Energy-Autonomous Picocell Remote Antenna Unit for Radio-Over-Fiber System Using the Multiservices Concept

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 8, APRIL 15, 2012

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Energy-Autonomous Picocell Remote Antenna Unit for Radio-Over-Fiber System Using the Multiservices Concept Christophe Lethien, David Wake, Bernard Verbeke, Jean-Pierre Vilcot, Christophe Loyez, Malek Zegaoui, Nathan Gomes, Nathalie Rolland, and Paul-Alain Rolland

Abstract— The study reported in this letter deals with the extension of the multiservices concept to radio-over-fiber systems with energy-autonomous picocell remote antenna units. Continuous-power, radio-frequency, and digital signals have been combined in a single multimode fiber for the first time. The results clearly demonstrate no impairment of the optically powered remote antenna unit compared to an electrically powered version. The proposed system complies with the classical baseband Ethernet high-data-rate network (10 GbE – bit error rate 10−12 ). The measured error vector magnitude for the radio-frequency (IEEE802.11g) signal transmission through the designed system stays around 2%, including both the optical transmission over 100-m OM3 multimode fiber and a wireless coverage of 5 m. Index Terms— Energy-autonomous access point, multimode fiber, radio-over-fiber (ROF), 10 GbE, wavelength multiplexing, zero-power devices.

I. I NTRODUCTION

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HE aim of this study is to propose a multi-services system based on glass multimode fiber and wavelength multiplexing devices. As the bandwidth capacities of multimode fibers is not fully exploited in the case of a classical home network, we propose to use the same fiber distribution system for a fixed IP/Ethernet digital signal and a radio frequency (RF) wireless signal. Additionally, the wireless access points used to process the RF signal are supplied by optical means, ie through the distribution fiber, which means they do not

Manuscript received October 20, 2011; revised December 23, 2011; accepted January 10, 2012. Date of publication January 23, 2012; date of current version March 28, 2012. This work was supported in part by the European Regional Development Fund (ERDF) and in part by the Nord-Pasde-Calais Region, France, under the CPER CIA Fund. C. Lethien is with the History of Science Department, Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN)-IRCICA, Villeneuve d’Ascq 59652, France (e-mail: [email protected]). D. Wake and N. Gomes are with the Department of Electronics, Broadband and Wireless Communications Group, University of Kent, Canterbury CT2 7NT, U.K. (e-mail: [email protected]; [email protected]). B. Verbeke, J.-P. Vilcot, C. Loyez, M. Zegaoui, N. Rolland, and P.-A. Rolland are with IEMN, UMR 8520, Université des Sciences et Technologies de Lille, Villeneuve d’Ascq 59652, France, and also with the Research Federation IRCICA USR 3380, Villeneuve d’Ascq 59652, France (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). 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.2012.2185224

need any electrical power supply. With the reported topology based on energy autonomous Remote Antenna Units (RAUs), link cost is reduced owing to additional radio over fiber deployment. The energy autonomy of wireless devices is a key issue to get a self sufficient wireless network. Energy harvesting is a potential solution to supply wireless systems with ambient energy such as solar, vibration, thermoelectricity [1, 2]. In the field of picocell radio over fiber systems, the power consumption (∼250 mW) of a RAU [3] is relatively high to achieve the bidirectional communication with electronic components (amplifier, laser, photodiodes) only powered by some harvesting sources (100 µW). Primary or secondary batteries have to be charged or replaced, leading to a human maintenance on the picocell access points. To reach a zero power radio over fiber network where the RAUs do not integrate either coin, battery or energy harvesting devices, the developed concept proposed in this Letter is to distribute the required power by optical fiber. A high power laser, localized in the central office, is used to distribute optical power through a fiber network to supply RAUs which incorporate a passive fiber pigtailed photovoltaic converter (fig. 1). In each RAU, the optical power is converted to the electrical domain and is used to supply a laser, a photodiode and a RF amplifier, to be able to achieve the wireless communication. In [4], the RF and the power signals are transmitted respectively on singlemode and multimode fibers. Wake et al [3, 5] have developed a radio over multimode fiber system which included an optically powered RAU. A dual transmission over a single multimode fiber has already been reported [6]. In this letter, three types of signal (digital, RF and continuous power) have been transmitted through one single glass multimode fiber (GMMF – 50/125 µm – 100m length – OM3 type – downlink) using coarse wavelength division multiplexing (CWDM) devices. To the best of the authors’ knowledge, such a topology is reported for the first time leading to an energy autonomous picocell RAU. The proposed concept is an evolution of the recent works published in [7, 8] where the simultaneous transmission of digital and RF signals (IEEE 802.11g) on a single OM4 glass multimode fiber is studied. The first part of this Letter describes the methodology and the design of an energy autonomous remote antenna unit (RAU). The last part of this Letter reports on a comparative study between electrically and optically powered RAU; spectrum masks and error vector magnitude (EVM)

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 8, APRIL 15, 2012

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measurements are presented for the IEEE 802.11g wireless local area networking standard. II. T OPOLOGY OF THE E NERGY AUTONOMOUS RO F S YSTEMS Fig. 1a presents the overall description of the energy autonomous radio over fiber system for home applications. The signal distribution between the set-top-box and several RAUs located in different rooms is proposed to enhance the indoor coverage of RF signals. Contrary to the work reported in [6], the power over fiber transmission is achieved at 980 nm, ie far away from the classical telecommunication windows close to 850 nm (RoF link) and 1300 nm (digital 10GbE link or high definition TV - HDTV) as depicted in fig. 1b. An additional digital transmission in also considered as compared to [6]. The uplink and downlink directions of the proposed bidirectional RoF system both use the classical intensity modulation/direct detection technique (IMDD). The key component of the RAU is a photovoltaic fiber pigtailed converter (passive device) allowing the electrical power generation inside the RAU. A voltage regulator (3V), voltage doubler and inverter (−6V) and step down converter (2V) are proposed to supply respectively the amplifiers, the photodiode and the laser. For the RoF link, an 850 nm fiber pigtailed vertical cavity surface emitting laser (VCSEL) and a PIN photodiode are used to perform the transmission. Low consumption (20 mW) RF amplifiers operating in the 2.4 GHz band have been specially designed (using the Avago Technologies ATF-54143 transistor) to reduce the overall consumption of the RAU. The gain of the RF amplifier at 2.4 GHz is ∼15 dB and the noise figure is ∼0.5 dB). Concerning the digital link, classical 10GbE transceivers operating at 1300 nm are selected to demonstrate the proof of concept. The power signal used to

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supply the energy autonomous RAU is transmitted through the downlink as shown in fig. 1b. The optical multiplexing of the three transmitted downlink signals is performed with WDM devices based on 50 µm glass multimode fiber. The uplink is only composed of the digital and the RF signals (up streams). Only the energy autonomous RAUs are supplied with the high power laser. In fact, the 10GbE transceivers are electrically powered owing to their high consumption (400mA under 12 volts for the evaluation board including a 1300 nm XFP transceiver). The needed optical power to supply the transceivers will be too high and so incompatible with the power handling of the CWDM devices. The locations of the RAUs are relatively remote in the proposed topology, ie at the top of the wall as depicted in fig.1a. In this way, the optical power needed to supply the RAU is confined in the fiber, ie at a location where the eye safety is secured. Contrary to the RAU, the digital transceivers able to achieve the HDTV or 10GbE distribution inside the home network are not optically supplied. In fact, the Ethernet/IP locations are more likely to be close to electrical power outlets: no continuous power at 980 nm is sent inside the house to propose an eye-safe solution. III. T RANSMISSION OT THE M ULTI -S IGNALS ON THE OM3 MMF The WDM devices are firstly characterized (fig. 2) in order to check the isolation (roughly 25 dB) between the 3 ports and the insertion losses. In particular, the optical loss (3 dB) at 980 nm is a crucial point as the optical power sent by the high power laser needs to be adjusted to supply the RAU at the right power level by taking the loss into account. The test setup used to characterize the proposed concept is depicted in fig. 3. The measurements are carried out over

LETHIEN et al.: ENERGY-AUTONOMOUS PICOCELL RAU FOR RADIO-OVER-FIBER SYSTEM 7

Error Vector Magnitude (%rms)

the OM3 multimode fiber (MMF – 100m). A vector signal generation and analysis solution issued from Agilent (ESG 4438c and PSA E4440a) allows the RF generation and analysis of the IEEE 802.11g standard (carrier frequency: 2.4 GHz, modulation scheme: 64QAM, data rate: 54Mbps). The link gain [9] of the radio over fiber link is close to −30 dB at 850 nm (without any amplifier) after taking account of the optical losses. The wireless range is set to 5m to be compatible with lab-based experiments and an additional RF amplifier (using the Avago Technologies ATF-54143 transistor) with similar performance to those already reported in this letter is inserted at the mobile station (fig. 3) before the vector modulation analysis. The 10GbE analysis is performed using the Serial Bert N4906b from Agilent. Concerning the digital analysis (1300 nm link), only the insertion loss (1.5 dB loss) due to the WDM devices decreases the optical power budget (OPB). Even if this OPB is reduced, the “no error free” case is obtained with such a topology, leading to a highly open eye diagram and a bit error rate close to 10−12. The required electrical power to supply the laser, the photodiode and the RF amplifier of the proposed RAU is approximately 60 mW. According to the conversion efficiency of the photovoltaic fiber pigtailed converter (50%), the optical power needed to supply one RAU is roughly 120 mW. Regarding the fiber loss (1 dB/km at 980 nm) and the CWDM insertion losses at 980 nm (−3 dB per CWDM device), the required optical power at the central unit needed to supply the RAU is close to 27 dBm (∼500 mW). An additional loss due to the core mismatching between the fiber used to pigtail the high power laser and the OM3 MMF leads to a 3 dB additional penalty. This optical power level (30 dBm) is compatible with the power handling of the CWDM devices. At the 1300 nm photoreceiver, the optical power level of the 980 nm signal is close to −3 dBm (30 dB loss owing to the CWDM devices localized both in the central unit and close to the RAU and 3 dB additional penalty owing to the core mismatching). Nevertheless, due to the relatively low responsivity of the 1300 nm photoreceiver at 980 nm (0.1 A/W), the resulting photocurrent of the 980 nm signal reaches a value close to 50 µW. The calculated photocurrent of the 980 nm signal in the 1300 nm digital photodetector should be considered as noise penalty since the received optical power level of the 10GbE signal is approximately 0 dBm and the responsivity of the photodetector is 0.7 A/W at 1300 nm (10GbE signal photocurrent ∼700 µA). The results of the vector modulation analysis are shown in fig. 4. The EVM has been measured as a function of the RF input power level of the IEEE802.11g signal. Similar curves are obtained when the RAU is electrically powered. Actually, a high EVM is measured either when the signal to noise ratio is low (weak RF input power level) or when high distortion (high input power level) occurring both in the laser and the RF amplifier are exhibited. The minimum value of the EVM reaches the 2.5% threshold over a large RF input power level range, ie from −20 dBm up to –5 dBm as shown in fig. 4. Similar behavior is exhibited either with a DC supply or with an optical power supply, which confirms the potential of the proposed topology based

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on multi-service distribution inside a multimode fiber. To confirm the transmission quality of the IEEE802.11g standard (OM3 fiber and 5m wireless coverage), the spectrum mask measurement has been performed and shown in fig. 5. No significant difference is observed between the electrical and optical power supplied RAU. IV. C ONCLUSION In this Letter, an energy autonomous remote antenna unit is proposed in a topology where digital, RF and continuous power signals are transmitted over a single OM3 multimode glass fiber. From the presented results, it is clear that no impairments in the digital or RoF domains resulting from the high power transmitted over the multimode fiber are highlighted. R EFERENCES [1] A. Harb, “Energy harvesting: State-of-the-art,” Renewable Ener., vol. 36, no. 10, pp. 2641–2654, Oct. 2011. [2] C. O. Mathuna, T. O’Donnell, R. V. Martinez-Catala, J. Rohan, and B. O’Flynn, “Energy scavenging for long-term deployable wireless sensor networks,” Talanta, vol. 75, no. 3, pp. 613–623, May 2008. [3] D. Wake, A. Nkansah, and N. Gomes, “Optical powering of remote units for radio over fiber links,” in Proc. IEEE Int. Microw. Photon., Oct. 2007, pp. 29–32. [4] D. H. Thomas, G. V. de Faria, and J. P. Weid, “Fully powered-over-fibre remote antenna unit,” in Proc. Int. Microw. Photon., 2008, pp. 102–105. [5] D. Wake, A. Nkansah, N. J. Gomes, C. Lethien, C. Sion, and J. P. Vilcot, “Optically powered remote units for radio-over-fiber systems,” J. Lightw. Technol., vol. 26, no. 15, pp. 2484–2491, Aug. 1, 2008. [6] D. Wake, N. J. Gomes, C. Lethien, C. Sion, and J. P. Vilcot, “An optically powered radio over fiber remote unit using wavelength division multiplexing,” in Proc. Int. Microw. Photon., 2008, pp. 197–200. [7] C. Lethien, C. Loyez, J. P. Vilcot, and P. A. Rolland, “Multi-service applications on high modal bandwidth glass multimode fibre,” Electron. Lett., vol. 45, no. 18, pp. 951–952, Aug. 2009. [8] C. Lethien, C. Loyez, J.-P. Vilcot, and P. A. Rolland, “Potential of the high modal bandwidth OM4 glass multimode fiber for the multi-services concept,” Opt. Commun., vol. 284, no. 2, pp. 585–589, Jan. 2011. [9] C. Lethien, et al., “Review of glass and polymer multimode fibers used in a wimedia ultrawideband MB-OFDM radio over fiber system,” J. Lightw. Technol., vol. 27, no. 10, pp. 1320–1331, May 15, 2009.