Why Do You Need to Measure Both BER and MER on Digital Signals by HC111211094146


									         Why Do You Need to Measure Both BER
              and MER on Digital Signals

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            Most digital analysers measure Bit Error Ratio
             (BER) and Modulation Error Ratio (MER).
            BER and MER each have their limitations.
            This seminar explains why it is important to
             measure both BER and MER and tells you what
             types of impairments will be missed if you only
             measure one or the other.
            Viewing of the Constellation Made Easy
             Seminar and the Modulation Error Ratio Made
             Easy Seminar is recommended prior to viewing
             this seminar.
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         Modulation Error Ratio

                                                   RMS error magnitude
            MER is defined as follows: 10 log
                                                 average symbol magnitude
            MER is expressed in dB.

                                                          RMS error


                                                          Average symbol

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            MER effectively assigns a value to the fuzziness of the
             symbol cluster.
            The larger or fuzzier the cluster, the poorer the MER.
            The further from the ideal locations, the poorer the

              Constellation with           Constellation with “poor”
                “good” MER                            MER
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         How Errors Occur

            Each symbol on the constellation is
             framed by decision boundaries.           5

            When the signal falls inside the         3
             decision boundaries the information
             is transmitted error-
                                     -7  -5   -3 -1     1   3     5   7
             free.                                    -1


                   Correct locations fall
                     within decision                  -7

                                Locations in error fall outside
                                    decision boundaries
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         Comparison Between Three
         Error Free Constellations
             All constellations below have a perfect BER with no errors, because
              the cluster always falls within the decision boundaries.
                      7                              7                          7
             The constellations to the right, however, have significantly better MER
              with less noise.
                      5                              5                          5
             When the cluster falls within the decision boundaries, BER is not an
              effective measurement of quality because the BER is perfect.
                        3                                   3                                  3

                         1                                   1                                  1

-7       -5   -3   -1     1   3   5   7 -7   -5   -3   -1     1   3   5    7
                                                                          -7    -5   -3   -1     1   3     5      7
                        -1                                  -1                                 -1

                        -3                                  -3                                 -3

                        -5                                  -5                                 -5

                        -7                                  -7                                 -7

      Poor MER                               Good MER                           Best MER
     Perfect BER                             Perfect BER                       Perfect BER
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         Theoretical MER vs BER With
         Only Gaussian Noise
         Impairing a 64 QAM Signal
                             Note there are no errors
                              in this range of MER.
         BER   No errors






                           35 34 33 32 31 30 29 28 27 26 25 24 23 22 21

          In practice errors will tend to occur at higher MERs due to other
                   forms of impairments besides Gaussian Noise.
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         Why Measure BER?

            Since MER can quantify signal quality when no
             errors exist the question can be raised, why
             measure BER at all if MER will do?
            The major limitation of MER is the inability of
             the measurement to capture fast intermittent
            A signal can have a very good MER, but poor
             BER due to intermittent interference.

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         Examples of Intermittent
         Interference That Cause
         Poor BER But Good MER

            Loose Connections
             - Corroded or loose connections.
            Sweep System Interference
             - Sweep pulses from a sweep system set up to sweep empty
            Laser Clipping
             - Occasional overload of the laser due to analogue sync pulses
               lining up.
            Microphonics
             - Vibration of digital origination equipment can cause
               intermittent errors.

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     Troubleshooting By Measuring
     Both MER and BER

         One way to determine if you have intermittent
          problems is to measure both MER and BER.
         If the MER is high, but you still see errors, then the
          errors are probably caused by an intermittent

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          Intermittent Errors on a
          Constellation Display

          Intermittent errors will show up on a
           constellation display as lone dots away from the
           main cluster.

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      BER vs MER vs C/R              Q&A

         Q. What is MER?
         A. MER is a measurement of the modulation impairment that
          affects the ability of a digital receiver to recover data bits.
          It accounts for modulation problems such as non linearity,
          group-delay and flatness variations, filter mismatching, and

         Q. When is MER useful considering?
         A. When considering digital video system margin or SNR issues.

         Q. What is the difference between MER and C/N?
         A. A good C/N accounts for the effect of noise only while MER
          accounts for modulation problems.

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          BER vs MER vs C/R          Q&A

          Q. Can we calculate MER by measuring C/N (for instance using a
           spectrum analyser) and then applying the relationship between
           C/N and MER(SNR)?
          A. No, because C/N accounts for noise ONLY while MER accounts
           for ALL kinds of signal impairments —noise, signal leakage, IQ
           level and quadrature imbalance etc —.

          Q. What minimum value of MER is measurable?
          A. To all practical purposes, down to 18 dB for DVB-C systems,
           and down to 5dB for DVB-S systems. This is due to the fact that a
           test equipment must be able to demodulate the signal before
           MER measurement. If the signal is so poor that this process fails,
           the measurement result cannot be trusted.

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          BER vs MER vs C/R         Q&A

          Q. What are we going to use for faultfinding when MER is below
           18 dB (DVB-C) or 5 dB (DVB-S)?

          A. At this point C/N (measured as channel power/noise power)
           can be used instead of no (MER) measurement. Noise only will
           be accounted for this way, but it is better than nothing.

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          BER vs MER vs C/R           Q&A
          Q. What is the difference between BER and MER?
          A. BER reflects noise and modulation impairments severe
           enough to cause bit errors, remaining insensitive to subtle
           trends in the digital modulation. MER allows the installer to
           detect modulation impairments being concealed by the system’s
           equalisation and error correction (before they become errors). In
           other words BER starts counting when problems surface as
           errors. MER is useful to detect impairments underneath the
           surface that is, while impairments are still within the reach of
           the correcting capabilities of the system.
          Q. What does a good BER mean?
          A. It indicated proper service delivery.

          Q. What does a bad BER mean?
          It highlights impaired service, but does not identify the cause of
           the problem.
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      BER vs MER vs C/R
      Comparison Table

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         In order to see the effects of
          all types of impairments on a
          QAM signal you need to
          measure both BER and MER.
         MER can quantify the quality
          of a digital signal that does
          not have any errors.
         MER has the limitation of not
          being able to see
          intermittent errors so a
          signal can have a good MER
          but a poor BER (Bit Error
                                                UNAOHM EP3000
         The most common type of
          intermittent errors are          Digital signal analyzer with
          caused by laser clipping.        COFDM/QAM/QPSK testing

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