Phase-shift keying (PSK) is a method of digital communication in which the phase of a transmitted signal is varied to convey information. There are several methods that can be used to accomplish PSK.The simplest PSK technique is called binary phase-shift keying (BPSK). It uses two opposite signal phases (0 and 180 degrees). The digital signal is broken up time wise into individual bits (binary digits). The state of each bit is determined according to the state of the preceding bit. If the phase of the wave does not change, then the signal state stays the same (0 or 1). If the phase of the wave changes by 180 degrees -- that is, if the phase reverses -- then the signal state changes (from 0 to 1, or from 1 to 0). Because there are two possible wave phases, BPSK is sometimes called bi-phase modulation.

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Consider a sinusoidal carrier, if it is modulated by a bi-polar bit stream according to the scheme illustrated in figure above. Its polarity will be reversed every time the bit stream changes polarity, thus, for a sine wave s equivalent to a phase reversal (shift). The multiplier output is a BPSK signal. The information about the bit stream is contained in the changes of phase of the transmitted signal.

ADVANTAGES:

• System complexity is simple.
• Improved Power efficiency

DISADVANTAGES:

• Low bandwidth efficiency.