ICTP-ITU/BDT-URSI School on Radio-Based Computer Networking for Research and Training in Developing Countries
The Abdus Salam International Centre for Theoretical Physics ICTP, Trieste (Italy), 7th February - 4th March 2005
Radiocommunication Channel
and Digital Modulation: Basics
Prof. Dr. R. Struzak
r.struzak@ieee.org
Note: These are preliminary notes, intended only for
distribution to participants. Beware of misprints!
Outline
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•
•
•
Radiocommunication channel
Modulation
Modulation spectra & Intermodulation
Summary
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Microwave radio link
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Wireless Local Loop
BSS
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Radio Link model
Original message/ data
Transmitter
Coding/
Processing
Time series
Environment
T-antenna
Noise
Propagation medium
EM waves:
timedistancedirectionpolarization
R-antenna
Receiver
Reconstructed message/ data
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Time series
Processing/
De-coding
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Transmitting station
Electrical current
Original
signal
Transmitter
RF cable
(signal processing)
(signal attenuation)
EM wave
Transmitting
antenna
Radio
wave
Focus of the school
Electrical signal is represented by a function of time.
Radio wave transmitted is represented by a function of
time, distance, direction, and polarization.
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Receiving station
EM wave
Radio
wave
Electrical current
Receiving
antenna
RF cable
Receiver
(signal attenuation)
(signal processing)
Recovered
signal
Focus of the school
Radio wave received is represented by a function of time,
distance, direction, and polarization that depends on signalpath environment
Electrical signal is represented by a function of time.
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Modern radio: details
ANTENNA
RFFILTER
RFFILTER
RF UP/ DOWN
CONVERSION
RFAMPLIFICATION
SWITCH
SYNTHETIZER
MODULATION
&DEMODULATION
IF GAIN & SELECTIVITY
IF
FILTER
SYNTHETIZER
FILTER
DAC
TRANSMITTER
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CRYSTAL
REFERENCE
FILTER
BASEBAND PROCESSING
& PC INTERFACE
COMMON PART
ADC
RECEIVER
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Beamforming
Beamforming
Freq. spread
Freq. despread
Modulation
Demodulation
Multiplex
Demultiplex
Format
Format
Encryption
Decryption
Encoding
Decoding
Analog/Digital
Digital/ Analog
Information source
Information sink
To other destinations
From other sources
RADIO WAVE PROPAGATION PATH
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• Modern radio =
combination of
radio and computer
hardware &
software
– Software-defined
radio
• Systems with
most functions
defined by
software
• Automatically
and/or at distance
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Outline
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•
•
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Radiocommunication channel
Modulation
Modulation spectra & Intermodulation
Summary
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Modulation
RADIO ENVIRONMENT
s'(t)
s(t)
Modulator/
Signal Processing
m(t)
Demodulator/
Signal Processing
f(t)
m'(t)
Carrier Generator
TRANSMITTER
• = process of translation
the message from
baseband signal to
bandpass (modulated
carrier) signal at
frequencies that are very
high compared to the
baseband frequencies.
• Demodulation is the
reverse process
– Note: An information-bearing
signal is non-deterministic, i.e.
it changes in an unpredictable
manner.
RECEIVER
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Modulation Process
f  f  a1 , a2 , a3 ,...an , t  (= carrier)
a1 , a2 , a3 ,...an (= modulation parameters)
t (= time)
• Modulation implies varying one or more
characteristics (modulation parameters a1, a2, … an) of
a carrier f in accordance with the information-bearing
(modulating) baseband signal
• Each of the parameters a, b, c... carrying information
can be modulated independently, increasing
communication capacity at a cost of complexity.
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• The carrier is generated in the transmitter
• It may be a continuous (e.g. sinusoidal) current of
radio frequency, a sequence of short pulses, or
noise
– Systems using pulse sequences are also called
carrierless or impulse systems
• It may also be a number of carriers, such as in
Orthogonal Frequency Division Multiplexing
(OFDM) systems.
• For instance, one of standards Wireless Local Area Networks
(WLANs) foresees 52 carriers spaced 312.5 kHz apart
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Why Carrier?
• To radiate EM waves effectively
– Radiation efficiency requires antenna dimensions to be
comparable with the radiated wavelength
• Antenna for 30 kHz would be 10 km long
• Antenna for 3 GHz carrier is 10 cm long
• To assure signal orthogonality (avoiding mutual
interference by using orthogonal frequencies)
– Note: There are also other methods of avoiding
interference (e.g. time- or code-orthogonality)
• Standards and RR impose limitations on carrier
frequencies (interference, intercommunications)
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Continuous carrier
• In the case of sinusoidal carrier, three modulation
parameters can be varied: the amplitude, the
frequency, and the phase of the sinusoid. This
generates three distinct modulation types: the
amplitude modulation (AM), the frequency
modulation (FM) and the phase modulation (PM)
• Each of these may be continuous, when the
instantaneous amplitude, frequency and phase of
the sinusoid are continuous functions of time, or
may be pulsed, when the variations occur
instantaneously
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Carrier: A sin[t +] + polarztn.; A, , , polarztn. = const
• Amplitude modulation (AM)
– A = A(t)
–  = const
–  = const
• Frequency modulation
(FM)
– A = const
–  = (t)
–  = const
• Phase modulation (PM)
– A = const
–  = const
–  = (t)
• Polarization
modulation
– Used in optical
communications
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Amplitude Shift Keying (ASK)
Baseband
Data
1
0
0
1
0
ASK
modulated
signal
Acos(t)
Acos(t)
• Pulse shaping can be employed to remove spectral spreading
• ASK demonstrates poor performance, as it is heavily affected by noise,
fading, and interference
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Frequency Shift Keying (FSK)
Baseband
Data
1
BFSK
modulated
signal
f1
0
0
1
f0
f0
f1
where f0 =Acos(c-)t and f1 =Acos(c+)t
• Example: The ITU-T V.21 modem standard uses FSK
• FSK can be expanded to a M-ary scheme, employing multiple
frequencies as different states
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Phase Shift Keying (PSK)
Baseband
Data
1
BPSK
modulated
signal
s1
0
s0
0
1
s0
s1
where s0 =Acos(ct) and s1 =Acos(ct + )
•
•
Major drawback – rapid amplitude change between symbols due to phase
discontinuity, which requires infinite bandwidth. Binary Phase Shift Keying
(BPSK) demonstrates better performance than ASK and BFSK
BPSK can be expanded to a M-ary scheme, employing multiple phases and
amplitudes as different states
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PSK graphic representation
Im(s)
-A
A Re(s)
Decision: s = s0
Decision: s = s1
0
1
The two signals
s0 =Acos(t)
s1 =Acos(t + )
can be represented by
two vectors (or points)
in the signal plane
[Re(s), Im(s)]
• Noise & interference
can change positions of
the points and modify
decision: 0 or 1
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Differential Modulation
• In the transmitter, each symbol is modulated
relative to the previous symbol and modulating
signal, for instance in BPSK 0 = no change,
1 = +1800
• In the receiver, the current symbol is demodulated
using the previous symbol as a reference. The
previous symbol serves as an estimate of the
channel. A no-change condition causes the
modulated signal to remain at the same 0 or 1 state
of the previous symbol.
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• DPSK = Differential phase-shift keying: In the
transmitter, each symbol is modulated relative to
the phase of the immediately preceding signal
element tranbsmitted
• Differential modulation is theoretically 3dB poorer
than coherent. This is because the differential
system has 2 sources of error: a corrupted symbol,
and a corrupted reference (the previous symbol)
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Pulse trains as ‘Carrier’
• A ‘carrier” = a train of identical
pulses regularly spaced in time
• Example 2003 : Ultra Wideband
(UWB) systems
– Systems that use time-domain
modulation and signal processing
methods (e.g., pulse-position
modulation)
– Used for sensing, short-range radar,
and telecommunication applications
– Employ short pulses (duration of ~1 to
10 ns), occupying the bandwidth of
more than 1.5 GHz (or more than 25%
of the center frequency)
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• In pulse-frequency modulation (PFM), the pulse
repetition rate is varied in accordance with the
modulating signal; in Pulse-Amplitude Modulation
(PAM), the amplitude of individual pulses in the
pulse train is varied
• In pulse-time modulation (PTM) generic class, the
time of occurrence of some characteristic of the
pulsed carrier is varied, eg. Duration (PDM) or
position (PPM)
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• In pulse-position
modulation (PPM),
the temporal
positions of
individual pulses are
varied in relation to
the reference
positions, in
accordance the
modulating signal
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• Noise (random processes), and pseudo-random
processes can also be used as ‘carriers’
– Example: spread-spectrum systems
– In some systems, the carrier and modulation format
change during the transmission
• Independently of the modulation type, spectra of
signals used in radiocommunications are,
contained between 9 kHz and 275 GHz, as defined
in ITU Radio Regulations
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Demodulation & Detection
• Demodulation
– Is process of removing the carrier signal to
obtain the original signal waveform
• Detection – extracts the symbols from the
waveform
– Coherent detection
– Non-coherent detection
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Coherent (synchronous) Detection
• Signal change introduced by the channel (phase
and attenuation) is estimated. It is then possible to
reproduce the transmitted signal and demodulate.
• Requires a replica carrier wave of the same
frequency and phase to be delivered at the
receiver.
• The received signal and replica carrier are crosscorrelated using information contained in their
amplitudes and phases.
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• Carrier recovery methods include
– Pilot Tone (such as Transparent Tone in Band)
• Less power in the information bearing signal, High peak-tomean power ratio
– Carrier recovery from the information signal
• E.g. Costas loop
• Applicable to
– Phase Shift Keying (PSK)
– Frequency Shift Keying (FSK)
– Amplitude Shift Keying (ASK)
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Non-Coherent Detection
• Requires no reference wave; does not exploit
phase reference information (envelope detection)
• Applicable to
– Differential Phase Shift Keying (DPSK)
– Frequency Shift Keying (FSK)
– Amplitude Shift Keying (ASK)
• Non coherent detection is less complex than
coherent detection (easier to implement), but has
worse performance.
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Geometric Representation
• Digital modulation involves choosing a particular
signal si(t) form a finite set S of possible signals.
• For binary modulation schemes a binary
information bit is mapped directly to a signal and
S contains only 2 signals, representing 0 and 1.
• For M-ary keying S contains more than 2 signals
and each represents more than a single bit of
information. With a signal set of size M, it is
possible to transmit up to log2M bits per signal.
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• Any element of set S can be represented as a point
in a vector space whose coordinates are basis
signals j(t) such that

   t    t  dt  0, i  j; (= are orthogonal)
i
j


E


i  t   dt  1; ( normalization)
2
N
Then
si  t    sij j  t 
j 1
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Example: BPSK Constellation Diagram
 
 
2 Eb
2 Eb
S BPSK    s1  t  
cos  2 f ct   ,  s2  t   
cos  2 f ct  ;
Tb
Tb
 
 
Eb  energy per bit; Tb  bit period
For this signal set, there is a single basic signal
1  t  

 
  ; 0  t  Tb
 
Q
2
cos  2 f c t  ; 0  t  Tb
Tb

S BPSK   Eb 1  t   ,   Eb 1  t  
-Eb
Eb
I
Constellation diagram
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Constellation diagram
= graphical representation of the complex
envelope of each possible symbol state
– The x-axis represents the in-phase component
and the y-axis the quadrature component of the
complex envelope
– The distance between signals on a constellation
diagram relates to how different the modulation
waveforms are and how easily a receiver can
differentiate between them.
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QPSK
• Quadrature Phase Shift Keying (QPSK) can
be interpreted as two independent BPSK
systems (one on the I-channel and one on
Q), and thus the same performance but
twice the bandwidth efficiency
• Large envelope variations occur due to
abrupt phase transitions, thus requiring
linear amplification
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QPSK Constellation Diagram
Q
Q
I
I
Carrier phases
{0, /2, , 3/2}
Carrier phases
{/4, 3/4, 5/4, 7/4}
• Quadrature Phase Shift Keying has twice the bandwidth efficiency of
BPSK since 2 bits are transmitted in a single modulation symbol
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Types of QPSK
Q
Q
I
I
Conventional QPSK
•
•
•
•
Q
Offset QPSK
I
/4 QPSK
Conventional QPSK has transitions through zero (i.e. 1800 phase transition). Highly
linear amplifiers required.
In Offset QPSK, the phase transitions are limited to 900, the transitions on the I and Q
channels are staggered.
In /4 QPSK the set of constellation points are toggled each symbol, so transitions
through zero cannot occur. This scheme produces the lowest envelope variations.
All QPSK schemes require linear power amplifiers
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Multi-level (M-ary) Phase and
Amplitude Modulation
16 QAM
•
•
•
16 PSK
16 APSK
Amplitude and phase shift keying can be combined to transmit several bits per symbol.
(Often referred to as linear as they require linear amplification. More bandwidthefficient, but more susceptible to noise.)
For M=4, 16QAM has the largest distance between points, but requires very linear
amplification. 16PSK has less stringent linearity requirements, but has less spacing
between constellation points, and is therefore more affected by noise.
Java simulation: http://www.educatorscorner.com/index.cgi?CONTENT_ID=2478
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Decision region
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Distortions
Decision region
Perfect channel
White noise
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Phase jitter
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Eye Diagram
Magnitude
• Eye pattern is an oscilloscope
display in which digital data
signal from a receiver is
repetitively superimposed on
itself many times
Time (symbols)
•If the “eye” is not
open at the sample
point, errors will occur
due to signal
corruption
– (sampled and applied to the
vertical input, while the data
rate is used to trigger the
horizontal sweep).
• It is so called because the
pattern looks like a series of
eyes between a pair of rails.
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Outline
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•
•
•
Radiocommunication channel
Modulation
Modulation spectra & Intermodulation
Summary
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Relative Magnitude (dB)
Modulation Spectra
Nyquist Minimum
Bandwidth
Adjacent
Channel
• The Nyquist bandwidth is the
minimum bandwidth that can
carry a given volume of
information
• The spectrum occupied by a
signal is usually larger and
spill over adjacent channels
causing interference
• The spectrum occupied by a
signal can be reduced by
application of filters
• Technical standards and RR
impose limits on spectral
masks
Frequency
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Intermodulation
• Intermodulation signals can be generated when
two or more RF signals are applied to a non-linear
device
• They could produce interference
• The magnitude of the spurious signals depends on
the power of the original signals and on the degree
of device nonlinearity
• Technical standards and RR impose limits on outof-band (spurious) radiations
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• Three potential origins of intermodulation interference:
– Receiver RF input/ mixing stage
– Transmitter output stages
– Vicinity of the equipment (usually of the transmitter)
• Five types of interference:
–
–
–
–
–
adjacent signal interference,
transmitter spurious radiations,
receiver spurious responses,
transmitter intermodulation
receiver intermodulation.
• Note: Several interactions are likely to occur simultaneously
» (Source: ITU/ CCIR Rep. 524-1, Vol. 1, p. 30, 1986)
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• Non-ideal wideband
memory-less linear devices
are often treated by
expressing the output (Y) of
the system as a power series
Y  a0  a1 X  a2 X 2  a3 X 3 ...an X n ...
of the total input signal X:
X(t)
Y(t)
X(t) = A1sin(w1t) + A2sin(w2t) + …
The coefficients a are presumed to be real and
independent on X.
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Intermodulation products
• The frequency (Fi) of an intermodulation
product Fi = C1*F1+C2*F2+ .. +Cn*Fn
• {C1, C2, ...,Cn} are positive or negative integers or
zero, and
• {F1, F2, ..., Fn} are the frequencies of the signals
applied to the device
• The order of the intermodulation product is
the sum: {|C1| + |C2| + ... + |Cn|}
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• Usually, the most important intermodulation
product are those of the 3rd order, but also of
the 5th, and 7th order
– Reason: they are close to the frequency
spectrum of the original ‘real’ signals and often
cannot be rejected by tuned filters.
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Example: 3rd order intermodulation
• Two ‘real’ signals of frequencies F1 and F2 when applied
to a nonlinearity, produce six ‘false signals’ (3-rd order
intermodulation products) at the following frequencies:
–
–
–
–
–
–
Fia = 2*F1 - F2
Fib = 2*F2 - F1
Fic = 2*F1 + F2
Fid = 2*F2 + F1
Fie = 3F1
Fif = 3F2
Even if F1 and F2 do not interfere
one with another, intermodulation
products can interfere with one or
another.
Frequencies Fia, Fib, can be close
to F1 or F2.
More ‘real’ signals, more the
number of ‘false signals’
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2F1-F2
2F2-F1
2F1+F2
2F1
2F2+F1
2F2
3F1
3F2
F1 F2
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Summary
•
•
•
•
Introduction
Radiocommunication channel
Modulation
Modulation spectra & Intermodulation
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Any question?
Thank you for your attention
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