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Re: [Discuss-gnuradio] Reminder about call today

From: Johnathan Corgan
Subject: Re: [Discuss-gnuradio] Reminder about call today
Date: Thu, 16 Feb 2012 11:53:37 -0800

On Thu, Feb 16, 2012 at 10:24, Tom Rondeau <address@hidden> wrote:

> Powerline communications have been discussed for quite
> a while, but they tend to be horrible channels (old copper, long distances,
> bad shielding, etc.). So yes, I'd expect OFDM for the equalization and
> multipath (or 'echos' in this case) issues and heavy channel coding are
> required.

OFDM for the equalization and FEC for the channel errors has been a
successful technique used in residential power-line communications.
However, commercial buildings tend to have long and unpredictable
power delay profiles, requiring long symbol times.  In addition, there
are enormous sources of interference (primarily impulse noise) from
things like elevator motors and paper shredders (!), as well as strong
shortwave RF induced signals.  Finally, some installations use
multiple transformers between floors, with their own very
unpredictable channel response.

As an alternative, one commercial R&D contract I did a while back was
to use DSSS/CDMA with long codes to reduce the interference by the
coding gain, then use additional delay correlators to receive and
combine echos (a basic RAKE receiver).  This worked extremely well for
interference rejection, without FEC at the data level, but at the
expense of a low bit rate and long synchronization times.  Since the
communication requirement was a unidirectional broadcast, this worked
out well.

The initial implementation was done in GNU Radio with a chipping rate
of 2 Mcps, then ported to the USRP2 FPGA (minus the rake) for chipping
rates up to 25 Mcps.

I think this goes to the point that GNU Radio is an excellent tool for
experimentation, R&D, prototyping, etc., and not so much for chasing
existing standards.

> My main question regarding this topic is to figure out what would be
> necessary to create a test bed to try it out on? Obviously, we aren't going
> to stick the Rx/Tx port of the USRP into a wall socket :)

That company made some generic 50-ohm to wall socket power couplers
for the project, but I don't know if they ever made them commercially.
 I still have a pair in my lab to play with :)


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