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