Phase-shift keying facts for kids

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.

The Gray code for Binary-phase-shift keying (2PSK)

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.

Images for kids

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  Conceptual transmitter structure for QPSK. The binary data stream is split into the in-phase and quadrature-phase components. These are then separately modulated onto two orthogonal basis functions. In this implementation, two sinusoids are used. Afterwards, the two signals are superimposed, and the resulting signal is the QPSK signal. Note the use of polar non-return-to-zero encoding. These encoders can be placed before for binary data source, but have been placed after to illustrate the conceptual difference between digital and analog signals involved with digital modulation.

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  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.

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  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.

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  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.

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  Timing diagram for DBPSK and DQPSK. The binary data stream is above the DBPSK signal. The individual bits of the DBPSK signal are grouped into pairs for the DQPSK signal, which only changes every Ts = 2Tb.

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  Differential encoding/decoding system diagram

See also

In Spanish: Modulación por desplazamiento de fase para niños