[EFM] Modulation and Digital Duplexing (DD)
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 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
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@broadcom.com>
To: "'Behrooz Rezvani'" <brezvani@ikanos.com>; "'Wei, Dong'"
<wei@tri.sbc.com>; <daun@metanoia-technologies.com>
Cc: <stds-802-3-efm-copper@ieee.org>; <stds-802-3-efm@ieee.org>; "'Zagalsky,
Nelson'" <Nelson_Zagalsky@adc.com>
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.
>
>