indeed
commercial 4FSK implementations are more about bandwidth than
sensitivity. Frequency spacing between to tones is bandwidth optimizated
not system sensitivity optimized.
if we consider each of the 4 tones of 4FSK being received by a separate
receiver (easier to conceptualize) , each tone will have half the
information rate of each tone in a 2FSK system.
And with half the information rate we have half the noise bandwidth, and
twice the sensitivity.
Our optimal 4FSK receiver, being four filters on four frequencies only
has a noise bandwidth of the information rate in one tone. A commercial
implementation of 4FSK (P25, DMR) has a noise bandwidth >= than the
information rate of the total payload, this costs the modem significant
performance.
On 5/12/2017 10:00 AM, Gullik Webjorn wrote:
Hmm, if we think of a modulation index around 1, Carsons rule says Bt
= 2*(deltaF + fm ).
If it is the modulation index that governs SNR in the recovered
baseband, the same index
will occur at 1/2 deltaF if we reduce fm by 2, i.e. symbol rate.
Thus 4FSK would require half the swing ( for same Beta ) and that
would imply half deltaF.
We could reduce the bandwith to half, and gain 3 dB of reduced noise.
Of course there is
also a loss since our baseband must be discriminated with twice as
many levels.
So, IS 4-FSK more power efficient than 2-FSK? Commercial FM has a
Large Beta. ( to exploit SNR )
Gullik / SM6FBD
On 2017-12-04 21:04, David Rowe wrote:
Hi Adrian,
Fine business on your experiments. My thoughts:
+ I like 4FSK as it's constant amplitude, simplifying the PA and rx
(limiting amps can be used at IF), and the modems are easy to get
working at ideal performance. It's reasonably bandwidth and power
efficient, especially as we have better voice compression than
incumbent DV modes.
+ OFDM isn't needed unless your bandwidth is significantly wider, as
its a flat fading channel. Single carrier PSK/QAM is fine and has a
better PAPR. M-QAM only needed if you want wideband data, not needed
for voice as we have good compression.
+ Can't seen any good reason for FDMA, I'm shooting for TDMA with the
SM2000 and Brady's work. TDMA has been used for decades on GSM so PA
design issues aren't a practical issue (I'm not interested in re-use
of legacy FM radios).
-/-
I've recently started playing with a Red Pataya board. I'm
considering it as a prototyping platform for VHF/UHF FreeDV in
combination with my SM2000 prototypes. It has a FPGA to handle tight
TDMA timing and digital up and down conversion, VHF ADCs and DACs for
direct sampling at 2M, and plenty of CPU for FreeDV. Could combine
this with the LNA/BPF, PA, and tx/rx switch of the SM2000 design.
Cheers,
David
On 05/12/17 06:04, Adrian Musceac wrote:
Hello,
Yesterday I commited support for same-mode Codec2 repeater (digital to
digital). Apart from voice frames, now station identifiers and text
messages also get digipeated. For now the Codec2 audio gets decoded if
forwarded over the VoIP connection and re-encoded at the other end for
RF transmission. I intend to change that in the future and send direct
Codec2 frames over the wire to avoid loss of quality.
The roadmap has mixed-mode repeating (analog to digital) and
multi-channel repeater using either FDMA or TDMA, with user control
over VoIP channels similar to DMR talkgroups.
Would be curious to know what people think of the alternative between
these two access modes. Here's what I came up with:
TDMA cons:
- hardware needs to support burst timestamping
- fast tx/rx cycle limits amplifier choice drastically
OFDMA cons:
- high PAPR leads to net reduction of transmitted power and more
complex equalization
- frequency correction and Doppler are an issue
FDMA pros:
- can do mixed mode repeating
Best regards,
Adrian
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
------------------------------------------------------------------------------
Check out the vibrant tech community on one of the world's most
engaging tech sites, Slashdot.org! http://sdm.link/slashdot
_______________________________________________
Freetel-codec2 mailing list
Freetel-codec2@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/freetel-codec2
