ABSTRACT. Keywords: OFDMA, optical beating interference, balanced detection, optical access network 1. INTRODUCTION

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1 Optical Beat Iterferece Noise Reductio i OFDMA Optical Access Lik Usig Self-Homodye Balaced Detectio Sag-Mi Jug a, Yog-Yuk Wo b, Sag-Kook Ha* a Departmet of Electrical ad Electroic Egieerig, Yosei Uiversity, 50 Yosei-ro, Seodamugu, Seoul, South Korea; b Yosei Istitute of Covergece Techology, Yosei Uiversity, 6- Sodo-dog, Yeosu-gu, Icheo, South Korea ABSTRACT A Novel techique for reducig the OBI oise i optical OFDMA-PON uplik is preseted. OFDMA is a multipleaccess/multiplexig scheme that ca provide multiplexig operatio of user data streams oto the dowlik sub-chaels ad uplik multiple access by meas of dividig OFDM subcarriers as sub-chaels. The mai issue of high-speed, sigle-wavelegth upstream OFDMA-PON arises from optical beatig iterferece oise. Because the sub-chaels are allocated dyamically to multiple access users over same omial wavelegth, it geerates the optical beatig iterferece amog upstream sigals. I this paper, we proposed a ovel scheme usig self-homodye balaced detectio i the optical lie termial (OLT) to reduce OBI oise which is geerated i the uplik trasmissio of OFDMA-PON system. Whe multiple OFDMA sub-chaels over the same omial wavelegth are received at the same time i the proposed architecture, OBI oises ca be removed usig balaced detectio. Usig discrete multitoe modulatio (DMT) to geerate real valued OFDM sigals, the proposed techique is verified through experimetal demostratio. Keywords: OFDMA, optical beatig iterferece, balaced detectio, optical access etwork. INTRODUCTION I recet years, orthogoal frequecy divisio multiple access (OFDMA) based passive optical etwork (PON) system has emerged i the area of the optical commuicatios with its various advatages such as ease of maiteace, trasparecy of data format, robust security, ad its strog scalability with wavelegth divisio multiplexig (WDM) []. A OFDMA is a multiple-access/multiplexig scheme based o orthogoal frequecy divisio multiplexig (OFDM), so it ca provide multiplexig operatio by allocatig subcarrier subsets for data streams of users. These subcarrier sub-chaels are cosists of multiple orthogoal subcarriers of OFDM frames ad could be modulated idividually for each users. Because these modulated user data streams are orthogoal to each other i the frequecy domai, cross-talks betwee the sub-chaels are elimiated ad make the arrow subcarrier spacig. Ad with chagig subcarrier allocatio of each user as time, it ca provide the dyamic badwidth allocatio withi the overall OFDMA badwidth. With the trasparecy of subcarriers, the adaptive modulatio techique which depeds o the idepedet chael respose of each subcarriers makes this system has flexible ad provide efficiet badwidth usage. However, optical beat iterferece (OBI) oise is a issue i the realizatio of multiple access operatio i the uplik scheme of OFDMA PON. A OBI oise is the optical iterferece amog the multiple lightwaves, ad it is geerated whe the differet user s multiple access sigals are trasmitted o the lightwaves of same omial wavelegth. I the uplik trasmissio of OFDMA PON, this is a critical issue, because the OFDM sub-chaels for access users are dyamically allocated over the same omial wavelegth. I the same way, the OBI oise was the mai obstacle i the developmet of a subcarrier multiplexed (SCM) PON because SCM PON also uses several radio frequecy (RF) subcarriers over the same wavelegth i order to distiguish differet users [3]. The techique of avoidig OBI oise usig optical carrier suppressio ad coheret detectio has bee proposed [6]. However, wavelegth stability is required i the optical etwork uit (ONU) due to the optical carrier suppressio, ad it is difficult to implemet a cost-effective ONU because it uses a Mach-Zehder modulator. I additio, a additioal optical device such as a optical iterleaver, is required to avoid the OBI effect. The techique of usig a optical source with differet wavelegths at each ONU has bee proposed to avoid OBI oise [7-8]. Ufortuately, it would make the burde too heavy for the etwork provider because the repair ad maiteace costs of the etwork would icrease due Optical Metro Networks ad Short-Haul Systems VI, edited by Werer Weiershause, Bejami B. Digel, Achyut K. Dutta, Atul K. Srivastava, Proc. of SPIE Vol. 9008, 90080M 04 SPIE CCC code: X/4/$8 doi: 0.7/ Proc. of SPIE Vol M-

2 to the ivetory of differet optical ad electrical devices. Similarly, it is a well-kow fact that the OBI oise has bee prevetig the developmet of a subcarrier multiplexed (SCM)-PON because a SCM-PON also uses several radio frequecy (RF) subcarriers over a sigle light with the same wavelegth i order to distiguish differet users [3-6]. I this paper, we proposed that a ovel scheme usig self-homodye balaced detectio i the optical lie termial (OLT) to reduce OBI oise which geerated i the uplik trasmissio of OFDMA-PON system. Whe multiple OFDMA sub-chaels over a same omial wavelegth are received at the same time i the proposed architecture, OBI oises, which are geerated amog the all of OFDMA chaels, ca be removed usig balaced detectio, while the itesity of OFDMA chael are icreased by 3-dB. We use a discrete multitoe modulatio (DMT) to geerate real valued OFDM sigals. The proposed techique is verified through experimetal demostratio.. SCHEMATICS Fig. shows the cocept of reducig OBI oise i a OFDMA-PON uplik usig self-homodye balaced detectio. As show i Fig., we ca describe the OFDM sigal icomig from the th ONU as E ( t) = A ( t) exp( jω t) (.) s where A (t) is amplitude of the th uplik OFDM sigal ad ω s is the optical frequecy. Similarly, the optical field of the local laser prepared at the balaced receiver ca be writte as E () t = A exp( jω t) (.) LO LO LO where A LO is the amplitude of the local laser ad ω LO is the optical frequecy of the local laser. The, the OFDM sigal power of the th ONU P ad the local laser power P LO could be preseted with complex amplitudes A ad A LO as P = A / (.3) P LO LO = A / (.4) At the OLT, upstream sigals are detected by the balaced receiver. Whe the uplik optical OFDM sigals ad the cotiuous wave (CW) light of the local laser are co-polarized by a polarizatio cotroller (PC), the optical fields icidet o the upper ad lower photodiodes at the balaced photo diodes ca be give as E () t + E () t = E () t + E () t E () t + E () t at upper photodiode (.5) s LO LO E () t E () t = E () t + E () t E () t E () t at lower photodiode (.6) s LO LO pap.i obpc biji owl Figure. OBI oise reductio i the OFDMA-PON lik usig self-homodye balaced receiver Proc. of SPIE Vol M-

3 where E s (t) is the total upstream optical field from the ONUs. The, the output curret of each photo detector ca be writte as (.7) Ak ( t)exp( jωst) + ALO exp( jωlot) I() t = RRe R = Pk() t+ PLO + Pk() tpj() t+ Pk() tplo cos {( ωs ωlo) t θk() t θlo() t} + j= k+ (.8) Ak ()exp( t jωst) ALO exp( jωlot) I() t = RRe R = Pk() t+ PLO + Pk() tpj() t Pk() tplo cos {( ωs ωlo) t θk() t θlo() t} + j= k+ where Re deotes the real part, ad θ k (t) ad θ LO (t) are phases of the OFDM sigal ad local laser, respectively. R is the resposibility of the photodiode. I each equatio, the beatig terms betwee the uplik ONU sigals are the OBI oise, as show i Eq. (.7) ad (.8) P ( tp ) ( t) k j= k+ j (.9) which is expressed i the product betwee multiple ONUs. Accordigly, the output of the balaced receiver ca the be give as {( ω ω ) θ θ } It () = I() t I() t = R Pk() tplo cos s LO t+ k() t LO() t where P LO is always costat ad θ LO (t) icludes oly the phase oise that varies i time. I the proposed scheme, Eq. (.0) ca be chaged ito Eq. (.) because self-homodye detectio (ω s = ω LO ) is employed. k LO ( θk θlo ) It () = I() t I() t = R P() tp cos () t () t As show i Eq. (.), OBI oises, which are geerated amog multiple ONUs, are reduced perfectly, while the sigal to oise ratio (SNR) of each OFDM sigal ca be improved sigificatly usig balaced detectio with the help of a local laser. (.0) (.) 3. EXPERIMENTAL SETUP Fig. shows the experimetal setup for the proposed scheme. The DMT sigal with quadrature phase shift keyig (QPSK) symbol mappig for each subcarriers was geerated by offlie processig. The umber of DMT subcarriers was 56 ragig from DC to GHz. Some of DMT subcarriers (from the 0 th subcarrier to the 0 th subcarrier) were allocated for the first ONU, ad the others (from the 35 th subcarrier to the 40 th subcarrier) were allocated for the secod ONU. There was guard bad betwee the ONUs (from the th subcarrier to the 34 th subcarrier). The fast Fourier trasform (FFT) size was 5 due to Hermitia symmetry. Total trasmissio throughput was 3.Gb/s. The calculated DMT sigal was loaded ito a -Gs/s arbitrary waveform geerator (AWG: Tektroix 7B) ad the a reflective semicoductor optical amplifier (RSOA) was droved by the DMT sigal of AWG 7B with the modulatio depth of 00 %. A exteral cavity laser (ECL) with a ceter wavelegth of m provided the CW light for the ONU. Its liewidth was 50 khz with the output optical power was 8 dbm. A self-homodye apparatus was used to geerate the OBI oise usig light from a ECL[4]. This method causes the effect of iterferece betwee two ucorrelated sources Proc. of SPIE Vol M-3

4 which are geerated by oe optical source. The legth of the upper arm i the self-homodye apparatus (5-km fiber legth) was adequately adjusted to break the coherece betwee the DMT sigals over the delayed light ad the directly passed light. Aother self-homodye apparatus, which is composed of a 0-km sigle mode fiber (SMF) ad a 0-km SMF, was added i order to check the OBI effect as the umber of ONU icreased to four. As show i Fig., a uidirectioal optical lik was implemeted i order to elimiate the Rayleigh backscatterig effect so that oly the OBI effect is geerated i the proposed scheme. The measured 3-dB badwidth of the used RSOA was less tha GHz. The uplik sigal modulated by the DMT sigal at the RSOA was trasmitted to the 40-GHz coheret receiver (ut: CPRV4A) through a 0-km SMF. They were captured by a digital phosphor oscilloscope (DPO: Tektroix 7004B) with a samplig speed of 50 Gs/s. izois4ol EDEV owes izopf4ol CaNsL' Kov,,xsdaw,,xsdaw,xsdw Figure. Experimetal setup for the verificatio of OBI oise reductio i a OFDMA-PON lik based o self-homodye balaced detectio 4. RESULTS AND DISCUSSION To verify the proposed scheme for suppressig the OBI oise i a OFDMA-PON lik, it is ecessary to measure the performace differece betwee a direct detectio ad balaced detectio. Fig. 3 shows the measured bit error rate (BER) curves as the itesity of ECL chages from -3 dbm to 5 dbm. Filled squares show the BER results of oe-arm detectio of balaced receiver. I here, oe-arm detectio meas that oly upper photodiode of balaced receiver was used. Filled circles show the BER curves of balaced detectio. As show i Eq. (.), it was expected that the SNR of DMT sigal would be improved whe the output power (P LO ) of the local laser icreased. However, it was observed that the BER values of the oe-arm chael as well as the balaced chael decreased to a BER of less tha 0-5 as the itesity of the ECL icreased to 5 dbm as expected i Eqs. (.7) ad (.8). I case of self-homodye balaced detectio, the BER was also reduced compared to that of a oe-arm chael. This reductio occurred because the OBI oise of each chael was suppressed due to the self-homodye balaced detectio, as show i Eqs. (.7), (.8), ad (.). Proc. of SPIE Vol M-4

5 TTT.TU GP Log0(BER) qbu Figure 3. Measured BER curves agaist output power of exteral cavity laser Fig. 4 shows the measured BER results agaist received optical power whe the output power of ECL was 5 dbm. The BER of uplik sigal was measured repeatedly usig a sigle RSOA after the allocated DMT subcarriers for each ONU were loaded oto the AWG. I case of direct detectio usig a sigle photo-diode (filled circles), it was impossible to recover a DMT sigal due to the OBI oise. Also, it was see that the performace of balaced detectio (ope squares, ope triagles) was always better tha that of oe-arm chael (filled squares, filled triagles) irrespective of received optical power. More specifically, the power pealty of about db was reduced due to the use of self-homodye balaced detectio at the BER of 0-3. This reductio occurred because the OBI oise was suppressed effectively ad the SNR of DMT sigal was improved s- -o- -A- Oe-arm Ch_ (BB) Balaced Ch. (BB) Oe-arm Ch_ (0km) Balaced Ch. (0km) Direct Detectio (0km) -4- oi) o I I I I I Received optical power (dbm) Figure 4. Measured BER results as a fuctio of received optical power Fig. 5 shows the differece betwee the BERs of the oe-arm chael ad balaced chael as the umber of chael goes up to four. Four chaels were implemeted by addig aother self-homodye apparatus to the OFDMA- PON lik with a sigle self-homodye apparatus, as show i Fig.. I case of the oe-arm chael (filled squares ad filled triagles), the BER deteriorated because the SNR of the DMT sigal was reduced due to the accumulatio of OBI Proc. of SPIE Vol M-5

6 oise as the umber of chaels icreased from two to four. Moreover, the BER results of the balaced chael (ope squares ad ope triagles) idicate that the self-homodye balaced detectio allows the proposed techique to suppress the OBI oise i OFDMA-PON lik, eve though the umber of ONUs goes up to four. Accordigly, we ca expect that the proposed scheme has a high resistace to OBI oise idepedet of the umber of OFDMA-PON chaels. Oe-arm Ch_ Rx power of -3 dbm Balaced Rx power of -3 dbm Oe -arm Rx power of -0 dbm Balaced Rx power of dbm - - á -3- w m ó -4 - o sç AE: -4 A o o The umber of chael Figure 5. Measured BER as a fuctio of the umber of OFDMA chaels for the first ONU. 5. CONCLUSION I this paper, a ew scheme for suppressig the OBI oise i a OFDMA-PON lik was proposed. A selfhomodye balaced detectio ot oly reduced the OBI oise but also icreased the SNR of the DMT sigal i the proposed scheme. I the case of balaced detectio, the receiver sesitivity was improved by db compared with the detectio of a oe-arm chael due to the destructive iterferece of the OBI oise, which is geerated betwee two chaels for balaced detectio. I spite of the growth of the OFDMA-PON chael, a balaced detectio did ot chage the BER, while the oe-arm chael did chage the BER. Cosequetly, we ca expect that this techique would be effective i suppressig OBI oise i OFDMA-PON systems. I additio, it ca provide the cetralized cotrol of OBI oise i OLTs. ACKNOWLEDGMENT This work was supported by the Idustrial Strategic techology developmet program KEIT (004775, Research o 5Gbps optical access etwork based o discrete multitoe eablig dyamic etwork resource maagemet) fuded By the Miistry of Trade, idustry & Eergy (MI, Korea), ad i part by MSIP(Miistry of Sciece, ICT & Future Plaig), Korea i the ICT R&D Program 03. REFERENCES [] J. Armstrog, OFDM for optical commuicatios, J. Lightw. Techol., 7(), (009). [] I. B. Djordjevic, ad B. Vasic, Orthogoal frequecy divisio multiplexig for high-speed optical trasmissio, Opt. Express 4, 3767 (006). [3] W. Shieh, H. Bao, ad Y. Tag, Coheret optical OFDM: Theory ad desig, Opt. Express 6, 96 (008). Proc. of SPIE Vol M-6

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