Multicore-Fiber-Enabled WSDM Optical Access Network ... - IEEE Xplore

Multicore-Fiber-Enabled WSDM Optical Access Network With Centralized Carrier Delivery and RSOA-Based Adaptive Modulation Volume 7, Number 4, August 2015 Zhenhua Feng Borui Li Ming Tang, Senior Member, IEEE Lin Gan Ruoxu Wang Rui Lin Zhilin Xu Songnian Fu Lei Deng Weijun Tong Shengya Long Lei Zhang Hongyan Zhou Rui Zhang Shuang Liu Perry Ping Shum

DOI: 10.1109/JPHOT.2015.2445103 1943-0655 Ó 2015 IEEE

IEEE Photonics Journal

MCF-Enabled WSDM Optical Access Network

Multicore-Fiber-Enabled WSDM Optical Access Network With Centralized Carrier Delivery and RSOA-Based Adaptive Modulation Zhenhua Feng,1 Borui Li,1 Ming Tang,1 Senior Member, IEEE, Lin Gan,1 Ruoxu Wang,1 Rui Lin,1,2 Zhilin Xu,1 Songnian Fu,1 Lei Deng,1 Weijun Tong,3 Shengya Long,3 Lei Zhang,3 Hongyan Zhou,3 Rui Zhang,3 Shuang Liu,1 and Perry Ping Shum4 1

Next Generation Internet Access National Engineering Lab (NGIA), School of Optical and Electronic Information, Huazhong University of Science and Technology (HUST), Wuhan 430030, China 2 School of Information and Communication Technology (ICT), Royal Institute of Technology (KTH), 164 40 Kista, Sweden 3 State Key Laboratory of Optical Fiber and Cable Manufacture Technology, Yangtze Optical Fibre and Cable Company Ltd. (YOFC), Chengdu 610072, China 4 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 DOI: 10.1109/JPHOT.2015.2445103 1943-0655 Ó 2015 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received May 6, 2015; revised June 10, 2015; accepted June 10, 2015. Date of publication June 15, 2015; date of current version June 25, 2015. This work was supported in part by the 863 High Technology Plan of China under Grant 2013AA013402; by the National Natural Science Foundation of China under Grant 61331010, Grant 61275069, Grant 61205063, and Grant 61307091; by the Fundamental Research Funds for the Central Universities, HUST, under Grant 2013TS052; and by the Program for New Century Excellent Talents in University under Grant NCET-13-0235. Zhenhua Feng and Borui Li contributed equally to this work. Corresponding author: M. Tang (e-mail: [email protected]).

Abstract: We proposed and experimentally demonstrated a wavelength-space division multiplexing (WSDM) optical access network architecture with centralized optical carrier delivery utilizing multicore fibers (MCFs) and adaptive modulation based on reflective semiconductor amplifier (RSOA). In our experiment, five of the outer cores are used for undirectional downstream (DS) transmission only, whereas the remaining outer core is utilized as a dedicated channel to transmit upstream (US) signals. Optical carriers for US are delivered from the optical line terminal (OLT) to the optical network unit (ONU) via the inner core and then transmitted back to the OLT after amplification and modulation by the RSOA in the colorless ONU side. The mobile backhaul (MB) service is also supported by the inner core. Wavelengths used in US transmission should be different from that of the MB in order to avoid the Rayleigh backscattering effect in bidirectional transmission. With quadrature phase-shift keying–orthogonal frequency-division multiplexing (QPSK-OFDM) modulation format, the aggregation DS capacity reaches 250 Gb/s using five outer cores and ten wavelengths, and it can be further scaled to 1 Tb/s using 20 wavelengths modulated with 16 QAM-OFDM. For US transmission, 2.5 Gb/s QPSK-OFDM transmission can be achieved just using a lowbandwidth RSOA, and adaptive modulation is applied to the RSOA to further enhance the US data rate to 3.12 Gb/s. As an emulation of high-speed MB transmission, 48 Gb/s inphase and quadrature (IQ) modulated popularization division multiplexing (PDM)-QPSK signal is transmitted in the inner core of MCF and coherently detected in the OLT side. Both DS and US optical signals exhibit acceptable performance with sufficient power budget. Index Terms: Optical access network, wavelength-space division multiplexing (WSDM), multicore fiber (MCF), advanced modulation formats, adaptive modulation.

Vol. 7, No. 4, August 2015

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1. Introduction Recently, with the advent of bandwidth-intensive applications such as business internet protocol (IP) traffic, video/photo sharing, super HD video, mobile traffic backhaul, cloud computing, social networking, etc. [1], [2], the bandwidth demand of the end-users has witnessed an exponential growth; thus, the access network faces great challenges of being upgraded [3]. With the further evolution of access network and the increasing bandwidth demand, the future optical access network should be able to converge heterogeneous services and support more than 1000 users with more than 1 Gb/s access rate in a system distance of 60–100 km [4]. Multiple candidates have been proposed to satisfy the requirements, such as passive optical networks (PON) based on wavelength division multiplexing (WDM) [5], [6], time-wavelength division multiplexing (TWDM) [7], and orthogonal frequency division multiplexing (OFDM) [8], [9]. However, the access capacity, transmission distance and subscriber number are still limited by conventional techniques. On the other hand, the space division multiplexing (SDM) technique based on few mode fibers (FMFs) or multi-core fibers (MCFs) has been proposed as a favorable solution to tackle the fiber capacity crunch in both long-haul transmission [10] and short-reach access network [11]–[13]. Although the FMF based access network example has been reported very lately [11], [12], the differential modal dispersion and modal interference may hinder its deployment in the access network region while the MCF is considered to be a better choice owing to its wellcontrolled inter-core crosstalk and almost identical transmission quality compared with standard single mode fibers (SSMFs). Zhu et al. demonstrated a 7-core fiber based optical access network using traditional TDM-PON technologies [13]. However, the access data rate and the fiber link distance are quite limited (2.5 Gb/s and 11.3 km). In order to keep pace with the requirements imposed by NG-PON2 and beyond, the MCF-based SDM scheme needs to incorporate with the standardized WDM technology and affordable advanced modulation formats to fully unveil its potential in the optical access network. Moreover, as a universal platform for wired/wireless data services, the optical access network plays an even more important role in the 4G/5G mobile data transmission [14] and it is also interesting to envision the application of MCFs in the fiber/wireless converged networks. Very recently, we have reported a proof-of-concept experimental demonstration of optical access network architecture based on hybrid wavelength and space division multiplexing (WSDM) technique using MCF and intensity modulated and directly detected (IM/DD) OFDM modulation formats [15]. In [15], 300 Gb/s aggregation DS rate and 20 Gb/s coherently detected QPSK based mobile backhaul (MB) transmission have been achieved in a 58 Km 7-core fiber link, together with 5 Gb/s OOK upstream (US) transmission. Unfortunately, the data rate for US and MB transmission are still limited since less spectral efficient modulation format is used. Also, the accumulated fiber dispersion in high-speed longreach optical access network is another tough problem if OOK is used for US. In addition, in order to alleviate the Rayleigh backscattering effect in bidirectional transmission, wavelengths used in US and DS directions are offset, which sacrifices the valuable wavelength resources especially in ultra-dense WDM PON system. More importantly, in [15], a tunable laser and an external modulator are needed for each ONU to support US transmission, which may increase the cost and consequently prevent the commercial deployment. Therefore, to further enhance the capacity of the WSDM based optical access network, not only the modulation formats should be scaled to higher level for both DS and US, but also polarization division multiplexing (PDM) technique should be adopted in MB transmission. Furthermore, it is essential to realize colorless US transmission in the optical access network using cost-effective methods while eliminating the dispersion induced distortion and Rayleigh backscattering noise. In this paper, we further proposed a modified hybrid wavelength-space division multiplexing (WSDM) optical access network with centralized optical carrier delivery utilizing multicore fibers and adaptive modulation based on RSOA for bi-directional transmission. In our proposed architecture, the inner core of the MCF is employed to deliver optical carriers used for US transmission from the OLT to ONUs and simultaneously transmit the IQ modulated MB service in the opposite direction. One of the outer cores is utilized to transmit the US OFDM signal modulated


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Fig. 1. Modified WDM/SDM optical access network architecture using MCF and adaptive modulation based on RSOA. (PS: power splitter, mod: modulator, MUX/DEMUX: WDM multiplexer/de-multiplexer, Tx: transmitter, Rx: receiver, EDFA: erbium doped fiber amplifier, ECL: external cavity laser, DSP: digital signal processing, RSOA: reflective semiconductor optical amplifier, BPD: balanced photo detector, PBS: polarization beam splitter, LO: local oscillator).

by a colorless reflective semiconductor optical amplifier (RSOA) from the ONUs to the OLT side. Other 5 outer cores act as parallel channels to realize IM/DD QPSK-OFDM based DS transmission. The traffic for DS and US are transmitted via independent physical channels, so the Rayleigh backscattering noise can be eliminated even though the same wavelengths are reused for both DS and US. For the bidirectional transmission in the inner core, wavelengths are offset to eliminate the Rayleigh backscattering induced interference. To demonstrate the scalability of the proposed access network, 16QAM-OFDM and adaptive modulation are employed in DS and US transmission, respectively, indicating potential capacity expansion capability. Moreover, polarization division multiplexing (PDM) technique is used in MB transmission to double the data rate without increasing much complexity or cost.

2. Proposed WSDM Access Network Architecture Our proposed MCF based WSDM optical access network architecture is shown in Fig. 1. With the spatially isolated fiber channels in the same fiber, one of the cores within MCF (the inner core of a typical 7-core MCF for example) can be allocated to the high-speed wireless data transmission such as the mobile backhaul transmission, considering the booming mobile Internet demand. An outer core of MCF is utilized to transmit US signal while the other 5 cores are employed as the parallel channels for DS transmission, thus the Rayleigh backscattering noise can be eliminated even though the same wavelengths are reused for both DS and US. In the OLT block, m wavelengths are utilized as the laser source. For each wavelength in one subset OLT, it is power split by N 1 in which N representing the number of cores of MCF. 1=ðN 1Þ of the signal power is assigned as the optical carrier for US signal modulation which is delivered to the ONU side via the inner core. In this way, this configuration can support ðN 2Þ m subscribers by only employing m wavelengths. In contrast to WDM-PON that support the same user scale, our proposed WSDM optical access network consumes lower expense per subscriber. To further enhance the capacity with affordable cost and complexity, downstream signal on each wavelength is suggested to be intensity-modulated with directly detected optical (DDO) OFDM modulation format, which is spectral efficient, bandwidth flexible, and robust against fiber dispersion. After modulation, the ðN 2Þ m branches from 1 to m are multiplexed respectively by N 2 m-wavelength Mux devices like array waveguide gratings (AWGs). After amplified by erbium-doped fiber amplifiers (EDFAs), the N 2 sets of DS WDM signals are injected into the N 2 outer cores of MCF with the aid of the fan-in device. Subsequently, the DS signals are transmitted in the MCF, and output to N 2 independent single mode fibers by the fan-out device. Signals from each core containing wavelength from 1 to m are demultiplexed respectively and each wavelength is dedicated to one ONU for DS transmission. For US


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Fig. 2. Experimental setup schematic diagram. (PS: power splitter, PC: polarization controller, WSS: wavelength selective switch, AWG: arbitrary waveform generator, IM: intensity modulator, VOA: variable optical attenuator, EDFA: erbium doped fiber amplifier, ECL: external cavity laser, TOF: tunable optical filter, DSO: digital storage oscilloscope, PD: photo detector, PPG: pulse pattern generator, PBS: polarization beam splitter).

transmission, the optical carriers distributed from the OLT side using the inner core can be amplified and modulated by a RSOA at the ONU side and then transmitted to the OLT side via the reserved outer core. The optical carriers allocated to the ONUs and the modulated US signals are transmitted in separate physical channels in order to avoid performance degradation induced by the Rayleigh backscattering which severely deteriorates the US signal especially in the long-reach case. All the ONUs served by one subset OLT will share the same wavelength for US transmission, in a TDMA or OFDMA manner, as for typical asymmetric access network scenarios. As OFDM signal with adequate cyclic prefix (CP) is robust against fiber dispersion, it is unnecessary to compensate for the accumulated fiber dispersion even after long distance transmission in high data rate systems. As for the MB transmission, the incoming broadband mobile backhaul traffic are IQ modulated with polarization division multiplexing (PDM) QPSK format. After transmission in the inner core, the MB traffic data are wavelength demultiplexed and amplified before detected by a polarization and phase diversity coherent receiver in the OLT side. The wavelengths used for MB transmission need to be distinguished from that of the optical carriers used for US transmission so that Rayleigh backscattering noise is negligible. This may decrease the efficiency of wavelength resource but it is acceptable because the number of wavelengths for MB transmission is very limited compared with that of DS and US, due to the fact that the data rate per wavelength can be very high when coherent detection and advanced modulation or multiplexing methods are used in MB transmission. Therefore our proposed WSDM optical access network has the potential to deliver multi-giga-bit services to a substantial number of subscribers using spectral and spatial domain combined with multicore fiber and advanced modulation formats.

3. Experimental Results and Discussion To verify the feasibility of our proposed WSDM access network architecture, we conducted a proof-of-concept experiment using the setup depicted in Fig. 2. The low-crosstalk MCF (with average loss of about 0.25 dB/Km for the inner core and 0.3 dB/Km for the outer cores) we developed and fabricated has seven cores in a hexagonal array. The low-loss fan-in/fan-out devices shown as inset in Fig. 2 are in-house developed using chemical etching process and fiber bundles manufacturing technique with an average loss of 3 dB per core for a pair of fan-in/ fan-out devices. The detailed descriptions about the MCF (geometrical and optical parameters) and fan-in/fan-out devices can be found in [15]. For DS transmission, ten wavelengths with 25 GHz channel spacing from an optical frequency comb generator (OFCG) seeded by an ECL centered at 1550.12 nm, are selected by a WSS (Finisar WaveShaper 4000S). Then the ten continuous waves (CWs) are intensity modulated


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Fig. 3. BER curve for DS transmission with (a) OFCG based 5 Gb/s QPSK-OFDM and (b) single wavelength based 10 Gb/s 16 QAM-OFDM.

with 5 Gb/s baseband QPSK-OFDM signal. The transmitted signal is generated by MATLAB program originated from 215j1 pseudorandom binary sequence (PRBS), and then mapped into QPSK modulation formats. For real-valued OFDM signal generation, Hermitian symmetry has been ensured and the first subcarrier is abandoned for eliminating noises near DC component. We adopt 128 points for the IFFT/FFT process. Thus the effective number of subcarriers in our system is 63. The length of cyclic prefix (CP) and frame are 13 and 139, respectively. The training sequence includes 11 OFDM symbols, in which one is used for frame synchronization and others are used for channel estimation. The time-domain OFDM signal is D/A converted by an arbitrary waveform generator (AWG 7122C, Tektronix, operated at sampling rate of 5 GS/s) to drive the optical intensity modulator (MX-LN-20, Photline Technologies, biased at the quadrature point) after anti-aliasing filtering and RF amplification. Boosted by an EDFA, the optical OFDM signals are power split by a 1:8 power splitter and simultaneously injected into five outer cores of the MCF through the fan-in device, and the optical spectra of amplified optical OFDM signals is shown as inset in Fig. 2. After about 58 km MCF transmission, the signals are output to five single mode fibers through fan-out device. At the receiver side of every single mode fiber, after pre-amplification and de-multiplexing, one wavelength is selected and directly detected by a photodetector (PD) with 2.4 GHz bandwidth and then sampled by a 20 GS/s digital sampled oscilloscope (DSO, Tektronix CSA7404B). Demodulation and bit error ratio (BER) counting are implemented offline. During the procedure of offline DSP similar to [16], down-sampling of the received signal, frame and frequency synchronization are implemented before CP removing and FFT operation. Afterwards, de-mapping is followed by least-square (LS) based channel estimation and one tap equalization. In OB2B configuration, the fiber is replaced by a variable optical attenuator (VOA) with equal loss, so the comparison of the transmission performance is under equal condition. The BER performance of QPSK based OFDM DS signal centered at 1550.12 nm from 5 outer cores at various received optical power after MCF transmission and in OB2B setup is shown in Fig. 3(a), with constellation diagrams of received power at j14 dBm, j17 dBm and j21 dBm from core 1 as the inset. The detected optical signals at other wavelengths in different cores present similar performance. The BER can be kept under 7% Forward Error Correction (FEC) limit at BER ¼ 3:8 103 with the received optical power as low as j16 dBm. No significant transmission performance deterioration is observed between OB2B and 58.7 km MCF transmission for all five DS spatial channels, because the adequate cyclic prefix (CP) is used to conquer the impact of chromatic dispersion (CD) and polarization mode dispersion (PMD) of MCF. Therefore an aggregated 250 Gb/s DS capacity has been realized with 10 wavelengths and 5 cores, through a combination of spectral and spatial dimensions. A BER floor has been observed when the received optical power excess j15 dBm, which is mainly due to the relatively poor optical signal-to-noise ratio (OSNR, about 20 dB) of the OFCG. In order to further increase the data rate, higher level modulation formats or more wavelengths are needed. However, more wavelengths from the OFCG means lower OSNR for each


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Fig. 4. RSOA based US transmission performance. (a) Estimated SNR for each subcarrier. (b) BER curve for US transmission using QPSK-OFDM.

comb line and higher modulation formats will require higher OSNR in return. To demonstrate the scalability of DS capacity, we use a single wavelength laser with higher OSNR to replace the OFCG and employ 16 QAM-OFDM modulation format in the same setup using the same configuration parameters. As shown in Fig. 3(b), with higher order modulation format, the DS data rate doubles to reach 10 Gb/s per wavelength while the receiver sensitivity of 16 QAM-OFDM based DS transmission is about j11 dBm, showing about 5 dB power penalty compared with QPSKOFDM owing to the closer distance for 16 QAM constellations. In addition, according to our previously research on OFCG [17], more than 20 comb lines with good flatness can be generated. In fact, several approaches have been proposed to improve the OSNR of the comb lines from OFCG, such as techniques based on two phase modulators cascaded with an electro-absorption modulator [18], optical injection locking of semiconductor-based harmonically mode-locked lasers [19], RF driving voltage optimization in recirculating frequency shifter (RFS) [20], and phase-managed continuous wave seeded parametric frequency comb with nonlinear pulse shaping [21]–[23]. Therefore, with the improvement of the OSNR of each comb line, 20 wavelengths from a high quality OFCG and 16 QAM-OFDM modulation format can be combined thus a Terabit (20105) DS transmission optical access network capacity can be expected based on MCFs. For the US transmission from the ONU, a single wavelength centered at 1550.12 nm is transmitted from the OLT to the ONU via the inner core. After getting through the circulators, the CW optical carrier is launched into a RSOA (CIP, SOA-RL-OEC-1550-03861) in the ONU side. After modulated and amplified by RSOA, the US signal is then transmitted to the OLT, where the US signal is amplified and filtered before detection, through the sixth outer core of the MCF. As the DS and US traffic are transmitted in physically isolated channels, the transmission performances in DS and US directions will not be influenced by the Rayleigh backscattering noise, even though wavelengths are reused in US transmission. Based on this US transmission scheme, 2.5 Gb/s uniform QPSK modulation on every subcarrier is firstly applied to obtain the channel state information. As is often the case, signal-to-noise ratio (SNR) can be chosen as the figure of merit to evaluate the channel condition. In order to acquire the SNR for each subcarrier, error vector magnitude (EVM) based SNR estimation method [16], [24] is introduced in our experiment and the estimated SNR for individual subcarrier is shown in Fig. 4(a). As can be seen, after a round-trip transmission of about 58 Km MCF, the 3 dB bandwidth of the RSOA is much less than 1 GHz when the driven current is 60 mA and the injection optical power to RSOA is j10 dBm. Using QPSK-OFDM modulation format, the US data rate can reach as high as 2.5 Gb/s with BER under FEC limit at received optical power of j12 dBm, leading to over 16 dB power budget and the transmission performance is summarized in Fig. 4(b). Subsequently, in order to enhance the US data rate restrained by the low bandwidth RSOA and imperfect channel responses distorted by the chromatic dispersion induced power fading and bandwidth-limited optoelectronic components, OFDM with adaptive modulation technique is used. During the adaptive modulation, the bit number and power allocation on each subcarrier is rearranged based on Chow's rate-adaptive bit-loading algorithm [25] using the estimated


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Fig. 5. Results for adaptive modulation based US transmission. (a) Bit and power allocation for every subcarrier with adaptive modulation. (b) Received constellation diagrams for US transmission using adaptive modulation at received optical power of j10 dBm.

Fig. 6. BER performance for MB transmission (inset figures: constellations on two polarization directions at received optical power of j10 dBm and eye diagrams at received optical power of j5 dBm).

sub-channel SNR information. The results of number of bits per subcarrier and power allocation using the estimated SNR are shown in Fig. 5(a). With adaptive modulation, 3.12 Gb/s OFDM signal is successfully transmitted in the 58.7 Km MCF fiber link. Constellations of different modulation formats after MCF transmission are shown in Fig. 5(b), leading to 1.25 times increase in US data rate with BER under 3.8e3 . As the OFDM modulation format is inherently robust to the fiber dispersion induced inter-symbol interference (ISI), the US transmission performance will not be deteriorated even without dispersion compensation in long-reach access criteria, which is superior to OOK based transmission. As proposed in the architecture shown in Fig. 1, the inner core of the MCF is reserved for large capacity wireless data service and we thus performed a US transmission of 48 Gb/s PDMQPSK IQ modulated signal from ONU to OLT as an emulation of the high speed MB application. A CW laser from ECL centered at 1556.55 nm is modulated by a PDM-IQ modulator with 12 Gbaud binary signal generated by BER tester (BERT). After amplification and filtering, the signal is coherently detected (Tektronix OM4006D) at the OLT. After coherent detection, the output electrical signal is digitalized by a real-time oscilloscope (DSA 72504D) and then offline digital signal processing is implemented using the traditional DSP flow [26]. The MB transmission result with enough power budget is shown in Fig. 6, where the constellation diagram for two polarization directions at received optical power of j10 dBm and the eye diagrams for the in-phase component in x-direction and quadrature component in y-direction at received optical power of j5 dBm are inserted as insets. We also confirm that no significant deterioration occurs


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when the optical carriers used for US are co-transmitted with the MB signal, because the wavelengths used in the two directions differ greatly thus Rayleigh backscattering noise can be neglected. The coherent receiver is placed in the OLT side thus its cost and power consumption can be shared by many ONUs or mobile base stations. With the wide deployment of the commercial 100 Gb/s coherent systems, 100 Gb/s PDM-QPSK based transponders can be directly used in our proposed MB service compatible optical access network, which will further decrease the shared MB cost.

4. Conclusion We proposed and experimentally demonstrated a duplex WSDM optical access network capable with centralized optical carrier delivery based on DDO-OFDM utilizing our in-house developed low-crosstalk 7-core MCFs and fan-in/fan-out devices. The proof-of-concept experiment proves the capability of the MCF based access network in terms of long reach transmission (58.7 km), large capacity(potential terabit aggregation DS data rate) and massive count of users (50 ONUs), compatible with 48 Gb/s coherent PDM-QPSK MB transmission and 3.13 Gb/s RSOA based adaptively modulated US signal. It is obvious that by using the affordable DDOOFDM signal transmission together with multiple spatial channels in one fiber, optical access data rate could be significantly enhanced without adding much complexity in the ONUs. At the same time, in the colorless ONU, the low cost RSOA based adaptive modulation can not only offer flexibility to subscriber bandwidth allocation but also enhance the total data rate. The centralized carrier sources delivered from OLT to ONU with a dedicated core of MCF further reduce the overall system cost and maintenance complexity. Therefore, the additional spatial dimension with the help of OFDM based advanced modulation formats and efficient digital signal processing we introduced in this architecture is prominent for future evolution of flexible and manageable optical access networks. As a universal platform for wired/wireless data access, our proposed architecture provides additional dimension for high speed mobile signal transmission, which emerges as an ideal platform for future data-driven multi-service featured communication systems such as content delivery metropolitan area network (MAN) and traffic-rich data center networks (DCN).

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