The newly released FreeDV 700C mode uses the Coherent PSK (COHPSK) modem which I developed in 2015. This post describes the challenges of building HF modems for DV, and how the COHPSK modem evolved from the FDMDV modem used for FreeDV 1600.
HF channels are tough. You need a lot of SNR to push bits through them. There are several problems to contend with:
When the transmit signal is reflected off the ionosphere, two or more copies arrive at the receiver antenna a few ms apart. These echoes confuse the demodulator, just like a room with bad echo can confuse a listener.
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