Quadrature Phase-shift Keying (QPSK)

Quadrature Phase-shift Keying

• Sometimes known as quaternary or quadriphase PSK or 4-PSK, QPSK uses four points on the constellation diagram, equispaced around a circle.
• With four phases, QPSK can encode two bits per symbol-twice the rate of BPSK.
• This is used either to double the data rate compared to a BPSK system while maintaining the bandwidth of the signal or to maintain the data-rate of BPSK but halve the bandwidth needed.
• Although QPSK can be viewed as a quaternary modulation, it is easier to see it as two independently modulated quadrature carriers.
• With this interpretation, the even (or odd) bits are used to modulate the in-phase component of the carrier, while the odd (or even) bits are used to modulate the quadrature -phase component of the carrier.
• BPSK is used on both carriers and they can be independently demodulated.