simple-minded non-OFDM signal design question

Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:

> Randy Yates wrote:
>
>
>>>>So I guess I have two questions: 1) Is it possible to design
>>>>single-carrier systems (including the receiver) which are just as
>>>>immune to multipath as multi-carrier systems?
>>>
>>>Certainly. Although it will require a huge amount of computation.
>> Why? What sort of things would you have to do to a single-carrier
>> signal to "get out the multipath?"
>
> The single carrier receiver will have to use an adaptive model of the
> channel and apply some sort of the maximum likelihood algorithm for
> the optimal demodulation. This is a heavy computational task.

Once you've got the channel identified, would you then proceed
to form a whitening filter or form an ISI equalizer?
-- 
%  Randy Yates                  % "Though you ride on the wheels of tomorrow,
%% Fuquay-Varina, NC            %  you still wander the fields of your
%%% 919-577-9882                %  sorrow."
%%%% <yates@ieee.org>           % '21st Century Man', *Time*, ELO
http://home.earthlink.net/~yatescr

Randy Yates wrote:
>>>
>>> What sort of things would you have to do to a single-carrier
>>> signal to "get out the multipath?"
>>
>>The single carrier receiver will have to use an adaptive model of the
>>channel and apply some sort of the maximum likelihood algorithm for
>>the optimal demodulation. This is a heavy computational task.
> 
> 
> Once you've got the channel identified, would you then proceed
> to form a whitening filter or form an ISI equalizer?

Neither. Both ways are suboptimal.

I will try to find the most likely sequence at the input of the channel 
  by matching the received signal with the output of the model of the 
channel. For that purpose, I can employ Viterbi algorithm, for example.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

On Tue, 26 Sep 2006 03:31:14 GMT, Randy Yates <yates@ieee.org> wrote:

>Randy Yates <yates@ieee.org> writes:
>> [...]
>
>OK, let me tip my hand. 
>
>There are some folks/circles that are ragingly debating
>whether the COFDM used in DVB/T is better than the US's
>8VSB. I'm wondering if there is any real difference IF
>the end-to-end systems are designed "properly."

Oh, gag a maggot.

I can't believe anybody still thinks that 8VSB is a good idea for
anything, anywhere, under any circumstances.

I've got some opinion on this, which I'll offer FWIW, but it's really
just my opinion.   I've worked with both 8VSB and OFDM and am pretty
familiar with both the ATSC and DVB-T standards.

As far as I can tell the Grand Alliance, from which we've been
afflicted with 8VSB, was populated mostly by old farts who hadn't been
exposed to much radio technology other than NTSC and other television
standards.   The only two modulation techniques that were seriously
considered, by my understanding, were 16-QAM and 8VSB.   The
conclusion was that 8VSB was "superior" at least partly because it
only needed a real-valued equalizer in the receiver and it was,
therefore, simpler to implement.  And since 8VSB looks pretty familiar
to somebody who's been looking at NTSC signals through their career,
it probably was very comfortable for them to accept it.

Remember, this decision was made a long time ago before gates got
nearly as cheap as they are now.   A full-on, complex EQ for 16QAM is
gonna be mouse nuts in a modern receiver at the signal bandwidths used
by 8VSB and DVB, so there's really no benefit IMHO to using 8VSB and a
bunch of avoidable downsides (like restriced data rate, efficiency,
channel survivability, etc., etc.).   And 8VSB has some added
complexity (and performance reduction) due to the vestigial sideband.
IMHO it's a really crappy modulation method and shouldn't be
considered for any modern system given the alternatives available;
essentially, I've never been able to find a condition under which it's
better than other common methods.  16QAM is always, in my experience,
a better choice, and OFDM even better than that.

DVB-T, on the other hand, is a pretty well-designed system with some
really clever stuff in it.  It's efficient and robust and works well.
It was pretty new, I think still under development, but available to
the Grand Alliance when they did their work, but it wasn't even
considered by my understanding.

So from a real comm engineering standpoint 8VSB doesn't really offer
any advantages over even 16QAM, (it's worse, generally), and certainly
not over an OFDM system in frequency-selective fading.   And broadcast
channels are VERY frequency selective in general so OFDM is a really
good choice for that application.

There's a reason you don't see n-VSB showing up anywhere else.   It
pretty much just sucks, IMHO.  The Grand Alliance really didn't do us
any favors by selecting that.

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

Eric Jacobsen wrote:
> On Tue, 26 Sep 2006 03:31:14 GMT, Randy Yates <yates@ieee.org> wrote:
> 
> 
>>Randy Yates <yates@ieee.org> writes:
>>
>>>[...]
>>
>>OK, let me tip my hand. 
>>
>>There are some folks/circles that are ragingly debating
>>whether the COFDM used in DVB/T is better than the US's
>>8VSB. I'm wondering if there is any real difference IF
>>the end-to-end systems are designed "properly."
> 
> 
> Oh, gag a maggot.
> I can't believe anybody still thinks that 8VSB is a good idea for
> anything, anywhere, under any circumstances.
> 
> [... a long and accurate rant about the dumbness of 8-VSB clipped ....]

Well, I find the rant accurate, but the statement before misses a key 
issue. 8-VSB seems like its about to become the dominant approach across 
the world, because the Chinese just chosen a variant of it. They did 
this because they believe they have sidestepped the patents for which 
Americans are paying heavy royalties. The believed they couldn't do this 
with DVB/T. In a world of heavy royalties, any crappy technology which 
can sidestep them is often a much preferred solution.

Steve

Randy Yates skrev:
> Is it possible to design
> single-carrier systems (including the receiver) which are just as
> immune to multipath as multi-carrier systems?

The answer is no.

The reason is that the physics of any multipath, narrow-band system
determines that there will be zones of destructive interefernce.

Consider a transmitter and reciever placed in an infinite half-space
above a plane reflecting surface, which has an arbitrary reflection
coefficient R. Assume the transmitter's position is (0,z) and the
receiever's position is (r,s)

The recieved signal x(t) will become

x(r,s,z,t) = x'(r,z,s,,t) + x"(r,z,s,t)

where x' is the direct signal and x" is the signal that has been
reflected from the ground.

The key is to realize that no matter how you choose r,z, and s,
there will always exist some geometrical configuration of transmitter
and reciever where x' + x" = 0, and no transmission is possible.
If you want to look at the dteails of the computations, search for
"Lloyd's mirror" in the acoustic literature. That's valid for
monochromatic signals, but the general idea is valid for narrow-band
signals as well.

The reason why one uses multi-carrier systems, is that if
one channel suffers from destructive interference, some other
channel might come through.

Destructive interference is based on the physics of the
set-up. No computational power in the world will be able
to detect a transmitted signal where none can be 
measured.

Rune

Rune Allnor said the following on 01/10/2006 06:29:
> Randy Yates skrev:
>> Is it possible to design
>> single-carrier systems (including the receiver) which are just as
>> immune to multipath as multi-carrier systems?
> 
> The answer is no.
> 
> The reason is that the physics of any multipath, narrow-band system
> determines that there will be zones of destructive interefernce.
> 
> Consider a transmitter and reciever placed in an infinite half-space
> above a plane reflecting surface, which has an arbitrary reflection
> coefficient R. Assume the transmitter's position is (0,z) and the
> receiever's position is (r,s)
> 
> The recieved signal x(t) will become
> 
> x(r,s,z,t) = x'(r,z,s,,t) + x"(r,z,s,t)
> 
> where x' is the direct signal and x" is the signal that has been
> reflected from the ground.
> 
> The key is to realize that no matter how you choose r,z, and s,
> there will always exist some geometrical configuration of transmitter
> and reciever where x' + x" = 0, and no transmission is possible.

This can only be true if you ignore the inverse-square law, surely?  The 
reflected beam will always travel further, and therefore incur more 
attentuation, and therefore will never be a 0dB echo.

(Obviously when the transmitter-receiver separation is very great, the 
difference in attenuation will be negligible.)


> The reason why one uses multi-carrier systems, is that if
> one channel suffers from destructive interference, some other
> channel might come through.

Randy's question was referring to a comparison between single-carrier 
and multi-carrier systems with the same bandwidth (it wouldn't be a fair 
comparsion otherwise).  If this is the case, then in identical multipath 
conditions, they should each experience identical interference (which is 
obvious if you consider that the spectral response of the channel will 
be identical in each case).

In a situation where the entire single-carrier bandwidth is nulled out 
as you describe, then the entire multi-carrier bandwidth would be nulled 
out as well, so there's no disadvantage.



-- 
Oli

Oli Charlesworth skrev:
> Rune Allnor said the following on 01/10/2006 06:29:
> > Randy Yates skrev:
> >> Is it possible to design
> >> single-carrier systems (including the receiver) which are just as
> >> immune to multipath as multi-carrier systems?
> >
> > The answer is no.
> >
> > The reason is that the physics of any multipath, narrow-band system
> > determines that there will be zones of destructive interefernce.
> >
> > Consider a transmitter and reciever placed in an infinite half-space
> > above a plane reflecting surface, which has an arbitrary reflection
> > coefficient R. Assume the transmitter's position is (0,z) and the
> > receiever's position is (r,s)
> >
> > The recieved signal x(t) will become
> >
> > x(r,s,z,t) = x'(r,z,s,,t) + x"(r,z,s,t)
> >
> > where x' is the direct signal and x" is the signal that has been
> > reflected from the ground.
> >
> > The key is to realize that no matter how you choose r,z, and s,
> > there will always exist some geometrical configuration of transmitter
> > and reciever where x' + x" = 0, and no transmission is possible.
>
> This can only be true if you ignore the inverse-square law, surely?  The
> reflected beam will always travel further, and therefore incur more
> attentuation, and therefore will never be a 0dB echo.

What the transmission of useful information is concerned, it is
sufficient that the destructive interference results in a "very weak"
signal.

> (Obviously when the transmitter-receiver separation is very great, the
> difference in attenuation will be negligible.)

You would be surprised by the ranges where these effects occur.

Some years ago, a colleague of mine tested a radio link between
a transmitter inside buoy at sea and a reciever onboard a small
boat. The buoy was well within visual range (< 2km) when they lost
the link. When they tested the link over land, it worked at ranges
up to some 8-10 km.

My colleaugue had tested and tried absolutely everything to
get the thing to work at sea, to no avail. Only when I modeled this
interference effect and showed him that they lost the signal
exactly where the simulation predicted they would, did he accept
that the radio instruments worked. The problem was the physics
of the multipath scenario.

> > The reason why one uses multi-carrier systems, is that if
> > one channel suffers from destructive interference, some other
> > channel might come through.
>
> Randy's question was referring to a comparison between single-carrier
> and multi-carrier systems with the same bandwidth (it wouldn't be a fair
> comparsion otherwise).  If this is the case, then in identical multipath
> conditions, they should each experience identical interference (which is
> obvious if you consider that the spectral response of the channel will
> be identical in each case).

OK. Maybe one should substitute "narrow-band" for "single-carrier"
in my post.

> In a situation where the entire single-carrier bandwidth is nulled out
> as you describe, then the entire multi-carrier bandwidth would be nulled
> out as well, so there's no disadvantage.

Again, I discussed narrow-band propagation and may have
misunderstood the exact nature of Randy's question.

I was thinking of the case where a pilot tone is used by the reciever.
In this situation it suffices to null the (very narrow-band) pilot tone
to
kill the link. You are right if we discuss modulation schemes where
the pilot tone is only internal, and is not  actually transmitted.

Regardless of what terminology one decides to use, my argument
still is that interference effects due to the multipath propagation
are the causes why one want to use broadband[*] transmission links.

Rune

[*] Links with significantly broader bands than the bandwidth of the
    transmitted signal.

Oli Charlesworth <catch@olifilth.co.uk> writes:
> [...]
> Randy's question was referring to a comparison between single-carrier
> and multi-carrier systems with the same bandwidth (it wouldn't be a
> fair comparsion otherwise). 

Exactly.

> In a situation where the entire single-carrier bandwidth is nulled out
> as you describe, then the entire multi-carrier bandwidth would be
> nulled out as well, so there's no disadvantage.

Right.
-- 
%  Randy Yates                  % "...the answer lies within your soul
%% Fuquay-Varina, NC            %       'cause no one knows which side
%%% 919-577-9882                %                   the coin will fall."
%%%% <yates@ieee.org>           %  'Big Wheels', *Out of the Blue*, ELO
http://home.earthlink.net/~yatescr

"Rune Allnor" <allnor@tele.ntnu.no> writes:
> [...]
> OK. Maybe one should substitute "narrow-band" for "single-carrier"
> in my post.

But that wasn't the intent of my question either, Rune. As I tipped my
hand, I am essentially asking questions about terrestrial COFDM versus
8VSB for television channels, i.e., 6 MHz channels. I don't believe
the coherence bandwidth of such a channel is anywhere near 6 MHz, but
someone correct me if I'm wrong. In essence, I'm asking questions
about "wideband" channels, i.e., channels in which the channel
bandwidth is much greater than the coherence bandwidth.

>> In a situation where the entire single-carrier bandwidth is nulled out
>> as you describe, then the entire multi-carrier bandwidth would be nulled
>> out as well, so there's no disadvantage.
>
> Again, I discussed narrow-band propagation and may have
> misunderstood the exact nature of Randy's question.

No problem. 

> I was thinking of the case where a pilot tone is used by the reciever.
> In this situation it suffices to null the (very narrow-band) pilot tone
> to
> kill the link. You are right if we discuss modulation schemes where
> the pilot tone is only internal, and is not  actually transmitted.
>
> Regardless of what terminology one decides to use, my argument
> still is that interference effects due to the multipath propagation
> are the causes why one want to use broadband[*] transmission links.

Agreed.
-- 
%  Randy Yates                  % "My Shangri-la has gone away, fading like 
%% Fuquay-Varina, NC            %  the Beatles on 'Hey Jude'" 
%%% 919-577-9882                %  
%%%% <yates@ieee.org>           % 'Shangri-La', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:

> Randy Yates wrote:
>
>
>>>>So I guess I have two questions: 1) Is it possible to design
>>>>single-carrier systems (including the receiver) which are just as
>>>>immune to multipath as multi-carrier systems?
>>>
>>>Certainly. Although it will require a huge amount of computation.
>> Why? What sort of things would you have to do to a single-carrier
>> signal to "get out the multipath?"
>
> The single carrier receiver will have to use an adaptive model of the
> channel and apply some sort of the maximum likelihood algorithm for
> the optimal demodulation. This is a heavy computational task.

But is the computational complexity significantly greater than
O(N*log(N))? That is the computational load an OFDM system must
have from the git-go.
-- 
%  Randy Yates                  % "With time with what you've learned, 
%% Fuquay-Varina, NC            %  they'll kiss the ground you walk 
%%% 919-577-9882                %  upon."
%%%% <yates@ieee.org>           % '21st Century Man', *Time*, ELO
http://home.earthlink.net/~yatescr