Signal 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. For example, in cable television, 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 is a circuit that performs demodulation, the inverse of modulation. A modem, 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

Category	Modulation Type	Key Features	Example Uses
Analog Modulation	AM	Varies Amplitude of Carrier	AM Radio
	FM	Varies Frequency of Carrier	FM and Two-way Radio
	PM	Varies Phase of Carrier	Analog TV and Satellite
Digital Modulation	ASK	Amplitude represents binary data	RFID, Optical Comm
	FSK	Frequency shift encodes data	Modems, Bluetooth
	PSK	Phase of carrier encodes bits	Wi-Fi, Satellite
	QPSK	4 phase states: two bits per symbol	DVB, LTE
	QAM	Both amplitude and phase vary; can transmit multiple bits	Cable TV, Wi-Fi
	OFDM	Multiple carriers, each modulated separately	4G/5G, Wi-Fi
Pulse Modulation	PWM	Pulse width represents amplitude	Motor Control, Audio
	PPM	Pulse position within a time slot represents data	Optical Comm., Radar
Spread Spectrum	DSSS	Signal bandwidth spread using code sequence	CDMA, GPS
	FHSS	Carrier hops between different frequencies	Bluetooth, Military

Amplitude Shift Keying : 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 : 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 : Modifies the phase of the carrier signal based on data. Common forms include Binary PSK and Quadrature PSK, used in Wi-Fi, Bluetooth, and cellular networks. Offers good spectral efficiency and robustness against interference.
Quadrature Amplitude Modulation : 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 : Splits the data across multiple, closely spaced sub-carriers, each modulated separately. 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 : Combines features of PSK and QAM, mainly used in satellite communications for improved power efficiency.
Spread Spectrum : 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 in which the amplitude of the carrier wave is varied by the modulation signal, and frequency modulation 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, 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 modulation. A more complicated digital modulation method that employs multiple carriers, orthogonal frequency-division multiplexing, is used in WiFi networks, digital radio stations and digital cable television transmission.

Analog modulation methods

In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include:

Amplitude modulation
* Double-sideband modulation
** Double-sideband modulation with carrier
** Double-sideband suppressed-carrier transmission
** Double-sideband reduced-carrier transmission
* Single-sideband modulation
** Single-sideband modulation with carrier
** Single-sideband modulation suppressed carrier modulation
* Vestigial-sideband modulation
* Quadrature amplitude modulation
Angle modulation, which is approximately constant envelope
* Frequency modulation
* Phase modulation
* Transpositional Modulation, in which the waveform inflection is modified, resulting in a signal where each quarter cycle is transposed in the modulation process. TM is a pseudo-analog modulation. Where an AM carrier also carries a phase variable phase f. TM is f
Digital modulation methods

In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols.

A simple example: A telephone line is designed for transferring audible sounds, for example, tones, and not digital bits. Computers may, however, communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols, the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or 1000 baud. Since each tone represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second.

According to one definition of digital signal, the modulated signal is a digital signal. According to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission.

Fundamental digital modulation methods

The most fundamental digital modulation techniques are based on keying:

PSK : a finite number of phases are used.
FSK : a finite number of frequencies are used.
ASK : a finite number of amplitudes are used.
QAM : a finite number of at least two phases and at least two amplitudes are used.

In QAM, an in-phase signal and a quadrature phase signal are amplitude modulated with a finite number of amplitudes and then summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK.
In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase, frequency or amplitude.
If the alphabet consists of alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate is symbols/second, the data rate is bit/second.
For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate.
In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis for each symbol.