Re: [EFM] Modulation and Digital Duplexing (DD)
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).
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
> 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 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'"
> <wei@xxxxxxxxxxx>; <daun@xxxxxxxxxxxxxxxxxxxxxxxxx>
> Cc: <firstname.lastname@example.org>; <email@example.com>; "'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
> > 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.