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
Sent: 02 December 2017 18:38
To: freetel-codec2@lists.sourceforge.net
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 <kk4...@gmail.com> 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
> kk4...@gmail.com
>
> From: Samuel Hunt
> Sent: Saturday, December 2, 2017 10:35 AM
> To: freetel-codec2@lists.sourceforge.net
> 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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