Sunday, November 22, 2009

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.

Binary Phase Shift Keying (BPSK)

BPSK is the simplest form of PSK.

  • It uses two phases which are separated by 180° and so can also be termed 2-PSK.
  • It does not particularly matter exactly where the constellation points are positioned
  • It is, only able to modulate at 1bit/symbol and so is unsuitable for high data-rate applications.

Modulation

Modulation is the process by which a carrier wave is able to carry the message or digital signal (series of ones and zeroes).
There are three basic methods to this:-

  • Amplitude,
  • Frequency
  • Phase shift keying.

Higher orders of modulation allow us to encode more bits per symbol or period (time).



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