Re: [EFM] Modulation and Digital Duplexing (DD)
AGREE..........
Vladimir
Behrooz Rezvani wrote:
> 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.
> > > > > >
> > > > > >
> > > >
> > > >
> >
> >