VeEX VePal CX350, Руководство пользователя

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Modulation type (QPSK, QAM-16, QAM-64, QAM-128)
Digital Carrier Level in dBmV or dBuV
Modulation Error Ratio (MER) in dB
Pre-FEC rate
Post FEC rate
Pre Errored Seconds count
Post Errored Seconds count
Severely Errored Seconds count
QAM Constellation (see below)
Return Path QAM Constellation Measurement
Interpreting the Constellation Diagram
In a constellation diagram, all the possible combinations of “I” and "Q" produce a “grid” making them easier to interpret. The constellation diagram can be thought of as an
array of “boxes”, with each box representing one state or “symbol”.
Perfect data transmission conditions will result in each received bit landing right in the center of its “box”, or the “nominal” position for that particular symbol. In the real
world, noise, ingress and reflections push the bit away from its nominal or optimal position, towards the boundary of an adjacent box.
The boundary is called the “Decision Threshold”. If a signal disturbance pushes a symbol across this Threshold, it becomes a “bit error”. Symbols that are not disturbed
enough to be pushed across the Threshold are always interpreted correctly.
A constellation diagram, therefore, is a good troubleshooting aid which provides valuable clues concerning the source and nature of a disturbance.
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7.8.1.3 RP Measurement and Troubleshooting:
Reliable QAM transmisison in the return path requires attention to several details:
The Cable Modem Termination System (CMTS) configuration must be optimized for QAM modulation.
The entire cable network—headend, distribution network, and subscriber drops, must be DOCSIS-compliant.
Recommended forward and reverse performance parameters are detailed in the DOCSIS Radio Frequency Interface Specification
http://www.cablemodem.com/specifications
Verify that the upstream digitally modulated carrier amplitude at the input to the CMTS upstream port is within specification - typically 0dBmV
Due to the bursty nature of upstream digitally modulated carriers, it is difficult to measure the average power level - switch the spectrum analyzer to zero-span to
obtain an accurate amplitude measurement
Check the upstream carrier-to-noise, carrier-to-ingress, and carrier-to-interference ratios. The DOCSIS specification assumes a minimum of 25dB for all three
parameters measured at the CMTS upstream port.
Group delay may be a problem even when frequency response is flat. Group delay is worse near band edges and diplex filter roll-off areas.
Verify MER is within acceptable limits. Depending on the CMTS brand, QAM-16 and QAM-64 modulated signals typically require a minimum MER of 17~20dB.
Check for Microreflections which are caused by impedance mismatches:
Connectors, amplifiers inputs and outputs, passive device inputs and outputs, and even the cable itself
Upstream cable attenuation is lower than downstream cable attenuation, so upstream micro-reflections tend to be worse
Micro-reflections and group delay are compensated by using adaptive equalization. Use the adaptive equalizer stress graph to identify these problems.
A typical DOCSIS operating system is depicted in the diagram below. In summary:
DOCSIS cable modems have a maximum output level of + 58.0dBmV when operating in QPSK mode, + 55.0dBmV in QAM-16 and +54dBmV in QAM-64 modes.
The CMTS located at the Headend requires a signal level of 0dBmV.
The maximum link loss without any amplification is 58.0dB with QPSK or 55.0dB with QAM-16.
Cable modems should never operate at their maximum output because that would put the system at risk when signal degradations are present.
The level of the return signal (5 to 65 MHz) should be 15.0dBmV at the input of all amplifiers and NODES on the system.
The desired 15.0dBmV input signal level should include the signal loss created by the RF couplers, the loss of the drop cables (RG-6 or RG-59), the loss of the
multi-tap and some headroom at the output signal of the cable modem.
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7.8.2 Sweep Operations
Overview:
Sweep is used to identify improper balance of amplifiers, tilts, or suck outs.
Tilt and Suck Out
CX350 e-Manual D07-00-037 RevC01 Page 43 of 81