S. Brand, Philips Semiconductors, PCALE
QAM Demodulation
QAM Demodulation
o
o
o
o
o
Application area
What is QAM?
What are QAM Demodulation Functions?
General block diagram of QAM demodulator
Explanation of the main function
(Nyquist shaping, Clock & Carrier Recovery, AGC, Adaptive Equaliser)
o
o
Performance
Conclusion
Wireless Communications
0
1
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Example Application Area
“Wireless Cable” Digital TV using Microwave Transmission
QAM
Modulation
Multiplexing
Set-top Box
Radio
Channel
Compression
• Compression = bit rate reduction
• Multiplexing = assembly of multiple programs
• Modulation = conversion to transmission format
• Set-top Box = Integrated Receiver Decoder (IRD),
provides a subscriber access to a wide range of programs
Wireless Communications
2
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
What is QAM?
o
Amplitude Modulation of
o
Two Orthogonal Carriers
+7
+5
+3
+1
-1
-3
-5
-7
+7
+5
+3
+1
-1
-3
-5
-7
ai-1=+3
ai=+1
xi ( t ) =
2E o
2Eo
-------a
cos
(
ω
t
)
+
---------b
c
Ts i
Ts i sin ( ω c t )
Q
ai+1=-7
7
I
bi-1=-5
5
Ts
3
bi=+5
1
bi+1=-1
Tc
√Εο
I
√Εο
−1
Q
−3
−5
−7
time
64QAM in time domain
−7
−5
−3
−1
1
3
5
7
64QAM Constellation diagram
Wireless Communications
M-ary QAM
I
Q
{
{
Satellite
b1b0
3
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Cable
b5b4b3b2b1b0
Noise power
signal power
S/N > 21 dB for M=64
S/N > 27 dB for M=256
S/N > 3 dB for M=4
Wireless Communications
S. Brand, Philips Semiconductors, PCALE
QAM Demodulation
What to do to recover the information?
Functions
Result
Automatic Gain Control
Optimal position of constellation diagram in reception window
Quadrature
down conversion
I & Q base band signals
(Half) Nyquist Filtering
Pulse shaping
Clock Recovery
Sampling reference for A/D Converter
Carrier Recovery
Carrier frequency reference
Adaptive Equaliser
Compensate for channel distortion
Demapping
Representation of received data in bits
Wireless Communications
4
5
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
System Block Diagram
f
f
BPF
Tuner
f
LPF
QAM DEMODULATOR
I2C
fs
fine
AGC
I
√Ν
Complex
Equaliser
1,0,-1,0
ADC
0,-1,0,1
Carrier Recovery
AGC
VCXO
Clock Recovery
Cable Connection
VCO
Q
√Ν
4fs
demapping
IF
AGC
detect
clock
detect
DAC
DAC
DTO
Digital
loop
filter
carrier
detect
Analogue
Wireless Communications
DAC
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Automatic Gain Control
IF down
conversion
ADC
* 2 loops AGC
n Filtering &
Equalisation
I
Q
coarse
AGC
fine
AGC
* Coarse AGC to prevent ADC
from overloading
* After Nyquist filtering and
Equalisation ‘small’ QAM
remains.
* Fine AGC to position contellation diagram to decision window
I
I
Tuner output
Q
Q
Q
Equaliser output
I
Fine AGC output
Wireless Communications
6
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
7
(Half) Nyquist Filtering
√Ν
* Pulse Shaping required to realise
ISI=0 in limited BW
I
1,0,-1,0
ADC
* ISI=0 when zero crossings occur at
multiples of Ts=1/fs
0,-1,0,1
√Ν
4fs
Q
* Achieved with Nyquist Criterion
Ts=1/fs
Sn
(DVB: α = 15%)
Sn+3
Sn+6
S
Ts n+1
* Cascade of Transmitter & Receiver
fulfil Nyquist Criterion
BW=∞
time
Sn+2 S Sn+5
n+4
0
freq
* Digital implementation
(1+α)fs
fs
(Tdelay = 9 Tsymbol)
BW=8MHz
time
0
(Half Nyquist each )
freq
* This delay is in the loops and thus
influences the demodulator architecture
Wireless Communications
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
8
Clock Recovery
* Recovery with 2nd order PLL
1,0,-1,0
√Ν
ADC
* Clock Detector
- Energy Maximization algorithm
- After Half Nyquist Filter to achieve
ISI=0 at detector input
0,-1,0,1
DAC
vcxo
clock
det.
* Half Nyquist Filter in loop is allowed
- Received clock has crystal accuracy
(100 ppm at 7 Msym/s))
- Loop BW may be small
- Delay in loop is allowed (no instability)
I, Q signal
time
Ts
Recovered
clock
* Quadarture Demodulation
- fclock = 4 fsymbol
- Simple with j-n (n=0,1,2,3,...)
Wireless Communications
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
9
Carrier Recovery
* Recovery with 2nd order PLL
delay
* Carrier Detector
- Decision directed
- After equaliser
- PD (lock) and PFD (unlock)
* PFD for large acquisition range (100 kHz)
* PD for stable behaviour once in lock
IF
LPF
√Ν
ADC
equaliser
4fs
vco
vcxo
DAC
I or Q
carrier
det.
* Half Nyquist & Equaliser in loop
- Large delay causes problems for disturbances like:
* phase noise
* microphonics (mechanical vibrations)
time
Tcarrier
* Alternative solution required
Recovered
carrier
Wireless Communications
QAM Demodulation
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10
Carrier Phase Disturbances (1)
Additive White
Gaussian Noise
* AWGN Disturbance
- Random distribution
- Mainly inserted in the cable
channel
* Result
- Enlarged constellation points
Implementation Loss
Cable
s(t)
Tuner
+
n(t)
QAM
demod
r(t)
BW
Wireless Communications
* PLL Properties
- Average the noise
- Loop BW small
- Low IL
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QAM Demodulation
11
Carrier Phase Disturbances (2)
* Phase Noise & Microphonics
Phase Noise/
-No random distribution
Microphonics
Q
-Mainly inserted in the tuner by
LC oscillators which are sensative for mechanical vibrations (Microphonics)
I
Implementation Loss
Cable
s(t)
Tuner
X
QAM
demod
r(t)
* Result
-Rotation of constellation diagram.
* PLL Properties
-Follow the phase disturbance
-Loop BW large
-Low IL
* PLL properties for AWGN and
BW phase noise are in contradiction
Wireless Communications
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
12
Implementation Loss [dB]
Phase noise versus AWGN
* Loop BW trade of between:
a. Ability to follow phase noise
b. Ability to average AWGN
AWGN
AWGN+Phase noise
* Rule of thumb:
1
BW = ----------1000 fsymbol
* Simulations show this is
approximately correct
* Optimum depends on S/N and
amount of phase noise
OPTIMUM
* Problem: Optimum loop BW
instable due to large delay in
the loop (Half Nyquist + Equaliser).
Loop BW [kHz]
Wireless Communications
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
13
Double Loop Carrier Recovery
* Introduction of second loop with
(relatively) small delay
large delay
IF
LPF
√Ν
ADC
equaliser
carrier
det.
4fs
vco
dto
vcxo
DAC
I or Q
Loop
filter
inner loop
outer loop
* Inner Loop
- Optimum loop BW as trade off
between phase noise & AWGN
- Large Loop BW due to small delay
- PD only
time
Tcarrier
* Outer loop
-Adjust (static) frequency offset
-Small loop BW due to large delay
-PD/PFD
* Conclusion: optimum loop BW can
be selected and causes no instability
Recovered
carrier
Wireless Communications
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Equalisation
* Nyquist Criterion specifies a frequency domain condition
on the received pulses to achieve ISI=0
* Generally this is NOT satisfied unless the channel is equalised
* Equalise the channel = compensate for channel distortion
* Unfortunately, any equalisation enhances noise from the channel
* Tradeoff between:
Accurately minimising ISI
Minimising the noise
* Different types of Equaliser
Wireless Communications
14
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
15
Multipath Distortion
* Multipath distortion causes ISI
Multipath
Reflection
Φ
* Each original point consists of M
new points in the shape of constellation diagram
* Amplitude, delay and phase of the
echo determine shape/size of the
small constellation diagrams
A
Cable
Tuner
* Varying channel requires
Adaptive Equaliser
QAM
demod
s(t)
+
r(t)
Amplitude, delay, phase
Wireless Communications
16
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Equaliser Structure
Decision Feedback Equaliser (DFE)
Linear Equaliser (LE)
Symbol
Decision
Symbol
Decision
Iout
Coefficients
Iin
Qin
Complex
FIR filter
Qout
Coefficients
Coefficients
Iout
Iin
Qout
Qin
FFE
Wireless Communications
+
DFE
17
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Equaliser Structure
Linear Equaliser (LE)
H(z)
in
G(z)
+
A
Decision Feedback Equaliser (DFE)
out
in
Z-τ
+
-A
in
G(z)
+
A
out
in
out
+
Z-τ
Z-τ
A
(+) No residual ISI
–τ
G ( z) = 1 – A z
H(z)
-
Z-τ
(-) Residual ISI (A2,2τ)
H ( z) = 1 + A z
out
1
G ( z ) = ----------------------–τ
1 + Az
–τ
2 –2τ
H ( z) G ( z) = 1 – A z
H ( z) G ( z) = 1
(+) ‘Fast’ acquisition
(-) ‘High’ noise amplification
zeroes
(-) ‘Slow’ acquisition
(+) ‘Low’ noise amplification
poles
noise
Wireless Communications
noise
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
18
Equaliser Adaptation Algorithm
Zero Forcing (ZF)
Mean Square Error (MSE)
(+) Complete elimination of ISI
(+) Minimize sum of ISI and noise
(-) Penalty = Noise amplification
(+) Less noise amplification by
(-) Allowing residual ISI
Wireless Communications
19
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Adaptive Equaliser
LE
ZF (Zero Forcing)
Suited for QAM with M≤64
Suited for QAM with M≤64
(-) residual ISI does not allow higher M
(-) residual ISI does not allow higher M
(+) Fast acquisition
(+) Fast acquisition
(+) High stability
(+) High stability
No suitable solution
Required for QAM with M>64
(-) Because of complete elimination
of ISI system is instable when zero
in spectrum
DFE
MSE (Mean Square Error)
(+) Stablity guaranteed when
zero in spectrum
Equaliser
Equaliser
channel
channel
(-) ‘Slow’ acquisition
Wireless Communications
20
QAM Demodulation
S. Brand, Philips Semiconductors, PCALE
Measurement Results
BER
Implementation Loss
(2) 0.6 dB
(3) 1.1 dB
(4) 1.6 dB
(5) 1.9 dB
(6) 2.0 dB
(1) Theory
(2) AWGN (single car loop)
(3) AWGN (double car loop)
(4) 1 ray echo
(5) 2 ray echo
(6) 3 ray echo
1
2
3
4
56
S/N
Wireless Communications
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QAM Demodulation
21
Conclusion
Single Chip QAM Demodulator with low Implemenation Loss
- Double Loop AGC for optimum usage of A/D Converter
- Delay in half Nyquist filter and equaliser require double carrier
recovery loop structure to achieve high performance on phase
noise & microphonics
- Adaptive equaliser
* LE/ZF or LE/MSE preferred for QAM withM≤64
* DFE/MSE required for QAM with M>64
Wireless Communications