dB per integration
doubling, yields how many doublings are required.
To keep the implementation easy, we only want to
have integration counts which are powers of 2. So
here we round the number of doublings up to the
next biggest integer.
2 raised to the number of doublings gives the
needed integration count
And finally (!!) after all that, we
have the answer we want: How
much acquisition header do we
need? It’s in the green box!
Submission
The integration count times the symbol repetition
period equals the number of ms of header time
needed for leading edge characterization.
Slide 43
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
What does the spreadsheet say?
“Life is tough!”
This number
is okay for
indoor
channels.
Okay
to
meet
our
targets
.
Submission
Slide 44
Brethour, Time Domain
May, 2005
doc.: IEEE 802.15-05-0246-01-004a
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) looks
doable.
• 20 meter positioning to 1 meter accuracy in under 2 ms
(per round trip, with a small allowance for overhead) also
looks doable.
• 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.
Submission
Slide 45
Brethour, Time Domain