A variation on the QPSK modulation technique. /4 DQPSK can be viewed as the superposition of two QPSK constellations offset by 45 degrees relative to each other. /4 DQPSK is specified in cordless telephony standards, including Personal Wireless Telecommunications (PWT and PWT/E), Personal Handyphone Service (PHS), and Personal Communications Service (PCS). /4 DQPSK also is specified in cellular radio standards, including Digital Advanced Mobile Phone Service (D-AMPS) and Personal Digital Cellular (PDC). See also cellular radio, cordless telephone, D-AMPS, modulation, PCS, PDC, PHS, PWT, PWT/E, and QPSK.