s. brand, philips semiconductors, pcale qam...
TRANSCRIPT
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S. Brand, Philips Semiconductors, PCALE 0QAM Demodulation
Wireless Communications
QAM Demodulation
o Application areao What is QAM?o What are QAM Demodulation Functions?o General block diagram of QAM demodulatoro Explanation of the main function
(Nyquist shaping, Clock & Carrier Recovery, AGC, Adaptive Equaliser)
o Performanceo Conclusion
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S. Brand, Philips Semiconductors, PCALE 1QAM Demodulation
Wireless Communications
Example Application Area
Multiplexing
Compression
Set-top Box
Radio
• Compression = bit rate reduction• Multiplexing = assembly of multiple programs• Modulation = conversion to transmission format
• Set-top Box = Integrated Receiver Decoder (IRD), wide range of programs
QAM Modulation
“Wireless Cable” Digital TV using Microwave Transmission
Channel
provides a subscriber access to a
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S. Brand, Philips Semiconductors, PCALE 2QAM Demodulation
Wireless Communications
What is QAM?o Amplitude Modulation of
o Two Orthogonal Carriers xi t( )2Eo
Ts---------ai ωct( )cos
2Eo
Ts---------bi ωct( )sin+=
+7+5+3+1-1-3-5-7
ai-1=+3 ai=+1 ai+1=-7
+7+5+3+1-1-3-5-7
Ts
Tc
bi-1=-5 bi=+5 bi+1=-1
time1
1√Εο
3 5 7−7 −5 −3 −1
3
5
7
−7
−5
−3
−1
√Εο
64QAM in time domain 64QAM Constellation diagram
I
Q
I
Q
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S. Brand, Philips Semiconductors, PCALE 3QAM Demodulation
Wireless Communications
M-ary QAM
Satellite Cable
S/N > 21 dB for M=64S/N > 27 dB for M=256
S/N > 3 dB for M=4
signal power
Noise power
b5b4b3b2b1b0b1b0
{ I { Q
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S. Brand, Philips Semiconductors, PCALE 4QAM Demodulation
Wireless Communications
What to do to recover the information?
Functions ResultAutomatic Gain Control Optimal position of constellation diagram in reception window
Quadrature down conversion
I & Q base band signals
(Half) Nyquist Filtering Pulse shapingClock Recovery Sampling reference for A/D ConverterCarrier Recovery Carrier frequency reference
Adaptive Equaliser Compensate for channel distortionDemapping Representation of received data in bits
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S. Brand, Philips Semiconductors, PCALE 5QAM Demodulation
Wireless Communications
System Block Diagram
Tuner LPF ADCBPF
Cab
le C
onne
ctio
n VCXOVCO
√Ν
√Ν
ComplexEqualiser
clockdetect
DAC
AGCdetect
DAC
carrierdetect
DAC
1,0,-1,0
0,-1,0,1
loopfilterDTO
fineAGC
QAM DEMODULATOR
I
Q
AG
C
Car
rier R
ecov
ery
Clo
ck R
ecov
ery
4fs
f f f
IF fs
dem
appi
ng
AnalogueDigital
I2C
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S. Brand, Philips Semiconductors, PCALE 6QAM Demodulation
Wireless Communications
Automatic Gain Control* 2 loops AGC
* Coarse AGC to prevent ADC from overloading
* After Nyquist filtering and Equalisation ‘small’ QAM remains.
* Fine AGC to position contella-tion diagram to decision window
IF downconversion ADC
Filtering &Equalisation
coarseAGC
fineAGC
IQ
n
Tuner output Equaliser output Fine AGC output
I
Q
I
I
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S. Brand, Philips Semiconductors, PCALE 7QAM Demodulation
Wireless Communications
(Half) Nyquist Filtering
BW=∞
Sn Sn+3
Sn+2 Sn+4
time
Ts
Ts=1/fs
BW=8MHz
time
Sn+1
Sn+5
Sn+6
freq
freq
0
0
fs
(1+α)fs
* Pulse Shaping required to realise ISI=0 in limited BW
* ISI=0 when zero crossings occur at multiples of Ts=1/fs
* Achieved with Nyquist Criterion(DVB: α = 15%)
* Cascade of Transmitter & Receiver fulfil Nyquist Criterion
(Half Nyquist each )
* Digital implementation(Tdelay = 9 Tsymbol)
* This delay is in the loops and thus influences the demodulator archi-tecture
ADC
√Ν
√Ν
1,0,-1,0
0,-1,0,1
4fs
I
Q
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S. Brand, Philips Semiconductors, PCALE 8QAM Demodulation
Wireless Communications
Clock Recovery
* Recovery with 2nd order PLL
* Clock Detector- Energy Maximization algorithm- After Half Nyquist Filter to achieve
ISI=0 at detector input
* 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)
* Quadarture Demodulation - fclock = 4 fsymbol- Simple with j-n (n=0,1,2,3,...)
I, Q signal
Ts
Recovered
time
clock
vcxo
ADC
clockdet.
DAC
√Ν
1,0,-1,0
0,-1,0,1
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S. Brand, Philips Semiconductors, PCALE 9QAM Demodulation
Wireless Communications
Carrier Recovery
I or Q
Tcarrier
Recovered
time
carrier
* Recovery with 2nd order PLL
* 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
* Half Nyquist & Equaliser in loop- Large delay causes problems for distur-bances like:* phase noise* microphonics (mechanical vibrations)
* Alternative solution required
ADC
vco
4fs
delay
LPF
vcxo
IF
√Ν equaliser
carrierdet.
DAC
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S. Brand, Philips Semiconductors, PCALE 10QAM Demodulation
Wireless Communications
Carrier Phase Disturbances (1)Additive WhiteGaussian Noise
TunerCable QAMdemod
+s(t)
n(t)
r(t)
* AWGN Disturbance
- Random distribution- Mainly inserted in the cable channel
* Result- Enlarged constellation points
* PLL Properties- Average the noise- Loop BW small- Low ILImplementation Loss
BW
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S. Brand, Philips Semiconductors, PCALE 11QAM Demodulation
Wireless Communications
Carrier Phase Disturbances (2)Phase Noise/
TunerCable QAMdemod
Microphonics
Xs(t) r(t)
* Phase Noise & Microphonics-No random distribution-Mainly inserted in the tuner by LC oscillators which are sen-sative for mechanical vibra-tions (Microphonics)
* Result-Rotation of constellation dia-gram.
* PLL Properties-Follow the phase disturbance-Loop BW large-Low IL
* PLL properties for AWGN and phase noise are in contradic-tion
I
Q
BW
Implementation Loss
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S. Brand, Philips Semiconductors, PCALE 12QAM Demodulation
Wireless Communications
Phase noise versus AWGN* Loop BW trade of between:
a. Ability to follow phase noiseb. Ability to average AWGN
* Rule of thumb:
* Simulations show this is approximately correct
* Optimum depends on S/N and amount of phase noise
* Problem: Optimum loop BW instable due to large delay in the loop (Half Nyquist + Equal-iser).
BW1
1000------------fsymbol=AWGN
AWGN+Phase noise
Loop BW [kHz]
Impl
emen
tatio
n Lo
ss [d
B]
OPTIMUM
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S. Brand, Philips Semiconductors, PCALE 13QAM Demodulation
Wireless Communications
Double Loop Carrier Recovery* Introduction of second loop with
(relatively) small delay
* Outer loop-Adjust (static) frequency offset-Small loop BW due to large delay-PD/PFD
* Inner Loop- Optimum loop BW as trade off between phase noise & AWGN
- Large Loop BW due to small delay- PD only
* Conclusion: optimum loop BW can be selected and causes no instability
I or Q
Tcarrier
Recovered
time
carrier
equaliser
Loopdto inner loop
ADC
vco
4fs
large delay
LPF
vcxo
IF
√Νcarrierdet.
DACouter loop
filter
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S. Brand, Philips Semiconductors, PCALE 14QAM Demodulation
Wireless Communications
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
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S. Brand, Philips Semiconductors, PCALE 15QAM Demodulation
Wireless Communications
Multipath DistortionMultipathReflection
TunerCable QAMdemod
s(t) r(t)
Amplitude, delay, phase
+
* Multipath distortion causes ISI
* Each original point consists of M new points in the shape of constella-tion diagram
* Amplitude, delay and phase of the echo determine shape/size of the small constellation diagrams
* Varying channel requires Adaptive Equaliser
Φ
A
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S. Brand, Philips Semiconductors, PCALE 16QAM Demodulation
Wireless Communications
Equaliser Structure
Decision Feedback Equaliser (DFE)Linear Equaliser (LE)
ComplexFIR filter
Coefficients
SymbolDecision
Iout
Qout
Iin
QinFFE
Coefficients Coefficients
DFE+
Iout Qout
Iin
Qin
Symbol Decision
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S. Brand, Philips Semiconductors, PCALE 17QAM Demodulation
Wireless Communications
Equaliser Structure
Linear Equaliser (LE) Decision Feedback Equaliser (DFE)
H z( ) 1 Azτ–
+=
G z( ) 1 Azτ–
–=
H z( ) G z( ) 1 A2z
2τ––=
G z( )1
1 Azτ–
+-----------------------=
H z( ) G z( ) 1=
zeroes poles
noise noise
A Z-τ
+in out
-A Z-τ
+in out
H(z) G(z)
A Z-τ
+in out
AZ-τ
+outin
H(z) G(z)
(-) Residual ISI (A2,2τ)
(+) ‘Fast’ acquisition(-) ‘High’ noise amplification
(+) No residual ISI
(-) ‘Slow’ acquisition(+) ‘Low’ noise amplification
-
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S. Brand, Philips Semiconductors, PCALE 18QAM Demodulation
Wireless Communications
Equaliser Adaptation Algorithm
Zero Forcing (ZF) Mean Square Error (MSE)
(+) Complete elimination of ISI
(-) Penalty = Noise amplification (+) Less noise amplification by
(-) Allowing residual ISI
(+) Minimize sum of ISI and noise
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S. Brand, Philips Semiconductors, PCALE 19QAM Demodulation
Wireless Communications
Adaptive EqualiserL
ED
FE
ZF (Zero Forcing) MSE (Mean Square Error)
(-) residual ISI does not allow higher M
Equaliser
channel
(+) Stablity guaranteed when
Suited for QAM with M≤64
(-) Because of complete elimination
Equaliser
channel
(-) ‘Slow’ acquisition
(+) High stability
Required for QAM with M>64
(+) Fast acquisition(-) residual ISI does not allow higher M
Suited for QAM with M≤64
(+) High stability
(+) Fast acquisition
No suitable solution
of ISI system is instable when zeroin spectrum
zero in spectrum
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S. Brand, Philips Semiconductors, PCALE 20QAM Demodulation
Wireless Communications
Measurement Results
(1) Theory(2) AWGN (single car loop)
(4) 1 ray echo(5) 2 ray echo(6) 3 ray echo
BE
R
1 2 3 4 5
S/N
Implementation Loss(2) 0.6 dB(3) 1.1 dB(4) 1.6 dB(5) 1.9 dB(6) 2.0 dB
(3) AWGN (double car loop) 6
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S. Brand, Philips Semiconductors, PCALE 21QAM Demodulation
Wireless Communications
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 with M≤64* DFE/MSE required for QAM with M>64