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How Many Frequency Bands Are There?
FoxIVX asks: "What is the carrying capacity of earth's atmosphere, in terms of pure bandwith? With radio, TV, HAM, citizens band, cellular, and countless other radio frequencies, each of them taking up space on the proverbial 'dial' what is left for the 'Wireless Revolution'? I know that, for now, radio-based data is slow and isolated, but what about the future, when everyone goes with cell phones instead of land-lines, and people start carrying around next-gen PDAs with full screen video capabilities and gigabytes of magnetic RAM? Does the spectrum of radio frequencies give enough room for this kind of data transfer? I know that with factors like distance/wattage, and various kinds of multiplexing you can squeeze more out of a certain wireless band, but there has to be some sort of a ceiling to it all. This could be an important new field as more and more areas and people go wireless. And this doesn't even touch on the issue of who owns the airwaves and who is going to regulate it all." Would the International equivalent of the FCC need to be formed to handle these kinds of issues on a global basis?
Not directly, but we need only look to cell phone to see part of the solution: more towers with lower power. Lets say there is a limit of 1 gigabit/second. (Obviously low). That is more then enough for me and a few neightbors. All I need is some way to get it to land lines which don't suffer the bandwidth problem.
In other words, I want high speed wireless, but I'd be content with a many cell phone like towers scattered around. In fact I prefer this model to others.
Even if someone invents technology that would allow my equipment to talk to anything else in the world via short wave I wouldn't want it. To power a signal around the world needs more watts then to send it to a local tower. There is no gain for me in the US use direct wireless to get to someone in Autrillia. I would much prefer much lower powered transmitters that can only go a short distance. Now if I was in the middle of the ocean there would be.
Remember our usage: lap/palmtops in the backyard covers most people. Sailors will need more, but there are not many of them (and they will probably want a bigger transmitter on the ship acting as a repeator to small ones onboard). Atsronaughts will need more, but they should be considered like sailors. (I'm being optimistic here and assuming that in 20 years more people have will have walked on the moon then currently drive a car)
Of course my point is that we don't need to worry because low power/distance transmittors have limits well byond our needs, and high power transmittors can be directional and in any case are not needed very much. Just think, we can get rid of the entire FM and AM dials in the future because eveyrone will have a digital device getting streams from the local tower. (Accually In propose that we keep the old AM towers for diaster - crystal sets are easy to make from junk and can be valuable in some cases)
The radio spectrum is a natural resource, nobody owns it.
Bands are a synthetic thing, what you actually want to know is how much bandwidth you can use. Essentially, we don't run out if we manage it well. The best way to manage it we know of so far is by using cellular techniques, which allow you to re-use the same spectrum every few miles, to connect wireless devices to the wired Internet. When spectrum gets tight, you build more cells, closer together, and reuse spectrum within smaller areas.
Where is the ceiling? Currently, it is defined by how high a frequency you can build an effective radio for. We can get into the milimeter waves, extremely high frequencies which theoreticaly contain much more bandwidth than we are using today. Current equipment for these frequencies is very primitive and tends to be wasteful of bandwidth, that will improve. Eventually we hit a ceiling defined by how well very-high-frequency radio propogates through objects - if it won't go through walls or windows, etc., its use may be limited to in-building use. There are also new technologies like spread-spectrum and ultrawideband that may allow us some additional frequency reuse.
The way the FCC is currently managing spectrum could be improved. They tried auctioning license rights off, and are still doing it, and this has resulted in 5 redundant bands for cellular phones, with about the same thing going on in each of those bands. If they'd worked out a way to better share the costs of the cellular infrastructure between vendors, we could have been doing the same thing in one band, building more cells as usage increased instead of adding more frequencies. .
Thanks
Bruce (K6BP)
Bruce Perens.
Penetration distance of radio waves through a non-conducting substance (like concrete) is proportional to the wavelength of the signal (very roughly). This means that ordinary radio has no problem going trough walls and floors, but that things like cell phone signals in the GHz range are more easily blocked if there are a couple of buildings between you and the tower. This problem will get much, much worse as frequency increases. Expect your 20 GHz wireless PDA to stop working indoors (unless you have a repeater).
Radio of conventional wavelengths will pass through rain, smog, and clouds with little difficulty. Higher frequencies, however, have problems. Again, this is just a question of there being a lot of matter between the transmitter and the receiver. This means that as wireless transmission moves higher up the microwave scale, you'll either have to space the towers more closely or have signal cut out whenever it rains.
IMO, the practial limit is going to be in the 10-30 GHz range, with degradation setting in long before that. This is more than enough for rural areas. In cities, the best approach IMO is to provide wireless service on a per-building basis, with a short-range wireless hub inside the building connected to a fiber grid networking the city. The frequency is practical, and the hubs will serve few enough users that everyone will still be able to download all the video clips and pr0n they want.
FM, AM, visible spectrum, and audible sound are mere blips in the size of the spectrum. You're talking about Ghz of space available, while these take up mere Khz.
Um, no.
The FM and AM spectra take up on the order of a few MHz, not kHz. Each station needs several kHz to sound decent, and there are many stations.
TV needs about 10 MHz per station to transmit video data, and there are many stations on your UHF dial.
Visible light runs from around 700 nm to 400 nm - a bandwidth of about 3.2e14 Hz (320 THz).
The question being asked is, "what is the total usable bandwidth within Earth's atmosphere for carrying digital data". Ignoring other things that use bandwidth, this ranges from 0 Hz up to the frequency range where rain and fog and walls block your broadcast data - somewhere in the double-digit GHz range.
This bandwidth has to be shared with all users within a tower's transmission radius. In a city, this will be a lot of users.
The nice thing is it's mostly packet data, meaning you can have many devices use the same frequency if you throw in some collision avoidance, same that's used for Ethernet.
Collision avoidance works by _reducing_ the data rate on each device when too many devices are trying to use a data pipe at once. It does NOT give you more total bandwidth - it just makes sure that any bandwidth available is allocated fairly and not wasted in an electronic shouting match.
For a bandwidth of "foo" GHz, you will have _roughly_ "foo" gigabits of _shared_ bandwidth between all users in range of one tower. The only way to pack in more data is to use analog transmission, and the power required to get more bits grows exponentially with the number of bits per sample (gets impractical very quickly).
This is a more detailed chart which lists the users/uses of the spectrum between 137MHz and 10GHz in the US. Here's one from the UK. And here is a more general chart posted as reply in this thread.
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http://www.naval.com/radio-bands.htm
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