On 11 Aug 2005 07:23:34 -0700, jaco.versfeld@gmail.com wrote:
>Hi,
>
>What is the difference between DMT and OFDM. Why isn't OFDM used in
>the xDSL technologies?
>
>If I have to guess the difference, I think that OFDM has several
>carrier frequencies that are orthogonal. DMT I think uses carriers
>seperated by some space on the frequency spectrum.
>
>Kind Regards,
>Jaco
They're essentially the same thing. When people use the term "DMT"
they're almost always referring to the ADSL wireline standard, since
that's the terminology used there. Plunk the exact same thing down
anywhere else and it would be called OFDM.
So it's marketing, more or less. ;)
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org
Reply by Randy Yates●August 11, 20052005-08-11
porterboy76@yahoo.com writes:
>> What is the difference between DMT and OFDM. Why isn't OFDM used in
>> the xDSL technologies?
>
> OFDM and DMT are mostly the same thing.
> [...]
Nice response!
--
% 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
Reply by Steve Underwood●August 11, 20052005-08-11
porterboy76@yahoo.com wrote:
> In fact latest multicarrier schemes relax the perfect reconstruction
> constraint on the filterbanks being used, and implement near-perfect
> reconstruction multicarrier modulation. Some have even taken to calling
> this NOFDM (non-orthogonal frequency division multiplexing) but why
> they dont just call this FDM is beyond me. Filtered Multitone, from IBM
> is another example of marketeers hijacking a simple concept and selling
> their spin on it. This is just good old FDM which has existed (perhaps
> not digitally) for over half a century, if not more.
Actually it was bad old FDM before DSP. The analogue systems were so
troublesome, due to drift, that the first civilian comms systems to go
fully DSP were the last generation of PSTN FDM stacks, around 1980.
Regards,
Steve
Reply by ●August 11, 20052005-08-11
> What is the difference between DMT and OFDM. Why isn't OFDM used in
> the xDSL technologies?
OFDM and DMT are mostly the same thing. They both refer to
multi-carrier communication implemented digitally by means of a DFT. In
both cases, all subchannels are orthogonal, and they both offer the
useful trick that a short cyclic prefix on the transmit signal allows
perfect distortionless data recovery at the transmitter, even when
transmitting over a dispersive channel.
The two main differences between DMT and OFDM are:
1. OFDM refers to wireless bandpass multicarrier communication, while
DMT is baseband wireline multicarrier communication.
2. DMT, because of the slowly varying nature of the channel, allows
spectral shaping by bit-loading, to take advantage of the measured
channel characteristics.
These are the conventional and common-use interpretations of OFDM and
DMT. However, they are of course arbitrary, and have taken their
respective meanings by default, rather than by design... wireless
engineers went for OFDM , while John Cioffi's crew over at Stanford,
and Bellcore went for DMT for ADSL (think they slapped all sorts of
patents over it).
There is no reason why DMT and OFDM couldnt be interchanged in their
usage, since wireless multicarrier communications implemented by a DFT
is of course "discrete multi-tone" and baseband wireline multicarrier
comms implemented by a DFT is of course "orthogonal FDM". In fact there
is no reason why OFDM shouldnt refer to discrete wavelet multitone,
wavelet packet modulation, and any modulation based on orthogonal
filterbanks since these all implement "orthogonal frequency division
multiplexing". Of course this is a heresy in some circles.
In fact latest multicarrier schemes relax the perfect reconstruction
constraint on the filterbanks being used, and implement near-perfect
reconstruction multicarrier modulation. Some have even taken to calling
this NOFDM (non-orthogonal frequency division multiplexing) but why
they dont just call this FDM is beyond me. Filtered Multitone, from IBM
is another example of marketeers hijacking a simple concept and selling
their spin on it. This is just good old FDM which has existed (perhaps
not digitally) for over half a century, if not more.
There is real need for a convergence of nomenclature here, before the
whole thing gets totally confusing. Personnally, I refer to all digital
FDM schemes as multicarrier communications, and then classify schemes
within this framework as orthogonal or not, oversampled or critically
sampled, lapped or block transform, spectrally uniform or not, coded or
uncoded, baseband or bandpass, wireline or wireless and so on.
To answer your question, xDSL technically does use "OFDM" if you mean
what the letters stand for and not what convention tells you OFDM means.
Reply by Steve Underwood●August 11, 20052005-08-11
jaco.versfeld@gmail.com wrote:
> Hi,
>
> What is the difference between DMT and OFDM. Why isn't OFDM used in
> the xDSL technologies?
>
> If I have to guess the difference, I think that OFDM has several
> carrier frequencies that are orthogonal. DMT I think uses carriers
> seperated by some space on the frequency spectrum.
>
> Kind Regards,
> Jaco
>
Maybe you could arrange the spacing of DMT carriers so they are
orthogonal, and DMT would be a lot like OFDM? :-)
Regards,
Steve
Reply by ●August 11, 20052005-08-11
Hi,
What is the difference between DMT and OFDM. Why isn't OFDM used in
the xDSL technologies?
If I have to guess the difference, I think that OFDM has several
carrier frequencies that are orthogonal. DMT I think uses carriers
seperated by some space on the frequency spectrum.
Kind Regards,
Jaco