I wondered what this “Smart Power” code was all about, so I did a bit of research.

In the process I tripped over a really interesting article dated May 15th, 2014 by Bob Cringely, called “Did the NSA help kill UWB?“.

The UWB startup that got the most press back then was called Time Domain and the name says a lot about how the technology worked. Rather than using specific frequencies UWB transmitted on all frequencies at the same time. The key was knowing when and where in the frequency band to expect a bit to appear. Two parties with synchronized clocks and codebooks could agree that at 10 nanoseconds after the hour at a certain frequency or range of frequencies a bit would appear if one was intended. The presence of that signal at that time and place was a 1 and the absence was a 0. But if you didn’t know when to listen where — if you weren’t a part of the conversation — it all looked just like noise.

The whole article is full of fascinating tidbits, but this is the story I liked the most, which really hasn’t much to do with this notebook entry.

There was a San Diego startup called PulseLINK that came up with the idea to try UWB not only through the air but also over wires. They reasoned that RF travelled through copper as well as it travelled through air. So they injected their UWB signal into the local cable TV system (without permission of course) just to see what would happen. Could they establish point-to-point and point-to-multipoint communications as an Over-The-Top network on the local cable plant? It worked. They created a gigabit network atop established cable infrastructure without the cable company even noticing it was happening.

One fascinating aspect of the PulseLINK test was that UWB, which is an electrical signal, was able to propagate throughout the Cox cable plant even though sections of the Cox network used optical signaling. They went from electrons to photons back to electrons again and the fact that was even possible came down to the fact that it was CATV, not IP. Had the cable system been IP-based every electrical to optical conversion point would have involved capturing packets, fixing them as needed, then retransmitting them, which would have foiled UWB. But in the rotgut world of cable there were no packets — just an analog signal carrying digital and analog data alike which was block converted from one medium to another and back. So the UWB data was converted and retransmitted throughout the cable plant as noise.

This aside, the bottom line is that the FCC proposed that UWB should be constrained to operate as follows:

For emissions from UWB devices other than GPRs and, possibly, through-wall imaging systems, it proposed that emissions that appear below approximately 2 GHz be attenuated by at least 12dB below the general emission limits.

The “general emission limits” they talk about refer to part 15 of  FCC Regulations for Low-power, Non-licensed Transmitters.   There is a large table of emission limits (p9-26), and I’m not sure which part of the table is appropriate for UWB.  But assuming we are talking about  3.5-6.5GHz (per DW), then the emission limits range from 500 µV/m @ 3 m to 500,000 µV/m @ 3m.  Eyeballing the tables, the median looks to be about 12,500 µV/m @ 3m.  There is also a simple 1 watt limit on many frequencies.

2-UWB-1This diagram on the left I found helpful.

All of this get’s more pertinent to my original “Smart Power” question when we start looking at how these tiny signals are to be measured.  Their rules go on for pages and pages, and I admit I got lost. It seems that the thing they care about is having so much extra noise injected into the ether, that traditional radios find the signal-to-noise ratios so poor that they can’t extract signals as easily.

So, as best I can figure out, they seem to have come up with a way or specifying the signal strength in terms of the amount of noise it injects into a specified frequency band, over a given amount of time.  This limits the number of packets that can be sent, at a certain power, over a certain set of frequencies (or portion of the radio spectrum).

So you can play games.  If you only send a few bits every so often, then you can crank up the power, and vice versa.  That’s what all this “Smart Power” thing is about.

Apparently for UWB the transmit power has to be -41dBm in each 1MHz of channel bandwidth, over a 1 ms period.  This “1 millisecond” thing crops up all over the place in the DW code.  DW can send data at 110Kbs, or 6.8Mbs.  At 6.8 Mbs it can transmit a short packet within 1 ms, and so the transmit power can be jacked up (to something) providing it doesn’t transmit again for another 1ms.  So, in “Smart Power Mode”, if DW notices that the packet is short enough, it cranks up the power automatically.

For us this might not be desirable if we want to save power.

Some other things I discovered

Except for toys, we are permitting indoor systems to be used for any type of application, including communication systems, provided emissions are not intentionally directed outside, e.g., through a window or doorway to perform an outside function such as the detection of persons about to enter a building. We also are prohibiting the use of fixed outdoor antennas, such as antennas mounted on the side or top of a building, or other outdoors infrastructure.

DarthVaderMentor May 15, 2014 at 5:42 pm  UWB is still alive and kicking and actually thriving in some places. Water, gas and electrical utilities are actually the preferred medium, now that they have overcome the transformer and pumping station issue. Ever wonder why all of a sudden DHS was intensely interested in utilities? There is even a project, very in similar in nature to the SETI@Home project actively seeking UWB transmissions for counter CISR using massively parallel computing (Sensor SNAP) techniques. Can you imagine what this would do to the government-industry unholy alliance with the carriers if this replaces economically WAN communications? They made a mistake with Bitcoin (it’s traceable) but with UWB you may just really be invisible.

Leave a Reply