What would be the disadvantages of using analog amplitude modulation [similar to AM radio] at 300 MHz frequency for wireless internet connections? Let's say the modulator signal is attenuated [to prevent clipping due to excess signal amplitude and to prevent interference with nearby station] prior to D-A conversion and transmission. At the receiving end, the carrier signal is amplified [so it can be recognized by the receiving computer] prior to demodulation and A-D conversion. In addition, the receiver is DXed. DX is a radio technique to receiving distant stations.
What would be the disadvantages of such a wireless internet connection?
At 2.4GHz, Wi-Fi requires about 22Mhz occupied bandwidth to move
54Mbits/sec raw data rate. At 300MHz, there is no service that will allow you to monopolize 22Mhz bandwidth. The bandwidth problem also applies to antennas, where keeping the VSWR reasonable over the 22Mhz bandwidth at 300MHz is difficult.
Also, if you're planning to do this without a license, on top of existing users, I'll be the first to turn you in to the FCC enforcement burro.
Ummm... This is gibberish. Try again please.
802.11 at 1 and 2 Mbits/sec is pure PM (phase modulation). There's no AM component. However, all other higher speeds combine a mixture PM and AM to maximize the spectral efficiency in bits/Hz. If you were to try this with pure AM, in the same 22Mhz bandwidth, you would waste
50% of your power on the carrier, and half again in each of the two symmetrical sidebands. I'm too lazy to calculate the exact number, but my guess is that you would get about 4Mbits/sec maximum data rate with pure AM in 22MHz occupied bandwidth.
Note that there are systems that actually do use pure AM and its close relative on/off keying. The advantage is that it's fairly easy to compensate for doppler shift effects. RFID is a good example.
Nope. DX is the art of exchanging useless information with a distant station, via a weak signal, out of a mess of noise, using rather expensive equipment, in the presense of interference from hundreds of other DX'ers, for the sole purpose of obtaining QSL cards, that substitute for wallpaper. More generally, DX is a sport.
It won't have the capacity of a system that uses both PM and AM.
It won't be legal.
It will have bandwidth restrictions due to the lower frequency that will substantially lower thruput.
It will interfere with existing services on the frequencies you plan to trash.
It will not be backed by a reputeable standards group.
It pisses me off.
No. Doing this under the cover of darkness will not make it work or prevent getting caught.
What if the carrier frequency is 300 GHz instead of 300 MHz?
Also, if all wireless hot-spots were to use 300 GHz analog amplitude modulation, would this be a problem?
So most wireless telecommunications use PM?
The on/off you describe is digital. I am asking about analog AM.
DXing increases the reception of heterodynes. How do heterodynes affect wireless networks running on analog amplitude modulation radio signals?
Okay.
Okay. So use 300 GHz AM instead.
Not if the signals are strongly attenuated prior to transmission.
I am asking about the scientific disadvantages, not the social, legal, or political drawbacks.
It's not a matter of getting caught. I am asking because I notice heterodynes to be louder on the AM radio at night while much softer [sometimes even absent] during the day.
300Ghz is almost optical. Officially, it's sub-millimeter microwave. You could not afford the hardware. It's mostly MASER and LASER based. The FCC setup (and sold) the 30GHz LMDS band and service for local distribution. Very few systems are deployed because of the limited range, very expensive hardware, and inability to penetrate anything. The good news is that you probably would not interfere with anyone at
300GHz. Oh yeah, the world ham DX record at 300GHz is about 10km.
No problem if you don't mind carrying a rather large pile of waveguide, a pair of dish antennas, and can tolerate a typical range of a few cm.
Most wireless starts with PM. It's cheap, easy, cheap, reliable, cheap, and by the way, cheap. However, to squeeze more data in the same occupied bandwidth, an amplitude component is added on top of the PM. Start reading here:
Wrong. It is very difficult to distinguish between analog and digital modulation. For example, I have a class E linear amplifier design for AM and SSB. Digital techniques for analog modes. See the list of modes at:
in the box on the right. Note that they are divided into Analog, Digital, MUX, and Spread Spectrum. Note that some modes, such as QAM, appear as both digital and analog. I'm not sure I agree that OFDM should be classified as MUX instead of Spread Spectrum. Anyway, don't worry about whether it's digital or analog.
You seem to have some attachment to AM modultion. Let me just say that there's a reason that AM was first to be invented. It's very easy to generate and detect, but has serious limitations. The worst is that half the power is wasted in the carrier. That puts AM at a serious disadvantage to other methods over spectral efficiency and power efficiency. It's no accident that FM and SSB were invented shortly after AM was determined to inadequate. The various digital modes followed soon after in order to improve spectral efficiency even more. See:
Trying to run a wireless system on AM would be like turning back the clock of progress 80 years.
Hetrodynes are a method of mixing two frequencies to produce a 3rd frequency. It has nothing to do with channel carrying capacity, the modulation mode, or the distances (DX) involved. More specifically, absolutely NOTHING inherent in the modulation or occupied bandwidth has any relation to the distances (DX) involved.
Well, feel free to ignore the social, legal, and political issues and see how far you get. We had a local bootleg microwave link that was trashing communications. We also has a clown running about 10 watts ERP on his 2.4GHz cordless phone in the downtown area. I've also seen overpowered 2.4GHz 802.11b/g systems. I was involved in taking them off the air.
Sure. At 1MHz, propagation issues are paramount. That's why AM broadcast stations vary their power during daylight and nightime operation. Different frequency bands have different characteristics during different times of the day. They are also affected by atmospheric ionization depending on whether the sun is visible or below the horizon. Start reading about RF propagation here:
However, ionospheric effects disappear above the MUF (maximum usable frequency) or about 25MHz maximum. For VHF, UHF, and various microwave frequencies, propagation is mostly affected by simple inverse square law and atmospheric oxygen and water absorption. See curves at:
The trick is to pick a frequency that doesn't get easily absorbed. That's not easy as all the good ones are already taken.
You haven't bothered to disclose what you're trying to accomplish, but that ok. I can answer your questions in general terms. Before you attempt to do anything new in the area of wireless, methinks you should assemble a suitable background and experience level using existing technology. That will save you the frustration of building something that can't be deployed due to technical or legal limitations.
If you strongly attenuate your signal prior to transmission, unless you use a super cooled receiver, the range of your system would be useless. In other words, you would not have a viable system.
Useless. Once again, you can't use techniques that work at AM broadcast frequencies at microwave frequencies.
You can get away with attenuating the receive signal at frequencies below about 7MHz because the atmospheric noise (mostly from lightning hits) is so much higher than the receiver front end noise level. It makes not sense to have a very low noise front end when all you're amplifying is atmospheric noise. At higher frequencies, the front end noise and later the thermal noise increases faster than the gain of the front end amplifier. The result is that at microwave frequencies, the bulk of design effort is overcoming the base line noise level, while still retaining enough dynamic range to produce a useful product.
Jeff...you remember the fanfare about 30 years ago with the idea of replacing FM with SSB AM for mobilephones, public safety, and general business frequencies?
best bet would be to use an SHF frequency. High enough for sufficient bandwidth, low enough not to be opaque to the atmosphere. Am I on the right track?
Yep. I lost about a years income on ACSSB (amplitude companding single sideband) projects and investments. The major player 30 years ago was SEA (Stevens Engineering Associates).
I was working for Intech Inc (Santa Clara) manufacturing conventional marine SSB radios for Dick Stevens while his company was working on ACSSB. There were plans to also build ACSSB equipment when it finally arrived. I'll spare you my bad attitude and simply say that a somewhat related project was what finally made me quit Intech in disgust after 9.5 years.
Over the years, ACSSB technology as improved little. It has some major advantages for dealing with weak signals, something that few users have any real interest in doing. Equipment is expensive. It's also fairly useless for data transmission, something that every service is interested in doing. The best you can do is Telex speeds. There were some systems crammed into the VHF commercial band, which were later moved to the former 220-222Mhz ham band. Most of these were attempts to use ACSSB as if it were a replacement for land mobile FM. These systems were hardship cases, where ACSSB was the only type of license that they could obtain from the FCC and their frequency coordinators. ACSSB didn't work too well. There were a few local repeaters and trunking systems operating on 220MHz a few years ago, but all the licenses expired on Aug 1, 2003. I don't recall what happened to them.
Fairly recent story on 220MHz band status:
Incidentally, the FCC re-assigned the 220MHz ham band primarily for the benifit of UPS (United Parcel Service).
When UPS discovered that the system would cost them something like
5-10 times the cost of a NBFM system and that it wouldn't do data (for tracking and AVL), they backed out of the deal.
So, what is your application? You haven't bothered to disclose what you're trying to accomplish. My crystal ball is quite good at guessing, but with zero input, it's not doing too well.
If you're going to do something new, it's always best to prototype it on the various license free bands as described in FCC Part 15.205. This covers the limitations and explains the legalese better than the the original FCC docs:
If you know a politician or are able to provide a suitable bribe, err... donation to a politician, you can possibly get an STA (special temporary authority) which gives you a get out of jail card to use when your contivance clobbers the local licensed services.
I have no idea. You haven't described what you are trying to accomplish. Obviously, the choice of frequency is important. The problem is that you may not have much choice in the way of frequency selection. At this time, the FCC is engaged in cannibalizing the UHF TV frequencies and passing out the remains to various services that have been begging for them for years. Many of their applications could best be done at other frequencies. In general, design and construction is cheaper and easier at lower frequencies. However, it's easier to get approval for higher frequencies.
The FCC will probably give you the same line that they hand to everyone with a new idea. Prototype your idea on one of the license-free frequencies or ham radio frequencies. When it works and looks like there's a demand for your contrivance, come back and they'll give you a more suitable frequency.
Ummm.... are you planning on launching your own satellite? Sorry, but that's not very clear or even useful. You also didn't quite answer my question. I asked "what are you trying to accomplish?", not how you plan to do it. If it helps, that's the same as "what is all this going to do?"
Also, be advised that the "AM carrier" doesn't deliver any information. It's just there wasting 50% of the power doing nothing. The information is in the side bands.
Incidentally, here are the ham radio microwave DX records all the way up to 411GHz.
None of these use AM modulation.
Here's a clue what do-it-thyself equipment looks like for 10/24GHz.
That's a transverter so the exciter, modulator, receiver, and antenna system are not shown.
Another version of wireless internet. Note that this is not something I actually plan to do -- too expensive. I am asking about a theoretical scenario when analog AM is used for wireless internet access.
Okay.
One of them uses AM. The one that is 2nd up from the bottom.
Fine, but theoretical scenario to do what? You've got satellite, internet, and AM mixed together, with no clue on how they tie together. Sorry, but I can't help you with either the theory or the objective.
Incidentally, mm microwave is expensive if you use brand new equipment, but rather tolerable if you have a ready source of surplus junk.
Sure. Look carefully at the "frequency". It's red light and they're modulating the light intensity optically.
Incidentally, the range figures shown are anything but typical. Your mileage may vary substantially.
A few more photos at:
for typical do-it-thyself equipment. This one should give you some clue as to what I think a 24GHz(?) system looks like:
The rule is in RF is "the uglier the mess, the better it works".
Oh, I forgot to mention that you'll need to buy, borrow, rent, or steal a rather substantial amount of expensive test equipment in order to make anything work. I don't want to add up how much I've been buying for fear of precipitating a coronary crisis.
Incidentally, you don't really need a satellite to provide internet access to a large area. It can be done with a tethered balloon (aerostat) or a solar powered airplane flying donuts in the sky. There's even a frequency band (about 70GHz) allocated for the purpose. It's been tried a few times, but never quite gets off the ground. I think there's hope, but it's going to take someone that doesn't know the limitations of the technology, and can stomach the politics, to make it happen. Details if you're interested.
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