QAM Demodulation

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					S. Brand, Philips Semiconductors, PCALE                  QAM Demodulation
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                        QAM Demodulation
o     Application area
o     What is QAM?
o     What are QAM Demodulation Functions?
o     General block diagram of QAM demodulator
o     Explanation of the main function
      (Nyquist shaping, Clock & Carrier Recovery, AGC, Adaptive Equaliser)

o     Performance
o     Conclusion



                                          Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                 QAM Demodulation
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                                                                      a
                                                Example Application Are

     “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




                                                         Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                  QAM Demodulation
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                                                      What is QAM?
o     Amplitude Modulation of
                                                                                 2E o                        2Eo
o     Two Orthogonal Carriers                                      xi ( t ) =    -------- ai cos ( ω c t ) + ---------bi sin ( ω c t )
                                                                                        -
                                                                                   Ts                          Ts




       +7
                   ai-1=+3         ai=+1    ai+1=-7                                                                    Q
       +5
       +3                                                                        7
       +1
       -1
       -3
                                                               I                 5
       -5                            Ts
       -7
                                                                                 3
                                                                                                               √Εο
                 bi-1=-5          bi=+5    bi+1=-1                               1                                                               I
       +7                                                                                                                √Εο
       +5                                     Tc                                −1
       +3
       +1
                                                                                −3
       -1
       -3
                                                               Q
       -5                                                                       −5
       -7
                                                                                −7
                                                        time
                                                                                     −7     −5      −3      −1          1      3         5   7

                   64QAM in time domain                                           64QAM Constellation diagram



                                                     Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                QAM Demodulation
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                                                          M-ary QAM
                                                                                I             Q




                                                                           {
                                                                           {
                                Satellite                                                 Cable   b5b4b3b2b1b0
     b1b0
                                          Noise power



                                           signal power




                    S/N > 3 dB for M=4                                            S/N > 21 dB for M=64
                                                                                  S/N > 27 dB for M=256



                                                        Wireless Communications
S. Brand, Philips Semiconductors, PCALE                          QAM Demodulation
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                                                                       ?
                                   What to do to recover the information

                 Functions                                           Result
      Automatic Gain Control              Optimal position of constellation diagram in reception window
              Quadrature                                    I & Q base band signals
            down conversion
       (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
S. Brand, Philips Semiconductors, PCALE                                                                            QAM Demodulation
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                                                                          System Block Diagram
                                                                                                                          QAM DEMODULATOR
                                                      IF             fs                                   I2C                                                      fine
                                                                                                                                                                   AGC
                       f                                   f                               f                                     I
                                                                                                                   √Ν




                                                                                                                                                                            demapping
                                                                                                        1,0,-1,0                                      Complex
               Tuner         BPF                               LPF        ADC                                                                         Equaliser
                                                                                                        0,-1,0,1
                                                                                                                                 Q
                                                                          4fs                                      √Ν
Cable Connection




                                    VCO                                   VCXO
                                                                                               AGC                      clock        DTO               loop       carrier
                                                                                               detect                   detect                         filter     detect
                                                                                                                                           Digital
                                   Carrier Recovery




                                                                          Clock Recovery



                                                                                                                                           Analogue
                                                                                                DAC                      DAC                                       DAC
                       AGC




                                                                                           Wireless Communications
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                                              Automatic Gain Control
                                                                                 * 2 loops AGC
                                           n Filtering &
                 IF down            ADC                                      I
                 conversion                  Equalisation                    Q   * Coarse AGC to prevent ADC
                                                                                   from overloading
                      coarse                                         fine
                      AGC                                            AGC         * After Nyquist filtering and
                                                                                   Equalisation ‘small’ QAM
                                                                                   remains.
                                                                                 * Fine AGC to position contella-
                                                                                   tion diagram to decision window
                Q                         Q                      Q




                               I                       I                     I




          Tuner output              Equaliser output       Fine AGC output




                                                       Wireless Communications
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                                                   (Half) Nyquist Filtering
                                                                                  * Pulse Shaping required to realise
                                                       √Ν   I                      ISI=0 in limited BW
                                            1,0,-1,0
                        ADC                                                       * ISI=0 when zero crossings occur at
                                            0,-1,0,1
                                                                                   multiples of Ts=1/fs
                      4fs                              √Ν   Q
                                                                                  * Achieved with Nyquist Criterion
                                                                                     (DVB: α = 15%)
    Ts=1/fs
    Sn           Sn+3           Sn+6
                                                                                  * Cascade of Transmitter & Receiver
       S
     Ts n+1                                  BW=∞                                  fulfil Nyquist Criterion
                                          time                                       (Half Nyquist each )
                                                                0      freq
            Sn+2 S Sn+5                                     (1+α)fs               * Digital implementation
                  n+4
                                                              fs                     (Tdelay = 9 Tsymbol)


                                          BW=8MHz
                                                                                  * This delay is in the loops and thus
                                                                                   influences the demodulator archi-
                                          time                  0     freq         tecture


                                                        Wireless Communications
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                                                   Clock Recovery
                               1,0,-1,0                        * Recovery with 2nd order PLL
                ADC                       √Ν                   * Clock Detector
                               0,-1,0,1                          - Energy Maximization algorithm
                                                                 - After Half Nyquist Filter to achieve
                                                clock
                vcxo                      DAC                      ISI=0 at detector input
                                                det.


                                                               * Half Nyquist Filter in loop is allowed
                                                                 - Received clock has crystal accuracy
                                                                   (100 ppm at 7 Msym/s))
                                                                 - Loop BW may be small
    I, Q signal                                                  - Delay in loop is allowed (no instability)
                                                        time
                         Ts
                                                               * Quadarture Demodulation
                                                                 - fclock = 4 fsymbol
                                                                 - Simple with j-n (n=0,1,2,3,...)
    Recovered
    clock




                                                 Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                           QAM Demodulation
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                                                              Carrier Recovery
                                                delay                        * Recovery with 2nd order PLL
   IF                                                                        * Carrier Detector
                 LPF   ADC                      √Ν        equaliser
                                                                               - Decision directed
                           4fs                                                 - After equaliser
                                                                               - PD (lock) and PFD (unlock)
         vco           vcxo
                                                                                 * PFD for large acquisition range (100 kHz)
                                                                                 * PD for stable behaviour once in lock
                                                        carrier
                                          DAC
                                                        det.
                                                                             * Half Nyquist & Equaliser in loop
                                                                               - Large delay causes problems for distur-
                                                                                 bances like:
                                                                                 * phase noise
        I or Q
                                                                                 * microphonics (mechanical vibrations)
                                                                      time
                       Tcarrier                                              * Alternative solution required
    Recovered
    carrier




                                                              Wireless Communications
S. Brand, Philips Semiconductors, PCALE                         QAM Demodulation
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                                                                      )
                                          Carrier Phase Disturbances (1
         Additive White                                                   * AWGN Disturbance
         Gaussian Noise
                                                                           - Random distribution
                                                                           - Mainly inserted in the cable
                                                                             channel

                                                                          * Result
                                                                            - Enlarged constellation points

                                                                          * PLL Properties
                                                                            - Average the noise
                                                                            - Loop BW small
                                               Implementation Loss          - Low IL
    Cable          Tuner            QAM
                                    demod
           s(t)         +          r(t)


                       n(t)                                      BW



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



                                              Wireless Communications
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                                          Phase noise versus AWGN
                                                                      * Loop BW trade of between:
                                                                        a. Ability to follow phase noise
     Implementation Loss [dB]



                                                                        b. Ability to average AWGN

                                                                      * Rule of thumb:
                                                                                    1
                                                                                         -
                                                                          BW = ----------- f
                                                                                  1000 symbol
                                                     AWGN
                                AWGN+Phase noise                      * 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-
                                                    Loop BW [kHz]   iser).
                                   OPTIMUM



                                              Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                            QAM Demodulation
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                                                                      y
                                            Double Loop Carrier Recover
                                             large delay
                                                                                       * Introduction of second loop with
                                                                                         (relatively) small delay
  IF
                                                                         carrier
                LPF    ADC                  √Ν             equaliser
                                                                         det.          * Outer loop
                          4fs                                                            -Adjust (static) frequency offset
                                                    dto      Loop
                                                                       inner loop        -Small loop BW due to large delay
       vco             vcxo
                                                             filter
                                                                                         -PD/PFD
                                             outer loop
                                      DAC                                              * Inner Loop
                                                                                         - Optimum loop BW as trade off
                                                                                           between phase noise & AWGN
                                                                                         - Large Loop BW due to small delay
                                                                                         - PD only
       I or Q
                                                                             time      * Conclusion: optimum loop BW can
                           Tcarrier                                                      be selected and causes no instability
       Recovered
       carrier




                                                       Wireless Communications
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                                            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
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                                               Multipath Distortion
                                                                       * Multipath distortion causes ISI
                Multipath
                Reflection                                             * Each original point consists of M
                                                                         new points in the shape of constella-
                                                                         tion diagram
                                                                   Φ
                                                                       * Amplitude, delay and phase of the
                                                                         echo determine shape/size of the
                                                                         small constellation diagrams
                                                            A
                                                                       * Varying channel requires
                                                                         Adaptive Equaliser



     Cable           Tuner           QAM
                                     demod
    s(t)                                  +   r(t)

              Amplitude, delay, phase




                                                     Wireless Communications
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                                          Equaliser Structure


                   Linear Equaliser (LE)                    Decision Feedback Equaliser (DFE)


                           Symbol                                               Symbol
                           Decision                                             Decision

                                                                                Iout       Qout

                        Coefficients                           Coefficients                       Coefficients


    Iin                   Complex              Iout   Iin
                                                                  FFE                  +            DFE
    Qin                   FIR filter          Qout    Qin




                                           Wireless Communications
S. Brand, Philips Semiconductors, PCALE                                          QAM Demodulation
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                                                          Equaliser Structure
                 Linear Equaliser (LE)                                          Decision Feedback Equaliser (DFE)
                 H(z)                                  G(z)                          H(z)                                              G(z)
        in                    out           in                      out
                          +                                     +
                                                                           in                        out                in                    out
                                                                                              +                              +

                                                                                                                             -
             A     Z-τ                            -A     Z-τ                     A     Z-τ                                       Z-τ          A


    (-) Residual ISI (A2,2τ)                                                    (+) No residual ISI
                                  –τ
               H ( z) = 1 + A z                                                                                     1
                                                                                             G ( z ) = -----------------------
                                                                                                                        –τ
                                  –τ                                                                   1 + Az
              G ( z) = 1 – A z
                                          2 –2τ                                              H ( z) G ( z) = 1
             H ( z) G ( z) = 1 – A z

    (+) ‘Fast’ acquisition                                                      (-) ‘Slow’ acquisition
    (-) ‘High’ noise amplification                                              (+) ‘Low’ noise amplification
                          zeroes                                                      poles
                                                        noise                                                                                 noise




                                                               Wireless Communications
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                                                                      m
                                          Equaliser Adaptation Algorith


                   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
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                                            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
   LE




             (+) Fast acquisition                          (+) Fast acquisition
             (+) High stability                            (+) High stability

             No suitable solution                         Required for QAM with M>64
            (-) Because of complete elimination           (+) Stablity guaranteed when
                of ISI system is instable when zero           zero in spectrum
                in spectrum
                                                                                   Equaliser
   DFE




                                          Equaliser
                                                                                    channel
                                           channel
                                                          (-) ‘Slow’ acquisition




                                             Wireless Communications
S. Brand, Philips Semiconductors, PCALE                   QAM Demodulation
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                                                           s
                                          Measurement Result
                                                                      Implementation Loss
                                                                      (2) 0.6 dB
     BER


                                                                      (3) 1.1 dB
                                                                      (4) 1.6 dB
                                                                      (5) 1.9 dB
                                                                      (6) 2.0 dB

                       (1) Theory
                       (2) AWGN (single car loop)                 1          2   3   4    56
                       (3) AWGN (double car loop)
                       (4) 1 ray echo
                       (5) 2 ray echo
                       (6) 3 ray echo

                                                                                         S/N

                                           Wireless Communications
S. Brand, Philips Semiconductors, PCALE                  QAM Demodulation
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                                             Conclusion

Single Chip QAM Demodulator with low Implemenation Loss
   - Double Loop AGC for optimum usage of A/D Converte       r
   - 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>6        4




                                          Wireless Communications

				
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