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Phase Shift
In its simplest form, digital phase modulation, or phase shift keying modulation, changes the phase of the carrier wave by using digital data to switch between two signals of the same frequency, but opposite phase. However, the number of phase shifts is not limited to just two states and in QPSK modulation the carrier wave undergoes four changes in phase, corresponding to the 0, 90, 180 and 270 degree position within the waveform.
Bandwidth
In PSK, in each time period the phase of the carrier wave can change only once while the amplitude remains constant. However, in QPSK, four phases -- with each finite phase change representing unique digital data -- are possible, so two binary digits, or ̶0;bits," of information can be transmitted within each time period. In other words, the rate of change of the signal in QPSK allows the carrier wave to transmit two bits of information rather than one and effectively doubles the bandwidth, or transmission capacity, of the carrier wave.
Gray Coding
QPSK modulation takes input bits, two at a time, and creates a symbol that represents one of four phases. However, the performance of QPSK modulation can be enhanced by the use of a technique known as Gray coding. Gray coding maps every two input bits to one of four unique symbols in such a way that the pairs of bits vary by just a single bit from symbol to symbol. If a symbol is received in error, it will contain just one erroneous bit if it was received in error to an adjacent symbol.
Applications
QPSK modulation is one of the most popular digital modulation techniques for satellite communication, including Digital Video Broadcast - Satellite, and cable networks because it is resilient, easy to implement and less susceptible to noise than other modulation techniques.. QPSK is also widely used in code-division multiple access -- a digital technology used in cellular phones -- and other forms of digital communication over a radio frequency carrier wave.