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Showing posts from November, 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).