www.aeroflex.com FEE Conference Brussels October 2013 An
Introduction to Digital Modulation Presenter: Barry Hack, Aeroflex
UK Slide 2 www.aeroflex.com What do you know about Digital Comms ?
2 AM, Pulse FM PM V= A(t) sin[ (t)] (Sine wave modulation signal)
V= A(t) sin[2 f(t) + (t)] Is it really different from analog
modulation? Name some early systems using digital modulation? Slide
3 www.aeroflex.com What do you know about Digital Comms ? Early
systems employed: Sonar Morse code Semaphore Smoke signals 3 Slide
4 www.aeroflex.com What do you know about Digital Comms ? Acronyms
GSM TETRA UMTS BPSK /4 DQPSK CODEC LTE RBER BCCH MNC 4 Other terms
Interleaving Chipping Rate Rake Receiver Multi-path Spreading
Factor Slide 5 www.aeroflex.com 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 Cheaper to produce, more reliable, easier to align
5 Slide 6 www.aeroflex.com 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+) Immunity to
interference Capability to send voice and/or data 6 Slide 7
www.aeroflex.com One technical reason to move to Digital 7
Comparison with analog FM FM Low background noise High background
noise Quality Range Digital Slide 8 www.aeroflex.com 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 8 Slide 9
www.aeroflex.com The analog implementation Information source
modulates carrier directly 9 ModulatorTx Filter Demod- ulator Rx
Filter Baseband filter Duplexer V.C.O. Baseband filter Low Noise
Amp Power Amplifier Analog Slide 10 www.aeroflex.com Simplified
digital transceiver 10 A to DSpeech Coder Channel Coder ModulatorTx
Filter D to ASpeech Decoder Channel Decoder Demod- ulator Rx
FilterFilters & Equalizer Baseband filters Duplexer V.C.O.
Note: TX output PA and RX LNA removed for clarity AnalogDigital
Slide 11 www.aeroflex.com Modulation: where is the information 11
AM, Pulse FM PM V= A(t) sin[ (t)] (Sine wave modulation signal) V=
A(t) sin[2 f(t) + (t)] Slide 12 www.aeroflex.com Basic Digital
Modulation 12 Amplitude Frequency (FSK) Phase Both Amplitude and
Phase Slide 13 www.aeroflex.com IQ Diagram: Phase and Amplitude 13
Phase Mag 0 I+ Q+ 90 Reference Phase Origin Magnitude is an
absolute value from the origin Phase is relative to a reference
signal (from I + axis) Slide 14 www.aeroflex.com Digital
Modulation: Signal vector 14 Phase Mag 0 deg Amplitude Modulation
Phase 0 deg Phase Modulation Frequency Modulation Amplitude and
Phase 0 deg Slide 15 www.aeroflex.com 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) 15 Slide 16 www.aeroflex.com
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. Some causes of
EVM Component variations PCB track layout Phase Noise Spurious
signals Modulator errors 16 Q I Magnitude error (IQ error
magnitude) Measured signal Ideal (reference signal) Phase Error (IQ
Phase error) Error vector Slide 17 www.aeroflex.com Causes of EVM -
Example 1 Carrier Leakage Some of the un-modulated local oscillator
bleeds across to the output Poor screening Poor PCB layout 17 I Q
IQ Modulator /2 Carrier I: Q: Carrier Leakage Slide 18
www.aeroflex.com 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 18 IQ
Modulator /2 Carrier I: Q: Cos ( + 90 + skew ) I Q skew Slide 19
www.aeroflex.com 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 ! 19 IQ Modulator /2 Carrier I: Q: I Q Slide 20
www.aeroflex.com EVM Example 1 20 Noise can be seen because of the
spread of constellation points Gain imbalance present because I and
Q values are not symmetrical This angle is not 90 degrees showing
skew Slide 21 www.aeroflex.com EVM Example 2 Minor issues with
carrier leak and phase noise BUT the quality is well inside the
measurement limits 21 Vector DiagramConstellation Diagram Rotated
Vector Slide 22 www.aeroflex.com 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) 22 Slide 23
www.aeroflex.com 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 23 If you use a
1kHz test tone, SINAD and BER can give very similar answers for
receiver sensitivity .. Rx Sens = -119dBm Slide 24 www.aeroflex.com
Vector representation of AM and FM Remove carrier phase changes
Indicate relative phase changes only 24 I QQQ I I unmodulated
carrier, f c, arbitrary phase carrier, f c, with FMcarrier, f c,
with AM Slide 25 www.aeroflex.com Vector representation of AM -
Slow 25 Slide 26 www.aeroflex.com Vector representation of AM
Faster ! 26 Slide 27 www.aeroflex.com IQ Modulation Explained 27
serial/parallel conversion 1000 0111 Q I A B C D I(t) Q(t) t Dibits
vector phase states serial data stream t Bit period A B C D Symbol
period Slide 28 www.aeroflex.com IQ Modulation Explained 28 A I(t)
Q(t) t Dibits 1000 0111 Q I -sin(I), +cos(Q) 0 1 t A 1 0 Bit period
Symbol period L.O. 90 I signal Q signal fcfc I.sin(f c ) Q.cos(f c
) Slide 29 www.aeroflex.com IQ Modulation Explained 29 1000 0111 Q
I B I(t) Q(t) t Dibits 0 1 sin(I), -cos(Q) t B 0 1 Bit period
Symbol period Q signal L.O. 90 I signal fcfc I.sin(f c ) Q.cos(f c
) Slide 30 www.aeroflex.com IQ Modulation Explained 30 10 0111 Q I
00 t C 1 Bit period C I(t) Q(t) t Dibits Symbol period 1 1 -sin(I),
-cos(Q) L.O. 90 I signal Q signal fcfc I.sin(f c ) Q.cos(f c )
Slide 31 www.aeroflex.com IQ Modulation Explained 31 1000 0111 Q I
sin(I), cos(Q) t D 0 Bit period D I(t) Q(t) t Dibits 0 0 Symbol
period L.O. 90 I signal Q signal fcfc I.sin(f c ) Q.cos(f c ) Slide
32 www.aeroflex.com IQ Modulation Explained 32 1000 0111 Q I Slide
33 www.aeroflex.com Vector Timing and Synchronisation 33 To decide
when the vector is at a symbol point you need to apply timing and
synchronisation Slide 34 www.aeroflex.com QPSK Modulation Noisy but
perfect ! 34 Slide 35 www.aeroflex.com Alternative Approaches to
QPSK 35 Q I Q I Time offset QPSK Phase offset QPSK Avoid zero
crossings and so minimise AM Real signals are filtered Slide 36
www.aeroflex.com 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 36 Audio 300-3.4kHz8,000 13-bit samples / sec (103kbit / sec)
13kbit / sec A to DCODEC Slide 37 www.aeroflex.com The Channel
Coder 37 Speech Coder Channel Coder D to ASpeech Decoder Channel
Decoder Filters & Equalizer A to D Slide 38 www.aeroflex.com
Interleaving Loss of a single data frame makes the entire message
meaningless. How do we fix this? 38 COLD FEET NEED HEAT Slide 39
www.aeroflex.com Interleaving Demonstration 39 Slide 40
www.aeroflex.com Equalisation 40 Speech Coder Channel Coder D to
ASpeech Decoder Channel Decoder Filters & Equalizer A to D
Slide 41 www.aeroflex.com Equalisation Mobile communications often
rely on multi- path signals How often do you actually have sight of
the base- station 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...... a training sequence 41 Slide 42
www.aeroflex.com The End 42 Any questions ?
