32APSK Narrowband Modem for the QO-100 GEO Amateur Radio Transponder

Since 2019, the QO-100 transponder on the GEO satellite Es’hail 2 has been available to radio amateurs from one third of the world, covering from Brazil to Thailand. It uses a 2.4 GHz uplink and 10.5 GHz downlink.

One of the transponder channels has 500 kHz of bandwidth and is intended for narrowband signals, including SSB voice, telegraphy, and digital communications.

The band planning constraints for each signal in this transponder are 2.7 kHz bandwidth and 50 dB·Hz CN0. Here we present a work-in-progress design and implementation of a software modem that tries to get the maximum data rate with these limitations. It draws heavily from DVB-S2, and uses 32APSK and LDPC codes to transmit ~11 kbps. It also includes some more original ideas for synchronization.