OT: Technical Question for Jeff Liebermann

Discussion in 'Wireless Internet' started by GreenXenon, Oct 13, 2011.

  1. GreenXenon

    GreenXenon Guest

    Mr. Liebermann:

    Here is a speculative situation involving a hypothetical wireless
    internet access system. This system does not exist but is merely
    theoretical.

    A wireless router is connected to a cable modem. The wireless adapter
    is external and connects to the computer via USB. The adapter and
    router have their own own power supplies for maximum amplifications
    upon reception and demodulation. There is a single radio frequency
    used by the router and adapter to communicate with each other -- 2
    GHz. The modulation is AM. Both the carrier and modulation signals are
    completely linear. The carrier is always analog and amplitude-
    modulated. The modulation signal is initially digital but is converted
    to analog -- via a DAC -- prior to being transmitted on the carrier.
    Prior to entering the DAC, the digital modulation signal has it's
    amplitude attenuated until it is just strong enough to be clearly
    recognized by the DAC and subsequently the 2 GHz AM carrier generator.
    In the last step before transmission, the resulting AM carrier signal
    is also attenuated, until as weak as possible while still being
    intelligible.

    Obviously, both the router and adapter have their transmitting and
    receiving ends.

    On the receiving end, the analog AM carrier signal is amplified [using
    a 2 GHz amplifier] and is subsequently demodulated. After
    demodulation, the resulting modulator signal further amplified.
    Finally, the modulation signal, is converted back to digital -- via an
    ADC so that the computer can recognize it.

    On the receiving ends, all of the following entities are built in such
    as way that they are both omnidirectional AND as sensitive as
    physically-possible to weak signals:

    1. The antennas attached to the receivers
    2. The receivers
    3. The demodulators

    Given all of the above specs, what would be the benefit -- if any --
    of using these devices to wirelessly-connect a computer to a cable
    modem?


    Thanks,

    GX
     
    GreenXenon, Oct 13, 2011
    #1
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  2. GreenXenon

    Rob Sutter Guest

    Are you trying to dazzle and amaze your college professor?
     
    Rob Sutter, Oct 13, 2011
    #2
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  3. GreenXenon

    who where Guest

    (snip description of convoluted hypothetical system)
    Benefit? Obfuscation - reduced prospect of interception.

    Drawbacks? Analog noise and ADC/DAC quantisation errors -> errors in
    recovered digital stream. Cost. If you really meant 2GHz and not the
    "normal" 2.4GHz band, probable illegality.

    Just my 2c worth.
     
    who where, Oct 13, 2011
    #3
  4. GreenXenon

    GreenXenon Guest


    Even if the bit-resolution of the ADC and DAC are high-enough?


    Ok, let's say I use 2.4 GHz. Is it still illegal?
     
    GreenXenon, Oct 14, 2011
    #4
  5. GreenXenon

    miso Guest

    This part makes little sense. First of all, I don't think you have a
    handle on what AM means. For instance, do you mean OOK (on off keying)?
    Are you taking bytes of data and PCMing? Just strong enough to be
    clearly recognized by the DAC? What does that mean? You feed a DAC
    digital signals. There is nothing ambiguous in the data.

    Intrinsic in any communications scheme is data framing and clock
    recovery, not to mention having to whiten the data (scrambler).
     
    miso, Oct 14, 2011
    #5
  6. GreenXenon

    GreenXenon Guest

    1. Why does the data have be scrambled?

    2. No, I'm talking about OOK.
     
    GreenXenon, Oct 14, 2011
    #6
  7. GreenXenon

    GreenXenon Guest


    Typo! I meant to say I'm NOT talking about OOK
     
    GreenXenon, Oct 14, 2011
    #7
  8. GreenXenon

    atec77 Guest

    ook ?
    is that like a foot fetish ?
     
    atec77, Oct 14, 2011
    #8
  9. GreenXenon

    GreenXenon Guest


    Ah, what the hell, let's make the carrier frequency 2.5 GHz
     
    GreenXenon, Oct 14, 2011
    #9
  10. GreenXenon

    who where Guest


    There's ALWAYS quantisation error in the process. It gets less as the
    # of bits increases, but remains finite for finite #bits.

    The analog modulation and demod also add noise which contributes to
    error in the recovered data.
    The national authority in *your* country, in association with others
    at WARC's, determine the band usage and permissible modulation
    methods. , What they allow in your country I don't know off the top of
    my head, but I'll bet it's not "open slather" (*)

    (*) unless you are in China, where from what I observed it seems users
    assign their own operating frequencies and modes.
     
    who where, Oct 14, 2011
    #10
  11. GreenXenon

    miso Guest

    Since we haven't nailed down the modulation, I really can't go into the
    scrambler much, but basically data could be a bazillion zeroes or one.
    That can lead to modulation that doesn't really "flog" the system (use
    all possible symbols). For instance, if you had a simple 4 point
    constellation, it would only use two out of the 4 locations. You run the
    data through a scrambler to whiten it, i.e. make it look more random. It
    is generally hard to do clock recovery on a signal that isn't scrambled.
    Note this is not encryption. I'm having a hard time describing this
    because whitening the data is generally job 1 so I have to think hard
    about the bad things that happen if you don't whiten it. Actually this wiki looks good.

    Let's take the case of simple FSK with no scrambler and worse yet, no
    framing. (Note FSK doesn't really need clock recovery, though in
    practice, you dejitter the recovered data.) If you sent a long string of
    zeros, it would correspond to one frequency of the FSK pair. Your data
    recovery would have a hard time locking on the data. Now if yo framed
    it, say with start and stop bits, you could now lock on the data. But
    for analysis purposes, say estimating a bit error rate, you rather have
    both frequencies used. Better yet, most of the comm schemes have already
    been analyzed, so you can plug and chug, but the analysis generally
    assumes white noise and randomized data.

    OOK doesn't need clock recovery either. You could look at how an
    aircraft transponder works if you need an example. You do need apriori
    knowledge of the data rate to recover the data, and usually it is
    oversampled to aid in recovery.
     
    miso, Oct 15, 2011
    #11
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