下行信號比特誤碼率的測試方法

1、 Interface Practices Subcommittee AMERICAN NATIONAL STANDARD ANSI/SCTE 121 2006Test Method forDownstream Bit Error RateNOTICEThe Society of Cable Telecommunications Engineers (SCTE Standards are intended to serve the public interest by providing specifications, test methods and procedures that promo

2、te uniformity of product, interchangeability and ultimately the long term reliability of broadband communications facilities. These documents shall not in any way preclude any member or non-member of SCTE from manufacturing or selling products not conforming to such documents, nor shall the existenc

3、e of such standards preclude their voluntary use by those other than SCTE members, whether used domestically or internationally.SCTE assumes no obligations or liability whatsoever to any party who may adopt the Standards. Such adopting party assumes all risks associated with adoption of these Standa

4、rds, and accepts full responsibility for any damage and/or claims arising from the adoption of such Standards. Attention is called to the possibility that implementation of this standard may require the use of subject matter covered by patent rights. By publication of this standard, no position is t

5、aken with respect to the existence or validity of any patent rights in connection therewith. SCTE shall not be responsible for identifying patents for which a license may be required or for conducting inquiries into the legal validity or scope of those patents that are brought to its attention. Pate

6、nt holders who believe that they hold patents which are essential to the implementation of this standard have been requested to provide information about those patents and any related licensing terms and conditions. Any such declarations made before or after publication of this document are availabl

7、e on the SCTE web site at .All Rights Reserved© Society of Cable Telecommunications Engineers, Inc.140 Philips RoadExton, PA 19341TABLE OF CONTENTS1.0 SCOPE (12.0 DEFINITIONS AND ACRONYMS (23.0 NORMATIVE REFERENCES (34.0 INFORMATIVE REFERENCES (35.0 TEST SETUP (46.0 PROCEDURE

8、(9APPENDIX A: TEST INTEGRATION TIME (11APPENDIX B: ERRORED AND ERROR-FREE SECONDS (13LIST OF FIGURESFIGURE 1 QAM ONLY DISPLAY 7 FIGURE 2 QAM & AWGN DISPLAY 7 FIGURE 3 DOWNSTREAM BER TEST DIAGRAM 61.0SCOPEThe purpose of this test is to measure Bit Error Rate (BER of downstream (forward path broad

9、band telecommunications QAM signals. This procedure will address mainly pre-Forward Error Correction BER results for 64 and 256 QAM.It is important to rely upon both the pre-FEC and post-FEC BER results. In NorthAmerica, most 64 and 256QAM signals incorporate Annex B, FEC encoding. It istherefore im

10、portant to reference the post-FEC result, but extremely dangerous to relysolely upon it. BER suffers from what is commonly known as a “cliff effect” in whichthe post-FEC BER remains error free up to a point and then begins to rapidly decline.Referring to the pre-FEC BER result can yield insight as t

11、o how much margin is available prior to reaching the cliff.Although ITU J.83 Annex B is the standard of choice for forward error correction inNorth America, it can lead to an overoptimistic pre-FEC BER result. When utilizing ITU J.83 Annex B, the pre-FEC measurement is after the Trellis decoder, but

12、 before the Reed Solomon decoder. Therefore, the Annex B pre-FEC test results will include theimprovements of the Trellis coding.ITU J.83 Annex C is similar to Annex B in that it relies on Reed Solomon ErrorCorrection and occupies a 6MHz channel; however, Annex C does not incorporate Trellis coding.

13、 Since Annex C does not contain Trellis coding, it allows the test to yield a result closer to the true pre-FEC performance. Annex C is however, not likely to be the FECencoding used in the field in North America.This test is a static measurement at a single, specified DUT (Device Under Test inputle

14、vel. It does not provide performance over an input operating range. Also, the user will have the option of either testing with Annex B or Annex C. The final test results mustclearly indicate which standard was utilized.In addition to a Bit Error Rate measurement, Appendix B refers to the parameters,

15、Errored and Error-Free Seconds, which can be a useful tool in determining theimpairments resulting in the bit errors.Appendix A: TEST INTEGRATION TIMEAppendix B: ERRORED AND ERROR-FREE SECONDS2.0DEFINITIONS AND ACRONYMS2.1Adaptive Equalizer Real-time signal compensation for the linear distortiongrou

16、p delay.2.2Bit Error Ratio (BER - The number of erroneous bits divided by the totalnumber of bits transmitted, received, or processed over some stipulated period.2.3Errored Second A one second period with one or more Errored blocks.2.4Forward Error Correction (FEC - A system of error control for dat

17、atransmission wherein the receiving device has the capability to detect and correctany character or code block that contains less than a predetermined number ofsymbols in error. ITU J.83 Annex B (North America standard FEC is composedof four processing layers2.5Interleaving Evenly disperses the symb

18、ols, protecting against a burst of symbolerrors being sent to the RS decoder.2.6International Telecommunications Union An international organizationcoordinate global telecom networks and services.2.7ITU-T J.83 T refers to the Telecommunications Standardization sector of theITU. Series J applies to t

19、he transmission of television, sound programme andother multimedia signals. Recommendation J.83 is specific to digital multi-programme systems for television, sound and data services for cable distribution.2.8Quadrature Amplitude Modulation (QAM - Method of combining twobandwidth. In a QAM signal, t

20、here are two carriers, each having the samefrequency but differing in phase by 90 degrees (one quarter of a cycle, fromwhich the term quadrature arises. One signal is called the I (in phase signal, andthe other is called the Q (quadrature signal. Mathematically, one of the signalscan be represented

21、by a sine wave, and the other by a cosine wave. The twomodulated carriers are combined at the source for transmission. At thedestination, the carriers are separated, the data is extracted from each, and then thedata is combined to recreate the original modulating information.2.9Randomization Randomi

22、zes the data on the channel to allow effective QAM2.10Reed-Solomon Coding Provides block encoding and decoding to correct onesymbol for each pair of R-S symbols within an R-S block (correction of up to 3symbols in J-83 Annex B and up to 8 symbols in Annex C.2.11Symbol Rate The number of symbols tran

23、smitted over a give time.2.12Trellis Coding Increases the symbol constellation (redundancy withoutincreasing the symbol rate or power spectrum. The redundancy allows for animproved Signal-to-Noise threshold.3.0NORMATIVE REFERENCESThe following documents contain provisions, which, through reference i

24、n this text,constitute provisions of this standard. At the time of publication, the editions indicatedwere valid. All standards are subject to revision, and parties to agreement based on thisstandard are encouraged to investigate the possibility of applying the most recent editions of the documents

25、listed below.3.1ITU-T Recommendation J.83 (1997, Series J: Transmission of Television, SoundProgramme and other Multimedia Signals. Digital Transmission of TelevisionSignals.4.0INFORMATIVE REFERENCESThe following documents may provide valuable information to the reader but are notrequired when compl

26、ying with this standard.4.1NCTA Engineering Committees Subcommittee for Standards of GoodEngineering Practices, 2002. NCTA Recommended Practices for Measurementson Cable Television Systems, Third Edition Washington, DC: NCTA, 2002.4.2Edgington, Francis and Thomas, Jeffrey. Digital Basics For Cable T

27、elevisionSystems. Upper Saddle River: Prentice Hall PTR, 1999.5.0TEST SETUP5.1Test Equipment List5.1.1Digital modulator(s. Quantity to be determined by the amount ofchannel loading being tested.64 and/or 256 QAMITU J.83(B and/or ITU J.83(CVariable Interleaver controls5.1.2RF up-converter(s. Quantity

28、 to be determined by the amount ofchannel loading being tested.Variable from 54 to 860MHz, minimumAbove 860MHz desirable5.1.3Simulated Digital Loading (optionalAside from the QAM channels under test, the remaining digitalloading may be simulated digital loading vs. actual QAM channelsAWGN GeneratorV

29、ariable output power or incorporate anexternal variable attenuatorAppropriate bandpass (high/low pass filter to shape the noiseloading in accordance with the specified channel plan16dB minimum return loss, 75ohm+/- 10MHz from edge frequency, >40dBrejection+/- 0.25dB flatness< 1.5dB insertion l

30、ossNotch filters (channel deletion filtersCenter frequency = Center frequency of testchannel16dB minimum return loss, 75ohm6MHz wide, >50dB rejection+/- 2MHz from bandedge, <3.0dB insertion loss+/- 0.25dB flatness in the passband< 1.5dB insertion loss5.1.4One lab amplifier (optionalGain as

31、needed in order to meet the DUT specified input, minimaldistortion or noise contribution75ohm5.1.5One variable, RF attenuator16dB minimum return loss, 75ohm5.1.6One multi-channel signal generatorCapability to produce signals on all the nominal visual carrierfrequencies for all the channels in the fr

32、equency band to be tested Ability to adjust the power of each individual channelAbility to turn individual channels on/off.CSO, CTB, or spurious signals generated within the signalsource(s should be at least 20dB below those of the DUTFor testing with non-coherent carrier frequencies, the capability

33、 tomaintain individual non-coherent frequencies to within +/- 5kHz ofthe nominal carrier frequencies.5.1.7Two RF combiners, may be RF splitter/combiner or a directionalcoupler.Minimum isolation of 20dB, 75ohm5.1.8One 6MHz wide, bandpass filter with the center frequency set to thatof the channel unde

34、r test+/- 10MHz from center frequency, > 25dB rejection75ohm, 18dB minimum return lossMay not be required depending on the model ofdemodulator/analyzer used.5.1.9One QAM analyzer.64 and 256 QAM capabilitiesITU J.83(B and/or ITU J.83(CQAM analyzer should be able to display pre-FEC and post-FECBER

35、resultsNote: The QAM analyzer pre-FEC results are to be strictly basedon an errored bit count relative to the total # of transmitted bits. Itis not to be derived based on a theoretical value relative to asignal to noise measurement.5.1.10Spectrum Analyzer (optional5.2Follow all calibration requireme

36、nts recommended by the manufacturers of the testequipment, including adequate warm-up and stabilization time.5.3Setup Verification5.3.1Connect the equipment as depicted in Figure 1. Bypass the DUT witha straight coaxial connection from the combined analog and digital tothe input of the bandpass filt

37、er. requiredFigure 1 Downstream BER Test Diagram5.3.2Digital ModulatorSet the modulation to the proper format (i.e. 64 or 256 QAMSet the Symbol Rate accordingo Annex Bo64 QAM 5.056941Mspso256 QAM 5.360537Mspso Annex Co64 QAM 5.309734Mspso256 QAM 5.309734MspsInformation bit rate / Transmitted bit rat

38、eo Annex Bo64 QAM 26.97035Mbps / 30.342Mbpso256 QAM 38.81070Mbps / 42.8843Mbpso Annex Co64 QAM 29.359708Mbps / 31.644Mbpso256 QAM 39.146278Mbps / 42.192MbpsSet the Interleaver accordinglyo Annex Bo64 QAM I=128, J=4o256 QAM I=128, J=4o Annex Co64 QAM I=12, J=17o256 QAM I=12, J=17Set the Roll-off Fact

39、or accordinglyo Annex Bo64 QAM 18%o256 QAM 12%o Annex Co64 QAM 13%o256 QAM 13%5.3.3RF UpconverterSet the RF center frequency as required per the channel plano As a minimum, the first and last digital channel performanceshould be verified for BER performance.The RF output level should be adjusted suc

40、h that all digitalchannels are at an equal level as indicated in Figure 2. Figure 2 QAM only Display5.3.4Simulated Digital Loading (optionalWhen using a combination of Simulated Digital Loading (AWGN and QAM channels, the actual QAM channel will appear to behigher in level than the SDL when viewed o

41、n a spectrum analyzer.This is due to the fact that the total channel power is within a5.06MHz (64 QAM, Annex B or 5.36MHz (256 QAM, Annex Bspectrum versus the 6MHz of the SDL. The total power within a6MHz channel is to be equal. This may be accomplished bysetting the spectrum analyzer to measure pow

42、er per 6MHz channelor measuring in power/Hz.Figure 3 QAM & AWGN Display5.3.5Step AttenuatorAdjust the step attenuator in order to achieve the DUT specified input for the digital channel loading relative to the analog input. 5.3.6Multi-channel generatorEach channel to be operated as a CW tone (Co

43、ntinual Wave at its specified operating level.Set the output to provide all of the signals need for the applicable frequency plan.The analog carrier frequencies for noncoherent frequency plans should be randomly dispersed with a +/-5 kHz about the nominalvisual carrier frequency, in order to obtain

44、the most stable,repeatable result.5.3.7Input the combined analog and digital channel loading to the 6MHzbandpass filter. Tune or select the center frequency of the filter to thecenter frequency of the channel under test.5.3.8Input the filtered test channel to the QAM analyzer.Ensure that the input o

45、f the QAM analyzer is at the proper RF level. Refer to the manufacturers specification sheet.5.3.9QAM AnalyzerModeo J.83, Annex B or J.83, Annex CModulation formato64 or 256 QAMSet the Symbol Rate accordingo Annex Bo64 QAM 5.056941Mspso256 QAM 5.360537Mspso Annex Co64 QAM 5.309734Mspso256 QAM 5.3097

46、34MspsSaw Filtero6MHzSet the Roll-off Factor accordinglyo Annex Bo64 QAM 18%o256 QAM 12%o Annex Co64 QAM 13%o256 QAM 13%Adaptive Equalizero OnCenter frequency tuned to the channel under test5.3.10Set the QAM analyzer for a 1 minute averaged BER test. Enable themodulator forward error correction (Ann

47、ex B or Annex C.5.3.11Per the test equipment manual, reset the BER results and begin the 1minute averaged BER test. Ensure that both the post-FEC and pre-FECBER are error free.5.3.12If the test fails to meet error free performance, verify the setupconnections, the tuning of the Bandpass filter, prop

48、er RF levels,modulator/demodulator settings, as well as the QAM Analyzersettings. If pre-FEC error free performance is unobtainable, ensurethat the setup has sufficient performance margin (at least two orders ofmagnitude such that it will not impact the measurement of the DUT.Refer to the equipment

49、manufacturers manuals as needed.6.0PROCEDURE6.1Connect the DUT into the test setup as indicated in Figure 1. Verify that the DUTinput is per specification.6.2Adjust the levels of the DUT optical link and/or RF amplifier(s as specified.6.3Ensure that the input of the QAM analyzer is at the proper RF

50、level. Refer to themanufacturers specification sheet.6.4Set the QAM analyzer, for a 1 minute averaged BER test. Enable the modulatorforward error correction (Annex B or Annex C.Note: A sequential grouping of the individual 1-minute tests will be performed inorder to complete the overall test. See Ap

51、pendix A for a detailed explanation asto the overall integration time.6.4.1Per the test equipment manual, reset the BER results and begin the 1minute averaged BER test.6.4.2At the conclusion of the 1-minute test, record the pre-FEC BER resultsand immediately reset the BER measurement in order to beg

52、in the nextseries of the data compilation.Note: The accuracy of the pre-FEC BER results have been compromised onceany post-FEC bit errors have occurred. If any post-FEC errors are recorded,disregard the pre-FEC BER results.6.4.3Repeat 6.4.2 until the entire data set is completed. The final resultsmu

53、st clearly indicate whether Annex B or Annex C FEC encoding wasutilized.Note: Some test instruments can be set up to achieve minute by minute averageBER results during a multiple minute test. Provided the BER test sample periodis less than or equal to one minute, any sub-minute test period BER resul

54、ts canbe averaged in order to determine the average BER per each one minute timeperiod. For example, if BER is measured in one second samples, the average of60 contiguous samples would equal the average BER for a one minute period. Inthis case, the analyzer test duration is set for the total, desire

55、d test time. The one second samples are extracted and the individual 1-minute results are created at the conclusion of the entire test.Consult the test equipment manufacturers procedures for information on setting up the instrument.APPENDIX A: TEST INTEGRATION TIMEThe BER test integration time is me

56、rely the length of time that the test is allowed to run before taking a measurement or considering the test complete.Determining the test integration time (length of the test involves a couple of parameters. First, the length of the test is directly related to how small of a BER value is attempting

57、to be measured. For example:Assuming that the desired measurement is 1e-12 with a 256 QAM signal1 / (1e-12 = 1e12 bits1e12 bits / 42.8843Mbps (transmitted bit rate = 23.3186e3 seconds23.3186e3 seconds = 6.4774 hoursAssuming that the desired measurement is 1e-9 with a 256 QAM signal1 / (1e-9 = 1e9 bits1e9 bits / 42.8843Mbps = 23.3186 secondsThe next parameter to consider is an averaged BER measurement over 15 minutes, or longer, can mask a short duration anomaly by averaging its affects across the entire 15+ minute test. Linking several shorter tests together