Simultaneous Multiplexing and Demultiplexing of Wavelength ...

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 9, SEPTEMBER 2006

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Simultaneous Multiplexing and Demultiplexing of Wavelength-Interleaved Channels in DWDM Millimeter-Wave Fiber-Radio Networks Masuduzzaman Bakaul, Student Member, IEEE, Ampalavanapillai (Thas) Nirmalathas, Senior Member, IEEE, Christina Lim, Member, IEEE, Dalma Novak, Senior Member, IEEE, and Rod B. Waterhouse, Senior Member, IEEE

Abstract—A simultaneous multiplexing and demultiplexing (MUX/DEMUX) scheme for wavelength-interleaved millimeterwave 37.5-GHz-band fiber-radio channels spaced at 25 GHz has been proposed. The proposed MUX/DEMUX technique potentially realizes simple, compact, and low-cost central office and remote nodes by avoiding the use of wavelength-selective preand postprocessing hardware. The novel scheme incorporates an arrayed-waveguide grating with multiple loop-backs between the input and the output ports, in addition to multiple optical circulators and optical isolators. The multiplexing functionality of the proposed technology enables a carrier subtraction technique and consequently reduces the carrier-to-sideband ratios of the multiplexed channels. Multiplexing of the uplink channels generated via several methods is demonstrated experimentally. These techniques include generation of the channels by using the optical carriers that correspond to wavelengths spaced at the free spectral range (FSR) or multiples of the FSR from the downlink (DL) optical carriers and reuse of the DL optical carriers that are recovered by applying a wavelength reuse technique (λUL = λDL ± n × FSR, where n = 0, 1, 2, 3, . . .). The demultiplexing functionality of the proposed scheme that separates the 37.5-GHz-band wavelength-interleaved DL channels spaced at 25 GHz is also demonstrated. In addition, the effect of optical crosstalk on the transmission performance of the demultiplexed channels is also characterized experimentally. Index Terms—Adjacent channel crosstalk, arrayed-waveguide grating (AWG), carrier-to-sideband ratio (CSR), loop-back (LB), millimeter-wave (mm-wave) fiber-radio system, nonadjacent channel crosstalk, optical demultiplexing, optical multiplexing, optical single-sideband modulation, remote antenna base station (BS), simultaneous multiplexing and demultiplexing (MUX/DEMUX), wavelength interleaving (WI), wavelength-interleaved dense wavelength-division multiplexing (WI-DWDM), wavelength reuse.

Manuscript received November 25, 2005; revised May 15, 2006. This work was supported by the Australian Research Council’s Discovery Project 0452223. M. Bakaul and A. Nirmalathas are with the Victoria Research Laboratory, National ICT Australia Ltd., and also with the Department of Electrical and Electronic Engineering, University of Melbourne, Parkville, Vic. 3010, Australia (e-mail: [email protected]; [email protected]. edu.au). C. Lim is with the Department of Electrical and Electronic Engineering, University of Melbourne, Parkville, Vic. 3010, Australia (e-mail: c.lim@ee. unimelb.edu.au). D. Novak and R. B. Waterhouse are with the Department of Electrical and Electronic Engineering, University of Melbourne, Parkville, Vic. 3010, Australia, and also with Pharad, LLC, Glen Burnie, MD 21061 USA (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/JLT.2006.880591

I. I NTRODUCTION

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ILLIMETER-WAVE (mm-wave) fiber-radio systems, which have the potential to resolve the spectral congestion and the scarcity of transmission bandwidth at lower microwave frequencies, are considered promising technologies for the distribution of future broadband wireless access (BWA) services [1]–[3]. In these systems, multiple remote antenna base stations (BSs), which are suitable for providing untethered connectivity for BWA services, are directly interconnected to a central office (CO) via an optical fiber feeder network (FN) dedicated for performing all the switching and signal processing functionalities [4], [5]. The higher propagation losses of mmwave frequency signals, however, shrink the radio coverage of the BSs to microcells and picocells, which increase the number of antenna BSs required to cover a certain geographical area. Therefore, it is imperative that the BS architecture be simplified and be cost effective, whereas the fiber FN must be capable of supporting the large number of BSs required to service a certain geographical area. BSs incorporating multifunctional transceivers were proposed in [6] and [7]; however, these architectures were susceptible to the adverse effects of fiber chromatic dispersion since they were based on double-sideband modulation schemes, thereby requiring additional dispersion compensation [8]. Dispersion-tolerant BSs can be realized by employing a wavelength reuse technique in conjunction with optical single sideband with carrier (OSSB+C) modulation [9], [10]. The wavelength reuse technique simplifies the BS by removing the uplink (UL) light source completely, since a percentage of the downlink (DL) optical carrier is extracted and reused as a carrier for the UL signals. Meanwhile, OSSB+C modulation mitigates the impact of fiber chromatic dispersion, in addition to enabling increased spectral efficiency by reducing the required bandwidth for the transmitted channel [10]–[12]. The capacity of the fiber FN can also be increased by incorporating wavelength-division multiplexing (WDM), which enables the network to transport a large number of optical mm-wave channels that are routed to BSs through a single CO [13]. The introduction of wavelength interleaving (WI) compatible with dense WDM (DWDM) technology can further increase the capacity of the FN by transporting additional channels to support more BSs serviced by a common CO [14]–[16]. The WI technique utilizes the unused spectral bands

0733-8724/$20.00 © 2006 IEEE

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Fig. 1. Schematic depicting the optical spectra of the desired wavelength-interleaved channels in a DWDM mm-wave fiber-radio network.

available “in-between” the optical carriers and the respective modulation sidebands of the optical mm-wave channels, interleaving the neighboring DWDM channels via a spectral overlapping arrangement. The successful implementation of such wavelength-interleaved DWDM (WI-DWDM) FNs in mm-wave fiber-radio systems, however, is largely dependent on suitable and effective multiplexing and demultiplexing (MUX/DEMUX) schemes. Simple multiplexing schemes that efficiently interleave DWDM mm-wave fiber-radio channels separated by 25 GHz were proposed in [17]–[19]. A demultiplexing scheme for 25-GHz-separated DWDM mm-wave fiber-radio channels was proposed in [20], however, this scheme requires additional wavelength-selective pre- and postprocessing hardware, in addition to custom-developed arrayed-waveguide gratings (AWGs). An alternative approach to realizing a demultiplexing functionality that also avoids the aforementioned limitations is the introduction of multifunctional WDM optical interfaces [optical add/drop multiplexer (OADM) interfaces]. Such an interface effectively adds/drops the desired channel to/from the WI-DWDM FN [21] and also simplifies the BS by removing the light source from the UL path [22]. However, these interfaces are cascaded in the CO and the remote nodes (RNs) of such systems to enable demultiplexing of the multiple channels together, which may cause significant performance degradation and impose limitations in network dimensioning [23]. Moreover, the use of separate demultiplexer/cascaded OADM interfaces as well as multiplexers in the CO and RN makes the system complex, bulky, and expensive. If the multiplexing and demultiplexing functionality in the CO and the RN can instead be combined into a single device, cost-effective architectures with reduced complexity can be realized. In addition, it is important that passive WDM components in the COs and RNs are transparent to the UL channels generated by reusing the downlink optical carrier, which allows the BS to be simplified by removing the light source from the UL path [22]. In this paper, we propose and demonstrate a simultaneous MUX/DEMUX scheme for the CO and the RN in a fiberradio system, which effectively multiplexes and demultiplexes the 37.5-GHz-band WI-DWDM mm-wave fiber-radio channels spaced at 25 GHz. The incorporation of such a scheme in WI-DWDM mm-wave fiber-radio systems will offer efficient

multiplexing with improved overall link performance due to a reduction in the carrier-to-sideband ratio (CSR) [18]–[20], [24], [25]. Our technique will also provide effective demultiplexing by avoiding wavelength-selective pre- and postprocessing hardware as well as cascaded WDM interfaces. Our proposed method potentially results in the realization of simplified, consolidated, and cost-effective CO and RN hardware. In addition, the proposed scheme ensures the transparency of the CO and the RN to UL channels generated by reusing the DL optical carriers, which enables a simple, compact, and low-cost BS through the complete removal of the UL light source. This paper is organized as follows: Section II describes the architecture and the working principle of the proposed MUX/DEMUX scheme, whereas the experimental setup used to demonstrate the technique is described in Section III. Section IV then summarizes the experimental results obtained from demultiplexing the WI-DWDM DL channels via the proposed scheme. Experimental results obtained from the demonstration of the multiplexing functionality of the technique are presented in Sections V and VI. Here, the UL channels are generated by using optical carriers spaced at multiples of the free spectral range (FSR) of the AWG from the DL optical carriers (Section V) and by reusing the DL optical carrier recovered via the implementation of a wavelength reuse technique (Section VI). In Section VI, the effect of optical crosstalk while demultiplexing using the proposed scheme is characterized experimentally. Finally, in Section VIII, we present our conclusions. II. P ROPOSED MUX/DEMUX S CHEME Fig. 1 shows a schematic of the optical spectra of N optical mm-wave channels before and after interleaving, with a DWDM channel spacing and mm-wave carrier frequency of 2∆f and 3∆f , respectively. The optical carriers C1 , C2 , . . . , CN and their respective modulation sidebands S1 , S2 , . . . , SN (in OSSB+C modulation format) are interleaved in such a way that the adjacent channel spacing, irrespective of carrier or sideband, becomes ∆f . Fig. 2(a) shows the schematic of the novel MUX/DEMUX scheme that simultaneously enables multiplexing and demultiplexing of the proposed WI technique. MUX/DEMUX comprises a

BAKAUL et al.: MUX/DEMUX OF WAVELENGTH-INTERLEAVED CHANNELS IN DWDM FIBER-RADIO NETWORKS

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Fig. 2. Simultaneous multiplexing and demultiplexing of wavelength-interleaved channels in a DWDM mm-wave fiber-radio network. (a) Proposed DEMUX/MUX scheme. (b) Input–output characteristic matrix of the AWG.

(2N + 2) × (2N + 2) AWG with a channel bandwidth ≤ ∆f and a channel spacing of ∆f , in conjunction with multiple optical circulators (OCs) and optical isolators (OIs). The input (A) and output (B) ports of the AWG, which are reciprocal in nature, are numbered from 1 to 2N + 2. The characteristic matrix of the AWG that governs the distribution of different channels at various ports is tabulated in Fig. 2(b). For clarity, the proposed scheme is considered to be located at an RN where the UL channels are multiplexed and the DL channels are demultiplexed simultaneously. As shown in Fig. 2(a), the DL WI-DWDM channels from the FN enter the RN, are split by a 3-dB coupler, and pass through circulators OCD1 and OCD2 before entering the AWG via the ports A1 and A4 . The input

ports A1 and A4 were selected in such a way that the optical carriers CD1 , CD2 , . . . , CDN and their respective modulation sidebands SD1 , SD2 , . . . , SDN are demultiplexed together and exit the AWG via the odd-numbered output ports B1 − B2N −1 followed by OCM1 , . . . , OCMN , respectively. The circulators OCD1 , OCD2 , and OCM1 , . . . , OCMN work as the means of combining/separating the DL and UL channels to/from a specific port of the AWG and routing them to the destination accordingly. In the UL direction, OSSB+C-modulated optical mm-wave channels (SU1 , CU1 ), (SU2 , CU2 ), . . . , (SUN , CUN ), which are generated by either using the optical carriers that correspond to wavelengths spaced at multiples of the FSR of the AWG

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Fig. 3. Experimental setup for demonstration of the proposed DEMUX/MUX scheme for wavelength-interleaved channels in a DWDM mm-wave fiberradio network.

from the DL optical carriers or by reusing the DL optical carriers recovered by applying a wavelength reuse technique (λUL = λDL ± n × FSR, where n = 0, 1, 2, 3, . . .), are applied to the AWG via the ports B1 − B2N −1 followed by the circulators OCM1 , . . . , OCMN . Due to the reciprocal and cyclic characteristics of the AWG, the UL optical carriers and their respective modulation sidebands combine at ports A4 and A1 , respectively. The composite UL carriers CU1 , CU2 , . . . , CUN at A4 are then passed through OCD2 and looped back to the AWG through port B2 , which redistributes the carriers, respectively, to the odd-numbered A3 th, A5 th, A7 th, . . . , A(2N +1) th ports, starting with A3 th. To realize the desired interleaving for the UL channels, the distributed UL carriers CU1 , CU2 , . . . , CUN are again looped back to the AWG via the even-numbered B4 th, B6 th, B8 th, . . . , B(2N +2) th ports, starting with B4 th and the resulting outcome comprises the UL carriers and their respective modulation sidebands interleaved at port A1 (similar to the spectrum after multiplexing, shown in Fig. 1), which are then routed to the fiber FN via the OCD1 . The multiple loop-backs (LBs) of the UL carriers through the AWG reduce the CSR of the interleaved UL channels by as much as twice the insertion loss (2 × IL) of the AWG (typical IL: 4–5 dB), which is 8–10 dB. To minimize the effects of the unwanted signals from the even-numbered ports B4 to B2N +2 , the LB paths of the redistributed optical carriers were provided with directional OIs that route only the redistributed UL carriers to the AWG and suppress the remaining unwanted signals. Thus, the proposed simultaneous MUX/DEMUX scheme enables efficient multiplexing for the WI-DWDM mm-wave channels in the UL direction, whereas in the DL direction, the circuit also demultiplexes the WI-DWDM channels very effectively. III. E XPERIMENTAL D EMONSTRATION Fig. 3 shows the experimental setup for the demonstration of the multiplexing and demultiplexing operations of our proposed technique. In the DL direction, three narrow linewidth

tunable light sources LS1 , LS2 , and LS3 at the corresponding wavelengths CD1 (1556.0 nm), CD2 (1556.2 nm), and CD3 (1556.4 nm) followed by separate polarization controllers (PC) were combined using two 3-dB optical couplers and used as the input to a dual-electrode Mach–Zehnder modulator (DE-MZM). A 37.5-GHz mm-wave signal carrying 155 Mb/s binary-phase-shift-keyed (BPSK) data was generated by mixing an 18.75-GHz local oscillator (LO) signal followed by a frequency doubler, with 155 Mb/s pseudorandom-bit-sequence (PRBS) data. The mixer output was then amplified, then split by a quadrature coupler into two equal components with a relative 90◦ phase difference, and used to drive the two RF ports of the DE-MZM. The bias voltage for the DE-MZM was selected for quadrature operation. Therefore, the resultant output of the DE-MZM was an OSSB+C-modulated signal comprising the three optical carriers and their respective modulation sidebands, which are interleaved together. This interleaved output signal can be seen in the measured optical spectrum shown in the inset of Fig. 3. Here, a CSR of 13 dB has been achieved with a 29-dB suppression of undesired sidebands. The output spectrum also shows the 12.5-GHz adjacent channel spacing (irrespective of carrier or sideband) in addition to the DWDM channel spacing and the mm-wave carrier frequency of 25 and 37.5 GHz, respectively. The DL WI-DWDM channels were amplified by an erbiumdoped fiber amplifier (EDFA) and then filtered using a 4-nm optical bandpass filter (BPF) to minimize out-of-band amplified spontaneous emission (ASE) noise. The filtered signal was transported over 10 km of single-mode fiber (SMF) to the proposed DEMUX/MUX located at an RN, which comprised an 8 × 8 AWG in conjunction with multiple OCs and OIs as described in the previous section. The AWG transmission profile was tuned to match the transported channels by increasing its operating temperature to 72 ◦ C. The AWG transmission profile before tuning the center frequency to the transported channels is shown in Fig. 4(a), whereas the drift of the transmission profile with temperature is shown in Fig. 4(b). The transmission


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Fig. 5. Modified portion of the experimental setup that generates an optically modulated UL channel by recovering a portion of the DL optical carrier via a wavelength reuse technique.

Fig. 4. Measured characteristic properties of the AWG. (a) Transmission profile before tuning the center frequency to the transmitted set of channels. (b) Frequency drift versus temperature.

profile demonstrates a 3-dB channel bandwidth of approximately 10 GHz and a channel spacing of 12.5 GHz, which is equal to the adjacent channel spacing of the desired WI scheme. Therefore, the interleaved DL channels are demultiplexed to (SD1 , CD1 ), (SD2 , CD2 ), and (SD3 , CD3 ), which can be recovered by using a suitable PD and data recovery circuit through the respective output ports, B1 , B3 , and B5 . The multiplexing functionality in the UL direction, which is due to resource limitations, is demonstrated by transporting one UL channel. However, we have previously demonstrated the multiplexing of three channels using a similar scheme [18], [19]. A narrow linewidth tunable light source LSFSR at the operating wavelength CU3 (1552.4 nm) was used as the input to another DE-MZM located at the BS. The UL 37.5-GHz mm-wave signal was generated by mixing a 37.5-GHz LO signal with 155-Mb/s BPSK data, amplified and applied to the DE-MZM. The RF inputs and biasing of the DE-MZM were controlled in a similar way as was done in the DL direction, which results in (SU3 , CU3 ), which is an optically modulated mm-wave signal in OSSB+C modulation format. The carrier

CU3 is separated from the DL optical carrier CD3 by 500 GHz [CU3 = CD3 − 5 × FSR, where FSR = 100 GHz]; therefore, the UL channel (SU3 , CU3 ) will enter the proposed scheme via port B5 of the AWG. A similar use of FSR of AWG in mm-wave fiber-radio system can be found in [26]. The AWG then distributes the carrier and the sideband to ports A4 and A1 , respectively. To realize the desired multiplexing, the optical carrier is looped back to the AWG as per the proposed scheme, and the resulting multiplexed signal (the optical carrier and the respective sideband together) exits the AWG via port A1 . The multiplexed signal was then routed over 10 km of SMF via the circulator OCD1 and transported to the CO, where it is amplified by an EDFA and filtered by a BPF, before being detected and the data recovered by a high-speed photodetector (PD) and data recovery circuit. To demonstrate the compatibility of the proposed scheme with UL channels generated by reusing the DL optical carriers, a portion of the experimental setup shown in Fig. 3 (indicated by double dotted lines) was modified as shown in Fig. 5. Fig. 5 shows that the three-port OCM3 (in Fig. 3) is replaced with a four-port OCMR3 , in addition to a 50% reflective fiber Bragg grating (FBG) at port 3 of OCMR3 that corresponds to the demultiplexed (SD3 , CD3 ). The center frequency and the 3-dB bandwidth of the FBG are 1556.4 nm (= CD3 ) and 12.5 GHz, respectively. The demultiplexed channel (SD3 , CD3 ) exits the AWG via the output port B5 and enters OCMR3 via port 2, and encounters the 50% reflective FBG at port 3, where 50% of CD3 is reflected. The remaining part of the channel (SD3 , 50% CD3 ) is transmitted through the FBG from which DL data can be easily recovered via the use of a suitable PD and data recovery circuit. The reflected 50% of CD3 is then recovered at port 4 of OCMR3 and routed to the BS to drive the UL DE-MZM. The UL radio signal, which is used in the earlier technique to generate the optically modulated UL channel, was applied to the UL DEMZM, which results in the wavelength reused UL (SU3 , CU3 )

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Fig. 6. Measured optical spectra of the DL channels. (a) WI-DWDM channels before entering the AWG. Measured optical spectra of the demultiplexed channels. (b) (SD1 , CD1 ). (c) (SD2 , CD2 ). (d) (SD3 , CD3 ) after the proposed scheme.

having an OSSB+C modulation format. The UL (SU3 , CU3 ) then enters the AWG via port B5 and exits as a multiplexed signal through port A1 before being transported over 10 km of SMF to the CO. At the CO, the signal is amplified by an EDFA, followed by a BPF, and then detected and the data recovered with a high-speed PD and data recovery circuit.

IV. R ESULTS FOR D EMULTIPLEXED DL C HANNELS Fig. 6(a) shows the measured optical spectrum of the WIDWDM DL channels before entering the AWG via ports A1 or A4 , whereas Fig. 6(b)–(d) shows the optical spectra for the demultiplexed (SD1 , CD1 ), (SD2 , CD2 ), and (SD3 , CD3 ), respectively. The recovered spectra show the presence of optical crosstalk in the demultiplexed channels, which is defined here as the ratio of the undesired optical carriers to the desired optical carriers at the demultiplexed channels. Crosstalk levels of −19 to −25 dB were observed. The impact of the optical crosstalk on the transmission performance of the DL channels is characterized in Section VII. The measured spectra also confirm that the AWG in the proposed DEMUX/MUX scheme exhibits an IL of 3.3–5.8 dB, whereas the circulators OCD1 and OCD2 exhibit an average IL of 1.5 dB in each of the paths in the transmission direction. The higher differences in port-toport ILs of the AWG can be attributed to the fabrication errors as well as the limited tuning offered by the device.

Fig. 7. Measured BER curves as a function of received optical power for the demultiplexed channels (SD1 , CD1 ), (SD2 , CD2 ), and (SD3 , CD3 ).

To measure the bit-error-rate (BER) performance, the demultiplexed channels were detected using a 45-GHz PD, amplified, downconverted to an intermediate frequency (IF) of 2.5 GHz, and electrically filtered using a BPF with a bandwidth of 400 MHz, from which the baseband data was recovered using a 2.5-GHz electronic phase-locked loop (PLL). Fig. 7 shows the measured BER curves as a function of received optical


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Fig. 8. Measured optical spectra of the UL channel. (a) Before entering to the scheme. (b) Immediately after the scheme. (c) After suppression of the unwanted reflection crosstalk at the CO, which is located at a 10-km SMF distance from the proposed scheme.

power for the demultiplexed channels. Although demultiplexed (SD1 , CD1 ) experiences higher IL through the AWG [as shown in Fig. 6(a)–(d)], it exhibits 0.3–0.7 dB better sensitivity, which can be attributed to the lower crosstalk contribution from the neighboring channels. The results thus confirm the successful demonstration of demultiplexing functionality of the proposed DEMUX/MUX scheme, with the receiver sensitivity approximately equal to −15 dBm at a BER of 10−9 . However, due to resource constraint, the generation of the wavelengthinterleaved DL channels in this case is restricted by the possible data correlation, and therefore, the exhibited BER performances for the demultiplexed signals may contain some contribution from the possible data correlation, in addition to the dominant optical crosstalk effects, which need to be taken care while deploying practically.

V. R ESULTS FOR M ULTIPLEXED UL C HANNEL : UL O PTICAL C ARRIER AT DL O PTICAL C ARRIER S UBTRACTED BY M ULTIPLES OF FSR Fig. 8(a) shows the measured optical spectrum of the carrier and sideband pair (SU3 , CU3 ), before entering the proposed

Fig. 9. Measured BER curves as a function of received optical power for the multiplexed UL channel (SU3 , CU3 ) transported over 10 km of SMF with the back-to-back curve as reference.

DEMUX/MUX scheme. Fig. 8(b) shows the optical spectrum immediately after the proposed scheme, whereas Fig. 8(c) shows the signal after 10 km of SMF (followed by an EDFA

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Fig. 10. Measured optical spectra of (a) DL (SD3 , 50% CD3 ) after recovering 50% of the carrier, (b) the recovered carrier to be used in the UL path, (c) UL (SU3 , CU3 ) generated by the recovered carrier, (d) the multiplexed UL (SU3 , CU3 ) at the CO after transport over 10 km of SMF, and (e) the undesired optical crosstalk at the CO generated by reflections from the DL path.

and an optical BPF). As expected, the measured spectra exhibit a CSR of 14 dB before the proposed DEMUX/MUX scheme, which is reduced to 5 dB after the DEMUX/MUX scheme. This additional reduction in CSR improves the overall link performance significantly [18]–[20], [24], [25], although reduction in CSR also reduces the link’s optical power for which make up is necessary. In addition, the optical spectrum in Fig. 8(b) shows that the UL channel is contaminated by the out-of-band reflected crosstalk from the DL direction, which is approximately −17 dB. This unwanted power can be removed by the suitable selection of a BPF that follows the EDFA in

order to minimize the out-of-band ASE noise as shown in Fig. 8(c). In a practical network, each of the WI-DWDM UL channels will be demultiplexed at the CO before detection, therefore, the out-of-band crosstalk from the DL path does not require any special attention and will merge with the typical crosstalk caused by the filtering effects of the demultiplexer. To measure the BER, the filtered UL channel was subsequently detected, and data were recovered using the circuit previously described in the DL path. Fig. 9 shows the measured BER curves for the back-to-back condition (with the AWG but no transmission fiber) and after transmission over 10 km


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of SMF for the channel (SU3 , CU3 ). The result exhibits a negligible 0.3 dB power penalty at a BER of 10−9 , which can be attributed to experimental errors. Therefore, the recovered optical spectra and the BER curves clearly demonstrate the functionality of the proposed DEMUX/MUX scheme in multiplexing the UL channels with optical carriers at wavelengths equal to the difference between the DL optical carriers and 5 × FSR. VI. R ESULTS FOR M ULTIPLEXED UL C HANNELS : UL O PTICAL C HANNEL I S G ENERATED BY R EUSING DL O PTICAL C ARRIER Fig. 10(a) shows the measured optical spectrum of the DL channel (SD3 , 50% CD3 ) after recovering 50% of the carrier, whereas Fig. 10(b)–(e) presents the optical spectra for the recovered optical carrier, the generated UL (SU3 , CU3 ) before entering the proposed scheme, the multiplexed UL channel at the CO after transportation over 10 km of SMF, and the unwanted reflected crosstalk with the UL channel at the CO from the DL path, respectively. As expected, due to recovering half of the optical carrier, the CSR of the DL (SD3 , 50% CD3 ) channel was reduced by 3 dB. Furthermore, the CSR of the UL (SU3 , CU3 ) was reduced to 5 dB after the proposed scheme, whereas it was 14 dB before the proposed scheme. As stated before, these reductions in CSRs improve the sensitivity of the DL and UL path significantly [18]–[20], [24], [25], although the recovery of 50% carrier from the demultiplexed downlink signal may restrict the receiver power budget, as no optical amplification is done after such recovery. Fig. 10(d) and (e) also confirms that, due to traversing through the AWG, the UL channel is contaminated by the unwanted reflected in-band and out-of-band crosstalk from the DL path, which is approximately −10 dB here. As before, the out-of-band crosstalk from the DL path does not require any special attention and will merge with typical crosstalk levels caused by the filtering effects of the demultiplexer. However, the dominant in-band crosstalk may need to be addressed and managed when deploying such systems in practical networks. To quantify the signal degradation due to transmission over 10 km of SMF, UL (SU3 , CU3 ) was detected and BER curves were measured, both at the beginning (back-to-back) and at the end of the fiber link using the same PD and data recovery circuit described earlier. The recovered BER curves are presented in Fig. 11, and it can be seen that the UL channel (SU3 , CU3 ) experiences a negligible 0.4-dB power penalty at a BER of 10−9 , which can be attributed to experimental errors. The presented recovered optical spectra and the BER curves clearly demonstrate the functionality of the proposed DEMUX/MUX scheme in multiplexing UL channels that are generated by employing a wavelength reuse technique, which simplifies the BS by eliminating the light source from the UL path while realizing compact, low-cost, and lightweight BSs. VII. O PTICAL C ROSSTALK D UE TO THE P ROPOSED DEMUX/MUX S CHEME As discussed earlier, the DEMUX/MUX scheme under investigation comprises an 8 × 8 AWG with multiple OCs and

Fig. 11. Measured BER curves as a function of received optical power for the multiplexed UL channel (SU3 , CU3 ) generated by applying a wavelength reuse technique and transported over 10 km of SMF with the back-to-back curve as reference.

OIs, and therefore, there is the potential to incur performance degradation through optical crosstalk. Fig. 12(a) shows the measured transmission spectrum of eight optical carriers (unmodulated) multiplexed by the AWG used in the DEMUX/ MUX scheme. Fig. 12(b) and (c) then shows the optical spectra of the adjacent and nonadjacent channel crosstalk, respectively. The adjacent channel crosstalk varies from −16 to −25 dB, whereas the nonadjacent channel crosstalk varies from −29 to −46 dB. These differences in various adjacent and nonadjacent channel crosstalk can be attributed to Gaussian characteristics of the AWG under investigation, which are highly sensitive to the wavelength tolerances of the laser sources used in the demonstration. The limited tunability of the laser sources used in the demonstration also contributes significantly here. Fig. 13 shows the simplified experimental setup developed for characterizing the effects of optical crosstalk while demultiplexing. Three OSSB+C-modulated optical mm-wave signals, each carrying 37.5-GHz-band 155-Mb/s BPSK data, were generated by using three optical carriers at the wavelengths C1 (1556.0 nm), C2 (1556.2 nm), and C3 (1556.4 nm). The independently generated modulated channels were then combined using 3-dB couplers, where channels (S1 , C1 ) and (S2 , C2 ) follow two variable optical attenuators (VOAs) before being combined; a similar scheme can be found in [20]. The combined optical signal was then transported to the AWG, where it is demultiplexed and the channel (S3 , C3 ) is recovered via output port B5 . The VOAs are inserted to vary the optical powers of (S1 , C1 ) and (S2 , C2 ) that result in variable optical crosstalk with the demultiplexed channel (S3 , C3 ). In order to observe the effects of optical crosstalk, we measured the power penalties incurred by channel (S3 , C3 ) (at a BER of 10−9 ) as a function of optical crosstalk level, and the results are plotted in Fig. 14. This graph shows that a power penalty of 0.5 dB is observed for an optical crosstalk level of −16 dB, whereas the WI-DWDM channels demultiplexed by the proposed scheme (described in Section IV) experience a crosstalk of −19 to −25 dB. These measurements indicate that the effects of optical crosstalk are

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Fig. 12. Measured optical spectra when eight individual channels (unmodulated) are transported through the AWG. (a) Transmission spectra. (b) Adjacent channel crosstalk. (c) Nonadjacent channel crosstalk.

Fig. 13. Experimental setup for characterizing the effects of optical crosstalk on the WI-DWDM channel while being demultiplexed by the proposed DEMUX/MUX scheme.

negligible when using the proposed MUX/DEMUX scheme for demultiplexing the WI-DWDM channels. VIII. C ONCLUSION We have proposed and demonstrated a simultaneous MUX/DEMUX scheme for 37.5-GHz-band 25-GHz-channelspaced WI-DWDM mm-wave fiber-radio systems to be located at the CO and the RN. The proposed scheme offers a consolidated architecture by combining both functionalities together and can potentially realize a simplified and cost-effective CO

and RN by avoiding the use of wavelength-selective narrowband hardware. The scheme is based on standard AWG technology and is suitable for integration with the other conventional technologies found in the optical access or metro domain. In addition, when multiplexing, the proposed scheme reduces the CSR of the multiplexed channels, thereby significantly improving the overall link performance. Moreover, the proposed scheme supports wavelength reuse enabled BSs that eliminate the light source in the UL path and realize a simple, compact, and low-cost BS architecture. The error-free (at a BER of 10−9 ) recovery of data when both multiplexing and demultiplexing


Fig. 14. Measured power penalty as a function of optical crosstalk.

the respective UL and DL channels confirms the functionality of our proposed MUX/DEMUX scheme without any noticeable power penalty observed when transporting the signals over 10 km of SMF. R EFERENCES [1] H. Schmuck, R. Heidemann, and R. Hofstetter, “Distribution of 60 GHz signals to more than 1000 base stations,” Electron. Lett., vol. 30, no. 1, pp. 59–60, Jan. 1994. [2] R. Heidemann and G. Veith, “MM-wave photonics technologies for Gb/swireless-local-loop,” in Proc. OECC, Chiba, Japan, 1998, pp. 310–311. [3] J. O’Reilly and P. Lane, “Remote delivery of video services using mmwaves and optics,” J. Lightw. Technol., vol. 12, no. 2, pp. 369–375, Feb. 1994. [4] W. I. Way, “Optical fibre-based microcellular systems: An overview,” IEICE Trans. Commun., vol. E76-B, no. 9, pp. 1078–1090, 1993. [5] H. Ogawa, D. Polifko, and S. Banba, “Millimeter wave fiber optics systems for personal radio communication,” IEEE Trans. Microw. Theory Tech., vol. 40, no. 12, pp. 2285–2293, Dec. 1992. [6] D. Wake, D. Johansson, and D. G. Moodie, “Passive pico-cell— New in wireless network infrastructure,” Electron. Lett., vol. 33, no. 5, pp. 404–406, Feb. 1997. [7] K. Kitayama, T. Kuri, R. Heinzelmann, A. Stöhr, D. Jäger, and Y. Takahashi, “A good prospect for broadband millimeter wave fiber-radio access system—An approach to single optical component at antenna base station,” in Proc. IEEE MTT-S Microw. Symp. Dig., Jun. 2000, vol. 3, pp. 1745–1748. [8] H. Schmuck, “Comparison of optical millimetre-wave system concepts with regard to the chromatic dispersion,” Electron. Lett., vol. 31, no. 21, pp. 1848–1849, Oct. 1995. [9] A. Nirmalathas, D. Novak, C. Lim, and R. Waterhouse, “Wavelength reuse in the WDM optical interface of a millimeter wave fiber–wireless antenna base station,” IEEE Trans. Microw. Theory Tech., vol. 49, no. 10, pp. 2006–2012, Oct. 2001. [10] G. H. Smith, D. Novak, and Z. Ahmed, “Overcoming chromaticdispersion effects in fiber–wireless systems incorporating external modulators,” IEEE Trans. Microw. Theory Tech., vol. 45, no. 8, pp. 1410–1415, Aug. 1997. [11] K. Kitayama, “Highly spectrum efficient OFDM/PDM wireless networks by using optical SSB modulation,” J. Lightw. Technol., vol. 16, no. 6, pp. 969–976, Jun. 1998. [12] A. Narasimha, X. J. Meng, M. C. Wu, and E. Yablonovitch, “Tandem single sideband modulation scheme to double the spectral efficiency of analog fiber links,” Electron. Lett., vol. 36, no. 13, pp. 1135–1136, Jun. 2000. [13] G. H. Smith, D. Novak, and C. Lim, “A millimeter wave full-duplex fiberradio star-tree architecture incorporating WDM and SCM,” IEEE Photon. Technol. Lett., vol. 10, no. 11, pp. 1650–1652, Nov. 1998.

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[14] C. G. Schaffer, M. Sauer, K. Kojucharow, and H. Kaluzni, “Increasing the channel number in WDM mm-wave systems by spectral overlap,” in Proc. IEEE Top. Meet. MWP, Oxford, U.K., 2000, pp. 164–167. [15] C. Lim, A. Nirmalathas, D. Novak, R. S. Tucker, and R. B. Waterhouse, “Technique for increasing optical spectral efficiency in millimetre-wave WDM fibre-radio,” Electron. Lett., vol. 37, no. 16, pp. 1043–1045, Aug. 2001. [16] H. Toda, T. Yamashita, K.-I. Kitayama, and T. Kuri, “A DWDM MM-wave fiber radio system by optical frequency interleaving for high spectral efficiency,” in Proc. IEEE Top. Meet. MWP, Long Beach, CA, 2002, pp. 85–88. [17] H. Toda, T. Yamashita, T. Kuri, and K. Kitayama, “25-GHz channel spacing DWDM multiplexing using an arrayed waveguide grating for 60-GHz band radio-on-fiber systems,” in Proc. MWP, 2003, pp. 287–290. [18] M. Bakaul, A. Nirmalathas, C. Lim, D. Novak, and R. Waterhouse, “Simplified multiplexing scheme for wavelength-interleaved DWDM millimeter-wave fiber-radio systems,” in Proc. ECOC, Glasgow, U.K., 2005, vol. 4, pp. 809–810. [19] ——, “Efficient multiplexing scheme for wavelength-interleaved DWDM millimeter-wave fiber-radio systems,” IEEE Photon. Technol. Lett., vol. 17, no. 12, pp. 2718–2720, Dec. 2005. [20] H. Toda, T. Yamashita, T. Kuri, and K. I. Kitayama, “Demultiplexing using an arrayed-waveguide grating for frequency-interleaved DWDM millimeter-wave radio-on-fiber systems,” J. Lightw. Technol., vol. 21, no. 8, pp. 1735–1741, Aug. 2003. [21] C. Marra, A. Nirmalathas, C. Lim, D. Novak, B. Ashton, L. Poladian, W. S. T. Rowe, T. Wang, and J. A. Besley, “Wavelength-interleaved OADMs incorporating optimized multiple phase-shifted FBGs for fiberradio systems,” J. Lightw. Technol., vol. 21, no. 1, pp. 32–39, Jan. 2003. [22] M. Bakaul, A. Nirmalathas, and C. Lim, “Multifunctional WDM optical interface for millimeter-wave fiber-radio antenna base station,” J. Lightw. Technol., vol. 23, no. 3, pp. 1210–1218, Mar. 2005. [23] ——, “Performance characterization of single as well as cascaded WDM optical interfaces in millimeter-wave fiber-radio networks,” IEEE Photon. Technol. Lett., to be published. [24] R. D. Esman and K. J. Williams, “Wideband efficiency improvement of fiber optic systems by carrier subtraction,” IEEE Photon. Technol. Lett., vol. 7, no. 2, pp. 218–220, Feb. 1995. [25] M. Attygalle, C. Lim, G. J. Pendock, A. Nirmalathas, and G. Edvell, “Transmission improvement in fiber wireless links using fiber Bragg grating,” IEEE Photon. Technol. Lett., vol. 17, no. 1, pp. 190–192, Jan. 2005. [26] H. Toda, T. Nakasyotani, T. Kuri, and K.-I. Kitayama, “A full-duplex WDM millimeter-wave-band radio-on-fiber system using a supercontinuum light source,” in Proc. IEEE Top. Meet. MWP, Seoul, Korea, 2005, pp. 111–114.

Masuduzzaman Bakaul (S’02) received the B.Sc.Eng. degree in electrical and electronic engineering from the Bangladesh University of Engineering and Technology (BUET), Dhaka, Bangladesh, and the Ph.D. degree from the Centre for Ultra-Broadband Information Networks (CUBIN), University of Melbourne, Parkville, Australia, in 2006. His Ph.D. research focused on the development of system technologies for broadband wireless transport over optical access infrastructure. His dissertation was complemented with more than 20 publications in renowned journals and conference proceedings. He was with Fiber Optic Network Solutions Bangladesh Ltd. as an Optical Engineer until 2001. He is currently with the National ICT Australia (NICTA), University of Melbourne, where he explores the design and the development of cost-effective optical performance monitoring systems for next-generation wavelength-division-multiplexed long-haul and metro networks. Dr. Bakaul was one of the recipients of the LEOS/Newport/Spectra-Physics Research Excellence Awards in 2005.

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Ampalavanapillai (Thas) Nirmalathas (S’96– M’97–SM’03) received the B.E.(Hons.) degree in electrical and electronic engineering and the Ph.D. degree in electrical and electronic engineering from the University of Melbourne, Parkville, Australia, in 1993 and 1997, respectively. Since 1997, he has been with the Department of Electrical and Electronic Engineering, University of Melbourne, where he is an Associate Professor and Reader. He is also the Program Leader for the Network Technologies Research Program, Victoria Research Laboratory, National ICT Australia (NICTA), University of Melbourne. He held the positions of Research Fellow, Senior Research Fellow, and Senior Lecturer at the University of Melbourne before moving to his current position. Between 2001 and 2005, he was also the Director of the Photonics Research Laboratory, University of Melbourne, and the Program Manager of the Telecommunications Technologies Research Program in the Australian Photonics CRC. In 2004, he was a Guest Researcher of the UltraFast Photonic Network Group of NICT, Japan, and a Visiting Scientist at the Lightwave Department of I2 R, Singapore. His current research interests include microwave and terahertz photonics, optical access networks, optical performance monitoring, photonic packet switching technologies, and ultrafast optical communications systems.

Christina Lim (S’98–M’00) received the B.E. (first-class honors) and Ph.D. degrees in electrical and electronic engineering from the University of Melbourne, Parkville, Australia, in 1995 and 2000, respectively. In 1999, she joined the Photonics Research Laboratory (a member of the Australian Photonics Cooperative Research Centre), University of Melbourne. She is currently a Senior Research Fellow with the ARC Special Research Centre for Ultra-Broadband Information Networks (CUBIN), Department of Electrical and Electronic Engineering, University of Melbourne. Her research interests include fiber–wireless access technology, modeling of optical and wireless communication systems, microwave photonics, application of modelocked lasers, optical network architectures, and optical signal monitoring. Dr. Lim was also one of the recipients of the 1999 IEEE Lasers and ElectroOptics Society (LEOS) Graduate Student Fellowship. She was also the recipient of the 2004 Australian Research Council Australian Research Fellowship.


Dalma Novak (S’90–M’91–SM’02) received the B.E. degree (first-class honors) in electrical engineering and the Ph.D. degree from the University of Queensland, Brisbane, Australia, in 1987 and 1992, respectively. She is a Vice President at Pharad, LLC, Glen Burnie, MD, which develops advanced wireless communications, sensors, and antenna products. From 1992 to 2004, she was a Faculty Member in the Department of Electrical and Electronic Engineering, University of Melbourne, Parkville, Australia, where she is now a Professorial Fellow. From January 1992 to August 1992, she was a Lecturer in the Department of Electrical and Computer Engineering, University of Queensland, and in September 1992, she joined the University of Melbourne. From July 2000 to January 2001, she was a Visiting Researcher at the Department of Electrical Engineering, University of California, Los Angeles, and at the Naval Research Laboratory, Washington, DC. From June 2001 to December 2003, she was a Technical Section Lead with Dorsál Networks, Inc. and later with Corvis Corporation, Columbia, MD. From January to June 2004, she was a Professor and the Chair of Telecommunications at the University of Melbourne. Her research interests include hybrid fiber radio systems, microwave photonics applications, high-speed optoelectronic devices and systems, wavelength-division multiplexing networks, and wireless communications. She has authored or coauthored more than 200 papers in these and related areas, including three book chapters. Dr. Novak is an Associate Editor (Systems/Networks) for the IEEE/OSA JOURNAL OF LIGHTWAVE TECHNOLOGY and a member of the IEEE LEOS and MTT-S Microwave Photonics Technical Subcommittees. She is also the Chair of the 2007 Optical Fiber Communications Conference Technical Program Subcommittee on Optical Processing and Analog Systems and the Chair of the 2007 MTT-S International Microwave Symposium Technical Program Subcommittee on Microwave Photonics.

Rod B. Waterhouse (S’90–M’94–SM’01) received the B.Eng., M.S., and Ph.D. degrees in electrical engineering from the University of Queensland, Brisbane, Australia, in 1987, 1989, and 1994, respectively. In 1994, he joined RMIT University, Melbourne, Australia, as a Lecturer and became a Senior Lecturer in 1997 and an Associate Professor in 2002. In 2001, he took a leave of absence from RMIT and joined the venture-backed Dorsal Networks, which was later acquired by Corvis Corporation, Columbia, MD. In 2003, he left Corvis and resigned from his position at RMIT and worked for Photonic Systems Inc. as a Principal Engineer. In 2004, he cofounded Pharad, Glen Burnie, MD which is a broadband wireless communications company, where he is now a Vice President. He is an adjunct Senior Fellow within the Department of Electrical and Electronic Engineering, University of Melbourne, Parkville, Australia. His 2003 book on printed antenna design follows more than 180 publications and seven patent applications in the field of antennas, electromagnetics, microwave, and photonics engineering. Dr. Waterhouse is an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION. He chaired the IEEE Victorian MTTS/APS Chapter from 1998 to 2001.