QAM Demodulation. Performance Conclusion. o o o o o. (Nyquist shaping, Clock & Carrier Recovery, AGC, Adaptive Equaliser) o o. Wireless Communications

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1 0 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

2 1 Example Application Area Wireless Cable Digital TV using Microwave Transmission QAM Modulation Set-top Box Multiplexing 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

3 2 o Amplitude Modulation of What is QAM? o Two Orthogonal Carriers x i () t = 2E o a i cos ( ω c t) T s + 2E o b i sin ( ω c t) T s a i-1 =+3 a i =+1 a i+1 =-7 T s bi-1=-5 bi=+5 bi+1=-1 Tc 64QAM in time domain time I Q Q Ε ο 1 Εο QAM Constellation diagram I

4 3 b1b0 Satellite Noise power M-ary QAM I { Cable Q { b 5 b 4 b 3 b 2 b 1 b 0 signal power S/N > 3 db for M=4 S/N > 21 db for M=64 S/N > 27 db for M=256

5 4 What to do to recover the information? Functions Automatic Gain Control Quadrature down conversion (Half) Nyquist Filtering Clock Recovery Carrier Recovery Adaptive Equaliser Demapping Result Optimal position of constellation diagram in reception window I & Q base band signals Pulse shaping Sampling reference for A/D Converter Carrier frequency reference Compensate for channel distortion Representation of received data in bits

6 5 System Block Diagram IF f s I 2 C QAM DEMODULATOR fine AGC f f f Tuner BPF LPF ADC 4f s 1,0,-1,0 0,-1,0,1 Ν Ν I Q Complex Equaliser demapping Cable Connection AGC VCO Carrier Recovery VCXO Clock Recovery AGC detect DAC clock detect DAC DTO Digital Analogue loop filter carrier detect DAC

7 6 Automatic Gain Control IF down conversion ADC n Filtering & Equalisation I Q * 2 loops AGC * Coarse AGC to prevent ADC from overloading coarse AGC fine AGC * After Nyquist filtering and Equalisation small QAM remains. Q Q Q * Fine AGC to position contellation diagram to decision window I I I Tuner output Equaliser output Fine AGC output

8 7 (Half) Nyquist Filtering Ν I * Pulse Shaping required to realise ISI=0 in limited BW ADC 4fs Ts=1/fs S n S n+3 S T s n+1 Sn+6 1,0,-1,0 0,-1,0,1 BW= time Ν Q 0 freq * ISI=0 when zero crossings occur at multiples of T s =1/f s * Achieved with Nyquist Criterion (DVB: α = 15%) * Cascade of Transmitter & Receiver fulfil Nyquist Criterion (Half Nyquist each ) Sn+2 S n+5 Sn+4 (1+α)f s f s * Digital implementation (T delay = 9 T symbol ) BW=8MHz time 0 freq * This delay is in the loops and thus influences the demodulator architecture

9 8 Clock Recovery 1,0,-1,0 * Recovery with 2 nd order PLL ADC vcxo 0,-1,0,1 Ν DAC clock det. * Clock Detector - Energy Maximization algorithm - After Half Nyquist Filter to achieve ISI=0 at detector input I, Q signal * 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) Recovered clock T s time * Quadarture Demodulation - f clock = 4 f symbol - Simple with j -n (n=0,1,2,3,...)

10 9 IF LPF vco I or Q Recovered carrier ADC 4fs vcxo T carrier DAC delay Ν Carrier Recovery equaliser carrier det. time * Recovery with 2 nd 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 disturbances like: * phase noise * microphonics (mechanical vibrations) * Alternative solution required

11 10 Additive White Gaussian Noise Carrier Phase Disturbances (1) * AWGN Disturbance - Random distribution - Mainly inserted in the cable channel * Result - Enlarged constellation points Cable Tuner QAM demod Implementation Loss * PLL Properties - Average the noise - Loop BW small - Low IL s(t) + r(t) n(t) BW

12 11 Cable s(t) Phase Noise/ Microphonics Tuner X Carrier Phase Disturbances (2) QAM demod r(t) Q Implementation Loss I BW * Phase Noise & Microphonics -No random distribution -Mainly inserted in the tuner by LC oscillators which are sensative for mechanical vibrations (Microphonics) * Result -Rotation of constellation diagram. * PLL Properties -Follow the phase disturbance -Loop BW large -Low IL * PLL properties for AWGN and phase noise are in contradiction

13 12 Implementation Loss [db] AWGN+Phase noise Phase noise versus AWGN AWGN * Loop BW trade of between: a. Ability to follow phase noise b. Ability to average AWGN * Rule of thumb: 1 BW = f symbol * Simulations show this is approximately correct * Optimum depends on S/N and amount of phase noise OPTIMUM Loop BW [khz] * Problem: Optimum loop BW instable due to large delay in the loop (Half Nyquist + Equaliser).

14 13 Double Loop Carrier Recovery IF vco LPF ADC vcxo 4f s large delay Ν dto equaliser Loop filter carrier det. inner loop * Introduction of second loop with (relatively) small delay * Outer loop -Adjust (static) frequency offset -Small loop BW due to large delay -PD/PFD DAC outer loop * Inner Loop - Optimum loop BW as trade off between phase noise & AWGN - Large Loop BW due to small delay - PD only I or Q T carrier time * Conclusion: optimum loop BW can be selected and causes no instability Recovered carrier

15 14 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

16 15 Multipath Reflection Multipath Distortion A Φ * Multipath distortion causes ISI * 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 * Varying channel requires Adaptive Equaliser Cable Tuner QAM demod s(t) + r(t) Amplitude, delay, phase

17 16 Equaliser Structure Linear Equaliser (LE) Decision Feedback Equaliser (DFE) Symbol Decision Symbol Decision Iout Qout Coefficients Coefficients Coefficients I in Qin Complex FIR filter I out Qout I in Qin FFE + DFE

18 17 Equaliser Structure Linear Equaliser (LE) Decision Feedback Equaliser (DFE) in H(z) + out in G(z) + out in H(z) + out in + G(z) out A Z -τ -A Z -τ A Z -τ - Z -τ A (-) Residual ISI (A 2,2τ) H ( z) = 1 + Az τ G( z) = 1 Az τ H( z) G( z) = 1 A 2 z 2τ (+) Fast acquisition (-) High noise amplification zeroes noise (+) No residual ISI G( z) = Az τ H( z) G( z) = 1 (-) Slow acquisition (+) Low noise amplification poles noise

19 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

20 19 LE Adaptive Equaliser ZF (Zero Forcing) MSE (Mean Square Error) 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 DFE No suitable solution (-) Because of complete elimination of ISI system is instable when zero in spectrum Equaliser channel Required for QAM with M>64 (+) Stablity guaranteed when zero in spectrum Equaliser channel (-) Slow acquisition

21 20 BER Measurement Results 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 S/N

22 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 with M 64 * DFE/MSE required for QAM with M>64