Process of varying one or more properties of a periodic waveform
This article is about the electronic method to imprint data on a transmitted carrier wave. For other uses of 'Modulation', seeModulation (disambiguation).
Categorization for signal modulation based on data and carrier types
Signal modulation is the process of varying one or more properties of a periodicwaveform inelectronics andtelecommunication for the purpose of transmitting information.
This carrier wave usually has a much higherfrequency than the message signal does. This is because it is impractical to transmit signals with low frequencies. Generally, receiving aradio wave requires aradio antenna with a length that is one-fourth of the wavelength of the transmitted wave.[2] 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 multiplechannels of information through a singlecommunication medium, usingfrequency-division multiplexing (FDM). For example, incable television (which uses FDM), many carrier signals, each modulated with a differenttelevision 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 isdemodulated to extract the information bearing modulation signal.
Amodulator is a device orcircuit that performs modulation. Ademodulator (sometimesdetector) is a circuit that performsdemodulation, the inverse of modulation. Amodem (frommodulator–demodulator), used in bidirectional communication, can perform both operations. The lower frequency band occupied by the modulation signal is called thebaseband, while the higher frequency band occupied by the modulated carrier is called thepassband.[citation needed]
Inanalog modulation, ananalog modulation signal is "impressed" on the carrier. Examples areamplitude modulation (AM) in which theamplitude (strength) of the carrier wave is varied by the modulation signal, andfrequency modulation (FM) in which thefrequency of the carrier wave is varied by the modulation signal. These were the earliest types of modulation[citation needed], and are used to transmit anaudio signal representing sound in AM and FMradio broadcasting. More recent systems usedigital modulation, which impresses adigital signal consisting of a sequence ofbinary digits (bits), abitstream, 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 orcomplex numbers, or sequences, like oscillations of different frequencies, so-calledfrequency-shift keying (FSK) modulation. A more complicated digital modulation method that employs multiple carriers,orthogonal frequency-division multiplexing (OFDM), is used inWiFi networks,digital radio stations and digital cable television transmission.
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. Themodulation 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.
Inanalog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques include:
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)
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)
Indigital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion and the correspondingdemodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (themodulation 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, thesymbol rate is 1000 symbols/second, or 1000baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, thebit rate is twice the symbol rate, i.e. 2000 bits per second.
In 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 ofbinarybits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises thesymbol that is represented by the particular phase, frequency or amplitude.
If the alphabet consists of alternative symbols, each symbol represents a message consisting ofN bits. If thesymbol rate (also known as thebaud rate) is symbols/second (orbaud), 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 aconstellation 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.
These are the general steps used by themodulator 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.
Adaptpulse 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 usingdigital 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 waveform with the I and Q signals, resulting in the equivalent low pass signal being frequency shifted to the modulatedpassband signal or RF signal. Sometimes this is achieved using DSP technology, for exampledirect digital synthesis using awaveform 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, thedemodulator typically performs:
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 thesuperheterodyne receiver principle).
Sampling and analog-to-digital conversion (ADC) (sometimes before or instead of the above point, for example by means ofundersampling).
Equalization filtering, for example, amatched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoidintersymbol 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 isphase synchronized with the sendercarrier signal. In this case, modulation symbols (rather than bits, characters, or data packets) areasynchronously transferred. The opposite issynchronous modulation.
MSK andGMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known ascontinuous-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 offrequency-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 sub-carriers 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 sequence of so-called OFDM symbols. OFDM can be extended to multi-userchannel access method in theorthogonal frequency-division multiple access (OFDMA) andmulti-carrier code-division multiple access (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers orspreading codes to different users.
Of the two kinds ofRF power amplifier,switching amplifiers (Class D amplifiers) cost less and use less battery power thanlinear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) andCDMA, but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers.
Automatic digital modulation recognition in intelligent communication systems is one of the most important issues insoftware-defined radio andcognitive radio. According to incremental expanse of intelligent receivers, automatic modulation recognition becomes a challenging topic in telecommunication systems and computer engineering. Such systems have many civil and military applications. Moreover, blind recognition of modulation type is an important problem in commercial systems, especially insoftware-defined radio. Usually in such systems, there are some extra information for system configuration, but considering blind approaches in intelligent receivers, we can reduce information overload and increase transmission performance. Obviously, with no knowledge of the transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information, etc., blind identification of the modulation is made fairly difficult. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels.[4]
There are two main approaches to automatic modulation recognition. The first approach uses likelihood-based methods to assign an input signal to a proper class. Another recent approach is based on feature extraction.
Digital baseband modulation changes the characteristics of a baseband signal, i.e., one without a carrier at a higher frequency.
This can be used as equivalent signal to be laterfrequency-converted to a carrier frequency, or for direct communication in baseband. The latter methods both involve relatively simpleline codes, as often used in local buses, and complicated baseband signalling schemes such as used inDSL.
Pulse modulation schemes aim at transferring a narrowband analog signal over an analog baseband channel as a two-level signal by modulating apulse wave. Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e., as aquantizeddiscrete-time signal) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example, someline code. These are not modulation schemes in the conventional sense since they are notchannel coding schemes, but should be considered assource coding schemes, and in some cases analog-to-digital conversion techniques.
