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Update: DTV antenna on VHF
On Jun 14, 10:03*am, Richard Clark wrote:
In the same graph, Shannon reveals how, if you code your bits (I will leave it to the student to discover the meaning of that), you could achieve the same 1:1000000 advantage with the addition of less than 1 dB of power boost. 73's Richard Clark, KB7QHC Until fairly recently, hams didn't do much coding, for a variety of reasons. Computational horsepower is probably a big reason. Coding's easy, decoding not so easy, at least in a "parts readily available from Radio Shack" sort of sense. Obviously, today, one could do all sorts of coding on a laptop PC, particularly at low bit rates, but you'd still need to have an unusual convergence of someone who knows how to implement the coding algorithms who's also interested in amateur microwave operating. It's not anything like a turnkey thing, or even a "go get gnuradio" thing. Where you see coding in common ham use, it's buried in an application (PSK31, JT65, and the like) The other problem is the frequency control issue. If you want low rates and ragged edge of Shannon, you need good frequency stability and control (and to a lesser extent, good phase noise). Until recently (with GPS disciplined oscillators and surplus Rb sources) this was a real challenge. As Rich commented with respect to antenna pointing, you also have to be right on for frequency, and that's hard, especially in a field situation. Tuning to 10Hz accuracy at 10GHz implies 1E-9 frequency accuracy, which is challenging. To a certain extent, processing power in a PC helps (get close, do parallel demodulation, find the signal), but just like for coding, it requires finding a person (or small group) who can deal with building low phase noise stable oscillators AND with developing software that is somewhat complex, compared to the usual "whack it out in a weekend of coding" stuff. I suspect there ARE hams experimenting with this, but it's a long way from critical mass wide acceptance. You need something that you can write an article in QST, and offer $100 widgets to make that happen. There's not much cheap surplus gear either, since commercial equipment these days tends to be more specialized and isn't as amenable to hackery. There are also proprietary rights issues with some coding techniques (e.g. Turbo) but I suspect that legal issues aren't what's holding hams back. For things like LDPC, there are published software implementations that are free to use. I haven't looked but I imagine that various convolutional codes and decoders are also publicly available, along with Viterbi soft-decision decoders. |
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