Re: [EFM] Modulation and Digital Duplexing (DD)
Vlad,
I think we both agreed that DD and DF are not the same
DD requires synchronization and the other one does not and the effects are
also different
we both covered the quantization noise and that is also independent of DD or
DF
folks I think this is the last e-mail from our sides on this subject
Behrooz
----- Original Message -----
From: "Vladimir Oksman" <voksman@xxxxxxxxxxxx>
To: "Behrooz Rezvani" <brezvani@xxxxxxxxxx>
Cc: <aidan@xxxxxxxxxxxx>; "'Wei, Dong'" <wei@xxxxxxxxxxx>;
<daun@metanoia-technologies.com>; <stds-802-3-efm-copper@ieee.org>;
<stds-802-3-efm@ieee.org>; "'Zagalsky, Nelson'" <Nelson_Zagalsky@adc.com>
Sent: Tuesday, July 09, 2002 12:48 PM
Subject: Re: [EFM] Modulation and Digital Duplexing (DD)
> Behrooz,
>
> indeed, less ADC bits didn't cut the transmission, but cut the
performance
> just because the quantization noise grows to high. For short reach (high
speeds)
> this has much less impact than for long reach, because FEXT is very high
and
> masks quantization noise. For long reach all changes: FEXT is attenuated
by the
> loop and quantization noise turns to be the main issue. So, it doesn't
surprise
> me that people can demonstrate digital duplexing even today.
>
> Regarding orthogonality, I think that both SCM and DMT are "almost
> orthogonal". In SCM that's because the leakage, in DMT due to timing
issues
> (except we assume that all if fully synchronize). I still think the
> orthogonality is not really an issue.
>
> Regarding your question - we are definitely going to implement
digital
> duplexing in SCM at the moment ADC became cheap enough. It is just the
same as
> for today EC systems (HDSL, ISDN, SHDSL, HDSL2) which use digital EC
duplexing.
>
> Vladimir
>
> Behrooz Rezvani wrote:
>
> > Vladimir,
> >
> > we looked at some of the points you brought up as well as some others
and
> > evaluated the problem from all angles and we concluded that it is still
> > better to do digital duplexing. By the way we are doing digital dulexing
> > with a lot lower number of A/D bits and we are getting very high
performance
> > level. In DMT, digital duplexing does not add anything to the existing
cost.
> > The DAC or ADC resolution because of it does not increase and no extra
> > hardware is required.
> >
> > Digital Duplexing and Digital filtering have some differences. In
digital
> > duplexing, the signals are orthogonal whereas in digital filtering they
are
> > all most orthogonal.
> > There are some merits to do it this way.
> > 1) the efficient use of frequency, because of orthogonality property of
> > FFT/IFFT there is very little loss in guard bands.
> > 2) Flexibility, there is no added filtering for Tx and bands can be
easily
> > divided in any way you choose,
> >
> > Do you think you digital duplexing makes sence for SCM?
> > In any event There are very efficient ways of implementing digital
duplexing
> > in DMT. This does not mean that QAM needs to do that. I am sure digital
> > filtering can be used but I think they have different role
> >
> > Kind regards
> > Behrooz
> >
> > ----- Original Message -----
> > From: "Vladimir Oksman" <voksman@xxxxxxxxxxxx>
> > To: "Behrooz Rezvani" <brezvani@xxxxxxxxxx>
> > Cc: <aidan@xxxxxxxxxxxx>; "'Wei, Dong'" <wei@xxxxxxxxxxx>;
> > <daun@metanoia-technologies.com>; <stds-802-3-efm-copper@ieee.org>;
> > <stds-802-3-efm@ieee.org>; "'Zagalsky, Nelson'"
<Nelson_Zagalsky@xxxxxxx>
> > Sent: Monday, July 08, 2002 8:20 AM
> > Subject: Re: [EFM] Modulation and Digital Duplexing (DD)
> >
> > > Behrooz,
> > >
> > > the upstream and downstream carriers are always almost
orthogonal
> > > otherwise one cannot separate them. The grade of this "almost" in the
case
> > of
> > > FDD both MCM and SCM currently use depends on filtering technique. If
we
> > assume
> > > that ADC and DAC has unlimited resolution, there could be always found
a
> > > suitable bandpass filter to provide small enough crosstalk between
> > upstream and
> > > downstream. DMT is doing that using FFT and windowing as a filter, SCM
> > usually
> > > uses digital filters. I don't know exactly which are more complex, but
for
> > now
> > > neither is a problem of digital duplexing we really face.
> > >
> > > The today problem is lack of resolution of ADC and DAC to deal
with
> > echo
> > > (assuming relevant implementation for the hybrid). Thus all mostly
> > depends on
> > > the dynamic range of the transmit signal, where SCM due to small
number of
> > bands
> > > usually has 5-6 dB advantage. You can see some calculations for
ADC/DAC
> > > requirements for DMT in T1E1.4 contributions (Ottawa, 1999).
> > >
> > > Kind regards,
> > >
> > > Vladimir
> > >
> > > Behrooz Rezvani wrote:
> > >
> > > > Aiden,
> > > >
> > > > Thanks for touching on the subject of Digital Duplexing and I would
like
> > to
> > > > add few words to it. As you correctly pointed out Digital Duplexing
is
> > > > independent of modulation and I hope to see the use of this
technique
> > for
> > > > PHY devices to become more readily available.
> > > >
> > > > On the technical issue I think you covered the "necessary" condition
and
> > I
> > > > would like to discuss the "sufficient" condition in order to have
> > digital
> > > > duplexing done
> > > >
> > > > I will focus mostly on DMT base modulation and may be you may want
to
> > take a
> > > > crack at QAM based modulation on this is accomplished.
> > > >
> > > > In DMT based modulation one has many sub-carriers similar to OFDM
based
> > > > techniques of 802.11a. The sub-carrier bandwidth is typically 4.135
KHz
> > (and
> > > > other options also exist) The number of these sub-carriers (or
tones)
> > can be
> > > > as much as 4196. Digital Duplexing (DD)in DMT fundamentally works
on
> > the
> > > > principal of orthogonalizing the downstream transmission to upstream
> > > > transmission. Remember that having large dynamic range A/D or D/A is
not
> > > > sufficient to perform DD. In VDSL-dmt frame there is function that a
dds
> > > > Cyclic Prefix or Suffix to the symbol itself to allow the movement
the
> > > > downstream waveform wrt upstream waveform. During the timing
recovery
> > and
> > > > training mode symbol boundary is obtained using x-correlation
methods.
> > Based
> > > > on these information then upstream and downstream waveform are
> > synchronized
> > > > to each other. Cyclic prefix and suffix are used to adjust the
waveforms
> > in
> > > > time domain. (This is somewhat similar to ranging algorithm used in
PON
> > type
> > > > of network where based on the distance of a CPE and CO a time
reference
> > is
> > > > obtained)
> > > >
> > > > Upon completion of this task then ANY of the tones can transmit in
> > upstream
> > > > or downstream direction. In other words one can have as many
bandplans
> > that
> > > > one can imagine [combinatory problem: Normally there is no need for
> > large
> > > > number of bandplans. In VDSL we have 4-bands + an optional band.
This is
> > > > however designed to address a large population of loops with one
shot.
> > If
> > > > one allows more of local optimization view then one can imagine the
need
> > for
> > > > other band plans or more bands]
> > > >
> > > > I assume this can be done in QAM based methods, however the number
of
> > bands
> > > > are limited to the number of modulated carriers.
> > > >
> > > > Best
> > > > Behrooz
> > > >
> > > > ----- Original Message -----
> > > > From: "Aidan O'Rourke" <aidan@xxxxxxxxxxxx>
> > > > To: "'Behrooz Rezvani'" <brezvani@xxxxxxxxxx>; "'Wei, Dong'"
> > > > <wei@xxxxxxxxxxx>; <daun@xxxxxxxxxxxxxxxxxxxxxxxxx>
> > > > Cc: <stds-802-3-efm-copper@ieee.org>; <stds-802-3-efm@ieee.org>;
> > "'Zagalsky,
> > > > Nelson'" <Nelson_Zagalsky@xxxxxxx>
> > > > Sent: Tuesday, July 02, 2002 10:49 AM
> > > > Subject: RE: [EFM-Copper] RE: [EFM] (forward) progress in EFM copper
> > > >
> > > > > Behrooz,
> > > > >
> > > > > In an attempt to provide some clarification I would like to point
out
> > to
> > > > > members of the reflector, that digital duplexing and line coding
are
> > > > > entirely separate issues.
> > > > >
> > > > > Digital duplexing is the concept of digitizing a region of
spectrum
> > and
> > > > > employing digital filtering techniques to ensure that out of band
> > signal
> > > > and
> > > > > noise is not applied to the receiving demodulator. Whether or not
this
> > > > > demodulator employs FFT or QAM demodulation is an entirely
separate
> > > > matter.
> > > > >
> > > > > The main requirement for digital duplexing is that you have an A
to D
> > > > > converter that has enough dynamic range and low enough
quantization
> > noise
> > > > to
> > > > > enable digital techniques to be employed. I am not aware of any
> > advantage
> > > > > that DMT has in its ability to be digitized more readily, or with
any
> > less
> > > > > complexity than single carrier. If you are aware of these
advantages I
> > > > would
> > > > > be very interested to learn more.
> > > > >
> > > > >
> > > > > Best Regards,
> > > > >
> > > > > Aidan.
> > > > >
> > > > >
> > >
> > >
>
>