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Wireless Growth & Wireless Interference
windowpain writes "An article in Monday's Washington Post says "The explosive growth of the mobile phone industry has crowded and tangled the nation's airwaves to such an extent that wireless company signals are increasingly interfering with emergency radio frequencies used by police and firefighters, public safety agencies said." Wifi is not a problem, evidently. Understandable, given its short range."
I'm not surprised it's the Washington Post who is reporting on this. I live in Washington, DC as a consultant, and travel a lot to other cities. I have had cell phones with three carriers, and with all of them I've had much worse trouble here than in cities elsewhere, without exception. On one hand, we have some serious density of equipment here, but on the other hand there are zones where one knows one will lose their cell phone signal; the worst two zones are adjacent to the CIA's facility and the Pentagon, for example. This sort of thing can't be helping the matter.
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802.11a and b currently operate primarily at 2.4 Ghz.
Actually, only 802.11b operates at 2.4 GHz. 802.11a operates at 5 GHz.
The emerging 802.11g standard is intended to deliver the same data rate as 802.11a but on the 2.4-GHz band.
The coolest voice ever.
the cutoff you describe is actually dependant upon the ability of the receive to discern between the signals. Has absoulutely nothing to do with anything on the broadcasting end. More expensive receivers may be able to tell the difference between 800.0000001 and 800.0000002, but you regular cell might static up from both of these trying to receive on 800.
Speak for yourself.
It's a combination of frequency, bandwidth, and a few other factors. For example, when something is at 800mhz, that's the center frequency. It will have a bandwidth associated that may be a few khz or a few mhz, depending on the application.
Then you get a combination of different technologies. Someone else mentioned TDMA, CDMA, GSM, and some others. Some of these operate in the same bandwidth without significant interference because the way they operate is so different. CDMA for example, doesn't suffer from the same kind of interference that TDMA, GSM and a few others do.
There's a technology called Ultra Wide Band that may take off over the next decade. While it covers a very large bandwidth (as the name would imply) that would normally interfere with other technologies, because of the way it operates, there's little, if any interference. The nice thing about it is, you can operate a number of different devices, each paid of sender and receivers has a "code" that allows them to communicate. Nothing else will interfere and many devices with different codes can operate in proximity.
Most technologies operate on the basic principal of sending a signal via the actual radio wave. They do this by modulating some part of the wave, usually frequency or amplitude (hence FM and AM radio).
UWB doesn't do that. Instead, it sends pulses very quickly (in picosecond lengths) and the pattern created by the pulses (think binary, on and off), are what are used to create teh signal.
Another advantage of UWB is that it works at a lower power (given the same range). Also, unlike the higher frequency applicatons (particularly when you get up to 5+ ghz), it doesn't have the same issues penetrating walls and such. Nice technology. Hope it makes headway.
IANARFE (Radio Freqency Enginerd)
I think this is caused because the FCC specs say that a device X must use frequency y, and less than backgound noise on other frequencies.
Now, every RF transmitter is a analog device and as such it's impossible to transmit perfectly in the mandated frequency, so some leak to the neighbouring frequencies is inevitable. Still OK, a single device X will not produce noticable interference, as it produces less than background noise.
Now, instead of one device, you get thousands of RF transmitters, and the interference starts adding up, finally surpassing the background noise.
I believe this is also the reason why people are worried of UWB tech as well.
Instead of measuring interference produced by a single device, FCC will probably have to move measuring the interference created by an operational, full network.
Ofcourse this begs to ask why FCC allocated 800Mhz for mobile phones while knowing public safety departments use it AND the whole fucking entire other world uses 900Mhz. Thus making american phones incompatible with the entire other world for a long time...
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The basic argument, like the article makes for first responders, is that the military band for communications is being encroached upon by civilian use. Having fewer frequencies directly impacts the military's ability to conduct training operations and exercises.
The Navy (Department of Defense) has a page which educates visitors and range spectrum users on how to defend against civilian encroachment of DoD frequency spectrum.
The Electromagnetic Spectrum Training Chart shows military uses of certain frequencies and the competing civilian use.
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I really don't know about that. Just as receivers have tolerances, transmitters do too. So if I have a crappy transmitter and I tell it to broadcast at a certain frequency, there will be a certain tolerance there, unless I'm using a laser. They're certainly not. Even with a good transmitter, there's a certain +/- to the frequency distribution, although presumably less.
So ultimately, his question was quite a fair one - for someone to tell the difference between two signals 800.0000001 and 800.0000002, the two transmitters will have to be good enough to send out precise, narrow signals, and his receiver will have to be good enough to tell the difference.
-Looking for a job as a materials chemist or multivariat
that really depends on the bandwidth you need.
but, short answer, is no.
if you have a bandwidth of 1Hz, as you are suggesting, this is much too small to transmit any useful inf at any speed. for analogue voice, you need at least a bandwidth of ~30 Khz, and thats without the overheads of initiating communication, etc. now, GSM uses digital communication, which has a higher bandwidth, but means that you can multplex (which is essential, othewise you would just run out of spectrum).
add on top of that more BW for housekeeping and you can see why you are running out of space.
spread spectrum, now theres a thought.......
-----im billy troll----- im better than you at everything you do.
The amount of bandwidth basically depends on you much noise there is around- you can pack much more data into a narrow channel if the channel has hardly any noise, whereas if the channel is very noise you'll need a wide channel to send the same data.
Also in practice you need a gap between neighbouring channels- the receivers need to filter out the other channels and they don't do this perfectly (although the better the receivers are, the closer you can pack them in.)
However Shannons law only deals with broadcast communications where everyone can 'hear' everyone else equally, if you use directional antennas then it doesn't necessarily apply- two communications could use the very same channels.
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"NEXTEL is the prime culprit. The problem stems from what NEXTEL is.
a NEXTEL phone appears to be a cell phone, but legally it is a handheld, trunked two-way radio with full-duplex capability and access to a phone patch. While this accurately describes a cell phone, the evolution differs.
"Cell phones" that follow the CDMA, GSM, TDMA or even AMPS (analog) standards are all using standards that were originally developed for use as telephones. They are licensed as telephones, and use portions of the spectrum that are reserved for telephones.
Cell phones that follow the iDEN standard (this would be NEXTEL) use a protocol that evolved from a half-duplex digital trunked two-way radio system.
What NEXTEL used to be in the business of (under a different name, which I can't recall at the moment) was providing nationwide fleets with trunked digital two-way radio service. Someone in NEXTEL had this brilliant idea that if you could add full-duplex capability and a massive banked phone patch that you could compete with the cellular companies, and this is what they did.
The problem with this is that NEXTEL, being an operator of a Commercial Land Mobile Radio Service (CLMRS), gets to license portions of the spectrum intended for Land Mobile Radio Service users.
...and there is the rub. Police, fire departments, ambulances, tow trucks, taxis, and just about every business except sometimes aviation and maritime businesses, are users of the Land Mobile Radio Service. Like NEXTEL, they license portions of the spectrum that are there for the Land Mobile Radio Service (there is no distinction in spectrum allocation between a commercial and a private LMRS license).
The biggest pain here is involved in that NEXTEL does not operate on the same frequencies everywhere. They go from location to location, licensing 5-20 frequencies in the 800MHz LMRS band (and I think sometimes in the 900MHz LMRS band) in any given location, but the frequency that they allocate in one city may be the same frequency that a police department in another adjacent city is using.
Yes, the FCC probably should have prevented this. However, their allocation scheme was good enough for systems that had a low duty cycle, and it worked for decades. Introduction of a high duty cycle system such as NEXTEL broke the system.
Incidentally, I strongly advocate that police, fire and other services should not move to 800MHz trunked systems if they don't have a serious need to. The fire department in my home town of Selkirk still uses the same frequency (46.06MHz) with no repeater, no trunking, nothing except for a remote base (which communicates to the three firehouses on 460.6375 and 455.6375MHz) and provides complete coverage for the fire district with minimal vulnerabilities, including immunity from interference from NEXTEL.
BTW, WiFi is not a problem because the frequencies are nowhere near 800MHz.
www.wavefront-av.com
Interference is a relatively new part of this problem. For decades, public safety has had "holes" in coverage caused by hilly terrain blocking the signals. Same thing happens with cell phones, but the cell phone companies can afford many more transmitter sites to fill in the gaps. Put another way, public safety coverage will never be as good as cell phone coverage, for reasons that have nothing to do with interference. Reallocate the spectrum all you want -- it won't cure this problem, but it could help with the newer interference issue. What might also help is independent third-party review of public safety radio system designs that are originally drawn up by the same companies that are selling radio systems to the public safety agencies. Some agencies have no idea of what they ordered or what performance to expect or, moreover, the tradeoffs between cost and performance.
I'm going to ask a question that may sound snide, but is actually very serious: Why did you go to 800MHz? Were you using VHF-Lo, VHF-Hi or UHF before? What was the coverage like?
I ask this because in my hometown of Selkirk, NY, they use a system that is a hybrid VHF-Lo and UHF system. VHF-Lo (46.06MHz) covers the town, and the UHF is only used as a remote base, with the base radios at the three fire companies talking on 455.6375 and listening on 460.6375. It works very well. My father, who is a member of this fire department, has been fighting a movement to "upgrade" to 800MHz.
At my current locationin Schenectady, NY, they use 460.6000. It works well and has no dead spots that I am aware of. I will fight any plan the city puts forth to "upgrade" to 800MHz.
www.wavefront-av.com