Re: [EFM] Modulation and Digital Duplexing (DD)
On these duplexing/modulation methods -- there are a couple of
book Chapters (4 and 5) at a class web site (its my site, and
the book is free to those with a printer):
(click on "course reader" category)
The method to do digital duplexing involves a modification of the
QAM/DFE structure that is called a "Circulant DFE" there.
Behrooz is correct it is limited to number of carriers, and
the procedure there shows how to get correct sampling rates
and transmit/receive filters.
At 06:16 PM 7/7/2002 -0700, Behrooz Rezvani wrote:
>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
>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 adds
>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
>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.
>----- Original Message -----
>From: "Aidan O'Rourke" <aidan@xxxxxxxxxxxx>
>To: "'Behrooz Rezvani'" <brezvani@xxxxxxxxxx>; "'Wei, Dong'"
>Cc: <email@example.com>; <firstname.lastname@example.org>; "'Zagalsky,
>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
> > noise is not applied to the receiving demodulator. Whether or not this
> > demodulator employs FFT or QAM demodulation is an entirely separate
> > 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
> > 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
> > be very interested to learn more.
> > Best Regards,
> > Aidan.
John M. Cioffi
363 Packard Electrical Engineering Bldg.
350 Serra Mall
Stanford, CA 94305-9515
+1-650-723-2150 Fax: +1-650-724-3652