Tomas Härdin
2016-02-11 11:14:58 UTC
Hi list
I've been looking at designing modems for what you might call QRPP for
the last few days, and I've been following Mike's designing his open VHF
FSK modem. What I'm thinking about may follow along lines similar to the
goals of the FreeDV project, and feels more relevant than whatever drama
I saw someone try to stir up on here earlier today
My question is simple: how low can you go in terms of Joules per user
bit, if you're allowed arbitrarily high capacity-to-bandwidth ratios
(say 1:10 or 1:100)?
Terms more relevant to this list might be: what if we ran the 700 bit/s
system in something like a 12.5 kHz channel? Could that improve the
range we get out of a 5W PA? Our old friend the channel capacity theorem
supports this. Letting S/N = (d0/d)^2*SNR_d0 and solving for d/d0 for a
700 bit/s channel gives:
d/d0 = [(2^(700/B)-1)/SNR_d0]^(-1/2)
In the 12.5 kHz example above, assuming we have some range d0 for an SNR
of 0 dB in a 700 Hz band, this suggests we should be able to get five
times the range out of the same amount of energy(!)
Anywho, I'm also in the process of designing an
as-simple-and-compact-as-possible BPSK (really DSB-SC)
modulator+transmitter since BPSK seems to have the best chance in terms
of Eb/No. Part of that will include writing simulations in Octave to try
and work some of this out, so quite a bit of perspiration involved. The
plan is to add on FEC and crank up the bitrate. I'm trying to ensure at
this early stage that I'm not making any fundamental mistakes however,
and good ideas are welcome
73 de sa2tms
/Tomas
I've been looking at designing modems for what you might call QRPP for
the last few days, and I've been following Mike's designing his open VHF
FSK modem. What I'm thinking about may follow along lines similar to the
goals of the FreeDV project, and feels more relevant than whatever drama
I saw someone try to stir up on here earlier today
My question is simple: how low can you go in terms of Joules per user
bit, if you're allowed arbitrarily high capacity-to-bandwidth ratios
(say 1:10 or 1:100)?
Terms more relevant to this list might be: what if we ran the 700 bit/s
system in something like a 12.5 kHz channel? Could that improve the
range we get out of a 5W PA? Our old friend the channel capacity theorem
supports this. Letting S/N = (d0/d)^2*SNR_d0 and solving for d/d0 for a
700 bit/s channel gives:
d/d0 = [(2^(700/B)-1)/SNR_d0]^(-1/2)
In the 12.5 kHz example above, assuming we have some range d0 for an SNR
of 0 dB in a 700 Hz band, this suggests we should be able to get five
times the range out of the same amount of energy(!)
Anywho, I'm also in the process of designing an
as-simple-and-compact-as-possible BPSK (really DSB-SC)
modulator+transmitter since BPSK seems to have the best chance in terms
of Eb/No. Part of that will include writing simulations in Octave to try
and work some of this out, so quite a bit of perspiration involved. The
plan is to add on FEC and crank up the bitrate. I'm trying to ensure at
this early stage that I'm not making any fundamental mistakes however,
and good ideas are welcome
73 de sa2tms
/Tomas