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).
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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.
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.
Below
shows the basic representation. With binary FSK, the center 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 frequency.
It 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 equaled 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 centers the signal at frequency. The choice f a controls the
frequency deviation, away from the center 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.
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 signaling 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
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In the past FSK was used in the
Bell 103 and Bell 202. These were the first data modem but due to their 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 by frequency, 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 condenser
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:
- Amplitude modulation
(AM) (here the amplitude of the modulated signal is varied)
- Double-sideband modulation (DSB)
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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
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 high
frequency 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”)
Analog 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:
- Amplitude modulation
(AM) (here the amplitude of the modulated signal is varied)
- 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:
- In the case of PSK,
a finite number of phases are used.
- In the case of FSK,
a finite number of frequencies are used.
- In the case of ASK,
a finite number of amplitudes are used.
- In the case of QAM,
a finite number of at least two phases and at least two amplitudes are
used.
In QAM, an in phase 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 4bits. 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 is the imaginary unit). The resulting so called equivalent
low pass signal or equivalent baseband signal is a complex-valued
representation of the real-valued modulated physical signal (the so called pass
band signal or RF signal).
- These
are the general steps used by the modulator to transmit data:
1. Group the incoming data bits
into code words, one for each symbol that will be transmitted.
2. Map the code words 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 pass band 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 band
pass filtering to avoid harmonic distortion and periodic spectrum
At the receiver side, the
demodulator typically performs:
1.
Band pass 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
- Scillator sine wave
and cosine wave frequency (see the super heterodyne receiver principle).
- Sampling and
analog-to-digital conversion (ADC) (Sometimes before or instead of the
above point, for example by means of under sampling).
- Equalization
filtering, for example a matched filter, compensation for multipath
propagation, time spreading, phase distortion and frequency selective
fading, to avoid inter symbol 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 code words (bit groups).
- Parallel-to-serial
conversion of the code words 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 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 (rather than 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
(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 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 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. low pass 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-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 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)
Differential PCM (DPCM)
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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