Modulation_ Demodulation and Coding Course - pessoal.utfpr.edu by hcj

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									  Digital Communications I:
Modulation and Coding Course


        Period 3 - 2007
      Catharina Logothetis
           Lecture 7
          Last time we talked about:
   Another source of error due to filtering
    effect of the system:
       Inter-symbol interference (ISI)
   The techniques to reduce ISI
       Pulse shaping to have zero ISI at the
        sampling time
       Equalization to combat the filtering effect of
        the channel



                         Lecture 7                   2
    Today, we are going to talk about:

   Some bandpass modulation schemes used
    in DCS for transmitting information over
    channel
       M-PAM, M-PSK, M-FSK, M-QAM
   How to detect the transmitted information
    at the receiver
       Coherent detection
       Non-coherent detection


                        Lecture 7         3
         Block diagram of a DCS



           Source   Channel      Pulse      Bandpass
Format
           encode   encode      modulate    modulate

                                 Digital modulation




                                                           Channel
                                Digital demodulation

           Source   Channel                  Demod.
Format                           Detect
           decode   decode                   Sample


                    Lecture 7                          4
                Bandpass modulation
   Bandpass modulation:           The process of converting
    data signal to a sinusoidal waveform where its
    amplitude, phase or frequency, or a combination of
    them, is varied in accordance with the transmitting data.
   Bandpass signal:




    where          is the baseband pulse shape with energy     .
   We assume here (otherwise will be stated):
             is a rectangular pulse shape with unit energy.
       Gray coding is used for mapping bits to symbols.
          denotes average symbol energy given by
                                Lecture 7                          5
        Demodulation and detection
   Demodulation: The receiver signal is converted to
    baseband, filtered and sampled.
   Detection: Sampled values are used for detection
    using a decision rule such as ML detection rule.




                                         Decision
                                         circuits
                                       (ML detector)




                         Lecture 7                      6
               Coherent detections

   Coherent detection
       requires carrier phase recovery at the
        receiver and hence, circuits to perform
        phase estimation.
       Source of carrier-phase mismatch at the
        receiver:
          Propagation delay causes carrier-phase offset in
           the received signal.
          The oscillators at the receiver which generate
           the carrier signal, are not usually phased locked
           to the transmitted carrier.

                            Lecture 7                      7
          Coherent detection ..
   Circuits such as Phase-Locked-Loop (PLL) are
    implemented at the receiver for carrier phase
    estimation (       ).
                                               I branch


                               PLL
                                                   Used by
           Oscillator                90 deg.       correlators


                                                    Q branch




                        Lecture 7                          8
        Bandpass Modulation Schemes

   One dimensional waveforms
       Amplitude Shift Keying (ASK)
       M-ary Pulse Amplitude Modulation (M-PAM)
   Two dimensional waveforms
       M-ary Phase Shift Keying (M-PSK)
       M-ary Quadrature Amplitude Modulation (M
        -QAM)
   Multidimensional waveforms
       M-ary Frequency Shift Keying (M-FSK)


                         Lecture 7                 9
       One dimensional modulation,
       demodulation and detection
   Amplitude Shift Keying (ASK) modulation:



                                 On-off keying (M=2):
                                     “0”      “1”


                                     0




                     Lecture 7                          10
          One dimensional mod.,…

   M-ary Pulse Amplitude modulation (M-PAM)


                       4-PAM:
                       “00”     “01”       “11”   “10”


                                       0




                    Lecture 7                            11
Example of bandpass modulation:
          Binary PAM




             Lecture 7            12
     One dimensional mod.,...–cont’d
   Coherent detection of M-PAM



                               ML detector
                     (Compare with M-1 thresholds)




                   Lecture 7                         13
       Two dimensional modulation,
    demodulation and detection (M-PSK)
   M-ary Phase Shift Keying (M-PSK)




                     Lecture 7         14
   Two dimensional mod.,… (MPSK)
  BPSK (M=2)

  “0”     “1”
                                     8PSK (M=8)

                                             “011”
                             “010”                   “001”

  QPSK (M=4)
                                                        “000”
                     “110”
“01”      “00”


                        “111”                         “100”


                                     “101”


  “11”   “10”
                 Lecture 7                                      15
     Two dimensional mod.,…(MPSK)
   Coherent detection of MPSK




                                Compute   Choose
                                          smallest




                    Lecture 7                    16
    Two dimensional mod.,… (M-QAM)
   M-ary Quadrature Amplitude Mod. (M-QAM)




                    Lecture 7          17
Two dimensional mod.,… (M-QAM)

                   16-QAM

      “0000”   “0001”      “0011”   “0010”
                          3

      “1000”   “1001”     “1011”    “1010”

                          1
        -3        -1           1         3

                          -1
      “1100”   “1101”     “1111”    “1110”


                          -3
      “0100”   “0101”     “0111”    “0110”



                        Lecture 7            18
    Two dimensional mod.,… (M-QAM)

   Coherent detection of M-QAM


                        ML detector



                                      Parallel-to-serial
                                         converter

                        ML detector




                   Lecture 7                           19
Multi-dimentional modulation, demodulation &
                  detection
 M-ary Frequency Shift keying (M-FSK)




                  Lecture 7            20
Multi-dimensional mod.,…(M-FSK)




                            ML detector:
                                 Choose
                           the largest element
                         in the observed vector




             Lecture 7                            21
             Non-coherent detection
   Non-coherent detection:
       No need in a reference in phase with the
        received carrier
       Less complexity as compared to coherent
        detection at the price of higher error rate.




                           Lecture 7                   22
                Non-coherent detection …
      Differential coherent detection
          Differential encoding of the message
               The symbol phase changes if the current bit is
                different from the previous bit.




Symbol index:             0 1 2 3 4 5 6 7
Data bits:                  1 1 0 1 0 1 1
Diff. encoded bits        1 1 1 0 0 1 1 1              0
Symbol phase:                   0 0

                                Lecture 7                        23
          Non-coherent detection …
   Coherent detection for diff encoded mod.
      assumes slow variation in carrier-phase mismatch during
       two symbol intervals.
      correlates the received signal with basis functions

      uses the phase difference between the current received
       vector and previously estimated symbol




                           Lecture 7                   24
        Non-coherent detection …
   Optimum differentially coherent detector


                                                    Decision

                                 Delay
                                  T
   Sub-optimum differentially coherent detector

                                                 Decision

                 Delay
                  T

             Performance degradation about 3 dB by using sub-
              optimum detector

                               Lecture 7                         25
         Non-coherent detection …
   Energy detection
        Non-coherent detection for orthogonal signals
         (e.g. M-FSK)

             Carrier-phase offset causes partial correlation between
              I and Q braches for each candidate signal.

              The received energy corresponding to each candidate
              signal is used for detection.




                              Lecture 7                           26
         Non-coherent detection …

   Non-coherent detection of BFSK




                                 +   Decision stage:


                                 -




                     Lecture 7                         27

								
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