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Text of the page (random words):
erforms modulation a demodulator sometimes detector is a circuit that performs demodulation the inverse of modulation a modem from mod ulator dem odulator 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 4 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 5 6 7 8 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 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 9 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 9 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 9 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 9 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 9 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 citation needed 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 analog modulation methods edit a low frequency message signal top may be carried by an am or fm radio wave waterfall plot of a 146 52 mhz radio carrier with amplitude modulation by a 1 000 hz sinusoid two strong sidebands at and 1 khz from the carrier frequency are shown a carrier frequency modulated by a 1 000 hz sinusoid the modulation index has been adjusted to around 2 4 so the carrier frequency has small amplitude several strong sidebands are apparent in principle an infinite number are produced in fm but the higher order sidebands are of negligible magnitude in analog modulation the modulation is applied continuously in response to the analog information signal common analog modulation techniques include 3 105 168 10 125 181 184 amplitude modulation am here the amplitude of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal double sideband modulation dsb double sideband modulation with 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 single sideband modulation with carrier ssb wc single sideband modulation suppressed carrier modulation ssb sc vestigial sideband modulation vsb or vsb am quadrature amplitude modulation qam angle modulation which is approximately constant envelope frequency modulation fm here the frequency of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal phase modulation pm here the phase shift of the carrier signal is varied in accordance with the instantaneous amplitude of the modulating signal transpositional modulation tm 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 am where an am carrier also carries a phase variable phase f ǿ tm is f am ǿ digital modulation methods edit 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 the modulation alphabet schematic of 4 baud 8 bit s data link containing arbitrarily chosen values 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 1000 baud since each tone i e symbol 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 11 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 citation needed fundamental digital modulation methods edit the most fundamental digital modulation techniques are based on keying 12 psk phase shift keying a finite number of phases are used fsk frequency shift keying a finite number of frequencies are used ask amplitude shift keying a finite number of amplitudes are used in quadrature amplitude modulation qam an in phase signal or i with one example being a cosine waveform and a quadrature phase signal or q with an example being a sine wave 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 m 2 n displaystyle m 2 n alternative symbols each symbol represents a message consisting of n bits if the symbol rate also known as the baud rate is f s displaystyle f_ s symbols second or baud the data rate is n f s displaystyle nf_ s 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 or ask 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 edit 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 complex valued 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 and form the spectrum of the equivalent low pass signal typically using digital signal processing perform 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 generate a high frequency sine carrier waveform and perhaps also a cosine quadrature component carry out the modulation for example by multiplying the sine and cosine waveforms with the i and q signals resulting in the equivalent low pass signal being frequency shifted to the modulated passband signal or rf signal sometimes this is achieved using dsp technology for example direct digital synthesis using a waveform table instead of analog signal processing in that case the above dac step should be done after this step amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum at the receiver side the demodulator typically performs bandpass filtering automatic gain control agc to compensate for attenuation for example fading frequency shifting of the rf signal to the equivalent baseband i and q signals or to an intermediate frequency if signal by multiplying the rf signal with a local oscillator sine wave and cosine wave frequency see the superheterodyne receiver principle sampling and analog to digital conversion adc sometimes before or instead of the above point for example by means of undersampling equalization filtering for example a matched filter compensation for multipath propagation time spreading phase distortion and frequency selective fading to avoid intersymbol interference and symbol distortion 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 symbol values mapping of 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 has 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 asynchronous methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier signal in this case modulation symbols rather than bits characters or data packets are asynchronously transferred the opposite is synchronous modulation list of common digital modulation techniques edit the most common digital modulation techniques are phase shift keying psk binary psk bpsk using m 2 symbols quadrature psk qpsk using m 4 symbols 8psk using m 8 symbols 16psk using m 16 symbols differential psk dpsk differential qpsk dqpsk offset qpsk oqpsk π 4 qpsk frequency shift keying fsk audio frequency shift keying afsk multi frequency shift keying m ary fsk or mfsk dual tone multi frequency signaling dtmf amplitude shift keying ask on off keying ook the most common ask form m ary vestigial sideband modulation for example 8vsb quadrature amplitude modulation qam a combination of psk and ask polar modulation like qam a combination of psk and ask citation needed continuous phase modulation cpm methods minimum shift keying msk gaussian minimum shift keying gmsk continuous phase frequency shift keying cpfsk orthogonal frequency division multiplexing ofdm modulation discrete multitone dmt including adaptive modulation and bit loading wavelet modulation trellis coded modulation tcm also known as trellis modulation spread spectrum techniques direct sequence spread spectrum dsss chirp spread spectrum css according to ieee 802 15 4a css uses pseudo stochastic coding frequency hopping spread spectrum fhss applies a special scheme for channel release msk and gmsk are particular cases of continuous phase modulation 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 multiplexing fdm but the multiplexed streams are all parts of a single original stream the bit stream is split into several parallel data streams each transferred over its own sub carrier using some conventional digital modulation scheme the modulated subcarriers are summed to form an ofdm signal this dividing and recombining help with handling channel impairments ofdm is considered as a modulation technique rather than a multiplex technique since it transfers one bit stream over one communication channel using one ...
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