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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 18, SEPTEMBER 15, 2012 1621
Generation of Multiband Signals in a BidirectionalWireless Over Fiber System With High Scalability
Using Heterodyne Mixing TechniqueLiang Zhang, Chenhui Ye, Xiaofeng Hu, Zhihua Li, Shu-Hao Fan, Yu-Ting Hsueh, Qingjiang Chang,
Yikai Su, Senior Member, IEEE, and Gee-Kung Chang, Fellow, IEEE
Abstract— We propose and experimentally demonstrate abidirectional radio over fiber system to simultaneously generateand transmit downstream multiband signals and upstream data.To the best of our knowledge, this is the first time thatthe multiband signals, including baseband, 24-GHz microwave(MW), and 42- and 60-GHz millimeter-wave (MMW) signals, arerealized through multicarrier generation and heterodyne mixingtechniques. The frequencies of the MW and MMW signals arecontinuously tunable, thus high scalability can be achieved. Inthe base station, part of the continuous wave light is used as anoptical carrier for upstream data, which is transmitted back tothe central station though the same fiber. Error-free performancesare achieved for all the signals after 25-km single-mode fiber and5 ft air link transmission for wireless data.
Index Terms— Mach–Zehnder modulator (MZM), microwave(MW), millimeter wave (MMW), multiband, optical heterodynemixing, radio over fiber (RoF).
I. INTRODUCTION
THE NEXT generation access networks are expected todeliver both wired and wireless services, and operate
at multi-gigabit per second for data intensive multimediaand real-time applications. Radio over fiber (RoF) technologyis an attractive candidate to serve both fixed and mobileusers with its high capacity, large bandwidth and increased
Manuscript received May 22, 2012; revised July 6, 2012; accepted July 26,2012. Date of publication August 1, 2012; date of current version August 29,2012. This work was supported in part by the NSF I/UCRC Center for OpticalWireless Applications, in part by the NSFC under Grant 61077052 and Grant61125504), in part by the MoE under Grant 20110073110012, and in partby the Science and Technology Commission of Shanghai Municipality underGrant 11530700400.
L. Zhang is with the State Key Laboratory of Advanced OpticalCommunication Systems and Networks, Department of Electronic Engineer-ing, Shanghai Jiao Tong University, Shanghai 200240, China, and also withthe School of Electrical and Computer Engineering, Georgia Institute of Tech-nology, Atlanta, GA 30332 USA (e-mail: [email protected]).
C. Ye, Z. Li, S.-H. Fan, Y.-T. Hsueh, and G.-K. Chang are with the School ofElectrical and Computer Engineering, Georgia Institute of Technology,Atlanta, GA 30332 USA (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]).
X. Hu and Y. Su are with the State Key Laboratory of AdvancedOptical Communication Systems and Networks, Department of ElectronicEngineering, Shanghai Jiao Tong University, Shanghai 200240, China (e-mail:[email protected]; yikaisu@sjtu. edu.cn).
Q. Chang is with the Research and Innovation Center, Alcatel-LucentShanghai Bell, Shanghai 201206, China (e-mail: [email protected]).
Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2211004
mobility [1]–[8]. Among them, efficient modulation schemeof multiband signals is a desirable choice that simultaneouslytransmits baseband, microwave (MW) and millimeter wave(MMW) signals in an integrated platform [5]–[8]. Multibandsignals were realized with dual-arm modulator driven by20-GHz and 40-GHz clocks, requiring high-frequency synthe-sizer and high-speed electronic devices [6]. In Ref. [7], theauthors demonstrated an RoF system for the generation ofmultiband signals using a dual-parallel Mach–Zehnder modu-lator (DPMZM) followed by a single-drive MZM, resulting ina complex architecture and high insertion loss. Recently, weproposed a cost-effective method to realize multiband signals,however, only on-off-keying (OOK) format can be achievedsince data was loaded on the bias port of the MZM [8].
In this letter, we propose and experimentally demonstratea novel full-duplex RoF system simultaneously transmittingwired and multiband wireless signals. A simple and cost-effective central station (CS) is realized, where only asingle-drive MZM is used to generate a three-tone signal. Inthe base station (BS), the three-tone signal is coupled witha local oscillator (LO) and heterodyne mixed within a high-speed photo-detector (PD), where high-tolerance of dispersionis obtained since the beating signals in the PD are single sideband (SSB)-like signals. To the best our knowledge, this isthe first time that baseband, 24-GHz MW, 42-GHz MMW and60-GHz MMW signals are simultaneously achieved using onlyone 10-GHz modulator and electronic devices, and thus signif-icantly reducing the cost of the system. Part of the LO is splitto be the carrier of upstream data, which is transmitted throughthe same fiber to the CS with the downlink signal. In Ref[6]–[8], the frequencies of the wireless signals were fixed, andcould be a limitation on selecting a particular frequency forcertain applications. In our scheme, the frequencies of the MWand MMW signals can be easily adjusted on a large scale bycontrolling the frequency of clock source and the wavelengthof the laser in the CS, thus high scalability can be achieved. Inthe proposed structure, all bands carry the same data, whichcould be used in multiband wireless broadcast scenarios.
II. PRINCIPLE
The schematic diagram of the proposed RoF system isdepicted in Fig. 1. In the CS, OOK modulation format isrealized using a direct modulation laser (DML) driven by anelectrical data Sd = A (t). A single-drive MZM is biased atthe peak point of the transmission curve and driven by a radio
1041–1135/$31.00 © 2012 IEEE
1622 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 18, SEPTEMBER 15, 2012
Fig. 1. Schematic diagram of the proposed multiband RoF system.
frequency (RF) clock with an angular frequency of ωRF . Theoutput of the MZM has three tones with a frequency spacingof 2ωRF , which are loaded with the same data and can beexpressed as [11]:
Eout = A(t)E0
[e j (ω1−2ωRF )t + e jω1t + e j (ω1+2ωRF )t
](1)
where ω1 and E0 are the angular frequency and the amplitudeof the DML. After the transmission, at the BS, an opticalcoupler is used to combine the three-tone signal and a LOoriginated from a continual wave (CW) light EL O = E2e jω2t .ω2 and E2 are the angular frequency and the amplitude ofthe LO. A high-speed PD is employed to detect the combinedsignals and the output of PD can be written as [11]:
r(t) ≈ (|Eout + EL O |)2
≈ 3[A(t)E0]2 + (E2)2
+A(t)E0 E2 cos(ω2 − ω1 − 2ωRF )t
+A(t)E0 E2 cos(ω2 − ω1)t
+A(t)E0 E2 cos(ω2 − ω1 + 2ωRF )t (2)
where self-beating and heterodyne mixings between LO andthe three-tone signal are considered, while the beatingsbetween different carriers of the three-tone signal are negligi-ble since LO has much higher power than the three-tone signal.Eq. (2) shows that the baseband component and three wirelesssignals are generated. The power of the wireless signals canbe improved with the increase of the optical power of the LO,which should not exceed the saturated input power of the PD.After the PD, a low pass filter (LPF) is required to select thebaseband signal for the wired user and three pairs of antennasare employed to broadcast the wireless signals to differentmobile users.
The scalability of the system can be achieved by adjustingthe frequency of the clock ωRF and the angular frequency ofthe DML ω1. One can choose any two frequencies of the threebands, and the third one is the consequence of the other two.For example, if ωRF = 9.75G H z and ω2 − ω1 = 37.5G H zare chosen, one can achieve 18-GHz MW, 37.5-GHz MMW(low bound of 40-GHz band) and 57-GHz MMW signals(low bound of 60-GHz band); and if ωRF = 10.75G H z andω2 − ω1 = 42.5G H z are set, 21-GHz MW, 42.5-GHz MMW(high bound of 40-GHz band) and 64-GHz MMW signals(high bound of 60-GHz band) would be generated. Further-more, 2.4-GHz and 5.6-GHz WiFi signals can be obtainedif appropriate frequencies are selected for ωRF and ω1. As aresult, our proposed scheme has a high scalability and one canflexibly select a particular frequency for required applications.
It should be noted that these adjustments are realized in theCS, no wavelength controls are needed in the BS.
III. EXPERIMENTAL SETUP AND RESULTS
Fig. 2 shows the experimental setup for the proposedmultiband optical-wireless access network over fiber and airlinks. At the CS, a 1.25-Gb/s pseudo-random bit sequence(PRBS) data with a word length of 231−1 is loaded on aDML with a wavelength of 1552.480 nm (Fig. 3(a)) to realizeOOK modulation. The OOK signal is injected to a MZMbiased at the peak of the transmission curve and driven byan amplified 9-GHz RF clock. The output of the MZM hasthree tones (Fig. 3(b)) with 18-GHz frequency spacing [11],which carry the same data. The three-tone signal from theMZM is amplified to reach a power level of 6 dBm anda tunable optical filter (TOF: 3-dB bandwidth of 1.5 nm)is employed to suppress the amplifier spontaneous emission(ASE) noise. After transmission through 25-km SMF, at theBS, an optical coupler is used to combine the three-tonesignal and a un-modulated CW light with a wavelength of1552.816 nm. In that case, the frequency spacing betweenthe three tones and the CW light are 24 GHz, 42 GHz and60 GHz, as shown in Fig. 3(c). The combined signals areinput to a high-speed PD (u2t XPDV 2020R), where self-beating and heterodyne mixings are realized. The spectrumafter the PD is shown in Fig. 3(d), consisting of base-band, 24-GHz MW, 42-GHz and 60-GHz. The 18-GHz and36-GHz components are considered as the beating noise andcan be removed by a band-pass filter since they are out-of-bandinterferences. After the PD, a 1:4 splitter is used to divide thesignal into four parts. One part of them is input to an amplifier(Narda West NW 06-0023) with a 5-GHz bandwidth centeredat 60 GHz. A pair of rectangular horn antennas (DucommunARH-1525-62) with a gain of 25 dBi at the rangeof 50–75 GHz are employed to broadcast and receive the60-GHz wireless signal. After transmission over 5-ft wirelesslink, at user terminal, the received 60-GHz signal is down-converted through envelope detection (ED). Compared withconventional down-conversion scheme, the ED is less efficient,however, it can eliminate local clock source and phase control.Moreover, the ED can be used not only for OOK signal, butalso for the wireless signals, such as intermediate frequency-quadrature phase shift keying (IF-QPSK) and so on. The wire-less transmission and down-conversion of the 42-GHz MMWare realized with the same architecture as 60-GHz MMW.Due to the lack of 20-GHz antennas and envelope detector,the wireless transmission of the 24-GHz MW signal is notdemonstrated. An amplifier with a 3-dB bandwidth of 4 GHzfollowed by a low pass filter is used to receive the basebandsignal. For the upstream signal, part of the CW light is splitas the upstream optical carrier and modulated by a 1.25-GbpsPRBS data with a word length of 231−1. After an opticalcirculator, the upstream signal is transmitted to the CS throughthe same fiber. At the CS, after passing through anothercirculator, the upstream signal is detected by a low-speed PD.Although only single feeder fiber is used for bidirectionaltransmission, back scattering is negligible since downstream
ZHANG et al.: GENERATION OF MULTIBAND SIGNALS IN A BIDIRECTIONAL WIRELESS OVER FIBER SYSTEM 1623
Fig. 2. Experimental setup of the proposed multiband RoF system.
1551.5 1552.0 1552.5 1553.0 1553.5-70
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Fig. 3. Spectra taken at different positions as indicated in Fig. 2. (a) Mod-ulated signal. (b) Three-tone signal after the MZM. (c) Combined three-tonesignal and LO. (d) Electrical spectrum after PD.
Fig. 4. BER curves and electrical eye diagrams. (a) Downstream basebandsignal. (b) Downstream 42-GHz MMW signal. (c) Downstream 60-GHzMMW signal. (d) Upstream data.
and upstream signals are carried on different wavelengths. Inthe experimental demonstration, the bandwidths of devices andthe interferences between the three tones of the multibandsignals could limit the operational bit rates.
The BER measurements of the downstream multiband sig-nals and upstream data are provided in Fig. 4, where thepower of LO is set to be 6 dBm. The electrical eye diagramsare measured after 25-km transmission with the BER valueof 10−9. For the baseband signal, after transmission of 25-km
SMF, the power penalty is 0.2 dB. Error-free performances arealso achieved for the 42-GHz and 60-GHz MMW signals withonly ∼0.3-dB power penalties, which can be attributed to thehigh tolerance of dispersion of the SSB-like signals. For the60-GHz MMW signal, when the received power is increasedto −14.2 dBm, an error-free performance can be achievedwith 10-ft wireless transmission. This could be extended toa longer distance with a higher received power. Since tiny CDand no nonlinear effects are induced, the upstream data hasnegligible power penalty after 25-km SMF transmission. Thedownstream baseband signal shows a worse BER than thatof the upstream signal, which can be attributed to more ASEnoise and the interferences from self-beatings of the multibandsignals. For the 42-GHz MMW signals, the BER performanceof the single band is ∼1.5 dBm better than that of multiband.The interferences between different bands will be the topic ofour next investigation.
IV. CONCLUSION
We have proposed and experimentally demonstrated anovel bidirectional multiband RoF transmission scheme deliv-ering 1.25-Gbps baseband, 24-GHz MW, 42-GHz MMWand 60 GHz-MMW signals over 25-km SMF using only a10-GHz single-drive MZM in the CS. Wireless transmissionsover 5-ft free space are also demonstrated for 42-GHz MMWand 60-GHz MMW signals. Error-free transmissions havebeen achieved for both baseband and wireless signals withnegligible power penalties due to the SSB-like modulationstructure. Symmetric upstream data is transmitted throughthe same fiber, with mitigated Rayleigh backscattering sincedownstream and upstream signals are carried on the differentwavelengths. The scalability of the proposed scheme is alsoanalyzed, which can cover the whole 40-GHz and 60-GHzMMW bands. The experiment results validate our scheme asa desirable candidate for future converged wired and wirelessaccess networks.
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