Multi-Carrier Modulation and MIMO Principle Application on Subscriber Lines

Transcription

1 RADIOENGINEERING, VO. 16, NO. 4, DECEMBER Mult-Carrer Modulaton and MIMO Prncple Applcaton on Subscrber nes Jří VODRÁŽKA Dept. of Telecommuncaton Technology, Czech Techncal Unversty, Techncá, Praha, Czech Republc vodraza@fel.cvut.cz Abstract. The mult-carrer modulaton s used n many applcatons, prmary for a wreless transmsson, for example W-F and WMAX networs or DVB-T. But the same physcal prncple can be used also for metallc lnes n access or local networs, for example ADS and VDS. The mult-carrer modulaton n these cases s called. The domnant source of nose n mult-par metallc cables s crosstal when the nformaton capacty s lmted dramatcally. However, nformaton capacty of metallc lnes can be ncreased, f the system s usng MIMO prncples, concrete V modulaton and lne boundng concept. The methods for V modulaton and partal crosstal cancellaton are dscussed and smulaton results are presented. Keywords Mult-carrer Modulaton, Dgtal subscrber lne, Twsted par, Crosstal cancellaton, V. 1. Applcatons of Mult-Carrer Modulaton The mult-carrer modulaton (MCM) s used n many applcatons, prmary for wreless transmsson, for example n local networs (IEEE 80.11, W-F), metropoltan networs (IEEE 80.16, WMAX) or vdeo broadcast (DVB-T). The mult-carrer modulaton n these cases s called orthogonal frequency multplex (OFDM) or ts varants. But the same physcal prncple can be used for metallc lnes (twsted pars) n access or local networs too, for example for asymmetrc dgtal subscrber lnes (ITU-T G.99, ADS), very hgh speed dgtal subscrber lnes (ITU-T G.993, VDS). The mult-carrer modulaton s then called dscrete mult- (). The ADS modems [5] use mult-carrer modulaton n base-band wth 56 sub-channels, sub-channel space Hz and adaptve SNR(f) dependence bt allocaton from 0 to 15 bts for each sub-cannel. The total frequency band s from 4 (or 5, 138) to 1104 Hz. The VDS [5] modems use mult-carrer modulaton wth maxmum 4096 sub-channels and sub-channel space or 8.65 Hz. The total frequency band s from 4 (or 5, 138) Hz to maxmal 30 MHz. The second generaton dgtal subscrber lnes ADS+ and VDS gradually replace the frst generaton of ADS systems n homes and small offces. Snce there s a need to estmate the avalable bt rate, we have used MATAB Web Server to desgn a smulator of xds lnes that s avalable at our web pages [1].. Transmsson Envronment Metallc lnes n access networs begn at the man dstrbuton frame n local exchange, then they run as a part of mult-par cables to lne dstrbuton frames, from there they branch nto smaller groups of subscrber lnes or to ndvdual lnes leadng to the subscrbers premses. In ths networ varous types of cables wth copper core (mostly 0.4 mm n dameter) and varous numbers of pars, bascally arranged n quads, are commonly used. The prncpal factors lmtng the transmsson of hgh-bt-rate sgnals are attenuaton of the lne and crosstal between the pars that s domnant source of nose n mult-par metallc cables..1 Smple Crosstal Modelng The domnant source of nose n mult-par metallc cables s crosstal representng the essental nformaton capacty lmt. However, the nformaton capacty of metallc lnes can be ncreased, f the system s usng MIMO prncples, concrete vectored modulaton (V) and lne boundng concept. The descrbed method for summarzng of contrbutve near-end and far-end crosstal (NEXT and ) has been recommended by the FSAN (Full Servce Access Networ) consortum [5]. The smplfcaton of crosstal computaton s n crosstal parameters (for NEXT and ). These parameters are averaged over the total length of the subscrber lne, not consderng the real cascade structure. In addton, the poston (n the same group, n dfferent groups) of dsturbng and dsturbed pars s gnored. The values of crosstal constants are preventvely calculated for the worst-case dsturbance envronment scenaro. However, ths worst case results n more pess-

2 34 J. VODRÁŽKA, MUTI-CARRIER MODUATION AND MIMO PRINCIPE APPICATION ON SUBSCRIBER INES mstc level of nose [9]. The typcal attrbute of crosstal s very hgh varance of values [3]. It s clear that f we nterleave the mnmum attenuaton values, we obtan the worst-case dsturbance scenaro, whch s approxmately 10 db worse than average attenuaton values. The maxmum attenuaton values are 1 or 15 db (or more) hgher than the worst-case scenaro.. Dvdng Pars to Crosstal Groups The frequency dependence of crosstal transmsson functon has a random character. Wth respect to the constructon of standard cables used n access networs, t s possble to dvde the symmetrc pars nto several groups: Neghborng pars the pars wthn a quad and those n the neghborng quads of the same subgroup; Near pars the pars of the far quads n the same or neghborng subgroup; Farther pars the pars n quads of the farther subgroups (separated by at least one subgroup); Far pars the pars n other groups; Pars n other branch of a cable tree. The nowledge of crosstal transfer functon between all pars wll be necessary for Dynamc Spectrum Management (DSM) purposes and for transmsson usng Vectored modulaton [7], [9]. Downstream Central Node DSAM m1 par bounded m par bounded Mult par cable Upstream modem 1 modem A smple method of crosstal cancellaton s used n the Ggabt Ethernet nterface for cables of the 5E category for dstances up to 100 m. For a longer dstance t s sutable to dspose of crosstal at the near-end usng frequency dvson duplex technque (FDD) and then to deal wth crosstal cancellaton at the far-end (see Fg. 1). The modems on provder sde are usually concentrated n the central node n a devce called dgtal subscrber lne access multplexer (DSAM). The bt rate of metallc lnes n access or local area can be ncreased, f the lne boundng s used. s very effectve n cancelng the crosstal, partcularly between sub-channels. Ths modulaton s then called vectored (V) and can be classfed as one from the group of MIMO systems (Multple Input Multple Output) [8], [9]. The crosstal can be cancelled out only n bounded mult-par groups or bounded mult-par groups and between ts groups too for better performance. 3.1 Modulaton The mult-carrer modulaton (MCM) s selected for modern dgtal subscrber lnes le ADS/ADS+ and VDS/VDS. It s the Dscrete Mult Modulaton () n partcular. The can solve the problems of the poor characterstcs of the transmsson channel and unfavorable nfluence from other transmsson systems. The Inter-Channel Interference (ICI) s restrcted by usng wea and ndependent s and by usng the Dscrete Fourer Transformaton (DFT). The Inter-Symbol Interference (ISI) s restrcted by usng Cyclc Prefx (CP) and by usng a FIR flter for shortenng of the channel mpulse respond. For example 51 samples of symbol and 40 samples of CP are used for ADS. The Decson Feedbac equalzer (DFE) s also beng used for receved symbol adaptaton. The DFE purpose s to reduce the ISI as well. 3. Vectored Modulaton The Vectored (V) modulaton s an extenson of the classcal modulaton. The crosstal has man dsturbng mpact on DS transmsson performance n the metallc access networ. It s possble to reduce NEXT by usng frequency dvson duplex (FDD) prncple for data transmsson n the upstream (from user to networ provder) and downstream (from networ provder to user) drecton. The V s prmarly desgned to reduce the. Fg. 1. The crosstal for downstream drecton n multpar cable wth par boundng between bounded pars and between pars from other boundng groups, too. Tx s no. 1 from other lnes Downstream pre-compensaton Subscrber modem 3. Usng MIMO Concept for Crosstal Cancellaton Tx QAM DSAM Rx QAM wthout Upstream compensaton Rx s no. 1 from other lnes Mod. Demod. Down stream Upstream Mult par cable Demod. Mod. Rx QAM wthout Tx QAM Fg.. The prncple of V compensaton on recever sde (Rx) for upstream and pre-compensaton on transcever sde (Tx) for downstream. The advantage of V s a possblty to create multpont networs, where the cancellaton for both drectons s done n a central networ element. For the upstream drecton there s compensaton at the recever sde and for downstream the pre-compensaton s used at the transmtter

3 RADIOENGINEERING, VO. 16, NO. 4, DECEMBER sde (see Fg. ). Generally, user modems are connected to DSAM by twsted pars. It s clear, that output symbol (wthout CP) of the lne depends not only on nput symbol of the lne but also on past nput. And even more, the output symbol of the lne depends not only on nput of the lne but also on the nput of all lnes. For smplfcaton t s useful to consder that the character of the channel nose s Gaussan and negatve nfluence of the channel mpulse response s reduced both by CP and synchronzaton of beng sent and receved. For receved symbol we can state: y K x + z (1) = H, 1 x1 + + H, x + + H, where y s the vector of the output symbol for the dsturbed lne, x s the vector of the nput symbol for the dsturbng lne, H,j s the matrx of the channel transmsson functon (for = j) or the crosstal transmsson functon (for j and j = 1..) of the dsturbng lne, z s the nose vector for the dsturbng lne, ncludng AWGN and crosstal from other dgtal subscrber lnes n the cable wthout V. Equaton (1) can be generalzed for all lnes to MIMO system: y = H x + z () where y s the matrx of the output of lnes, x s the matrx of the nput of lnes, H s the matrx contanng channel transmsson functons and transmsson functons, z s the nose matrx. For correct V operaton t s necessary to now transmsson path parameters, along wth crosstal from surroundng (neghborng) lnes, whch are placed n dentcal metallc cables. Transmsson path parameters are dentfed durng the process of establshng connecton between subscrber modem and access DSAM. The relatvely statonary value of transmsson parameters s one advantage of channels n metallc envronment. To compensate the crosstal completely t s requred to send sgnals on all lnes. They are present n DSAM devce, but not n subscrber s modem. Therefore, compensaton has to be performed n both drecton of transmsson n DSAM, or more precsely for the drecton upstream the crosstal s beng compensated on the recevng sde and for the drecton downstream sgnal pre-compensaton s appled on the transmtter sde. To carry out coordnaton of each symbol n all cables s computatonally very challengng. In order to decrease ths demand, the factor QR method can be used. Ths method decomposes above derved matrx H nto the untary matrx and the upper trangular matrx. Sgnfcance of QR decomposton les n decreasng computaton complexty wth crosstal represson. Instead of tang nto consderaton dsturbng lnes -1 for each lne, after QR decomposton for lne n- no compensaton sgnal wll be used, for lne -1 only one and for the 1st lne -1 compensaton sgnals must be used. Thereby the total number of operatons wll be decreased by half. The orgnal x can be estmated from receved y after QR decomposton [7] for : xˆ 1 = dec y r j xˆ, r, j= + 1 j where r are the elements of R matrx and dec denotes the decson operaton. 3.3 Channel Capacty For channels n metallc cable and VDS lnes the receved sgnal s less then crosstal. The approxmaton of the matrx elements can be used for : r h, (3), (4) where h are the elements of H matrx (). The bt-load for lne on can be estmated: b s r, log 1 + σ Γ = where s s mean power of sgnal and σ s mean power of resdual nose on. The parameter Γ represents Shannon gap [5] and s a functon of the target bt error rate (BER), code gan (CG) and nose margn (NM). The maxmum achevable bt rate of the one twsted par C and total bt rate of mult-par channel C s calculated from (5): N C = Δ b f = 1 C = Δ N f b = 1 = 1 where Δ f s spacng, practcally equal to the symbol rate (for ADS and VDS 4 or 8 Bd). 3.4 Partal Crosstal Cancellaton As was mend above, many of research wors were concerned wth prncples, analyss and presumed results wthn the vector modulaton V applcaton. The coordnaton of lmted number of lnes or more precsely of lmted number of sub-channels should be performed wth regard to the tme-consumng calculaton [7], [8]. The partal crosstal cancellaton n the upstream drecton (compensaton n the DSAM on the recevng sde) as well as n the downstream drecton (precompensaton n the DSAM on the transmttng sde) s realzed by ths approach. Thus, a frequency, space selecton or both can be performed. The selecton can be performed on bass of the transmsson functons analyss durng the ntalzaton process. The maxmal avalable cancellaton rato of crosstal specfed by gven techncal resources (maxmzaton of throughput) can be used as a startng pont. On the other sde the necessary reducton range of the crosstal based on the bt rate requests s specfed n accordance wth provded nature of servces. Certan uncoordnated number of (5) (6)

4 36 J. VODRÁŽKA, MUTI-CARRIER MODUATION AND MIMO PRINCIPE APPICATION ON SUBSCRIBER INES lnes (not ntegrated to the coordnated spectrum management and crosstal cancellaton system) should be nearly always allowed n practce. Prmary presumptons resultng from the group cables structure that s used n the Czech Republc were followng: 1. The bggest source of crosstal can be expected between the lnes that are part of one quad;. The most sgnfcant crosstal s wthn the subgroup; therefore V space selecton wll be used only wthn the gven subgroup; 3. The lowest crosstal can be expected between the lnes of the dfferent subgroups that are separated by other subgroup n the cable profle; 4. Results acheved wth the full crosstal cancellaton can be approxmated wth the partal crosstal cancellaton (wthn less than 50 % of the profle). 3.5 Expermental Results Analyss and measurements of the local cables were performed and followng results were dscovered: 1. The frst presumpton was not confrmed relatvely low unbalances and crosstal are acheved as a consequence of correct quad elements balancng;. Just a part of the second presumpton was correct. There are cables and group of cables for whch the crosstal cancellaton wthn the subgroup reduces the most sgnfcant crosstal sources. However, a hgh resdual crosstal remans n most cases; 3. The thrd presumpton was correct. Remote subgroups have very low nfluence on the dsturbance compared to neghborng subgroups; 4. The fourth presumpton was not confrmed. The avalable throughput approxmaton wthn the full crosstal cancellaton s possble after crosstal cancellaton wthn gven subgroup and neghborng subgroups (t corresponds to 3/5 of the profle); often also several domnant dsturbance sources from the remote subgroups should be nvolved n cancellaton. [dbm] Fg. 3. Example of the total crosstal power level from the sngle pars of the cable subgroup. The mend fndngs can be demonstrated on measurement and smulaton results. Frstly, a crosstal nfluence n the cable group profle s presented. A total crosstal performance s calculated up to the 30 MHz from lne 1 to 49 nto the reference lne 0 that s a part of the subgroup 0 9. The neghborng subgroups are and Examples of the results are shown n Fg. 3. After further analyss and combnaton of the crosstal performances data for ths case t can be concluded that the cancellaton from the lne no. 8 and no. 9 maes 1.9 db nose reducton of the total nose, the cancellaton from the next four lnes maes 3.5 db total reducton and cancellaton from other four lnes (total cancellaton from ten pars) 4. db total reducton only. Smlar results were acheved from all over the measurement sute. Concluson s that the partal coordnaton wthn the subgroup or more precsely wthn the 10 lnes s defcent and does not lead to the requred results n majorty of cases. 4. Smulaton Outputs The avalable bt rate for systems wthout cancellaton, wth partal and complete crosstal cancellaton s calculated. Transmsson functons and crosstal of the lne are the subjects of the measurements of the local cable (400 m length, 0.4 mm n dameter) n the frequency band from 138 Hz to 30 MHz (VDS extended band). The choce of the transmsson lnes for cancellaton s based on the lmt determnaton (nterval n db) wth regard to the worst case of the crosstal. Tab. 1. shows the results for space selecton for the dfferent threshold of the crosstal cancellaton. Also the avalable bt rate that s calculated through a whole frequency band wthout transmsson drecton resoluton and number of the lnes that are above the gven threshold and that should be added to the crosstal cancellaton systems are presented n Tab. 1. Cancellaton threshold [db] Number of selected lnes [-] Bt rate [Mb/s] Tab. 1. Smulaton results for space selecton wthn partal cancellaton. Cancellaton threshold [db] Number of selected s [thousands] Tones converson to number of lnes [-] Bt rate [Mb/s] Tab.. Smulaton results for frequency selecton wthn partal cancellaton. The bt rate gradual growth wth ncreasng number of the coordnated lnes s evdent, but the approach to the full cancellaton (n the last column) s very slow. Better results can be acheved by frequency selecton ( selecton from all lnes) as s shown n Tab.. The converson s performed wth regard to the total number of s. The cancellaton effect can be demonstrated through Fg. 4 and Fg. 5, where the bt rate dstance dependence for upstream and downstream s showed for V wth full crosstal cancellaton and wthout crosstal cancellaton for 0% used lnes n the cable profle and for addtve Gaussan whte nose (AWGN) level -136 dbm/hz. Fg. 4 shows dependence for frequency plan 998, when optmal for asymmetrc applcaton (drecton downstream s speeder them upstream) and Fg. 5 shows dependence for fre-

5 RADIOENGINEERING, VO. 16, NO. 4, DECEMBER quency plan 997, when optmal for symmetrc applcaton. The bt rate 100 Mb/s wll be used for ndvdual lne wth VDS lnes wth V on dstance of hundreds meters n local and access networ. Through lne assocaton (par boundng wth nverse multplexng) the bt rate near 1 Gb/s can be acheved for example ten lnes bundle. demands great calculaton capacty from sgnal processors. The partal crosstal cancellaton wth the most dsturbng lnes or s selecton can be the soluton. But ths method of the soluton s not the best for all cables that are used n the Czech Republc n accordance to performed experments. However, bt rate 100 Mb/s can be acheved for ndvdual lne for mddle-range lengths and wth utlzaton of VDS physcal layer up to 30 MHz (band plan 997). Acnowledgements Ths wor has been supported by the Grant Agency of the Academy of Scences of the Czech Republc under project 1ET Fg. 4. Bt rate for frequency plan 998 (VDS) wth full crosstal cancellaton (wth V) and wthout t. Fg. 5. Bt rate for frequency plan 997 (VDS) wth full crosstal cancellaton (wth V) and wthout t. 5.Conclusons The demands n the local and access networs for hgher bt rates can be satsfed usng VDS lnes, but the dstance must not exceed 1 m. The ablty of precse topology modelng and nose modelng of access networs s very mportant. Wth respect to the ncreasng number of systems n access networs, the mplementaton of new modulaton and MIMO methods s necessary. Dynamc spectrum management (DSM) methods are also necessary to provde up-to-date nformaton on the access networ operatonal status and dfferent transmsson systems confguratons mang best of potentals of access networs. V modulaton utlzaton, ale the OFDM modulaton for wreless systems, evoes the huge ncrease of the bt rate n the cables that are already engaged by subscrber lnes compared wth common transmsson methods. It References [1] VODRÁŽKA, J., JAREŠ, P., HUBENÝ, T. xds smulator. In Praha, 005. [] VODRÁŽKA, J. Downstream power-bac-off used for ADS. In Proceedngs EC-SIP-M 005. Bratslava, Slova Unversty of Technology, 005, pp [3] VODRÁŽKA, J., JAREŠ, P., PROKOP, T. Modelng of mddlerange metallc lnes for Ethernet wth V. In IWSSIP 007 & EC-SIPMCS 007. Unversty of Marbor, 007, s [4] VODRÁŽKA, J., HRAD, J., JAREŠ, P. Modelng of access networ structure. In Proceedngs of the 6th Conference on Telecommuncatons. Insttuto de Telecomuncaçoes sboa, 007, s [5] RAUSCHMAYER, D. J. ADS/VDS Prncples: A Practcal and Precse Study of Asymmetrc Dgtal Subscrber nes and Very Hgh Speed Dgtal Subscrber nes. Indanapols, USA: Macmllan Techncal Publshng, [6] CENDRION, R., MOONEN, M. Iteratve spectrum balancng for dgtal subscrber lnes. Communcatons. ICC Vol.: 3, p [7] CENDRION, R., GINIS, G., BOGAERT, E., MOONEN, M. A near-optmal lnear crosstal canceller for VDS. IEEE Transactons on Sgnal Processng [8] CENDRION, R., GINIS, G., MOONEN, M., ACKER, K. Partal crosstal precompensaton n downstream VDS. Sgnal Processng 84. Elsever (004), pp [9] BRADY, M. H., CIOFFI, J. M. The worst-case nterference n DS systems employng dynamc spectrum management. Hndaw Publshng Corporaton EURASIP Journal on Appled Sgnal Processng. Vol. 006, Artcle ID 7854, pp About Author Jří VODRÁŽKA was born n Prague n He joned the Department of Telecommuncaton Technology CTU of Prague FEE n 1996 as a research assstant. Ph.D. n electrcal engneerng n 001, a head of the Transmsson meda and system scence group snce 005, partcpates n a number of projects n cooperaton wth external organzatons.