FPGA Central

Hi,

I'm somewhat of a newbie to the whole
orthogonal frequency division multiplexed
world, having come from the GSM, DCS, PCS,
and broadcast band power amplifier worlds,
as well as some 2-18 GHz DLVA work.

It's very interesting that they use
53 carriers: 48 data, 4 pilots, and one
nulled carrier.

My question is that it seems that
after our product does the A/D conversion on
the baseband signals, that we are left with 24
positive frequencies and 24 "negative" ones.

I understand that the negative
frequencies are something like the
imaginary portions of the FFT, or
something like that.

But if all the carriers are carrying
different data, then the negative
frequencies are not just a mirror image
of the positive carriers, so it's not
like a dual-sideband signal at all, where
you can get rid of one side and still
retain all the info.

And how do they down-convert in 802.11,
and extract "negative" frequencies?

Thanks for your help.

S.

On 1 May 2005 23:43:41 -0700, [email protected] wrote:

>Hi,
>
> I'm somewhat of a newbie to the whole
>orthogonal frequency division multiplexed
>world, having come from the GSM, DCS, PCS,
>and broadcast band power amplifier worlds,
>as well as some 2-18 GHz DLVA work.
>
> It's very interesting that they use
>53 carriers: 48 data, 4 pilots, and one
>nulled carrier.
>
> My question is that it seems that
>after our product does the A/D conversion on
>the baseband signals, that we are left with 24
>positive frequencies and 24 "negative" ones.
>
> I understand that the negative
>frequencies are something like the
>imaginary portions of the FFT, or
>something like that.
>
> But if all the carriers are carrying
>different data, then the negative
>frequencies are not just a mirror image
>of the positive carriers, so it's not
>like a dual-sideband signal at all, where
>you can get rid of one side and still
>retain all the info.
>
> And how do they down-convert in 802.11,
>and extract "negative" frequencies?

It's exactly the same process as you've likely done in
GSM/PCS/whatever; a complex mix to baseband provides I and Q (aka real
and imaginary) channels that are orthogonal and remove any ambiguity
between positive and negative frequencies. The only difference with
OFDM is that you take a DFT (usually via an FFT) of the baseband
signal rather than processing it in the time domain.


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

Hi,


I'm somewhat of a newbie to the whole
orthogonal frequency division multiplexed
world, having come from the GSM, DCS, PCS,
and broadcast band power amplifier worlds,
as well as some 2-18 GHz DLVA work.


It's very interesting that they use
53 carriers: 48 data, 4 pilots, and one
nulled carrier.


My question is that it seems that
after our product does the A/D conversion on
the baseband signals, that we are left with 24
positive frequencies and 24 "negative" ones.


I understand that the negative
frequencies are something like the
imaginary portions of the FFT, or
something like that.


But if all the carriers are carrying
different data, then the negative
frequencies are not just a mirror image
of the positive carriers, so it's not
like a dual-sideband signal at all, where
you can get rid of one side and still
retain all the info.


And how do they down-convert in 802.11,
and extract "negative" frequencies?


Thanks for your help.


S.

On 4 May 2005 22:37:57 -0700, [email protected] wrote:

....

> My question is that it seems that
>after our product does the A/D conversion on
>the baseband signals, that we are left with 24
>positive frequencies and 24 "negative" ones.
>
>
> I understand that the negative
>frequencies are something like the
>imaginary portions of the FFT, or
>something like that.
>
>
> But if all the carriers are carrying
>different data, then the negative
>frequencies are not just a mirror image
>of the positive carriers, so it's not
>like a dual-sideband signal at all, where
>you can get rid of one side and still
>retain all the info.
>
>
> And how do they down-convert in 802.11,
>and extract "negative" frequencies?

Complex values. Most implementations use direct-conversion to
baseband with a quadrature mix and two ADCs, one for the I channel and
one for the Q. The FFT/IFFT is complex, so the output subcarriers
will be complex and therefore not necessarily symmetric about zero Hz.




Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

Eric Jacobsen wrote:
> On 4 May 2005 22:37:57 -0700, [email protected] wrote:
>
> ...
>
> > My question is that it seems that
> >after our product does the A/D conversion on
> >the baseband signals, that we are left with 24
> >positive frequencies and 24 "negative" ones.
> >
> >
> > I understand that the negative
> >frequencies are something like the
> >imaginary portions of the FFT, or
> >something like that.
> >
> >
> > But if all the carriers are carrying
> >different data, then the negative
> >frequencies are not just a mirror image
> >of the positive carriers, so it's not
> >like a dual-sideband signal at all, where
> >you can get rid of one side and still
> >retain all the info.
> >
> >
> > And how do they down-convert in 802.11,
> >and extract "negative" frequencies?
>
> Complex values. Most implementations use direct-conversion to
> baseband with a quadrature mix and two ADCs, one for the I channel
and
> one for the Q. The FFT/IFFT is complex, so the output subcarriers
> will be complex and therefore not necessarily symmetric about zero
Hz.
>

I understand the addition of the 90 degree out
of phase of the quadrature component. What i don't
get is what a "negative" frequency looks like in the
baseband. Like what it would look like in the time-domain
on an oscilloscope.


S.

<[email protected]> wrote in message
news:[email protected] oups.com...
>
> Eric Jacobsen wrote:
> > On 4 May 2005 22:37:57 -0700, [email protected] wrote:
> >
> > ...
> >
> > > My question is that it seems that
> > >after our product does the A/D conversion on
> > >the baseband signals, that we are left with 24
> > >positive frequencies and 24 "negative" ones.
> > >
> > >
> > > I understand that the negative
> > >frequencies are something like the
> > >imaginary portions of the FFT, or
> > >something like that.
> > >
> > >
> > > But if all the carriers are carrying
> > >different data, then the negative
> > >frequencies are not just a mirror image
> > >of the positive carriers, so it's not
> > >like a dual-sideband signal at all, where
> > >you can get rid of one side and still
> > >retain all the info.
> > >
> > >
> > > And how do they down-convert in 802.11,
> > >and extract "negative" frequencies?
> >
> > Complex values. Most implementations use direct-conversion to
> > baseband with a quadrature mix and two ADCs, one for the I channel
> and
> > one for the Q. The FFT/IFFT is complex, so the output subcarriers
> > will be complex and therefore not necessarily symmetric about zero
> Hz.
> >
>
> I understand the addition of the 90 degree out
> of phase of the quadrature component. What i don't
> get is what a "negative" frequency looks like in the
> baseband. Like what it would look like in the time-domain
> on an oscilloscope.

The signals just run backwards on the scope...
Sorry, couldn't resist the wise-crack on a Friday eve.

I hope you aren't starting another thread that starts an argument over real
and complex signals - we've been over several rounds of "real signals can
be sent over a wire, complex is just math and cannot be sent over wires".

>
>
> S.
>

On Fri, 6 May 2005 17:38:43 -0700, "Bhaskar Thiagarajan"
<[email protected]> wrote:

><[email protected]> wrote in message
>news:[email protected] roups.com...
>>
>> Eric Jacobsen wrote:
>> > On 4 May 2005 22:37:57 -0700, [email protected] wrote:
>> >
>> > ...
>> >
>> > > My question is that it seems that
>> > >after our product does the A/D conversion on
>> > >the baseband signals, that we are left with 24
>> > >positive frequencies and 24 "negative" ones.
>> > >
>> > >
>> > > I understand that the negative
>> > >frequencies are something like the
>> > >imaginary portions of the FFT, or
>> > >something like that.
>> > >
>> > >
>> > > But if all the carriers are carrying
>> > >different data, then the negative
>> > >frequencies are not just a mirror image
>> > >of the positive carriers, so it's not
>> > >like a dual-sideband signal at all, where
>> > >you can get rid of one side and still
>> > >retain all the info.
>> > >
>> > >
>> > > And how do they down-convert in 802.11,
>> > >and extract "negative" frequencies?
>> >
>> > Complex values. Most implementations use direct-conversion to
>> > baseband with a quadrature mix and two ADCs, one for the I channel
>> and
>> > one for the Q. The FFT/IFFT is complex, so the output subcarriers
>> > will be complex and therefore not necessarily symmetric about zero
>> Hz.
>> >
>>
>> I understand the addition of the 90 degree out
>> of phase of the quadrature component. What i don't
>> get is what a "negative" frequency looks like in the
>> baseband. Like what it would look like in the time-domain
>> on an oscilloscope.
>
>The signals just run backwards on the scope...
>Sorry, couldn't resist the wise-crack on a Friday eve.
>
>I hope you aren't starting another thread that starts an argument over real
>and complex signals - we've been over several rounds of "real signals can
>be sent over a wire, complex is just math and cannot be sent over wires".

Ha. But complex *CAN* be sent over wires
if you have two wires.

See Ya',
[-Rick-]

"Bhaskar Thiagarajan" <[email protected]> writes:

> <[email protected]> wrote in message
> news:[email protected] oups.com...
>>
>> Eric Jacobsen wrote:
>> > On 4 May 2005 22:37:57 -0700, [email protected] wrote:
>> >
>> > ...
>> >
>> > > My question is that it seems that
>> > >after our product does the A/D conversion on
>> > >the baseband signals, that we are left with 24
>> > >positive frequencies and 24 "negative" ones.
>> > >
>> > >
>> > > I understand that the negative
>> > >frequencies are something like the
>> > >imaginary portions of the FFT, or
>> > >something like that.
>> > >
>> > >
>> > > But if all the carriers are carrying
>> > >different data, then the negative
>> > >frequencies are not just a mirror image
>> > >of the positive carriers, so it's not
>> > >like a dual-sideband signal at all, where
>> > >you can get rid of one side and still
>> > >retain all the info.
>> > >
>> > >
>> > > And how do they down-convert in 802.11,
>> > >and extract "negative" frequencies?
>> >
>> > Complex values. Most implementations use direct-conversion to
>> > baseband with a quadrature mix and two ADCs, one for the I channel
>> and
>> > one for the Q. The FFT/IFFT is complex, so the output subcarriers
>> > will be complex and therefore not necessarily symmetric about zero
>> Hz.
>> >
>>
>> I understand the addition of the 90 degree out
>> of phase of the quadrature component. What i don't
>> get is what a "negative" frequency looks like in the
>> baseband. Like what it would look like in the time-domain
>> on an oscilloscope.
>
> The signals just run backwards on the scope...
> Sorry, couldn't resist the wise-crack on a Friday eve.

Actually that's pretty close to the truth. In order to see a complex
signal on a scope, you'd use x-y mode, and in that mode a positive
frequency will move counterclockwise while a negative frquency moves
clockwise.

> I hope you aren't starting another thread that starts an argument over real
> and complex signals - we've been over several rounds of "real signals can
> be sent over a wire, complex is just math and cannot be sent over wires".

Then all those orthogonal signalling schemes (all PSKs, e.g.) are
being transmitted "illegally"???

Yeah, OK, so we use modulation to get two signals onto one wire. Still,
the point is that it isn't "just math."
--
% Randy Yates % "...the answer lies within your soul
%% Fuquay-Varina, NC % 'cause no one knows which side
%%% 919-577-9882 % the coin will fall."
%%%% <[email protected]> % 'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

Randy Yates wrote:

...

> Yeah, OK, so we use modulation to get two signals onto one wire. Still,
> the point is that it isn't "just math."

I can multiplex two signal of any type if I modulate. Call me back when
you can send a baseband complex signal on a single wire in the same
direction. (Hybrids allow two baseband signals to be transmitted on the
same wire if they travel in opposite directions.)

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Randy Yates wrote:
> "Bhaskar Thiagarajan" <[email protected]> writes:
>
> > <[email protected]> wrote in message
> > news:[email protected] oups.com...
> >>
> >> Eric Jacobsen wrote:
> >> > On 4 May 2005 22:37:57 -0700, [email protected] wrote:
> >> >
> >> > ...
> >> >
> >> > > My question is that it seems that
> >> > >after our product does the A/D conversion on
> >> > >the baseband signals, that we are left with 24
> >> > >positive frequencies and 24 "negative" ones.
> >> > >
> >> > >
> >> > > I understand that the negative
> >> > >frequencies are something like the
> >> > >imaginary portions of the FFT, or
> >> > >something like that.
> >> > >
> >> > >
> >> > > But if all the carriers are carrying
> >> > >different data, then the negative
> >> > >frequencies are not just a mirror image
> >> > >of the positive carriers, so it's not
> >> > >like a dual-sideband signal at all, where
> >> > >you can get rid of one side and still
> >> > >retain all the info.
> >> > >
> >> > >
> >> > > And how do they down-convert in 802.11,
> >> > >and extract "negative" frequencies?
> >> >
> >> > Complex values. Most implementations use direct-conversion to
> >> > baseband with a quadrature mix and two ADCs, one for the I
channel
> >> and
> >> > one for the Q. The FFT/IFFT is complex, so the output
subcarriers
> >> > will be complex and therefore not necessarily symmetric about
zero
> >> Hz.
> >> >
> >>
> >> I understand the addition of the 90 degree out
> >> of phase of the quadrature component. What i don't
> >> get is what a "negative" frequency looks like in the
> >> baseband. Like what it would look like in the time-domain
> >> on an oscilloscope.
> >
> > The signals just run backwards on the scope...
> > Sorry, couldn't resist the wise-crack on a Friday eve.
>
> Actually that's pretty close to the truth. In order to see a complex
> signal on a scope, you'd use x-y mode, and in that mode a positive
> frequency will move counterclockwise while a negative frquency moves
> clockwise.
>

I was thinking it might actually be something like
this, is the phasor moving clock-wise or counter-clock-wise?



> > I hope you aren't starting another thread that starts an argument
over real
> > and complex signals - we've been over several rounds of "real
signals can
> > be sent over a wire, complex is just math and cannot be sent over
wires".
>
> Then all those orthogonal signalling schemes (all PSKs, e.g.) are
> being transmitted "illegally"???
>
> Yeah, OK, so we use modulation to get two signals onto one wire.
Still,
> the point is that it isn't "just math."


Sigh... I don't think anyone here's really
gonna help me.....


S

[email protected] wrote:
> Randy Yates wrote:
>
>>"Bhaskar Thiagarajan" <[email protected]> writes:
>>
>>
>>><[email protected]> wrote in message
>>>news:[email protected] egroups.com...
>>>
>>>>Eric Jacobsen wrote:
>>>>
>>>>>On 4 May 2005 22:37:57 -0700, [email protected] wrote:
>>>>>
>>>>>...
>>>>>
>>>>>
>>>>>> My question is that it seems that
>>>>>>after our product does the A/D conversion on
>>>>>>the baseband signals, that we are left with 24
>>>>>>positive frequencies and 24 "negative" ones.
>>>>>>
>>>>>>
>>>>>> I understand that the negative
>>>>>>frequencies are something like the
>>>>>>imaginary portions of the FFT, or
>>>>>>something like that.
>>>>>>
>>>>>>
>>>>>> But if all the carriers are carrying
>>>>>>different data, then the negative
>>>>>>frequencies are not just a mirror image
>>>>>>of the positive carriers, so it's not
>>>>>>like a dual-sideband signal at all, where
>>>>>>you can get rid of one side and still
>>>>>>retain all the info.
>>>>>>
>>>>>>
>>>>>> And how do they down-convert in 802.11,
>>>>>>and extract "negative" frequencies?
>>>>>
>>>>>Complex values. Most implementations use direct-conversion to
>>>>>baseband with a quadrature mix and two ADCs, one for the I
>
> channel
>
>>>>and
>>>>
>>>>>one for the Q. The FFT/IFFT is complex, so the output
>
> subcarriers
>
>>>>>will be complex and therefore not necessarily symmetric about
>
> zero
>
>>>>Hz.
>>>>
>>>> I understand the addition of the 90 degree out
>>>>of phase of the quadrature component. What i don't
>>>>get is what a "negative" frequency looks like in the
>>>>baseband. Like what it would look like in the time-domain
>>>>on an oscilloscope.
>>>
>>>The signals just run backwards on the scope...
>>>Sorry, couldn't resist the wise-crack on a Friday eve.
>>
>>Actually that's pretty close to the truth. In order to see a complex
>>signal on a scope, you'd use x-y mode, and in that mode a positive
>>frequency will move counterclockwise while a negative frquency moves
>>clockwise.
>>
>
>
> I was thinking it might actually be something like
> this, is the phasor moving clock-wise or counter-clock-wise?
>
>
>
>
>>>I hope you aren't starting another thread that starts an argument
>
> over real
>
>>>and complex signals - we've been over several rounds of "real
>
> signals can
>
>>>be sent over a wire, complex is just math and cannot be sent over
>
> wires".
>
>>Then all those orthogonal signalling schemes (all PSKs, e.g.) are
>>being transmitted "illegally"???
>>
>>Yeah, OK, so we use modulation to get two signals onto one wire.
>
> Still,
>
>>the point is that it isn't "just math."
>
>
>
> Sigh... I don't think anyone here's really
> gonna help me.....
>

These should be considered meaningful and useful answers to your
question. A bit whimsical, maybe, but to the point. If you don't find
them helpful, maybe you need to be asking more basic questions.

Regards,
Steve

Jerry Avins <[email protected]> writes:

> Randy Yates wrote:
>
> ...
>
>> Yeah, OK, so we use modulation to get two signals onto one wire. Still,
>> the point is that it isn't "just math."
>
> I can multiplex two signal of any type if I modulate. Call me back
> when you can send a baseband complex signal on a single wire in the
> same direction. (Hybrids allow two baseband signals to be transmitted
> on the same wire if they travel in opposite directions.)

You might as well be saying, "Let me know when you can get a complex
value into a real value." Of course the answer is "you can't," but it
is just as ludicrous to imply that complex signals aren't significant
and aren't in very real [no pun intended] ways used in many signal
processing and communication systems.

I think you know that, but I still get the impression that you want to
blow off the importance of this mathematical system.
--
% Randy Yates % "Bird, on the wing,
%% Fuquay-Varina, NC % goes floating by
%%% 919-577-9882 % but there's a teardrop in his eye..."
%%%% <[email protected]> % 'One Summer Dream', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

[email protected] writes:

> Sigh... I don't think anyone here's really gonna help me.....

You may be right, in which case I'd ask for my money back.
--
% Randy Yates % "My Shangri-la has gone away, fading like
%% Fuquay-Varina, NC % the Beatles on 'Hey Jude'"
%%% 919-577-9882 %
%%%% <[email protected]> % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

On Sat, 07 May 2005 01:24:18 GMT, R.Lyons@_BOGUS_ieee.org (Rick Lyons)
wrote:


>
>Ha. But complex *CAN* be sent over wires
>if you have two wires.

Hi,
actually, as Clay Turner reminded me onetime,
complex signals can be transmitted over a
wire if you have two "channels".

See Ya',
[-Rick-]

On 6 May 2005 23:35:59 -0700, [email protected] wrote:


(snipped)
>
>
> Sigh... I don't think anyone here's really
>gonna help me.....

Hi,

your questions are simple, straightforward, and
easy to ask. But believe me they're not simple to answer.
I'm guesing that you might find some useful
answers at:

http://www.dspguru.com/info/tutor/quadsig2.htm

Good Luck,
[-Rick-]

Rick Lyons wrote:
> On Sat, 07 May 2005 01:24:18 GMT, R.Lyons@_BOGUS_ieee.org (Rick Lyons)
> wrote:
>
>
>
>>Ha. But complex *CAN* be sent over wires
>>if you have two wires.
>
>
> Hi,
> actually, as Clay Turner reminded me onetime,
> complex signals can be transmitted over a
> wire if you have two "channels".

So what do the channels consist of? Grooves that run the length of the
wire? Instead of getting tricky with a definition of "send", let's
discuss signals that can be displayed with a single oscilloscope probe.
Or is there a way to sidetrack that discussion also?

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Randy Yates wrote:
> Jerry Avins <[email protected]> writes:
>
>
>>Randy Yates wrote:
>>
>> ...
>>
>>
>>>Yeah, OK, so we use modulation to get two signals onto one wire. Still,
>>>the point is that it isn't "just math."
>>
>>I can multiplex two signal of any type if I modulate. Call me back
>>when you can send a baseband complex signal on a single wire in the
>>same direction. (Hybrids allow two baseband signals to be transmitted
>>on the same wire if they travel in opposite directions.)
>
>
> You might as well be saying, "Let me know when you can get a complex
> value into a real value." Of course the answer is "you can't," but it
> is just as ludicrous to imply that complex signals aren't significant
> and aren't in very real [no pun intended] ways used in many signal
> processing and communication systems.
>
> I think you know that,

I do.

> but I still get the impression that you want to
> blow off the importance of this mathematical system.

If by "importance" you mean "intrinsically unavoidable conceptual
necessity", then yes; I think it's not important. If instead you mean
"beautifully simple way to summarize and calculate" then it is certainly
important.

I see the complex representation of signals as a, well, representation.
You see it as signal. I think that the issue is important for beginners.
I know at least two people with IQs over 150 who can't face algebra
without tears. When they were told that "minus times minus is plus" is a
self-evident truth, and they were never able to figure out why, they
both concluded that math was beyond their ken and turned to other
topics. I got to one of them (the other was my first wife) early enough
to save him by telling him that it is not logically necessary, but a
definition adopted for convenience and elegance. I gave examples where
this definition simplified an otherwise complicated analysis (and some
systems where it doesn't apply).

When you say of a signal represented by a complex number, "This is what
it is" rather than "This is how we chose to represent it", you create an
unnecessary hurdle for those who naturally look deep into relations.
Some of them don't get over it.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Randy Yates wrote:

...

> Actually that's pretty close to the truth. In order to see a complex
> signal on a scope, you'd use x-y mode, and in that mode a positive
> frequency will move counterclockwise while a negative frquency moves
> clockwise.

Another point: If there's one wire, there's no X-Y display.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

On 6 May 2005 23:35:59 -0700, [email protected] wrote:

>Randy Yates wrote:
>> "Bhaskar Thiagarajan" <[email protected]> writes:
>>
>> > <[email protected]> wrote in message
>> > news:[email protected] oups.com...
>> >>
>> >> Eric Jacobsen wrote:
>> >> > On 4 May 2005 22:37:57 -0700, [email protected] wrote:
>> >> >
>> >> > ...
>> >> >
>> >> > > My question is that it seems that
>> >> > >after our product does the A/D conversion on
>> >> > >the baseband signals, that we are left with 24
>> >> > >positive frequencies and 24 "negative" ones.
>> >> > >
>> >> > >
>> >> > > I understand that the negative
>> >> > >frequencies are something like the
>> >> > >imaginary portions of the FFT, or
>> >> > >something like that.
>> >> > >
>> >> > >
>> >> > > But if all the carriers are carrying
>> >> > >different data, then the negative
>> >> > >frequencies are not just a mirror image
>> >> > >of the positive carriers, so it's not
>> >> > >like a dual-sideband signal at all, where
>> >> > >you can get rid of one side and still
>> >> > >retain all the info.
>> >> > >
>> >> > >
>> >> > > And how do they down-convert in 802.11,
>> >> > >and extract "negative" frequencies?
>> >> >
>> >> > Complex values. Most implementations use direct-conversion to
>> >> > baseband with a quadrature mix and two ADCs, one for the I
>channel
>> >> and
>> >> > one for the Q. The FFT/IFFT is complex, so the output
>subcarriers
>> >> > will be complex and therefore not necessarily symmetric about
>zero
>> >> Hz.
>> >> >
>> >>
>> >> I understand the addition of the 90 degree out
>> >> of phase of the quadrature component. What i don't
>> >> get is what a "negative" frequency looks like in the
>> >> baseband. Like what it would look like in the time-domain
>> >> on an oscilloscope.
>> >
>> > The signals just run backwards on the scope...
>> > Sorry, couldn't resist the wise-crack on a Friday eve.
>>
>> Actually that's pretty close to the truth. In order to see a complex
>> signal on a scope, you'd use x-y mode, and in that mode a positive
>> frequency will move counterclockwise while a negative frquency moves
>> clockwise.
>>
>
> I was thinking it might actually be something like
>this, is the phasor moving clock-wise or counter-clock-wise?

Whatever they do, the rotation is opposite for positive or negative
frequencies. In practice the baseband signal is always mixed up to
some IF for transmission. It's only at baseband that this conceptual
problem exists for some people.

>> > I hope you aren't starting another thread that starts an argument
>over real
>> > and complex signals - we've been over several rounds of "real
>signals can
>> > be sent over a wire, complex is just math and cannot be sent over
>wires".
>>
>> Then all those orthogonal signalling schemes (all PSKs, e.g.) are
>> being transmitted "illegally"???
>>
>> Yeah, OK, so we use modulation to get two signals onto one wire.
>Still,
>> the point is that it isn't "just math."
>
>
> Sigh... I don't think anyone here's really
>gonna help me.....

Everybody that's posted has been trying to help, I think. I hope
you're getting an appreciation that there are some signficant
conceptual and philosophical issues buried under the question you've
asked, and that's what's fueling the discussion.

To attempt to simplify a bit, though, each subcarrier just has a
magnitude and a phase. With complex input values the FFT can
transform the received baseband signal into independent magnitude and
phase values for each subcarrier. Since the modulations used all
manipulate magnitude and phase to modulate the data, this allows full
recovery in the receiver. As long as the FFT input is complex all of
the subcarriers can be modulated independent of each other.


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

Rick Lyons wrote:
> >
> > Sigh... I don't think anyone here's really
> >gonna help me.....
>
> Hi,
>
> your questions are simple, straightforward, and
> easy to ask. But believe me they're not simple to answer.
> I'm guesing that you might find some useful
> answers at:
>
> http://www.dspguru.com/info/tutor/quadsig2.htm
>

Are you really the Richard Lyons that
wrote the book i was reading last night at
Barnes and Nobles?

Excellent. A pleasure to hear from you.
You chapter on the difference between linear
and non-linear in the DSP world was VERY
clearly written. There are some people who
are extremely knowledgable about a subject,
but it's even rarer to find one of them who
can explain concepts CLEARLY to other people.
I'm an analog RF/ADS/Smith Chart/phase-locked-loop engineer, so i'm
more used to talking about
Class C or Class A gate-biasing on n-channel
enhancement-mode mosfets, and how IP3s grow with non-linearities, etc.
But it was good to see non-linearity
discussed from a DSP point of view.

It looks like the 18 page link
you wrote includes some discussion of
the negative frequency baseband concept,
so i'll absorb that and get back to you.

Funny how the old Digital vs. Analog
divide is getting fuzzier and fuzzier by
the year, eh? This forces strictly RF guys
like me to bug you DSP guys so we know at
least a little bit about what happpens
before/after the D/A/A/D converters. And
also forces the digital people to learn
co-planar waveguides and RF layout techniques.

I was thinking of buying your book,
it seems to be what i want. $80???

Damn! this reminds me of college!

Could i order one from you directly?
Maybe a first-timer discount? hehe!


S.

Jerry Avins <[email protected]> writes:

> When you say of a signal represented by a complex number, "This is
> what it is" rather than "This is how we chose to represent it", you
> create an unnecessary hurdle for those who naturally look deep into
> relations. Some of them don't get over it.

How would you, then, represent the square root of -1 by a real number?
--
% Randy Yates % "And all that I can do
%% Fuquay-Varina, NC % is say I'm sorry,
%%% 919-577-9882 % that's the way it goes..."
%%%% <[email protected]> % Getting To The Point', *Balance of Power*, ELO
http://home.earthlink.net/~yatescr

Randy Yates wrote:
> Jerry Avins <[email protected]> writes:
>
>
>>When you say of a signal represented by a complex number, "This is
>>what it is" rather than "This is how we chose to represent it", you
>>create an unnecessary hurdle for those who naturally look deep into
>>relations. Some of them don't get over it.
>
>
> How would you, then, represent the square root of -1 by a real number?

I would not claim that a voltage's fundamental representation involved
the square root of -1 in any way. I show anyone who needs trig for any
purpose that complex arithmetic can derive all the trig identities and
manipulate all the trig relations with great ease. That's an entirely
different matter. You can sell me j volts when your grocer sells you j
pounds of potatoes.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins <[email protected]> writes:

> Randy Yates wrote:
>> Jerry Avins <[email protected]> writes:
>>
>>>When you say of a signal represented by a complex number, "This is
>>>what it is" rather than "This is how we chose to represent it", you
>>>create an unnecessary hurdle for those who naturally look deep into
>>>relations. Some of them don't get over it.
>> How would you, then, represent the square root of -1 by a real
>> number?
>
> I would not claim that a voltage's fundamental representation involved
> the square root of -1 in any way.

Who said we were constraining ourselves to a "voltage's fundamental
representation?"

> You can sell me j volts when your grocer sells you j
> pounds of potatoes.

And the only systems we're interested in here are potato sales systems?
--
% Randy Yates % "So now it's getting late,
%% Fuquay-Varina, NC % and those who hesitate
%%% 919-577-9882 % got no one..."
%%%% <[email protected]> % 'Waterfall', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

Randy Yates wrote:
> Jerry Avins <[email protected]> writes:
>
>
>>Randy Yates wrote:
>>
>>>Jerry Avins <[email protected]> writes:
>>>
>>>
>>>>When you say of a signal represented by a complex number, "This is
>>>>what it is" rather than "This is how we chose to represent it", you
>>>>create an unnecessary hurdle for those who naturally look deep into
>>>>relations. Some of them don't get over it.
>>>
>>>How would you, then, represent the square root of -1 by a real
>>>number?
>>
>>I would not claim that a voltage's fundamental representation involved
>>the square root of -1 in any way.
>
>
> Who said we were constraining ourselves to a "voltage's fundamental
> representation?"

You have claimed that in the past. Have you shifted your ground?

>>You can sell me j volts when your grocer sells you j
>>pounds of potatoes.
>
> And the only systems we're interested in here are potato sales systems?

Potatoes are interesting because they use the same enumeration as volts.

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Jerry Avins wrote:
> I would not claim that a voltage's fundamental representation involved
> the square root of -1 in any way. I show anyone who needs trig for any
> purpose that complex arithmetic can derive all the trig identities and
> manipulate all the trig relations with great ease. That's an entirely
> different matter. You can sell me j volts when your grocer sells you j
> pounds of potatoes.

Well, Jerry, most people think selling potatoes is pretty simple, but
signal processing is really really complex. :-)

Regards,
Steve