Re: [Discuss-gnuradio] Power line communications

[Johnathan Corgan]

On Sun, Feb 19, 2012 at 17:33, George Nychis <address@hidden> wrote:

>> 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.
>
> This was a super informative post, it's really interesting to hear about
> experience of actually building PLC in practice.

Thanks.

> I haven't seen very many
> studies on it, but it seems like different buildings have greatly varying
> profiles.  I didn't think of things like elevator motors in office
> buildings, but that's got to introduce a lot of interference.

Cheap switching power supplies (wall warts) are issues as well, if
they are plugged in close to the power coupler.

> If you ever do this again, I would love to see some of the channels over
> time.  I ordered some basic PLC equipment, but all I really have access to
> are packets :P

I'll try to grab a spectrum analyzer plot from my lab.  If I find time
I'll see if I can get a channel sounding.

>> 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.
>
> Nice, I guess with unidirectional broadcast that does work out well for you.
>  How do you actually get echoes in a PL?  I never thought about this.

Any time there is a discontinuity in impedance on a transmission line,
there will be a reflection, including at the end of an open circuit.
Power lines have lots of open branches that signals can go down, get
reflected back from, and form a delayed sum with the original signal.
This can include, for example, traveling out to pole transformers and
back.

One idea would be to measure the impulse response between two power
line endpoints, then use that as a baseline to detect changes which
could mean damage or unauthorized access.

> It would be really cool to be able to hook up the USRP to a PL and
> understand various channels in different types of buildings.  Totally not
> conducive to my dissertation, but I find it interesting.

Happy to distract you :-)

Johnathan