Pulse-amplitude modulation

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Image:PAM neutral.png

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 (4-PAM) 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Pulse-amplitude modulation is now rarely used, having been largely superseded by pulse-code modulation, and, more recently, by pulse-position modulation.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. 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 PAM5 for each pair.

PAM is being considered in the proposed IEEE 802.3an standard for 10 Gigabit Ethernet over copper, dubbed 10GBase-T. PAM with 12 discrete levels (PAM12) or 8 levels (PAM8) are being considered. PAM12 for 10 Gigabit Ethernet would operate at 825 million pulses per second. It has an advantage of using a lower pulse rate than PAM8, which would operate at 1000 million pulses per second. PAM8, however, has the advantage of more separation between discrete levels, making it more immune to noise.

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, though some newer extensions to Ethernet, such as HomePlug bring this ability back through the use of more advanced Orthogonal frequency-division multiplexing.

See also

• Pulse-code modulation
• Pulse-position modulation
• Pulse-width modulation
• Pulse-density modulation
• Pulse forming network
• quadrature amplitude modulation

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