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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?
Example: Newtonian kinetic energy is mv^2/2. Special relativity shows that, yes, mv^2/2 is correct if v is small. As v gets larger, a different expression (not going to look it up now) is more accurate. So the implicit assumption in the statement "kinetic energy is mv^2/2" is "if v is small."
Similarly, technological advancements often occur not because someone changed their answer to the question of "how fast can you go", but rather someone changed the question because they didn't like the answer.
That's where the real genius is. Answering questions is one thing, but realizing that your are asking the wrong question to begin with is another.Bruce
Bruce Perens.
Bruce
Bruce Perens.
To state it simplisticaly, you can get something less than 1/2 symbol per second per Hertz, and if you use phase for encoding, you can have more than two symbols, so this is more than 1/2 bit per second but in practice less than 15 bits per second.
The key is reuse, not carrying capacity.
Bruce
Bruce Perens.
A while back there was a /. story about Ultra Wideband radio technology. According to Time Domain's webpage, the FCC has recently (May 10, 2000) "adopted a proposal to consider permitting the operation of Ultra-wideband (UWB) technology." If the US government ever decides to stop strangling this technology, there wouldn't be nearly as much of a need to move into the higher gigahertz frequencies.
The problem is that, while UWB transmitters might be easier to build than conventional transmistters, they still _use_ the higher spectrum frequencies (data is just spread out over the time and frequency domains instead of just the time domain).
If atmospheric and obstruction effects cut off everything above, say, 30 GHz, and your wideband transmitter makes use of parts of the spectrum above the cutoff point, the received data will be garbled (what will actually happen is that the pulses will smear out and start interfering with each other).
UWB is an interesting technology, but the data rate limits imposed by bandwidth limits are independent of the encoding of the data (see my posts re. analog transmission for the caveat to this).
Put another way, 1 MHz of radio bandwith does not equal 1 million bits per second, at least not as far as my limited knowlege implies.
While this is true, there are strong practical limits to how many bits per sample you can have.
The problem is that to encode n bits in one sample, you need to have 2^n distinguishable analog levels in your sample. You also can't space these levels arbitrarily closely - noise from your electronics and fundamental limits to the certainty with which you can count the number of radio photons in your sample both limit your spacing. As spacing grows exponentially with the number of bits, you soon reach a limit for any given power level.
In principle, you can just increase the power to compensate, but the power required goes up exponentially once you hit your level spacing limit.
In practice, you typically have only a handful of bits per sample to keep the power requirements sane.
In theory, the electromagnetic spectrum spans from zero Hz to infinity Hz, but it's not practical to use it all.
Low frequencies need large antennas. Nobody wants to hang a 160 meter dipole off their web-pad, do they? No. And high frequencies become extremely line-of-site and easily attenuated or blocked. You don't want to have to precisely aim a laser out the window at your ISP either. You probably don't want more than a few cm of antenna so a minimum freq. of what, 2 GHz? And anything over maybe 25 GHz will be absorbed by a heavy shower of rain, so maybe that will be a practical top limit.
Antennas (and your radio-connected web-pad will need one) are designed to operate at a resonant frequency. They will function when operated off-frequency, but with reduced efficiency. You probably won't get practical operation at about more than +/-10% from your resonant frequency, so if you have a resonant frequency of 20 GHz your web-pad won't want to transmit any lower than 18 GHz or higher than 22 GHz, so you have a useable radio bandwidth of 4 GHz, and that is very line-of-site and with lots of path-loss. Drop your carrier to 10 GHz will improve on the path-loss and directionality, but halve the radio bandwidth.
When you modulate a carrier it occupies more bandwidth as the data rate increases. Someone (Nyquist?) says that your bandwidth usage is twice your data rate. At a 20 GHz carrier you only have 4 GHz useable radio bandwidth (the antenna won't handle anything wider) so your data-rate can only be 2Gbit/sec. That 2Gbit/sec has to be shared by everyone within range of your transmitters. Using CSMA/CD (Carrier Sense Multiple Access/Collision Detection) you can share this bandwidth, but there is a theoretical maximum data rate which is less than the unshared maximum. The figure 18% is ringing a bell - someone please correct me! Anyway, 18% of 2 Gbit/sec is 360 Mbit/sec, assuming that nobody else is using sharing the bandwidth. Multiple users will cause interference with each other, pushing the actual, practical data rates way down.
Reducing the size of "cells" and increasing their number will help. The cells can be linked via fibre. The range to the heart of the cell will be smaller, so path-loss/attenuation will not be as important a factor, allowing the use of higher carrier frequencies, giving higher data rates.
Who knows? Some of what I just said might even be correct!
73, de Gus
Eight Papa Six Sly Mongoose
I was with you until the last paragraph. I'll assume that in your last paragraph you are refering to the frequencies allocated for PCS use (ie digital cell phones). There are not 5 redundant bands, but one band in the 1900 Mhz range that has been defined for use with PCS. This band has then been split up into several different blocks that can be auctioned off individually to different operators. The reason you want to have several blocks available in the same band is to promote competition and to allow the "Mom and Pop" shops a chance to enter the market. This is no different than how things are done for, say, GSM in the rest of the world.
I'm not sure that I understand your comment about costs of cellular infrastrcuture though. Vendors are the ones that build the equipment (usually called manufacturers), you are probably referring to operators here. Assuming you mean operators, why would they want to share the cost of the cellular infrastructure? They each have to build their own network in order to accomodate their own customers? Why would I let a competitor use my base station? Most of the signalling and such that takes place on the ground is done using leased-lines, so the cost for that portion of the network is already shared anyway. As I already mentioned, they are already all using the same band, so that isn't a problem. And when usage increases, they do just as you suggest: add more cells. Adding more cells is possible for all operators using the blocks that they have licensed using frequency reuse patterns.
Disclaimer: I work for Ericsson, however these views are my own and have not been endorsed by my employer.
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I know the question already touched on this somewhat, but multiplexing is basically the way to go with this... CDMA can already squeeze more than 12 times the bandwidth out of a frequency and it's bound to only get better.
I'd say the only problem with this is that it makes the hardware more intricate and more expensive. CDMA (Code Division Multiple Access) requires precise power regulation because nothing can be louder than another sender... This means that your power has to be ramped as your distance from a cell changes and handshaking with new cells is more complex as well.
I also think that there are a lot of bands which are currently allocated that should be scrapped for newer tech or at least re-appropriated... Nextel, the wireless company, for example operates on what used to be a 2-way business radio band. Because of this they're in almost every major market but didn't have to bother with licensing. At the same time, their frequencies aren't necessarily guaranteed either. I could definitely see a lot of the PDA stuff getting into this band if a standard's ever developed.
Check out alt.cellular for a lot of good info on this stuff...
-- atomly
It's called the International Telecommunications Union, or ITU for short.
It's homepage is here.
It's purpose is to develop and foster global standards for bandwidth usage, among other things. Most modern countries have communications ministries or bureaus that abide by them (the FCC for example).
ozone pilot
Finding and common band is a real problem. The Bluethooth folks assumed that they had it all worked out and spend billions on infrastructure, ASICS, etc. Bluetooth just got a nasty shock. The French military refuses to open up their portion of the 2.45 MHz band required for Bluetooth. They could very well make it illeagal to have a bluetooth device in France. Imagine getting your laptop nicked at the airport because it has Bluetooth! The truth is, their is no universal chunk of bandwidth in the world and the death of Bluetooth is going to prove it.
I wholeheartedly disagree...
Our understanding of physics may change...and we may find clever ways to get around limitations... but this does not change or invalidate physics.
Normally "limits" that are broken are NOT defined by physics but by other things...the need to interoperate with existing systems is a big one. Current day manafactuing technology is another.
These are not physics. If you push something beyond the limit that current physical laws dictate it must have...then you have undeniable proof that those laws are wrong and must be further researched and modified to meet the new data. THAT is the very essence of science.
"I opened my eyes, and everything went dark again"
As for a "global FCC," well that's just a huge stinker of a solution. After all, look at the marvelous job they do here in the US, holding back low-power FM for years so that the mega-media could dominate/satuarate/placate the masses....
sig not found
There already is an organization, the Iternational Telecommunications Union (ITU) that administers international RF frequency allocation on a nation by nation basis, among other things. It mostly deals with surface to space and long-range bands, and adjudicates international bandwidth disputes. It is then up to national governments to administer their spectrum as they see fit.
Of course, I'm not sure this answers the question posed -- it just shows how frequencies are used, but doesn't show how much "bandwidth" is available.
I'm not sure of the easiset way to answer that question, anyway -- think about telephone lines, for example. Used to be, everyone figured that they had an "audio bandwidth" of about 3000 Hz (or am I way off here?) So you might figure that means about 3kbps total maximum throughput. However, we're getting 56k (or so) over those same lines, through clever use of multiple channels, multiple bits per baud, etc, etc.
Put another way, 1 MHz of radio bandwith does not equal 1 million bits per second, at least not as far as my limited knowlege implies.
So, maybe, the question is really this: If we scrapped all existing modulation systems (FM, AM, whatever), turned all communications into digital bits, and selected the best (most efficient, best range, etc.) scheme for modulating and encoding those bits, what's the maximum bandwidth available? Interesting question, but basically academic, 'cause I don't see everyone throwing out all their TVs, radios, and cell phones for a maximum-efficiency digital system.
And, besides, isn't sub-space communicaiton right around the corner? :-) david.
IIRC, the carrying capacity of our atmosphere is about 12.
Is that an African Atmosphere or a European atmosphere?
Let's ask Mr. Owl!
Mr. Owl: One... two... three... **CRUNCH**
Three.
-Denor
Low-grade primer on EM radiation frequency and wavelength: Speed == Wavelength * Frequency. Travelling electromagnetic waves all have the same speed (3x10^8 meters/second), but different frequencies. Different colors of visible light (that you can see with your eye) have frequencies on the order of 10^-15 seconds, hence wavelengths on the order of 500x10^-9 meters == 500 nm. UV light is around 300 nm, blue is around 450 nm, green is about 530 nm, red is 700 nm or so, infrared starts around 800 nm, etc. So visible light frequencies are around a petahertz (a million gigahertz). As another poster mentioned, this means that really high-frequency EM waves, like visible light, don't transmit through walls and trees (nor even curtains) very well, but you already knew that was true. :) ) This problem is overcome by sending the light down fibers that can make it bend around walls.
So here's my main point: We should worry about how many different wavelengths (or frequencies, or colors) we can discriminate among within a certain frequency band. The reason people like the guy down the hall from netcurl use visible light (and near-visible light like ultraviolet and infrared) to send and receive signals is that one can get amazingly monochromatic light out of a laser. For example, I used to use a (yttrium vanadate) laser that emitted light at 532.40 nm plus or minus 0.03 nm (I forget the exact figures). That means, in principle, that we could send signals at 532.4 nm, 532.6 nm, 532. 8 nm, etc. simultaneously down the same fiber.
However, discriminating among these different colors is kind of hard because of color filters not having sharp cutoffs and because of frequency-spreading that can occur in fibers. The cutting edge of research in fibers, then, is largely in a) making fibers that prevent or correct for spreading, and b) finding clever ways of distinguishing between two nearly identical colors.
I've probably forgotten something, but hope this helps.
--jd
One of these days/I'm going to cut you into little pieces.
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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