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Modulation techniques
Analog modulation
AM SSB FM PM SM
Digital modulation
OOK FSK ASK PSK QAMMSK CPM PPM TCM OFDM
Spread spectrum
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See also: Demodulation
For other uses, see Modulation (disambiguation).
In telecommunications, modulation is the process of varying a periodic waveform, i.e. a tone, in order to use that signal to convey a message, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Normally a high-frequency sinusoid waveform is used as carrier signal. The three key parameters of a sine wave are its amplitude (“volume”), its phase (“timing”) and its frequency (“pitch”), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal.
A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (short for “Modulator-Demodulator”).
Contents
1 Aim
2 Analog modulation methods
3 Digital modulation methods
3.1 Fundamental digital modulation methods
3.2 Modulator and detector principles of operation
3.3 List of common digital modulation techniques
4 Digital baseband modulation or line coding
5 Pulse modulation methods
6 Miscellaneous modulation techniques
7 See also
8 References
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Aim
The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a filter limits the frequency range to between 300 and 3400 Hz) or a limited radio frequency band.
The aim of analog modulation is to transfer an analog lowpass signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel.
Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate bandpass channels.
The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a lowpass channel, typically a non-filtered copper wire such as a serial bus or a wired local area network.
The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband lowpass channel or, in some of the schemes, as a bit stream over another digital transmission system.
Analog modulation methods
In analog modulation, the modulation is applied continuously in response to the analog information signal.
A low-frequency message signal (top) may be carried by an AM or FM radio wave.
Common analog modulation techniques are:
Amplitude modulation (AM) (here the amplitude of the modulated signal is varied)
Double-sideband modulation (DSB)
Double-sideband modulation with unsuppressed carrier (DSB-WC) (used on the AM radio broadcasting band)
Double-sideband suppressed-carrier transmission (DSB-SC)
Double-sideband reduced carrier transmission (DSB-RC)
Single-sideband modulation (SSB, or SSB-AM),
SSB with carrier (SSB-WC)
SSB suppressed carrier modulation (SSB-SC)
Vestigial sideband modulation (VSB, or VSB-AM)
Quadrature amplitude modulation (QAM)
Angle modulation
Frequency modulation (FM) (here the frequency of the modulated signal is varied)
Phase modulation (PM) (here the phase shift of the modulated signal is varied)
Digital modulation methods
In digital modulation, an analog carrier signal is modulated by a digital bit stream. 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 (the modulation alphabet).
A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). 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 (corresponding to a musical instrument that can generate four different tones, one at a time), 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 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 bit per second.
According to one definition of digital signal, the modulated signal is a digital signal, and 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
These are the most fundamental digital modulation techniques:
In the case of PSK, a finite number of phases are used.
In the case of FSK, a finite number of frequencies are used.
In the case of ASK, a finite number of amplitudes are used.
In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.
In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and 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.
If the alphabet consists of M = 2N alternative symbols, each symbol represents a message consisting of N bits. If the symbol rate (also known as the baud rate) is fS symbols/second (or baud), the data rate is NfS 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.
Modulator and detector principles of operation
PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I+jQ (where j is the imaginary unit). The resulting so called equivalent lowpass signal or equivalent baseband signal is a representation of the real-valued modulated physical signal (the so called passband signal or RF signal).
These are the general steps used by the modulator to transmit data:
Group the incoming data bits into codewords, one for each symbol that will be transmitted.
Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values.
Adapt pulse shaping or some other filtering to limit the bandwidth…
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