Optical Fiber Technology 15 (2009) 222225

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10doIntroduction

The mm-wave bands would be utilized to meet the requirementr broadband service and overcome the frequency congestion ine future ROF-based optical-wireless network. In ROF system anter station (CS) is connected to many functionally simple baseations (BSs) via optical ber. Almost all processing includingodulation, demodulation, coding, routing are performed at the[14]. The main function of the BS is to realize optical/wirelessnversion and broadcasting by antenna. Novel schemes of wave-ngth reuse or centralized lightwave in the central oce (CO)ve been proposed and experimentally demonstrated [2,59].rthogonal frequency division multiplexing (OFDM) system canovide excellent tolerance towards multipath delay spread andequency-dependent channel distortion. In recent research, it ismonstrated that OFDM will become a strong candidate for trans-ission signals in the next generation long-haul and access net-orks because of its high spectrum eciency and the resistancechromatic dispersion and polarization mode dispersion [1016].the combination of OFDM and ROF is naturally suitable for

tical-wireless systems to increase the bandwidth and extend theansmission distance of mm-wave over both ber and air links.e generation of low-cost mm-wave for carrying OFDM signal ise of the key technologies for OFDM-ROF system [1214]. Thetical millimeter generation by frequency quadrupling techniqueas proposed in Ref. [9]. Because a low RF oscillator can be usedgenerate optical millimeter-wave signal with frequency quadru-

Corresponding author at: School of Computer and Communication, Hunan Uni-rsity, Changsha 410082, China.E-mail address: liliuchen12@vip.163.com (L. Chen).

pling and sextupling, it has been considered a cost-effective so-lution. In this paper, we utilized a full-duplex ROF architectureas shown in Ref. [8] to transmit 2.5 Gbit/s 16QAM OFDM sig-nals on 40 GHz millimeter-wave generated by multiple double-frequency technique. The constellation diagrams of the receivedsignal before and after transmission over the ber are obtained.Both down-stream and upstream signals transmission over 20-kmconventional single-mode ber (SMF-28) have been experimentallydemonstrated.

2. System architecture

Fig. 1 shows the principle of frequency quadrupling and wave-length reuse scheme for up-link connection in OFDM-ROF system.An intensity modulator (IM) and a cascaded optical lter are em-ployed to generate optical mm-wave and provide the lightwavesource for upstream data modulation. To realize optical mm-wavecarrier with four times of LO frequency, the IM needs to be theDC-biased at the top peak output power when the LO signal is re-moved [7]. If the repetitive frequency of the radio-frequency (RF)microwave source is f, the rst-order modes are suppressed andthe frequency spacing between the second-order modes is equalto 4f. Then an optical lter is employed to separate the optical car-rier from the two second-order sidebands. The OFDM analog dataare carried by the second-order sidebands via another intensitymodulator (IM). Then the modulated mm-wave signals are com-bined with the optical carrier by using an optical coupler (OC).After transmission over the ber, the optical mm-wave signals areseparated from the optical carrier by an optical lter. The opti-cal mm-wave signals are detected by a high-speed receiver. In thebase station, the down-converted upstream data are modulated byContents lists availab

Optical Fiber

www.elsevier.co

radio-over-ber system with photonic gavelength reuse for upstream data conne

. Chen a,b,, J. Lu a,b, J. He a,b, Z. Dong a,b, J. Yu a,b

ey Laboratory for Micro/Nano Opto-Electronic Devices of Ministry of Education, Hunan Univchool of Computer and Communication, Hunan University, Changsha 410082, China

r t i c l e i n f o a b s t r a c t

ticle history:ceived 17 July 2008vised 19 February 2009ailable online 28 March 2009

ywords:dio-over-bertical mm-waveDM signal

We have experimentally demover-ber (ROF) system with2.5 Gbit/s 16QAM OFDM sigare generated with four timedown- and up-stream signal68-5200/$ see front matter 2009 Elsevier Inc. All rights reserved.i:10.1016/j.yofte.2009.02.005at ScienceDirect

echnology

locate/yofte

erated 16QAM OFDM signals andtion

y, Changsha 410082, China

strated a wavelength reuse scheme for up-link connection in a radio-otonics generated 2.5 Gbit/s 16QAM OFDM signals. In this architecture,are carried by the optical millimeter-wave (mm-wave) carriers which

equency of the local oscillator (LO) signal. The power penalties for bothvery over 20 km ber are less than 1 dB.

2009 Elsevier Inc. All rights reserved.

chno

Fi02

anth

3.

ligLDbyulcaanseFig. 1. Principle diagram of wavelength reuse scheme for up-link connection in mm-wave OFDM-ROF system.

g. 2. Experimental setup for OFDM-ROF system. The resolution for optical spectrum insets (i)(iii), (v) is 0.5 nm. The resolution for optical spectrum insets (iv), (vi), (vii) is5 nm.

other IM before the upstream optical signals are transmitted toe CS.

Experimental setup and results

Fig. 2 shows the experimental setup for OFDM-ROF system. Thehtwave generated by a distributed-feedback laser diode (DFB-) at 1541.54 nm is modulated by an intensity modulator drivena 10 GHz RF microwave signal. The optical spectrum after mod-

ation is inserted in Fig. 2 as inset (i). After modulation, it isn be seen that the odd-order sidebands are almost suppressed,

second-order sidebands is 0.32 nm (40 GHz). The fourth-ordersidebands is 20 dB lower than the second-order sidebands. An op-tical interleavers (IL) with 50/25 GHz channel spacing is employedto separate the optical carrier and the second-order sidebands. Weused a tunable laser with 2 nm tunable range to enable the cor-rect separation. This interleaver has a 3 dB bandwidth of 0.15 nm,therefore, the wavelength tolerance of the interleaver cannot bevery large.

The spectrum of the separated carrier is shown in Fig. 2 as in-set (ii). Then the optical mm-wave is modulated by the second IML. Chen et al. / Optical Fiber Ted the power of optical carrier is 12 dB larger than that of thecond-order sidebands. The wavelength spaceing between the two

dratlogy 15 (2009) 222225 223iven by the 2.5 Gbit/s OFDM baseband signal which are gener-ed oine by Matlab program. The OFDM is baseband signal. The

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OinerbyinplnotoelpahiissiacaletwnatrresearPDFig. 4. BER curves for (a) upstream and (b) downstream data.

FDM baseband signals are calculated oine with Matlab programcluding mapping 2151 PRBS into 256 16QAM-encoded subcarri-s, subsequently converting the OFDM symbols into time domainusing IFFT and then adding 32 pilot signal in notch. Guard

terval length is 1/4 OFDM period. 10 training sequences are ap-ied for each 150 OFDM-symbol frame in order to enable phaseise compensation. The digital waveforms are then downloadedan arbitrary waveform generator (AWG) to generate 2.5 Gbit/s

ectrical OFDM signal waveform. At the output of the AWG low-ss lters (LPF) with 5 GHz bandwidth are used to remove thegh-spectral components. The optical spectrum after modulationshown in Fig. 2 as inset (iii). The modulated optical mm-wavegnals are combined with the separated optical carrier by using3-dB OC before they are transmitted over 20-km SMF-28. If werefully choose the ber length which connect the OC and inter-aver between the two separated signal, the delay between theo separated signals after the interleaver does not affect the sig-l performance because the delay is very short. The optical spec-um after OC and EDFA is shown in Fig. 2 as insets (iv) and (v),spectively. After transmission, the optical mm-wave signals areparated from the optical carrier by using another IL. The sep-

signal is amplied by an electrical amplier (EA) with a band-width of 10 GHz centered at 40 GHz. An electrical local oscillator(LO) signal at 40 GHz is generated by using a frequency multi-plier from 10 to 40 GHz. We use the electrical LO signal and amixer to down-convert the electrical mm-wave signal to retrievethe downlink baseband signals, while the separated optical carrieris re-modulated by a 2.5 Gbit/s upstream signal. The optical spec-tra from the two ports of the second IL are shown in Fig. 2 asinsets (vi) and (vii). The eye diagram of the upstream data aftertransmission over 20 km SMF is shown in Fig. 2 as inset (viii). Thedown-converted signals are sampled with a real-time digital os-cilloscope. The received data are processed and recovered off-linewith a Matlab program as an OFDM receiver and obtain the BERperformance. Fig. 3 (a) and (b) show the constellation diagram ofthe received signal before and after transmission over the ber,respectively. The effect of ber dispersion can be neglected by us-ing the electrical OFDM signals. Compared with the B-T-B case, theconstellation diagram performance is still good. We measure theBER performance for both up- and down-stream signals in Fig. 4(a) and (b), respectively. Fig. 4 shows the up- and down-stream4 L. Chen et al. / Optical Fiber Tec

Fig. 3. The constellation diagram of the demodulated sigated optical mm-wave is detected by O/E conversion via a PINwith a 3-dB bandwidth of 60 GHz. The converted electrical

sipelogy 15 (2009) 222225

(a) before transmission and (b) after transmission.gnal after delivery over 20 km ber has 1 dB and 0.5 dB powernalty, respectively. It should be pointed out that the BER mea-

L. Chen et al. / Optical Fiber Technology 15 (2009) 222225 225

surement for OFDM signal is based on off-line processing. For apractical system, a real-time processing will be needed.

4. Conclusion

We have proposed and experimentally demonstrated a wave-length reuse scheme for up-link connection in a radio-over-ber(ROF) system with photonics generated 2.5 Gbit/s 16QAM OFDMsignals. The 2.5 Gbit/s electrical OFDM signals is transmitted overthe 40 GHz optical millimeter wave signals which are generatedby using multiple double-frequency techniques. In this scheme, therepetitive frequency of the RF source and the bandwidth of opticalmodulator are largely reduced. The separated high power opticalcarrier is re-modulated in the base station; hence the all opticalpower can be eciently utilized. The power penalty of down-stream signal delivery over 20 km ber is less than 1 dB. The effectof ber dispersion can be neglected by using the OFDM signals. Webelieve that this multiple double-frequency technique to generatemm-wave signal to carry OFDM signal is a practical scheme forfuture broadband ROF network.

Acknowledgments

This work is partially supported by the National 863 high-te20oforMTe

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[1

[1

LimBevepacoch research and development program of China under Grant07AA01Z263, the Hunan Provincial Natural Science FoundationChina (Grant No. 06JJ50108) and the Open Fund of Key Lab-atory of Optical Communication and Lightwave Technologies,inistry of Education, PR China (Beijing University of Posts andlecommunications).

ferences

1] J. Ma, J. Yu, C. Yu, Z. Jia, G.K. Chang, The inuence of ber dispersion onthe code from of the optical mm-wave signal generated by single sidebandintensity-modulation, Opt. Commun. 271 (2006) 396403.

2] L. Chen, H. Wen, S. Wen, A radio-over-ber system with a novel scheme formillimeter-wave generation and wavelength reuse for up-link connection, IEEEPhoton. Technol. Lett. 19 (2006) 20562058.(2007) 483485.5] A. Kim, H.J. Yong, K. Yungsoo, 60 GHz wireless communication systems with

radio-over-ber links for indoor wireless LANs, IEEE Trans. Consum. Elec-tron. 50 (2004) 517520.

6] L. Chen, J. He, Y. Li, et al., Simple ROF conguration to simultaneously realizeoptical millimeter-wave signal generation and source-free base station opera-tion, ECOC 2 (2007) 4546.

n Chen was born in 1968. He received the Ph.D. degree in optical com-unications from the Beijing University of Posts and Telecommunications,ijing, China, in June 2004. He is currently a Professor at Hunan Uni-rsity, Changsha, China. He has authored or coauthored over 40 journalpers, His current research interests include polarization mode dispersionmpensation, new modulation format techniques, and radio over ber.

A radio-over-fiber system with photonic generated 16QAM OFDM signals and wavelength reuse for upstream data connectionIntroductionSystem architectureExperimental setup and resultsConclusionAcknowledgmentsReferences