Which of the following is characteristic of QPSK31?

QPSK31 (Quadrature Phase Shift Keying at 31 baud) is a digital mode related to the popular BPSK31. It uses four phase shifts instead of two, allowing it to send two bits of data per symbol. This extra data capacity is used to provide Forward Error Correction (FEC), making the signal more robust against noise. Despite having four phase states, QPSK31 maintains approximately the same narrow 31 Hz bandwidth as BPSK31. However, because it relies on specific phase relationships that are inverted if the sideband is flipped, it is sideband sensitive, meaning both transmitting and receiving stations must use the exact same sideband (usually USB).