VDSL is a transmission system conceived to operate on the last portion of the copper access network,where the physical lines are constituted by copper pairs (twisted or untwisted) whose physical properties are typically very poor.The physical channel is attenuation limited and depending on the chosen duplexing method and frame structure,the most common channel impairments are both self noise (Far End crossTalk-FEXT,Near End crossTalk-NEXT),crosstalk from other transmission systems,impulsive noise and noise induced by radio systems utilizing common frequency bands.In particular,the source of the radio noise is the unperfected balance of the copper pairs that convert the common mode radio signal onto a differential one that is perceived as true signal by the differential VDSL receiver.The same physical principle converting the radio common mode signal into a VDSL receiver.The same physical principle converting the radio common mode signal into a VDSL differential mode signal reverts to generate a common mode radio signal as a result of the conversion in the unbalanced copper pair of the VDSL differential signal.Both immunity and emission issues must be properly covered by the relevant international standard bodies: the main recommendations on emissions are contained in EN 55022 (for Class B operation) for Europe and Part 15 of Title 47 of the Code of Federal Regulation for USA.immunity aspects can be found in EN 58802-1 where the minimum set of requirements for residential and commercial equipment is shown.The concepts of transmitted power (Transmit PSD) and noise model and mask have been considered by the relevant standard committees (with reference to ETSI TM6 and ANSI T1E1.4 committees) and the draft resolutions are described in the draft standard documents (ETSI VDSL DTR.DTR/TM-06003 and ANSI T11.4/96-153R3).The reference value of + 10 dBm,measured over the frequency range DC to 30 MHz has been considered and two different Tx PSD masks have been suggested in ETSI,depending on whether the radio interference is strong or weak (these concept have not been yet precisely quantified.) Typically,a Tx PSD of -60 dBm seems appropriate in correspondence of those frequency bands not used by radio interfering systems while the value - 80 dBm seems appropriate in any other case.
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