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What is digital modulation?

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Published: Mon, 5 Dec 2016

Introduction

Digital modulation schemes transform digital signals like the one shown below into waveforms that are compatible with the nature of the communications channel. There are two major categories of digital modulation. One category uses a constant amplitude carrier and the other carries the information in phase or frequency variations (FSK, PSK). The other category conveys the information in carrier amplitude variations and is known as amplitude shift keying (ASK). The past few years has seen a major transition from the simple amplitude modulation (AM) and frequency modulation (FM) to digital techniques such as Quadrate Phase Shift Keying (QPSK), Frequency Shift Keying (FSK), Minimum Shift Keying (MSK) and Quadrate Amplitude Modulation (QAM). For designers of digital terrestrial microwave radios, their highest priority is good bandwidth efficiency with low bit-error-rate. They have plenty of power available and are not concerned with power efficiency. They are not especially concerned with receiver cost or complexity because they do not have to build large numbers of them. On the other hand, designers of hand-held cellular phones put a high priority on power efficiency because these phones need to run on a battery. Cost is also a high priority because cellular phones must be low-cost to encourage more users. Accordingly, these systems sacrifice some bandwidth efficiency to get power and cost efficiency. Every time one of these efficiency parameters (bandwidth, power or cost)is increased, another one decreases, or becomes more complex or does not perform well in a poor environment. Cost is a dominant system priority. Low-cost radios will always be in demand. In the past, it was possible to make a radio low-cost by sacrificing power and bandwidth efficiency. This is no longer possible. The radio spectrum is very valuable and operators who do not use the spectrum efficiently could lose their existing licenses or lose out in the competition for new ones. These are the tradeoffs that must be considered in digital RF (Radio Frequency) communications design. If you understand the building blocks, then you will be able to understand how any communications system, present or future, works.

Introduction – Cont

http://digitalmodulation.net/indrot1.gif

Why use Digital?

The move to digital modulation provides more information capacity, compatibility with digital data services, higher data security, better quality communications, and quicker system availability. Developers of communications systems face these constraints:

  • available bandwidth
  • permissible power
  • inherent noise level of the system

The RF spectrum must be shared, yet every day there are more users for that spectrum as demand for communications services increases. Digital modulation schemes have greater capacity to convey large amounts of information than analogue modulation schemes. The Fundamental Trade-off:

Introduction – Cont.

Industry trends over the past few years a major transition has occurred from simple analogue Amplitude Modulation (AM) and Frequency/Phase Modulation (FM/PM) to new digital modulation techniques. Examples of digital modulation include:

  • FSK (Frequency Shift Keying)
  • QPSK (Quadrature Phase Shift Keying)
  • QAM (Quadrature Amplitude Modulation)
  • MSK (Minimum Shift Keying)

Now that we understand the basic principles of modulation you should be ready to take the first tutorial.

Frequency Shift Keying – FSK

What is FSK?

The two binary states, logic 0 (low) and 1 (high), are each represented by an analogue waveform. Logic 0 is represented by a wave at a specific frequency, and logic 1 is represented by a wave at a different frequency.

http://digitalmodulation.net/fsk_files/image002.gif

http://digitalmodulation.net/fsk_files/image004.gif

Below shows the basic representation. With binary FSK, the centre or carrier frequency is shifted by the binary input data. Thus the input and output rates of change are equal and therefore the bit rate and baud rate equal. The frequency of the carrier is changed as a function of the modulating signal (data), which is being transmitted. Amplitude remains unchanged. Two fixed-amplitude carriers are used, one for a binary zero, the other for a binary one. You can see from the movie below how the FSK wave form is generated. Note when the edge of a new logic level enters the transmitter the frequency of the output. Frequency Shift Keying – Cont.If two or more of the same logic level are received in secession the frequency will remain the same until the logic level changes

As illustrated below

Frequency Shift Keying – Cont.

How the Waveform is Generated. The general analytic expression for FSK is; si(t) = Acos2p Æ’i t 0 = t = T and i = 1,….,M Where; Æ’i = (Æ’0 + 2i – M)Æ’d Æ’0 denotes the carrier frequency. Generation of these waveforms may be accomplished with a set of M separate oscillators, each tuned to the frequencyIt can be observed below that the error probability for a given signal-to-noise ratio decrease as M increases, contrary to other modulation scheme (i.e. PSK and QAM), but on the other hand the bandwidth efficiency decrease as M increases, it value being given by; Below shows error probability of coherently demodulated FSK where P (e) is the probability of error.

Frequency Shift Keying – Cont.

The FSK Transmitter. Below shows a block diagram of a FSK modulator where the input signal M equalled to either 2-,4-or 8-level impulses separated by the baud period, T. It is first filtered by v(t) to control the bandwidth of the base band signal which, in turn, partially controls the FSK signal spectrum. The filter output signal level is then adjusted and input to a phase modulator. The phase modulator centres the signal at frequency. The choice f a controls the frequency deviation,away from the centre frequency for each symbol.Different choices of the low-pass filter characteristic and signal gain, a, control the signal bandwidth and inter symbol interference (ISI) on the base band signal. A common filter characteristic uses a rectangular pulse shape. It does not cause ISI but the bandwidth is relatively wide. Another choice is to use a Nyquist filter that introduces controlled ISI but complicates the demodulator timing recovery. More aggressive filtering, such as Gaussian filters, provide very good bandwidth control but require ISI compensation in the demodulator. Note that base band-filtering-induced ISI is different from multi-path-induced ISI that causes distortion on the FM signal rather than the base band.

  • http://digitalmodulation.net/fsk3.h1.gif

    Frequency Shift Keying – Cont.

    Uses of FSK.

    Today FSK Modems are used for short haul data communication over private lines or any

    dedicated wire pair. These are many used for communication between industrial applications

    like railroad signalling controls and mobile robotic equipment. The short haul modem offers

    the following specs;

    – Speeds of up to 9600 bps

    – Full-duplex or half duplex operation.

    – Distance up to 9.5 miles

    In the past FSK was used in the Bell 103 and Bell 202. These were the first data modem but due to there low bit rate there not being used any more. The Bell 103 had a data rate of only 300 bauds. This modem was predominant until the early 1980s Analog modulation methods In analog modulation, the modulation is applied continuously in response to the analog information signal.

    Analog signal An Analog or analogue signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are meaningful. Analog is usually thought of in an electrical context; however, mechanical, pneumatic, hydraulic, and other systems may also convey analog signals.

    An analog signal uses some property of the medium to convey the signal’s information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltage followed closely byfrequency, current, and charge.Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer. For example, in sound recording, fluctuations in air pressure (that is to say, sound) strike the diaphragm of a microphone which induces corresponding fluctuations in the current produced by a coil in an electromagnetic microphone, or the voltage produced by a condensor microphone. The voltage or the current is said to be an “analog” of the sound. An analog signal has a theoretically infinite resolution. In practice an analog signal is subject to noise and a finite slew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth. As analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such an extent that the performance of digital systems may surpass it. Similarly, as digital systems become more complex, errors can occur in the digital data stream. A comparable performing digital system is more complex and requires more bandwidth than its analog counterpart. In analog systems, it is difficult to detect when such degradation occurs. However, in digital systems, degradation can not only be detected but corrected as well. A low-frequency message signal (top) may be carried by an AM or FM radio wave.

    Common analog modulation techniques are:-

    1. Amplitude modulation (AM) (here the amplitude of the modulated signal is varied)
    2. Double-sideband modulation (DSB)

    Double-sideband modulation with unsuppressed 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),

    SSB with carrier (SSB-WC)

    SSB suppressed carrier modulation (SSB-SC)

    1. Vestigial sideband modulation (VSB, or VSB-AM)
    2. Quadrature amplitude modulation (QAM)
    3. Angle modulation
    4. Frequency modulation (FM) (here the frequency of the modulated signal is

      varied)

    5. Phase modulation (PM) (here the phase shift of the modulated signal is varied

    Modulation is the process of varying one waveform in relation to another waveform. In telecommunications, modulation is used to convey a message, or a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Often a highfrequency sinusoid waveform is used as carrier signal to convey a lower frequency 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 (short for “Modulator-Demodulator”) http://upload.wikimedia.org/wikipedia/commons/a/a4/Amfm3-en-de.gifAnalog modulation methods In analog modulation, the modulation is applied continuously in response to the analog information signal. A low-frequency message signal (top) may be carried by an AM or FM radio wave.

    Common analog modulation techniques are:

    1. Amplitude modulation (AM) (here the amplitude of the modulated signal is varied)
    2. Double-sideband modulation (DSB)

    Double-sideband modulation with unsuppressed 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),

    SSB with carrier (SSB-WC)

    SSB suppressed carrier modulation (SSB-SC)

    • Vestigial sideband modulation (VSB, or VSB-AM)
    • Quadrature amplitude modulation (QAM)
    • Angle modulation
    • Frequency modulation (FM) (here the frequency of the modulated signal is

      varied)

    • Phase modulation (PM) (here the phase shift of the modulated signal is varied)

    Digital modulation methods In digital modulation, an analog carrier signal is modulated by a digital bit stream. 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).

    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 sequence00, 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 bits per second. According to one definition of digital signal, the modulated signal is a digital signal, and 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.

    Fundamental digital modulation methods

    These are the most fundamental digital modulation techniques:

    1. In the case of PSK, a finite number of phases are used.
    2. In the case of FSK, a finite number of frequencies are used.
    3. In the case of ASK, a finite number of amplitudes are used.
    4. In the case of QAM, a finite number of at least two phases, and at least two amplitudes are used.

    In 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, and 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.

    If the alphabet consists of M = 2N

    symbol rate

    alternative symbols, each symbol represents a message

    consisting of N bits. If the (also known as the baud rate) is fS

    baud

    symbols/second (or

    ), the data rate is NfS

    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.

    bit/second.

    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 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 PSK and ASK, and sometimes also FSK, are often generated and detected using the principle f QAM. The I and Q signals can be combined into a complex-valued signal I+jQ (where j isthe 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:

    1. Group the incoming data bits into codewords, one for each symbol that will be transmitted. 2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values. 3. 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. 4. 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). 5. Generate a high-frequency sine wave carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine wave form with the I and Q signals, resulting in that the equivalent lowpass signal is frequency shifted into a 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.

    6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum

    At the receiver side, the demodulator typically performs:

    1. Bandpass filtering.
    2. Automatic gain control, AGC (to compensate for attenuation, for example fading).
    3. 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
    4. scillator sinewave and cosine wave frequency (see the superheterodyne receiver principle).
    5. Sampling and analog-to-digital conversion (ADC) (Sometimes before or instead of

      the above point, for example by means of undersampling).

    6. 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.
    7. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal.
    8. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values.
    9. Mapping of the quantized amplitudes, frequencies or phases to codewords (bitgroups).
    10. Parallel-to-serial conversion of the codewords into a bit stream.
    11. 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.

    Non-coherent modulation methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier wave. In this case, modulation symbols (ratherthan bits, characters, or data packets) are asynchronously transferred. The opposite is coherent modulation.

    List of common digital modulation techniques 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)
    • p/4-QPSK
    • Frequency-shift keying (FSK):
    • Audio frequency-shift keying (AFSK)
    • Multi-frequency shift keying (M-ary FSK or MFSK)
    • Dual-tone multi-frequency (DTMF)
    • Continuous-phase frequency-shift keying (CPFSK)
    • 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.
    • Continuous phase modulation

    [citation needed]

    (CPM) methods:

    • Minimum-shift keying (MSK)
    • Gaussian minimum-shift keying (GMSK)
    • 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 pseudostochastic 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 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 modulated sub-carriers are summed to form an OFDM signal. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream overone 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-CDMA schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users. Of the two kinds of RF power amplifier, switching amplifiers (Class C amplifiers) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and 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.

    Digital baseband modulation or line coding

    The term digital baseband modulation (or digital baseband transmission) is synonymous to line codes. These are methods to transfer a digital bit stream over an analog baseband channel (a.k.a. lowpass channel) using a pulse train, i.e. a discrete number of signal levels, by directly modulating the voltage or current on a cable. Common examples are unipolar, non-return-tozero (NRZ), Manchester and alternate mark inversion (AMI) coding. Pulse modulation methods Pulse modulation schemes aim at transferring a narrowband analog signal over an analog baseband channel as a two-level signal by modulating a pulse wave. 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. These 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.

    Analog-over-analog methods:

    • Pulse-amplitude modulation (PAM)
    • Pulse-width modulation (PWM)
    • Pulse-position modulation (PPM)

    Analog-over-digital methods:

    Pulse-code modulation (PCM)

    1. Differential PCM (DPCM)
    2. Adaptive DPCM (ADPCM)
    • Delta modulation (DM or .-modulation)
    • Sigma-delta modulation (S.)
    • Continuously variable slope delta modulation (CVSDM), also called Adaptive-delta modulation (ADM)
    • Pulse-density modulation (PDM) Miscellaneous modulation techniques
    • The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation.
    • Adaptive modulation
    • Space modulation A method whereby signals are modulated within airspace, such as that used in Instrument landing systems

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