Modulation and multiplexing in optical communication systems

Transcription

1 Research Highlighs Modulaion and muliplexing in opical communicaion sysems Peer J. Winzer Bell Labs, Alcael-Lucen, Holmdel, NJ Digial elecronics and opical ranspor The rapid ransiion from analog o digial sysems over he pas ~50 years has enabled universal processing of all kinds of informaion, fundamenally wihou loss of qualiy []. Breakhroughs in digial semiconducor echnologies and heir enormous abiliy o scale [2] have enabled cos-effecive mass-producion of richly funcional ye highly reliable and power-efficien microchips ha are found in virually any elecronic device oday, from high-end inerne rouers o low-end consumer elecronics. Closely coupled o he generaion, processing, and sorage of digial informaion is he need for daa ranspor, ranging from shor on-chip [3] and board-level [4,5] daa buses all he way o long-haul ranspor neworks spanning he globe [6,7] and o deep-space probes collecing scienific daa [8], cf. Fig. [5,0]. Each of hese very differen applicaions brings is own se of echnical challenges, which can be addressed using elecronic, radio-frequency (RF), or opical communicaion sysems. Among he differen communicaion echnologies, opical communicaions generally has he edge over baseband elecronic or RF ransmission sysems whenever high aggregae bi raes and/or long ransmission disances are involved. Boh advanages are deeply rooed in physics: Firs, he high opical carrier frequencies allow for high-capaciy sysems a small relaive bandwidhs. For example, a mere 2.5% bandwidh a a carrier frequency of 93 THz (.55 µm wavelengh) opens up a 5-THz chunk of coninuous communicaion bandwidh. Such narrow-band sysems are much easier o design han sysems wih a large relaive bandwidh. Second, ransmission losses a opical frequencies are usually very small compared o baseband elecronic or RF echnologies. Today s opical elecommunicaion fibers exhibi losses of less han 0.2 db/km; he loss of ypical coaxial cables supporing ~ GHz of bandwidh is 2 o 3 orders of magniude higher. In free-space sysems opical beams have much smaller divergence angles han in he microwave regime, a he expense of significanly exacerbaed anenna poining requiremens, hough. The narrow beam widh favorably ranslaes ino he sysem s link budge, in paricular in space-based sysems where amospheric absorpion is less of a problem. Apar from he above wo major advanages, oher consideraions someimes come ino play, such as he unregulaed specrum in he opical regime or he absence of elecromagneic inerference. The gradual replacemen of elecronic ranspor The suiabiliy of opical communicaions for differen sysem scenarios can be furher analyzed using he hree basic ransponder characerisics shown in Fig. 2: A ransponder s sensiiviy measures he minimum power (or he minimum signal-o-noise raio) required by he receiver o close a digial communicaion link, which impacs he link disance ha may be bridged. In his loosely defined conex, he erm sensiiviy also includes he effec of linear and nonlinear signal disorions due o he ransmission channel. The capaciy of a sysem measures he amoun of daa ha can be ransmied over he communicaion medium. Here, we hink of he capaciy per waveguide, wih he undersanding ha parallel lanes (buses) are likely o be used in applicaions ha require high aggregae capaciies a igh ransponder inegraion requiremens. In many applicaions, implemenaion aspecs of a ransponder (including is physical dimensions, power consumpion, cos, and reliabiliy) are he mos criical parameers and ofen delay he enrance of opics ino a paricular applicaion space. The figure roughly indicaes he relaive The divergence angle of an anenna of diameer D operaing a wavelengh l is given by l/d. A µm, a elescope (=anenna) of 0 cm diameer has a divergence angle of 0 µrad (50.6 mdeg). GEO, LEO Terresrial Neworks Deep-Space Submarine Access Rack-o-Rack Backplanes Chip-o-Chip On-Chip 00,000 km,000 km 0 km 00 m m cm Figure. Digial communicaion disances can be over 00,000 km in deep-space missions and below mm on-chip. (GEO: Geosaionary saellie orbi; LEO: Low-Earh saellie orbi.) Figures reproduced wih permission. From lef o righ, couresy of () NASA/JPL-Calech; (2) European Space Agency (ESA); (3) Alcael-Lucen; (4) Alcael-Lucen []; (5) Corning, Inc. [9]; (6) (9) IBM [3]. 4 IEEE LEOS NEWSLETTER February 2009

2 imporance of he hree performance merics for differen communicaion applicaions. As bandwidh demands have coninuously increased and as opo-elecronic device and inegraion echnologies have advanced, opical communicaions has gradually replaced elecronic (and o some exen direcional 2 microwave) soluions. This process sared on a large scale in he lae 970s and 980s a he mos demanding high-bandwidh/long-disance applicaions of erresrial [6] and submarine [7] ranspor. Wih massive fiber-o-he-home (FTTH) deploymens now under - way world-wide, opics is currenly capuring he access space [9], and rack-o-rack inerconnecs are saring o become opical [3]. The red applicaion areas in Fig. 2 indicae well esablished opical communicaion echnologies. The applicaions marked orange denoe areas where opics can be found bu is no ye used on a massive scale. The blue applicaions are sill dominaed by elecronics, wih research on opical successors being acively pursued. Despie he coninuing improvemen in elecronic ransmission echniques [2], opical soluions are expeced o ener backplanes, paving he way o opical chip-o-chip and, evenually, on-chip communicaions once elecronic ransmission can no longer keep pace wih he growing need for communicaion capaciy, power consumpion, or escape bandwidh, i.e., he inerconnec capaciy per uni of inerface area [3,4,5]. A he same ime, areas where opical communicaions is already well esablished have o coninue supporing ever-increasing capaciy demands. Orhogonal dimensions and muliplexing In order o mee he applicaion-specific requiremens on sensiiviy and capaciy under he respecive implemenaion consrains, one has o choose he bes suied modulaion and muliplexing echniques based on he available physical dimensions shown in Fig. 3 [3]. Of paricular imporance in his conex is he noion of orhogonaliy [5]. Loosely speaking 3, wo signals are orhogonal if messages sen in hese wo dimensions can be uniquely separaed from one anoher a he receiver wihou impacing each oher s deecion performance. This way, independen bi sreams can share a common ransmission medium, which is referred o as muliplexing. The amoun of individual bi sreams ha can be packed ono a single ransmission medium deermines a sysem s aggregae capaciy. The mos advanced muliplexing echniques are herefore found in capaciy-consrained sysems, such as long-haul fiber-opic ranspor (cf. Fig. 2). Muliplexing is performed by exploiing orhogonaliy in one or more of he physical dimensions shown in Fig. 3. Sending signals in disjoin frequency bins on differen opical carrier frequencies is called wavelengh-division muliplexing (WDM), cf. Fig. 4. Such signals are orhogonal, and individual bi sreams can be recovered using opical bandpass filers or elecronic filers following a coheren receiver fron-end 2 Owing o he inherenly high direcionaliy of opical anennas, microwave sysems will likely coninue o be he soluion of choice for mobile environmens requiring omni-direcional recepion and ransmission. 3 A rigorous definiion of orhogonaliy in he conex of opical communicaions is given in, e.g., [3,4]. Capaciy Terresrial Long-Haul Mero and Regional Submarine Access LAN, SAN Chip-o-Chip Backplane On-Chip Implemenaion Rack-o-Rack Sensiiviy Deep-Space GEO, LEO Terresrial Free-Space Figure 2. Sensiiviy, capaciy, and implemenaion aspecs (physical dimensions, power consumpion, and cos) are key facors behind he success of any communicaion echnology. Saring from high sensiiviy / high capaciy applicaions (erresrial and submarine long-haul), opical communicaions is seadily replacing elecronic ransmission echnologies. [6]. If signals leak energy ino neighboring frequency bins, orhogonaliy is degraded and perfec reconsrucion is no longer possible ( WDM crossalk ). As shown in Fig. 4, a possible couner-measure, which has been used in some research demonsraions, is alernaing he polarizaion of adjacen channels o re-esablish orhogonaliy in he polarizaion dimension ( polarizaion inerleaving ). Using rue polarizaion-division muliplexing (PDM, cf. Fig. 4), one sends wo independen signals on boh orhogonal polarizaions suppored by a single-mode opical fiber. In order o recover hese polarizaion-muliplexed bi sreams, one eiher uses a polarizaion beam splier whose axes are consanly kep aligned wih he signal polarizaions ( polarizaion conrol ), or one deecs wo arbirary orhogonal polarizaions ( polarizaion diversiy ) using coheren deecion. Since upon fiber ransmission he polarizaion axes a he receiver will be randomly roaed compared o he ransmier, one elecronically back-roaes he deeced signals using he (esimaed) inverse Jones marix of he ransmission channel. This is he approach aken by modern coheren receivers [6]. Anoher way of achieving orhogonaliy in he frequency domain is by leing he signal specra a adjacen wavelenghs overlap bu choosing he frequency spacing o be exacly /T S, where T S is he symbol duraion, synchronized across he individual (sub)carriers. This approach is visualized in ime and frequency domain in Fig. 5. Alhough he superposiion of he hree modulaed signals (examples shown are 23 and 223 ) looks uninelligible a a firs glance, a receiver can uniquely filer ou he informaion ranspored by each subcarrier by firs muliplying he superposiion wih a sine wave of he desired subcarrier s frequency and hen inegraing over he symbol duraion. This operaion can be paricularly efficienly done in he elecronic domain using he fas Fourier ransform (FFT). This kind of muliplexing is known as orhogonal frequency division muliplexing (OFDM) [7] or coheren WDM February 2009 IEEE LEOS NEWSLETTER 5

3 PPM ETDM OTDM Mod Separae Fibers Muliple Modes Space PolSK Mod Pol.Muliplexing Pol.Inerleaving Polarizaion Physical Dimensions for Modulaion and Muliplexing Quadraure FSK, MSK Ampliide / Phase Modulaion Mod Code WDM OFDM CoWDM ocdma Im{E x } Im{E x } Im{E x } Re{E x } Re{E x } Re{E x } QPSK 8-PSK 6-QAM Figure 3. Physical dimensions ha can be used for modulaion and muliplexing in opical communicaions. (OTDM: Opical imedivision muliplexing; ETDM: Elecronic ime-division muliplexing; ocdma: Opical code-division muliple access; PPM: Pulse posiion modulaion; PolSK: Polarizaion shif keying; FSK: -shif keying; MSK: Minimum-shif keying; WDM: Wavelengh-division muliplexing; CoWDM: Coheren WDM; OFDM: Orhogonal frequency-division muliplexing; PSK: Phase shif keying; QPSK: Quadraure PSK; QAM: Quadraure ampliude modulaion; E x : Opical field (x polarizaion).) WDM /T s WDM wih Polarizaion Inerleaving x-pol y-pol T s 2 3 WDM wih Polarizaion Muliplexing x-pol y-pol Figure 4. Orhogonaliy hrough disjoin frequency bins (WDM) can be combined wih orhogonaliy in he polarizaion dimension. (CoWDM) [8,9], depending on wheher he (de)muliplexing operaions are performed elecronically or opically (equivalen o he disincion beween ETDM and OTDM in he ime domain). If he orhogonal waveforms are no sine waves bu orhogonal sequences of shor pulses ( chips ), we arrive a opical code-division muliple access (ocdma) [20]. Finally, one can make use of he spaial dimension, in is mos obvious form by sending differen signals on parallel opical waveguides, someimes referred o as spaial muliplexing. Using parallel waveguides is paricularly aracive for Figure 5. Orhogonal frequency spacings of /T S lead o OFDM or CoWDM. implemenaion-consrained sysems (rack-o-rack inerconnecs and shorer), where frequency sable lasers and filers operaing over a significan emperaure range lead o bulky and power-consuming soluions, and coheren signal processing becomes problemaic for he same reasons. Here, coarse WDM (CWDM) wih uncooled componens allows for channel spacings of ypically 20 nm and can be an aracive muliplexing soluion. In conras, for long-haul ranspor sysems, which are he mos capaciy-consrained sysems exising oday, spaial muliplexing is no cos efficien, and dense WDM is a requiremen, recenly even in combinaion wih PDM. The key parameer characerizing such sysems is he specral 6 IEEE LEOS NEWSLETTER February 2009

4 efficiency (SE), defined as he raio of per-channel bi rae o WDM channel spacing. Modulaion and coding Modulaion denoes he mehod by which digial informaion is imprined ono an opical carrier, and in is mos general sense also includes coding o preven ransmission errors from occurring ( line coding ) or o correc for already occurred ransmission errors ( error correcing coding ). Uncoded on/off keying (OOK, cf. Fig. 6) in is various flavors [2] has been used in opical communicaions for decades because i is by far he simples forma in erms of hardware implemenaion and inegraion and exhibis a good compromise beween complexiy and performance. Those applicaions in Fig. 2 ha are idenified o be implemenaion-consrained, especially if inegraion and power efficiency weigh heavily, are likely o employ uncoded OOK unil capaciy or sensiiviy requiremens dicae he use of more sophisicaed formas or compuaionally inensive error correcing coding. For sensiiviy-dominaed applicaions, in paricular for space-based laser communicaions, binary phase shif keying (PSK, cf. Fig. 6) was sudied inensively and se several s ensiiviy records [22,23,24]. Furher sensiiviy improvemens can be obained a he expense of modulaion bandwidh, eiher by M-ary orhogonal modulaion or by coding. Orhogonal modulaion formas employ M. 2 orhogonal signal dimensions, such as M non-overlapping ime slos per symbol duraion ( pulse posiion modulaion, PPM, cf. Fig. 6 for M 5 4) [8,4,25] or M orhogonal frequencies (M-ary frequencyshif keying, FSK) [4]. In PPM, an opical pulse is ransmied in one ou of M slos per symbol. The occupied slo posiion denoes he bi combinaion conveyed by he symbol. Boh PPM and FSK expand he signal bandwidh by M/log 2 M compared o OOK. For example, using 64-PPM, sensiiviy is improved by 7.5 db a a bi error raio (BER) of 0 6 a he expense of a 0-fold increase in modulaion bandwidh [5]. Wih error correcing coding ( forward error conrol, FEC), redundancy is inroduced a he ransmier and is used o correc for deecion errors a he receiver [26]. Typical FECs for erresrial fiber-opic sysems oday operae a up o 40 Gb/s wih 7% overhead and are able o correc a channel BER of of o 0 6, yielding a sensiiviy improvemen of ~9 db a a mere 7% bandwidh expansion. FECs wih more han db of coding gain a BER and a a 25% bandwidh overhead have been implemened a 0 Gb/s [26]. These high sensiiviy gains achieved by FEC a a low bandwidh expansion in comparison wih orhogonal modulaion come a he expense of a significan increase in implemenaion complexiy for FEC processing. Through he combinaion of modulaion and coding, sensiiviies of phoon/bi have been repored using PPM [27]. In conras, capaciy-consrained sysems employ modulaion formas ha avoid an increase in modulaion bandwidh o allow for dense WDM channel packing (high specral efficiency). Narrow modulaion specra are accomplished by sicking o he wo-dimensional quadraure signal space, i.e., by using muliple levels of real and imaginary pars (or magniude and phase) of he complex opical field, as shown by he hree examples in Fig. 3. In addiion, low-overhead FEC (~7% o Inensiy Phase Inensiy π Symbol OOK PSK PPM Figure 6. Waveforms associaed wih some opical modulaion formas. Specral Efficiency [b/s/hz] 0 0. Capaciy- Consrained 0 Shannon PPM 8 [23] QAM 28 PSK [38] [37] [3] Required Signal-o-Noise Raio E b /N 0 [db] ~25%) is used o improve sensiiviy. A currenly invesigaed 00-Gb/s single-channel raes, quadraure phase shif keying (QPSK) [28,29], 8-PSK [30], and 6-QAM [3] have been repored, boh on a single carrier and using CoWDM [32]. Figure 7 visualizes he rade-off beween sensiiviy and specral efficiency for he linear addiive whie Gaussian noise (AWGN) channel 4 [5]. The ulimae limi is given by Shannon s 4 Differen limis are obained for oher channels, for example for he sho noise limied case. While he AWGN channel is he mos relevan for opically amplified ransmission sysems [33], free-space sysems can be sho-noise limied [25,34] [30] [29] OOK 256 [35] [36] Sensiiviy- Consrained Figure 7. Trade-off beween specral efficiency (per polarizaion) and sensiiviy of various modulaion formas limied by AWGN. Modulaion formas (brigh: heoreical limis; fain: experimenal resuls) are repored for a 7% overhead code a a pre-fec BER of (Squares: PPM; riangles: PSK; circles: QAM; diamonds: OOK.) February 2009 IEEE LEOS NEWSLETTER 7

5 capaciy. The lower porion of he figure belongs o he realm of sensiiviy-consrained sysems while he upper porion applies o capaciy-consrained sysems. The heoreically achievable sensiiviy for four classes of modulaion formas (OOK, PSK, QAM, PPM) are also shown, assuming he above menioned 7% overhead FEC ( pre-fec BER). The performance of some recen experimenal resuls is capured by he fainer colored symbols. I is eviden ha hardware implemenaion difficulies preven he formas from performing a heir heoreical limis, boh in erms of sensiiviy and specral efficiency. WDM sysem evoluion Fiber-opic ranspor sysems are he mos capaciy-consrained of all opical communicaion sysems. To assess echnological progress a he forefron of ransmission capaciy, Fig. 8 compiles research experimens repored a he Opical Fiber Communicaion Conferences (OFC) and he European Conferences on Opical Communicaions (ECOC). The green daa poins show he experimenally achieved bi raes of elecronically ime-division muliplexed (ETDM) single-channel sysems, which reflec he hisoric growh rae of he speed of semiconducor elecronics. By 2005/2006, ETDM bi raes had reached 00 Gb/s [39,40]. By he mid 990s, he erbium-doped fiber amplifier (EDFA) had made WDM highly aracive because i could simulaneously amplify many WDM channels. This allowed he capaciy of fiber-opic communicaion sysems o scale in he wavelengh domain by wo orders of magniude compared o single-channel sysems, as indicaed by he red daa poins. Up unil ~2000, achieving a closer WDM channel spacing was a maer of improving he sabiliy of lasers and of building highly frequency selecive opical filers; pre-2000, he increase in specral efficiency, represened by he yellow daa poins in Fig. 8, was herefore due o improvemens in device echnologies. When 40-Gb/s sysems sared o ener opical neworking a he urn of he millennium, opical modulaion formas [2,4] and coding 5 [26] became very imporan, firs o improve sensiiviy so ha he reach of 40-Gb/s sysems would no fall oo shor of ha of legacy 0-Gb/s sysems. Wih he simulaneous developmen of sable 00-GHz and 50-GHz spaced opics, he modulaed opical signal specra quickly approached he bandwidh allocaed o a single WDM channel, which ook he increase of specral efficiency from a device design level o a communicaions engineering level, and made specrally efficien modulaion imporan, as i had radiionally been he case in elecronic and RF communicaion sysems. Using advanced communicaion echniques such as coheren deecion (presenly sill wih off-line signal processing insead of real-ime bi error couning), PDM, OFDM, and pulse shaping, specral efficiencies have coninued o increase a muli-gb/s raes, wih oday s records being a 4.2 b/s/hz a 00 Gb/s [30, 3], 5.6 b/s/hz a 50 Gb/s [37], and 9.3 b/s/hz a 4 Gb/s [38]. Furher scaling of specral efficiency becomes increasingly more difficul, requiring expo nenially more Sysem Capaciy Tb/s Gb/s Specral Efficiency [b/s/hz] Muli-Channel Single Channel (ETDM) Specral Efficiency 200 Figure 8. Progress in fiber-opic ransmission capaciies, as repored a pos-deadline sessions of ECOC and OFC. (Green: Singlechannel ETDM raes; red: WDM aggregae capaciies on a single fiber; yellow: specral efficiency.) consellaion poins per modulaion symbol 6. Recen sudies on he fundamenal capaciy limis of opical ransmission sysems over sandard single-mode fiber predic a maximum capaciy of abou b/s/hz over 2000 km [33,43], assuming ha PDM doubles capaciy compared o he repored singlepolarizaion case. The experimenally demonsraed record for he aggregae capaciy over a single opical fiber is currenly a 25.6 Tb/s a a specral efficiency of 3.2 b/s/hz [42]. As eviden from he red daa poins in Fig. 8, repored capaciies have noiceably sared o saurae over he las few years. Wih coninuously increasing specral efficiencies, his can be aribued, a leas in par, o he slower growh rae of single-channel ETDM bi raes, which necessiaes a large increase in he number of WDM channels o achieve record capaciies and makes such experimens boh ime consuming and expensive. For example, he above menioned 25.6-Tb/s experimen [42] used a oal of 320 ETDM channels (2 opical amplificaion bands, 80 wavelenghs per band, and 2 polarizaions per wavelengh, modulaed a 80 Gb/s each). All he above daa indicae ha WDM is sill scaling in specral efficiency and capaciy a presen bu will likely reach fundamenal as well as pracical limis in he near fuure. Therefore, new approaches have o be explored in order o coninue he scaling of capaciy-consrained sysems. Such approaches could include he use of lower nonlineariy or lower-loss opical ransmission fiber [43], ransmission over exended wavelengh ranges, or even he use of muli-core or muli-mode opical fiber [44]. WDM Channels 5 In submarine sysems, coding was inroduced well before 2000 [7,26]. 6 Transporing k bis of informaion per symbol (and hence per uni bandwidh in quadraure space) requires 2 k modulaion symbols. 8 IEEE LEOS NEWSLETTER February 2009

6 Conclusions The success of digial informaion processing over he las cenury has riggered he demand o ranspor massive amouns of digial informaion, ranging from on-chip daa buses all he way o iner-planeary disances. Opical communicaion sysems have been replacing elecronic and RF echniques saring a he mos demanding capaciy-consrained and sensiiviy-consrained applicaions and are seadily progressing owards more implemenaion-consrained shorer-reach sysems ha require dense inegraion, low power consumpion, and low cos. Modulaion and muliplexing echniques are key design elemens of sensiiviy-consrained and capaciyconsrained sysems, used o harves he bandwidh advanages ha opical echnologies fundamenally offer. Specrally efficien modulaion will say a key area of research for capaciy-consrained sysems. As WDM capaciies over convenional fibers are approaching heir fundamenal limis, breakhroughs in fiber design and in complemenary muliplexing echniques are expeced o furher scale capaciy. Acknowledgmen The auhor is graeful for discussions wih many colleagues in he opical communicaions communiy, including R.-J. Essiambre, A. Gnauck, G. Raybon, C. Doerr, H. Kogelnik, A. Chraplyvy, R. Tkach, J. Foschini, G. Kramer, A. Leven, F. Fidler, T. Kawanishi, M. Nakazawa, D. Caplan, P. Pepeljugoski, Y. Vlasov, S. Jansen, S. Savory, and many ohers. References. C. E. Shannon, A mahemaical heory of communicaion, Bell Sys. Tech. J., vol. 27, no. 3, pp , G. E. Moore, Cramming more componens ino inegraed circuis, Elecron. Mag., vol. 38, no. 8, Y. Vlasov, Silicon phoonics for nex generaion compuing sysems, in Proc. 34h European Conf. Exhibiion Opical Communicaion (ECOC), 2008, Paper Tu..A.. [Online]. Available: hp:// 4. J. A. Kash, F. E. Doany, C. L. Schow, R. Budd, C. Baks, D. M. Kucha, P. Pepeljugoski, L. Schares, R. Dangel, F. Hors, B. J. Offrein, C. Tsang, N. Ruiz, C. Pael, R. Horon, F. Libsch, J. U. Knickerbocker, Terabus: Chip-ochip board level opical daa buses, in Proc. 2s Annu. Meeing IEEE Lasers Elecro-Opics Soc. (LEOS), 2008, Paper WM, pp A. F. Benner, M. Ignaowski, J. A. Kash, D. M. Kucha, and M. B. Rier, Exploiaion of opical inerconnecs in fuure server archiecures, IBM J. Res. Dev., vol. 49, no. 4/5, pp , H. Kogelnik, On opical communicaion: Reflecions and perspecives, in Proc. European Conf. Exhibiion Opical Communicaion (ECOC), 2004, Paper Mo S. Abbo, Review of 20 years of undersea opical fiber ransmission sysem developmen and deploymen since TAT-8, in Proc. 34h European Conf. Exhibiion Opical Communicaion (ECOC), 2008, Paper Mo.4.E.. 8. Sephen A. Townes, Bemard L. Edwards, Abhiji Biswas, David R. Bold, Roy S. Bonduran, Don Boroson, Jamie W. Bumside, David O. Caplan, Alan E. DeCew, Ramon DePaula, Richard J. Fizgerald, Farzana I. Khari, Alexander K. McInosh, Daniel V. Murphy, Ben A. Parvin, Alen D. Pillsbury, William T. Robers, Joseph J. Scozzafava, Jayan Sharma, Malcolm Wrigh, The Mars Laser communicaion demonsraion, in Proc. Conf. Aerospace, 2004, pp R. E. Wagner, Fiber-based broadband access echnology and deploymen, in Opical Fiber Telecommunicaions V, vol. B, I. P. Kaminov, T. Li, and A. E. Willner, Eds. New York: Academic, pp , R. E. Wagner, Opporuniies in elecommunicaions neworks, in Proc. Opoelecronics Communicaions Conf. (OECC), 2005, Paper 5B-.. S. K. Koroky, Nework global expecaion model: A saisical formalism for quickly quanifying nework needs and coss, J. Lighwave Technol., vol. 22, no. 3, pp , A. Adamiecki, M. Duelk, and J. H. Sinsky, 25 Gbi/s elecrical duobinary ransmission over FR-4 backplanes, Elecron. Le., vol. 4, no. 4, pp , P.J. Winzer and R.-J. Essiambre, Advanced opical modulaion formas, in Opical Fiber Telecommunicaions V, vol. B, I. P. Kaminov, T. Li, and A. E. Willner, Eds. Academic, pp , D. O. Caplan, Laser communicaion ransmier and receiver design, in Free-Space Laser Communicaions: Principles and Advances, A. Majumdar and J. Ricklin, Eds. New York: Springer-Verlag, pp , J. G. Proakis, Digial Communicaions. New York: McGraw- Hill, K. Kikuchi, Coheren opical communicaion sysems, in Opical Fiber Telecommunicaions V, vol. B, I. P. Kaminov, T. Li, and A. E. Willner, Eds. New York: Academic, pp , S. L. Jansen, Opical OFDM, a hype or is i for real? in Proc. European Conf. Opical Communicaion (ECOC), 2008, Paper Mo.3.E H. Sanjoh, E. Yamada, and Y. Yoshikuni, Opical orhogonal frequency division muliplexing using frequency/ ime domain filering for high specral efficiency up o bi/s/hz, in Proc. Opical Fiber Communicaion Conf. (OFC), 2002, Paper ThD. 9. A. D. Ellis, F. C. G. Gunning, B. Cueno, T. C. Healy, and E. Pincemin, Towards TbE using Coheren WDM, in Proc. Opoelecronics Communicaions Conf and 2008 Ausralian Conf. Opical Fibre Technology (OECC/ACOFT), Paper We-A. 20. P. R. Prucnal, Ed., Opical Code Division Muliple Access: Fundamenals and Applicaions, Boca Raon, Fl: CRC, P. J. Winzer and R.-J. Essiambre, Advanced opical modulaion formas, Proc. IEEE, vol. 94, no. 5, pp , February 2009 IEEE LEOS NEWSLETTER 9

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