MILLIMETER-WAVE (mm-wave) fiber-radio systems

1210 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 3, MARCH 2005 Multifunctional WDM Optical Interface for Millimeter-Wave Fiber-Radio Antenna Base Station Masuduzzaman Bakaul, Student Member, IEEE, Ampalavanapillai Nirmalathas, Senior Member, IEEE, Member, OSA, and Christina Lim, Member, IEEE Abstract A wavelength-division-multiplexed (WDM) optical interface has been proposed and demonstrated with the capacity of adding and dropping wavelength interleaved fiber-radio WDM channels spaced at 25 GHz and also enabling wavelength reuse, which eliminates the need for a light source at the base station. The proposed WDM optical interface is realized by the use of a multiport optical circulator in conjunction with multinotch fiber Bragg grating (FBG) filters. Its functionality is demonstrated both by experiment and simulation. The effects of optical impairments on the transmission performance of WDM channels were studied in detail through simulation for single and cascaded configurations of the interface. Index Terms Broad-band wireless access, cascaded optical add-drop multiplexing (OADM), dual-electrode Mach Zehnder modulator, fiber-radio base station, millimeter-wave fiber-radio system, multinotch fiber Bragg grating (FBG), multiport optical circulator, optical crosstalk, optical single-sideband (OSSB) modulation, remote antenna base station (BS), wavelength interleaving, wavelength reuse. I. INTRODUCTION MILLIMETER-WAVE (mm-wave) fiber-radio systems have been considered as one of the future means of providing broad-band radio services to customers. Due to the higher propagation losses of mm-wave (25 to 100 GHz) signals, the propagation distance is usually limited to few tens of meters to few tens of kilometers. Consequently, the broad-band wireless access (BWA) network architecture incorporating mm-wave radio transmission requires a micro- or pico cell, which implies the need for a large number of remote antenna base stations (BSs) within a small geographical area. In such BWA network architectures, the BSs that provide the wireless access to the users are interconnected to a central office (CO) dedicated for performing all the switching and signal processing functionalities [1] [3]. If the CO and the BSs are interconnected via an optical fiber feeder network, with optical links directly transporting the wireless mm-wave signals to and from CO, cost-effective BS architectures with reduced complexity can be realized. The use of wavelength-division multiplexing (WDM) in such networks Manuscript received August 16, 2004; revised November 16, 2004. M. Bakaul and C. Lim are with the Australian Photonics Cooperative Research Centre, Photonics Research Laboratory, Department of Electrical and Electronic Engineering, The University of Melbourne, VIC 3010, Australia (e-mail: mbakaul@ee.mu.oz.au). A. Nirmalathas is currently with the Ultra-fast Photonic Network Group, Information and Network Systems Department, National Institute of Information and Communications Technology, Koganei, Tokyo 184-8795, Japan. Digital Object Identifier 10.1109/JLT.2005.843445 can provide a high-capacity feed network required in the BWA applications [4], [5]. A simplified BS architecture incorporating electroabsorption modulators (EAMs) was initially proposed in [6] and was further extended and demonstrated in [7] and [8]. One disadvantage of the EAM-based techniques is the need for additional dispersion compensation as they are often based on the double-sideband modulation scheme [9]. To overcome this, optical single sideband with carrier modulation (OSSB C) scheme can be combined with wavelength reuse technique [10] to realize simple BSs. The wavelength reuse scheme eliminates the need for a separate light source at the BS by providing the optical carrier for uplink transmission where the uplink optical signal is generated by recovering a portion of the optical carrier used in the downlink transmission [10]. When the mm-wave radio-frequency (RF) signals are imposed onto the optical carrier, sidebands are generated at the spacings equal to the modulating mm-wave frequency away from the optical carrier. This causes the interchannel spacing of a WDM feeder network for a mm-wave fiber-radio system to rise. Wavelength interleaving has been proposed to increase the spectral efficiency in the optical domain [11], [12]. It is achieved by multiplexing the mm-wave-modulated WDM signals with channel spacings smaller than the modulating mm-wave signal frequency. By applying this technique, channel separations of 50 or 25 GHz can be easily realized [11], [12]. In addition, if the optical feeder network for mm-wave fiber-radio system can be developed by accessing the existing optical network infrastructure in the access and metro network domains, it will allow a fast route for realizing BWA systems based on the fiber-radio concept. We recently introduced such a BS optical interface that is dispersion-tolerant (based on the OSSB C modulation technique) and supports wavelength interleaving and wavelength reuse techniques where both the uplink and downlink signals were optimized independently (irrespective of the link length) [13]. The incorporation of such interfaces in future fiber-based millimeter-wave communication systems for BWA will offer higher spectral efficiency, increased wavelength utilization, and the possibility of merging with optical network infrastructure in the access or metro domain while realizing simple, compact, and low-cost BSs. However, within the interface, there are ultra-narrow-band fiber Bragg gratings (FBGs), which are capable of handling closely spaced dense-wdm (DWDM) channels. Therefore, the performance of signals passing through the interface may suffer from additional crosstalk and chromatic dispersion (CD) introduced by the elements of the interface. Consequently, this may limit the cascadability of multiple 0733-8724/$20.00 2005 IEEE

BAKAUL et al.: MULTIFUNCTIONAL WDM OPTICAL INTERFACE FOR mm-wave FIBER-RADIO ANTENNA BS 1211 Fig. 1. system. Proposed WDM optical interface enabling the wavelength recovery and optical add drop functionality for a wavelength-interleaved DWDM fiber-radio units of the proposed interface when applied to an optical fiber feeder network in a ring or bus configuration [14]. The effects of such impairments need to be characterized and managed. In this paper, a multifunctional wavelength-interleaved WDM optical interface is proposed and demonstrated that supports a wavelength reuse technique that provides optical carrier for the uplink transmission. The effects of optical impairments on the wavelength-interleaved and reused WDM channels passing through the proposed interface in single as well as cascaded configuration are then characterized. The paper is organized as follows. Section II describes the architecture and the working principle of the proposed WDM optical interface. Section III describes the experimental setup used to demonstrate the functionality of the proposed interface, both in downlink and uplink direction, and also presents the results obtained from the experiment. Section IV models a simulation setup similar to the experiment and characterizes the effects of optical impairments involving the proposed interface. The simulation results are discussed in Section V, and finally, in Section VI, results are summarized. II. PROPOSED WDM OPTICAL INTERFACE Fig. 1 shows the schematic of the proposed WDM optical interface of an antenna BS with the corresponding input, output, drop, and add spectra shown as insets. The input spectrum shows the three-wdm wavelength-interleaved channels (25-GHz spacing), namely,, and with their respective modulation sidebands at,, and with a modulation frequency of 37.5 GHz in OSSB C modulation format. After interleaving, the spacing between an optical carrier and adjacent sidebands is 12.5 GHz. The interface consists of a seven-port optical circulator (OC) connected to a two-notch FBG (FBG1) between port 2 and port 6 and a single-notch FBG (FBG2) at port 3 of the OC with a notch bandwidth of 12.5 GHz 0.1 nm each. The transmission profiles of the FBGs can be seen from the inset of Fig. 1. FBG1 is designed in such a way that it reflects 100% of a specific downlink optical carrier (e.g., ) with its modulation sideband from the input wavelength-interleaved WDM signals. The reflected signal is received at port 3 while the transmitted signals (the through channels) are routed to port 6 of the OC, where they will exit the interface via port 7 (OUT). FBG2 at port 3 was designed to reflect only 50% of the carrier at, while the remaining 50% of the carrier and the corresponding sideband ( ) of the downlink signal will be dropped at port 3 (DL Drop) that can be detected using a high-speed photodetector (PD). The reflected 50% carrier at is recovered at port 4 ( -re-use) of the OC and will be reused at the BS as the optical carrier for the up-

1212 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 3, MARCH 2005 Fig. 2. system. Experimental setup for the demonstration of a simplified WDM optical interface for wavelength reuse in a wavelength-interleaved DWDM fiber-radio link path. In the uplink direction, a dispersion-tolerant OSSB C formatted optical signal is generated using the recovered carrier and the uplink radio signal at the RF frequency equal to the downlink RF frequency. The optically modulated uplink signal is then added to the interface via port 5 of the OC. The added signal will be routed to port 6, where it will be reflected by FBG1 and combines with the remaining through wavelength-interleaved channels (the through channels) before being routed out of the interface via port 7 (OUT). The output spectrum along with the spectra of the downlink drop, the wavelength-reuse carrier, and the uplink signal are shown in the inset of Fig. 1. III. EXPERIMENTAL DEMONSTRATION The experiment to demonstrate the downlink/uplink transmission incorporating the proposed multifunctional WDM optical interface at the BS is shown in Fig. 2, which is split into Fig. 2(a) and (b) for simplicity. In this experiment, three

BAKAUL et al.: MULTIFUNCTIONAL WDM OPTICAL INTERFACE FOR mm-wave FIBER-RADIO ANTENNA BS 1213 narrow-linewidth optical carriers from three tunable lasers at the wavelengths 1556.0, 1556.2, and 1556.4 nm were used as the optical sources. The three optical carriers, followed by separate polarization controllers (PCs) were multiplexed together using two 3-dB optical couplers. The multiplexed optical carriers were then launched into a dual-electrode Mach Zehnder modulator (DE-MZM). A 37.5-GHz mm-wave signal with binary phase-shift-keying (BPSK) format was generated by mixing a 37.5-GHz local oscillator (LO) signal with a 155-Mb/s pseudorandom bit sequence (PRBS) data. The DE-MZM was biased at a quadrature bias (QB) point, and the amplified mm-wave signal was used to drive the two RF ports of the DE-MZM with a 90 phase shift maintained between the two drive signals. The resultant output of the modulator was an modulated signal with the three optical carriers and their respective sidebands interleaved. The interleaved optical signal was amplified by using an erbium-doped fiber amplifier (EDFA) followed by an optical bandpass filter (BPF) to minimize out-of-band amplified spontaneous emission (ASE) noise. The filtered signal was then transported over 10 km of single-mode fiber (SMF) to a remote BS comprising the proposed WDM optical interface, as described in Section II, in addition to the optical-to-electrical (O/E) and electrical-to-optical (E/O) interfaces and the RF electronics (RF interface). The interleaved spectrum entering the proposed interface can be seen from the inset of Fig. 2(a). An eight-port OC was used to construct the proposed WDM interface in conjunction with the FBGs. FBG1 has center wavelengths at 1556.2 and 1556.5 nm, bandwidths of approximately 0.11 nm, and reflectivity of 99.9%. FBG2 has a center wavelength of 1556.2 nm with a bandwidth of approximately 0.11 nm and reflectivity of 50%. The desired downlink signal for the specific BS which comprised 50% of the carrier at 1556.2 nm with its corresponding sideband at 1556.5 nm was recovered from the DL Drop port of the WDM optical interface, and the optical spectrum is shown in the inset of Fig. 2(b). The downlink signal was then directed to the O/E interface of the BS, where it was detected using a 45-GHz PD. After photodetection, the downlink signal was amplified using an amplifier chain of low-noise amplifier (LNA) and medium power amplifier (MPA) and then downconverted to an intermediate frequency (IF) of 2.5 GHz. Subsequently, the baseband data was recovered using a 2.5-GHz electronic phase-locked loop (PLL). To verify the functionality of the proposed interface in the uplink direction, 50% of the carrier at 1556.2 nm was recovered from the -re-use port of the WDM optical interface, and the optical spectrum is shown in the inset of Fig. 2(b). The recovered carrier was used to drive the uplink DE-MZM, part of the O/E interface in the BS. The uplink radio signal was generated using a BPSK generator at 37.5 GHz with a data rate of 155 Mb/s, similar to the one used in the downlink transmission. The generated signal was then amplified before being applied to the uplink DE-MZM to generate an formatted optical signal. The resultant uplink optical spectrum is shown in the inset of Fig. 2(b). The modulated optical uplink signal was then routed to the interface via the ADD port, combined with the through channels at port 6, and left the interface via the OUT port. Also shown in the insets of Fig. 2(b) are the Fig. 3. Measured BER curves as a function of received optical power for (a) downlink transmission and (b) uplink transmission. optical spectrum of the through channels after the channel 2 (carrier at 1556.2 nm with the corresponding sideband at 1556.5 nm) is dropped with the achieved suppression of more than 30 db and also the optical spectrum at the output of the interface depicting the through channels and the added uplink channel 2. In comparison with the input spectrum, the through channels experience an insertion loss of around 3.5 db and added channel 2 suffers a 14-dB loss. This higher insertion loss of the drop add channel can be attributed to the poor frequency response of the DE-MZM used to generate the uplink signal at 37.5 GHz, which is much higher than the specified maximum bandwidth of the DE-MZM at 20 GHz [15]. To measure the system performance for uplink communication, the generated uplink optical signal was transmitted through another 10 km of SMF, before it was optically amplified, and the data was recovered using the same data recovery setup as for the downlink path. Fig. 3(a) and (b) shows the measured bit-error rate (BER) curves as a function of received optical power for the downlink and uplink paths. The downlink BER curve shows a transmission power penalty of 0.90 db with receiver sensitivity of 5.2 dbm at a BER of for transmission over 10 km of

1214 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 3, MARCH 2005 Fig. 4. Simulation setup to characterize the effects of optical impairments in single and cascaded WDM optical interfaces in a wavelength-interleaved DWDM fiber-radio system. SMF. The uplink curve shows the power penalty of 0.60 db with receiver sensitivity of 7.7 dbm at BER of for the same transmission distance. The incurred power penalty in downlink path is greater by 0.3 db, compared with the uplink direction. This can be attributed to the effects of crosstalk from the neighboring channels and the impact of lower sideband suppression of the OSSB C signals generated by the downlink DE-MZM used in the CO. The receiver sensitivity in the downlink path is lower by more than 2.5 db, compared with the uplink direction, which can be attributed to the lower modulation depth of the DE-MZM used in the downlink path in comparison with that of the uplink path, which is more than 5 db after 50% of the power is removed at the interface for carrier reuse. This can be overcome by replacing the DE-MZM of the downlink path with a similar one used in the uplink direction. Therefore, the optical spectra in the insets of Fig. 2 and the measured BER curves at Fig. 3 clearly demonstrate the functionality of the proposed multifunctional WDM optical interface that offers a practical solution for future high-capacity BWA networks incorporating wavelength interleaving and wavelength reuse techniques. IV. CHARACTERIZATION OF IMPAIRMENTS FOR SINGLE AND CASCADED INTERFACES Practical WDM networks, configured in ring/bus architectures, are promising technologies to achieve high-capacity transparent optical networks that offer advanced routing functionality through optical add drop multiplexing (OADM) interfaces. OADMs are indispensable to realize wavelength routing in the optical domain [16]. In the WDM ring/bus networks, optical signals will be transmitted through several WDM OADM in cascade [14], [17]. In the previous section, we have proposed and demonstrated a multifunctional WDM optical interface for DWDM mm-wave fiber-radio base stations. The concatenation of the interfaces will make the effective passband of the cascade narrower due to the variations in the passband roll-off and ripple in each individual FBG transfer functions [16], [17]. The required wavelength stability and accuracy in these systems becomes more stringent with the number of cascaded stages. All the above will give rise to signal waveform distortion, which can lead to eye closure and can introduce significant network performance degradation [18]. Hence, in this section, we will investigate and characterize via simulation studies the effects of optical impairments on the transmitted optical signals after traversing through single as well as cascaded interfaces. The simulation was carried out using commercially available photonic simulator platform VPITransmissionMaker. The schematic of the simulation setup to carry out the characterization of the effects of optical impairments in an optical link incorporating the proposed WDM optical interfaces is shown in Fig. 4. The simulation setup shows three OSSB C generators, combined and interleaved using a 4 1 combiner, amplified by an EDFA followed by a BPF. The filtered output was transported over 10 km of SMF to the two wavelength-interleaved WDM optical interfaces (OADM1 and OADM2). The OSSB C generators consist of three narrow-linewidth optical carriers with a channel spacing of 25 GHz and their modulation sidebands at 37.5 GHz with a data rate of 155 Mb/s. Each interface is shown as a block with five ports, namely, the input (IN), downlink drop (DL Drop), wavelength reuse drop ( -re-use), add (ADD) and output (OUT). The effects of the impairments were characterized based on the relative power penalties of the interleaved channels measured at the positions A, B, C, D, E, and F at the interfaces, as indicated in Fig. 4. For simplicity, the three interleaved channels with their modulation sidebands will be denoted as Ch1, Ch2, and Ch3, as shown in the inset of Fig. 4. The simulation models incorporated the observed experimental parameters such that the simulation study closely follows the experimental setup as far as possible. The BER curves for different channels at different positions were obtained by changing the center frequencies of the FBGs while keeping all other properties and parameters unchanged. V. SIMULATION RESULTS AND DISCUSSION As mentioned previously, the proposed interface comprises multiport OC and wavelength-selective FBGs. Therefore, both out-of-band and in-band crosstalk may cause performance

BAKAUL et al.: MULTIFUNCTIONAL WDM OPTICAL INTERFACE FOR mm-wave FIBER-RADIO ANTENNA BS 1215 Fig. 5. Measured BER curves at (a) point A showing downlink Ch2 with Ch 1 and Ch3 ON and OFF, respectively; (b) points A and B showing downlink Ch2; (c) point B showing downlink Ch2 without Ch1, uplink (UL) Ch2 and Ch3, with Ch1 and Ch3 but without uplink Ch2 and with all channels present; (d) points C and D showing uplink Ch2; and (e) point D showing Ch3 without Ch1, downlink Ch2 and uplink Ch2, with Ch1 and downlink Ch2 but without uplink Ch2 and with all channels present. degradation. In the downlink path, the double-notch FBG (FBG1) will result in a fraction of neighboring interleaved signal (out-of-band signal) to be reflected and pass through the DL Drop port of the interface along with the desired downlink signal. In addition, the uplink signal added to the interface uses the same FBG1 to be reflected to combine with the through channels before leaving the interface, a fraction of which is transmitted through the FBG1 and affects the downlink signal by in-band crosstalk. This crosstalk has significant effects on recovering the downlink data and results in power penalties in BER measurements [19], [20]. In the uplink direction, a fraction of the downlink signal will be transmitted due to

1216 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 3, MARCH 2005 Fig. 6. Measured BER curves at (a) points B and E showing downlink Ch2 and Ch1, respectively; (b) points A, D, and F showing downlink Ch3; and (c) points A, D, and E showing downlink Ch1. the imperfect property of the FBG1, which will eventually combine with the added uplink signal at the output of the interface and induces in-band crosstalk in the uplink path. Moreover, the imperfect isolations among the ports of the OC will result in additional crosstalk in both downlink and uplink directions. Fig. 5(a) shows the BER curves for downlink Ch2 at point A with two other channels (Ch1 and Ch3) ON and OFF, respectively. Recovered downlink Ch2 at point A experiences a negligible 0.15-dB power penalty that can be attributed to the effects of out-of-band crosstalk due to neighboring interleaved DWDM fiber-radio channels. To characterize the effects of crosstalk on the downlink Ch2 due to traversing the IN-DL Drop path of the interface, the BER curves were measured at points A and B, respectively, without the uplink signal, which can be seen from Fig. 5(b). It demonstrates an improvement of power penalty (negative power penalty) of 0.40 db. This can be attributed to the suppression of optical carrier by as much as 50% (as a result of wavelength reuse for the uplink path via FBG2) which, in turn, increases the modulation depth of downlink Ch2 after being dropped from the proposed interface [21]. To see the effects of added uplink Ch2 on the performance of downlink Ch2 at the DL Drop port, another set of BER curves were measured at point B under three different conditions: 1) removing Ch1 and Ch3 from the downlink interleaved channels along with the uplink Ch2 from the ADD port; 2) removing only the uplink Ch2 from the ADD port, but having Ch1 and Ch3 present; 3) having all the three interleaved channels along with the added uplink Ch2 present. The measured BER curves for these three conditions are shown in Fig. 5(c). It again shows that the downlink Ch2 at the DL Drop port experiences a negligible 0.15-dB power penalty without the presence of the uplink signal. However, the penalty increases to 0.30 db when the uplink signal is added to the interface, which is generated using part of the downlink carrier. The incurred additional penalty due to the added uplink Ch2 can be attributed to the effects of in-band crosstalk. For the uplink direction, BER curves were measured at points C and D for the uplink Ch2 to quantify the effects of crosstalk with all the other channels ON. The simulated BERs are shown in Fig. 5(d). It shows that the uplink Ch2 experiences 0.4-dB additional power penalty due to the in-band crosstalk from the downlink Ch2 as well as the out-of-band

BAKAUL et al.: MULTIFUNCTIONAL WDM OPTICAL INTERFACE FOR mm-wave FIBER-RADIO ANTENNA BS 1217 crosstalk from the neighboring interleaved channels. The effects of impairments on the through channels were investigated by measuring the BER curves for Ch3 at point D under three different conditions: 1) removing Ch1 and Ch2 from the downlink interleaved channels along with the uplink Ch2 from the ADD port of OADM1; 2) removing only the uplink Ch2 from the ADD port of OADM1 but having Ch1 and Ch2 present; 3) having all the three interleaved channels along with the added uplink Ch2 present. The measured BER curves can be seen from Fig. 5(e). It shows that the downlink channel or the added uplink channel does not have any significant effect on the through channels. To investigate and characterize the proposed interface in a cascade configuration, the BER curves for downlink Ch2 and Ch1 were recovered, respectively, at points B and E of the cascaded interfaces, where all the three interleaved channels along with the uplink Ch2 and uplink Ch1 were present. Here, we assume that OADM1 drops and adds Ch2, while OADM2 drops and adds Ch1. The measured BER curves can be seen from Fig. 6(a), which proves the functionality of the proposed interface in the cascaded configuration. To see the effects of the cascade on the through channels, BER curves for downlink Ch3 were measured at points A, D, and F with both downlink and uplink Ch2 and Ch1 active. The measured BER curves can be seen from Fig. 6(b). At each stage of concatenation, the observed additional power penalty is 0.20 db which again can be attributed to the effects of optical signal-to-noise ratio (OSNR) degradation due to the filtering effects of FBG1, insertion loss of the multiport OC, and the negligible contribution from the CD effects of FBG1 at each stage. To see the combined effects of crosstalk due to cascade and IN-DROP part of OADM2, BER curves for downlink Ch1 were measured at points A, D, and E, which can be seen from Fig. 6(c). It shows that Ch1 (at point E), after traversing through two cascaded OADMs, experiences an improvement of power penalty by 0.15 db relative to the signal before entering the OADM interfaces. This improvement can be attributed to the suppression of an optical carrier by as much as 50% by FBG2 in OADM2, as described previously. Despite this improvement, Ch1 experiences a power penalty of 0.2 db after propagating through a single OADM stage as per the analyses performed for the through channels. Therefore, the experiment as well as the simulation confirms the operation of a fiber-dispersion-tolerant optical interface for wavelength-interleaved mm-wave over fiber-radio signal transport. However, narrow-band FBG structures incorporated in the interface have the potential to incur additional penalties due their dispersion characteristics, as pointed out in [22]. During our studies, the effects of the impairments in single and cascaded interfaces were largely due to the effect of optical crosstalk arising from the interface, and dispersion effects due to FBGs had negligible contributions to the observed power penalties. In addition, the launched optical powers of the wavelength-interleaved WDM channels were maintained in such a way that any nonlinear interaction was minimized and had negligible contribution to the overall power penalties. VI. CONCLUSION This paper describes a proposed multifunctional WDM optical interface (OADM interface) for a future DWDM fiber-radio system that enables dispersion-tolerant OSSB C based wavelength-interleaved networks, which are capable of providing the optical carrier for the uplink transmission. The functionality of the proposed interface was verified experimentally for three-wavelength-interleaved DWDM channels with a channel spacing of 25 GHz, each of them carrying a 37.5-GHz RF signal with 155-Mb/s BPSK data transported over 10 km of fiber link. The use of the demonstrated interface in the future DWDM fiber-radio networks can improve the spectral efficiency, eliminate the need for a separate optical source for uplink, and ensure efficient wavelength utilization. To ensure the viability of the proposed interface in a network environment, the demonstrated system was modeled and studied using a VPITransmissionMaker simulator and characterized the effects of optical impairments with the proposed interface while in single and in cascade configuration. The simulation results confirm that the proposed interface experiences very negligible effects from optical crosstalk and can be used in cascade with an additional power penalty of less than 1 db. In the design process, we have taken the benefits of the matured and standard component technologies that enhance the possibility of merging the mm-wave fiber-radio-based BWA systems with existing optical network infrastructure in the access and metro domains. ACKNOWLEDGMENT The authors would like to thank M. Attygalle for some useful discussions and VPISystems for making VPITransmission Maker, used in the simulation studies, available to the authors. REFERENCES [1] R. Hofstetter et al., Dispersion effects in optical millimeter-wave systems using self heterodyne method for transport and generation, IEEE Trans. Microwave Theory Tech., vol. 43, no. 9, pp. 2263 2269, Sep. 1995. [2] W. I. Way, Optical fiber-based microcellular systems: An overview, IEICE Trans. Commun., vol. E 76-B, no. 9, pp. 1078 1090, 1993. [3] H. Ogawa et al., Millimeter wave fiber optics systems for personal radio communication, IEEE Trans. Microwave Theory Tech., vol. 40, no. 12, pp. 2285 2293, Dec. 1992. [4] G. H. Smith et al., A millimeter wave full-duplex fiber-radio star-tree architecture incorporating WDM and SCM, IEEE Photon. Technol. Lett., vol. 10, no. 11, pp. 1650 1652, Nov. 1998. [5] R. Heinzelmann et al., Optical add-drop multiplexing of 60 GHZ millimeter wave signals in a WDM radio-on-fiber ring, in Proc. Optical Fiber Communication Conf. (OFC), vol. 4, 2000, pp. 137 139. [6] D. Wake et al., Passive pico-cell New in wireless network infrastructure, Electron. Lett., vol. 33, pp. 404 406, 1997. [7] K. Kitayama et al., A good prospect for broadband millimeter wave fiber-radio access system-an approach to single optical component at antenna base station, in Proc. Microwave Symp. Dig. (IEEE MTT-S), vol. 3, Jun. 11 16, 2000, TH4C-3, pp. 1745 1748. [8] A. Stohr et al., Full duplex fiber optic RF subcarrier transmission using a dual- function modulator/photodetector, IEEE Trans. Microwave Theory Tech., vol. 47, no. 7, pp. 1338 1341, Jul. 1999. [9] H. Schmuck, Comparison of optical millimeter-wave system concepts with regard to the chromatic dispersion, Electron. Lett., vol. 31, pp. 1848 1849, 1995. [10] A. Nirmalathas et al., Wavelength reuse in the WDM optical interface of a millimeter wave fiber-wireless antenna base station, IEEE Trans. Microwave Theory Tech., vol. 49, no. 10, pp. 2006 2012, Oct. 2001.

1218 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 3, MARCH 2005 [11] C. Lim et al., Technique for increasing optical spectral efficiency in millimeter-wave WDM fiber-radio, Electron. Lett., vol. 37, no. 16, pp. 1043 1045, 2001. [12] H. Toda et al., A DWDM MM-wave fiber radio system by optical frequency interleaving for high spectra efficiency, in Proc. IEEE Topical Meeting Microwave Photonics (MWP 2001), 2002, pp. 85 88. [13] M. Bakaul et al., Dispersion tolerant novel base station optical interface for future WDM fiber-radio systems, in Proc. Optical Internet/ Australian Conf. Optical Fiber Technology (COIN/ACOFT 03), 2003, pp. 683 686. [14] C. Marra et al., Wavelength-Interleaved OADM s incorporating optimized multiple phase-shifted FBG s for fiber-radio systems, J. Lightw. Technol., vol. 21, no. 1, pp. 32 39, Jan. 2003. [15] A. Loayssa et al., Design and performance of the bidirectional optical single-sideband modulator, J. Lightw. Technol., vol. 21, no. 4, pp. 1071 1082, Apr. 2003. [16] T. Otani et al., Cascadability of passband-flattened arrayed waveguidegrating filters in WDM optical networks, IEEE Photon. Technol. Lett., vol. 11, no. 11, pp. 1414 1416, Nov. 1999. [17] M. Fukui et al., Experimental verification of cascadability of 12 channel 2 25 Gb/s WDM add/drop multiplexer employing unequally-spaced arrayed-waveguide grating, in Proc. 22nd Eur. Conf. Optical Communication (ECOC 1996), 1996, pp. 103 106. [18] N. N. Khrais et al., Performance degradation of multiwavelength optical networks due to laser and (de)multiplexer misalignment, IEEE Photon. Technol. Lett., vol. 7, no. 11, pp. 1348 1350, Nov. 1995. [19] D. Castleford et al., Optical crosstalk in fiber-radio WDM networks, IEEE Trans. Microwave Theory Tech., vol. 49, no. 10, pp. 2030 2035, Oct. 2001. [20] C. Yang et al., Apodised fiber Bragg gratings fabricated with uniform phase mask using low cost apparatus, Electron. Lett., vol. 36, pp. 655 657, 2000. [21] M. Attygalle et al., Simple, passive technique for performance enhancement of fiber wireless links, in Proc. of OptoElectronics and Communications Conf./Conf. Optical Internet (OECC/COIN2004), 2004, pp. 826 827. [22] K. Kitayama et al., Dispersion effects of FBG filter and optical SSB filtering in DWDM millimeter wave fiber-radio systems, J. Lightw. Technol., vol. 20, no. 8, pp. 1397 1407, 2002. Masuduzzaman Bakaul (S 02) received the B.Sc.Eng. degree in electrical and electronic engineering from Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh, in 1997. He is currently working toward the Ph.D. degree at the Photonics Research Laboratory, The University of Melbourne, Melbourne, Australia. He then joined Fiber Optic Network Solutions (FONS) Bangladesh Ltd., where he worked until 2001. His research focuses on base-station optical interfaces for future dense-wavelength-division-multiplexing fiber-radio systems. Ampalavanapillai Nirmalathas (S 96 M 97 SM 03) received the degrees of B.E. (Hons.) and Ph.D. degrees in electrical and electronic engineering from the University of Melbourne, Melbourne, Australia, in 1993 and 1997, respectively. In 1997, he joined the Photonics Research Laboratory (PRL), a member of the Australian Photonics Cooperative Research Centre (APCRC), at the University of Melbourne, where he held positions as a Research Fellow, Senior Research Fellow, and Senior Lecturer before moving to his current position as an Associate Professor and Reader in the Department of Electrical and Electronic Engineering. He is also the Director of PRL and the Program Manager of the Telecommunications Technologies Research Program in the APCRC. In 2004, he was a Guest Researcher of the Ultrafast Photonic Network Group of the National Institute of Information and Communication Technology (NICT), Koganei, Japan, and a Visiting Scientist at the Light Department of the Institute for Infocom Research (I R), Singapore. He has written more than 130 technical papers in his field and has given a number of invited presentations at leading international conferences. He also holds two international patents and one provisional patent. His current research interests include fibre-wireless networks, optical access networks, optical signal monitoring, photonic packetswitching technologies, ultrafast optical communications systems, and applications of mode-locked semiconductor lasers. Dr. Nirmalathas is a Member of the Optical Society of America (OSA) and the Telecommunications Society of Australia. He is one of the IEEE Lasers & Electro-Optics Society (LEOS) representatives on the Steering Committee of the Conference on Lasers and Electro-Optics (CLEO) Pacific Rim Conference and a Member of the Steering Committee of the international Conference on Optical Internet (COIN). He has also been a member of committees associated with a number of international conferences in his field of expertise. Christina Lim (S 98 M 00) received the B.E. (First-Class Hons.) and Ph.D. degrees in electrical and electronic engineering from the University of Melbourne, Australia in 1995 and 2000, respectively. In 1999, she joined the Photonics Research Laboratory (a member of the Australian Photonics Cooperative Research Centre) at the University of Melbourne, where she is now a Senior Research Fellow. She was also one of the recipients of the IEEE LEOS Graduate Student Fellowship in 1999. She has also received the Australian Research Council Australian Research Fellowship in 2004. Her research interests include fiber-wireless access technology, modeling of optical and wireless communication systems, microwave photonics, application of mode-locked lasers, optical network architectures and optical signal monitoring.