doc.: ieee 802.15-05-0335-00-004a submission june, 2005 brethour, time domainslide 1 project: ieee...

June, 2005

Brethour, Time DomainSlide 1

doc.: IEEE 802.15-05-0335-00-004a

Submission

Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)

Submission Title: [Crystal Offsets and UWB]

Date Submitted: [5 June, 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: [Signal tracking is needed to support long integrations.]

Purpose: [To promote discussion 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.

June, 2005

Brethour, Time DomainSlide 2

doc.: IEEE 802.15-05-0335-00-004a

Submission

Crystal offsets and UWB signals

(The need for tracking in 15.4a receivers.)

June, 2005

Brethour, Time DomainSlide 3

doc.: IEEE 802.15-05-0335-00-004a

Submission

We need process gain

• We are trying to keep our pulse amplitudes in 15.4a to voltages that we can generate with CMOS.

• Pulse Compression helps.

• But it will not usually be enough.

June, 2005

Brethour, Time DomainSlide 4

doc.: IEEE 802.15-05-0335-00-004a

Submission

We get process gain by integrating

• In Cairns, the 4a group decided to use path loss exponent = 3.5 for performance predictions.

• The spreadsheet for 4a signaling (0245r1) suggests the need for integrations as high as 8192 when path loss exponents of 3.5 are used.

June, 2005

Brethour, Time DomainSlide 5

doc.: IEEE 802.15-05-0335-00-004a

Submission

We must stay roughly aligned on the pulse while we’re integrating.

• But at 4.5 GHz, the pulses drift away in a hurry!

• Let’s suppose that we need 30 dB of processing gain from integration (reasonable for long links)

• That means we’re integrating 1024 pulses.

June, 2005

Brethour, Time DomainSlide 6

doc.: IEEE 802.15-05-0335-00-004a

Submission

Assume some typical values:

• Pulse compression code length = 19.

• Yet (even than) some folks still want large pulses.

• Makes the symbol get long. We’ll be looking at symbol times like 870 ns.

• An integration interval of 1024 of these symbols will span .89 ms.

June, 2005

Brethour, Time DomainSlide 7

doc.: IEEE 802.15-05-0335-00-004a

Submission

If we use 40 ppm crystals, we must budget for 80 ppm of total offset.

• Worst case is crystal #1 at + 40 ppm & crystal #2 at – 40 ppm.

• At 80 ppm we can drift 71 ps during a .89 ms integration interval.

• That’s a long way on a 4.5 GHz carrier!

June, 2005

Brethour, Time DomainSlide 8

doc.: IEEE 802.15-05-0335-00-004a

Submission

71 ps of drift!

At 4.5 GHz, a whole cycle is only 222 ps

71 ps of drift will cause our sample point to move fromhere at the start of the integration interval to here at the end.

June, 2005

Brethour, Time DomainSlide 9

doc.: IEEE 802.15-05-0335-00-004a

Submission

That’s not working!!

71 ps of drift will cause our sample point to move fromhere at the start of the integration interval to here at the end.

During this entire time (about a third of the integration interval), the integration was actually going the wrong way!

June, 2005

Brethour, Time DomainSlide 10

doc.: IEEE 802.15-05-0335-00-004a

Submission

What does this mean?

During this entire time (about a third of the integration interval), the integration was actually going the wrong way!

This says that with 40ppm crystals we cannot use integration of 1024 on a 4.5 GHz carrier unless we are tracking. (In this example)

June, 2005

Brethour, Time DomainSlide 11

doc.: IEEE 802.15-05-0335-00-004a

Submission

Does this mean we can’t do long integrations?

• Of course not!

• It means that we must expect to track the transmit signal with our receiver while we’re integrating.

• That’s just normal radio design.

June, 2005

Brethour, Time DomainSlide 12

doc.: IEEE 802.15-05-0335-00-004a

Submission

Tracking with a UWB receiver.

• This is the “reference receiver” shown in 05-0246r1:

I

Q

osc

90

lowpass

lowpassLNA A2D

A2D

Rectangle to

polar

Pha

seMag

Tracking logic here.

There are many options for implementing tracking in a receiver. It can be done either completely digitally or wholly in the analog world or as a combination.

June, 2005

Brethour, Time DomainSlide 13

doc.: IEEE 802.15-05-0335-00-004a

Submission

So fine! We do tracking. Does this mean we can use whatever crystals we like?

• We must be careful.

• We are not tracking during acquisition.

• Yet we need process gain for long links (even in acquisition).

June, 2005

Brethour, Time DomainSlide 14

doc.: IEEE 802.15-05-0335-00-004a

Submission

Conclusion.

• We the 15.4a project moves forward and defines the actual signaling that we will use we must pay attention to the amount of time bounded by an integration interval.

• That may put bounds on the quality of crystals we use.