Digital Modulation in Communications Systems

Agilent
Digital Modulation in
Communications Systems —
An Introduction
Application Note 1298
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This application note introduces the concepts of
digital modulation used in many communications
systems today. Emphasis is placed on explaining
the tradeoffs that are made to optimize efficiencies
in system design.
Most communications systems fall into one of three
categories: bandwidth efficient, power efficient, or
cost efficient. Bandwidth efficiency describes the
ability of a modulation scheme to accommodate
data within a limited bandwidth. Power efficiency
describes the ability of the system to reliably send
information at the lowest practical power level.
In most systems, there is a high priority on bandwidth
efficiency. The parameter to be optimized
depends on the demands of the particular system,
as can be seen in the following two examples.
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, 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 communications design.
This application note covers
• the reasons for the move to digital modulation;
• how information is modulated onto in-phase (I)
and quadrature (Q) signals;
• different types of digital modulation;
• filtering techniques to conserve bandwidth;
• ways of looking at digitally modulated signals;
• multiplexing techniques used to share the
transmission channel;
• how a digital transmitter and receiver work;
• measurements on digital RF communications
systems;
• an overview table with key specifications for
the major digital communications systems; and
• a glossary of terms used in digital RF communications.
These concepts form the building blocks of any
communications system. If you understand the
building blocks, then you will be able to understand
how any communications system, present
or future, works.
Introduction
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1. Why Digital Modulation?
1.1 Trading off simplicity and bandwidth
1.2 Industry trends
2. Using I/Q Modulation (Amplitude and Phase Control)
to Convey Information
2.1 Transmitting information
2.2 Signal characteristics that can be modified
2.3 Polar display—magnitude and phase represented
together
2.4 Signal changes or modifications in polar form
2.5 I/Q formats
2.6 I and Q in a radio transmitter
2.7 I and Q in a radio receiver
2.8 Why use I and Q?
3. Digital Modulation Types and Relative Efficiencies
3.1 Applications
3.1.1 Bit rate and symbol rate
3.1.2 Spectrum (bandwidth) requirements
3.1.3 Symbol clock
3.2 Phase Shift Keying (PSK)
3.3 Frequency Shift Keying
3.4 Minimum Shift Keying (MSK)
3.5 Quadrature Amplitude Modulation (QAM)
3.6 Theoretical bandwidth efficiency limits
3.7 Spectral efficiency examples in practical radios
3.8 I/Q offset modulation
3.9 Differential modulation
3.10 Constant amplitude modulation
4. Filtering
4.1 Nyquist or raised cosine filter
4.2 Transmitter-receiver matched filters
4.3 Gaussian filter
4.4 Filter bandwidth parameter alpha
4.5 Filter bandwidth effects
4.6 Chebyshev equiripple FIR (finite impulse response) filter
4.7 Spectral efficiency versus power consumption
5. Different Ways of Looking at a Digitally Modulated
Signal Time and Frequency Domain View
5.1 Power and frequency view
5.2 Constellation diagrams
5.3 Eye diagrams
5.4 Trellis diagrams
Table of Contents
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6. Sharing the Channel
6.1 Multiplexing—frequency
6.2 Multiplexing—time
6.3 Multiplexing—code
6.4 Multiplexing—geography
6.5 Combining multiplexing modes
6.6 Penetration versus efficiency
7. How Digital Transmitters and Receivers Work
7.1 A digital communications transmitter
7.2 A digital communications receiver
8. Measurements on Digital RF Communications Systems
8.1 Power measurements
8.1.1 Adjacent Channel Power
8.2 Frequency measurements
8.2.1 Occupied bandwidth
8.3 Timing measurements
8.4 Modulation accuracy
8.5 Understanding Error Vector Magnitude (EVM)
8.6 Troubleshooting with error vector measurements
8.7 Magnitude versus phase error
8.8 I/Q phase error versus time
8.9 Error Vector Magnitude versus time
8.10 Error spectrum (EVM versus frequency)
9. Summary
10. Overview of Communications Systems
11. Glossary of Terms
Table of Contents (continued)
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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 analog modulation
schemes.
1.1 Trading off simplicity and bandwidth
There is a fundamental tradeoff in communication
systems. Simple hardware can be used in transmitters
and receivers to communicate information.
However, this uses a lot of spectrum which limits
the number of users. Alternatively, more complex
transmitters and receivers can be used to transmit
the same information over less bandwidth. The
transition to more and more spectrally efficient
transmission techniques requires more and more
complex hardware. Complex hardware is difficult
to design, test, and build. This tradeoff exists
whether communication is over air or wire, analog
or digital.
Figure 1. The Fundamental Tradeoff
Complex
Hardware Less Spectrum
Simple
Hardware
Simple
Hardware
Complex
Hardware
More Spectrum
1. Why Digital Modulation?
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1.2 Industry trends
Over the past few years a major transition has
occurred from simple analog Amplitude Modulation
(AM) and Frequency/Phase Modulation
(FM/PM) to new digital modulation techniques.
Examples of digital modulation include
• QPSK (Quadrature Phase Shift Keying)
• FSK (Frequency Shift Keying)
• MSK (Minimum Shift Keying)
• QAM (Quadrature Amplitude Modulation)
Another layer of complexity in many new systems
is multiplexing. Two principal types of multiplexing
(or “multiple access”) are TDMA (Time Division
Multiple Access) and CDMA (Code Division
Multiple Access). These are two different ways to
add diversity to signals allowing different signals
to be separated from one another.
QAM, FSK,
QPSK
Vector Signals
AM, FM
Scalar Signals
TDMA, CDMA
Time-Variant
Signals
Required Measurement Capability
Signal/System Complexity
Figure 2. Trends in the Industry
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2.1 Transmitting information
To transmit a signal over the air, there are three
main steps:
1. A pure carrier is generated at the transmitter.
2. The carrier is modulated with the information
to be transmitted. Any reliably detectable
change in signal characteristics can carry
information.
3. At the receiver the signal modifications or
changes are detected and demodulated.
2.2 Signal characteristics that can be modified
There are only three characteristics of a signal that
can be changed over time: amplitude, phase, or frequency.
However, phase and frequency are just different
ways to view or measure the same signal
change.
In AM, the amplitude of a high-frequency carrier
signal is varied in proportion to the instantaneous
amplitude of the modulating message signal.
Frequency Modulation (FM) is the most popular
analog modulation technique used in mobile communications
systems. In FM, the amplitude of the
modulating carrier is kept constant while its frequency
is varied by the modulating message signal.
Amplitude and phase can be modulated simultaneously
and separately, but this is difficult to generate,
and especially difficult to detect. Instead, in
practical systems the signal is separated into
another set of independent components: I (Inphase)
and Q (Quadrature). These components are
orthogonal and do not interfere with each other.
Modify a
Signal
"Modulate"
Detect the Modifications
"Demodulate"
Any reliably detectable change in
signal characteristics can carry information
Amplitude
Frequency
or
Phase
Both Amplitude
Figure 3. Transmitting Information (Analog or Digital) and Phase
Figure 4. Signal Characteristics to Modify
2. Using I/Q Modulation to Convey Information
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2.3 Polar display—magnitude and phase represented
together
A simple way to view amplitude and phase is with
the polar diagram. The carrier becomes a frequency
and phase reference and the signal is interpreted
relative to the carrier. The signal can be expressed
in polar form as a magnitude and a phase. The
phase is relative to a reference signal, the carrier
in most communication systems. The magnitude is
either an absolute or relative value. Both are used
in digital communication systems. Polar diagrams
are the basis of many displays used in digital communications,
although it is common to describe the
signal vector by its rectangular coordinates of I
(In-phase) and Q (Quadrature).
2.4 Signal changes or modifications in
polar form
Figure 6 shows different forms of modulation in
polar form. Magnitude is represented as the distance
from the center and phase is represented as
the angle.
Amplitude modulation (AM) changes only the
magnitude of the signal. Phase modulation (PM)
changes only the phase of the signal. Amplitude
and phase modulation can be used together.
Frequency modulation (FM) looks similar to phase
modulation, though frequency is the controlled
parameter, rather than relative phase.
Phase
Mag
0 deg
Figure 5. Polar Display—Magnitude and Phase
Represented Together
Phase
Mag
0 deg
Magnitude Change
Phase
0 deg
Phase Change
Magnitude & Phase Change Frequency Change
0 deg
0 deg
Figure 6. Signal Changes or Modifications
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One example of the difficulties in RF design can
be illustrated with simple amplitude modulation.
Generating AM with no associated angular modulation
should result in a straight line on a polar
display. This line should run from the origin to
some peak radius or amplitude value. In practice,
however, the line is not straight. The amplitude
modulation itself often can cause a small amount
of unwanted phase modulation. The result is a
curved line. It could also be a loop if there is any
hysteresis in the system transfer function. Some
amount of this distortion is inevitable in any system
where modulation causes amplitude changes.
Therefore, the degree of effective amplitude modulation
in a system will affect some distortion
parameters.
2.5 I/Q formats
In digital communications, modulation is often
expressed in terms of I and Q. This is a rectangular
representation of the polar diagram. On a polar
diagram, the I axis lies on the zero degree phase
reference, and the Q axis is rotated by 90 degrees.
The signal vector’s projection onto the I axis is its
“I” component and the projection onto the Q axis
is its “Q” component.
{
{
0 deg
"I"
"Q"
Q-Value
I-Value
Project signal
to "I" and "Q" axes
Polar to Rectangular Conversion
Figure 7. “I-Q” Format
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2.6 I and Q in a radio transmitter
I/Q diagrams are particularly useful because they
mirror the way most digital communications signals
are created using an I/Q modulator. In the
transmitter, I and Q signals are mixed with the
same local oscillator (LO). A 90 degree phase
shifter is placed in one of the LO paths. Signals
that are separated by 90 degrees are also known as
being orthogonal to each other or in quadrature.
Signals that are in quadrature do not interfere
with each other. They are two independent components
of the signal. When recombined, they are
summed to a composite output signal. There are
two independent signals in I and Q that can be
sent and received with simple circuits. This simplifies
the design of digital radios. The main advantage
of I/Q modulation is the symmetric ease of
combining independent signal components into a
single composite signal and later splitting such a
composite signal into its independent component
parts.
2.7 I and Q in a radio receiver
The composite signal with magnitude and phase
(or I and Q) information arrives at the receiver
input. The input signal is mixed with the local
oscillator signal at the carrier frequency in two
forms. One is at an arbitrary zero phase. The other
has a 90 degree phase shift. The composite input
signal (in terms of magnitude and phase) is thus
broken into an in-phase, I, and a quadrature, Q,
component. These two components of the signal
are independent and orthogonal. One can be
changed without affecting the other. Normally,
information cannot be plotted in a polar format
and reinterpreted as rectangular values without
doing a polar-to-rectangular conversion. This conversion
is exactly what is done by the in-phase and
quadrature mixing processes in a digital radio. A
local oscillator, phase shifter, and two mixers can
perform the conversion accurately and efficiently.
90 deg
Phase Shift
Local Osc.
(Carrier Freq.)
Q
I
Composite
Output
Signal Σ
Local Osc.
(Carrier Freq.)
Quadrature Component
In-Phase Component
Composite
Input
Signal
90 deg
Phase Shift
Figure 8. I and Q in a Practical Radio Transmitter Figure 9. I and Q in a Radio Receiver
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2.8 Why use I and Q?
Digital modulation is easy to accomplish with I/Q
modulators. Most digital modulation maps the data
to a number of discrete points on the I/Q plane.
These are known as constellation points. As the signal
moves from one point to another, simultaneous
amplitude and phase modulation usually results.
To accomplish this with an amplitude modulator
and a phase modulator is difficult and complex. It
is also impossible with a conventional phase modulator.
The signal may, in principle, circle the origin
in one direction forever, necessitating infinite phase
shifting capability. Alternatively, simultaneous AM
and Phase Modulation is easy with an I/Q modulator.
The I and Q control signals are bounded, but infinite
phase wrap is possible by properly phasing
the I and Q signals.
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This section covers the main digital modulation
formats, their main applications, relative spectral
efficiencies, and some variations of the main
modulation types as used in practical systems.
Fortunately, there are a limited number of modulation
types which form the building blocks of any
system.
3.1 Applications
The table below covers the applications for different
modulation formats in both wireless communications
and video.
Although this note focuses on wireless communications,
video applications have also been included
in the table for completeness and because of their
similarity to other wireless communications.
3.1.1 Bit rate and symbol rate
To understand and compare different modulation
format efficiencies, it is important to first understand
the difference between bit rate and symbol
rate. The signal bandwidth for the communications
channel needed depends on the symbol rate, not
on the bit rate.
Symbol rate =
Modulation format Application
MSK, GMSK GSM, CDPD
BPSK Deep space telemetry, cable modems
QPSK, π/4 DQPSK Satellite, CDMA, NADC, TETRA, PHS, PDC, LMDS, DVB-S, cable (return
path), cable modems, TFTS
OQPSK CDMA, satellite
FSK, GFSK DECT, paging, RAM mobile data, AMPS, CT2, ERMES, land mobile,
public safety
8, 16 VSB North American digital TV (ATV), broadcast, cable
8PSK Satellite, aircraft, telemetry pilots for monitoring broadband video systems
16 QAM Microwave digital radio, modems, DVB-C, DVB-T
32 QAM Terrestrial microwave, DVB-T
64 QAM DVB-C, modems, broadband set top boxes, MMDS
256 QAM Modems, DVB-C (Europe), Digital Video (US)
bit rate
the number of bits transmitted with each symbol
3. Digital Modulation Types and Relative Efficiencies
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Bit rate is the frequency of a system bit stream.
Take, for example, a radio with an 8 bit sampler,
sampling at 10 kHz for voice. The bit rate, the basic
bit stream rate in the radio, would be eight bits
multiplied by 10K samples per second, or 80 Kbits
per second. (For the moment we will ignore the
extra bits required for synchronization, error
correction, etc.)
Figure 10 is an example of a state diagram of a
Quadrature Phase Shift Keying (QPSK) signal. The
states can be mapped to zeros and ones. This is a
common mapping, but it is not the only one. Any
mapping can be used.
The symbol rate is the bit rate divided by the number
of bits that can be transmitted with each symbol.
If one bit is transmitted per symbol, as with
BPSK, then the symbol rate would be the same as
the bit rate of 80 Kbits per second. If two bits are
transmitted per symbol, as in QPSK, then the symbol
rate would be half of the bit rate or 40 Kbits
per second. Symbol rate is sometimes called baud
rate. Note that baud rate is not the same as bit
rate. These terms are often confused. If more bits
can be sent with each symbol, then the same
amount of data can be sent in a narrower spectrum.
This is why modulation formats that are
more complex and use a higher number of states
can send the same information over a narrower
piece of the RF spectrum.
3.1.2 Spectrum (bandwidth) requirements
An example of how symbol rate influences spectrum
requirements can be seen in eight-state Phase
Shift Keying (8PSK). It is a variation of PSK. There
are eight possible states that the signal can transition
to at any time. The phase of the signal can
take any of eight values at any symbol time. Since
23 = 8, there are three bits per symbol. This means
the symbol rate is one third of the bit rate. This is
relatively easy to decode.
01 00
11 10
QPSK
Two Bits Per Symbol
QPSK
State Diagram
BPSK
One Bit Per Symbol
Symbol Rate = Bit Rate
8PSK
Three Bits Per Symbol
Symbol Rate = 1/3 Bit Rate
Figure 10. Bit Rate and Symbol Rate Figure 11. Spectrum Requirements
3.1.3 Symbol Clock
The symbol clock represents the frequency and
exact timing of the transmission of the individual
symbols. At the symbol clock transitions, the transmitted
carrier is at the correct I/Q (or magnitude/
phase) value to represent a specific symbol (a
specific point in the constellation).
3.2 Phase Shift Keying
One of the simplest forms of digital modulation is
binary or Bi-Phase Shift Keying (BPSK). One application
where this is used is for deep space telemetry.
The phase of a constant amplitude carrier signal
moves between zero and 180 degrees. On an I
and Q diagram, the I state has two different values.
There are two possible locations in the state diagram,
so a binary one or zero can be sent. The
symbol rate is one bit per symbol.
A more common type of phase modulation is
Quadrature Phase Shift Keying (QPSK). It is used
extensively in applications including CDMA (Code
Division Multiple Access) cellular service, wireless
local loop, Iridium (a voice/data satellite system)
and DVB-S (Digital Video Broadcasting — Satellite).
Quadrature means that the signal shifts between
phase states which are separated by 90 degrees.
The signal shifts in increments of 90 degrees from
45 to 135, –45, or –135 degrees. These points are
chosen as they can be easily implemented using an
I/Q modulator. Only two I values and two Q values
are needed and this gives two bits per symbol.
There are four states because 22 = 4. It is therefore
a more bandwidth-efficient type of modulation
than BPSK, potentially twice as efficient.
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BPSK
One Bit Per Symbol
QPSK
Two Bits Per Symbol
Figure 12. Phase Shift Keying
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3.3 Frequency Shift Keying
Frequency modulation and phase modulation are
closely related. A static frequency shift of +1 Hz
means that the phase is constantly advancing at
the rate of 360 degrees per second (2 π rad/sec),
relative to the phase of the unshifted signal.
FSK (Frequency Shift Keying) is used in many
applications including cordless and paging systems.
Some of the cordless systems include DECT
(Digital Enhanced Cordless Telephone) and CT2
(Cordless Telephone 2).
In FSK, the frequency of the carrier is changed as
a function of the modulating signal (data) being
transmitted. Amplitude remains unchanged. In
binary FSK (BFSK or 2FSK), a “1” is represented
by one frequency and a “0” is represented by
another frequency.
3.4 Minimum Shift Keying
Since a frequency shift produces an advancing or
retarding phase, frequency shifts can be detected
by sampling phase at each symbol period. Phase
shifts of (2N + 1) π/2 radians are easily detected
with an I/Q demodulator. At even numbered symbols,
the polarity of the I channel conveys the
transmitted data, while at odd numbered symbols
the polarity of the Q channel conveys the data.
This orthogonality between I and Q simplifies
detection algorithms and hence reduces power consumption
in a mobile receiver. The minimum frequency
shift which yields orthogonality of I and Q
is that which results in a phase shift of ± π/2 radians
per symbol (90 degrees per symbol). FSK with
this deviation is called MSK (Minimum Shift
Keying). The deviation must be accurate in order to
generate repeatable 90 degree phase shifts. MSK is
used in the GSM (Global System for Mobile
Communications) cellular standard. A phase shift
of +90 degrees represents a data bit equal to “1,”
while –90 degrees represents a “0.” The peak-topeak
frequency shift of an MSK signal is equal to
one-half of the bit rate.
FSK and MSK produce constant envelope carrier
signals, which have no amplitude variations. This
is a desirable characteristic for improving the
power efficiency of transmitters. Amplitude variations
can exercise nonlinearities in an amplifier’s
amplitude-transfer function, generating spectral
regrowth, a component of adjacent channel power.
Therefore, more efficient amplifiers (which tend to
be less linear) can be used with constant-envelope
signals, reducing power consumption.
MSK
Q vs. I
FSK
Freq. vs. Time
One Bit Per Symbol One Bit Per Symbol
Figure 13. Frequency Shift Keying
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MSK has a narrower spectrum than wider deviation
forms of FSK. The width of the spectrum is
also influenced by the waveforms causing the frequency
shift. If those waveforms have fast transitions
or a high slew rate, then the spectrum
of the transmitter will be broad. In practice, the
waveforms are filtered with a Gaussian filter,
resulting in a narrow spectrum. In addition, the
Gaussian filter has no time-domain overshoot,
which would broaden the spectrum by increasing
the peak deviation. MSK with a Gaussian filter is
termed GMSK (Gaussian MSK).
3.5 Quadrature Amplitude Modulation
Another member of the digital modulation family
is Quadrature Amplitude Modulation (QAM). QAM
is used in applications including microwave digital
radio, DVB-C (Digital Video Broadcasting—Cable),
and modems.
In 16-state Quadrature Amplitude Modulation
(16QAM), there are four I values and four Q values.
This results in a total of 16 possible states for the
signal. It can transition from any state to any other
state at every symbol time. Since 16 = 24, four bits
per symbol can be sent. This consists of two bits
for I and two bits for Q. The symbol rate is one
fourth of the bit rate. So this modulation format
produces a more spectrally efficient transmission.
It is more efficient than BPSK, QPSK, or 8PSK.
Note that QPSK is the same as 4QAM.
Another variation is 32QAM. In this case there are
six I values and six Q values resulting in a total of
36 possible states (6x6=36). This is too many states
for a power of two (the closest power of two is 32).
So the four corner symbol states, which take the
most power to transmit, are omitted. This reduces
the amount of peak power the transmitter has to
generate. Since 25 = 32, there are five bits per symbol
and the symbol rate is one fifth of the bit rate.
The current practical limits are approximately
256QAM, though work is underway to extend the
limits to 512 or 1024 QAM. A 256QAM system uses
16 I-values and 16 Q-values, giving 256 possible
states. Since 28 = 256, each symbol can represent
eight bits. A 256QAM signal that can send eight
bits per symbol is very spectrally efficient.
However, the symbols are very close together and
are thus more subject to errors due to noise and
distortion. Such a signal may have to be transmitted
with extra power (to effectively spread the
symbols out more) and this reduces power efficiency
as compared to simpler schemes.
16QAM
Four Bits Per Symbol
Symbol Rate = 1/4 Bit Rate
I
Q
32QAM
Five Bits Per Symbol
Symbol Rate = 1/5 Bit Rate
Vector Diagram Constellation Diagram
Figure 14. Quadrature Amplitude Modulation
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Compare the bandwidth efficiency when using
256QAM versus BPSK modulation in the radio
example in section 3.1.1 (which uses an eight-bit
sampler sampling at 10 kHz for voice). BPSK uses
80 Ksymbols-per-second sending 1 bit per symbol.
A system using 256QAM sends eight bits per symbol
so the symbol rate would be 10 Ksymbols per
second. A 256QAM system enables the same
amount of information to be sent as BPSK using
only one eighth of the bandwidth. It is eight times
more bandwidth efficient. However, there is a
tradeoff. The radio becomes more complex and is
more susceptible to errors caused by noise and distortion.
Error rates of higher-order QAM systems
such as this degrade more rapidly than QPSK as
noise or interference is introduced. A measure
of this degradation would be a higher Bit Error
Rate (BER).
In any digital modulation system, if the input signal
is distorted or severely attenuated the receiver
will eventually lose symbol lock completely. If the
receiver can no longer recover the symbol clock, it
cannot demodulate the signal or recover any information.
With less degradation, the symbol clock
can be recovered, but it is noisy, and the symbol
locations themselves are noisy. In some cases, a
symbol will fall far enough away from its intended
position that it will cross over to an adjacent position.
The I and Q level detectors used in the
demodulator would misinterpret such a symbol as
being in the wrong location, causing bit errors.
QPSK is not as efficient, but the states are much
farther apart and the system can tolerate a lot
more noise before suffering symbol errors. QPSK
has no intermediate states between the four
corner-symbol locations, so there is less opportunity
for the demodulator to misinterpret symbols.
QPSK requires less transmitter power than QAM
to achieve the same bit error rate.
3.6 Theoretical bandwidth efficiency limits
Bandwidth efficiency describes how efficiently the
allocated bandwidth is utilized or the ability of a
modulation scheme to accommodate data, within
a limited bandwidth. The table below shows the
theoretical bandwidth efficiency limits for the
main modulation types. Note that these figures
cannot actually be achieved in practical radios
since they require perfect modulators, demodulators,
filter, and transmission paths.
If the radio had a perfect (rectangular in the frequency
domain) filter, then the occupied bandwidth
could be made equal to the symbol rate.
Techniques for maximizing spectral efficiency
include the following:
• Relate the data rate to the frequency shift
(as in GSM).
• Use premodulation filtering to reduce the
occupied bandwidth. Raised cosine filters,
as used in NADC, PDC, and PHS, give the
best spectral efficiency.
• Restrict the types of transitions.
Modulation Theoretical bandwidth
format efficiency limits
MSK 1 bit/second/Hz
BPSK 1 bit/second/Hz
QPSK 2 bits/second/Hz
8PSK 3 bits/second/Hz
16 QAM 4 bits/second/Hz
32 QAM 5 bits/second/Hz
64 QAM 6 bits/second/Hz
256 QAM 8 bits/second/Hz
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Effects of going through the origin
Take, for example, a QPSK signal where the normalized
value changes from 1, 1 to –1, –1. When changing simultaneously
from I and Q values of +1 to I and Q values of –1,
the signal trajectory goes through the origin (the I/Q value
of 0,0). The origin represents 0 carrier magnitude. A value
of 0 magnitude indicates that the carrier amplitude is 0 for
a moment.
Not all transitions in QPSK result in a trajectory that goes
through the origin. If I changes value but Q does not (or
vice-versa) the carrier amplitude changes a little, but it
does not go through zero. Therefore some symbol transitions
will result in a small amplitude variation, while others
will result in a very large amplitude variation. The clockrecovery
circuit in the receiver must deal with this amplitude
variation uncertainty if it uses amplitude variations
to align the receiver clock with the transmitter clock.
Spectral regrowth does not automatically result from these
trajectories that pass through or near the origin. If the
amplifier and associated circuits are perfectly linear, the
spectrum (spectral occupancy or occupied bandwidth) will
be unchanged. The problem lies in nonlinearities in the
circuits.
A signal which changes amplitude over a very large range
will exercise these nonlinearities to the fullest extent.
These nonlinearities will cause distortion products. In continuously
modulated systems they will cause “spectral
regrowth” or wider modulation sidebands (a phenomenon
related to intermodulation distortion). Another term which
is sometimes used in this context is “spectral splatter.”
However this is a term that is more correctly used in association
with the increase in the bandwidth of a signal
caused by pulsing on and off.
3.7 Spectral efficiency examples in
practical radios
The following examples indicate spectral efficiencies
that are achieved in some practical radio
systems.
The TDMA version of the North American Digital
Cellular (NADC) system, achieves a 48 Kbits-persecond
data rate over a 30 kHz bandwidth or 1.6 bits
per second per Hz. It is a π/4 DQPSK based system
and transmits two bits per symbol. The theoretical
efficiency would be two bits per second per Hz and
in practice it is 1.6 bits per second per Hz.
Another example is a microwave digital radio using
16QAM. This kind of signal is more susceptible to
noise and distortion than something simpler such
as QPSK. This type of signal is usually sent over a
direct line-of-sight microwave link or over a wire
where there is very little noise and interference. In
this microwave-digital-radio example the bit rate is
140 Mbits per second over a very wide bandwidth
of 52.5 MHz. The spectral efficiency is 2.7 bits per
second per Hz. To implement this, it takes a very
clear line-of-sight transmission path and a precise
and optimized high-power transceiver.
19
Digital modulation types—variations
The modulation types outlined in sections 3.2 to
3.4 form the building blocks for many systems.
There are three main variations on these basic
building blocks that are used in communications
systems: I/Q offset modulation, differential modulation,
and constant envelope modulation.
3.8 I/Q offset modulation
The first variation is offset modulation. One example
of this is Offset QPSK (OQPSK). This is used in
the cellular CDMA (Code Division Multiple Access)
system for the reverse (mobile to base) link.
In QPSK, the I and Q bit streams are switched at
the same time. The symbol clocks, or the I and Q
digital signal clocks, are synchronized. In Offset
QPSK (OQPSK), the I and Q bit streams are offset
in their relative alignment by one bit period (one
half of a symbol period). This is shown in the diagram.
Since the transitions of I and Q are offset, at
any given time only one of the two bit streams can
change values. This creates a dramatically different
constellation, even though there are still just two
I/Q values. This has power efficiency advantages.
In OQPSK the signal trajectories are modified by
the symbol clock offset so that the carrier amplitude
does not go through or near zero (the center
of the constellation). The spectral efficiency is the
same with two I states and two Q states. The
reduced amplitude variations (perhaps 3 dB for
OQPSK, versus 30 to 40 dB for QPSK) allow a more
power-efficient, less linear RF power amplifier to
be used.
QPSK
Offset
QPSK
Q
I
Q
I
Eye Constellation
Figure 15. I-Q “Offset” Modulation
20
3.9 Differential modulation
The second variation is differential modulation as
used in differential QPSK (DQPSK) and differential
16QAM (D16QAM). Differential means that the
information is not carried by the absolute state, it
is carried by the transition between states. In some
cases there are also restrictions on allowable transitions.
This occurs in π/4 DQPSK where the carrier
trajectory does not go through the origin. A DQPSK
transmission system can transition from any symbol
position to any other symbol position. The π/4
DQPSK modulation format is widely used in many
applications including
• cellular
–NADC- IS-54 (North American digital cellular)
–PDC (Pacific Digital Cellular)
• cordless
–PHS (personal handyphone system)
• trunked radio
–TETRA (Trans European Trunked Radio)
The π/4 DQPSK modulation format uses two QPSK
constellations offset by 45 degrees (π/4 radians).
Transitions must occur from one constellation to
the other. This guarantees that there is always a
change in phase at each symbol, making clock
recovery easier. The data is encoded in the magnitude
and direction of the phase shift, not in the
absolute position on the constellation. One advantage
of π/4 DQPSK is that the signal trajectory does
not pass through the origin, thus simplifying transmitter
design. Another is that π/4 DQPSK, with root
raised cosine filtering, has better spectral efficiency
than GMSK, the other common cellular modulation
type.
QPSK π/4 DQPSK
Both formats are 2 bits/symbol
Figure 16. Differential Modulation
21
3.10 Constant amplitude modulation
The third variation is constant-envelope modulation.
GSM uses a variation of constant amplitude
modulation format called 0.3 GMSK (Gaussian
Minimum Shift Keying).
In constant-envelope modulation the amplitude of
the carrier is constant, regardless of the variation
in the modulating signal. It is a power-efficient
scheme that allows efficient class-C amplifiers to
be used without introducing degradation in the
spectral occupancy of the transmitted signal.
However, constant-envelope modulation techniques
occupy a larger bandwidth than schemes which are
linear. In linear schemes, the amplitude of the
transmitted signal varies with the modulating digital
signal as in BPSK or QPSK. In systems where
bandwidth efficiency is more important than
power efficiency, constant envelope modulation
is not as well suited.
MSK (covered in section 3.4) is a special type of
FSK where the peak-to-peak frequency deviation is
equal to half the bit rate.
GMSK is a derivative of MSK where the bandwidth
required is further reduced by passing the modulating
waveform through a Gaussian filter. The
Gaussian filter minimizes the instantaneous frequency
variations over time. GMSK is a spectrally
efficient modulation scheme and is particularly
useful in mobile radio systems. It has a constant
envelope, spectral efficiency, good BER performance,
and is self-synchronizing.
MSK (GSM)
Amplitude (Envelope) Varies
From Zero to Nominal Value
QPSK
Amplitude (Envelope) Does
Not Vary At All
Figure 17. Constant Amplitude Modulation
22
Filtering allows the transmitted bandwidth to be
significantly reduced without losing the content
of the digital data. This improves the spectral efficiency
of the signal.
There are many different varieties of filtering.
The most common are
• raised cosine
• square-root raised cosine
• Gaussian filters
Any fast transition in a signal, whether it be amplitude,
phase, or frequency, will require a wide occupied
bandwidth. Any technique that helps to slow
down these transitions will narrow the occupied
bandwidth. Filtering serves to smooth these transitions
(in I and Q). Filtering reduces interference
because it reduces the tendency of one signal or
one transmitter to interfere with another in a
Frequency-Division-Multiple-Access (FDMA) system.
On the receiver end, reduced bandwidth improves
sensitivity because more noise and interference
are rejected.
Some tradeoffs must be made. One is that some
types of filtering cause the trajectory of the signal
(the path of transitions between the states) to
overshoot in many cases. This overshoot can occur
in certain types of filters such as Nyquist. This
overshoot path represents carrier power and phase.
For the carrier to take on these values it requires
more output power from the transmitter amplifiers.
It requires more power than would be necessary
to transmit the actual symbol itself. Carrier
power cannot be clipped or limited (to reduce or
eliminate the overshoot) without causing the spectrum
to spread out again. Since narrowing the
spectral occupancy was the reason the filtering
was inserted in the first place, it becomes a very
fine balancing act.
Other tradeoffs are that filtering makes the radios
more complex and can make them larger, especially
if performed in an analog fashion. Filtering can
also create Inter-Symbol Interference (ISI). This
occurs when the signal is filtered enough so that
the symbols blur together and each symbol affects
those around it. This is determined by the timedomain
response or impulse response of the filter.
4.1 Nyquist or raised cosine filter
Figure 18 shows the impulse or time-domain
response of a raised cosine filter, one class of
Nyquist filter. Nyquist filters have the property
that their impulse response rings at the symbol
rate. The filter is chosen to ring, or have the
impulse response of the filter cross through zero,
at the symbol clock frequency.
0
0.5
1
-10 -5 0 5 10
h
i
t
i
One symbol
Figure 18. Nyquit or Raised Cosine Filter
4. Filtering
23
The time response of the filter goes through zero
with a period that exactly corresponds to the symbol
spacing. Adjacent symbols do not interfere
with each other at the symbol times because the
response equals zero at all symbol times except the
center (desired) one. Nyquist filters heavily filter
the signal without blurring the symbols together
at the symbol times. This is important for transmitting
information without errors caused by Inter-
Symbol Interference. Note that Inter-Symbol
Interference does exist at all times except the symbol
(decision) times. Usually the filter is split, half
being in the transmit path and half in the receiver
path. In this case root Nyquist filters (commonly
called root raised cosine) are used in each part, so
that their combined response is that of a Nyquist
filter.
4.2 Transmitter-receiver matched filters
Sometimes filtering is desired at both the transmitter
and receiver. Filtering in the transmitter
reduces the adjacent-channel-power radiation of
the transmitter, and thus its potential for interfering
with other transmitters.
Filtering at the receiver reduces the effects of
broadband noise and also interference from other
transmitters in nearby channels.
To get zero Inter-Symbol Interference (ISI), both
filters are designed until the combined result of
the filters and the rest of the system is a full Nyquist
filter. Potential differences can cause problems in
manufacturing because the transmitter and receiver
are often manufactured by different companies.
The receiver may be a small hand-held model and
the transmitter may be a large cellular base station.
If the design is performed correctly the results
are the best data rate, the most efficient radio, and
reduced effects of interference and noise. This is
why root-Nyquist filters are used in receivers and
transmitters as √ Nyquist x √ Nyquist = Nyquist.
Matched filters are not used in Gaussian filtering.
4.3 Gaussian filter
In contrast, a GSM signal will have a small blurring
of symbols on each of the four states because the
Gaussian filter used in GSM does not have zero
Inter-Symbol Interference. The phase states vary
somewhat causing a blurring of the symbols, as
shown in Figure 17. Wireless system architects
must decide just how much of the Inter-Symbol
Interference can be tolerated in a system and combine
that with noise and interference.
Actual Data
Root Raised
Cosine Filter
DAC
Detected Bits
Root Raised
Cosine Filter
Transmitter
Receiver
Demodulator
Modulator
Figure 19. Transmitter-Receiver Matched Filters
24
Gaussian filters are used in GSM because of their
advantages in carrier power, occupied bandwidth,
and symbol-clock recovery. The Gaussian filter is
a Gaussian shape in both the time and frequency
domains, and it does not ring like the raised cosine
filters do. Its effects in the time domain are relatively
short and each symbol interacts significantly
(or causes ISI) with only the preceding and succeeding
symbols. This reduces the tendency for
particular sequences of symbols to interact which
makes amplifiers easier to build and more efficient.
4.4 Filter bandwidth parameter alpha
The sharpness of a raised cosine filter is described
by alpha ( ). Alpha gives a direct measure of
the occupied bandwidth of the system and is
calculated as
occupied bandwidth = symbol rate X (1 + ).
If the filter had a perfect (brick wall) characteristic
with sharp transitions and an alpha of zero, the
occupied bandwidth would be
for = 0, occupied bandwidth = symbol rate X
(1 + 0) = symbol rate.
Hz
Ch1
Spectrum
LogMag
10
dB/div
GHz
0
0.2
0.4
0.6
0.8
1
0 0.2 0.4 0.6 0.8 1
α = 0.3
α = 0.5
α = 0
α = 1.0
Fs : Symbol Rate
Figure 20. Gaussian Filter Figure 21. Filter Bandwidth Parameters “ ”
25
In a perfect world, the occupied bandwidth would
be the same as the symbol rate, but this is not
practical. An alpha of zero is impossible to implement.
Alpha is sometimes called the “excess bandwidth
factor” as it indicates the amount of occupied
bandwidth that will be required in excess of the
ideal occupied bandwidth (which would be the
same as the symbol rate).
At the other extreme, take a broader filter with an
alpha of one, which is easier to implement. The
occupied bandwidth will be
for = 1, occupied bandwidth = symbol rate X
(1 + 1) = 2 X symbol rate.
An alpha of one uses twice as much bandwidth as
an alpha of zero. In practice, it is possible to implement
an alpha below 0.2 and make good, compact,
practical radios. Typical values range from 0.35 to
0.5, though some video systems use an alpha as
low as 0.11. The corresponding term for a Gaussian
filter is BT (bandwidth time product). Occupied
bandwidth cannot be stated in terms of BT because
a Gaussian filter’s frequency response does not go
identically to zero, as does a raised cosine. Common
values for BT are 0.3 to 0.5.
4.5 Filter bandwidth effects
Different filter bandwidths show different effects.
For example, look at a QPSK signal and examine
how different values of alpha effect the vector
diagram. If the radio has no transmitter filter as
shown on the left of the graph, the transitions
between states are instantaneous. No filtering
means an alpha of infinity.
Transmitting this signal would require infinite
bandwidth. The center figure is an example of a
signal at an alpha of 0.75. The figure on the right
shows the signal at an alpha of 0.375. The filters
with alphas of 0.75 and 0.375 smooth the transitions
and narrow the frequency spectrum required.
Different filter alphas also affect transmitted
power. In the case of the unfiltered signal, with an
alpha of infinity, the maximum or peak power of
the carrier is the same as the nominal power at the
symbol states. No extra power is required due to
the filtering.
QPSK Vector Diagrams
No Filtering α = 0.75 α = 0.375
Figure 22. Effect of Different Filter Bandwidth
26
Take an example of a π/4 DQPSK signal as used in
NADC (IS-54). If an alpha of 1.0 is used, the transitions
between the states are more gradual than for
an alpha of infinity. Less power is needed to handle
those transitions. Using an alpha of 0.5, the transmitted
bandwidth decreases from 2 times the symbol
rate to 1.5 times the symbol rate. This results
in a 25% improvement in occupied bandwidth. The
smaller alpha takes more peak power because of
the overshoot in the filter’s step response. This
produces trajectories which loop beyond the outer
limits of the constellation.
At an alpha of 0.2, about the minimum of most
radios today, there is a need for significant excess
power beyond that needed to transmit the symbol
values themselves. A typical value of excess power
needed at an alpha of 0.2 for QPSK with Nyquist
filtering would be approximately 5 dB. This is more
than three times as much peak power because of
the filter used to limit the occupied bandwidth.
These principles apply to QPSK, offset QPSK,
DQPSK, and the varieties of QAM such as 16QAM,
32QAM, 64QAM, and 256QAM. Not all signals will
behave in exactly the same way, and exceptions
include FSK, MSK, and any others with constantenvelope
modulation. The power of these signals
is not affected by the filter shape.
4.6 Chebyshev equiripple FIR (finite impulse
respone) filter
A Chebyshev equiripple FIR (finite impulse response)
filter is used for baseband filtering in IS-95 CDMA.
With a channel spacing of 1.25 MHz and a symbol
rate of 1.2288 MHz in IS-95 CDMA, it is vital to
reduce leakage to adjacent RF channels. This is
accomplished by using a filter with a very sharp
shape factor using an alpha value of only 0.113. A
FIR filter means that the filter’s impulse response
exists for only a finite number of samples. Equiripple
means that there is a “rippled” magnitude
frequency-respone envelope of equal maxima and
minima in the pass- and stopbands. This FIR filter
uses a much lower order than a Nyquist filter to
implement the required shape factor. The IS-95 FIR
filter does not have zero Inter Symbol Interference
(ISI). However, ISI in CDMA is not as important as
in other formats since the correlation of 64 chips
at a time is used to make a symbol decision. This
“coding gain” tends to average out the ISI and minimize
its effect.
Figure 23. Chebyshev Equiripple FIR Filter
27
4.7 Spectral efficiency versus power
consumption
As with any natural resource, it makes no sense
to waste the RF spectrum by using channel bands
that are too wide. Therefore narrower filters are
used to reduce the occupied bandwidth of the
transmission. Narrower filters with sufficient accuracy
and repeatability are more difficult to build.
Smaller values of alpha increase ISI because more
symbols can contribute. This tightens the requirements
on clock accuracy. These narrower filters
also result in more overshoot and therefore more
peak carrier power. The power amplifier must then
accommodate the higher peak power without distortion.
The bigger amplifier causes more heat and
electrical interference to be produced since the RF
current in the power amplifier will interfere with
other circuits. Larger, heavier batteries will be
required. The alternative is to have shorter talk
time and smaller batteries. Constant envelope
modulation, as used in GMSK, can use class-C
amplifiers which are the most efficient. In summary,
spectral efficiency is highly desirable, but there are
penalties in cost, size, weight, complexity, talk
time, and reliability.
28
There are a number of different ways to view a signal.
This simplified example is an RF pager signal
at a center frequency of 930.004 MHz. This pager
uses two-level FSK and the carrier shifts back and
forth between two frequencies that are 8 kHz apart
(930.000 MHz and 930.008 MHz). This frequency
spacing is small in proportion to the center frequency
of 930.004 MHz. This is shown in Figure
24(a). The difference in period between a signal at
930 MHz and one at 930 MHz plus 8 kHz is very
small. Even with a high performance oscilloscope,
using the latest in high-speed digital techniques,
the change in period cannot be observed or measured.
In a pager receiver the signals are first downconverted
to an IF or baseband frequency. In this
example, the 930.004 MHz FSK-modulated signal is
mixed with another signal at 930.002 MHz. The
FSK modulation causes the transmitted signal to
switch between 930.000 MHz and 930.008 MHz.
The result is a baseband signal that alternates
between two frequencies, –2 kHz and +6 kHz.
The demodulated signal shifts between –2 kHz
and +6 kHz. The difference can be easily detected.
This is sometimes referred to as “zoom” time or IF
time. To be more specific, it is a band-converted
signal at IF or baseband. IF time is important as
it is how the signal looks in the IF portion of a
receiver. This is how the IF of the radio detects
the different bits that are present. The frequency
domain representation is shown in Figure 24(c).
Most pagers use a two-level, Frequency-Shift-Keying
(FSK) scheme. FSK is used in this instance
because it is less affected by multipath propagation,
attenuation and interference, common in
urban environments. It is possible to demodulate it
even deep inside modern steel/concrete buildings,
where attenuation, noise and interference would
otherwise make reliable demodulation difficult.
Time-Domain
Baseband
Time-Domain
"Zoom"
Freq.-Domain
Narrowband
24 (a)
24 (c)
24 (b)
8 kHz
Figure 24. Time and Frequency Domain View
5. Different Ways of Looking at a Digitally Modulated Signal Time
and Frequency Domain View
29
5.1 Power and frequency view
There are many different ways of looking at a digitally
modulated signal. To examine how transmitters
turn on and off, a power-versus-time measurement
is very useful for examining the power level
changes involved in pulsed or bursted carriers. For
example, very fast power changes will result in frequency
spreading or spectral regrowth. This is also
known as frequency “splatter.” Very slow power
changes waste valuable transmit time, as the transmitter
cannot send data when it is not fully on.
Turning on too slowly can also cause high bit error
rates at the beginning of the burst. In addition,
peak and average power levels must be well understood,
since asking for excessive power from an
amplifier can lead to compression or clipping.
These phenomena distort the modulated signal
and usually lead to spectral regrowth as well.
5.2 Constellation diagrams
As discussed, the rectangular I/Q diagram is a
polar diagram of magnitude and phase. A twodimensional
diagram of the carrier magnitude and
phase (a standard polar plot) can be represented
differently by superimposing rectangular axes on
the same data and interpreting the carrier in terms
of in-phase (I) and quadrature-phase (Q) components.
It would be possible to perform AM and PM
on a carrier at the same time and send data this
way; it is easier for circuit design and signal processing
to generate and detect a rectangular, linear
set of values (one set for I and an independent set
for Q).
The example shown is a π/4 Differential Quadrature
Phase Shift Keying (π/4 DQPSK) signal as described
in the North American Digital Cellular (NADC)
TDMA standard. This example is a 157-symbol
DQPSK burst.
Frequency
Time
Amplitude
Time
Power vs.
Time
Freq. vs.
Time
DQPSK, 157 Symbols
and "Trajectory"
Constellation Diagram
DQPSK, 157 Symbol
Constellation with Noise
Polar Diagram
Q
I
Figure 25. Power and Frequency View Figure 26. Constellation Diagram
30
The polar diagram shows several symbols at a
time. That is, it shows the instantaneous value of
the carrier at any point on the continuous line
between and including symbol times, represented
as I/Q or magnitude/phase values.
The constellation diagram shows a repetitive
“snapshot” of that same burst, with values shown
only at the decision points. The constellation diagram
displays phase errors, as well as amplitude
errors, at the decision points. The transitions
between the decision points affects transmitted
bandwidth. This display shows the path the carrier
is taking but does not explicitly show errors at the
decision points. Constellation diagrams provide
insight into varying power levels, the effects of filtering,
and phenomena such as Inter-Symbol
Interference.
The relationship between constellation points and
bits per symbol is
M=2n where M = number of constellation points
n = bits/symbol
or n = log2 (M)
This holds when transitions are allowed from any
constellation point to any other.
5.3 Eye diagrams
Another way to view a digitally modulated signal is
with an eye diagram. Separate eye diagrams can be
generated, one for the I-channel data and another
for the Q-channel data. Eye diagrams display I and
Q magnitude versus time in an infinite persistence
mode, with retraces. The I and Q transitions are
shown separately and an “eye” (or eyes) is formed
at the symbol decision times. QPSK has four distinct
I/Q states, one in each quadrant. There are
only two levels for I and two levels for Q. This
forms a single eye for each I and Q. Other schemes
use more levels and create more nodes in time
through which the traces pass. The lower example
is a 16QAM signal which has four levels forming
three distinct “eyes.” The eye is open at each
symbol. A “good” signal has wide open eyes with
compact crossover points.
I-Mag Q-Mag
Time
QPSK
16QAM
I-Mag
Time
Figure 27. I and Q Eye Diagrams
31
5.4 Trellis diagrams
Figure 28 is called a “trellis” diagram, because
it resembles a garden trellis. The trellis diagram
shows time on the X-axis and phase on the Y-axis.
This allows the examination of the phase transitions
with different symbols. In this case it is for a
GSM system. If a long series of binary ones were
sent, the result would be a series of positive phase
transitions of, in the example of GSM, 90 degrees
per symbol. If a long series of binary zeros were
sent, there would be a constant declining phase of
90 degrees per symbol. Typically there would be
intermediate transmissions with random data. When
troubleshooting, trellis diagrams are useful in isolating
missing transitions, missing codes, or a blind
spot in the I/Q modulator or mapping algorithm.
Phase
Time
GMSK Signal
(GSM) Phase
vs.
Time
Figure 28. Trellis Diagram
32
The RF spectrum is a finite resource and is shared
between users using multiplexing (sometimes
called channelization). Multiplexing is used to
separate different users of the spectrum. This section
covers multiplexing frequency, time, code, and
geography. Most communications systems use a
combination of these multiplexing methods.
6.1 Multiplexing—frequency
Frequency Division Multiple Access (FDMA) splits
the available frequency band into smaller fixed frequency
channels. Each transmitter or receiver uses
a separate frequency. This technique has been used
since around 1900 and is still in use today. Transmitters
are narrowband or frequency-limited. A
narrowband transmitter is used along with a receiver
that has a narrowband filter so that it can demodulate
the desired signal and reject unwanted signals,
such as interfering signals from adjacent radios.
6.2 Multiplexing—time
Time-division multiplexing involves separating the
transmitters in time so that they can share the
same frequency. The simplest type is Time Division
Duplex (TDD). This multiplexes the transmitter
and receiver on the same frequency. TDD is used,
for example, in a simple two-way radio where a
button is pressed to talk and released to listen.
This kind of time division duplex, however, is very
slow. Modern digital radios like CT2 and DECT use
Time Division Duplex but they multiplex hundreds
of times per second. TDMA (Time Division Multiple
Access) multiplexes several transmitters or
receivers on the same frequency. TDMA is used in
the GSM digital cellular system and also in the US
NADC-TDMA system.
Narrowband
Transmitter
Narrowband
Receiver
TDMA Time Division Multiple-Access
1
2
3
TDD Time Division Duplex
Amplitude
Time
T R T R
A A A
B B B
C C C
A B C
Figure 29. Multiplexing—Frequency Figure 30. Multiplexing—Time
6. Sharing the Channel
33
6.3 Multiplexing—code
CDMA is an access method where multiple users
are permitted to transmit simultaneously on the
same frequency. Frequency division multiplexing
is still performed but the channel is 1.23 MHz
wide. In the case of US CDMA telephones, an additional
type of channelization is added, in the form
of coding.
In CDMA systems, users timeshare a higher-rate
digital channel by overlaying a higher-rate digital
sequence on their transmission. A different
sequence is assigned to each terminal so that the
signals can be discerned from one another by
correlating them with the overlaid sequence. This
is based on codes that are shared between the
base and mobile stations. Because of the choice of
coding used, there is a limit of 64 code channels on
the forward link. The reverse link has no practical
limit to the number of codes available.
6.4 Multiplexing—geography
Another kind of multiplexing is geographical or
cellular. If two transmitter/receiver pairs are far
enough apart, they can operate on the same frequency
and not interfere with each other. There
are only a few kinds of systems that do not use
some sort of geographic multiplexing. Clear-channel
international broadcast stations, amateur stations,
and some military low frequency radios are about
the only systems that have no geographic boundaries
and they broadcast around the world.
˜˜ Frequency
Amplitude
Time
F1
1
2
3
4
1
2
3
4
F1'
Figure 31. Multiplexing—Code
Figure 32. Multiplexing—Geography
34
6.5 Combining multiplexing modes
In most of these common communications systems,
different forms of multiplexing are generally
combined. For example, GSM uses FDMA, TDMA,
FDD, and geographic. DECT uses FDMA, TDD, and
geographic multiplexing. For a full listing see the
table in section ten.
6.6 Penetration versus efficiency
Penetration means the ability of a signal to be
used in environments where there is a lot of attenuation,
noise, or interference. One very common
example is the use of pagers versus cellular
phones. In many cases, pagers can receive signals
even if the user is inside a metal building or a
steel-reinforced concrete structure like a modern
skyscraper. Most pagers use a two-level FSK signal
where the frequency deviation is large and the
modulation rate (symbol rate) is quite slow. This
makes it easy for the receiver to detect and demodulate
the signal since the frequency difference is
large (the symbol locations are widely separated)
and these different frequencies persist for a long
time (a slow symbol rate).
However, the factors causing good pager signal
penetration also cause inefficient information
transmission. There are typically only two symbol
locations. They are widely separated (approximately
8 kHz), and a small number of symbols (500 to
1200) are sent each second. Compare this with a
cellular system such as GSM which sends 270,833
symbols each second. This is not a big problem for
the pager since all it needs to receive is its unique
address and perhaps a short ASCII text message.
A cellular phone signal, however, must transmit
live duplex voice. This requires a much higher bit
rate and a much more efficient modulation technique.
Cellular phones use more complex modulation
formats (such as π/4 DQPSK and 0.3 GMSK)
and faster symbol rates. Unfortunately, this greatly
reduces penetration and one way to compensate is
to use more power. More power brings in a host of
other problems, as described previously.
35
7.1 A digital communications transmitter
Figure 33 is a simplified block diagram of a digital
communications transmitter. It begins and ends
with an analog signal. The first step is to convert
a continuous analog signal to a discrete digital bit
stream. This is called digitization.
The next step is to add voice coding for data compression.
Then some channel coding is added.
Channel coding encodes the data in such a way as
to minimize the effects of noise and interference in
the communications channel. Channel coding adds
extra bits to the input data stream and removes
redundant ones. Those extra bits are used for error
correction or sometimes to send training sequences
for identification or equalization. This can make
synchronization (or finding the symbol clock) easier
for the receiver. The symbol clock represents the
frequency and exact timing of the transmission of
the individual symbols. At the symbol clock transitions,
the transmitted carrier is at the correct I/Q
(or magnitude/phase) value to represent a specific
symbol (a specific point in the constellation). Then
the values (I/Q or magnitude/phase) of the transmitted
carrier are changed to represent another
symbol. The interval between these two times is
the symbol clock period. The reciprocal of this is
the symbol clock frequency. The symbol clock
phase is correct when the symbol clock is aligned
with the optimum instant(s) to detect the symbols.
The next step in the transmitter is filtering. Filtering
is essential for good bandwidth efficiency.
Without filtering, signals would have very fast
transitions between states and therefore very wide
frequency spectra—much wider than is needed for
the purpose of sending information. A single filter
is shown for simplicity, but in reality there are
two filters; one each for the I and Q channels. This
creates a compact and spectrally efficient signal
that can be placed on a carrier.
The output from the channel coder is then fed into
the modulator. Since there are independent I and
Q components in the radio, half of the information
can be sent on I and the other half on Q. This is one
reason digital radios work well with this type of
digital signal. The I and Q components are separate.
The rest of the transmitter looks similar to a
typical RF transmitter or microwave transmitter/
receiver pair. The signal is converted up to a higher
intermediate frequency (IF), and then further upconverted
to a higher radio frequency (RF). Any
undesirable signals that were produced by the
upconversion are then filtered out.
A/D
I Mod I
Q Q
IF RF
Processing/
Compression/
Error Corr
Encode
Symbols
Figure 33. A Digital Transmitter
7. How Digital Transmitters and Receivers Work
36
7.2 A digital communications receiver
The receiver is similar to the transmitter but in
reverse. It is more complex to design. The incoming
(RF) signal is first downconverted to (IF) and
demodulated. The ability to demodulate the signal
is hampered by factors including atmospheric
noise, competing signals, and multipath or fading.
Generally, demodulation involves the following
stages:
1. carrier frequency recovery (carrier lock)
2. symbol clock recovery (symbol lock)
3. signal decomposition to I and Q components
4. determining I and Q values for each symbol
(“slicing”)
5. decoding and de-interleaving
6. expansion to original bit stream
7. digital-to-analog conversion, if required
In more and more systems, however, the signal
starts out digital and stays digital. It is never analog
in the sense of a continuous analog signal like
audio. The main difference between the transmitter
and receiver is the issue of carrier and clock
(or symbol) recovery.
Both the symbol-clock frequency and phase (or
timing) must be correct in the receiver in order to
demodulate the bits successfully and recover the
transmitted information. A symbol clock could be
at the right frequency but at the wrong phase. If
the symbol clock was aligned with the transitions
between symbols rather than the symbols themselves,
demodulation would be unsuccessful.
Symbol clocks are usually fixed in frequency and
this frequency is accurately known by both the
transmitter and receiver. The difficulty is to get
them both aligned in phase or timing. There are a
variety of techniques and most systems employ two
or more. If the signal amplitude varies during modulation,
a receiver can measure the variations. The
transmitter can send a specific synchronization
signal or a predetermined bit sequence such as
10101010101010 to “train” the receiver’s clock. In
systems with a pulsed carrier, the symbol clock can
be aligned with the power turn-on of the carrier.
In the transmitter, it is known where the RF carrier
and digital data clock are because they are being
generated inside the transmitter itself. In the
receiver there is not this luxury. The receiver can
approximate where the carrier is but has no phase
or timing symbol clock information. A difficult task
in receiver design is to create carrier and symbolclock
recovery algorithms. That task can be made
easier by the channel coding performed in the
transmitter.
AGC Demod Q
I I
Q
Adaption/
Process/
Decompress
D/A
RF IF
Decode
Bits
Figure 34. A Digital Receiver
37
Complex tradeoffs in frequency, phase, timing,
and modulation are made for interference-free,
multiple-user communications systems. It is necessary
to accurately measure parameters in digital
RF communications systems to make the right
tradeoffs. Measurements include analyzing the modulator
and demodulator, characterizing the transmitted
signal quality, locating causes of high
Bit-Error-Rate, and investigating new modulation
types. Measurements on digital RF communications
systems generally fall into four categories: power,
frequency, timing, and modulation accuracy.
8.1 Power measurements
Power measurements include carrier power and
associated measurements of gain of amplifiers and
insertion loss of filters and attenuators. Signals
used in digital modulation are noise-like. Bandpower
measurements (power integrated over a
certain band of frequencies) or power spectral
density (PSD) measurements are often made. PSD
measurements normalize power to a certain bandwidth,
usually 1 Hz.
8.1.1 Adjacent channel power
Adjacent channel power is a measure of interference
created by one user that effects other users in nearby
channels. This test quantifies the energy of a
digitally modulated RF signal that spills from the
intended communication channel into an adjacent
channel. The measurement result is the ratio (in
dB) of the power measured in the adjacent channel
to the total transmitted power. A similar measurement
is alternate channel power which looks at the
same ratio two channels away from the intended
communication channel.
TRACE A: Ch1 IQ Ref Time
A Ofs 38.500000 sym 3.43 dB 23.465 deg
100 uV
I-Q
20 uV/div
-100 uV
Amplitude
Frequency
GSM-TDMA
Signal
t
Figure 35. Power Measurement
Figure 36. Power and Timing Measurements
8. Measurements on Digital RF Communications Systems
38
For pulsed systems (such as TDMA), power measurements
have a time component and may have a
frequency component, also. Burst power profile
(power versus time) or turn-on and turn-off times
may be measured. Another measurement is average
power when the carrier is on or averaged over
many on/off cycles.
8.2 Frequency measurements
Frequency measurements are often more complex
in digital systems since factors other than pure
tones must be considered. Occupied bandwidth is
an important measurement. It ensures that operators
are staying within the bandwidth that they
have been allocated. Adjacent channel power is
also used to detect the effects one user has on
other users in nearby channels.
8.2.1 Occupied bandwidth
Occupied bandwidth (BW) is a measure of how
much frequency spectrum is covered by the signal
in question. The units are in Hz, and measurement
of occupied BW generally implies a power percent
age or ratio. Typically, a portion of the total power
in a signal to be measured is specified. A common
percentage used is 99%. A measurement of power
versus frequency (such as integrated band power)
is used to add up the power to reach the specified
percentage. For example, one would say “99% of
the power in this signal is contained in a bandwidth
of 30 kHz.” One could also say “The occupied
bandwidth of this signal is 30 kHz” if the desired
power ratio of 99% was known.
Typical occupied bandwidth numbers vary widely,
depending on symbol rate and filtering. The figure
is about 30 kHz for the NADC π/4 DQPSK signal
and about 350 kHz for a GSM 0.3 GMSK signal. For
digital video signals occupied bandwidth is typically
6 to 8 MHz.
Simple frequency-counter-measurement techniques
are often not accurate or sufficient to measure
center frequency. A carrier “centroid” can be calculated
which is the center of the distribution of
frequency versus PSD for a modulated signal.
fo
Figure 37. Frequency Measurements
39
8.3 Timing measurements
Timing measurements are made most often in
pulsed or burst systems. Measurements include
pulse repetition intervals, on-time, off-time, duty
cycle, and time between bit errors. Turn-on and
turn-off times also involve power measurements.
8.4 Modulation accuracy
Modulation accuracy measurements involve measuring
how close either the constellation states or
the signal trajectory is relative to a reference
(ideal) signal trajectory. The received signal is
demodulated and compared with a reference signal.
The main signal is subtracted and what is left
is the difference or residual. Modulation accuracy
is a residual measurement.
Modulation accuracy measurements usually involve
precision demodulation of a signal and comparison
of this demodulated signal with a (mathematically
generated) ideal or “reference” signal. The difference
between the two is the modulation error, and
it can be expressed in a variety of ways including
Error Vector Magnitude (EVM), magnitude error,
phase error, I-error, and Q-error. The reference
signal is subtracted from the demodulated signal,
leaving a residual error signal. Residual measurements
such as this are very powerful for troubleshooting.
Once the reference signal has been
subtracted, it is easier to see small errors that may
have been swamped or obscured by the modulation
itself. The error signal itself can be examined in
many ways; in the time domain or (since it is a
vector quantity) in terms of its I/Q or magnitude/
phase components.
A frequency transformation can also be performed
and the spectral composition of the error signal
alone can be viewed.
8.5 Understanding EVM (error vector magnitude)
Recall first the basics of vector modulation: Digital
bits are transferred onto an RF carrier by varying
the carrier’s magnitude and phase. At each symbolclock
transition, the carrier occupies any one of
several unique locations on the I versus Q plane.
Each location encodes a specific data symbol,
which consists of one or more data bits. A constellation
diagram shows the valid locations (i.e., the
magnitude and phase relative to the carrier) for all
permitted symbols of which there must be 2n, given
n bits transmitted per symbol. To demodulate the
incoming data, the exact magnitude and phase of
the received signal for each clock transition must
be accurately determined.
The layout of the constellation diagram and its
ideal symbol locations is determined generically by
the modulation format chosen (BPSK, 16QAM, π/4
DQPSK, etc.). The trajectory taken by the signal
from one symbol location to another is a function
of the specific system implementation, but is readily
calculated nonetheless.
At any moment, the signal’s magnitude and phase
can be measured. These values define the actual or
“measured” phasor. At the same time, a corresponding
ideal or “reference” phasor can be calculated,
given knowledge of the transmitted data stream,
the symbol-clock timing, baseband filtering parameters,
etc. The differences between these two
phasors form the basis for the EVM measurements.
40
Figure 38 defines EVM and several related terms.
As shown, EVM is the scalar distance between the
two phasor end points, i.e., it is the magnitude of
the difference vector. Expressed another way, it is
the residual noise and distortion remaining after an
ideal version of the signal has been stripped away.
In the NADC-TDMA (IS-54) standard, EVM is
defined as a percentage of the signal voltage at
the symbols. In the π/4 DQPSK modulation format,
these symbols all have the same voltage level,
though this is not true of all formats. IS-54 is
currently the only standard that explicitly defines
EVM, so EVM could be defined differently for
other modulation formats.
In a format such as 64QAM, for example, the symbols
represent a variety of voltage levels. EVM
could be defined by the average voltage level of all
the symbols (a value close to the average signal
level) or by the voltage of the outermost (highest
voltage) four symbols. While the error vector has a
phase value associated with it, this angle generally
turns out to be random because it is a function of
both the error itself (which may or may not be random)
and the position of the data symbol on the
constellation (which, for all practical purposes, is
random). A more useful angle is measured between
the actual and ideal phasors (I/Q phase error),
which contains information useful in troubleshooting
signal problems. Likewise, I-Q magnitude error
shows the magnitude difference between the actual
and ideal signals. EVM, as specified in the standard,
is the root-mean-square (RMS) value of the error
values at the instant of the symbol-clock transition.
Trajectory errors between symbols are ignored.
8.6 Troubleshooting with error vector
measurements
Measurements of error vector magnitude and related
quantities can, when properly applied, provide
much insight into the quality of a digitally modulated
signal. They can also pinpoint the causes for
any problems uncovered by identifying exactly
the type of degradation present in a signal and
even help identify their sources. For more detail
on using error-vector-magnitude measurements to
analyze and troubleshoot vector-modulated signals,
see Agilent Technologies product note 89400-14.
The literature number is 5965-2898E.
{
I
Q
Magnitude Error
(IQ error mag)
Error Vector
Ideal (Reference) Signal
Phase Error (IQ error phase)
Measured
Signal
φ
Figure 38. EVM and Related Qualities
41
EVM measurements are growing rapidly in acceptance,
having already been written into such important
system standards as NADC and PHS, and they
are poised to appear in several upcoming standards
including those for digital video transmission.
8.7 Magnitude versus phase error
Different error mechanisms affect signals in different
ways: in magnitude only, phase only, or both
simultaneously. Knowing the relative amounts of
each type of error can quickly confirm or rule out
certain types of problems. Thus, the first diagnostic
step is to resolve EVM into its magnitude and
phase error components (see Figure 38) and compare
their relative sizes.
When the average phase error (in degrees) is substantially
larger than the average magnitude error
(in percent), some sort of unwanted phase modulation
is the dominant error mode. This could be
caused by noise, spurious or cross-coupling problems
in the frequency reference, phase-locked loops, or
other frequency-generating stages. Residual AM is
evidenced by magnitude errors that are significantly
larger than the phase angle errors.
8.8 I/Q phase error versus time
Phase error is the instantaneous angle difference
between the measured signal and the ideal reference
signal. When viewed as a function of time (or
symbol), it shows the modulating waveform of any
residual or interfering PM signal. Sinewaves or
other regular waveforms indicate an interfering
signal. Uniform noise is a sign of some form of
phase noise (random jitter, residual PM/FM, etc.).
5
deg
Phase
–5
0 Sym 99 Sym
MSK1 Phs Error 1
Figure 39. Incidental (inband) PM sinewave is clearly
visible even at only three degrees peak-to-peak.
42
A perfect signal will have a uniform constellation
that is perfectly symmetric about the origin. I/Q
imbalance is indicated when the constellation is
not “square,” i.e., when the Q-axis height does not
equal the I-axis width. Quadrature error is seen in
any “tilt” to the constellation. Quadrature error is
caused when the phase relationship between the
I and Q vectors is not exactly 90 degrees.
8.9 Error Vector Magnitude versus time
EVM is the difference between the input signal and
the internally generated ideal reference. When
viewed as a function of symbol or time, errors may
be correlated to specific points on the input waveform,
such as peaks or zero crossings. EVM is a
scalar (magnitude-only) value. Error peaks occur
ring with signal peaks indicate compression or
clipping. Error peaks that correlate to signal minima
suggest zero-crossing nonlinearities.
An example of zero-crossing nonlinearities is in a
push-pull amplifier, where the positive and negative
halves of the signal are handled by separate
transistors. It can be quite a challenge (especially
in high-power amplifiers) to precisely bias and
stabilize the amplifiers such that one set is turning
off exactly as the other set is turning on, with no
discontinuities. The critical moment is zero crossing,
a well-known effect in analog design. It is also
known as zero-crossing errors, distortion, or nonlinearities.
5
deg
Real
–5
0 Sym 99 Sym
16QAM Phs Error 1
3
%
Magnitude
0
2
Magnitude
0
32QAM Err V Tim 1
40 Sym 80 Sym
32QAM Meas Time 1
40 Sym 80 Sym
Figure 40. Phase noise appears random in the
time domain.
Figure 41. EVM peaks on this signal (upper trace) occur
every time the signal magnitude (lower trace) approaches
zero. This is probably a zero-crossing error in an amplification
stage.
43
8.10 Error spectrum (EVM versus frequency)
The error spectrum is calculated from the Fast
Fourier Transform (FFT) of the EVM waveform
and results in a frequency-domain display that can
show details not visible in the time domain. In
most digital systems, nonuniform noise distribution
or discrete signal peaks indicate the presence
of externally coupled interference.
For more detail on EVM measurements, see
Agilent product note 89400-14, “Using Error-
Vector-Magnitude Measurements to Analyze and
Troubleshoot Vector-Modulated Signals,” literature
number 5965-2898E.
Communication system design requires the simultaneous
conservation of bandwidth, power, and
cost. In the past, it was possible to make a radio
low cost by sacrificing parameters such as power
and bandwidth efficiency.
This application note has presented the building
blocks of any communications system. With these
concepts, you will be able to understand how
communications systems work, and make more
informed decisions regarding the tradeoffs
required to optimize your system.
30
dB%
rms
Mag (dB)
-120
825.962 MHz 826.038 MHz
PI/4 Err V Spec 1
30
dB%
rms
Mag (dB)
-120
825.962 MHz 826.038 MHz
PI/4 Err V Spec 1
Figure 42. Interference from adjacent (lower) channel
causes uneven EVM spectral distribution.
Figure 43. Switching-power-supply interference appears
as EVM spur, offset from carrier by 10 kHz.
9. Summary
44
GSM900 NADC PDC CDMA
Geography Europe North America Japan North America,
Korea, Japan
Introduction 1992 1992 1993–1994 1995–1997
Frequency Range 935-960 MHz down 869-894 MHz down 810-826 MHz down 824-849 MHz (US)
890-915 MHz up 824-849 MHz up 940-956 MHz up 869-894 MHz (US)
EGSM 925-960 MHz 1777-1801 MHz down 832-834, 843-846,
880-915 MHz 1429-1453 MHz up 860-870 MHz (Japan)
887-889, 898-901,
915-925 MHz (Japan)
Data Structure TDMA TDMA TDMA CDMA
Channel per 8-16 3-6 3-6 32-64 (Dyn. adapt)
Frequency
Modulation 0.3 GMSK π/4 DQPSK π/4 DQPSK Mobile: QPSK
(1 bit/symbol) (2 bits/symbol) (2 bits/symbol) Base: OQPSK
(1 bit/symbol)
Speech CODEC RELP-LTP VSELP 8 Kbits/s VSELP 8 Kbits/s 8 Kbits/s var rate CELP
13 Kbits/s EFR 13 Kbits/s var rate CELP
Mobile Output 3.7mW to 20W 2.2mW to 6W 0.3W to 3W 10nW to 1W
Power
Modulation 270.833 Kbits/s 48.6 Kbits/s 42 Kbits/s 9600/14,400 bps data;
Data Rate (1 bit/symbol) (2 bits/symbol) (2 bits/symbol) 1.2288 Mb/s spreading
Filter 0.3 Gaussian SQRT raised cosine SQRT raised cosine Chebychev low
α = .35 α = .50 pass (FIR)
Channel Spacing 200 kHz 30 kHz 50 kHz 1.23 MHz
25 kHz interleave
Number of 124 frequency ch. 832 frequency ch. 1600 frequency ch. 19–20 frequencies
Channels w/8 timeslots per ch. w/3 users per ch. w/3 users per ch.
(1000) (2496) (4800)
Est # of 15-20 million 35-40 million 5 million
Subscribers by (8.9 million 9/92)
year 2000
Source GSM Standard IS-54 RCR Spec IS-95 spec
Std 27B
Service Public Cellular Public Cellular Public Cellular Public Cellular
10. Overview of Communications Systems
45
DCS1800 PHS DECT TETRA
Trans European
Trunked Radio
Geography Europe Japan/China Europe/China Europe
Introduction 1993 1993 Private office 1993 1995
1995 Public
Frequency Range 1.7 to 1.9 GHz 1895 to 1918 MHz 1.897 to 1.913 GHz 450 MHz
1710-1785 MHz down up/down <1 GHz
1805-1880 MHz up 1.9, 1.93 GHz (China) 1.9, 1.93 GHz (China)
Data Structure TDMA TDMA/TDD TDMA/TDD TDMA
Channel per 8-16 4-8 12 4
Frequency
Modulation 0.3 GMSK π/4DQPSK 0.5 GFSK π/4 DQPSK
(1 bit/symbol) (2 bits/symbol) ±202-403 kHz dev
(1 bit/symbol)
Speech CODEC RELP-LTP ADPCM ADPCM Includes channel
13 Kbits/s 32 Kbits/s 32 Kbits/s & speech coding
7.2 Kbits/s
Mobile Output 250mW to 2W 10mW 250mW
Power
Modulation Data 270.833 Kbits/s 384 Kbits/s 1.152 Mbit/s 19.2 Kb/s
Rate
Filter 0.3 Gaussian SQRT raised cosine 0.5 Gaussian α = 0.4 SQRT
α = .50 raised cosine
Channel Spacing 200 kHz 300 kHz 1.728 MHz 25 kHz
Number of 3000-6000 10 carrier frequencies
Channels w/12 users per
frequency (120)
Est # of Subscribers 4-13 million 6.5-13 million
by year 2000
Source prI-ETS 30 176 RCR spec Std 28 CI Spec., Part 1, Mobile Europe
prETS 300 175-2 China-First News Rev 05.2e Magazine 1/92
Release 8/15/96 China-First News
Release 8/15/96
Service Personal Cordless Telephone Wireless PBX Trunked system
Communications Personal Adj. ch. sel > 60 dB
Communications
46
ACP Adjacent Channel Power
ADPCM Adaptive Digital Pulse Code Modulation
AM Amplitude Modulation
AMPS Advanced Mobile Phone System
B-CDMA Broadband Code Division Multiple
Access
BER Bit Error Rate
BFSK Binary Frequency Shift Keying
BPSK Binary Phase Shift Keying
BW Bandwidth
CDMA Code Division Multiple Access
CDPD Cellular Digital Packet Data
COFDM Coded Orthogonal Frequency Division
Multiplexing
CRC Cyclic Redundancy Check
CT2 Cordless Telephone—2
DAB Digital Audio Broadcast
DCS 1800 Digital Communication System—
1800 MHz
DECT Digital Enhanced Cordless Telephone
DMCA Digital MultiChannel Access, similar to
iDEN
DQPSK Differential Quadrature Phase Shift
Keying
DSP Digital Signal Processing
DVB-C Digital Video Broadcast—Cable
DVB-S Digital Video Broadcast—Satellite
DVB-T Digital Video Broadcast—Terrestrial
EGSM Extended Frequency GSM
ERMES European Radio Message System
ETSI European Telecommunications
Standards Institute
EVM Error Vector Magnitude
FDD Frequency Division Duplex
FDMA Frequency Division Multiple Access
FER Frame Error Rate
FFSK Fast Frequency Shift Keying
FFT Fast Fourier Transform
FLEX 4-level FSK-based paging standard
developed by Motorola
FM Frequency Modulation
FSK Frequency Shift Keying
GFSK Gaussian Frequency Shift Keying
Globalstar Satellite system using 48 low-earth
orbiting satellites
GMSK Gaussian Minimum Shift Keying
GSM Global System for Mobile
Communication
HDTV High Definition Television
iDEN integrated Dispatch Enhanced Network
(Motorola designed system for dispatch,
cellular, and conference calling)
11. Glossary of Terms
47
IF Intermediate Frequency
I/Q In phase/Quadrature
Iridium Motorola voice/data 66-satellite system
worldwide
ISI Intersymbol Interference
IS-54 Interim Standard for US Digital Cellular
(NADC)
IS-95 Interim Standard for US Code Division
Multiple Access
IS-136 Interim Standard for NADC with Digital
Control Channels
LMDS Local Multipoint Distribution System
MFSK Minimum Frequency Shift Keying
MMDS Multichannel Multipoint Distribution
System
MPSK Minimum Phase Shift Keying
MSK Minimum Shift Keying
NADC North American Digital Cellular system
OFDM Orthogonal Frequency Division
Multiplexing
OQPSK Offset Quadrature Phase Shift Keying
PACS Personal Access Communications
Service
PCS Personal Communications System
PCM Pulse Code Modulation
PDC Pacific Digital Cellular System
(formerly JDC)
PHS Personal Handyphone System
(formerly PHP)
PRBS Pseudo-Random Bit Sequence
PSD Power Spectral Density
PSK Phase Shift Keying
QAM Quadrature Amplitude Modulation
QPSK Quadrature Phase Shift Keying
RAM Wireless data network
RF Radio Frequency
RMS Root Mean Square
SQRT Square Root
TDD Time Division Duplex
TDMA Time Division Multiple Access
TETRA Trans European Trunked Radio
TFTS Terrestrial Flight Telephone System
VSB Vestigal Side Band
WLL Wireless Local Loop
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Product specifications and descriptions in this
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Copyright © 2001 Agilent Technologies
Printed in U.S.A. March 14, 2001
5965-7160E