Phase modulation is a version of frequency modulation where the phase of the carrier wave is modulated to encode bits of digital information in each phase change.
The "PSK" in QPSK refers to the use of Phased Shift Keying. Phased Shift Keying is a form of phase modulation which is accomplished by the use of a discrete number of states. QPSK refers to PSK with 4 states. With half that number of states, you will have BPSK (Binary Phased Shift Keying). With twice the number of states as QPSK, you will have 8PSK.
The "Quad" in QPSK refers to four phases in which a carrier is sent in QPSK: 45, 135, 225, and 315 degrees.
QPSK Encoding
Because QPSK has 4 possible states, QPSK is able to encode two bits per symbol.
Phase Data 45 degrees Binary 00 135 degrees Binary 01 225 degrees Binary 11 315 degrees Binary 10
QPSK is more tolerant of link degradation than 8PSK, but does not provide as much data capacity.
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