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*Subject*: RE: [amsat-bb] DSP Possibilities ?*From*: "Howie DeFelice" <hdefelice@xxxxxxxxxxxxxx>*Date*: Wed, 20 Sep 2000 11:17:50 -0400

I don't know about the DSP aspects, but using higher order modulation schemes will require more power. Increasing the bits/hertz density will reduce the bandwidth requirement for a given data rate but this comes at the price of higher S/N ratios in order to accurately demodulate these more complex waveforms. In the case of 16QAM with no form of FEC, I estimate a S/N ratio of 20 dB or better would be required. Commercially, higher order modulation is almost always accompanied by enhanced FEC. The current popular method is encoding the data with Viterbi FEC then wrapping the encoded stream with interleaved Reed-Solomon. If not for this, those 18" DTV dishes would need to be at least a meter in diameter. A new form of FEC called Turbo Coding is now possible, which is more bandwidth efficient than the Viterbi/Reed-Solomon method. All the current commercial digital satellite modems I know of use DSP to modulate/demodulate IF at rates up to 155 MBPS. However, expect to pay $ 10,000 for a modulator or demodulator in this range. On the other hand, commercial modems in the sub 2 MBPS range are around $3,000. All make extensive use of ASIC and FPGA technology. The problem as I see it in adapting these technologies to amateur use are: 1) developing a pool of programmers and 2) getting sufficient quantity to make parts acquisition financially viable. I think #1 is a MUCH bigger problem than #2. Howie AB2S ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

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