The analog FM modulator/demodulator is going to be your main limit. Of
course, we don't have off-the-shelf handheld SDRs yet.
On Sat, Dec 2, 2017 at 1:27 PM, Samuel Hunt <[email protected]> wrote:
> Hi Adrian,
>
>
>
> This I understand, but then to double the air rate would then lead to a
> net reduction in sensitivity due to the wider bandwidth.
>
>
>
> What I trying to establish is if the coding gain from increasing the error
> correction would be more than the sensitivity loss for widening the
> bandwidth.
>
>
>
> It may be that you lose 6db of sensitivity from the wider bandwidth bit
> gain 8db of coding gain, therefore net 2db improvement.
>
>
>
> These are the figures I am just not familiar with.
>
>
>
> Sam
>
>
>
>
>
> *From: *Adrian Musceac <[email protected]>
> *Sent: *02 December 2017 18:38
> *To: *[email protected]
> *Subject: *Re: [Freetel-codec2] Ultra-reliable sensitive comms
>
>
>
> Hi,
>
>
>
> Channel coding will improve performance, depending on the type of code
>
> used. For example, in QRadioLink I use a rate 1/2 convolutional code,
>
> which leads to twice the symbol rate. This gives me an improvement of
>
> 3 dB over no code/half symbol rate mode. LDPC and puncturing would
>
> improve it even more with small increase in symbol rate or bandwidth.
>
>
>
> Regards,
>
> Adrian YO8RZZ
>
>
>
> On 12/2/17, Nino Carrillo <[email protected]> wrote:
>
> > Hi Sam,
>
> >
>
> > I’ll take a stab at your baud question.
>
> >
>
> > Seems to me that lower symbol rate equates to more energy per symbol, and
>
> > therefore higher probability of receiving said symbol at the distant
>
> > station. At the same received power level, a system with a lower symbol
> rate
>
> > will be more successful at synchronizing to the received symbol stream in
>
> > order to then (later in the receive chain) apply error correction to said
>
> > symbols.
>
> >
>
> > It’s outside the scope of this list, but I’ve been working on a related
> idea
>
> > using the ADF7021 transceiver chip. I’d be happy to chat with you about
> it,
>
> > feel free to email me.
>
> >
>
> > Nino Carrillo
>
> > KK4HEJ
>
> > [email protected]
>
> >
>
> > From: Samuel Hunt
>
> > Sent: Saturday, December 2, 2017 10:35 AM
>
> > To: [email protected]
>
> > Subject: [Freetel-codec2] Ultra-reliable sensitive comms
>
> >
>
> > I am hoping there will be some people on this list far more experienced
>
> > than me to be able to advise on this!
>
> >
>
> > Am playing with VHF voice, etc, and I am thinking what is the best way
>
> > to get ultra-reliable (super-sensitive) performance on a VHF modem.
>
> >
>
> > I am thinking for VHF, based on a radio where only a normal FM modulator
>
> > with a Class C PA is available, so it would have to be something like
>
> > 4FSK. Bandwidth isn't a huge problem (say 12.5khz is available which
>
> > gives loads of headroom for 1000 baud Codec2).
>
> >
>
> > Is it better to run 4FSK at a low baud rate (say using one of the low
>
> > baud rate versions of the codec), then have just a little error
>
> > correction, on the basis that low baud = narrow IF = sensitive, or is it
>
> > better to run at a high baud, really heavy on the error correction, such
>
> > as 4800 baud and then have loads of error correction such that the
>
> > actual throughput is only 1000 baud?
>
> >
>
> > Basically which would work better - 4800baud air with 1000 baud after
>
> > error correction, or 1200 baud air with 1000 baud after error correction?
>
> >
>
> > This would be assuming 4m band (70mhz) where Rayleigh fading isn't very
>
> > predominant.
>
> >
>
> >
>
> > Also, I know it becomes the law of diminishing returns between 4FSK,
>
> > 8FSK, 16FSK, etc, and it is generally felt that 4FSK is the most
>
> > reliable, but is there any advantage to spacing the tones further apart
>
> > except from a small "guard band" would help prevent interference between
>
> > the tones. Presumably if the tones start to get really spaced out then
>
> > there is absolutely no advantage because you just waste spectrum with
>
> > huge guard bands for the 4 filters on receive.
>
> >
>
> > I know that the work so far on Codec 2 has been based on low baud, low
>
> > error correction, but I am wondering if this equals best sensitivity or
>
> > if this is more about bandwidth efficiency.
>
> >
>
> >
>
> > I just wondering best way to get really sensitive comms, so a 5W HT
>
> > could go for miles and miles.
>
> >
>
> > Sam
>
> > M1FJB
>
> >
>
> >
>
> >
>
> >
>
> >
>
> > ------------------------------------------------------------
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>
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>
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>
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>
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>
> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>
> >
>
> >
>
>
>
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