As per December’s report I have a first pass on all the V2 algorithms and am now building up more comprehensive tests, which are showing up more bugs. This month I have been digging into a corner case with the timing estimator not playing nicely with the ML decoder. The symptom was poor speech quality on some channels tested in simulation, in response to different values from the timing estimator as the HF channel evolves.

We are breaking significant new ground with RADE V2, e.g. a ML network that deals with a variety of channel impairments like the multi-path channel and fine timing errors as well as performing speech error correction and compression. So it is not unexpected that we will have some challenges along the way. Much better to deal with any problems now, as they will be much harder to fix if left until later in the development cycle.

This was a complex problem, that took me most of this month to understand. My approach was to write up the work in a Latex document. As I am working alone on RADE development, I find it useful to clearly express complex problems using text and math.This led me to develop some maths to describe the OFDM timing and how it relates to the V2 decoder. This was an area that I didn’t understand well with RADE V1, where it was taken care of by the previous, pilot symbol approach. This months work also gives a greater depth of understanding around the performance of the V1 and V2 algorithms (V1 development was quite rushed). The Latex document (when edited) will also be useful for documenting the final V2 algorithms, which we plan to release in another paper.

The figure below attempts to describe the problem. The x-axis is time, the top bar represents the range of timing estimates the RADE decoder can successfully handle. Delta1 and Delta2 represent the timing estimates from the HF channel, which sometimes overlap the prohibited (grey) area.

I have now isolated the problem, can reproduce it with several experiments, and have two potential solutions which I am exploring.

Additional work for this month includes bringing the automated tests (ctests) up to date, and getting the over the air test script ready to run with RADE V2, to support subsequent stages of testing.