LDPC code design for my QO-100 narrowband modem

A couple months ago I presented my work-in-progress design for a data modem intended to be used through the QO-100 NB transponder. The main design goal for this modem is to give the maximum data rate possible in a 2.7 kHz channel at 50 dB·Hz CN0. For the physical layer I settled on an RRC-filtered single-carrier modulation with 32APSK data symbols and an interleaved BPSK pilot sequence for synchronization. Simulation and over-the-air tests of this modulation showed good performance. The next step was designing an appropriate FEC.

Owing to the properties of the synchronization sequence, a natural size for the FEC codewords of this modem is 7595 bits (transmitted in 1519 data symbols). The modem uses a baudrate of 2570 baud, so at 50 dB·Hz CN0 the Es/N0 is 15.90 dB. In my previous post I considered using an LDPC code with a rate of 8/9 or 9/10 for FEC, taking as a reference the target Es/N0 performance of the DVB-S2 MODCODs. After some performing some simulations, it turns out that 9/10 is a bit too high with 7595 bit codewords (the DVB-S2 normal FECFRAMEs are 64800 bits long, giving a lower LDPC decoding threshold). Therefore, I’ve settled on trying to design a good rate 8/9 FEC. At this rate, the Eb/N0 is 9.42 dB.

32APSK narrowband modem for QO-100

Some time ago I did a few experiments about pushing 2kbaud 8PSK and differential 8PSK through the QO-100 NB transponder. I didn’t develop these experiments further into a complete modem, but in part they served as inspiration to Kurt Moraw DJ0ABR, who has now made a QO-100 Highspeed Multimedia Modem application that uses up to 2.4 kbaud 8PSK to send image, files and digital voice. Motivated by this, I have decided to pick up these experiments again and try to up the game by cramming as much bits per second as possible into a 2.7 kHz SSB channel.

Now I have a definition for the modem waveform, and an implementation in GNU Radio of the modulation, synchronization and demodulation that is working quite well both on simulation and in over-the-air tests on the QO-100 NB transponder. The next step would be to choose or design an appropriate FEC for error-free copy.

In this post I give an overview of the design choices for the modem, and present the GNU Radio implementation, which is available in gr-qo100_modem.