Filtered symmetric differential phase-shift keying

In telecommunication, filtered symmetric differential phase-shift keying (FSDPSK) is a method of encoding information for digital transmission in which

• A 0 bit is encoded as a +90° change in the carrier phase, and a 1 bit is encoded as a -90° change in the carrier phase
• Abrupt phase transitions are smoothed by filtering or other functionally equivalent pulse shaping techniques.

Source: Federal Standard 1037C