Phase shift keying
The phase-shift keying ( English Phase Shift Keying , PSK ) is a digital modulation method in the communications engineering . A sinusoidal carrier oscillation is keyed in discrete phase steps by the digital data stream to be transmitted . In the simplest form, the binary PSK (BPSK) or 2-PSK, one bit can be transmitted per symbol . The PSK is rarely used in practice because of its low spectral efficiency , but it represents the basis of various digital modulation methods based on it. Such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), in which, in addition to the phase position, the amplitude of the carrier oscillation in discrete steps is modulated.
Binary phase shift keying
With binary phase shift keying, a switch is made between two phase positions, as shown in the constellation diagram on the right with the two symbol points. Due to the small number of symbols of only two symbols which are at the maximum distance from one another, there is the advantage of high interference immunity. The information from one bit, logical 0 and logical 1, is assigned to the two symbols and this meaning is agreed between sender and recipient. It does not matter where the constellation points are arranged in the plane, as long as the distance from the zero point is symmetrical and the connecting line of the two symbol points goes through the zero point.
Synchronization
The ambiguity of the phase position results in the fundamental problem for the receiver that without additional measures it cannot be recognized which phase position has which logical value. One possibility, which goes beyond the actual modulation process, is to agree a synchronization sequence between transmitter and receiver at the beginning of a transmission, with the aid of which the receiver can determine a reference oscillation in the phase position and thus set it to the same phase position as the transmitter. If necessary, this synchronization together with the necessary signaling must also be repeated during a transmission.
Another possibility that avoids matching or synchronizing the reference phase position is not to assign the information to the individual symbols (phase positions), but rather the information only in the change in the phase position for one logical state or in the missing change in the phase position for the second transferring the logical state. This method is called differential phase shift keying , abbreviated DPSK ( English Differential Phase Shift Keying ).
Palpation
When switching directly between the different phase positions, as shown in the figure opposite with the time signal curve of a data sequence modulated with BPSK, the sudden switching process and its high spectral components occupy a large bandwidth . To reduce the bandwidth, so-called pulse shaping filters are used, which meet the first Nyquist condition and avoid symbol crosstalk and make the immediate change between the phase positions more “gentle”.
Quadrature phase shift keying
Four symbols are used in QPSK, which means that two bits can be transmitted per symbol. This doubles the utilization of the available bandwidth. There are different variants of the QPSK such as the π / 4-QPSK, which is often used in practice, and the offset QPSK (OQPSK), which does not lead the transition between two transmission symbols in the complex plane through the zero point, which leads to a lowering of the carrier oscillation Equals zero. By avoiding excessive carrier lowering, interference effects triggered by non-linear transmission properties, for example in transmission amplifiers, can be avoided.
literature
- Rudolf Mäusl, Jürgen Göbel: Analog and digital modulation methods . Hüthig Verlag, 2002, ISBN 3-7785-2886-6 .
- John G. Proakis, Masoud Salehi: Digital Communications . 5th edition. McGraw Hill, 2008, ISBN 978-0-07-126378-8 , Chapter 3: Digital Modulation Schemes.