Digital modulation/demodulation formats
provide options in terms of bandwidth efficiency, power efficiency, and
complexity/cost when meeting a modern communications system’s data-transfer
needs.
Modulation and demodulation provide the means
to transfer information over great distances. As noted in the first part of
this article (see “Basics of Modulation and Demodulation”),
analog forms of modulation and demodulation have been around since the early
days of radio. Analog approaches directly encode information from changes in a
transmitted signal’s amplitude, phase, or frequency. Digital modulation and
demodulation methods, on the other hand, use the changes in amplitude, phase,
and frequency to convey digital bits representing the information to be
communicated.
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With growing
demands for voice, video, and data over communications networks of all kinds,
digital modulation and demodulation have recently replaced analog modulation
and demodulation methods in wireless networks to make the most efficient use of
a limited resource: bandwidth. In this second part, we explore how some
higher-order modulation and demodulation formats are created, and
how software and test equipment can help to keep different forms of
modulation and demodulation working as planned.
Enhancing Efficiency
Efficiency
is a common goal of all modulation/demodulation methods, whether they involve
conserving bandwidth, power, or cost. Digital modulation/demodulation formats,
in particular, have been found able to transfer large amounts of information
with minimal bandwidth and power. While increased data capacity tends toward
increased complexity in digital modulation/demodulation, high levels of
integration in modern ICs have made possible communications systems capable of
reliable, cost-effective operation with even the most advanced digital
modulation/demodulation formats.
Reasonable bandwidth efficiency is possible
with standard digital modulation formats, such as amplitude-shift keying (ASK),
frequency-shift keying (FSK), and phase-shift keying (PSK). By executing
additional variations, more complex digital modulation formats can be created
with improved data capacity and bandwidth efficiency, as measured in the number
of digital bits that can be transferred in a given amount of time per unit
amount of bandwidth (b/s/Hz).
For example,
with minimum-shift keying (MSK), essentially a form of FSK, peak-to-peak
frequency deviation is equal to one-half the bit rate. A further variation of
MSK is Gaussian MSK (GMSK), in which the modulated signal passes through a
Gaussian filter to minimize instantaneous frequency variations over time and
reduce the amount of bandwidth occupied by the transmitted waveforms. GMSK
maintains a constant envelope and provides good bit-error-rate (BER)
performance in addition to its good spectral efficiency.
By applying
some small changes, it is also possible to improve power efficiency. Quadrature
PSK (QPSK) is basically a four-state variation of simple PSK. It can be
modified in different ways—e.g., offset QPSK (OQPSK)—to boost efficiency. In
QPSK, the in-phase (I) and quadrature (Q) bit streams are switched at the same
time, using synchronized digital signal clocks for precise timing. A given
amount of power is required to maintain the timing alignment.
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In OQPSK, the I and Q bit streams are
offset by one bit period. Unlike QPSK, only one of the two bit streams can
change value at any one time in OQPSK, which also provides benefits in terms of
power consumption during the bit switching process. The spectral efficiency,
using two bit streams, is the same as in standard QPSK, but power efficiency is
enhanced due to reduced amplitude variations (by not having the amplitudes of
both bit streams passing at the same time). OQPSK does not have the same
stringent demands for linear amplification as QPSK, and can be transmitted with
a less-linear, more-power-efficient amplifier than required for QPSK.
The Role of Filtering
The
bandwidth efficiency of a modulation/demodulation format can be improved by
means of filtering, removing signal artifacts that can cause interference with
other communications systems. Various types of filters are used to improve the
spectral efficiency of different modulation formats, including Gaussian filters
(with perfect symmetry of the rolloff around the center frequency); Chebyshev
equiripple, finite-impulse-response (FIR) filters; and lowpass Nyquist filters
(also known as raised-cosine filters, since they pass nonzero bits through the
frequency spectrum as basic cosine functions).
The goal of
filtering is to improve spectral efficiency and reduce interference with other
systems, but without degrading modulation waveform quality. Excessive filtering
can result in increased BER due to a blurring of transmitted symbols that
comprise the data stream of a digital modulation format. Known as intersymbol
interference (ISI), this loss in integrity of the symbol states (phase,
amplitude, frequency) make it difficult to decode the symbols at the
demodulator and receiver in a digitally modulated communications system.
An ideal
filter is often referred to as a “brickwall” filter for its instant changeover
from a passband to a stopband. In reality, filters do not provide an ideal
reduction in signal bandwidth due to the need for some amount of transition
between a filter passband and its stopband; longer transitions require more
bandwidth.
Filters for
digital modulation/demodulation applications are regularly characterized by a
parameter known as “alpha,” which provides a measure of the amount of occupied
bandwidth by a filter. For example, a “brickwall” filter, with instant
transition from stopband to passband, would have an alpha value of zero.
Filters with longer transitions will maintain larger values of alpha. Smaller
values of filter alpha result in increased ISI, because more symbols can
contribute to the interference.
Modeling and Measuring
A wide range
of suppliers offer modulators and demodulators in various formats, from highly
integrated ICs to discrete components. A number of those highly integrated
transceiver ICs can be used for both functions—as transmitters/modulators and
receivers/demodulators. Some are even based on software-defined-radio (SDR)
architectures with sufficient bandwidths to serve multiple wireless
communications standards and modulation/demodulation requirements.
Modeling
software helps simplify the determination of requirements for a communications system’s
modulation/demodulation scheme. Some software programs provide general-purpose
modulation/demodulation analysis capabilities, allowing users to predict the
results of using different analog and digital modulation schemes. For example,
the Modulation Toolkit (Fig. 1) from National Instruments works with the
firm’s popular LabVIEW design software to simulate communications systems based
on different analog and digital modulation/demodulation formats. The software
makes it possible to experiment with different variables, such as carrier
frequency, signal strength, and interference; and predict different performance
parameters, such as BER, bandwidth efficiency, and power efficiency, under
different operating conditions.
In contrast, S1220 software from RIGOL Technologies USA simulates ASK and FSK
demodulation, in particular for Internet of Things (IoT) applications (Fig.
2). The software teams with the company’s spectrum analyzers to study
modulation/demodulation over a carrier frequency range of 9 kHz to 3.2 GHz (and
to 7.5 GHz with options). It provides an ASK symbol rate measurement range of 1
to 100 kHz and FSK deviation measurement range of 1 to 400 kHz.
Test instruments are an important part of achieving good
modulation/demodulation performance. Numerous test-equipment suppliers offer
programmable signal generators, such as arbitrary waveform generators, that can
create different modulation formats to be used with or without a carrier signal
generator for emulating modulated test signals. Spectrum analyzers provide
windows to the modulation characteristics of waveforms within their frequency
ranges. And some specialized measurement instruments have been developed for
the purpose of testing modulation and demodulation and associated components,
such as modulation domain analyzers (MDAs).
A number of different display formats provide ways to visualize modulated
signals—with both signal analyzers and software—including constellation
diagrams, eye diagrams, polar diagrams, and trellis diagrams (for trellis
modulation). For example, separate eye diagrams can be used to show the
magnitude versus time characteristics of two separate I and Q data channels,
with I and Q transitions appearing as “eyes” on a computer or instrument
display screen. Different modulation formats will show as different types of
displays; for instance, QPSK will appear as four distinct I/Q states, one in each
quadrant of the display screen. A high-quality signal creates eyes that are
open at each symbol.
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