FEE Conference – Brussels – October 2013
An Introduction to Digital Modulation
Presenter: Barry Hack, Aeroflex UK
www.aeroflex.com
What do you know about Digital Comms ?
Is it really different from analog modulation?
V= A(t) sin[2 p f(t) + f (t)]
AM, Pulse
FM
V= A(t) sin[ q(t)]
PM
(Sine wave modulation signal)
Name some early systems using digital modulation?
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What do you know about Digital Comms ?
Early systems employed:
Sonar
Morse code
Semaphore
Smoke signals
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What do you know about Digital Comms ?
▼
Acronyms

Other terms
– GSM
– Interleaving
– TETRA
– Chipping Rate
– UMTS
– Rake Receiver
– BPSK
– Multi-path
/4
– Spreading
Factor
–
DQPSK
– CODEC
– LTE
– RBER
– BCCH
– MNC
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Why do People want Digital Modulation ?
▼
Security
– Princess Diana and the Sun newspaper
– Prevent eaves dropping and ‘spoof’ or ‘rogue’
users….
▼
Capacity
– More users per piece of spectrum than analog
– Less congestion
– More revenue for operators
▼
Cost -> Seller makes more money
– Digital radios have less analog bits
▼
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Cheaper to produce, more reliable, easier to align
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Why do People want Digital Modulation ?
▼
Voice Quality
– Works better where signal is weak
▼
Roaming
– Can speak over larger geographies
– Emergency services can all communicate
directly
– GSM roaming across most countries (130+)
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▼
Immunity to interference
▼
Capability to send voice and/or data
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One ‘technical’ reason to move to ‘Digital’
Comparison with analog FM
Low background noise
Digital
Quality
High background noise
FM
Range
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Real radio systems
▼
They do not use one access method
– They combine techniques and attempt to get the
best of each (divide the users by space/location,
time, frequency and/or code)
▼
There is no “best” solution
– It depends on what you are trying to achieve
▼
voice, data
– Geography
– Regulatory constraints
– Spectrum availability
– Cost objectives
– Services needed
– User density
– Politics
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The analog implementation
▼ Information
source modulates carrier directly
Analog
Baseband Modulator
filter
Power
Amplifier
Tx Filter
Duplexer
V.C.O.
Baseband Demodfilter
ulator
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Low Noise Rx Filter
Amp
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Simplified digital transceiver
Digital
A to D
Speech
Coder
Analog
Channel
Coder
Baseband
filters
Modulator
Tx Filter
Duplexer
V.C.O.
D to A
Speech
Decoder
Channel
Decoder
Filters &
Equalizer
Demodulator
Rx Filter
Note: TX output PA and RX LNA removed for clarity
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Modulation: where is the information
V= A(t) sin[2 p f(t) + f (t)]
AM, Pulse
FM
V= A(t) sin[ q(t)]
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PM
(Sine wave modulation signal)
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Basic Digital Modulation
Amplitude
Frequency
(FSK)
Phase
Both Amplitude
and Phase
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IQ Diagram: Phase and Amplitude
Q+
90°
Reference Phase
Phase
0°
I+
Origin
– Magnitude is an absolute value from the origin
– Phase is relative to a reference signal (from I + axis)
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Digital Modulation: Signal vector
Phase
0 deg
Amplitude Modulation
0 deg
Amplitude and Phase
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Phase
0 deg
Phase Modulation
0 deg
Frequency Modulation
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Modulation Measurements
▼
Analog Systems
– Power
– Bandwidth
– Frequency error
– Modulation Accuracy (FM deviation / AM depth)
▼
Digital Systems
– Power
– Bandwidth
– Frequency error
– Modulation accuracy (Error Vector Magnitude)
– Burst Timing (Power)
– Symbol Timing (Data)
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Modulation Accuracy
▼
EVM is a good measure
– Some systems (i.e. FSK) would use phase error only
▼
Definition
– EVM is the difference between the actual signal vector and
an ideal signal vector.
Q
▼
Some causes of EVM
Magnitude
error (IQ
error
magnitude)
Measured signal
Error vector
– Component variations
– PCB track layout
– Phase Noise
Ideal (reference signal)
q
Phase Error (IQ Phase error)
I
– Spurious signals
– Modulator errors
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Causes of EVM - Example 1
▼
Carrier Leakage
– Some of the un-modulated local oscillator bleeds
across to the output
▼
Poor screening
▼
Poor PCB layout
Q
I:
p/2
Carrier
I
Carrier
Leakage
Q:
IQ Modulator
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Causes of EVM – Example 2
▼
IQ Skew
– The I and Q modulation paths are not exactly 90
degrees
▼
Component tolerances
▼
Different lengths for I and Q signal paths
▼
Poor PCB layout
Q
I:
p/2
Q:
Carrier
I
Cos (q + 90 + skew)
IQ Modulator
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Causes of EVM – Example 3
▼
IQ Gain Imbalance
– The I and Q modulation paths do not have the same
gain
▼
Component tolerances
– Note: Gain and skew can be seen as AM modulation in
the analog domain !
Q
I:
p/2
Carrier
I
Q:
IQ Modulator
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EVM
▼
Example 1
Noise can be seen
because of the spread
of constellation points
This angle is not 90
degrees showing skew
Gain imbalance
present because I and
Q values are not
symmetrical
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EVM
▼
Example 2
– Minor issues with carrier leak and phase noise
– BUT the quality is well inside the measurement limits
Vector Diagram
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Constellation Diagram
Rotated Vector
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Receiver Tests
▼
Analog Systems
– RSSI (Received Signal Strength Indicator)
– Rx sensitivity (using SINAD measurement)
▼
Digital Systems
– RSSI (Received Signal Strength Indicator)
– Rx sensitivity (using BER measurement)
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Receiver Tests
▼
Digital Systems
– Bit Error Rate is a measure of the received bits in
error as a ratio to the total received bits
▼
Other measurements include…
– RBER – Residual Bit Error Rate
– FER – Frame Error Rate
– MER – Message Error Rate
BER/MER v. Power Level
BER/MER
Rx Sens = -119dBm
-122.0
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-120.0
-118.0
-116.0 -114.0 -112.0
Power Level
45.00
40.00
35.00
30.00
25.00
20.00
15.00
10.00
5.00
0.00
-110.0
-108.0
-5.00
BER Class 0
BER Class 1
BER Class 2
RBER Class 0
RBER Class 1
MER
If you use a 1kHz test tone,
SINAD and BER can give very
similar answers for receiver
sensitivity …..
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Vector representation of AM and FM
▼ Remove
carrier phase changes
▼ Indicate
relative phase changes only
Q
Q
Q
I
I
unmodulated carrier,
fc, arbitrary phase
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I
carrier, fc, with
AM
carrier, fc, with FM
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Vector representation of AM - Slow
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Vector representation of AM – Faster !
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IQ Modulation Explained
A
B
C
serial data stream
D
10
t
Bit period
serial/parallel
conversion
I(t)
A
B
C
Q(t)
D
Q
00
I
11
01
vector phase states
t
Dibits Symbol period
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IQ Modulation Explained
I.sin(fc)
‘I’ signal
A
1 0
‘Q’ signal
t
Bit period
Q.cos(fc)
L.O.
fc
10
I(t)
Q(t)
1
A
0
Dibits Symbol period
Q
00
-sin(I), +cos(Q)
I
t
11
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90 º
01
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IQ Modulation Explained
I.sin(fc)
‘I’ signal
B
0 1
Q.cos(fc)
Bit period
t
‘Q’ signal
L.O.
fc
90 º
sin(I), -cos(Q)
10
I(t)
Q(t)
0
B
1
Q
00
I
t
Dibits Symbol period
11
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01
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IQ Modulation Explained
I.sin(fc)
‘I’ signal
C
1 1
Bit period
Q.cos(fc)
‘Q’ signal
t
L.O.
fc
90 º
-sin(I), -cos(Q)
10
1
C
1
I(t)
Q(t)
Q
00
I
t
Dibits Symbol period
11
30
01
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IQ Modulation Explained
I.sin(fc)
‘I’ signal
D
0 0
Bit period
t
Q.cos(fc)
‘Q’ signal
L.O.
fc
90 º
sin(I), cos(Q)
10
0
D
0
I(t)
Q(t)
Q
00
I
t
Dibits Symbol period
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11
01
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IQ Modulation Explained
10
Q
00
I
11
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Vector Timing and Synchronisation
▼
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To decide when the vector is at a symbol point
you need to apply timing and synchronisation
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QPSK Modulation – Noisy but perfect !
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Alternative Approaches to QPSK
Time offset QPSK
Q
Phase offset QPSK
Q
I
Real signals are filtered
I
Avoid zero crossings and so minimise AM
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Sampling and Speech Coding
▼ Standard
telecom data rates
– 300 to 3.4kHz audio band
– 8k samples / sec, 13 bits / sample (103kbit / sec)
– Compressed to 8 bits / sample (64kbit / sec) with A-law
or u-law compander
▼Even
64kbit / sec data is too high for wireless systems
▼ CODEC
– (COder-DECoder) reduces data rate by up to 80%
– Several approaches: model vocal tract; code book &
lookup table
A to D
Audio 300-3.4kHz
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CODEC
8,000 13-bit samples /
sec (103kbit / sec)
13kbit / sec
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The Channel Coder
A to D
D to A
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Speech
Coder
Channel
Coder
Speech
Decoder
Channel Filters &
Decoder Equalizer
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Interleaving
▼ Loss
of a single data frame makes the
entire message meaningless. How do we
fix this?
COLD
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FEET
NEED
HEAT
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Interleaving – Demonstration
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Equalisation
A to D
D to A
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Speech
Coder
Channel
Coder
Speech
Decoder
Channel Filters &
Decoder Equalizer
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Equalisation
▼ Mobile
communications often rely on multipath signals
– How often do you actually have sight of the basestation when using a mobile phone?
▼ The
EQUALISER in the receiver
– Overcomes the effects of delay spreading
– Using real-time adaptive filtering implemented in DSP
– Requires some prior knowledge of the received signal
▼i.e......
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a training sequence
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The End
Any
questions
?
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