Pulse-amplitude modulation

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Principle of PAM: (1) original signal, (2) PAM signal, (a) amplitude of signal, (b) time

Pulse-amplitude modulation (PAM), is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is an analog pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation is performed by detecting the amplitude level of the carrier at every symbol period.

Types[edit]

There are two types of pulse amplitude modulation:

  1. Single polarity PAM: In this a suitable fixed DC bias is added to the signal to ensure that all the pulses are positive.
  2. Double polarity PAM: In this the pulses are both positive and negative.

Pulse-amplitude modulation is widely used in modulating signal transmission of digital data, with non-baseband applications having been largely replaced 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).

The number of possible pulse amplitudes in analog PAM is theoretically infinite. Digital PAM reduces the number of pulse amplitudes to some power of two. For example, in 4-level PAM there are 2^2 possible discrete pulse amplitudes; in 8-level PAM there are 2^3 possible discrete pulse amplitudes; and in 16-level PAM there are 2^4 possible discrete pulse amplitudes.

Use in Ethernet[edit]

Some versions of the Ethernet communication standard are an example of PAM usage. In particular, the Fast Ethernet 100BASE-T2 medium (now defunct), running at 100 Mbit/s, uses five-level PAM modulation (PAM-5) running at 25 million pulses per second over two wire pairs. A special technique is used to reduce inter-symbol interference between the unshielded pairs.[citation needed] Current 100 Mbit/s networking technology is 100BASE-TX, which delivers 100 Mbit/s in each direction over a single twisted pair – one for each direction. The gigabit Ethernet 1000BASE-T medium uses four pairs of wire each running at 125 million pulses per second to achieve 1000 Mbit/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), ten levels (PAM-10), or eight levels (PAM-8), both with and without Tomlinson-Harashima Precoding (THP).

Use in photo biology[edit]

The concept is also used for the study of photosynthesis using a specialized instrument that involves a spectrofluorometric measurement of the kinetics of fluorescence rise and decay in the light-harvesting antenna of thylakoid membranes, thus querying various aspects of the state of the photosystems under different environmental conditions.

Use in electronic drivers for LED lighting[edit]

Pulse-amplitude modulation has also been developed for the control of light-emitting diodes (LEDs), especially for lighting applications. LED drivers based on the PAM technique offer improved energy efficiency over systems based upon other common driver modulation techniques such as pulse-width modulation (PWM) as the forward current passing through an LED is relative to the intensity of the light output and the LED efficiency increases as the forward current is reduced.

Pulse-amplitude modulation LED drivers are able to synchronize pulses across multiple LED channels to enable perfect colour matching. Due to the inherent nature of PAM in conjunction with the rapid switching speed of LEDs it is possible to use LED lighting as a means of wireless data transmission at high speed.

See also[edit]