Colorless Two Different Gigabit Data Access Transmissions Using Optical Double Sideband Suppressed Carrier and Optical Sideband Slicing

544 J. OPT. COMMUN. NETW./VOL. 5, NO. 6/JUNE 2013 Won et al. Colorless Two Different Gigabit Data Access Transmissions Using Optical Double Sideband Suppressed Carrier and Optical Sideband Slicing Yong-Yuk Won, Moon-Ki Hong, Yong-Hwan Son, and Sang-Kook Han Abstract A wavelength division multiplexed (WDM) radio over fiber access network architecture, capable of simultaneously transmitting both 63 GHz wireless and 2.5 Gb s wired data, is proposed in this paper. An optical carrier suppression effect and multiplexing of a 50 GHz spaced arrayed waveguide grating are employed to generate a 63 GHz millimeter-wave signal based on WDM. These techniques allow the proposed scheme to simultaneously transmit both wireless and wired data. For full colorless operation, a reflective semiconductor optical amplifier is used at the central office and base station. Error-free simultaneous transmissions [wired data: bit error rate (BER) of 10 12, wireless data: BER of 10 9 ]of2.5 Gb s wired data and 1.25 Gb s wireless data are achieved. Various impacts of the downlink transmission on the performance of uplink data are investigated with the proposed scheme. Index Terms Millimeter-wave; Optical carrier suppression; Radio over fiber; Wavelength division multiplexing. I. INTRODUCTION R ecently, the need for ubiquitous networks capable of continuously accessing information and services in all locations has greatly increased; the architecture of access networks has evolved according to social demands. Now, not only are different wireless services based on various mixed physical platforms but various wired ones are also being given to access users over diverse mediums, such as cable and fiber optics. Convergence among diverse services, except for the unified physical platforms, is eventually unable to offer users the economical and technical benefits and a system replacement or network is required in order to introduce the next-generation broadband multimedia services for the subscribers. As the demand for competitive systems for ubiquitousoriented services increases, various research around the world has focused on both a wavelength division Manuscript received November 12, 2012; revised February 9, 2013; accepted April 7, 2013; published May 9, 2013 (Doc. ID 179725). Yong-Yuk Won is with Yonsei Institute of Convergence Technology, Yonsei University, Incheon, South Korea. Moon-Ki Hong, Yong-Hwan Son, and Sang-Kook Han (e-mail: skhan@ yonsei.ac.kr) are with the Department of Electrical and Electronic Engineering, Yonsei University, Seoul, South Korea. http://dx.doi.org/10.1364/jocn.5.000544 multiplexed-passive optical network (WDM-PON) and a radio over fiber (RoF) access network. WDM-PON systems have been regarded as attractive broadband access networks with many advantages, such as huge bandwidth, bit rate independency, graceful upgradability, network flexibility, and excellent security [1 9]. It can also offer higher bandwidth per optical network unit (ONU), low splitting loss, and a longer access link over 80 km compared with Ethernet PON (EPON) and gigabit PON (GPON) systems [8,9]. Now, it is capable of supplying access users with broadband point to point (P2P) services with a data rate of 10 Gb s[10 13]. However, it is difficult to offer subscribers broadband wireless services including both broadcasting/video services as well as P2P services. An RoF technique capable of efficiently delivering gigabit wireless data allows the system providers to implement unified wireless access networks, independent of the type of wireless multimedia services. This is a result of radio frequency (RF) signals from direct current (dc) to 60 GHz millimeterwave bands being transmitted using fiber optic s broad bandwidth [14 30]. However, it may be difficult to connect an RoF access network with a conventional fiber to the home system because the WDM technique for enlarging the channel capacity is not easily applied at the millimeter-wave bands. Therefore, it has become very important to study the research on how the RoF technique can be effectively operated over a WDM-PON system in order to concurrently supply access users with both gigabit wireless and gigabit wired services [19]. Various schemes for simultaneous services have been proposed around the world [20 27]. A WDM-millimeter-wave band access network using a broadband optical source with optical bandwidths of greater than 30 nm was proposed [21]. A broadband optical source with 25 GHz spaced multimodes was used to generate several millimeter waves. The proposed system can be cost effective and simple because only a single optical source is required. However, it can be difficult to use the optical device in the real network because the broadband optical source with 25 GHz spaced multimodes is not available commercially. A WDM-RoF access network was also proposed employing a cyclic arrayed waveguide grating (AWG) to generate multimillimeter waves as well as to simplify the architecture of the proposed system [24]. However, 1943-0620/13/060544-10$15.00/0 2013 Optical Society of America

Won et al. VOL. 5, NO. 6/JUNE 2013/J. OPT. COMMUN. NETW. 545 in order to accomplish this, substantial optical losses, which can adversely affect the link budget, would be produced due to the complicated routing paths allocated for a single WDM-RoF channel. We have previously proposed RoF access networks supporting a millimeter-wave band signal using a selective injection locking and colorless WDM-RoF access network. This was based on a reflective semiconductor optical amplifier (RSOA) able to concurrently transmit both wired and wireless data [14,15,18, 26,27,31]. The technique of using nonlinear effects such as injection locking can make the architecture of the central office (CO) be complicated because many RF and optical devices would be required, and the stability of the proposed system can be reduced due to the perturbation of nonlinear effects [18,26,27]. Several schemes based on an RSOA are capable of simultaneously transmitting both gigabit wired and gigabit wireless data, based on WDM. However, these same schemes are unable to deliver millimeter-wave band signals to access users due to the WDM channel bandwidth limitation [14,15,31]. Therefore, an RoF access network based on WDM should be designed in consideration of the following: simplification, costeffectiveness, and easy and convenient maintenance of the system. This paper proposes a new WDM-RoF access network, capable of simultaneously transmitting two different types of data: 63 GHz gigabit wireless and gigabit wired. The network uses an RSOA as a wavelength-insensitive device at the optical line terminal (OLT) as well as the ONU. Both a millimeter-wave band of 1.25 Gb s data and a baseband of 2.5 Gb s data are generated. These are easily transmitted based on the WDM channels with the help of a 50 GHz spaced AWG and the optical carrier suppression (OCS) effect of a Mach Zehnder modulator (MZM). The remaining parts of this paper are organized as follows. In Section II, the operation principle of the proposed WDM-RoF access link is explained with an emphasis on the simultaneous optical transmission of 2.5 Gb s wired data and 1.25 Gb s wired data. The experimental setup for its verification is shown and described in Section III. In Section IV, along with the analytical discussions on the change of performance against the carrier to sideband ratio (CSR), various experimental results about downlink and uplink transmissions are presented in detail. The influence of the adjacent channel on the main channel is also investigated. Finally, this paper is summarized in Section V. II. PRINCIPLE OF OPERATION Figure 1 shows the WDM-RoF PON scheme, capable of simultaneously transmitting two different types of data: gigabit wired and gigabit wireless. At the CO, a continuous wave (CW) light with a center optical frequency of f 0 is split into two parts. One is injected into the MZM and then transformed into a double sideband suppressed carrier (DSB-SC) light after being modulated by an RF subcarrier of f 1, biased at V π. The frequency of the RF subcarrier is adjusted so that two optical sidebands fall into each channel bandwidth of a 50 GHz spaced AWG, as shown in Fig. 1. Two optical sidebands (DSB-SC light) are combined and then injected into RSOA 1 after being demultiplexed at the 50 GHz spaced AWG; they are modulated by the wireless data. The other CW light is fed to RSOA 2 and then directly modulated by the wired data after being demultiplexed at the 100 GHz spaced AWG. The modulated DSB- SC light and the modulated optical carrier are combined and then transmitted to the remote node (RN). At the 50 GHz spaced AWG of the RN, two demultiplexed lights are merged again and then fed to the ONU in order to detect a millimeter-wave band of wireless data at the optical Wireless Data RSOA 1 f 0-f 1 f 0+f 1 50-GHz spaced AWG 50-GHz RF subcarrier of f 1 f 0-f 1 f 0+f 1 2f 1 Base station CW f 0 Wired Data RSOA 2 MZM f 0 f 0 Rx 3 f 0-f 1 f 0-f 1 f 0+f 1 2f1 Rx 1 f 0+f 1 Rx 2 f 0-f 1 f 0 f 0+f 1 50-GHz spaced AWG RSOA 3 f 0 Wireless Data Wired Data Down converter Uplink wired data Uplink wireless data 100-GHz spaced 100-GHz AWG spaced AWG Uplink Data Central Office 100-GHz spaced AWG Remote node Fig. 1. Full colorless WDM-RoF PON supporting the simultaneous transmission of two data types: wired and wireless.

546 J. OPT. COMMUN. NETW./VOL. 5, NO. 6/JUNE 2013 Won et al. receiver (Rx 1), by the process of optical beating. Also, at the 100 GHz spaced AWG, the only optical carrier modulated by the wired data is filtered out and then fed to the baseband optical receiver (Rx 2) of the ONU. The wired data are recovered using the technique of baseband detection. For an uplink transmission, the modulated optical carrier, which is demultiplexed by the 100 GHz spaced AWG, is remodulated by the uplink data using the wavelength reusing the RSOA technique (RSOA 3 of the ONU) and then retransmitted back to the baseband optical receiver (Rx 3) of the CO. With an uplink it is difficult to directly modulate a millimeter-wave band wireless signal because of the relatively low frequency response of an RSOA (below 1.5 GHz). However, the wireless uplink data can be down-converted by a recovered millimeter-wave carrier from the downlink, when the downlink system is not activated, if time division multiplexing (TDM) is utilized in the proposed architecture. Discussion on the TDM technique will not be presented as part of this paper. III. EXPERIMENTAL SETUP Figure 2 shows the experimental setup for the proposed colorless WDM-RoF PON link supporting the simultaneous transmission of an independent millimeter-wave band of data and baseband data. Fourteen insets at the bottom of the experimental setup of Fig. 2 show the optical spectra measured at points, from A to L. A light from a tunable Fig. 2. Experimental setup for the proposed scheme. TLS, tunable light source; MZM, Mach Zehnder modulator; EDFA, erbium-doped fiber amplifier; AWG, arrayed waveguide grating; ATT, optical attenuator; OC, optical circulator; PC, polarization controller; 3 db, 3 db optical coupler; RSOA, reflective semiconductor optical amplifier; OBPF, optical bandpass filter; SMF, single-mode fiber; LNA, low noise amplifier; LPF, low pass filter. Insets: measured optical spectra at points A1, A2, B, C, D, E1, E2, F, G, H, I, J, K, and L.

Won et al. VOL. 5, NO. 6/JUNE 2013/J. OPT. COMMUN. NETW. 547 light source (TLS) was divided into two parts at a 3 db coupler. The light of the upper line was modulated by a 31.5 GHz RF signal using a single-armed MZM, biased at V π (4.7 V) and then converted into two optical sidebands with the DSB-SC format [see Fig. 2 inset (A1)]. An OCS ratio of approximately 5 db was measured because of the low slope efficiency of a used MZM. A polarization controller (PC 1) was used to maximize the coupling efficiency of the MZM. A Gaussian thermal AWG with 50 GHz channel spacing and 16 channels was used with an insertion loss of 2.5 db, a 3 db passband of 0.24 nm, adjacent crosstalk of 30 db, and nonadjacent crosstalk of 38 db. The first (1547.015 nm) and the second (1547.415 nm) channels of the AWG were used to demonstrate the proposed architecture. A DSB-SC light was optically amplified by erbiumdoped fiber amplifier (EDFA) 1 [see Fig. 2 inset (A2)] and then demultiplexed by the 50 GHz spaced AWG 1 through optical circulator (OC) 1. As shown in the insets (A1) and (A2) of Fig. 2, each carrier to noise ratio of the first sideband and seventh sideband was reduced compared with that of inset (A1), although the intensities of two sidebands increased from 52.2 dbm to 17.5 dbm. It was because amplified spontaneous emission (ASE) noises were added depending on the noise figure of EDFA after optical amplification. The spectra of each demultiplexed light are presented in Fig. 2 insets (B) and (C). They were merged at the 3 db optical coupler and were then directly modulated by the 1.25 Gb s baseband data with a 2 31 1 pseudorandom binary sequence (PRBS) and a 2 V p p swing depth, after being injected into RSOA 1. The measured 3 db frequency bandwidth of the RSOA was 1 GHz. The input optical power injected into the RSOA was measured at the (D) point of Fig. 2 and then the optical gain of the RSOA was 4 db, which was significantly lower than the small signal gain of 20 db at an input optical power of 25 dbm. This shows that the RSOA was operating with significant gain compression. The polarization-dependent gain of the RSOA was 1.5 db. The gain-peak wavelength ranged from 1530 to 1570 nm. PCs 2, 3, and 5 were used to maximize the gain efficiency of each RSOA. The modulated lights from RSOA 1 were multiplexed at the 50 GHz spaced AWG 1 and then amplified by EDFA 2 through OC 1 [see Fig. 2 insets (E1) and (E2)]. The light of the lower line was demultiplexed by the 100 GHz spaced AWG 1 through OC 2. It was fed to RSOA 2 and then directly modulated by the 2.5 Gb s baseband data with a 2 31 1 PRBS and 2 V p p swing depth. The modulated light from RSOA 2 was multiplexed at the 100 GHz spaced AWG 1 and then passed through OC 2 [see Fig. 2 inset (F)]. The lights (one with wireless data and the other with wired data), combined by a 3 db optical coupler, were transmitted to the RN after a 23 km optical transmission through OC 3 [see Fig. 2 inset (G)]. At the RN, they were equally split into two parts by a 3 db power splitter. One was injected into the 50 GHz spaced AWG 2 and then demultiplexed based on the channel bandwidth [see Fig. 2 insets (H) and (I)]. Two demultiplexed optical signals were merged and then fed to the ONU/base station (BS) [see Fig. 2 inset (J)]. A 63 GHz RF signal, generated by the optical mixing between two optical sidebands, was detected at the 60 GHz band Rx. The 1.25 Gb s wireless data was then recovered with an amplitude shift keying (ASK) receiver using direct detection with a Schottky diode. The other data were demultiplexed at the 100 GHz spaced AWG 2 and then also fed to the ONU/BS. The 2.5 Gb s wired data were detected with the baseband detection technique. The light was injected into RSOA 3 and directly remodulated by the 1.25 Gb s uplink data using the gain saturation effect of RSOA 3; it was also retransmitted back to the 1.25 GHz band Rx of the CO/OLT [see Fig. 2 inset (L)]. An optical bandpass filter (OBPF) with a 3 db bandwidth of 0.3 nm was utilized in place of a WDM demultiplexer. An input optical power injected into RSOA 3 was 13.5 dbm, providing a 15 db optical gain, considerably lower than the small signal gain of 25 db at an input optical power of 25 dbm, which had a polarization dependent gain of 2 db at this same optical power. This tells us that RSOA 3 was operated under a gain saturation region. In the small test-bed shown in Fig. 2, the bit error rate (BER) curves of both 1.25 Gb s wireless and 2.5 Gb s wired data were measured in a bidirectional optical transmission. Various relationships between the two date types were also investigated in terms of system performance. The BER curve of 1.25 Gb s uplink data was also repeatedly measured in the presence of a 23 km optical transmission. IV. RESULTS AND DISCUSSIONS This section has three parts. Part A investigates the qualities of both a 63 GHz millimeter-wave wireless signal with 1.25 Gb s data and one with 2.5 Gb s baseband data by repeatedly measuring each BER value against the received optical power. The receiver sensitivities of both 1.25 Gb s wireless data over a 63 GHz RF subcarrier and 2.5 Gb s wired data were repeatedly measured depending on the CSR and the swing depth of the wireless data in order to optimize the performance of the proposed architecture. Here, the CSR is defined as the intensity difference between two optical sidebands and the optical carrier modulated by wired data, as shown in the inset of Fig. 4. Part B shows the variation of the receiver sensitivity of the 1.25 Gb s uplink data, depending on the CSR. The change of the BER curve of the uplink data in the presence of the transmission of downlink data is also shown and analyzed. Finally, the impact of leaked adjacent channels on the main channel is investigated with the variation of receiver sensitivity in part C. A. Simultaneous Downlink Transmission of 1.25 Gb s ASK Data Over 63 GHz RF Subcarrier and 2.5 Gb s Baseband Data As shown in Fig. 1, a 63 GHz millimeter-wave band RF subcarrier for delivery of 1.25 Gb s wireless data is produced using the technique of optical beating between two optical sidebands, with a frequency spacing of 63 GHz due to the OCS. Generally, it has been reported that a DSB-SC light, which is generated due to the OCS, is able to deliver a gigabit of data in RoF access networks

548 J. OPT. COMMUN. NETW./VOL. 5, NO. 6/JUNE 2013 Won et al. with a 20 km optical transmission [9]. However, unlike other published schemes, the proposed architecture generates the processes of separation and combination between two optical sidebands. The phase noise of the millimeterwave band RF subcarrier is produced due to the imperfection of the 3 db optical couplers, located between RSOA 1 and the 50 GHz spaced AWG 1, as well as between the 50 GHz spaced AWG 2 and 60 GHz Rx. Therefore, the quality of the millimeter-wave band RF subcarrier needs to be evaluated as to whether or not the error-free transmission of 1.25 Gb s wireless data will be accomplished by the proposed scheme. Figure 3 shows the RF spectra of phase noises from a 63 GHz RF signal as well as an RF signal generator. The filled squares line is the phase noise of the 63 GHz tone generated by the proposed scheme, while the filled stars line corresponds to the RF signal generator. As shown, the phase noise of a 63 GHz RF subcarrier generated by the proposed architecture was higher than that of the RF signal generator, by approximately 30 db below the frequency offset of 100 khz. It was observed that there was little difference between the two phase noises near the frequency offset of 10 MHz. This is because the phase noise, generated due to the usage of an imperfect optical coupler, is attached to the 63 GHz RF subcarrier; its spectral density shows the highest values below the frequency offset of 100 khz. In the proposed scheme, it is very important to optimize the CSR, the intensity ratio of the modulated optical carrier with wired data to the two modulated optical sidebands with wireless data, in order to achieve a bidirectional error-free transmission. As shown in Fig. 4, in the case of a relatively low CSR, the receiver sensitivity of the 2.5 Gb s wired data may be degraded due to the crosstalk of the two modulated optical sidebands that are partly unsuppressed depending on the adjacent crosstalk of the 100 GHz spaced AWG 2 at the RN. In the case of a relatively high CSR, the performance of the 2.5 Gb s wired data can be improved because the intensity of 1.25 Gb s wireless data can be even lower than that of relatively low CSR. The performance of the 1.25 Gb s wireless data is in inverse Fig. 4. Performance variations of wired and wireless data depending on the CSR. proportion to that of the 2.5 Gb s wired data for each case. It is assumed that the peak intensity of the modulated optical carrier remains unchanged for a reasonable comparison of all cases. Figure 5 shows the variations in the receiver sensitivity of both the 2.5 Gb s wired data and the 1.25 Gb s wireless data against the CSR, when the peak intensity of the modulated optical carrier is unchanged. The filled squares line shows the measured BER values of the 2.5 Gb s wired data while the open circles line corresponds to that of the 1.25 Gb s wireless data. As shown in the figure, we observe that the receiver sensitivity of the 2.5 Gb s wired data was improved as the CSR increased to 22 db, consistent with the abovementioned theoretical comments concerning the CSR. We also checked that the simultaneous error-free transmission of the wired and wireless data could be achieved at a CSR of approximately 19 db. The system Fig. 3. Phase noises of 63 GHz RF subcarrier from the proposed scheme (filled squares) and RF signal generator (filled stars). Fig. 5. Measured BER curves of wired and wireless data as compared to the CSR.

Won et al. VOL. 5, NO. 6/JUNE 2013/J. OPT. COMMUN. NETW. 549 Fig. 6. Receiver sensitivity of wired and wireless data as compared to the modulation depth of 1.25 Gb s wireless data. performance may be different depending on the modulation depths even though the CSR values are the same. Figure 6 shows the receiver sensitivity variations of each data type against the modulation depth of the 1.25 Gb s wireless data at a CSR of 19 db. It was determined that the receiver sensitivity of the 2.5 Gb s wired data (filled squares line in Fig. 6) was degraded as the modulation depth of the 1.25 Gb s wireless data (open circles line in Fig. 6) increased, up to 100%. This indicates that a modulation depth of over 75% is required in order to achieve errorfree simultaneous transmissions of both the wired and wireless data. With the operational conditions (CSR of 19 db, modulation depth of 75%) obtained in Figs. 5 and 6, the BER curves of both the 1.25 Gb s wireless data and the 2.5 Gb s wired data were repeatedly measured before and after a 23 km optical transmission, as shown in Fig. 7. Each eye pattern was also measured at the lowest BER values after a 23 km optical transmission. The left inset of Fig. 7 shows the measured RF spectra of a 63 GHz wireless signal mixed with 1.25 Gb s ASK data; the right inset of Fig. 7 shows the 2.5 Gb s wired data. In the BER curve of the 2.5 Gb s wired data, there was a small power penalty below 0.5 db at the BER of 10 12, due to chromatic dispersion after the 23 km optical transmission. For the case of the demodulated 1.25 Gb s wireless data, there was no power penalty caused by the effect of dispersioninduced carrier suppression. These results indicate that there will be no serious issues due to optical transmissions for the case of a WDM-RoF access network system. It was also observed that the receiver sensitivity of the demodulated 1.25 Gb s wireless data was higher than that of the 2.5 Gb s baseband wired data, by approximately 5 db; this results from the ASK receiver, employed for recovering the 1.25 Gb s wireless data over the 63 GHz RF subcarrier, having a small input power dynamic range as compared to the 1.25 GHz band optical receiver. Fig. 7. Measured BER curves after simultaneous transmission of both 1.25 Gb s wireless data and 2.5 Gb s wired data. Left upper inset: RF spectrum of 63 GHz RF subcarrier. Right upper inset: RF spectrum of 2.5 Gb s wired data. sidebands, which are not fully suppressed according to the value of the adjacent crosstalk of the 100 GHz spaced AWG 2; as a result, the optimization of the CSR should be executed for the error-free transmission of 1.25 Gb s uplink data. The second degradation factor is the crosstalk of the 2.5 Gb s wired downlink data due to wavelength reuse. The third degradation factor is the Rayleigh backscattering noise caused by the bidirectional transmission based on a single optical fiber. Figure 8 shows the receiver sensitivity of uplink data as the CSR changes. As shown in Fig. 8, a CSR greater than 18 db is required for error-free transmission (BER of 10 9 ) of the 1.25 Gb s uplink data to be achieved by the proposed scheme. B. Optical Transmission of 1.25 Gb s Uplink Data In the proposed scheme, the receiver sensitivity of 1.25 Gb s uplink data may be degraded by three primary factors. The first degradation factor is the impact of the 1.25 Gb s wireless downlink data due to two optical Fig. 8. Variation of the receiver sensitivity of 1.25 Gb s uplink data as the CSR changes from 13 to 22 db.

550 J. OPT. COMMUN. NETW./VOL. 5, NO. 6/JUNE 2013 Won et al. -Log(BER) 2 3 4 5 6 7 Back to back 23 km transmission(with only 2.5 Gb/s wired downlink data) 23 km transmission(with only 1.25 Gb/s wireless downlink data) 23 km transmission(with wired/wireless downlink data) 8 10 9 11-26 -24-22 -20-18 -16-14 -12-10 -8-6 -4 Received optical power (dbm) Fig. 9. Measured BER curves of a 1.25 Gb s data uplink under bidirectional transmission. Insets: electrical eye patterns, in order from top to bottom (back to back, 23 m transmission with only the wired data, 23 km transmission with only the wireless data, and 23 km transmission with both wired and wireless data). The BER curves of a 1.25 Gb s data uplink, measured at a CSR of 19 db, are presented in Fig. 9, where open squares are for the case of no data transmissions, filled triangles are the measured BER values when only the 1.25 Gb s wireless data downlink is transmitted, open circles correspond to the measured BER values for the case of transmitting only the 2.5 Gb s wired data, and filled squares are for the case of the simultaneous transmission of both the 1.25 Gb s wireless and the 2.5 Gb s wired data. The insets of Fig. 9 show electrical eye patterns for each case (back to back, 23 km transmission with only the 2.5 Gb s wired data downlink, 23 km transmission with only the 1.25 Gb s wireless data downlink, and 23 km transmission with both wired and wireless data) in order, from top to bottom. It was observed that there was a power penalty of 4 db for the case of simultaneous downlink transmissions (filled squares) as compared to the case without any downlink transmissions (open squares). This can be attributed to the Rayleigh backscattering noise as well as the crosstalk of the 2.5 Gb s wired data downlink, where the former is caused by the interference between the remodulated uplink light and the amplified Rayleigh backscattered signal from the RSOA, and the latter is generated by the wavelength reuse based on the gain saturation effect of RSOA 3. C. Impact of Adjacent Channel on Multichannel Transmission It is important to investigate the influence of the adjacent channel on the main channel because the proposed system is based on the WDM technique. To analyze this, the second and third channels of the AWG were reallocated to the main channel while the first and fourth channels were given to two adjacent channels, respectively. Figure 10(a) shows Receiver sensitivity (dbm) -5-6 -7-8 -9-10 Without leaked adjacent channel 2.5 Gb/s wired data 1.25 Gb/s wireless data -11-55 -50-45 -40-35 -30-25 -20-15 Leaked adjacent channel power (dbm) Fig. 10. Change of receiver sensitivity of the main channel (ith channel) against the intensity of leaked adjacent signals (i 1th channel and i 1th channel) in the case of downlink transmission: (a) shows how the receiver sensitivities of the main channels can be degraded by two leaked adjacent channels and (b) shows their experimental results.

Won et al. VOL. 5, NO. 6/JUNE 2013/J. OPT. COMMUN. NETW. 551 how the leaked adjacent channels (i 1th and i 1th channel) can adversely affect the signal-to-noise ratio (SNR) of the main channel (ith channel) in the case of downlink transmission. In the case of wired data, the leaked modulated optical sidebands (i 1th and i 1th channel) can degrade the SNR of downlink wired data (ith channel). It can also be expected that there will be no impact of adjacent channels on downlink wireless data because the other crosstalk noises (at 1.25 and 37 GHz) would be blocked while downlink wireless data are passed through the bandpass filter (BPF) at 63 GHz. As shown in Fig. 10(b), the receiver sensitivities (at the BER of 10 12 in the case of 2.5 Gb s wired data and at the BER of 10 10 in the case of 1.25 Gb s wireless data) were measured repeatedly as the intensity of two leaked adjacent channels goes up to 20 dbm. Under the operating conditions presented in Figs. 5 and 6, we observed that the power penalty of 2.2 db was generated due to the crosstalk of two leaked modulated optical sidebands (i 1th and i 1th channel) in the case of the downlink wired data. The crosstalk of two adjacent modulated optical sidebands on a 63 GHz signal was rarely observed because only the 63 GHz signal was passed through the filter. Figure 11(a) shows how the leaked adjacent uplink channels (i 1th and i 1th channel) can degrade the SNR of the main uplink channel (ith channel). The power penalty of 2.4 db was observed because of the crosstalk of two leaked modulated optical sidebands (i 1th and i 1th channel) when the uplink data were recovered using the baseband detection at the CO/OLT as shown in Fig. 11(b). It can also be expected that the receiver sensitivities of downlink and uplink will be worse than the experimental results of Figs. 10 and 11 because the crosstalk of the leaked adjacent channel can be increased depending on the level of the adjacent crosstalk of the AWG in the case of channel spacing less than 50 GHz. V. CONCLUSION A new, fully colorless, bidirectional WDM-RoF access network architecture, supporting the simultaneous transmission of independent 1.25 Gb s wireless data and 2.5 Gb s wired data, was proposed in this paper. A 63 GHz millimeter-wave subcarrier, for delivery of the 1.25 Gb s wireless data, was generated with the help of the OCS effect of a MZM, as well as the multiplexing of a 50 GHz spaced AWG. The proposed technical methods allow the gigabit wired and wireless data to be simultaneously generated and then transmitted to the access user based on a WDM. An RSOA was utilized at the CO/OLT as well as at the ONU/BS, so that the proposed scheme is not only operated fully independent of wavelength but is also a cost-effective system. The error-free transmission of the simultaneous transmission of the 1.25 Gb s wireless data and the 2.5 Gb s wired data was accomplished after a 23 km optical transmission. In the case of the uplink transmission, it was observed that there was a 4 db power penalty due to the crosstalk of the 2.5 Gb s wired data as well as the Rayleigh backscattering noise. The experimental results indicate that the proposed system is a competitive model for an access network required to converge gigabit wired and gigabit wireless data for the implementation of ubiquitous networks. ACKNOWLEDGMENTS This work was partly supported by the MKE (The Ministry of Knowledge Economy), South Korea, under the IT Consilience Creative Program, a support program supervised by the NIPA (National IT Industry Promotion Agency; NIPA-2013-H0203-13-1002); by the Yonsei University Institute of TMS Information Technology, a Brain Korea 21 program, South Korea; and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A1012531). REFERENCES Fig. 11. Variationofreceiversensitivityofthemainuplinkchannel against the leaked adjacent uplink channel: (a) shows the deterioration of the SNR of the uplink data due to the crosstalk of two leaked adjacent channels and (b) shows its experimental result. [1] Z. Xu, Y. J. Wen, W. D. Zhong, C. J. Chae, X. F. Cheng, Y. Wang, C. Lu, and J. Shanker, High-speed WDM-PON using CW injection-locked Fabry Perot laser diodes, Opt. Express, vol. 15, no. 6, pp. 2953 2962, Mar. 2007. [2] M. Attygalle, T. Anderson, D. Hewitt, and A. Nirmalathas, WDM passive optical network with subcarrier transmission and baseband detection scheme for laser-free optical network units, IEEE Photon. Technol. Lett., vol. 18, no. 11, pp. 1279 1281, June 2006. [3] Y. Luo, X. Zhou, F. Effenberger, X. Yan, G. Peng, Y. Qian, and Y. Ma, Time- and wavelength-division multiplexed passive optical network (TWDM-PON) for next-generation PON stage 2 (NG-PON2), J. Lightwave Technol., vol. 31, no. 4, pp. 587 593, Feb. 2013.

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Won et al. VOL. 5, NO. 6/JUNE 2013/J. OPT. COMMUN. NETW. 553 transmission of bidirectional gigabit baseband signals and broadcasting signal, Opt. Express, vol. 17, no. 19, pp. 16571 16580, Sept. 2009. Yong-Yuk Won received the B.S. and M.S. degrees in electrical and electronic engineering at Yonsei University, Seoul, South Korea, in 1997 and 1999, respectively. From 1999 to 2002, he was with the Optoelectronics Lab, Samsung Electronics, involved the research and development of optical devices. He received a Ph.D. degree in electrical and electronic engineering from Yonsei University in 2008. He is currently a professor at Yonsei Institute of Convergence Technology. His current research interests are passive optical networks, optical devices, and optical systems for communications. Moon-Ki Hong received the B.S. and M.S. degrees in electrical and electronic engineering from Yonsei University, Seoul, South Korea, in 2005 and 2008, respectively. He is currently working toward the Ph.D. degree in electrical and electronic engineering at Yonsei University. From January 2011 to April 2011, he was with the COBRA Institute, Technical University of Eindhoven, The Netherlands, where he was involved in the European Commission FP7 programs ALPHA and EURO-FOS. His current research interests include discrete multitone and OFDM modulation for next-generation optical access networks, RoF systems with millimeter-wave photonics, and WDM PON. Yong-Hwan Son received the B.S., M.S., and Ph.D. degrees in electronic engineering from Hoseo University, Asan, South Korea, in 1999, 2001, and 2008, respectively. He is currently working as a postdoctor in electrical and electronic engineering at Yonsei University. His current research interests are radio over fiber systems, optical devices, optical sensors, visible light wireless communication, and optical systems for communications. Sang-Kook Han received the B.S. degree in electronic engineering at Yonsei University, Seoul, South Korea, in 1986 and his M.S. and Ph.D. degrees in electrical engineering from the University of Florida in 1994. From 1994 to 1996, he was with the System IC Lab, Hyundai Electronics, involved in the development of optical devices for telecommunications. He is currently a professor in the Department of Electrical and Electronic Engineering at Yonsei University. His current research interests include optical devices/systems for communications, optical switching, and microwave photonics technologies.