Modulation
Signal modulation is the process of varying one or more properties of a periodic waveform in electronics and telecommunication for the purpose of transmitting information.
The process encodes information in the form of a message signal modulated onto a carrier signal to be transmitted. For example, the message signal might be an audio signal representing sound from a microphone, a video signal representing moving images from a video camera, or a digital signal representing a sequence of binary digits, a bitstream from a computer.
Carrier waves are necessary when the frequency of the message is too low to practically transmit. Generally, receiving a radio wave requires a radio antenna with a length that is one-fourth of the wavelength of the transmitted wave. For low-frequency radio waves, wavelength is on the scale of kilometers and building such a large antenna is not practical.
Another purpose of modulation is to transmit multiple channels of information through a single communication medium, using frequency-division multiplexing (FDM). For example, in cable television (which uses FDM), many carrier signals, each modulated with a different television channel, are transported through a single cable to customers. Since each carrier occupies a different frequency, the channels do not interfere with each other. At the destination end, the carrier signal is demodulated to extract the information-bearing modulation signal.
A modulator is a device or circuit that performs modulation. A demodulator (sometimes detector) is a circuit that performs demodulation, the inverse of modulation. A modem (from modulator–demodulator), used in bidirectional communication, can perform both operations. The lower frequency band occupied by the modulation signal is called the baseband, while the higher frequency band occupied by the modulated carrier is called the passband.
Signal modulation techniques are fundamental methods used in wireless communication to encode information onto a carrier wave by varying its amplitude, frequency, or phase. Key techniques and their typical applications
Types of Signal Modulation
•Amplitude Shift Keying (ASK): Varies the amplitude of the carrier signal to represent data. Simple and energy efficient, but vulnerable to noise. Used in RFID and sensor networks.
•Frequency Shift Keying (FSK): Changes the frequency of the carrier signal to encode information. Resistant to noise, simple in implementation, often used in telemetry and paging systems.
•Phase Shift Keying (PSK): Modifies the phase of the carrier signal based on data. Common forms include Binary PSK (BPSK) and Quadrature PSK (QPSK), used in Wi-Fi, Bluetooth, and cellular networks. Offers good spectral efficiency and robustness against interference.
•Quadrature Amplitude Modulation (QAM): Simultaneously varies both amplitude and phase to transmit multiple bits per symbol, increasing data rates. Used extensively in Wi-Fi, cable television, and LTE systems.
•Orthogonal Frequency Division Multiplexing (OFDM): Splits the data across multiple, closely spaced sub-carriers, each modulated separately (often with QAM or PSK). Provides high spectral efficiency and robustness in multipath environments and is widely used in WLAN, LTE, and WiMAX.
•Other advanced techniques:
•Amplitude Phase Shift Keying (APSK): Combines features of PSK and QAM, mainly used in satellite communications for improved power efficiency.
•Spread Spectrum (e.g., DSSS): Spreads the signal energy across a wide band for robust, low probability of intercept transmission.
In analog modulation, an analog modulation signal is "impressed" on the carrier. Examples are amplitude modulation (AM) in which the amplitude (strength) of the carrier wave is varied by the modulation signal, and frequency modulation (FM) in which the frequency of the carrier wave is varied by the modulation signal. These were the earliest types of modulation, and are used to transmit an audio signal representing sound in AM and FM radio broadcasting. More recent systems use digital modulation, which impresses a digital signal consisting of a sequence of binary digits (bits), a bitstream, on the carrier, by means of mapping bits to elements from a discrete alphabet to be transmitted. This alphabet can consist of a set of real or complex numbers, or sequences, like oscillations of different frequencies, so-called frequency-shift keying (FSK) modulation. A more complicated digital modulation method that employs multiple carriers, orthogonal frequency-division multiplexing (OFDM), is used in WiFi networks, digital radio stations and digital cable television transmission.
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