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
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