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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
-
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
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
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
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