IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 15, AUGUST 1, 2011 1085

A Novel Lightwave Centralized Bidirectional HybridAccess Network: Seamless Integration of RoF With

WDM-OFDM-PONYu-Ting Hsueh, Student Member, IEEE, Ming-Fang Huang, Shu-Hao Fan, and Gee-Kung Chang, Fellow, IEEE

Abstract—We experimentally demonstrated a novel lightwavecentralized hybrid bidirectional access network for integration ofwavelength-division-multiplexing orthogonal frequency-divisionmultiplexing passive optical network (WDM-OFDM-PON) withradio-over-fiber systems employing multiwavelength generationand the carrier-reuse technique. In this proposed architecture, oneof the main impairments of bidirectional transmission over singlefiber link, Rayleigh backscattering, can be reduced, because of dif-ferent frequencies for downlinks and uplinks. In the wired trans-mission over 25-km single-mode fiber (SMF-28), power penaltiesare less than 0.5 dB for both 11.29-Gb/s OFDM-16 quadratureamplitude modulation (16QAM) downlink and 5.65-Gb/s OFDMquadrature phase-shift keying (QPSK) uplink. Moreover, suc-cessful access network transmissions have been demonstratedsince the error vector magnitude (EVM) measurements for world-wide interoperability for microwave access (WiMAX) 17.28-Mb/sOFDM-64QAM downstream and 11.52-Mb/s OFDM-16QAMupstream are always under the thresholds of IEEE 802.16e.

Index Terms—Radio-over-fiber (RoF), subcarrier-multiplexing(SCM), wavelength-division-multiplexing passive optical network(WDM-PON).

I. INTRODUCTION

T HE demand of bandwidth for end-to-end connectivity ofboth wired and wireless broadband access services is ex-

pected to grow continuously in the coming decades.Wavelengthdivision multiplexing-passive optical network (WDM-PON)has been widely investigated for high data rate wired servicesto provide triple-play services [1], while radio-over-fiber (RoF)technique has recently become a promising approach to meetthe requirements of the future super broadband wireless accesssystem [2]. However, taking account of the system cost andintegration complexity, the design of RoF system is expected tobe compatible with the interface requirements of WDM-PON.Thus it can provide both wired and wireless access servicessimultaneously on the same optical distributed infrastruc-ture. Moreover, orthogonal frequency-division multiplexing

Manuscript received October 11, 2010; revised April 15, 2011; accepted May07, 2011. Date of publication May 19, 2011; date of current version July 15,2011. This work was supported in part by Georgia Tech Broadband Institute(GTBI).Y.-T. Hsueh, S.-H. Fan, and G.-K. Chang are with the School of Elec-

trical and Computer Engineering, Georgia Institute of Technology, Atlanta,GA 30332 USA (e-mail: yhsueh3@mail.gatech.edu; shfan@gatech.edu;gkchang@ece.gatech.edu).M.-F. Huang is with the NEC Laboratories America, Princeton, NJ 08540

USA (e-mail: mhuang@nec-labs.com).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2011.2156402

Fig. 1. Schematic diagram of the hybrid bidirectional access networks.

(OFDM) with high spectrum efficiency and robust disper-sion tolerance are considered as a strong candidate for futurelong-haul and access networks [3]. Recently, simultaneousgeneration and transmission of baseband fiber-optic signalsand RoF wireless signals have been demonstrated [4]–[6].However, these results were achieved by expensive and com-plicated integrated modulators. To our best knowledge, none ofthe reports demonstrated bidirectional transmissions for bothwired and wireless signals utilizing wavelength-reuse schemeover single fiber span. The key challenge of bidirectional trans-mission lies in Rayleigh backscattering (RB) which can causeserious impairment for WDM-PON [7]. The carrier-RB [7] canbe attributed to crosstalk when downlink (DL) and uplink (UL)were transmitted through one fiber span at the same frequency.Although [8] also proposed a scheme to mitigate RB for thebidirectional transmission, it only demonstrated the two ULsfor wired and wireless services without the real DLs. In thiswork, a new architecture of lightwave centralized bidirectionalWDM-OFDM-PON and RoF access system is proposed. It cansimultaneously offer wireline and wireless OFDM services,such as WiMAX (Worldwide Interoperability for MicrowaveAccess) and Long Term Evolution (LTE), for both DL and ULtransmissions through a fiber link.

II. PROPOSED ARCHITECTURE

In our design, Fig. 1, the optical line terminal (OLT) designedconsists of channels to generate downstream (DS) signals anddetect upstream (US) data. The optical carrier in each channel

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1086 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 15, AUGUST 1, 2011

Fig. 2. Experimental setup of hybrid access networks. (i)–(iv) The optical spectra (0.01-nm resolution) and its corresponding measured points (a)–(j). (v) Opticaleye diagram for mm-wave signal. (vi)–(vii) Electrical spectra of the received WiMAX and Wired DS.

is modulated by the wireless DS from its base station (BS),and multiple sidebands are generated by an input radio-fre-quency (RF) signal at frequency, . An optical filter (OF) isthen used to separate two second-order sidebands from otherthree carriers for wired DL modulation. After a multiplexer(MUX) for combining signals, a demultiplexer (DEMUX) inthe remote note (RN) is used to split signals and deliver to eachoptical network unit/remote access unit (ONU/RAU). In theONU/RAU, two signals carried wireless and wired DL data areconverted to electrical domain by direct detection which is asimple, low-cost system and can be implemented with com-mercial, off-the-shelf components. Meanwhile, two first-ordersidebands without DS data are employed for wireless andwired US modulations, respectively. Therefore, no additionallight sources and wavelength management are required at theONU/RAU, which significantly reduces the component costand improves system stability. Using this scheme, we exper-imentally demonstrated a lightwave centralized bidirectionaltransmission of OFDM baseband and WiMAX signals over a25-km SMF-28 with low interference from the RB generatedby the carriers carrying DS data.

III. EXPERIMENTAL SETUP AND RESULTS

Fig. 2 illustrates the experimental setup for the proposedhybrid bidirectional access network: seamless integration ofRoF with WDM-OFDM-PON. At the OLT, a continuous-wave(CW) laser at 1551.15 nm is followed by a LiNbO dual-armMach–Zehnder modulator (MZM), which is driven by anamplified 17-GHz RF signal and WiMAX DL signal simultane-ously. The 17.28-Mb/s OFDM-64QAM (quadrature amplitudemodulation) WiMAX DL signal, with center frequency of2.66 GHz, bandwidth of 10 MHz, fast Fourier transform (FFT)size of 1024, oversampling rate of 28/25, subchannel spacingof 10.9375 kHz and guard period of 1/8, is synthesized bysoftware (Agilent signal studio for 802.16 WiMAX) and gen-erated by a vector signal generator (Agilent N5182A). Afterthe MZM, multicarriers with spacing of 17 GHz is generated(Fig. 2(i)), and only the central signal is carried by a WiMAXDL data; other subcarriers, including first and second-ordersidebands, are CW lightwaves [9]. After amplification, twooptical interleavers (50/100-GHz IL and 25/50-GHz IL ) areused to separate the second-order sidebands from other car-riers in Fig. 2(ii). The second-order sidebands are considered

Fig. 3. EVM versus RF power for WiMAX (a) downlink, and (b) uplink.

as carriers for the 11.29-Gb/s wired DL OFDM signal withHermitian symmetry, which is generated offline by MATLABand uploaded into a Tektronix arbitrary waveform generator(AWG) operating at 8 GS/s. Additionally, a 68-GHz opticalmillimeter-wave (mm-wave) can be realized at the same time.Due to the limitation of the experimental equipments, only theoptical eye diagram of 68-GHz mm-wave signal is displayed inFig. 2(v). For wired DL and UL signals, 16QAM and quadra-ture phase-shift keying (QPSK) modulation formats are usedfor baseband symbol mapping. The FFT size is 128, where96 subcarriers are mapped with random bit sequence (RBS)stream, 27 subcarriers for guard channels, and one subcarrierin the middle of the OFDM spectrum is set to zero for DC inbaseband. Cyclic prefix (CP) with length of 8 samples has beenapplied to resist chromatic dispersions. Four training sequencesare added every 100 OFDM data frames. In the ONU/RAU,four different signals are separated by OF , formed by a wave-length selective switch (WSS), as Fig. 2(iii). For the WiMAXDL carried by the central carrier, a 5-GHz PIN photodetector

HSUEH et al.: NOVEL LIGHTWAVE CENTRALIZED BIDIRECTIONAL HYBRID ACCESS NETWORK 1087

Fig. 4. BER versus received optical power for wired DL and UL transmissions.

Fig. 5. Throughput versus antenna input power for BTB WiMAX DL and ULtransmissions employing real WiMAX BS from NEC PasoWings.

is used, and then the detected signal is analyzed by an AgilentN9020A signal analyzer. Furthermore, the converted elec-trical wired OFDM DL signal is sent to a real-time scope forthe offline processing. The electrical spectra of the receivedWiMAX and wired DL are shown in Fig. 2(vi) and (vii),respectively For ULs, two first-order CW sidebands are modu-lated by 11.52-Mb/s OFDM-16QAM WiMAX and 5.65-Gb/sOFDM-QPSK, respectively. By using optical circulators (CLs),the two ULs are sent back to OLT over the same fiber span andthen separated by OF with 3 dB bandwidth of 0.1 nm, whichis also used to filter out the back reflection at the DL carriers inFig. 2(iv). The EVM measurements for WiMAX DL and UL atdifferent RF power are illustrated in Fig. 3. According to IEEE802.16e standard [10], the EVM thresholds are 3.1% and 6%for successful WiMAX communication with OFDM-64QAMand 16QAM formats, respectively. The experimental resultsshow that the EVM measurements of over 25-km SMF-28with RF power from to dBm are always under thethresholds of WiMAX. The clear constellations before andafter transmission for both WiMAX DL and UL are insertedin Fig. 3. Fig. 4 shows the bit error rate (BER) performancesversus received optical power for wired DL and UL. The powerpenalties caused by 25-km SMF-28 are less than 0.5 dB forboth transmissions. The constellations of the OFDM-16QAMand OFDM-QPSK are also in Fig. 4. In order to show realWiMAX traffic, we also demonstrate a successful back-to-backtransmission by utilizing a real WiMAX BS (NEC PasoWings),2.5G omni-directional antenna with gain of 3 dBi, and a laptopwith PCMCIA WiMAX transceiver as mobile subscribers(MS). The WiMAX DL and UL are 9-Mb/s OFDM-64QAMand 4-Mb/s OFDM-16QAM signals with 28/25 oversampling

factor at 2.66 GHz. Fig. 5 shows the throughputs with differentantenna input power for WiMAX DL and UL, exhibiting theRF operating range is 20 dB. The constellation for the DLsignal with EVM of dB and throughput of 9.18 Mb/sis also shown in Fig. 5 as inset.

IV. CONCLUSION

We proposed and experimentally demonstrated a novel light-wave centralized bidirectional access networks for the seamlessintegration of RoF with WDM-OFDM-PON system based onthe multiwavelength generation and wavelength- reuse scheme.The RB from the carriers with DL data can be reduced be-cause of different occupied frequencies for them andUL signals.The experimental results show that the WiMAX 17.28-Mb/sOFDM-64QAMDL and 11.52-Mb/s OFDM-16QAMUL trans-missions are always under the EVM thresholds of IEEE stan-dard. Moreover, for both wired 11.29-Gb/s OFDM-16QAM DSand 5.65-Gb/s OFDM-QPSK US signals, the penalties are lessthan 0.5 dB after 25-km SMF-28. The unique advantage of thisscheme is to utilize simple modulators to realize successful hy-brid bidirectional transmission with low RB interference andwithout additional light sources for USs. Therefore, the pro-posed scheme can be applied to provide multiple independentwired and wireless services for the next generation optical ac-cess networks.

REFERENCES

[1] E. Wang, K. L. Lee, and T. B. Anderson, “Directly modulated self-seeding reflective semiconductor optical amplifiers as colorless trans-mitters in wavelength division multiplexed passive optical networks,”J. Lightw. Technol., vol. 25, no. 1, pp. 67–74, Jan. 2007.

[2] A.M. J. Koonen and L. M. Garcia, “Radio-Over-MMF techniques-PartII: Microwave to millimeter-wave systems,” J. Lightw. Technol., vol.26, no. 15, pp. 2396–2408, Aug. 1, 2008.

[3] D. Qian, N. Cvijetic, J. Hu, and T. Wang, “108 Gb/s OFDMA-PONwith polarization multiplexing and direct detection,” J. Lightw.Technol., vol. 28, no. 4, pp. 484–493, Feb. 15, 2010.

[4] L. Chen et al., “A novel scheme for seamless integration of ROFwith centralized lightwave OFDM-WDM-PON system,” J. Lightw.Technol., vol. 27, no. 14, pp. 2786–2791, Jul. 15, 2009.

[5] B. Liu, X. Xin, L. Zhang, K. Zhao, and C. Yu, “Broad convergence of32QAM-OFDM ROF and WDM-OFDM-PON system using and in-tegrated modulator for bidirectional access networks,” in Proc. OFC/NFOEC 2010, San Diego, CA, Paper JThA26.

[6] P.-T. Shih et al., “Hybrid access network integrated with wireless mul-tilevel vector and wired baseband signals using frequency doubling andno optical filtering,” IEEE Photon. Technol. Lett., vol. 21, no. 13, pp.857–859, Jul. 1, 2009.

[7] C. W. Chow, G. Talli, and P. D. Townsend, “Rayleigh noise reductionin 10-Gb/s DWDM-PONs by wavelength detuning and phase-modu-lation-induced spectral broadening,” IEEE Photon. Technol. Lett., vol.19, no. 6, pp. 423–425, Mar. 15, 2007.

[8] C. W. Chow, C. H. Yeh, L. Xu, and H. K. Tsang, “Rayleigh backscat-tering mitigation using wavelength splitting for heterogeneous opticalwired and wireless access,” IEEE Photon. Technol. Lett., vol. 22, no.17, pp. 1294–1296, Sep. 1, 2010.

[9] Y.-T. Hsueh, Z. Jia, H.-C. Chien, J. Yu, and G.-K. Chang, “A novelbidirectional 60-GHz radio-over-fiber scheme with multiband signalgeneration using a single intensity modulator,” IEEE Photon. Technol.Lett., vol. 21, no. 18, pp. 1338–1340, Sep. 15, 2009.

[10] IEEE Standard for Local and Metropolitan Area Networks; Part 16:Air Interface for Fixed Broadband Wireless Access Systems, Amend-ment 2: Physical and MAC Layers for Combined Fixed and MobileOperation in Licensed Bands and Corrigendum 1, IEEE Standard 802.16e-2005, Feb. 2006.