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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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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.
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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A radio signal, unless it's a pure sine wave, uses a certain amount of bandwidth. Double the amount of data pushed through, and you double the bandwidth used, all other things being equal.
For example, my R/C plane uses 72.450 mhz, and the bandwidth used is 20 khz, so 72.441 mhz -> 72.459 mhz are what's in use. Well, that's what's allocated anyways -- the radio should use a little less. The transmitter should not transmit signifigant amounts of power outside of that range, and the receiver should ignore any signals outside of that range.
(For comparison, morse code can use less than one khz of bandwidth. FM voice transmissions (such as used by police radios) take up 3-6 khz. SSB and FM use less, but the sound quality is less too. More bandwidth = more quality, though certain modulation types work better with little bandwidth. I don't know how much bandwith a commercial FM radio station (88-108 mhz) uses, but it's probably more like 100 khz (after all, it's music.)
If you start reducing the bandwidth used, you lose data. If it's an AM signal, you'll start losing the higher frequencies. I'd have to think about what it would do to other modulation types ...
The real problem here is that the FCC has done some really wonky allocations of bandwidth. For example, the 72.450 mhz frequency I mentioned earlier -- the 72 mhz band is for R/C planes, but it's also shared with pagers. So 72.450 mhz is channel 33 for planes, but 72.460 mhz is probably used by a pager company. If they put their tower near our R/C field, and I fly my plane over near that end, and a page goes out ... their signal can overpower my wimpy little 1 watt transmitter, and guess what happens to the plane? (Actually, 1 watt isn't that wimpy -- R/C plane transmitters have a range of about 1.5 miles. But the pager tower may use something like 750 watts ...)
In any event, that exact same problem has happened in the 800 mhz band -- 801.4 mhz may be allocated to a police band, but 801.5 mhz may be used by a cell phone tower ...
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.
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