Modulation and the types

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Modulation and the types

Written by goldeneggs

Thursday, 06 November 2008 06:45

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 (a contraction of the two terms).

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. You could say that modems play music for each other. 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.

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 multiplex (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.

Common analog modulation techniques are:

Angular modulation
- Phase modulation (PM)
- Frequency modulation (FM)
Amplitude modulation (AM)
- Double-sideband modulation with unsuppressed carrier (used on the radio AM band)
- Double-sideband suppressed-carrier transmission (DSB-SC)
- Double-sideband reduced carrier transmission (DSB-RC)
- Single-sideband modulation (SSB, or SSB-AM), very similar to single-sideband suppressed carrier modulation (SSB-SC)
- Vestigial-sideband modulation (VSB, or VSB-AM)
- Quadrature amplitude modulation (QAM)

Digital modulation methods

In digital modulation, an analog carrier signal is modulated by a digital bit stream of either equal length signals or varying length signals. This can be described as a form of analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of alternative symbols (the modulation alphabet).
These are the most fundamental digital modulation techniques:

In the case of CW, groupings of on-off keying of varying length signals are used.
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, 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. It can be seen as a two channel system. The resulting signal is a combination of PSK and ASK, with a finite number of at least two phases, and a finite number of at least two amplitudes.

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 bit. Thus, the data rate is four times the baud rate.
In the case of PSK, ASK and QAM, 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.
PSK and ASK, and sometimes also FSK, can be generated and detected using the principle of QAM. The I and Q signals can be combined into a complex valued signal called the equivalent lowpass signal or equivalent baseband signal. This is a representation of the 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 into codewords;
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 and form the spectrum, typically using digital signal processing
Digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP). Sometimes the next step is also achieved using DSP, and then the DAC should be done after that.
Modulate the high-frequency carrier waveform, resulting in that the equivalent low pas signal is frequency shifted into a modulated passband signal or RF signal
Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum

At the receiver, the demodulator typically performs:

Bandpass filtering
Automatic gain control, AGC (to compensate for attenuation)
Frequency shifting of the RF signal baseband I and Q signals, or to an intermediate frequency (IF) signal, or
Sampling and analog-to-digital conversion (ADC) (Sometimes before the above point)
Equalization filtering
Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal;
Quantization of the amplitudes, frequencies or phases to the nearest allowed values, using mapping.
Map the quantized amplitudes, frequencies or phases to codewords (bit groups);
Parallel-to-serial conversion of the codewords into a bit stream
Pass the resultant bit stream on for further processing such as removal of any error-correcting codes.

As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations.
The most common digital modulation techniques are:

Phase-shift keying (PSK)
Frequency-shift keying (FSK) (see also audio frequency-shift keying (AFSK))
Amplitude-shift keying (ASK) and its most common form, on-off keying (OOK)
Quadrature amplitude modulation (QAM) a combination of PSK and ASK
Polar modulation like QAM a combination of PSK and ASK.
Continuous phase modulation (CPM)
- Minimum-shift keying (MSK)
- Gaussian minimum-shift keying (GMSK)
Orthogonal frequency division multiplexing (OFDM) modulation, also known as discrete multitone (DMT).
Wavelet modulation
Trellis coded modulation (TCM) also known as trellis modulation

MSK and GMSK are particular cases of continuous phase modulation (CPM). Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling).
OFDM is based on the idea of Frequency Division Multiplex (FDM), but is utilized as a digital modulation scheme. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The sub-carriers are summarized into a OFDM symbol. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the Orthogonal Frequency Division Multiple Access (OFDMA) and MC-OFDM schemes, allowing several users to share the same physical medium by giving different sub-carriers to different users.

Digital baseband modulation or line coding

The term digital baseband modulation is synonymous to line codes, which are methods to transfer a digital bit stream over an analog lowpass channel using a discrete number of signal levels, by modulating a pulse train (a square wave instead of a sinusoidal waveform). Common examples are unipolar, non-return-to-zero (NRZ), Manchester and alternate mark inversion (AMI) coding.

Pulse modulation methods

Pulse modulation schemes aim at transferring a narrowband analog signal over an analog lowpass channel as a two-level quantized signal, by modulating a pulse train. Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e. as a quantized discrete-time signal) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example some line code. They are not modulation schemes in the conventional sense since they are not channel coding schemes, but should be considered as source coding schemes, and in some cases analog-to-digital conversion techniques.