Pulse-amplitude modulation
From Wikipedia, the free encyclopedia
Pulse-amplitude modulation, acronym PAM, is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses.
Example: A two bit modulator (PAM-4) will take two bits at a time and will map the signal amplitude to one of four possible levels, for example −3 volts, −1 volt, 1 volt, and 3 volts.
Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.
Pulse-amplitude modulation is now rarely used, having been largely superseded by pulse-code modulation, and, more recently, by pulse-position modulation.
In particular, all telephone modems faster than 300 bit/s use quadrature amplitude modulation (QAM). (QAM uses a two-dimensional constellation).
It shall be noted, however, that the widely popular Ethernet communication standard is a good example of PAM usage. In particular, 100BASE-T2 (running at 100Mb/s) Ethernet medium is utilizing 5 level PAM modulation running at 25 megapulses/sec over two wire pairs. A special technique is used to reduce inter-symbol interference between the unshielded pairs. Later, 1000BASE-T medium raised the bar to use 4 pairs of wire running each at 125 megapulses/sec to achieve 1000Mb/s data rates, still utilizing PAM-5 for each pair.
The IEEE 802.3an standard defines the wire-level modulation for 10GBASE-T as a Tomlinson-Harashima Precoded (THP) version of pulse-amplitude modulation with 16 discrete levels (PAM-16), encoded in a two-dimensional checkerboard pattern known as DSQ128. Several proposals were considered for wire-level modulation, including PAM with 12 discrete levels (PAM-12), 10 levels (PAM-10), or 8 levels (PAM-8), both with and without Tomlinson-Harashima Precoding (THP).
To achieve full-duplex operation, parties must ensure that their transmitted pulses do not coincide in time. This makes use of bus topology (featured by older Ethernet implementations) practically impossible with these modern Ethernet mediums. This technique is called Carrier Sense Multiple Access and is used in some home networking protocols such as HomePlug. More modern protocols use Time Division Multiple Access instead, which performs better under heavy traffic loading conditions.
