Rpitx and 2FSK

This post describes tests to evaluate the use of rpitx as a 2FSK transmitter.

About 10 years ago I worked on the Village Telco – a system for community telephone networks based on WiFi. At the time we used WiFi SoCs which were open source at the OS level, but the deeper layers were completely opaque which led (at least for me) to significant frustration.

Since then I’ve done a lot of work on the physical layer, in particular building my skills in modem implementation. Low cost SDR has also become a thing, and CPU power has dropped in price. The physical layer is busy migrating from hardware to software. Software can, and should, be free.

So now we can build open source radios. No more chip sets and closed source.

Sadly, some other aspects haven’t changed. In many parts of the world it’s still very difficult (and expensive) to move an IP packet over the “last 100 miles”. So, armed with some new skills and technology, I feel it’s time for another look at developing world and humanitarian communications.

I’m exploring the use rpitx as the heart of HF and UHF data terminals. This clever piece of software turns a Raspberry Pi into a radio transmitter. Evariste (F5OEO) the author of rpitx, has recently developed the v2beta branch that has improved performance, and includes some support for FreeDV waveforms.

Running Tests

I have some utilities for the Codec 2 FSK modem that generate frames of test bits. I modified the fsk_mod_ext_vco utility to drive a utility Evariste kindly wrote for FreeDV experiments with rpitx. So here are the command lines that generate 600 seconds (10 minutes) of 100 bit/s 2FSK test frames, and transmit them out of rpitx, using a 7.177 MHz carrier frequency:

$ ./fsk_get_test_bits - 60000 | ./fsk_mod_ext_vco - ~/rpitx/2fsk_100.f 2 --rpitx 800 100

~/rpitx $ sudo ./freedv 2fsk_100.f 7177000

On the receive side I used my FT-817 connected to FreeDV to sample the signal as a wave file, then fed the signal into C and Octave versions of the demodulator. The RPi is top left at rear, the HackRF in the foreground was used initially as a reference transmitter:

Results

It works really well! One of the most important tests for any modem is adding calibrated noise and measuring the Bit Error Rate (BER). I tried Eb/No = 9dB (-5.7dB SNR), and obtained 1% BER, right on theory for a 2FSK modem:

 $ ./cohpsk_ch ~/Desktop/2fsk_100_rx_rpi.wav - -26 | ./fsk_demod 2 8000 100 - - | ./fsk_put_test_bits -
FSK BER 0.009076, bits tested 11900, bit errors 108
SNR3k(dB): -5.62

This line takes the sample wave file from the FT-817, adds some noise using the cohpsk_ch utility, then pipes the signal to the FSK demodulator, before counting the bit errors in the test frames. I adjusted the 3rd “No” parameter in cohpsk_ch by hand until I obtained about 1% BER, then checked the SNR against the theoretical SNR for an Eb/No of 9dB.

Here are some plots from the Octave version of the demodulator, with no channel noise applied. The first plot shows the time and frequency domain signal at the input of the demodulator. I set the shift at 800 Hz, so you can see one tone at 800 Hz, the other at 1600 Hz:

Here is the output the of the FSK demodulator filters (red and blue for the two filter outputs). We can see a nice separation, but the red “high” level is a bit lower than blue. Red is probably the 1600 Hz tone, the FT-817 has a gentle low pass filter in it’s output, reducing higher frequency tones by a few dB.

There is some modulation on the filter outputs, which I think is explained by the timing offset below:

The sharp jump at 160 samples is expected, that’s normal behaviour for modem timing estimators, where a sawtooth shape is expected. However note the undulation of the timing estimate as it ramps up, indicating the modem sample clock has a little jitter. I guess this is an artefact of rpitx. However the BER results are fine and the average sample clock offset (not shown) is about 50ppm which is better than many sound cards I have seen in use with FreeDV. Many of our previous modem transmitters (e.g. the first pass at Wenet) started with much larger sample clock offsets.

A common question about rpitx is “how clean is the spectrum”. Here is a sample from my Rigol DSA815, with a span of 1MHz around the 7.177 MHz tx frequency. The Tx power is actually 11dBm, but the marker was bouncing around due to FSK modulation. On a wider span all I can see are the harmonics one would expect of a square wave signal. Just like any other transmitter, these would be removed by a simple low pass filter.

So at 7.177 MHz it’s clean to the limits of my spec analyser, and exceeds spectral purity requirements (-43dBc + 10log(Pwatts)) for Amateur (and I presume other service) communications.