Tony, K2MO, has recently published some fine videos of FreeDV 1600, 700C, and 700D passing through simulated HF channels. The results are quite interesting.
This video shows the 700C mode having the ability to decode with 50% of it’s carriers removed:
This 700C modem sends two copies of the tx signal at high and low frequencies, a form of diversity to help overcome selective fading. These are the combined at the receiver.
Tony’s next video shows three FreeDV modes passing through a selective fading HF channel simulation:
This particular channel has slow fading, a notch gradually creeps across the spectrum.
Tony originally started testing to determine which FreeDV mode worked best on NVIS paths. He used path parameters based on VOACAP prediction models which show the relative time delay and signal power for the each propagation mode i.e., F1, F2:
Note the long delay paths (5ms). The CCIR NVIS path model also suggests a path delay of 7ms. That much delay puts the F-layer at 1000 km (well out into space), which is a bit of a puzzle.
This video shows the results of the VOCAP NVIS path:
In this case 700C does better than 700D. The 700C modem (COHPSK) is a parallel tone design, which is more robust to long multipath delays. The OFDM modem used for 700D is configured for multipath delays of up to 2ms, but tends to fall over after that as the “O” for Orthogonal assumption breaks down. It can be configured for longer delays, at a small cost in low SNR performance.
The OFDM modem gives much tighter packing for carriers, which allows us to include enough bits for powerful FEC, and have a very narrow RF bandwidth compared to 700C. FreeDV 700D has the ability to perform interleaving (Tools-Options “FreeDV 700 Options”), which is a form of time diversity. This feature is not widely used at present, but simulations suggest it is worth up to 4dB.
It would be interesting to combine frequency diversity, LDPC, and OFDM in a wider bandwidth signal. If anyone is interested in doing a little C coding to try this let me know.
I’ve actually seen long delay on NVIS paths in the “real world”. Here is a 40M 700D contact between myself and Mark, VK5QI, who is about 40km away from me. Note at times there are notches on the waterfall 200Hz apart, indicating a round trip path delay of 1500km:
Modems for HF Digital Voice Part 1, explaining the frequency diversity used in 700C
Testing FreeDV 700C, shows how to use some built in test features like noise insertion and interfering carriers.
FreeDV User Guide, including new 700D features like interleaving