Pulse amplitude modulation( PAM) is a pulse modulation( PM) in which the amplitude of a pulse-shaped carrier signal is changed according to the amplitude of the primary character oscillation (modulation signal).
In pulse amplitude modulation, the message signal is sampled by pulses according to the sampling theorem. These amplitude-variable pulses are transmitted and converted back into an analog signal at the receiver. The information content is exclusively in the pulse amplitude, the time intervals are given by the clock signal and are constant.
Pulse amplitude modulation is largely the same as analog signal transmission, but it occurs at discrete time intervals. In this type of modulation, the carrier signal is pulse-shaped and the modulation signal is analog.
Pulse amplitude modulation is also used in combination with multi- level coding ( MLC). In this case, the pulse amplitude is assigned several levels, each of which represents a binary code. If the pulse signal has the three levels +1, 0 and -1, i.e. if it is ternary, then two levels can be assigned to logic 0 and 1 and the third can be used for forward error correction, for example. The corresponding modulation coding technique is then called PAM3.
If the pulse signal has the five amplitude levels +2, +1, 0, -1 and -2, then it is a quinary coding, a binary numerical code of five bits. In practice, in the PAM5 method, the five level values are assigned the four bit combinations 00, 01, 10 and 11. The multi-level coding methods can also work with eight (PAM8) or twelve (PAM12) levels and have the advantage that the number of binary codes increases with the number of levels, but at the same time the symbol rates for transmission are reduced. With PAM8 and PAM12, which is used in 10 Gigabit Ethernet, the symbol rate is reduced to 1000 MSymbols/s and 825 MSymbols/s, respectively.
The PAM method is also used for light modulation in optical transmission technology. In this technique, the light is modulated in several intensity levels. With PAM4, there are four, with which two bits can be transmitted per modulation step.