If a carrier modulated by a digital bit stream has one of four possible phase shifts (0°, 90°, 180°, and 270°), such modulation is known as:

Which of the following statements is/are correct regarding the Zero Forcing Equalizer used in Digital communication? S1: It is a type of Linear equalizer. S2: It is typically used in channels which have a flat frequency response. S3: It completely removes inter-symbol interference but may enhance noise in the signal.