Digital Domain Power Division Multiplexed Dual Polarization Coherent Optical OFDM Transmission

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1 Dgtal Doma Power Dvso Multplexed Dual Polarzato Coheret Optcal OFDM Trasmsso Qog Wu 1, Zhehua Feg 1, Mg Tag 1*, Xag L 2, Mg Luo 2, Hub Zhou 1, Soga Fu 1, ad Demg Lu 1 1 Wuha Natoal Lab for Optoelectrocs (WNLO) & Natoal Egeerg Laboratory for Next Geerato Iteret Access System, School of Optcal ad Electroc Iformato, Huazhog Uversty of Scece ad Techology, Wuha, , Cha 2 State Key Laboratory of Optcal Commucato Techologes ad Networks, Wuha Research Isttute of Post ad Telecommucato, Wuha , Hube, Cha * tagmg@mal.hust.edu.c ABSTRACT Capacty s the eteral pursut for commucato systems due to the overwhelmg demad of badwdth hugry applcatos. As the backboe frastructure of moder commucato etworks, the optcal fber trasmsso system udergoes a sgfcat capacty growth over decades by explotg avalable physcal dmesos (tme, frequecy, quadrature, polarzato ad space) of the optcal carrer for multplexg. For each dmeso, strget orthogoalty must be guarateed for perfect separato of depedet multplexed sgals. To catch up wth the ever-creasg capacty requremet, t s therefore terestg ad mportat to develop ew multplexg methodologes relaxg the orthogoal costrat thus achevg better spectral effcecy ad more flexblty of frequecy reuse. Ispred by the dea of o-orthogoal multple access (NOMA) scheme, here we propose a dgtal doma power dvso multplexed (PDM) trasmsso techology whch s fully compatble wth curret dual polarzato (DP) coheret optcal commucato system. The coheret optcal orthogoal frequecy dvso multplexg (CO-OFDM) modulato has bee employed owg to ts great superorty o hgh spectral effcecy, flexble codg, ease of chael estmato ad robustess agast fber dsperso. Ad a PDM-DP-CO-OFDM has bee theoretcally ad expermetally demostrated wth 100Gb/s wavelegth dvso multplexg (WDM) trasmsso over 1440km stadard sgle mode fbers (SSMFs). Two basebad quadrature phase shft keyg (QPSK) OFDM sgals are overlad together wth dfferet power levels. After IQ modulato, polarzato multplexg ad log dstace fber trasmsso, the PDM-DP-CO-OFDM sgal has bee successfully recovered the typcal polarzato dversty coheret recever by successve terferece cacellato (SIC) algorthm. No-orthogoal overlad sgals dfferet power double the system spectral effcecy ad t eables flexble provsog of qualty of servce (QoS) by properly adjustg power ratos of o-orthogoal multplexed braches. Itroducto To meet the creasg demad of hgh capacty optcal fber trasmsso etwork, fve avalable physcal dmesos cludg tme, frequecy, quadrature, polarzato ad space have bee utlzed for modulato ad multplexg optcal commucatos 1. I these schemes, strget orthogoalty must be satsfed to avod ferece or crosstalk from other chaels so that sgals ca be separated dvdually wthout degradg each other s detecto performace. However, oe of the major problems these orthogoal multplexg systems such as orthogoal frequecy dvso multplexg (OFDM) or Nyqust WDM s that they do ot allow frequecy reuse of two depedet sgals wth the same physcal dmeso. Recetly, the o-orthogoal multple access (NOMA), also kow as power doma multple access, s proposed as a potetal caddate for the upcomg 5G wreless commucato stadard due to ts superor spectral effcecy 2. I NOMA, multple users are multplexed wth dfferet power levels usg superposto codg at the trasmtter sde ad successve terferece cacellato (SIC) based mult-user detecto algorthms at the recevers. The o-orthogoal feature sgfcatly mproves the capacty ad throughput both wreless ad vsble lght commucato systems by allocatg the etre badwdth to dfferet users smultaeously 3,4. Ispred by the dea of NOMA, we have recetly proved the feasblty of usg a ew multplexg dmeso to ehace the capacty of drect detecto optcal OFDM (DDO-OFDM) system by multplexg the spectrally overlad sgals wth dfferet power levels dgtal doma 5,6.

I ths paper, we further develop the dea of dgtal doma power dvso multplexg (PDM) to dual polarzato coheret optcal OFDM (DP-CO-OFDM) trasmsso systems, whch s promsg for ultra-large capacty log-haul

2 I ths paper, we further develop the dea of dgtal doma power dvso multplexg (PDM) to dual polarzato coheret optcal OFDM (DP-CO-OFDM) trasmsso systems, whch s promsg for ultra-large capacty log-haul coheret optcal fber commucatos. After theoretcal aalyss ad umercal smulatos, we expermetally demostrated a 100 Gb/s WDM trasmsso over a stadard sgle mode fber (SSMF) lk of 1440km. The results show that system capacty ca be early doubled whe two basebad quadrature phase shft keyg (QPSK) OFDM sgals are overlad together wth proper power rato before optcal modulato process ad decoded after phase recovery process usg carefully desged SIC algorthm, whch s proved to show better performace tha covetoal herarchcal de-mappg approach our expermet. Besdes the capacty upgrade, the PDM scheme s also flexble to allocate customzed qualty of servce (QoS) for dfferet subscrbers by properly settg sutable power levels of o-orthogoal multplexed braches. Results ad dscusso Theory Fg 1. Schematc of our proposed dgtal doma power dvso multplexed dual polarzato coheret optcal OFDM trasmsso system: (a) system model, (b) dgtal doma power dvso multplexg module, (c) OFDM frame structure, (d) SIC algorthm based power dvso de-multplexg module. Fg 1(a) gves a global vew of the system model of our proposed dgtal doma power dvso multplexed dual polarzato coheret optcal OFDM (PDM-DP-CO-OFDM) trasmsso system. The dgtal sgal processg (DSP) module at the trasmtter sde of covetoal OFDM modulato cludes seral-to-parallel (S/P) coverso, symbol mappg, dscrete Fourer trasform (DFT) spreadg, verse fast Fourer trasform (IFFT), ad the parallel-to-seral (P/S) coverso. Two braches of OFDM basebad sgal are overlad together wth dfferet power levels before dual polarzato modulato process. After trasmsso alog the optcal fber chael, at the recever sde, the sychrozato cludg frame symbol alasg ad frequecy offset estmato/compesato s coducted frst. The, seral-to-parallel (S/P) coverso s completed before the fast Fourer trasform (FFT), after whch multple put multple output (MIMO) processg s performed to separate sgals wth dfferet polarzato states. Fally, verse DFT spreadg ad phase recovery are carred out before de-multplexg overlad sgals wth dfferet power levels. Suppose D ad F are the DFT spreadg matrx ad FFT matrx respectvely, ( )ad ( x ) are the basebad QAM symbols ad OFDM symbols respectvely, the the DFT-spread OFDM modulato process could be expressed as Where P x F Z DX ( 1,2,..., ) (1) P Z s the zero-paddg matrx used to pad the data-abset subcarrers wth zeros. As show Fg 1(b), multple basebad OFDM sgals ( x )are learly combed together after codg ad modulato usg power dvso multplexg dgtal doma to form oe ew basebad verso x p x X

3 where p s the power of th p F Z DX P F 1 Z D p X P (2) sgal, ad the total power s ormalzed, amely p 1. The dea of our o-orthogoal scheme s to utlze power dvso multplexg terms of usg the same frequecy but explotg dfferet power levels. Therefore, the multplexed sgals should have dfferet power levels. Wthout loss of geeralty, we assume that the power levels are arraged descedg order, amely p1 p2 p. The OFDM frame structure s depcted Fg 1(c) to show how two basebad OFDM sgals are overlad together. After trasmsso through the commucato chael, suppose h ad H are the temporal ad frequecy respose of the basebad sgal respectvely, s the phase rotato matrx caused by phase ose, the the receved power multplexed sgal ca be expressed as r h* x N F HFx N F HZ PD p N 1 X (3) where N s the addtve whte Gaussa ose (AWGN). The the receved sgal after OFDM demodulato, ca be expressed as T T y D Z( H H) H Fr Where Z R R ( ) p T T ( R ) ( P ) D Z H H H F F HZ D p X N p X D Z( H H ) H FN T T R HX kn 1 (4) s the zero-removg matrx used to remove the data-abset subcarrers, th H s the chael respose of the multplexed basebad QAM symbols, s a costat ose coeffcet. Evetually, the detected sgal wll be demodulated ad decoded oe by oe accordg to the descedg order of power levels usg SIC algorthm, whose flow chart s show Fg 1(d). It s worth metog that the overlad sgals ca also be separated by herarchcal de-mappg 7. Smulato k Fg 2. BER of both QPSK braches versus power dvso rato

Fg 3. Costellato dagram: (a) 2:1, (b) 4:1, (c) 6:1, (d) 8:1 Wthout loss of geeralty, we study the dgtal doma power dvso multplexg scheme the scearo of two braches of QPSK-OFDM sgals ths work.

SNR= 8 db. Two basebad QPSK-OFDM sgals are geerated from PRBS 2 15 usg 512 pots IFFT amog whch 232 subcarrers are used to carry data formato.

We also show the evoluto of the costellato dagram vs dfferet PDR Fg 3.

4 Fg 3. Costellato dagram: (a) 2:1, (b) 4:1, (c) 6:1, (d) 8:1 Wthout loss of geeralty, we study the dgtal doma power dvso multplexg scheme the scearo of two braches of QPSK-OFDM sgals ths work. To vestgate the relatoshp betwee the power dvso rato (PDR, defed as P1/P2) ad BER performace of each power multplexed brach, we frstly coduct Matlab based smulato uder the codto of AWGN chael wth SNR= 8 db. Two basebad QPSK-OFDM sgals are geerated from PRBS 2 15 usg 512 pots IFFT amog whch 232 subcarrers are used to carry data formato. From Fg 2, we ca coclude that the optmzed PDR to mmze the BER of the secod brach wth lower power s 4:1, whch case the two superposed QPSK could be emulated by a 16QAM costellato. We also show the evoluto of the costellato dagram vs dfferet PDR Fg 3. Expermet OOFDM Trasmtter Offle DSP AWG I Q(12G Sa/s) RF Polarzato MUX 40km SMF Fber Lk 40km SMF DELAY PC π/2 PC ECL1 I/Q Modulator Itesty Modulator PBS PBC EDFA1 VOA1 Rama Amplfer Rama Amplfer OOFDM Recever PBS DSP Recever 50 G sa/s DSO 90⁰ Hybrd 90⁰ Hybrd OSA 10:90 EDFA3 TOF EDFA2 VOA2 PBS ECL2 After TOF Before TOF Fg 4. Expermetal setup. ECL: Exteral cavty laser. AWG: arbtrary waveform geerator. PC: Polarzato cotroller. PBS/C: Polarzato beam spltter/comber. EDFA: Erbum doped fber amplfer. VOA: Varable optcal atteuator. TOF: Tuable optcal flter. OSA: Optcal spectrum aalyzer. To verfy the proposed dgtal doma power dvso multplexg scheme, we performed further expermets usg the setup show Fg. 4. At the trasmtter, two basebad QPSK-OFDM sgals are geerated Matlab orgated from PRBS usg 512 pots IFFT amog whch 232 subcarrers are used to carry data formato. To combat the fber dsperso, the cyclc prefx (CP) s set to 64. Each OFDM frame has 140 OFDM

5 BER symbols ad at the begg of each OFDM frame are 15 trag symbols (TSs), oe of whch s employed for OFDM tmg sychrozato ad the rest fourtee are used for polarzato de-multplexg ad chael estmato. Two braches of QPSK-OFDM sgals wth power dvso rato 4:1 are overlad together dgtal doma ad the D/A coverted by a AWG (Tektrox, 7122C) at a samplg rate of 12GSa/s. After upcoverso the modulated optcal sgal s the duplcated to three copes by a testy modulator order to esure orthogoal bad multplexg wth chael spacg of GHz 8. Therefore, the total data rate s / /140 = Gb/s after sub-bad multplexg. Afterward, the mult-bad optcal sgal s polarzato multplexed by a par of polarzato beam spltter/comber wth oe brach delayed by oe OFDM symbol. The trasmsso lk cotas several spas of 40km SSMF whose loss s fully compesated by Rama amplfers. At the recever, a VOA followed by a EDFA s used to adjust the optcal sgal ose rato (OSNR) of the total optcal lk, whch ca be measured by optcal spectrum aalyzer. A tuable optcal flter (TOF) s used to select the requred bad before the dual polarzato coheret recever, after whch two IQ compoets are acqured by a osclloscope at 50GSa/s ad the spectrally overlad OFDM sgal s power demultplexed by a SCI recever after phase recovery of both polarzato trbutares durg the offle DSP Brach1 W/ SIC Brach2 W/ SIC Brach1 W/O SIC Brach2 W/O SIC Lauchg Power(dBm) Fg 5. BER versus lauch power wth 480km trasmsso both power dvso multplexed braches usg SIC algorthm ad herarchcal de-mappg BER Brach1 W/ SIC Brach2 W/ SIC Brach1 W/O SIC FEC Lmt 3.8e-3 Brach2 W/O SIC OSNR(dB) Fg 6. BER versus OSNR wth 480km trasmsso both power dvso multplexed braches usg SIC algorthm ad herarchcal de-mappg. I our expermet, herarchcal de-mappg s also employed to demodulate the two multplexed QPSK braches ad ts performace s compared wth that demodulated by SIC algorthm. I coveece, W/O SIC s used to represet herarchcal de-mappg the followg fgures. We frst vestgated the trasmsso performace of our proposed system wth fber lk of 480km. As show Fg 5, the optmal lauchg power s foud to be -7dBm, regardless of the power dvso de-

BER multplexg method we use. The we vary OSNR of the receved optcal sgal to check the BER performace of both QPSK braches wth lauchg power fxed at -7dBm.

6 BER multplexg method we use. The we vary OSNR of the receved optcal sgal to check the BER performace of both QPSK braches wth lauchg power fxed at -7dBm. As ca be see from Fg 6, the QPSK Brach1 allocated wth hgher power ejoys a OSNR redudacy of about 2dB tha the QPSK Brach2 to reach the FEC lmt of BER=3.8e-3, whle SIC algorthm shows a OSNR mprovemet of about 0.5dB compared wth herarchcal de-mappg. We also cofrmed the robustess of our proposed system wth exteded fber lks Fg 7, whch proves the feasblty of dgtal doma power dvso multplexg log-reach optcal coheret trasmsso systems compatble wth curret wavelegth dvso multplexg (WDM) ad space dvso multplexg (SDM) techologes. Obvously, SIC algorthm ca mprove the trasmsso dstace of both multplexed QPSK braches, whle the QPSK Brach1 wth hgher power level ejoys a trasmsso redudacy of about 250km tha QPSK Brach2 wth lower power level to reach the same FEC lmt of BER=3.8e-3. To esure detcal BER performace of the two power multplexed sgals, stroger FEC codg such as soft-decso FEC ca be used for weaker sgals. The costellato dagrams after 480km SSMF trasmsso are depcted Fg 8, whch agrees well wth the smulato predctos FEC Lmt 3.8e Brach1 W/ SIC Brach2 W/ SIC Brach1 W/O SIC Brach2 W/O SIC Fber Legth(km) Fg 7. BER versus fber legth both power dvso multplexed braches usg SIC algorthm ad herarchcal de-mappg. Fg 8. Costellato dagrams at legth=480km. (a) Brach1 Pol-X, (b) Brach2 Pol-X, (c) Brach1 Pol-Y, (d) Brach2 Pol-Y. Cocluso Whle chasg hgh capacty to meet the creasg traffc demads, the ext geerato optcal etwork features the flexblty. Tradtoal physcal multplexg dmesos lke tme, frequecy ad space have lttle room to adjust multplexed sgal qualty due to the physcal strget orthogoal codto thus sacrfce the flexblty. I ths paper, we propose a DP-CO-OFDM trasmsso scheme based o dgtal doma o-orthogoal power dvso multplexg, ad verfed t a 100-Gb/s trasmsso over 1440km fber lk. Our scheme proves to be

7 feasble wth a early doubled system capacty whe two basebad QPSK-OFDM sgals of dfferet power levels are overlad together before optcal modulato process ad successvely decoded after phase recovery usg ether SIC algorthm or herarchcal de-mappg, whle SIC algorthm turs to be a better choce to mprove BER performace. Besdes, o-orthogoal dgtal doma power dvso multplexg s compatble wth curret dual polarzato WDM ad SDM systems. Moreover, classfed qualty of servce (QoS) ca also be realzed aytme by smply adjustg the power ratos ad modulato formats of dfferet subscrbers wth varous badwdth requremets ad trasmsso dstaces. Above all, t s a promsg techque to mprove the system capacty ad flexblty smultaeously for future etwork costructo, expaso, mateace ad operato. Methods Power de-multplexg usg SIC Algorthm As show Fg 1(c), The SIC process maly cossts of three steps: (1) estmate the chael respose ad demodulate the stroger sgal x whle treatg all the other sgals x x as terferece ose; (2) re-modulate the estmated sgal x ˆ ad multply t by the chael respose H before subtract the product from the receved sgal ad the decode the weaker sgal x 1 ; (3) cotue wth the aforemetoed steps utl all the sgals are decoded. As SIC algorthm suppresses terferece from stroger (earler decoded) sgals for relatvely weaker (yet to be demodulated) sgals, eve the sgals wth lower power ca be correctly recovered. Herarchcal de-mappg of two overlad QPSK braches Fg 9 draws the mappg costellato of sgle QPSK brach ad two overlad QPSK braches. Whe demodulate wth SIC algorthm, two overlad QPSK braches are separated frst ad the both de-mapped usg sgle QPSK mappg costellato o the left. However, whe demodulate wth herarchcal de-mappg, two overlad QPSK braches are straghtly demapped usg the herarchcal mappg costellato o the rght, the the frst two bts are allocated to the QPSK brach wth hgher power level whle the rest two bts are allocated to the lower oe. y Refereces Fg 9. Mappg costellato of sgle QPSK brach ad two overlad QPSK braches 1. Peter J. Wzer. Makg spatal multplexg a realty. Nature Photo. 8, , (2014). 2. Yuk Edo et al. Uplk o-orthogoal access wth MMSE-SIC the presece of ter-cell terferece. Iteratoal Symposum o Wreless Commucato Systems (ISWCS),pp , Aug Husey Hac et al. Performace of No-orthogoal Multple Access wth a Novel Iterferece Cacellato Method. IEEE ICC 2015, Lodo, UK, Jue H. Marshoud et al. No-Orthogoal Multple Access for Vsble Lght Commucatos. Photo. Techo. Lett., vol.28, o.1, 51-54, Ja Zhehua Feg et al. Dgtal Doma Power Dvso Multplexg DDO-OFDM Trasmsso wth Successve Iterferece Cacellato. CLEO: Scece ad Iovatos,2016. pp. SF2F Zhehua Feg et al. Spectrally Overlad DDO-OFDM Trasmsso Eabled by Optcal Power Dvso Multplexg. ICOCN pp.t3-o-02.

8 7. Jag H, Wlford P A. A herarchcal modulato for upgradg dgtal broadcast systems. IEEE Tras. Broadcastg, 2005, 51(2): Y, Xgwe, et al. Tb/s coheret optcal OFDM systems eabled by optcal frequecy combs. J. Lghtwave Techol. 28, (2010).

9 Ackowledgemets (ot compulsory) Ths work was supported by the Natoal Natural Scece Foudato of Cha (NSFC) uder Grat No , the 863 Hgh Techology Pla of Cha (2013AA013402) ad the Program for New Cetury Excellet Talets Uversty (NCET ). Author cotrbutos statemet M.T., Q.W., ad Z.F. coceved the study. X.L., M.L., H.Z. ad Q.W. desged ad dd the expermet. Q.W., Z.F. ad M.T. developed the theory ad aalyzed the data. Q.W., Z.F., M.T., S.F. ad D.L. prepared the mauscrpt. Competg facal terests: The authors declare o competg facal terests.

Module 6. Channel Coding. Version 2 ECE IIT, Kharagpur

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