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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005

Efficient Multiplexing Scheme for Wavelength-Interleaved DWDM Millimeter-Wave Fiber-Radio Systems M. Bakaul, Student Member, IEEE, A. Nirmalathas, Senior Member, IEEE, C. Lim, Member, IEEE, D. Novak, Senior Member, IEEE, and R. Waterhouse, Senior Member, IEEE

Abstract—A simple multiplexing scheme is proposed and demonstrated with the capability to interleave optically modulated 37.5-GHz radio channels in a dense-wavelength-division-multiplexed fiber-radio system with 25-GHz wavelength spacing, and also enable a carrier subtraction technique that improves the overall link performance by reducing the carrier-to-sideband ratio of the multiplexed channels. The proposed scheme is realized by the use of an arrayed waveguide grating having multiple optical loop-backs between the input and the output ports. Index Terms—Arrayed waveguide grating (AWG), carrierto-sideband ratio (CSR), millimeter-wave (mm-wave) fiber-radio system, optical loop-back (LB), optical multiplexing, optical single sideband (OSSB) modulation, remote antenna base station (BS), wavelength interleaving (WI).

I. INTRODUCTION

M

ILLIMETER-WAVE (mm-wave) fiber-radio systems are being considered as a potential candidate for the distribution of future broad-band wireless access (BWA) services. In these systems multiple remote antenna base stations (BSs), suitable for untethered connectivity for BWA applications, can be directly serviced by a central office (CO) via an optical fiber feeder network [1]. The excessive propagation loss at such high radio frequencies however, shrinks the radio coverage of the BSs to micro- and pico-cells, which brings forth the need for a large number of antenna BSs to cover a certain geographical area. The use of wavelength-division-multiplexing (WDM) in such fiber distribution systems can improve the capacity of these systems by increasing the number of BSs serviced via a single CO [2]. The introduction of wavelength interleaving (WI) also enables these systems to support a dense-WDM (DWDM) feeder network that can further increase the number of BSs serviced through a single CO [3]. A demultiplexing scheme for 25-GHz separated DWDM mm-wave fiber-radio

Manuscript received June 27, 2005; revised September 11, 2005. This work was supported by the Australian Research Council’s Discovery Project 0452223. M. Bakaul is with the ARC Special Research Centre for Ultra Broadband Information Networks (CUBIN), The University of Melbourne, Melbourne, VIC 3010, Australia (e-mail: [email protected]). A. Nirmalathas is with National ICT Australia Limited, The University of Melbourne, Melbourne, VIC 3010, Australia. C. Lim, D. Novak, and R. Waterhouse are with the Department of Electrical and Electronic Engineering, The University of Melbourne, Melbourne, VIC 3010, Australia. Digital Object Identifier 10.1109/LPT.2005.859518

channels was proposed in [4], which requires additional preprocessing and postprocessing devices. An alternative approach is the introduction of a multifunctional WDM optical interface, which effectively adds and drops the desired channels to and from the wavelength-interleaved-DWDM (WI-DWDM) feeder network, in addition to enabling a wavelength reuse technique at the BSs [5]. The successful deployment of such systems supporting a WI-DWDM feeder network however, is largely dependent on suitable and efficient multiplexing schemes. In mm-wave fiber-radio systems, electrooptic intensity modulators (e.g., a dual-electrode Mach–Zehnder modulator, DE-MZM), suitable for generating optical single sideband modulation, often exhibit smaller with carrier OSSB dB are modulation depths (carrier-to-sideband ratio, CSR typical) which lead to poor overall link performances. This can be overcome by employing modulation depths where the CSRs of the modulated channels are reduced by some external means [6], [7]. However, most of these techniques require additional wavelength-selective components, which unfortunately are inherently susceptible to performance degradation. If a reduction in CSR can be combined with the multiplexing of WI-DWDM channels and additional hardware can be avoided, an efficient and simplified CO or remote node supporting WI-DWDM channels, can be realized. In this letter, we propose and demonstrate a wavelength-interleaved-multiplexer (WI-MUX) for 37.5-GHz band DWDM mm-wave fiber-radio channels spaced at 25 GHz. Unlike the scheme proposed in [8], the incorporated optical loop-backs (LBs) in our scheme reduce the CSRs by enabling the suppression of the optical carriers while leaving the respective modulation sidebands unchanged. II. PROPOSED WI-MUX Fig. 1(a) shows the desired WI scheme for optical mm-wave channels with a DWDM channel spacing and and , respectively. mm-wave carrier frequency of The optical carriers and their respective mod(in OSSB modulation ulation sidebands format) are interleaved in such a way that the adjacent channel . Fig. 1(b) spacing, irrespective of carrier or sideband, is shows the schematic of the proposed WI-MUX that realizes such a WI scheme with the input and output spectra shown in arrayed wavethe insets. It comprises a and a guide grating (AWG) with a channel bandwidth,

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BAKAUL et al.: EFFICIENT MULTIPLEXING SCHEME FOR WAVELENGTH-INTERLEAVED DWDM

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Fig. 2. Experimental setup for the demonstration of a WI-MUX that also reduces the CSR while interleaving the DWDM mm-wave channels in a WI-DWDM fiber-radio system.

and the modulation sidebands interleaved. The interleaved spectrum can be seen in the insets of Fig. 1(b). Due to the LBs, the optical carriers are suppressed by as much as twice the insertion – dB) compared to the loss (2 IL) of the AWG (typical IL modulation sidebands. Thus, the proposed WI-MUX enables a reduction in the CSR of the WI-DWDM channels by 8 to 10 dB which significantly improves the overall link performance. III. EXPERIMENTAL DEMONSTRATION Fig. 1. Proposed multiplexing scheme for WI-DWDM mm-wave fiber-radio systems that also reduces the CSR: (a) the desired interleaving scheme, (b) the proposed WI-MUX, and (c) the input–output characteristic matrix of the AWG.

channel spacing, , equal to the adjacent channel spacing of the desired WI scheme. The input ( ) and output ( ) ports of . The characteristic the AWG are numbered from to matrix of the AWG that governs the allocation and distribution of different channels at different ports is tabulated in Fig. 1(c). formatted input signals [shown as insets of The OSSB Fig. 1(b)] enter the AWG via the odd-numbered input ports, to . The AWG combines all the modulation sidebands at the output port . Due to the cyclic characteristics of the AWG as tabulated in Fig. 1(c), the optical carriers also exit as a composite signal via the output . The composite carriers are then looped back to the port that redistributes the carriers AWG through the input port to the odd-numbered output ports starting with . To realize the desired interleaving, the distributed carriers are again looped back to the AWG via the even-numbered input ports starting is the optical carriers with , and the resultant output at port

Fig. 2 shows the experimental setup used to demonstrate the proposed scheme, and is divided into Fig. 2(a) and (b) for clarity. In this experiment, three narrow linewidth tunable (1556.0 nm), (1556.2 nm), light sources at wavelengths (1556.4 nm) followed by separate polarization conand trollers (PCs) were used as the input to the three DE-MZMs. Three 37.5-GHz mm-wave signals with 155 Mb/s binary phase-shift-keyed data were generated by mixing 37.5- and 18.75-GHz (followed by a frequency doubler) local oscillator signals respectively with 155-Mb/s pseudorandom bit sequence data. The mixer outputs were amplified (one divided into two to provide the third mm-wave signal) and then applied to modulated optical the DE-MZMs that generate OSSB mm-wave channels. The modulated channels were then applied to an 8 8 AWG with a channel separation of 12.5 GHz and a channel bandwidth of 10 GHz. The allocation of the input ports and the selection of the LB paths are shown in Fig. 2(a), which result in the desired WI output. Fig. 3(a)–(c) shows the measured spectra of the modulated optical mm-wave channels, , and before multiplexing, with observed CSRs of 17.8, 13.5, and 13.4 dB, respectively. Also, Fig. 3(d) shows the combined spectrum of the channels after

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 12, DECEMBER 2005

Fig. 5. Impact of number of loop backs: (a) the optical spectra, and (b) the BER curves of (C ; S ) while transmitted as a single channel through the AWG and the number of LBs increased from zero to three.

Fig. 3. Measured spectra of the optical mm-wave channels: (a)–(c) are the channels (C ; S ); (C ; S ), and (C ; S ). respectively. before entering to AWG, and (d) the multiplexed channels after AWG.

coupler and no LB was provisioned; 2) was allowed one LB and before combining with ; 3) was between ports allowed two LBs between ports and and and bewas allowed three LBs before combining with ; and 4) and and , and and before comtween ports bining with . The measured optical spectra and the respective BER curves can be seen in Fig. 5(a) and (b), respectively. The results indicate that the increase in the number of LBs from zero to three causes a reduction in CSR of 10.7 dB which improves the overall link performance by 7.2 dB. IV. CONCLUSION

Fig. 4. Measured performance of proposed multiplexing scheme: (a) the optical spectrum, and (b) the BER curves of (C ; S ) recovered from three WI-DWDM channels after 10-km SMF.

multiplexing, where the CSRs of the interleaved channels have been reduced to 9.3, 6.2, and 5.1 dB. The interleaved channels were then amplified by an erbiumdoped fiber amplifier and followed by a 4-nm optical bandpass filter (BPF) prior to transmission over 10 km of singlemode fiber (SMF) to the optical add–drop multiplexing interis recovered, the meaface of a BS where channel sured spectrum of which is shown in Fig. 4(a). The recovered was then detected using a 45-GHz photodechannel tector, amplified, down-converted to an intermediate frequency of 2.5 GHz, and filtered by using an electrical BPF with a bandwidth 400 MHz, from which the baseband data was recovered using a 2.5-GHz electronic phase-locked loop (PLL). Fig. 4(b) shows the measured bit-error-rate (BER) curves for the recovered channel for the back-to-back case (having the AWG, but no fiber) and after transmission over 10 km of SMF. The result exhibits a negligible power penalty of 0.2 dB at a BER of that can be attributed to experimental errors. To characterize the effects of the reduction in CSRs due to was transported through the AWG the LBs, channel and the data was recovered under four conditions: 1) carrier and sideband at the OUT ports were combined using a 3-dB

We have proposed and demonstrated a simplified multiplexing scheme for mm-wave (37.5 GHz) WI-DWDM (25-GHz channel spacing) fiber-radio systems. The proposed scheme also enables a reduction in CSR of the optical mm-wave channel by optimal selection of the LB paths which improves the overall link performance significantly while avoiding additional hardware. The demonstration of error-free data from the multiplexed channels after recovery BER transmission over 10 km of SMF with negligible power penalty, confirms the functionality of such a WI-MUX. REFERENCES [1] W. I. Way, “Optical fiber-based microcellular systems: An overview,” IEICE Trans. Commun., vol. E 76-B, no. 9, pp. 1078–1090, 1993. [2] G. H. Smith, D. Novak, and C. Lim, “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. [3] C. Lim, A. Nirmalathas, D. Novak, R. S. Tucker, and R. B. Waterhouse, “Technique for increasing optical spectral efficiency in millimeter-wave WDM fiber-radio,” Electron. Lett., vol. 37, no. 16, pp. 1043–1045, 2001. [4] H. Toda, T. Yamashita, T. Kuri, and K. 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. [5] 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. [6] 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. [7] 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. [8] 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 Int. Topical Meeting Microwave Photonics (MWP 2003), 2003, pp. 287–290.