QAM Technology by FKij3B

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									QAM Technology

Rev. A00
                                                                     CATV HFC Network




                 HDTV, MOD, VoIP, and Broadband data services
                   are made possible by digital cable services.

QAM Technology               Confidential & Proprietary Information of VeEX Inc.    2
                                                                                         Analog vs. Digital

                         Analog                                                             Digital


                 Video                                                                          Haystack

                                  Audio




      Video and audio channels are modulated                       Video and audio signals are digitized to Digital 0
       to three separated frequencies within a                       and 1, QPSK or QAM modulated, then
       6MHz bandwidth                                                transmitted in a 6MHz band.
      They are transmitted at different levels.                    Digital symbols (bits) embedded in the Haystack
       Normally, a video channel is about 10dB                      High Digital bit rates can be transmitted in a
       higher than the Audio channels                                6MHz band for up to 40Mbps suitable for
      Signals are Analog in nature, and                             Internet, VoIP, or HDTV services
       therefore, more resistant to noise                           Noise can affect the Digital bit streams
                                                                    Uses Forward Error Correction (FEC) to correct
                                                                     errors caused by noise

QAM Technology                            Confidential & Proprietary Information of VeEX Inc.                             3
                                                              Digital TV vs. Analog TV


                 Effect of noise on Analog systems (gradually poorer C/N)




                  45dB C/N        35dB C/N                      25dB C/N                 20dB C/N


                 Effect of noise on Digital systems (gradually poorer MER)




                 25dB MER        18dB MER                   17dB MER                    16dB MER

        Noise has very little affect on Digital systems until the system fails completely.

QAM Technology                    Confidential & Proprietary Information of VeEX Inc.               4
                                                                                                     QAM
                                                                                        Constellation Diagram


                 Each box in the Constellation diagram contains one symbol
                            QAM64: 6 bits per symbol, 64 boxes

                           QAM256: 8 bits per symbol, 256 boxes




             Quadrant 4                                                                 Quadrant 1




             Quadrant 3                                                                 Quadrant 2




QAM Technology                    Confidential & Proprietary Information of VeEX Inc.                       5
                                                                            HFC Forward Path


                           QAM64 or QAM256 are commonly used

                               Modulation                 Std. Symbol                  Max data rate
                               type                       Rate (MHz)                   (Mbps)
                 Annex A       QAM64                      6.952                        41.4
                 (8MHz)
                 Annex A       QAM256                     6.952                        55.2
                                                                                       (220 max 4 channel
                 (8MHz)                                                                bonding)

                 Annex B       QAM64                      5.057                        38
                 (6MHz)
                 Annex B       QAM256                     5.361                        43
                                                                                       (160 max 4 channel
                 (6MHz)                                                                bonding)




QAM Technology                   Confidential & Proprietary Information of VeEX Inc.                        6
                                                                                     HFC Return Path
                                                                                                                  DOCSIS


                  Data-Over-Cable Service Interface Specifications (DOCSIS)
                             Reverse Path/Upstream Data Rate

                 DOCSIS         Bandwidth                    Modulation                     Max data rate
                                (MHz)                        type                           (Mbps)
                 1.0            3.2                          QPSK                           5.12

                 1.1            3.2                          QPSK                           5.12
                                                             QAM16                          10.24
                 2.0            6.4                          QAM16                          10.24
                                                             QAM64                          30.72
                 3.0            6.4                          QAM64                          120
                                                                                            (4 channel bonding)
                                                             QAM128

          Standard symbol rate (bandwidth): 1.28 (1.6), 2.56 (3.2), 5.12 (6.4)MHz

QAM Technology                        Confidential & Proprietary Information of VeEX Inc.                              7
                                     Measuring Analog Channels


                                                                             More
                                                                           advanced
   Video and                                       Adjacent
                 Carrier to                                                  meter
  audio signal                                    channel and
                  Noise                                                    measures
     levels                                          HUM
                                                                           CSOs and
                                                                             CTB




QAM Technology       Confidential & Proprietary Information of VeEX Inc.              8
                                                Measuring Digital Channels
                                                                                    Easy?

   Signal Level, MER
   Checks for Pre and Post FEC errors = 0




QAM Technology                Confidential & Proprietary Information of VeEX Inc.       9
                                                                                                Tiling
                                                                                    What is the Problem?




What does the signal level meter and
spectrum analyzer tell us about the digitally
modulated signal on Channel 93 (639MHz)?

 The average power level is +4.6 dBmV
 The “Haystack” looks OK
 Hmmm, must be the STB!


QAM Technology                Confidential & Proprietary Information of VeEX Inc.                     10
                                                                                 What’s Missing?

   While a signal level meter and conventional spectrum analyzer are valuable
    tools, they do not tell the whole story about the health of downstream and
    upstream digitally modulated signals
   How, then, can one “look inside” the Haystack to see what’s going on?




                                              ?


QAM Technology                 Confidential & Proprietary Information of VeEX Inc.             11
                                                                                              QAM Analyzer

   QAM Analyzers support a suite of sophisticated measurements

            Analog channel signal level                          Some instruments with DOCSIS
                                                                  Cable Modem can measure the
            Digital channel average power                        upstream channels of their:
            Constellation display                                Upstream transmit level
                                                                  IP Ping
            Modulation error ratio (MER)                         Trace Route
            Pre- and post-FEC bit error rate                     Web browser
                                                                  Throughput
            Adaptive equalizer graph                             VoIP, IPTV

           Some instruments support other                         More advanced instruments support
           measurements such as:                                  additional measurements such as:
            In-channel frequency response,                        Symbol rate error
             group delay                                           Frequency error
            Ingress or interference under the                     Un-equalized MER
             carrier                                               Echo margin
            Phase jitter                                          Noise margin
            Max amplitude change                                  Equalizer stress
            HUM                                                   ASI MPEG
            EVM                                                   MPEG analysis


QAM Technology                          Confidential & Proprietary Information of VeEX Inc.              12
                             QAM Analyzer Block Diagram



                                                                       ASI




QAM Technology   Confidential & Proprietary Information of VeEX Inc.         13
                                           Downstream Performance
                                                                                   QAM Analyzer




MER
64-QAM: 27 dB min
256-QAM: 31 dB min

Pre- and post-
FEC BER




                                                                   Constellation



QAM Technology       Confidential & Proprietary Information of VeEX Inc.                     14
                                       Downstream Performance
                                                                       Pre/Post-FEC BER




                                        In this example, digital channel
                                        power, MER and constellation are
                                        fine, but pre- and post-FEC BER
                                        indicate a problem—perhaps Sweep
                                        transmitter interference, downstream
                                        laser clipping, an upconverter
                                        problem in the headend, or a loose
                                        connection.


QAM Technology   Confidential & Proprietary Information of VeEX Inc.                 15
                                                                 Modulation Error Ratio
                                                                                             Modulation Quality


   Modulation error = Transmitted symbol – Target symbol
                                         Q



                                                                                       Modulation error
                                          Target symbol
                                                                                 Transmitted (or received)
                                                                                          symbol


                                                                                              I




       Source: Hewlett-Packard


QAM Technology                   Confidential & Proprietary Information of VeEX Inc.                         16
                                                                 Modulation Error Ratio


MER = 10log (avg symbol power/avg error power)                                               N 2 2 
                                                                                                  I 
                                                                                             j j 
                                                                                                  Q
                                                                                                  
                                                                                         10 
                                                                                         log         
                                                                                               j1
                                                                                       MER 10 N
                                                                                              2
                                                                                                    
                                                                                                    2
                                                                                              j j
                                                                                              I Q
                                                                                              
                                                                                                     
                                                                                                     
                                                                                             j1      
  Q
                                                                            Q


                                                                                             A large “cloud” of
                                Average error                                                symbol points means
                                                                                         I
                                                                                             low MER—this is not
                                power                                                        good!



                    Average symbol                                          Q

                        power                                                                A small “cloud” of
                                                                                             symbol points
                                                                                         I   means high MER—
                                     I                                                       this is good!

      Source: Hewlett-Packard

QAM Technology                   Confidential & Proprietary Information of VeEX Inc.                               17
                                                                     Constellation Display


                 Poor CNR or Low MER                                       I-Q Imbalance




QAM Technology                  Confidential & Proprietary Information of VeEX Inc.        18
                                                                      Constellation Display


                 Phase Jitter/Noise                                    Coherent Interference




QAM Technology                   Confidential & Proprietary Information of VeEX Inc.           19
                                                                     Constellation Display


                                                                   Gain Compression
                 Gain Compression                                Upstream Laser Clipping




QAM Technology                  Confidential & Proprietary Information of VeEX Inc.        20
                                                                        Constellation Display


                 Quadrature Distortion                                         Zoom Function




QAM Technology                     Confidential & Proprietary Information of VeEX Inc.         21
                                                               Linear Distortions



                       Equalizer graph

                         In-channel
                    frequency response
                          In-channel
                          group delay




                 Un-equalized-equivalent
                  constellation and MER




QAM Technology   Confidential & Proprietary Information of VeEX Inc.            22
                                                                                          Linear Distortions


                                                              Micro-reflection at about 2.5 µs (2500 ns):
                                                              Assume ~1 ns per ft, 2500/2 = 1250 ft
                                                              (actual is 1.17 ns per ft: (2500/1.17)/2 = 1068 ft)




                 Frequency response ripple ~400 kHz p-p:
                 Distance to fault = 492 x (.87/.400) = 1070 ft



QAM Technology                              Confidential & Proprietary Information of VeEX Inc.                     23
                                                                                          Linear Distortions
                                                                                                  In-Depth Understanding

      ECHO MARGIN
      The Coefficients of the Equalizer will also reveal the presence of an Echo (a.k.a. micro-reflections).
      The Equalizer will cancel such an echo, and in doing so, the Equalizer coefficient which corresponds
      to the delay of the Echo will be much higher than the surrounding ones -- “it sticks out of the grass”.
      The relative amplitude of this Coefficient is an indication of the seriousness of the Echo, and its
      position gives the delay of the Echo, hence its roundtrip distance. The Echo Margin is the smallest
      difference between any Coefficients and a template defined by Cablelabs, as a safety margin before
      getting too close to the “cliff effect”. It is normal to notice relatively high Coefficients close to the
      Reference as this corresponds to the filters in the modulator/demodulator pair and to the shape of
      QAM signal.

      EQUALIZER STRESS
      The Equalizer Stress is derived from the Equalizer Coefficients and indicates how much the
      Equalizer has to work to cancel the Linear distortions -- it is a global indicator of Linear distortions.
      The higher the figure, the less stress.

      NOISE MARGIN
      We all know that the lower the MER, the larger the probability of errors in transmission (Pre-FEC and
      then Post-FEC); the MER degrades until errors are so numerous that adequate signal recovery is no
      longer possible (cliff effect). Since Noise is a major contributor to the MER, we define Noise Margin
      as the amount of Noise that can be added to a signal (in other words, how much we can degrade
      MER) before getting dangerously close to the cliff effect. Noise is chosen because on the one hand it
      is always present, and on the other hand it is mathematically tractable. Other impairments, such as
      an Interferer, are not easily factored into error probabilities.


QAM Technology                              Confidential & Proprietary Information of VeEX Inc.                       24
                                                                                         Linear Distortions
                                                                                                 In-Depth Understanding


      EQUALIZED MER vs. UN-EQUALIZED MER
      The Modulation Error Ratio (MER) is the ratio of the QAM signal to Non-Linear distortions of the
      incoming QAM signal. The MER should have included the Linear distortions to indicate the health of
      the signal; but the QAM demodulator cannot operate properly without the Equalizer and the Equalizer
      uses the MER as a tool to adaptively cancel the Linear distortions. Consequently it is convenient to
      distinguish the MER (non-Linear distortions only) from an Un-equalized MER (non-Linear and Linear
      distortions). The Un-equalized MER is calculated from the MER and Equalizer Stress.
      The Un-equalized MER is always worse than the MER. A small difference between the two indicates
      little Linear distortions, while a large difference shows that there are strong Linear distortions. Even if
      the Linear distortions are cancelled by the Equalizer, we have to keep in mind that the Equalization is a
      dynamic process as it tracks Linear distortions by trial and error even after converging. The larger the
      Linear distortions, the larger the tracking transients are, hence more probability of transmission error
      (Pre-FEC or Post-FEC BER).

      PHASE JITTER
      Phase Jitter is caused by instability of the carrier of the QAM signal at the demodulator. This instability
      could be found at the QAM modulator and up-converter or in the QAM receiver (Local Oscillators used
      in frequency conversions). Phase Jitter introduces a rotation of the constellation, where the symbol
      clusters elongate and get closer to the symbol’s boundary. Eventually some symbols will cross the
      boundary and cause an error in transmission. The QAM demodulator has a Phase lock loop to track
      phase variations of the carrier; it easily tracks long term drift as well as some short terms variations (up
      to 10 or 30kHz), but it cannot track very fast variations above its loop response. So in a QAM
      demodulator, the wideband Jitter is more damageable than short term Jitter.



QAM Technology                             Confidential & Proprietary Information of VeEX Inc.                       25
                                                                                            Linear Distortions
                                                                                                       Recommendations

                                    TABLE 1 DOCSIS SPECIFICATIONS, DOWNSTREAM

                                    Assumed Downstream RF Channel Characteristics
                                    DOCSIS Radio Frequency Interface Specifications
                 Parameter                                         Value
                 Carrier-to-noise ratio in a 6MHz band             Not less than 35 dB
                 Carrier-to-composite triple beat distortion       Not less than 41 dB
                 ratio
                 Carrier-to-composite second order                 Not less than 41 dB
                 distortion ratio
                 Carrier-to-any other discrete interference        Not less than 41 dB


                 Amplitude ripple                                  3 dB within the design bandwidth
                 Group delay ripple in the spectrum                75 ns within the design bandwidth
                 occupied
                 Micro-reflections bound for dominant echo         -10 dBc    @ <= 0.5 µs
                                                                   -15 dBc    @ <= 1.0 µs
                                                                   -20 dBc    @ <= 1.5 µs
                                                                   -30 dBc    @ > 1 .5 µs

                 Carrier hum modulation                            Not greater than -26 dB (5%)



QAM Technology                                Confidential & Proprietary Information of VeEX Inc.                   26
                                                                                            Linear Distortions
                                                                                                    Recommendations

                                       TABLE 2 DOCSIS SPECIFICATIONS, UPSTREAM
                                     Assumed Upstream RF Channel Characteristics
                                    DOCSIS Radio Frequency Interface Specifications
                 Parameter                                         Value
                 Carrier-to-interference plus ingress ratio        Not less than 25 dB
                 Amplitude ripple                                  0.5 dB/MHz
                 Group Delay ripple                                200 ns/MHz
                 Micro-reflections bound for dominant echo         -10 dBc @ <= 0.5 µs
                                                                   -20 dBc @ <= 1.0 µs
                                                                   -30 dBc @ > 1 .5 µs

                                                TABLE 3 SCTE STANDARDS
                                   Digital Channels RF Transmission Characteristics
                                                        SCTE 40 2004
                 Carrier-to-noise ratio, C/(N+I), in a 6MHz   Not less than:
                 band where C/(N+I) includes the              27 dB for 64QAM
                 simultaneous presence of all additive        33 dB for 256QAM
                 impairments in the 6MHz channel
                 bandwidth including CTB, CSO, other
                 discrete interference
                 Phase Noise                                  < -88 dBc/Hz @ 10kHz offset (relative to
                                                              the center of the QAM spectrum)

QAM Technology                                Confidential & Proprietary Information of VeEX Inc.                27
                                                                  Operational RF Levels


          DOCSIS recommends that the
       average power level of the digitally
       modulated signal be set at 6 dB to
       10 dB below what the visual carrier
        level of an analog TV channel on
          the same frequency would be.




          This ratio should be maintained
            throughout the entire cable
                      network.




QAM Technology                   Confidential & Proprietary Information of VeEX Inc.   28
                                                              Upstream Performance
                                                                                      Cable Modem

   Step-by-step Cable Modem link up process to clearly identified failed step.
   After link up, power level on forward and return paths are measured.




QAM Technology                  Confidential & Proprietary Information of VeEX Inc.            29
                                               Upstream Performance
                                                                       Cable Modem IP



                                                  Proper IP connection/throughput
                                                  should be verified at the Cable
                                                  Modem service location.




QAM Technology   Confidential & Proprietary Information of VeEX Inc.                30
                                                                                           Troubleshooting
                                                                                            Integrated Upconverter

   Verify correct average power level
       Integrated upconverter RF output should be set in the DOCSIS-specified +50 to +61 dBmV
        range
       Typical levels are +55 to +58 dBmV

   Also check BER, MER, and Constellation


                                    CMTS



                                                              To headend downstream
                                                                     combiner

                                                            Attenuator
                                                           (if required)

                            88-860 MHz downstream
                                   RF output
                            (+50 dBmV to +61 dBmV)




QAM Technology                       Confidential & Proprietary Information of VeEX Inc.                        31
                                                                                                  Troubleshooting
                                                                                                    External Upconverter

   Verify correct average power level, BER, MER, and Constellation
       CMTS downstream IF output
       External upconverter IF input
       External upconverter RF output



                                 CMTS

                                                       44 MHz IF input to
                                                           upconverter
                                                     (typ. +25 dBmV to +35                            88-860 MHz downstream
                                                             dBmV)                                   RF output to CATV network
                                                                                                      (+50 dBmV to +61 dBmV)
                                                 Attenuator                 RF upconverter
                   44 MHz downstream
                          IF output
                 (e.g., +42 dBmV +/-2 dB)




QAM Technology                              Confidential & Proprietary Information of VeEX Inc.                                  32
                                          Combiner Output and Fiber Link

   Check signal levels and BER at downstream laser input and node output
       Bit errors at downstream laser input but not at CMTS or upconverter output may indicate
        Sweep transmitter interference, loose connections, or combiner problems
       Bit errors at node output but not at laser input are most likely caused by downstream
        laser clipping




QAM Technology                       Confidential & Proprietary Information of VeEX Inc.        33
                                                                                      In the field…

   If everything checks out OK at the node, go to the affected subscriber’s
    premises.
   Measure downstream RF levels, MER and BER, and evaluate the constellation
    for impairments. Look at the adaptive equalizer graph, in-channel frequency
    response and group delay. If your QAM analyzer supports it, repeat these
    measurements in the upstream.
   Measure upstream transmit level and packet loss.
   Use the “divide-and-conquer” technique to locate the problem.




QAM Technology                  Confidential & Proprietary Information of VeEX Inc.               34
                                              Measurement & Troubleshooting
                                                                                                            Summary


      Constellation display                                        Linear distortions
                Low MER or CNR                                               Adaptive equalizer graph
                Phase noise                                                  In-channel frequency response
                I-Q imbalance
                                                                              In-channel group delay
                Coherent interference
                 (ingress, beats)                                             Constellation display (Unequalized)
                Gain compression                                             MER (Unequalized)
                Laser clipping                                     Transient impairments
                Sweep transmitter interference
                                                                              Pre- and post-FEC BER
      Pre- and post-FEC BER                                                  Constellation display zoom function
                Sweep transmitter interference
                                                                              Upstream packet loss
                Laser clipping
                Loose connections                                  Signal level problems
                Low MER or CNR                                               Analog TV channel signal level
      Equalizer graph                                                        Digital channel power
                Micro-reflections                                            Upstream transmit level
                                                                              Constellation display


QAM Technology                            Confidential & Proprietary Information of VeEX Inc.                        35
Thank you.
Any questions?




QAM Technology   Confidential & Proprietary Information of VeEX Inc.   36

								
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