On Tue, Aug 9, 2011 at 5:22 PM, Marcus D. Leech <mle...@ripnet.com> wrote:
> Keep in mind the old information theorist's adage: if you don't have >> bit errors, you're using too much power! (ok, I don't know how old >> that is; fred harris always quotes it, but he credits someone else >> with it, probably Tony Constantinides). >> >> In other words, we normally design our systems around having bit >> errors, and indeed we recognize that they are unavoidable except under >> extreme SNR conditions. To compensate, you really want to you some >> kind of channel coding. The way things are in our benchmark code, a >> single bit error means that an entire packet is lost. >> >> Tom >> >> In radio systems, I agree--bit errors are a fact of life, and you can > cope with them either with protocol design, or > "frame design". The trend in the last couple of decades for radio systems > has been to incorporate some sort of > FEC, to reduce the impact of channel distortions--the receiver can simply > reconstruct from the FEC data, or, > force a re-transmit. Systems that use FEC almost always assume that > there's a higher-layer protocol mechanism > in place for dealing with packets that were too damaged to decode, and > thus must be re-transmitted. > > On the other hand, there are plenty of extant *wired* communications > systems in which bit-errors are exceedingly rare. > The various Ethernet standards for example, assume that bit errors aren't > common, and there's no FEC (at least at > 100Mbit and 10Mbit levels--I'm not sure about 1000Mbit and 10000Mbit). > > The problem is that many communications/networking engineering types who > are new to radio don't really understand, on a > visceral level, that the radio channel environment is different from > wired, not just in degree, but in type, of channel distortions. > And further, their experience with a channel model for wireless may > include only simulations, rather than "real world". > > My very earliest internet connection at home, back in the mid-to-late > 1980s, was wireless. Over an amateur-radio 56KBps radio > link using a "split repeater" on 220Mhz and 432Mhz. It wasn't a very nice > environment. Various RFI issues, hidden-terminal issues. > Collisions. Multi-path. Receiver de-sense. And a complete lack of any > FEC. Given all of that, I'm stunned that LTE and WiFi and all its > modern friends work at all :-) > > > > -- > Marcus Leech > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > > > > ______________________________**_________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/**listinfo/discuss-gnuradio<https://lists.gnu.org/mailman/listinfo/discuss-gnuradio> > Also point to point wireless comm. with a single antenna is also more or less a solved problem these days. Unless you are trying to do something in a weird channel or with very very low power. I guess they have said "coding is dead" many times in the past and have been wrong. Or thought CDMA or TDMA is king, etc, etc. Things are more interesting when you look at PHY/MAC cross layer work. I'm also surprised they get LTE to work...16 b/s/hz spectral efficiency....thats pretty up there.
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