Signal modulation: Difference between revisions

Revision as of 19:27, 24 September 2024

Revision as of 14:48, 27 September 2024

Line 8:

In [[electronics]] and [[telecommunications]], '''modulation''' is the process of varying one or more properties of a periodic [[waveform]], called the ''[[carrier signal]]'', with a separate signal called the ''modulation signal'' that typically contains information to be transmitted.<ref>{{Cite web |date=2014-10-22 |title=How does modulation work? {{!}} Tait Radio Academy |url=https://www.taitradioacademy.com/topic/how-does-modulation-work-1-1/ |access-date=2024-06-17 |website=Tait Radio Academy}}</ref> For example, the modulation signal might be an [[audio signal]] representing [[sound]] from a [[microphone]], a [[video signal]] representing moving images from a [[video camera]], or a [[digital signal]] representing a sequence of binary digits, a [[bitstream]] from a computer.

This carrier wave usually has a much higher [[frequency]] than the message signal does. This is because it is impractical to transmit signals with low frequencies. Generally, to receive a [[radio wave]] one needs a [[Antenna (radio)|radio antenna]] with length that is one-fourth of wavelength.<ref>{{cite web |url=https://www.wimo.com/en/faq/post/what-antenna-length-do-i-need-for-which-frequency |title=General Antennas: What antenna length do I need for which frequency? |first=Rodrigo "Rod" |last=Herrera |date=April 23, 2024 |website=wimo.com |access-date=June 19, 2024 |lang=en}}</ref> For low frequency radio waves, wavelength is on the scale of kilometers and building such a large antenna is not practical. In [[radio communication]], the modulated carrier is transmitted through space as a [[radio wave]] to a [[radio receiver]].

Another purpose of modulation is to transmit multiple [[Communication channel|channels]] of information through a single [[transmission medium|communication medium]], using [[frequency-division multiplexing]] (FDM). For example, in [[cable television]] (which uses FDM), many carrier signals, each modulated with a different [[television 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 is [[demodulation|demodulated]] to extract the information bearing modulation signal.

Line 47:

<blockquote>'''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.</blockquote>

According to one definition of [[Digital signal (electronics)|digital signal]],<ref>{{Cite web|title=Modulation Methods {{!}} Electronics Basics {{!}} ROHM|url=https://www.rohm.com/electronics-basics/wireless/modulation-methods|website=www.rohm.com|access-date=2020-05-15}}</ref> 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]].{{cn|date=September 2024}}

===Fundamental digital modulation methods===

Line 60:

In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of [[Binary numeral system|binary]] [[bit]]s. 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 <math>M = 2^N </math> alternative symbols, each symbol represents a message consisting of ''N'' bits. If the [[symbol rate]] (also known as the [[Baud|baud rate]]) is <math>f_{S}</math> symbols/second (or [[baud]]), the data rate is <math>N f_{S}</math> 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.

Line 74:

# 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 waveform 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 synthesizer|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.

Line 106:

* [[Frequency-shift keying]] (FSK)

** [[Audio frequency-shift keying]] (AFSK)

** [[Multiple frequency-shift keying|Multi-frequency shift keying]] (M-ary FSK or MFSK)

** [[Dual-tone multi-frequency]] (DTMF)

* [[Amplitude-shift keying]] (ASK)

Line 118:

** [[Continuous-phase frequency-shift keying]] (CPFSK)

* [[Orthogonal frequency-division multiplexing]] (OFDM) modulation

** [[Discrete multitone modulation|Discrete multitone]] (DMT), including adaptive modulation and bit-loading

* [[Wavelet modulation]]

* [[Trellis coded modulation]] (TCM), also known as [[Trellis modulation]]

* [[Spread-spectrum|Spread spectrum]] techniques

** [[Direct-sequence spread spectrum]] (DSSS)

** [[Chirp spread spectrum]] (CSS) according to IEEE 802.15.4a CSS uses pseudo-stochastic coding

Line 128:

[[Minimum-shift keying|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).

[[Orthogonal frequency-division multiplexing|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 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-user [[channel access method]] in the [[orthogonal frequency-division multiple access]] (OFDMA) and [[multi-carrier code-division multiple access]] (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers or [[spreading code]]s to different users.

Of the two kinds of [[RF power amplifier]], [[switching amplifier]]s ([[Class D amplifier]]s) cost less and use less battery power than [[linear amplifier]]s of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and [[code-division multiple access|CDMA]], 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 (ADMR)===

Line 151:

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 later [[Frequency mixer|frequency-converted]] to a carrier frequency, or for direct communication in baseband. The latter methods both involve relatively simple [[line code]]s, as often used in local buses, and complicated baseband signalling schemes such as used in [[Digital subscriber line|DSL]].

==Pulse modulation methods==

Line 164:

;Analog-over-digital methods

* [[Pulse-code modulation]] (PCM)

** [[DPCM|Differential PCM]] (DPCM)

*** [[Adaptive differential pulse-code modulation|Adaptive DPCM]] (ADPCM)

* [[Delta modulation]] (DM or Δ-modulation)

** [[Delta-sigma modulation]] (ΣΔ)

Line 172:

==Miscellaneous modulation techniques==

* The use of [[on-off keying]] to transmit [[Morse code]] at [[radio frequency|radio frequencies]] is known as [[continuous wave]] (CW) operation.

* [[Adaptive modulation]]

* [[Space modulation]] is a method whereby signals are modulated within airspace such as that used in [[instrument landing system]]s.