Phase-shift keying
Phase shift keying is a way of transmitting information. This is done by modulating the phase of a carrier wave. In such a system, the amplitude of the wave does not carry any information; all information is present in the phase of the signal. In many cases, this allows to better use the bandwidth available.
If we think of a wave as a wiggly line, (like a sine wave), wiggling a specific number of times a second, and we can change at which part of the wiggle it is in. Say if it is at the top of its wiggle, and we immediately change it to the bottom of its wiggle, this is called a phase shift. We can use that change to carry information.
By either changing or not changing the wave every time it gets to the top of its wiggle, we can send either ones or zeros. This is called Binary Phase Shift Keying. If we change the phase of the wave as it reaches the top, we can have this represent a 1. If we don't change it at the top, we can have this represent a 0. We can use a computer and a radio to turn text into a wave like this and then send it. A radio and a computer listening to this wave changing or not changing can figure out the original message being sent and turn it back into text.
Binary Phase Shift Keying can be used to send computer data across radio waves quite efficiently. Certain Wireless LAN standards use Phase-shift keying, which they sometimes couple with Orthogonal frequency-division multiplexing, to get higher data rates.
Phase-shift Keying Media
Constellation diagram for QPSK with Gray coding. Each adjacent symbol only differs by one bit.
Receiver structure for QPSK. The matched filters can be replaced with correlators. Each detection device uses a reference threshold value to determine whether a 1 or 0 is detected.
Timing diagram for QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown at the top, and the total combined signal at the bottom. Note the abrupt changes in phase at some of the bit-period boundaries.
Signal doesn't pass through the origin, because only one bit of the symbol is changed at a time.
Difference of the phase between QPSK and OQPSK
Timing diagram for offset-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note the half-period offset between the two signal components.
Dual constellation diagram for π/4-QPSK. This shows the two separate constellations with identical Gray coding but rotated by 45° with respect to each other.
Timing diagram for π/4-QPSK. The binary data stream is shown beneath the time axis. The two signal components with their bit assignments are shown the top and the total, combined signal at the bottom. Note that successive symbols are taken alternately from the two constellations, starting with the "blue" one.
