Pulsoamplitudenmodulation

Pulsoamplitudenmodulation, abbreviated PAM, is a form of pulse modulation in which the amplitude of a train of fixed-width pulses is varied in accordance with the samples of an analog signal. It is one of the simplest baseband modulation schemes and serves as a building block for more complex signaling methods.

In a PAM system, the continuous-time signal is sampled at regular intervals, producing a sequence of discrete values. Each sample value is mapped to a pulse of fixed duration, with the pulse amplitude proportional to the quantized sample. The resulting pulse train is then often passed through a pulse-shaping filter to control its spectral content and improve performance over the transmission channel.

PAM can be implemented with different amplitude levels. Unipolar PAM uses nonnegative levels, while bipolar PAM uses symmetric levels around zero. The notation M-PAM denotes the use of M distinct amplitude levels (for example, PAM-4 uses four levels). For M = 2, PAM reduces to a two-level NRZ-like scheme.

The bandwidth of a PAM signal is influenced by the pulse shape and the symbol rate (the inverse of the sampling interval). Without shaping, the spectrum can be wide; with appropriate pulse shaping, such as a raised-cosine filter, the bandwidth is reduced to a practical value suitable for the channel. The choice of M affects data rate and noise tolerance: higher-level PAM increases potential data throughput but requires higher linearity and better signal-to-noise ratio.

Applications of PAM include digital-to-analog conversion, baseband data transmission, and as a fundamental component in higher-order modulation schemes such as QAM. PAM-4, in particular, is used in modern high-speed serial links and some optical and electrical data networks to achieve higher bit rates over limited bandwidth.