18 September 2023

[Crazy idea] HLK-LD1115H for digital transmission?

This is just an idea that needs to be verified once I receive the second HLK-LD1115H 24 GHz radar.

The radar module outputs a serial stream with the status of "mov"evement, "occ"upancy or null. "mov" and "occ" strings are followed by two numbers, the first being the "spectral line" and the "signal strength".

Now, if two of these guys face each other and their frequencies are not too far apart (TBD, but I'd say < 1 MHz), a receiving end should detect an on-off modulation of the counterpart as if it was a movement.

The module outputs 10 status lines per second on the serial port at 115200 baud. If each status line represents a bit, we can theoretically achieve 10 bps, or about 1 baud, that is one 8-bit symbol per second. We could speed up things a little bit using the Baudot code that uses 5-bit symbols; then you need a start and stop bit for a total of 7 bits or 1.4 baud.

Sounds slow? Yes, sure. But don't forget that WSPR runs at 1.5 baud. And that you would be operating on 24 GHz with the simple help of a microcontroller!

Another way to exploit this system would be to encode very slow Morse code, where the decoding brain (the grey matter one) can make up for decoding errors reducing the need of a robust(ish) data encoding or CRC or FEC or ...

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