Date: August 23,999 Dist'n: T1E1.4

Transcription

1 08/0/99 1 T1E1.4/99-49 Projet: T1E1.4: VDSL Title: Filtering elements to meet requirements on power spetral density (99-49) Contat: G. Cherubini, E. Eleftheriou, S. Oeler, IBM Zurih Researh Lab. Saeumerstr. 4, CH-8803 Rueshlikon, SWITZERLND phone: fax: bi@zurih.ibm.om, ele@zurih.ibm.om, oel@zurih.ibm.om J. Cioffi, Dept of EE, Stanford U., Stanford, C ioff@stanford.edu, phone: , fax: M. Sorbara, Globespan Tehnologies In., 1E Shulz Dr., Red Bank, NJ msorbara@globespan.net, , F: Date: ugust 3,999 Dist'n: T1E1.4 bstrat: This ontribution addresses filtering requirements to meet the ETSI and NSI speifiations on transmit signal power spetral density (PSD) for VDSL systems employing multiarrier modulation tehniques. It is shown that the speifiations an be met with simple fixed filtering elements. Conformane with the various PSD masks is ahieved by proper adjustment of the digital gains at the input of eah subhannel. Suh filter design is generally diffiult for QM systems unless they have a suffiient number of subhannels. NOTICE This ontribution has been prepared to assist Standards Committee T1 - Teleommuniations. This doument is offered to the Committee as a basis for disussion and is not a binding on any of the ompanies listed as authors. The requirements are subjet to hange after further study. The authors speifially reserve the right to add to, amend, or withdraw the statements ontained herein.

2 08/0/99 T1E1.4/99-49 Filtering elements to meet requirements on power spetral density (99-49) G. Cherubini E. Eleftheriou S. Oeler IBM Zurih Researh Lab. Phone: Fax: J. Cioffi Dept. EE Stanford University Stanford, C Phone: Fax: ioffi@stanford.edu M. Sorbara Globespan Teh., 1E Shulz Dr. Red Bank, NJ P: F: msorbara@globespan.net 1. Introdution The ETSI and NSI speifiations [1] and [] define various masks to limit the power spetral density (PSD) of transmitted VDSL signals. These spetral masks vary aording to the loation of the line termination modem (FTTCab or FTTEx) sine that determines the tolerable rosstalk and hene the allowable PSD levels for both the line termination and the network termination. Moreover, VDSL modems must be able to redue the PSD level of the transmitted signals to below -80 dbm/hz within the internationally standardized amateur radio bands. This operation, also known as nothing, is provided to prevent VDSL systems from beoming a soure of signifiant interferene for existing systems that utilize that portion of the spetrum. In this ontribution, we investigate the problem of meeting the ETSI/NSI speifiations on transmit PSD for VDSL modems employing multiarrier modulation tehniques. We show by way of an example that simple fixed filter setions are suffiient and onformane with the various PSD masks an be ahieved by proper adjustment of the digital gains at the input of eah subhannel. We treat the ases of DMT modulation, with and without windowing [3], as well as FMT [4] modulation. The main onlusion of the ontribution is that multiarrier modulation tehniques do not need omplex variable or swithable filter setions, neither in the analog nor in the digital domain, to satisfy the spetral mask onstraints.. System model The blok diagram of the transmitter of a multiarrier system is depited in Figure 1. The M modulation symbol streams k, i = 0, 1,, M-1, are multiplied by digital gains in parallel into the multiarrier modulator at the modulation rate of 1/T. The gains g and input g are usually employed to approximate the optimum, hannel-apaity ahieving PSD for the transmitted signals [5], subjet to a onstraint that a given PSD mask must not be exeeded. This latter aspet of the problem is of onern in this ontribution.

3 08/0/99 3 T1E1.4/99-49 The signal x n at the modulator output is generated at the rate of 1/T C, in general upsampled by a fator L, and input to a digital interpolation filter with frequeny response C D ( f) before being onverted to an analog signal by a digital-to-analog () onverter. The onversion to a stairase signal by a zero-order hold onverter orresponds to a filtering operation by a filter with frequeny response H = (T /L) sin( πft /L)/( πft /L). The onverter output signal is filtered by an analog transmit filter with transfer funtion G ( f) and input to the hannel. The power spetral density of the transmitted signal is given by PSD = 1 T PSD x C D H G, (1) where PSD x ( f), the power spetral density of the signal at the output of the modulator, an be expressed as PSD x M-1 i = 0 σ ( g ) H(f - i. () In this equation, σ denotes the variane of the symbol sequene transmitted on the ith subhannel and Df the spaing between the arriers. The expression of the frequeny response H depends on the partiular multiarrier sheme that is adopted. We onsider the following ases: - DMT: - Windowed DMT: sin( πf/ H π f/ f sin( πf/ H πf/ f os( απf/ 1 (αf/ (3) (4) - FMT: H 1 + e 1 + ρ e 0 -jπf/ f - jπf/ f if f f/ otherwise. (5) 3. Numerial results

4 08/0/99 4 T1E1.4/99-49 We now illustrate by way of an example how PSD requirements an easily be met with multiarrier modulation by using simple fixed transmit filters and by properly adjusting the digital gains for the subhannels. We onsider the PSD mask defined as Pex.D3.LT.M1 in [1]. This mask is hosen for the line termination in the FTTEx senario in the ase that no baseband servies are provided on the same wire pair, and baseband servies as well as DSL over ISDN are present in the bundle. Moreover, nothes are provided in the amateur radio bands as explained in the introdution. For the example, we assume a asade of digital interpolation filter, onverter, and analog transmitter front-end filter suh that the amplitude harateristi of the overall frequeny response is given by C D H G 1 sin( πft ) = T. (6) N 1 + (f/f ) πft 0 We note that (6) is ahieved by using Butterworth filtering of order N. The amplitude harateristi shown in Figure orresponds to (6) with N = 8, T = 1/ f 0 = 1/.08 µ s, and is obtained without digital interpolation filter. Figures 3 and 4 illustrate the PSD of the transmitted signals for the hoies of the frequeny response H given by (3) and (5). The PSDs have been obtained assuming (6) and proper adjustment of the digital gains. measurement resolution bandwidth of 10 khz was employed. In Figures 3 and 4, the PSD mask Pex.D3.LT.M1 is also plotted for omparison purposes. The PSD obtained with H given by (4) is as expeted very similar to the one shown in Figure 3 and has therefore not been plotted. Similar results an be derived for the other masks speified in [1],[]. 4. Conlusion In this ontribution we showed that the PSD requirements set by ETSI and NSI for VDSL systems an be met with simple fixed filter setions at the transmitter. The orresponding filter design problem for QM systems is generally more diffiult unless they employ a suffiient number of subhannels. Referenes [1] ETSI-TM6, ETSI VDSL Speifiations (Part 1) Funtional Requirements, Contribution D.535(WP1/15), ITU-T Study Group 15, Geneva, Switzerland, June 1-July, [] Texas Instruments et al., Very-high-speed Digital Subsriber Lines System Requirements (T1E1/98-036R1), Temporary Doument PO-03, ITU-T Study Group 15, Portland, Oregon, 18- Jan [3] latel et al., Physial medium speifiation for G.vdsl, Temporary Doument M-050R1, ITU-T Study Group 15, Melbourne, ustralia, 9 Marh pril [4] IBM et al., CP/QM, DMT, and filter-bank modulation tehniques for VDSL, Contribution 99-39, NSI T1E1.4, Ottawa, June 7, [5] J.M. Cioffi, symmetrial digital subsriber lines, in The Communiations Handbook, J.D. Gibson (Ed.), CRC Press In., pp , 1997.

5 08/0/99 5 T1E1.4/99-49 (0) k (0) g (1) k (1) g Multiarrier modulator x n L C D G (M-1) k (M-1) g Figure 1: Blok diagram of multiarrier transmitter.

6 08/0/99 6 T1E1.4/99-49 Figure : Frequeny response of the asade of onverter and analog Butterworth transmit filter of order N = 8 [see equation (6)]. Figure 3: Referene PSD mask and PSD of transmitted signals for a DMT system with M = 4096.

7 08/0/99 7 T1E1.4/99-49 Figure 4: Referene PSD mask and PSD of transmitted signals for an FMT system with = 0.1 and M = 56.