Slashdot Mirror

Attenuation, plain and simple. At 145MHz, the attenuation of 200' of ok coax (LMR-400 @ $0.86/ft) is almost exactly 3dB - that's 50% of your transmitted power wasted heating the coax. Additionally that makes your receiver 3dB more deaf to weak signals.

At 450MHz, it's even worse. Attenuation is 5.4dB, which steals about 71.3% of your signal.

Stepping up to 1/2" Andrews DF4-50A heliax, you find the price jumps to $1.69/ft but the attenuation at 450MHz drops to only 2.9dB for 200'. We're still losing 50% of our signal.

Stepping up again to 7/8" hardline (3.99/ft), attenuation is 1.44dB @ 450 MHz - you _still_ lose 30% of the signal.

By putting the transmitter, computer, etc at the top of the tower, you get the best of all worlds - very low attenuation in the RF path, no long computer cables to pick up stray RF or lightning-induced surges, and fewer vandals. The downside is that you have to run power up the tower (usually 120VAC, since running 12VDC hits the same sorts of snags as coax - lots and lots of loss. Imagine 200' of welding cable - $$$ and heavy!). As long as you have a decent AC filter up the tower, you're safer than separating the components. The downside, however, is a beastly climb if anything fails catastrophically.

I just happened to be involved in the university project that produced this patent. The patent was filed before I got involved, so I can't comment on the perceived obviousness at the time of filing (or any other aspect of the filing). From personal experience, in 1995 most people I spoke to about what I was doing didn't "get" it and questioned why anyone would bother doing such a thing. It's hard to tell how much of that was due to the technology being non-obvious, or how much was due to applications being non-obvious.

It's interesting that there is only one name in common between the list of authors on the patent and the paper, and that person isn't the lead author on the paper. I guess that might be because the paper is about the second implementation. The first implementation, on which the patent is presumably based, was done in software in non-real time (burst mode). If judging obviousness, it would be worth comparing with the HiperLAN project and the work that went into it.

It is only hard to distinguesh from the background noise because you don't really know what to look for. That does not mean the destined receiver doesn't know what to look for. According to this website, commercial SS radios can work with noise levels 5dB higher than the signal strength. It also explains a few techniqies used.

http://www.sss-mag.com/uwbp3.html

There's no doubt about it.

http://www.sss-mag.com/ss.html

Wideband (like FM) is wideband, that's different.

Actually Part 97 specifically talks about rules for spread spectrum operations,

http://www.sss-mag.com/hamss.html which basically state no more than 100 watts (and 100 watts is pretty leathal on 2.4ghz btw - thats where water resonates)

That whole 1500 watts thing is a huge misconception. To run that much power typically you need 220vac, also many bands prohibit anymore than 200 watts (30 meters and 60 meters are good examples). Never mind you'd be interfering with everyone in the neighborhood.

FredFnord's right. 802.11 is CSMA, just like ethernet. There is no collision detect, particularly because of the "hidden node" problem, and the lack of a full-duplex radio.

So, they've implemented a RTS/CTS handshake (which are themeselves packets) to avoid collisions.

Pretty good writeup here.

All of this is why you get 4Mb/s on a good day out of 11b, and a similar efficiency from 11a/g.

Here is a primer on spread spectrum. It covers some points you might find interesting. like: The military uses it because it is resistent to jaming AND when you use it you are less likely to jam another broadcast.

And why is it necessarily Nextel at fault

It's funny, I always wondered why cellphones had always been banned in RF-sensitive areas. Back when I had a Verizon cellphone on CDMA, my frequency sniffer never reported any activity coming out of my cellphone, being spread spectrum (aka "We live under the noise"). Then I got a Nextel, and when I walk in the room, my sniffer chirps to indicate that it's capturing freqs off my phone. Totally different beast. My Nextel phone puts noise bursts on my tv from 8 feet away. It also does it in the car but from less than 6 inches. Still, my CDMA phones *never* did that sort of thing, and I suddenly realized why cellphones were historically banned from Intensive Care Units and airplanes. Scary stuff. I don't even like putting it next to my head or carrying it on my hip too much.

What limits throughput on a wireless connection? Is it broadcast power, frequency, amount of spectrum, or "c"?

All of the above!

The most limiting factor is the amount of spectrum -- the bandwidth allocated to the signal. If you're restricted to a very narrow set of frequencies, you can't send as much data in a given amount of time.

But, once you've picked the bandwidth to use, broadcast power and frequency each affect how well the transmitted signal can be received. Lower frequencies pass through solid objects more easily, for example, but often require more broadcast power. There are myriad details affecting the quality of a signal as it passes from the transmitter to the receiver, not the least of which is interference from other transmitters.

And finally, "c" ultimately affects the latency of your signals. NASA's conversations with the Voyager spacecraft would be an extreme example: it takes about 25 hours now for a signal to make a round trip from Earth to Voyager 1 and back. Closer to home, even Earth satellites are far enough away that the speed of light becomes a limitation -- the round trip to a geosynchronous satellite is 0.239 seconds. If your signal is mostly one-way, this has very little impact. Television, for example, isn't hindered by this. But any kind of interactive connection like online gaming or even telephone conversations are adversely affected by that amount of latency.

I still think that the emerging spread spectrum technologies are our best bet for wireless computer networking.

- Peter

Jay J. Ely and team are pretty much the leaders in tearms of research in this area, as the NASA Langley Technical Reports Server shows.

You can get actual reports of incidents related to PEDs and aircraft events at The National Aviation Safety Data Analysis Center .

Also in Oct 2002, at the Digital Avionics Systems Conference in Irvine, CA, Session E addressed this topic:

Session E - The Electromagnetic Environment
Co-Chairs - Paul Cox, Honeywell Defense Avionics Systems Bill Larsen, Federal Aviation Administration

• "A Description of the Software Element of the NASA Portable Electronic Device Radiated Emissions Investigation" Sandra V. Koppen
• "Determination of Receiver Susceptibility to Radio Frequency Interference from Portable Electronic Devices" Truong X. Nguyen
• Avionics Interference from Portable Electronic Devices: Review of the Aviation Safety Reporting System Database" Bill Strauss
• "Ultrawideband Electromagnetic Interference To Aircraft Radios" Jay J. Ely
• "Electromagnetic Interference Assessment of CDMA and GSM Wireless Phones to Aircraft Navigation Radios" Jay J. Ely
• Investigation of RF Emissions from Wireless Networks as a Threat to Avionic Systems" Maria Theresa P. Salud

Parent post is plagiarized from http://www.sss-mag.com/hamss2.html.

(Don't bother to answer, it's obvious.)

1. From http://www.aatl.net/publications/contran.htm
   • "The mechanics of blocked transmissions are easily understood. In general, it may involve simultaneous transmission from two aircraft or simultaneous transmission from an aircraft and ATC. These transmissions interfere with each other, "blocking" both signals. There is a widespread belief that such blocked transmissions would always be detected (i.e heterodyne heard by the pilot and/or ATC). An equally widespread belief, held by controllers, is that ATC transmissions are always heard. Neither is necessarily true. However, even if detected, it is often too late to do anything and aircrew and ATC are usually too busy managing events in progress."

   The person who has the mic keyed isn't going to hear anything, and thus won't know their transmission wasn't received. The whole point is to notice that a communications breakdown happened and correct it before something bad happens.

2. I tried to make it clear that -41.25 was the very top end of allowed emissions for UWB. When GPS frequencies are at stake, the limits are much lower.

   Read the slides at http://www.sss-mag.com/uwbslides.html -- that's the "first link" I referred to before.

   When referring to GPS frequencies, the FCC limit for indoor & handheld outdoor UWB devices is -75 dBm/MHz. GPR, wall imaging, and medical imaging systems are limited to -65, and thru-wall imaging and surveilance systems can go all the way up to about -53 dBm/MHz -- but only law enforcement, fire, and rescue organizations get access to that equipment.

   The IEEE Powerpoint presentation has some interesting information on noise floors -- you might want to read that.

I must note one thing, though... you'd make an excellent scare-monger. You've got the Star Trek-style technobabble down pat. Have you considered a position as a lobbyist? How about running for congress?

This is the end of this thread from my end, I've made my point. All the info is there for those who care to read it.

I run Spread Spectrum Scene, a site devoted to RF and wireless technology. It was listed as an Online Sales site. We will see how effective the request for removal is!

Here are a few more articles on the subject that IMHO are a bit more substantive:

From BusinessWeek

From M Commerce Times

Some Information on its Problems:

A brief primer from the Ultra Wideband Working Group

And a very in depth look at the history of UWB

-john

I was looking for this before but couldn't find it. A much more detailed discussion of spread spectrum modulation (with pictures :-)
http://sss-mag.com/primer.html#c

Wow... Years without a single post. And now two in less than an hour

1 2 3 4 5 6 7

There's a few to start you off.