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OT: Technical Question for Jeff Liebermann

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

Reply to
GreenXenon
Loading thread data ...

Are you trying to dazzle and amaze your college professor?

Reply to
Rob Sutter

(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.

Reply to
who where

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?

Reply to
GreenXenon

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).

Reply to
miso

  1. Why does the data have be scrambled?
  2. No, I'm talking about OOK.
Reply to
GreenXenon

Typo! I meant to say I'm NOT talking about OOK

Reply to
GreenXenon

ook ? is that like a foot fetish ?

Reply to
atec77

Ah, what the hell, let's make the carrier frequency 2.5 GHz

Reply to
GreenXenon

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.

Reply to
who where

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.

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.

Reply to
miso

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