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

 

Are you trying to dazzle and amaze your college professor?

 

(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.

 

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?

 

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).

 

1. Why does the data have be scrambled?

2. No, I'm talking about OOK.

 

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

 

ook ?
is that like a foot fetish ?

 

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

 

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.

 

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.