Microphone Jack Modes

These modes can be used with almost any SSB or FM transceiver unmodified, and with any SignalLink soundcard interface (either with the old or new transformers).

Name	Usage	BW Hz	Baud	Carriers	Mod'	Bit/Sec	Datarate	FEC
OFDM 500F	SSB	500	62.5	4	4PSK	500	250	K=15 @ 1/2 rate
OFDM 750F	SSB	750	125	3	8PSK	1125	562	K=13 @ 1/2 rate

OFDM 500F /750F are intended for NVIS and regular HF-skip usage.
The OFDM 500F and 750F modes are tx/rx frequency-agile modes.

9600 Port Modes

These modes can be used with almost any 9600-capable FM transceiver unmodified.

Name	Usage	BW Hz	Baud	Carriers	Mod'	Bit/Sec	Datarate	FEC
OFDM 3500	FM 9600-port	3500	250	7	8 PSK	5,250	5,250	NONE

The mode OFDM 3500 is intended for VHF/UHF line of sight communications.
To use this mode, the soundcard interface must be connected to the radio's 9600-port (set JP4 on SignalLink).
To use this mode with a SignalLink soundcard interface it must be one that has the new transformers (built after June 2018).
DRA adapters from Masters Communications (or similar) can be used this mode.
The OFDM 3500 signal is far too wide to operate through FM repeaters.

Use of the radio's 9600-port bypasses the pre-emphasis, de-emphasis, and audio-bandpass filters giving far less distortions and a wider usable bandwidth.

Performance

OFDM 500F

This mode can survive with 100% decode the "Mid-Latitude Disturbed NVIS" simulation, including multiple 2 second deep-fades causing complete-losses of reception. Simulation results confirmed by real-world testing.

OFDM 750F

This mode can survive with 100% decode the "Mid-Latitude Disturbed NVIS" simulation including multiple 1.5 second deep-fades causing complete-losses of reception. Simulation results confirmed by real-world testing.

UTF-8 Support

The OFDM modes support sending all UTF-8 characters. This means the mode can be used to communicate in any language. Please ensure the font chosen in Fldigi has the characters for the language you are trying to use, otherwise Fldigi will just display every character as an empty box.

RSID Usage

OFDM modes require a high-degree of frequency accuracy to operate. This is achieved by utilizing RSID to set the receive frequency to ± 3Hz. RSID is mandatory when using these modes.

When received, the RSID triggers the receiver end to flush the long interleaver and soft-decision viterbi decoder. This prepares the viterbi decoder and interleaver for the data about to be received and is required for the long 3200 millisecond interleaver.

Flushing any received noise-bits before decoding the actual signal decreases the probability of error and provides FEC gain. This gain comes from not mixing the good data-bits with bad noise-bits by pre-filling interleaver and soft-decoder instead with punctures.

Technical Details

Vestigial AFC

OFDM sends a continuous-phase & continuous-frequency tone slightly lower in frequency than the data-signal. This tone is used to provide an accurate frequency/phase reference, allowing for denser PSK modulations on HF.

This vestigial tone is used for Automatic Frequency Control, after RSID.

Vestigial AFC improves decode by:

Compensating for frequency drift, common with older radios
Providing a stable AFC frequency reference: +- 0.5 Hz accuracy
Automatically adjusting for mobile doppler-shifts (up to around 900Mhz)
Adjusting for +- 5Hz instantaneous shifts mid-transmission
Allowing for drift-rates of up to 1Hz/second (60Hz/minute)
Maintaining receive orthogonality by keeping the center-frequency highly accurate.

Forward Error Correction

The forward error correction used by the OFDM modes are inspired by the longer constraint length codes used by NASA for deep-space communications.
Only in recent years have home computers become powerful enough to run these FEC algorithms realtime.

The long-codes provide extra FEC power needed to handle normal HF noise combined with NVIS-distortions or deep-fades.

Use of soft-decision Viterbi FEC and long-tracebacks help the OFDM FEC provide additional coding gain, allowing the modes work well even with highly-distorted signals and multiple complete losses of signal.

OFDM 500F
- A constraint length 15 convolutional FEC is used at 1/2 rate. This gives a free distance of 18 (can correct a 8-bit error). FEC achieves up to 8.6dB in coding gain (equivalent to using 7.2X more transmitter power)
OFDM 750F
- A constraint length 13 convolutional FEC is used at 1/2 rate. This gives a free distance of 16 (can correct a 7-bit error). This provides up to 7.8dB in coding gain (equivalent to using 6X more transmitter power)
OFDM 3500 - TBD

Long Interleaving

The OFDM interleaver length has been modeled after MT63 and tailored for multicast-usage. This allows for a high level of robustness to fades at the expense of having a quite long turnaround time.

For NBEMS usage, where transmit times are many seconds long, a long interleaver gives coding-gain without negatively impacting throughput.

Mode	Bits	msec	Survives fade of
OFDM 500F	2000	4000	< 2000 msec
OFDM 750F	3600	3200	< 1600 msec

Using interleavers this long provides additional coding-gain, but also requires the longer constraint length FEC in order to function properly.

This is due to the FEC having to handle both normal HF noise in addition to correcting over fades of up to 2-seconds from NVIS.

This allows for a better hybrid digital-mode robust enough for strong HF signals, but without excessive delays on VHF/UHF.

The OFDM FEC modes are NOT meant to be used with any stop-and-wait protocols, like AX.25 or PSKmail. Stop-and-wait protocols will timeout if trying to use with an OFDM FEC mode.

Intercarrier Spacing

For OFDM-500F and OFDM-750F each subcarrier is spaced to twice minimum orthogonality. This reduces inter-carrier-interference making the modes usable at the signal to low-to-medium signal-to-noise ratios common of the amateur HF bands. Increased intercarrier spacing also increases the modes resistance to HF doppler-spread.

Pulse Shaping

Raised-cosine pulse-shaping is used on each symbol within each OFDM subcarrier.
This reduces the sidelobes of each carrier, which reduces inter-carrier-interference (ICI). By lowering ICI, these modes become more sensitive and usable at lower signal to noise ratios.

Pulse-shaping also decreases Inter-Symbol Interference (ISI) within the same carrier. The point of zero-amplitude provided by this pulse shaping serves as a reference to recover accurate signal timing.

Due to the low baudrates used, the zero-points of the pulse-shaping also acts as a minimally effective guard interval.

Timing Recovery

Since each symbol is raised-cosine pulse-shaped, there is a tiny moment of silence between each symbol. This silence is used for timing-recovery on the receiving side.

The low baudrates used allow the intersymbol zero-amplitude moments to be long enough the be detected and used for accurate timing-recovery.

Even with a constant string of 1's or 0's accurate timing synchronization is maintained.

Side Lobes

The orthogonal stacking of carriers to create an OFDM based mode causes multiple sidelobes, with only the sidelobes closest to the fundamental signal being significant.

The first side lobe is below the fundamental signal by -18dB and the second side lobe by -22dB.

The side lobes only become an issue at higher power-levels of ~500 watts and above.

Below this power-level, the sidelobes are likely at or below the noise floor for HF.

Mode	Width	@ 25 watts	@ 250 watts	@ 500 watts	@ 1500 watts
OFDM-500F	62.5 Hz	0.0016 watts/Hz	0.016 watts/Hz	0.032 watts/Hz	0.096 watts/Hz
OFDM-750F	125 Hz	0.0008 watts/Hz	0.008 watts/Hz	0.016 watts/Hz	0.064 watts/Hz

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