Research On Noise And Clip That Impact On CM Signal And The Improvement Method Of Reverse Path Alignment

When bi-direction cable network carries out interactive data service, commmunicatingsignal ofen offline for excess reverse noise. The traditional solutions such asimproving construction quality, adding many filters in reverse path can not workwith satisfaction.These traditional solutions separate signal from reverse noise todiscuss ,neglect the reactive relationship of signal and reverse noise and the impacton both when reverse path being aligned by different method.The source and category of reverse noise are complicate. If the noise thataffect cable modem signal is nomalized, two kind noise---additive Gaussian whitenoise and fixed carrier noise can be singled out .Changing amplitude andfrequence of noise step by step at experiment path,it shows noise which insidesignal work frequence is the direct factor that impact reverse service indexes suchas sigal to noise ratio and success rate of ping docsis,and yet that noise whickoutside signal work frequence has no impact on service indexes.Through stimulating cable modem to critical state, the feature of all kinds ofnoise that generate laser clip can be made clear in detail,cable modem makealmost the same clip feature with fixed carrier noise.Thereof, it come into beingto reflect the traditional method called power per Hz based on noise power ratioof laser ,which used to decide the work level of cable modem.Then the utilitarianimprovement method for reverse path alignment is found out:1)the maximum lossof distruibution netwok and the maximum transmission level of cable modemdecide the work level of unit gain point;2)according to the laser clip point offixed carrier to decide input level range of cable modem into laser module;3)corroborating the output level of transceiver and the link loss betweentransceiver and cable modem terminal system in accordance with the maximumpath gain.Through comparing indexes such as sigal to noise ratio and carrier to noiseratio of traditional method with improvement method,it is confirmed thatimprovement method make sense.Using improvement method will simplify thetheory of aligning reverse path ,elevate the efficiency of everyday maintenance,improve the ability of cable modem signal that resist reverse noise.Thereby, indexes including carrier to convergence noise ratio,upsreamtransport gain difference and the upsream maximum distortion level depicted in"GY/T180-2001 technical specification of HFC network physical upstreamtransmission path" have to be revised.Upstream data packet,generated fromcable modem terminal system communicating with cable modem ,is a bettersignal resource for measuring carrier to noise ratio of reverse path ,because itmakes more measurable for RF indexes of reverse path. So the revisedspecification will have more propulsive value.