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For the musical use of "modulation", see modulation (music). 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 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 demodulation is known as a demodulator (sometimes detector). 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 transfering 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: Digital modulation methods In digital modulation, a digital bit stream by varying an analog carrier signal. This can be described as a form of digital-to-analog conversion. The changes in the carrier signal are chosen from a number of alternative symbols (the modulation alphabet). These are the most fundamental digital modulation techniques: 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 alternative symbols, each symbol represents a message consisting of bits. If the symbol rate (also known as the baud rate) is symbols/second (or baud), the data rate is 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 and ASK, the modulation alphabet is often conveniently represented on a constellation diagram. These are the general steps used by the modulator to transmit data: At the receiver, the demodulator 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: 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 sinusuid 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 allows 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. Miscellaneous modulation techniques See also | ||||||||
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