SIM31 (SIM BPSK) is a relatively new digital mode, based on a BPSK Varicode. A team led by Dany ON4NB (and developers Nizar Ben Rejeb / Tunisian SWL; ON2TSF Bernard, Didier ON5DVO, Michel ON7TV and Paco EA5GVJ / ON6LP) had set a goal to achieve good ham communication with lowest power even under bad conditions. So SIM, an abbreviation for "Structured Integrated Message" was born. The advantage of SIM over PSK is actually a higher robustness against noise and fading, as well as a more efficient decoding of weak signals. This allows faster data throughput or less decoding errors. SIM is therefore suitable for QRP operation with low power or DX contacts.
SIM-PSK is by its very nature, a hybrid of PSK31 and JT9 and also allows automatic contacts. The bandwidth of the signal is 45 Hz (USB). The software also supports multiple languages. On the Web site of ON4NB you will find a detailed description and a link to download the software: http://www.on4nb.be/sim31.htm#English.
The operation of the program is relatively easy, and with a little patience and experience in handling similar programs it might be possible to get quickly to the destination.
With all the positive properties the rude frequency selection must be mentioned. Thus, the default QRG in earlier versions in the 20-m band was 14073 kHz; now the developers have changed to 14067 kHz, a frequency primarily intended for the telegraphists (CW). SIM31 thus fits into the series of some other, more recent digital modes, which you can probably attest considered a reckless elbow mentality, for example as by ROS or JT-65HF, which are covering some frequencies usually used by morse code operators - and so provoking conflicts. Ham Spirit is different! - Tom DF5JL