Submission Title: [Crystal Offsets and UWB] June, 2005 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. Brethour, Time Domain

Crystal offsets and UWB signals June, 2005 Crystal offsets and UWB signals (The need for tracking in 15.4a receivers.) Brethour, Time Domain

June, 2005 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. Brethour, Time Domain

We get process gain by integrating June, 2005 We get process gain by integrating In Cairns, the 4a group decided to use path loss exponent = 3.5 for performance predictions. A spreadsheet for 4a signaling (0245r1) suggests the need for integrations as high as 8192 when path loss exponents of 3.5 are used. Brethour, Time Domain

We must stay roughly aligned on the pulse while we’re integrating. June, 2005 We must stay roughly aligned on the pulse while we’re integrating. But at 500 MHz, 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. Brethour, Time Domain

Assume some typical values: June, 2005 Assume some typical values: From the spreadsheet 0245r1 with the path loss exponent set to 3.5. The single “pulse event” repetition rate is 430 ns. An integration interval of 1024 of these symbols will span .44 ms. Brethour, Time Domain

If we use 10 ppm crystals, we must budget for 20 ppm of total offset. June, 2005 If we use 10 ppm crystals, we must budget for 20 ppm of total offset. Worst case is crystal #1 at + 10 ppm & crystal #2 at – 10 ppm. At 20 ppm we can drift 8.8 ns during a .44 ms integration interval. 8.8 ns is a long way on a 5 ns envelope! Brethour, Time Domain

June, 2005 8.8 ns of drift! With a drawing more or less to scale, that’s enough to carry our sample point from here to here. At 500 MHz, the whole envelope is only 5 ns long Brethour, Time Domain

June, 2005 That’s not working!! Red X’s are bad! For successful integration of the envelope, we want to have a situation more like drifting between the blue X’s We can handle 2 ns of drift! At 500 MHz, the whole envelope is only 5 ns long Brethour, Time Domain

June, 2005 What does this mean? We can handle 2 ns of drift! To only have 2.2 ns of drift (worst case) during an integration interval of .44 ms, we would have to use a pair of 2.5 ppm crystals. Brethour, Time Domain

Does this mean we can’t do long integrations? June, 2005 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. Brethour, Time Domain

Tracking with a UWB receiver. June, 2005 Tracking with a UWB receiver. This is the “reference receiver” shown in 05-0246r1: I Q osc 90 low pass LNA A2D Rectangle to polar Phase Mag 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. Brethour, Time Domain

We are not tracking during acquisition. So fine! We do tracking. Does this mean we can use whatever crystals we like? June, 2005 We must be careful. We are not tracking during acquisition. Yet we need process gain for long links (even in acquisition). Brethour, Time Domain

June, 2005 Can we use 40 ppm crystals? 40 ppm crystals makes acquisition tough. At 80 total ppm of relative drift, we are carried 4.4 ns during an integration interval of 128 pulses. That’s almost clear across the envelope! If we’re drifting that far during an integration, we didn’t need to add up all 128 samples: about half of them had low energy anyway! Brethour, Time Domain

Can we integrate 128 with 40 ppm crystals? June, 2005 Can we integrate 128 with 40 ppm crystals? It’s tough! Even if we’re perfectly aligned on the envelope: 4.4 ns of drift during the integration interval The good zone Samples added to our integration during these parts of the integration interval aren’t helping us much! Brethour, Time Domain

June, 2005 So acquisition is tough with 40 ppm crystals. What about Channel Sounding? Channel sounding needs even more process gain than acquisition. Fortunately, we can be tracking the pulse (and even the carrier) during channel sounding. We acquire in this part of the preamble. Integration 128? We do channel sounding in this part of the preamble. Integration 1024?, but we’re tracking! Brethour, Time Domain

June, 2005 Conclusion. As the 15.4a project moves forward we define the actual signaling that we will use, we must pay attention to the amount of time bounded by an integration interval. That may impact the quality of crystals we use when we are projecting our acquisition performance There is no question about needing to implement tracking for channel sounding (to do ranging). Brethour, Time Domain