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RE: DSP Possibilities ?



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

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