carrier wave

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Related to carrier waves: Carrier frequencies

modulation

, in communications

modulation, in communications, process in which some characteristic of a wave (the carrier wave) is made to vary in accordance with an information-bearing signal wave (the modulating wave); demodulation is the process by which the original signal is recovered from the wave produced by modulation. The original, unmodulated wave may be of any kind, such as sound or, most often, electromagnetic radiation, including optical waves. The carrier wave can be a direct current, an alternating current, or a pulse chain. In modulation, it is processed in such a way that its amplitude, frequency, or some other property varies.

Amplitude Modulation

Amplitude modulation (AM) is the modulation method used in the AM radio broadcast band. In this system the intensity, or amplitude, of the carrier wave varies in accordance with the modulating signal. When the carrier is thus modulated, a fraction of the power is converted to sidebands extending above and below the carrier frequency by an amount equal to the highest modulating frequency. If the modulated carrier is rectified (see rectifier) and the carrier frequency filtered out, the modulating signal can be recovered. This form of modulation is not a very efficient way to send information; the power required is relatively large because the carrier, which contains no information, is sent along with the information.

In a variant of amplitude modulation, called single sideband modulation (SSB), the modulated signal contains only one sideband and no carrier. The information can be demodulated only if the carrier is used as a reference. This is normally accomplished by generating a wave in the receiver at the carrier frequency. SSB modulation is used for long-distance telephony (such as in the amateur radio bands) and telegraphy over land and submarine cables.

Frequency and Phase Modulation

In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time. Its principal application is also in radio, where it offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. The FM band has become the choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast.

Digital radio is based on frequency division multiplexing (FDM), which allows transmission of multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (audio, video, etc.). Orthogonal frequency-division multiplexing (OFDM) takes this concept further, separating an individual transmission into multiple low-frequency signals with a high resistance to interference. A further extension of the technology, coded orthogonal frequency-division multiplexing (COFDM), is widely used in Europe and elsewhere where the digital audio broadcast (DAB) standard has been adopted. OFDM and COFDM offer the benefits of high spectral efficiency, resilience to radio-frequency (RF) interference, and lower multipath distortion.

Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave). The two methods are very similar in the sense that any attempt to shift the frequency or phase is accomplished by a change in the other.

Pulse Modulation

Pulse modulation involves modulating a carrier that is a train of regularly recurrent pulses. The modulation might vary the amplitude (PAM or pulse amplitude modulation), the duration (PDM or pulse duration modulation), or the presence of the pulses (PCM or pulse code modulation). PCM can be used to send digital data; audio signals on a compact disc use pulse code modulation. Developed in 1939 by the English inventor Alec H. Reeves, pulse code modulation is the most important form of pulse modulation because it can be used to transmit information over long distances with hardly any interference or distortion; for this reason it has become increasingly important in the transmission of data in the space program and between computers. Although PCM transmits digital instead of analog signals, the modulating wave is continuous. Digital modulation begins with a digital modulating signal. The two most common digital modulating techniques are phase-shift keying (PSK) and frequency-shift keying (FSK).
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carrier wave

[′kar·ē·ər ‚wāv]
(chemistry)
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

carrier wave

The component of a modulated wave that is independent of the modulation. Generally referred to as a carrier.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved

carrier frequency

The transmission of a fixed frequency that has been altered (modulated) to "carry" data. The frequency is measured in Hertz (cycles per second). See carrier, Hz, FDM and TDM.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
References in periodicals archive ?
In principle, this is no different than the carrier wave and transmitted information illustration.
If an SPWM-control strategy is used like in Figure 6, the output voltage can be in three different modes, depending on whether the reference signal is above, below, or inside the carrier wave band.
It has a certain wavevector and frequency as shown in Figure 1(b), and can propagate in space-time, which is accompanied with the carrier wave. Its feature of propagation depends on the concrete nature of the particles.
5, we used the sinusoidal PWM and DPWM control strategies and we took into account the following values: inductive load R=10[ohm], L=10mH, amplitude modulation index [m.sub.a] = 0.95, carrier wave frequency 50Hz, switching frequency 5kHz, and supply voltage amplitude [U.sub.d] = 310 V.
NTT DoCoMo plans to increase the number of carrier waves per relay station to up to 12 to enhance i-mode service connection capacity, the officials said.
Unfortunately, FM will work only for carrier waves of high frequency, and these cannot be transmitted much beyond the horizon.
"The electromagnetic waves used by television, radio, mobile communication and other types of wireless technology are carrier waves, which means that they carry information or media content over long distances with wide coverage," Karim writes.
Electronic phase modulation of this kind will work well with fairly long-term processes, but electronic carrier waves have fairly low frequencies.

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