Doc.: IEEE 802.15-05-0246-00-004a Submission May, 2005 Brethour, Time DomainSlide 1 Project: IEEE...

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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


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


doc.: IEEE 802.15-05-0246-00-004a

Submission


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


doc.: IEEE 802.15-05-0246-00-004a

Submission


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


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


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


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


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


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


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


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


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


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


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


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


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


doc.: IEEE 802.15-05-0246-00-004a

Submission

Chip time is an element of the computation.

May, 2005


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


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


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


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


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


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


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


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.

Doc.: IEEE 802.15-05-0246-00-004a Submission May, 2005 Brethour, Time DomainSlide 1 Project: IEEE...

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