JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, /$ IEEE

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1 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, Full Colorless WDM-Radio Over Fiber Access Network Supporting Simultaneous Transmission of Millimeter-Wave Band and Baseband Gigabit Signals by Sideband Routing Yong-Yuk Won, Hyun-Seung Kim, Yong-Hwan Son, and Sang-Kook Han, Member, IEEE Abstract A new wavelength division multiplexed-radio over fiber (WDM-RoF) access network scheme supporting the simultaneous transmission of a 1.25-Gb/s wired data as well as a 1.25-Gb/s wireless data is proposed in this paper. An optical carrier suppression effect and sideband routing using the multiplexing of arrayed waveguide grating (AWG) with 50-GHz channel spacing are utilized to generate a millimeter wave band carrier. These techniques make the proposed architecture transmit both a wired data and a wireless one at the same time. A reflective semiconductor optical amplifier (RSOA) is employed at both central office and base station so that this architecture is operated colorlessly. Error free transmissions (BER of 10 9 ) of both downlink and uplink are achieved simultaneously. A rare impact of downlink transmission on the performance of uplink data is observed due to the usage of the unmodulated light. Index Terms Arrayed waveguide grating (AWG), millimeter wave, optical carrier suppression, radio over fiber (RoF), reflective semiconductor optical amplifier (RSOA), wavelength division multiplexing (WDM). I. INTRODUCTION R CENTLY, it has been important for service providers to implement access networks supporting various broadband multimedia services irrespective of surroundings around customers. Among these systems, a wavelength division multiplexed-passive optical network (WDM-PON) has been regarded as the most promising system which is capable of transmitting gigabit wired data to customers effectively [1] [6]. A radio over fiber (RoF) technology has been well known as a competitive candidate one which can provide broadband wireless data with several subscribers using a millimeter wave [7] [11]. It is very important for the optical network unit (ONU) of WDM-PON to coexist with the base station (BS) of RoF system in order to implement RoF access networks based on WDM which is capable of transmitting wired and wireless data at the same time for various kinds of customers [12]. Consolidating two systems into one makes these kinds of access networks be cost-effective Manuscript received October 28, 2009; revised January 12, 2010; accepted February 12, Date of publication February 22, 2010; date of current version July 28, This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) funded by the Korea government (MEST) under Grant and in part by the Yonsei University Institute of TMS Information Technology, a Brain Korea 21 Program, Korea. The authors are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul , Korea ( skhan@yonsei.ac.kr). Digital Object Identifier /JLT system as well as an appropriate one for various in-building network. Also, the channel capacity of RoF access networks can be increased with the help of WDM technique. Various schemes for RoF access networks based on WDM have been proposed [13] [19]. A millimeter wave band RoF access network based on WDM using a broadband optical source was proposed [14]. A very broadband optical source with 25 GHz spaced multi-modes was used to generate several millimeter waves. The proposed system can be a cost-effective and simple one because of the utilization of single optical source. However, this optical device can be far away from the commercial product because it shows user-defined specifications. A WDM-RoF access network was also proposed employing a cyclic arrayed waveguide grating to generate multi millimeter waves as well as to simplify the architecture of the proposed system [17]. However, to do this, substantial optical losses, which can affect link budget adversely, would be produced because complicated routing paths should be allocated for single WDM-RoF channel. Also, architecture of central office is in a tangle because of the duplication of various RF and optical devices or the stability of the proposed system can be deteriorated due to the usage of the nonlinear effects like injection locking [18], [19]. Therefore, a RoF access network based on WDM should be designed according to the following technical issues; simplification, cost-effectiveness, and easy and convenient maintenance of system. In this paper, a novel RoF access networks scheme based on WDM supporting the simultaneous transmission of gigabit wired and wireless signals is proposed. A reflective semiconductor optical amplifier (RSOA) is utilized in the both central office (CO) and ONU/BS in order to implement a simple and costeffective bidirectional WDM-RoF access network. A millimeter wave is generated using a single-armed Mach Zehnder modulator and WDM-RoF channels are allocated for each wavelength using a simple routing path of 50-GHz spaced AWG. II. PRINCIPLE OF OPERATION Fig. 1 shows the scheme of WDM-RoF access network which is capable of transmitting gigabit wired and wireless data simultaneously. In a single WDM channel, a double sideband suppressed carrier (DSB-SC) light is generated by the modulated RF subcarrier using a Mach Zehnder modulator biased at (see the and ). The frequency of RF subcarrier is adjusted so that two optical sidebands fall into each channel bandwidth of 50 GHz spaced WDM multiplexer as shown in the inset /$ IEEE

2 2214 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, 2010 Fig. 1. Schematic of full colorless WDM-RoF access network supporting the simultaneous transmission of gigabit wired and wireless data. They are de-multiplexed at the 50 GHz spaced WDM multiplexer (the first WDM multiplexer) through optical circulator. One of the de-multiplexed lights is injected into RSOA and then modulated by gigabit data. The other is multiplexed at the second WDM multiplexer. The optical sideband modulated by gigabit data are multiplexed by the first WDM multiplexer and then routed via optical circulator. Each optical sideband ( and ) is combined and then transmitted to the remote node (RN) after optical transmission based on single mode fiber. At the RN, two de-multiplexed lights are merged again and then fed to the ONU/BS in order to detect millimeter-wave band wireless data as well as baseband wired data at the optical receiver (Rx) of ONU/BS. An unmodulated optical sideband is modulated by uplink data (wired data: baseband modulation, wireless data: down-mixing by RF mixer) at the RSOA of ONU/BS and then retransmitted back to the CO/OLT through WDM multiplexer. An uplink data is detected by the technique of baseband detection at an optical receiver (Rx) of CO/OLT. Here, in the uplink, it is difficult to modulate a millimeter wave band wireless signal directly because of relatively low frequency response of a RSOA (below 1.5 GHz). However, the wireless uplink data can be down-converted by a recovered millimeter wave carrier from downlink when a downlink system is not activated, if the time-division-multiplexing (TDM) is utilized in the proposed architecture. The discussion on TDM technique will not be presented because the aim of this paper is to demonstrate the proposed system conceptually. It is also meaningful to transmit simultaneously the wired and wireless data with the same data rate because the same kind of data should be transmitted irrespective of various customers (fixed and mobile subscribers) in the hot spot zones such as the airport, hospital, and so forth. III. EXPERIMENTAL SETUP Fig. 2 shows the experimental setup for the proposed colorless WDM-RoF access network link supporting the simultaneous transmission of millimeter wave band and baseband signals. Ten insets at the bottom of the experimental setup show the optical spectrum measured at each point. A light from a tunable laser source (TLS) was modulated by 31.5-GHz RF signal using a single-armed MZM biased at the (5 V) and then was converted into two optical sidebands with DSB-SC format (see the inset of (A)). The linewidth of TLS was 700 khz. An optical carrier suppression (OCS) ratio of 5 db was measured because of the low slop efficiency of a used MZM. A polarization controller (PC 1) was used to maximize the coupling efficiency of MZM. A Gaussian athermal AWG with 50-GHz channel spacing and 16 channels was used with the insertion loss of 2.5 db, 3-dB passband of 0.24 nm, adjacent crosstalk of 30 db, and non-adjacent crosstalk of 38 db. The first ( nm) and the second ( nm) channels of AWG were used to demonstrate the proposed architecture. A DSB-SC light was amplified by EDFA (see the inset of (B)) and then de-multiplexed by AWG 1 through optical circulator 1 (OC 1). One of de-multiplexed lights was injected into RSOA 1 and then directly modulated by 1.25-Gb/s baseband data with pseudorandom binary sequence (PRBS) and 2- swing depth (see the inset of (C)). The other was fed to the 3 db optical coupler via optical circulator 2 (OC 2) (see the inset of (F)). The modulated light at RSOA 1 was multiplexed by AWG 1 and then was routed to the 3 db optical coupler through OC 1 after the amplification of erbium doped fiber amplifier (EDFA) (see the insets of (D) and (E)). The lights combined by 3 db optical coupler were transmitted to the RN after 23-km optical transmission based on a standard single mode fiber (SSMF). No dispersion compensation was used. At the RN, two de-multiplexed optical signals were merged again and then fed to the ONU/BS (see the insets of (H), (I), and (J)). The unmodulated light, which is the output light of the second AWG channel, was modulated by a 1.25-Gb/s uplink signal at RSOA 2 and then retransmitted back to the 1.25-GHz band optical receiver (Rx) of CO/OLT. An optical bandpass filter (OBPF) was utilized instead of WDM multiplexer. An input optical power injected into RSOA 2 was dbm; it gaves a 10-dB optical gain, considerably lower than the small signal gain of 20 db at an input optical power of dbm, and had a polarization dependent gain of 2 db at this optical power. This tells us that a RSOA was operated under a gain saturation region. The polarization controller (PC 3) was used to maximize the optical gain of an input light injected into a RSOA. A 63-GHz wireless data, which was generated by the optical mixing between two optical sidebands, was detected at 60-GHz band PIN photo-detector (PD), and then 1.25-Gb/s wireless data was recovered by the receiver part of ASK transceiver (TR60AK1250) from comotech. Its principle of operation was as follows. A detected 63-GHz millimeter-wave signal was amplified by 60-GHz low noise amplifier (LNA) (frequency range: 61.9 GHz 64.1 GHz, noise figure: 8 db, RF gain: 30 db). After amplification, a 1.25-Gb/s wireless data was recovered using the envelop detector, where it was composed of schottky diode. Finally, a 1.25-Gb/s data was amplified using a 1.25-GHz low noise amplifier (noise figure: 3 db). A 1.25-Gb/s wired data

3 WON et al.: FULL COLORLESS WDM-RADIO 2215 Fig. 2. Experimental setup for the proposed scheme, MZM: Mach Zehnder modulator, EDFA: erbium doped fiber amplifier, AWG: arrayed waveguide grating, OC: optical circulator. 3 db: 3 db optical coupler, RSOA: reflective semiconductor optical amplifier, LNA: low noise amplifier, Measuerd optical spectra of each point (A, B, C, D, E, F, G, H, I, and J). was also detected by the technique of baseband detection. In this small scale test-bed, the bit error rate (BER) curves of both 1.25-Gb/s wired and wireless data were measured in bidirectional optical transmission. Also, the BER curves of the uplink data with 1.25-Gb/s data rate were measured repeatedly in the presence of the optical transmission of downlink data. In next section, various measured results will be discussed including the BER curves of uplink data and downlink one. IV. EXPERIMENTAL RESULTS AND DISCUSSIONS This section has three parts as follows. At the part A, the quality of a 63-GHz millimeter wave signal, which is generated using optical sideband routing as well as optical carrier suppression, is investigated by presenting RF phase noise and RF spectrum. The simultaneous transmission of 1.25-Gb/s downlink wireless data and 1.25-Gb/s wired one is evaluated based on the measurement of BER in the part B. At last, the change of BER curve of 1.25-Gb/s uplink data depending on the downlink data transmission is shown and analyzed in the part C. A. 63-GHz Millimeter Wave Signal Generation by Optical Carrier Suppression and Sideband Routing In the proposed scheme, the millimeter wave signal for the delivery of 1.25-Gb/s wireless data is generated by the optical mixing between two optical sidebands with frequency spacing of 63 GHz. They are also produced by sideband routing and optical carrier suppression. The quality of millimeter wave band carrier needs to be checked whether the optical transmission of 1.25-Gb/s wireless data will be achieved using this one or not. The parameters which can affect adversely the quality of this signal can be as follows. A noise can be generated due to the optical beat interference (OBI) caused by the mismatch of phase and polarization because the transmission lengths between two optical sidebands de-multiplexed by the first AWG can be different in the CO/OLT. The performance of millimeter wave band carrier can be degraded because of the mixing between it and OBI-noise generated in the photo-detection of 60-GHz PD. Fig. 3(a) shows the RF spectrum of 63-GHz carrier generated using a sideband routing and optical carrier suppression. Its resolution bandwidth as well as its video bandwidth was 300 khz. The maximum carrier to noise ratio of 50 db was measured by controlling the polarization state using polarization controller in the proposed system. The phase noise was measured to observe the performance of a measured 63-GHz carrier exactly as shown in the Fig. 3(b). A filled star line indicates the phase noise of 63-GHz signal generated at the RF signal generator used in the experimental setup. A filled square line shows the phase noise of a 63-GHz carrier obtained using two proposed methods (optical carrier suppression and sideband routing). As shown in the Fig. 3(b), the phase noise of 63-GHz carrier from the proposed scheme was higher than that of RF signal generator by 40 db below frequency offset of 100 khz. The difference between these phase noises became small up to 10 db over frequency offset of 100 khz. The large phase noise is generated by the two

4 2216 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, 2010 Fig. 3. (a) Measured RF spectrum of 63-GHz millimeter wave carrier, (b) Phase noises of 63-GHz signals from the proposed scheme and RF signal generator. following causes. In the experimental setup of Fig. 2, the optical path length of a routed optical sideband is different from that of an optical sideband which is not routed. The small perturbation due to the usage of the imperfect optical devices (optical circulator and 3 db optical coupler) is also generated. Accordingly, the phase delay between two optical sidebands is produced randomly because of two sorts of adverse effects. The phase noise of 63-GHz tone is increased inevitably because of their random phase delay. In the system based on the phase modulation, its performance can be degraded by the phase noise. It is necessary both to reduce the difference of optical path length and to improve the stability of the proposed scheme if it is developed as a real network. B. Simultaneous Downlink Transmission of 1.25-Gb/s Wired and 1.25-Gb/s Wireless Data The measured RF spectra and BER curves of both a 1.25-Gb/s wired data and a 63-GHz wireless one were presented in Fig. 4. Fig. 4(a) shows the measured RF spectra of 63-GHz wireless signal and its inset shows a demodulated 1.25-Gb/s data which is recovered at baseband because of the square-law detection characteristics of the Schottky diode into a 63-GHz band ASK receiver. The BER curves were measured repeatedly for both a 1.25-Gb/s wired data and a demodulated 63-GHz wireless one as shown in the Fig. 4(b). The electrical eye patterns before and after 23-km optical transmission are also shown in Fig. 4(b). In the BER curve of a demodulated 63-GHz wireless data, it was observed that there is a small power penalty below 0.5 db at the BER of due to the chromatic dispersion after 23-km transmission. As reported in [20], the RF fading effect caused by the dispersion induced carrier suppression (DICS) will be produced significantly at the frequency of 31.5-GHz RF signal because the phase of RF signal generated by the optical mixing Fig. 4. (a) Measured RF spectrum of 63-GHz carrier, Inset: RF spectrum of demodulated 1.25-Gb/s wireless data, (b) Measured BER curves after simultaneous transmission of 1.25-Gb/s wired and wireless data, Inset: eye patterns at the BER of 10. between optical carrier and upper optical sideband is different from that between optical carrier and lower optical sideband after optical transmission. When they stay in the state of out of phase, the intensity of a 31.5-GHz RF signal can be near to zero because of the destructive interference. Therefore, we can also expect that the RF fading effect of a 63-GHz millimeter-wave signal will be rarely generated because the signal is produced by the only optical mixing between two optical sidebands irrespective of the difference of phase between upper optical sideband and lower one. This result tells us that there will be no serious problem due to the optical transmission in case of WDM-RoF access network system. We also observed that the receiver sensitivity of a demodulated 63-GHz wireless data is much higher than that of a 1.25-Gb/s baseband wired one. This is because a used ASK receiver for recovering 1.25-Gb/s wireless data over 63-GHz carrier has a small input power dynamic range compared to 1.25-GHz band optical receiver. C. Transmission of 1.25-Gb/s Uplink Data Fig. 5(a) shows the measured RF noise floors of an unmodulated optical sideband corresponding to the second channel of the second AWG. They were measured repeatedly in the presence of a simultaneous downlink transmission of a 1.25-Gb/s wired and wireless data in order to check the impact of a downlink transmission on the performance of an uplink data as a result of wavelength reuse. The gray blank circle line shows the noise floor in case of not transmitting a downlink data. The filled

5 WON et al.: FULL COLORLESS WDM-RADIO 2217 Fig. 6. Variation of receiver sensitivity of main channel against leaked adjacent ones. Fig. 5. (a) Measured noise floors of unmodulated light corresponding to the second channel of 2nd AWG in the presence of a downlink transmission, Inset: RF spectrum of a 1.25-Gb/s uplink data with a downlink transmission, (b) Measured BER curves of 1.25-Gb/s uplink data under bidirectional transmission. black line shows that in the existence of the simultaneous transmission of downlink wired and wireless data. As shown in the Fig. 5(a), the noise floor in the presence of a downlink transmission showed a rise of 5 db over that without a downlink transmission below the frequency of 1 GHz. This is because an unfiltered optical sidebands including a downlink data remains in the second channel of the second AWG for an uplink transmission after de-multiplexing by the second AWG. The impact of a downlink transmission on the performance of an uplink data can be decreased using a MZM with high OCS ratio. The inset of Fig. 5(a) also shows the RF spectrum of a 1.25-Gb/s uplink data detected at an optical receiver in the CO/OLT under bidirectional transmission. We also observed that there was little influence of downlink transmission on the performance of uplink data. The measured BER curves of a 1.25-Gb/s uplink data were presented in the Fig. 5(b). The insets of Fig. 5(b) show eye patterns in the presence of a downlink transmission. There was a 2-dB power penalty in an uplink transmission after 23-km transmission (between filled squares and filled triangles). This is attributed to the Rayleigh backscattering noise caused by the interference between a remodulated uplink light and an amplified Rayleigh backscattered signal from the RSOA of ONU/BS. D. Analysis About the Impact of Adjacent Channel on Multi-Channel Transmission The proposed system is for the simultaneous transmission of wired and wireless data based on WDM technique. Fig. 6(a) shows how the leaked adjacent channel can degrade the signal to noise ratio (SNR) of main channel. In the proposed scheme, the adjacent channels of a downlink main channel ( th channel) can be a leaked modulated optical sideband ( th channel) as well as a leaked unmodulated one ( th channel). In case of an uplink transmission, the adjacent channel can be a leaked modulated sideband ( th channel). Therefore, it is necessary to check the impact of two kinds of adjacent channels on the performance of the main channel in the proposed system. As shown in the Fig. 6(b), the receiver sensitivities (at the BER of ) of downlink and uplink transmissions were measured repeatedly as the intensity of two leaked adjacent channels goes up to dbm. Under the operating conditions presented in the Fig. 2, the impact of two leaked optical sidebands on a millimeter-wave signal was rarely observed because the RF frequency spacing (63 GHz) between main channels was different from that (37 GHz) between main channel and adjacent one. We checked that the power penalty of 0.5 db was generated due to the crosstalk of a leaked modulated optical sideband ( th channel) in case of a downlink wired data. The power penalty of 1 db was also observed because of the crosstalk of a leaked modulated optical sideband ( th channel) when the uplink data was recovered using the baseband detection at the CO/OLT. In terms of the WDM channel spacing, the frequency of a millimeter-wave signal should be chosen so that the frequency spacing between main channels keeps within the region of each channel bandwidth. We can also easily expect that the receiver sensitivities of downlink data and uplink one will become worse than the experimental result of Fig. 6(b) because the crosstalk of adjacent channel gets large depending on the level of the adjacent crosstalk of AWG in case of narrow channel spacing below 50 GHz.

6 2218 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 28, NO. 16, AUGUST 15, 2010 V. CONCLUSION A new WDM-RoF access network architecture supporting the simultaneous transmission of gigabit wired and wireless data was proposed in this paper. A millimeter wave signal of 63 GHz was generated using both an OCS effect and sideband routing. These techniques enabled the proposed architecture to transmit simultaneously both a millimeter wave band wireless data and a baseband wired one based on WDM access link. A RSOA was employed at CO/OLT as well as ONU/BS so that the proposed scheme is operated irrespective of wavelength. Error free transmissions (BER of ) of both 1.25-Gb/s baseband data and wireless data were accomplished after 23-km transmission. It was observed that there was 2-dB power penalty of uplink transmission due to Rayleigh backscattering noise because of the usage of reflective ONU/BS. We also observed that there was a negligible influence of wavelength reuse on uplink transmission because an unmodulated light for an uplink was employed in the proposed architecture. These experimental results show understand that the proposed system can be a good model for near future WDM-RoF access network transmitting a gigabit wired data and a wireless one simultaneously. REFERENCES [1] N. J. Frigo, P. P. Iannone, P. D. Magil, T. E. Darcie, M. M. Downs, B. N. Desai, U. Koren, T. L. Koch, C. Dragone, H. M. Presby, and G. E. Bodeep, A wavelength-division multiplexed passive optical network with cost-share components, J. Lightwave Technol., vol. 6, pp , [2] S. L. Woodward, P. P. Iannone, K. C. Reichmann, and N. J. Frigo, A spectrally sliced PON employing Fabry Perot lasers, IEEE Photon. Technol. Lett., vol. 10, no. 9, pp , [3] Z. Xu, Y. J. Wen, W. D. Zhong, C. J. Chae, X. F. Cheng, Y. Wang, C. Lu, and J. 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Ahmed, Overcoming chromatic-dispersion effects in fiber-wireless systems incorporating external modulators, IEEE Trans. Microw. Theory Tech., vol. 45, no. 8, pp , Aug Yong-Yuk Won (S 06 M 08) received the B.S., M.S., and Ph.D. degrees in electrical and electronic engineering from Yonsei University, Seoul, Korea, in 1997, 1999, and 2008 respectively, where he is currently working as a research professor in electrical and electronic engineering. From 1999 to 2002, he was with Optoelectronics Laboratory, Samsung Electronics, where he was involved in the research and development of optical devices. His current research interests are in passive optical networks, optical devices, and optical systems for communications. Hyun-Seung Kim received the B.S. degrees in electrical and electronic engineering at Yonsei University, Seoul, Korea in He is currently studying in the doctor course in electrical and electronic engineering at Yonsei University. His current research interests are radio over fiber system, optical device, and optical system for communications. Yong-Hwan Son received the B.S., M.S., and Ph.D. degrees in electronic engineering from Hoseo University, Asan, Korea in 1999, 2001 and 2008 respectively. He is currently working as a Postdoctor in electrical and electronic engineering, Yonsei University. His current research interests are radio over fiber system, optical device, optical sensor, visible light wireless communication and optical system for communications. Sang-Kook Han (M 95) received the B.S. degrees in electronic engineering from Yonsei University, Seoul, Korea, in 1986 and the M.S. and Ph.D. degrees in electrical engineering from the University of Florida, Gainesville, in From 1994 to 1996, he was with the System IC Laboratory, Hyundai Electronics, where he was involved in the development of optical devices for telecommunications. He is currently a Professor with the Department of Electrical and Electronic Engineering, Yonsei University. His current research interests include optical devices/systems for communications, optical switching, and microwave-photonics technologies.