WSJT-X 2.4.0 will introduce Q65, a digital protocol designed for minimal two-way QSOs over especially difficult propagation paths

On paths with Doppler spread more than a few Hz, the weak-signal performance of Q65 is the best among all WSJT-X modes. Q65 is particularly effective for tropospheric scatter, ionospheric scatter, and EME on VHF and higher bands, as well as other types of fast-fading signals.

Q65 uses 65-tone frequency-shift keying and builds on the demonstrated weak-signal strengths of QRA64, a mode introduced to WSJT-X in 2016. Q65 differs from QRA64 in the following important ways:

A new low-rate Q-ary Repeat Accumulate code for forward error correction
User messages and sequencing identical to those in FT4, FT8, FST4, and MSK144
A unique tone for time and frequency synchronization. As with JT65, this “sync tone” is readily visible on the waterfall spectral display. Unlike JT65, synchronization and decoding are effective even when meteor pings or other short signal enhancements are present.
Optional submodes with T/R sequence lengths 15, 30, 60, 120, and 300 s.
A new, highly reliable list-decoding technique for messages that contain previously copied message fragments.

Q65 Quick Start Guide - https://physics.princeton.edu/pulsar/k1jt/Q65_Quick_Start.pdf