10Gbps Transmission over Standard Category-5 Copper Cable

Transcription

1 0Gbps Transmission oer Standard Category-5 Copper Cable Dr. Mohsen Kaehrad, Dr. John F. Doherty, Jun-Ho Jeong, Arnab Roy, Gaura Malhotra The Pennsylania State Uniersity Center for Information and Communiations Tehnology Researh (CICTR) Department of Eletrial Engineering Uniersity Par, PA 680 Tel.: (84) Abstrat - IEEE is interested in speifying e next generation 0Gbps opper able Eernet networ, termed 0000BASE-T, in e not too distant future. In searh of a proper solution, we are inestigating e inoled tradeoffs at eery design step. This paper presents oneptual designs of a 0,000BASE-T opper transmission system for distribution of a digital signal oer ~00 meters of a standard CAT-5 opper able. These designs mae it possible to ahiee is at a target aerage bit error rate (BER) of 0-0 after equalization and deoding, using standard CAT-5 able, if we adopt etored transmission on 4 pairs. This is done in presene of insertion-loss dispersion, NEXT, FEXT, Alien NEXT, Eho and all e assoiated noise. To ahiee is, e following tass hae been defined, MIMO anellation of all rosstal terms and iteratie etor equalization and deoding of e reeied signals. I. INTRODUCTION Unshielded twisted-pair (UTP) opper LANs proide an alternatie to e traditional LANs based on wireless and optial fiber. A opper LAN seres e same purpose as at of oer LANs: to enable exhange of information among deies attahed to e LAN. The maret for broadband LAN series seems set to ontinue growing at near exponential leels oer e next fie to ten years. Wi e inreasing popularity of multimedia series supplied oer a fixed networ, series suh as: web browsing, ideo onferening and ideo on demand, it is for sure only a matter of time before users will demand higher bandwid LAN aess. Adanes in signal proessing and fast CMOS proessing power hae also made it possible for users to afford highresolution isual series. The existing IEEE 000Base-T Gigabit Eernet [] operates oer e installed ategory 5 opper. This is a full duplex onfiguration, but unlie 00BASE-T, it uses all four opper twisted pairs in e able, simultaneously. Eah pair is responsible for supporting 50 Mbps of information. Multileel Pulse Amplitude Modulation (PAM-5) wi 4-leel signaling is used to lower e hannel bandwid requirement to 5 MHz, ough e maximum power density is well under 00 MHz, allowing it to run on 00 MHz-rated abling. IEEE is now interested in speifying e next generation 0Gbps UTP opper Eernet networ (0000BASE-T) in e not-too-distant future. One e signaling sheme for suh a new networ is agreed upon, measured data an be analyzed wi respet to e new networ requirements. II. 0 Gbps Transmission Design Considerations Below we speify e basi steps behind oerall transmission system designs. (a) Complete worst-ase haraterization of e transfer funtion of 00-meters of standard opper CAT-5 able up to.0 GHz frequeny is used, as shown in Fig.. (b) Performane limiting fators are lassified as: insertion loss, near-end rosstal (NEXT), far-end rosstal (FEXT), alien rosstal (A-NEXT) due to eletromagneti emissions, eho, propagation delay and any propagation sew. [db] Insertion Loss NEXT ECHO FEXT [MHz] Fig.- Frequeny Responses for Insertion Loss, NEXT, ECHO and FEXT In Fig.-, a transmission impairments model is shown at an be used to assess e system performane and to deelop ountermeasures. Reliable transmission of data

2 aross e four-pair UTP hannels is degraded by e presene of arious noise soures []. Cable attenuation (insertion loss) refers to e redution or loss of signal energy ourring between one end of e able and e opposite end of e same able. This attenuation inreases due to a law of physis alled sin effet and e proximity effet. At higher frequenies, e magneti field generated by flow of eletrons wiin e ondutor fores e eletrons to rowd to a in layer at e outer surfae of e ondutor. The higher e frequeny, e inner e onduting layer. The Insertion Loss impulse response oer Category 5 able has seere delay spread oer many symbol interals, resulting in inter-symbol interferene (ISI). Naturally, e solution to e ISI problem is equalization. Due to e superiority of e performane oer linear equalizer, espeially when e hannel introdues seere signal attenuation wiin speifi frequeny regions, Deision Feedba Equalizer (DFE) is preferred in arious digital ommuniation appliations. In e equation aboe, ˆX (.) is e input to e feedba setion of e DFE. To redue e simulation time, analyti solution for DFE tap oeffiients an be used, as follows. N Z( ) = ( n) r n= N ( ( ) ) ˆ T + n T + ( m) X ( m) = y m= where Z(.) is e input to e deision blo and, = [ ( N ) L () () L ( N )] T [ r ( + ( N ) T) L r( T) ) )] T y= L N The mean square error of e DFE an be expressed as: MSE = E {( X ( ) Z ( ) ) } T T { X ( ) } b + = E A T where E{ y y } = A, and { X ( ) y} = b E. The optimal DFE tap oeffiients an be expressed as = A b opt Fig. Transmission impairments model Hene, to begin wi, a MIMO DFE [3] will be used to ombat ISI resulted from delay spread in our appliation. The tap oeffiients for bo feedba and feed-forward filter of DFE an be updated using e normalized least mean square (NLMS) adaptation algorim as: + = + ε( where: and = µ ) 0 <µ < [ (, N ) L (, ) (, ) L (, N )] T [ r ( + N ) L r( ) ) N )] T = L Wi e optimal DFE tap oeffiients, e minimum MSE an be expressed as: T T MSE = E X ( ) b + A = E min { } opt opt opt T { X ( ) } b opt sine A T = A, and [ A ] T = A. Fig. 3 depits a omparison between omputer simulations and analyti results. The FEXT at e near-end transeiers is modeled as a multi-input, multi-output (MIMO) system. Maematially, e near-end signal is gien by e following equation () = () () + () () y t g t x t g t x t n nn n n = Desired omponent n FEXT 4 for n =, K, 4. The impulse response from e farend transmitter to e n near-end reeier is denoted as gn () t. The data stream transmitted by e n far-end transmitter is denoted as xn () t. Mitigating e effets of G ( f ), =, K, 4, is also alled equalization. The main problem wi mitigating e FEXT is at e near-end

3 reeier does not hae exat nowledge of e far-end transmitted waeforms. Similar to e FEXT model, e NEXT is also modeled as a MIMO system, exept at we hae aess to all e input signals. adaptie learning algorims [4-5] running simultaneously, one for eah near-end reeier. The learning best ours in e absene of far-end transmissions, so as not to onfuse e adaptie algorim wi e presene of FEXT signals. The entire system model at onsists of e Transmitter- Reeier (Transeier) units omprising all four twisted pairs and e assoiated iruitry at bo ends is emulated using MATLAB -SIMULINK. The assoiated MATLAB funtions are lined to ese. Operation of different transeiers has two modes; training mode and traing mode. When e system is initially turned on, it enters e Training Mode, during whih e Reeier learns e hannel parameters at are en used to anel e effets of rosstal and eho during e Traing Mode. The error between e transmitted signal and e estimate of e reeied signal is traed during e Traing Mode and if e error exeeds some pre-defined alue, en e system re-enters Training Mode to re-ealuate e hannel parameters. This is not expeted to happen too often in e present end-to-end system, as it is a fairly-stationary enironment. During e Training Mode, e eight transmitters transmit one after e oer in a sequene so at e parameters for eah transmitter-reeier ombination an be ealuated. During e Traing Mode, whih is e normal mode of operation, all transmitters transmit in parallel. (t) NEXT estimate - u (t) NR H (f) NT u (t) H (f) Fig.-3 Comparison of NLMS omputer simulations results and analytial results. Maematially, e near-end NEXT signal is gien by e following equation for n =, K, 4. 4 n nn n n 4443 = ECHO n NEXT () = () () + () () t h t u t h t u t The impulse response from e near-end transmitter to e h t. The data stream n near-end reeier is denoted as () transmitted by e u t. n () n n near-end transmitter is denoted as The ECHO may be interpreted as an auto-next omponent. The proposed mitigation approah is diagrammed in Fig. 4. The NEXT pa estimates, h n () t, for e n near-end reeier are estimated adaptiely by minimizing e meansquare of residual NEXT signal E e () t. There are four { n } Residual NEXT u (t) H (f) NT u 3 (t) H 3 (f) NT 3 u 4 (t) H 4 (f) NT 4 e (t) u (t) u 3 (t) u 4 (t) H (f) H 3 (f) H 4 (f) Adaptie Learning Algorim Fig.-4 The ECHO and NEXT is estimated and subtrated from e near-end reeption. A typial anellation result using LMS adaptation algorim is shown in Fig.-5 for a 0-PAM transmission system at a 0Gbps transmission rate. We hae been able to ahiee 60 db of interferene suppression, i.e., 60 db of signal-to-interferene ratio (SIR) wi is sheme, using an 833 Mbaud transmission symbol rate per eah of 4 UTP pairs. To analyze e performane in terms of aerage bit error rate (BER), simulations are performed in presene of residual FEXT/NEXT/ECHO orresponding to a 40 db

4 SIR. Results in terms of aerage BER performane oer 00 meters of CAT-5 able are summarized in Table-. We onsider arious fators and ompare different modulation shemes. Aerage BER alues orrespond to raw bit error rates, before any error orretion deoding is applied. Modulation, Bit Rate per 75m CAT-5 6-PAM, 500Mbaud* 4Pairs* 4bits/sym=8Gbps 6-PAM, 65Mbaud* 4Pairs* 4bits/sym=0Gbps 8-PAM, 833Mbaud* 4Pairs* 3bits/sym=0Gbps BER before Deoding 4.4 x x x PAM, 50Mbaud* 4Pairs* bits/sym=0gbps < 0-0 Table- DFE Performane oer 75meters of Standard CAT-5 Cable. III. Channel Coding Design Considerations Fig.-5 Typial MIMO Canellation Results Also, BER results in residual FEXT/NEXT/ECHO orresponding to a 40 db SIR oer 75 meters of CAT-5 able are summarized in Table-. Modulation, Bit Rate per 00m CAT-5 8-PAM, 500Mbaud* 4Pairs* 3bits/sym=6Gbps 8-PAM, 65Mbaud* 4Pairs* 3bits/sym=7.5Gbps 8-PAM, 833Mbaud* 4Pairs* 3bits/sym=0Gbps 4-PAM, 50Mbaud* 4Pairs* bits/sym=0gbps BER before Deoding.4 x x x 0-4. x 0-3 Table- DFE Performane oer 00 meters of Standard CAT-5 Cable. The pea eye-opening results in e two Tables orrespond to a pulse maximum oltage leel of olts pea-to-pea. Naturally, inreasing is alue improes e outomes. The urrent design uses a 0-PAM sheme (only 8 leels are alloated for information bits) in order to inrease e data rate from G to 0G. Indeed, when we use a multidimensional (4 in is ase) onstellation, we benefit from jointly sending symbols oer all 4 wires, ahieing a higher bandwid effiieny. Whereas, if eah of e 4 wires were separately enoded, we would need 6-PAM oer eah wire. Taing e symbol rate as 833 Mbaud and for a roughly 0% raised-osine roll-off fator, a maximum required transmission band of about 450 MHz is needed for a 0 Gbps transmission system. Inreasing e number of leels has a disadantage it redues e distane between adjaent signal points, us requiring a higher maximum pea-to-pea oltage for pulses. Conentional oding shemes, e.g., blo odes and onolutional odes, wor wi a bandwid expansion mehanism (to send redundany). In is appliation, we do not hae e luxury of using any bandwid expanding ode, sine we are exhausting all of e bandwid to inrease e transmit data rate. Thus, we see a oding sheme at will proide error protetion, at e same time presering e bandwid. TCM (Trellis Coded Modulation), inidentally, also e sheme used in 000- Base T, is nown to proide oding gains of up to 6 db for 4-dimensional sets (as used in 000 base T). Using e results aailable from e equalizer setion output, we hae found e BER at 4dB to be ~ 4dB. The is ~ 0-3 for 0-PAM in Gaussian noise.

5 E To design e ode, we must determine what S proides a BER equal to 0-0. To ahiee a 0-0 BER, we need ~9dB for 8-PAM and 3dB for 0-PAM. Thus, requiring ~ 7dB gain from e deoder for 0-PAM. Thus, an 8-state TCM ode, used in 000 Base-T, an ahiee a 0-0 BER at ~5.5dB for 0-PAM in Gaussian noise. If e atual falls below e 5.5 db alue (due to unompensated ISI), is TCM sheme annot ahiee an aerage BER of 0-0 oer 00 meters of CAT-5. Consequently, we are extending e MIMO DFE to inlude turbo (iteratie) equalization/deoding [6]. This is expeted to proide a few db of improement, whih in turn dereases e oerall bit error rate of e end-to-end system, so e target BER alue of 0-0 an be ahieed. IV. Conlusions IEEE is speifying e next generation 0Gbps UTP opper able Eernet networ (0000BASE-T). The apability to offer point to multi-point widespread dissemination of multimedia information in a LAN enironment lends itself to e requirements of many ommerial enterprises. The paper presents a rational for highspeed transmission signal proessing subsystems design. The paper also presents oneptual designs for a 0,000BASE-T opper transmission system for distribution of a digital signal oer ~00 meters of a standard CAT-5 UTP opper able. These designs mae it possible to ahiee is at a target BER of 0-0 after iteratie equalization/deoding, using standard CAT-5 able, if etored transmission on 4 pairs is adopted. Anowledgments This wor was funded in part by e Pittsburgh Digital Greenhouse (PDG) and e Penn-State CICTR. Referenes [] M. J. Riezenman, Subtle twists refine Lowly Twisted- Pair, IEEpetrum, pp.-9, Marh 00. [] M. Hawrylu, A. Yongaoglu, M. Kaehrad, "Simulation of a T-Based Disrete Multi-tone System for ADSL and VDSL Transmission," Proeedings of ISCC'98, Aens, June 998. [3] M. Kaehrad, J. Salz, "Cross-Polarization Canellation and Equalization in Digital Transmission Oer Dually- Polarized Multipa Fading Channels," AT&T Tehnial Journal, Vol. 64, No. 0, Deember 985. [4] J. F. Doherty and R. Poraya, A Robust Aousti Eho Caneler, IEEE Transations on Ciruits and Systems II: Analog and Digital Signal Proessing, ol. 44, no. 5, pp , May 997. [5] J. F. Doherty and R. J. Mammone, An Adaptie Algorim for Stable Deision-Feedba Filtering, IEEE Transations on Ciruits and Systems II: Analog and Digital Signal Proessing, ol. 40, no., pp. -9, Jan [6] F. Blamon, E. Sozer, M. Hagh, J.G. Proais, M. Salehi, Performane Comparison of Iteratie/Integral/Equalizer/Deoder Strutures for Underwater Aousti Channels, Pro. Oeans 0, Honolulu, Hawaii, Noember 00.