For a 16kHz wide or 10kHz wide IF filter with non linear demodulation, I
consider 2FSK the mode to choice where the bitrate can be satisfied .
4FSK takes a big hit in this implementation case, for reasons outlined
in many places and many times here and elsewhere. (DMR/P25 chaose 4FSK
to satisfy the bandwidth / implemenation simplifications)
For a linear demod receiver, 4FSK has the numbers on 2FSK. 2x the
numbers in fact. The 4FSK demod implementation is trivial, and arguably,
no more complex than a 2FSK system.
My preference is actually PSK, but that's another story, and has hefty
demod requirements to deal with adverse conditions, but my own
appslications have MFLOPS to burn....Like I want negative SNR dmeod
performance because I overlay a bunch of signals and subtract one from
the other.
*****
BUT like all these things, arguments on one modulation over another
should ALWAYS be accompanied by objectives , limitations and
restrictions on the application. There are always pluses and minues for
various applications.
******
glen
On 6/12/2017 8:49 AM, Adrian Musceac wrote:
OK Cool - OFDMA is new to me - I'll read up on it.
Hi David,
Consider this: with carrier per user instead of timeslot per user, you
don't have the distance limitation that TDMA introduces. Also, the
code is slightly more simple as it doesn't require tx burst timestamps
and hardware to support it. I've spent a week digging into the GSM
implementation of the osmo-trx software, and it's not straightforward
to do with a Gnuradio flowgraph. Reception is easy but transmission is
harder, and if you aim to maximize capacity you have very little time
to fit into a slot. Also, with OFDMA the PAPR issue is offset by
narrower receive bandwidth, unlike TDMA where you take the SNR hit for
all channels at the same time. I think it's worth looking into, I may
even have a demo by the end of the year if the holidays are
uneventful.
There are carefully tested Octave and C 4FSK modems that meet
theoretical performance in codec2-dev that you can use as a reference.
With open source, implementation complexity isn't a huge issue - someone
only has to build it once. mFSK isn't that much harder than 2FSK, and
certainly easier than a PSK modem.
I'd argue that 2FSK with convolutional encoding is way easier to
implement than 4FSK with same. But that's just my experience with
Gnuradio and may reflect my laziness. I'd say there are few people
writing code on this sort of stuff in the open, but I've seen open
source code for Yaesu's C4FM and the demodulation is much more complex
than generally believed. Meanwhile people tend to disregard open
source projects as inferior, so getting traction won't happen soon.
Best regards,
Adrian
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