Wireless AM analog internet connections?

Discussion in 'Wireless Internet' started by Radium, May 25, 2007.

  1. Radium

    Radium Guest

    Hi:

    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?

    Would there be any differences at night?


    Thanks,

    Radium
     
    Radium, May 25, 2007
    #1
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  2. 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.
    1. It won't have the capacity of a system that uses both PM and AM.
    2. It won't be legal.
    3. It will have bandwidth restrictions due to the lower frequency
    that will substantially lower thruput.
    4. It will interfere with existing services on the frequencies you
    plan to trash.
    5. It will not be backed by a reputeable standards group.
    6. It pisses me off.
    No. Doing this under the cover of darkness will not make it work or
    prevent getting caught.
     
    Jeff Liebermann, May 25, 2007
    #2
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  3. Radium

    Dana Guest

    This is an excellent description :)
     
    Dana, May 25, 2007
    #3
  4. Radium

    Radium Guest

    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. 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.
     
    Radium, May 26, 2007
    #4
  5. You're nudging the point at which the atmosphere is opaque.
    Atmospheric conditions will also start to mess seriously with your
    signal.
     
    Mark McIntyre, May 26, 2007
    #5
  6. 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:
    <http://en.wikipedia.org/wiki/Modulation>
    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:
    <http://en.wikipedia.org/wiki/Spectral_efficiency>
    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:
    <http://en.wikipedia.org/wiki/Radio_propagation>

    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:
    <http://www.microwaves101.com/encyclopedia/frequency.cfm>
    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.
     
    Jeff Liebermann, May 26, 2007
    #6
  7. Radium

    Dana Guest

    The on/off he described is not considered digital.
    Dxing, does no such thing.
    As described DXing is as described above.

    Keep thinking like you do.
    Than it would be useless
    You really have no clue.
    What you describe is a function of the frequencies used, not the
    "heterodynes"
     
    Dana, May 26, 2007
    #7
  8. Radium

    Radium Guest

    Not if the reciever sufficiently-amplifies the carrier wave prior to
    demodulation.
     
    Radium, May 26, 2007
    #8
  9. Radium

    Dana Guest

    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.
     
    Dana, May 26, 2007
    #9
  10. 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.

    This might help:
    <http://www.maxim-ic.com/appnotes.cfm/an_pk/1836>
     
    Jeff Liebermann, May 26, 2007
    #10
  11. Radium

    George Guest

    This guy appears to be the same troll who is also in the cellular groups
    with bizarre questions about AM.
     
    George, May 26, 2007
    #11
  12. Radium

    DTC Guest

    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?
     
    DTC, May 26, 2007
    #12
  13. Radium

    Radium Guest

    So what's best frequency for my application?

    500 MHz is too low, while 500 GHz is too high.
    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?
    Thanks for the link.
     
    Radium, May 26, 2007
    #13
  14. 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).
    <http://www.sea-dmi.com>
    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:
    <http://mrtmag.com/news/radio_why_mhz/>
    <http://220.mrtmag.com>

    Incidentally, the FCC re-assigned the 220MHz ham band primarily for
    the benifit of UPS (United Parcel Service).
    <http://www.cedmagazine.com/article.aspx?id=78374&terms=>
    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.
     
    Jeff Liebermann, May 26, 2007
    #14
  15. 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:
    <http://www.arrl.org/tis/info/part15.html>
    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.
     
    Jeff Liebermann, May 26, 2007
    #15
  16. Radium

    Radium Guest

    Radium, May 26, 2007
    #16
  17. 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.
    <http://www.g3pho.free-online.co.uk/microwaves/records.htm>
    None of these use AM modulation.

    Here's a clue what do-it-thyself equipment looks like for 10/24GHz.
    <http://www.ad6a.com/10G24G.html>
    That's a transverter so the exciter, modulator, receiver, and antenna
    system are not shown.
     
    Jeff Liebermann, May 26, 2007
    #17
  18. Radium

    Radium Guest

    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.
    One of them uses AM. The one that is 2nd up from the bottom.
    Thanks for the links.
     
    Radium, May 26, 2007
    #18
  19. 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:
    <http://www.50mhzandup.org/04tuneup/Tune-Up.html>
    for typical do-it-thyself equipment. This one should give you some
    clue as to what I think a 24GHz(?) system looks like:
    <http://www.50mhzandup.org/04tuneup/Picture 156.html>
    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.
     
    Jeff Liebermann, May 27, 2007
    #19
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