Polarization-Insensitive Remote Access
Unit for Radio-Over-Fiber Mobile Fronthaul
System by Reusing Polarization
Orthogonal Light Waves
Volume 8, Number 1, February 2016
Beilei Wu
Jiun-Yu Sung
Jhih-Heng Yan
Mu Xu
Jing Wang
Fengping Yan
Shuisheng Jian
Gee-Kung Chang, Fellow, IEEE
DOI: 10.1109/JPHOT.2015.2508426
1943-0655
Ó 2015 IEEE
Beilei Wu, Jiun-Yu Sung, Jhih-Heng Yan, Mu Xu, Jing Wang, Fengping Yan,1Shuisheng Jian,1 and Gee-Kung Chang,2Fellow, IEEE
1Institute of Lightwave Technology, Key Lab of All Optical Network and Advanced
Telecommunication of EMC, Beijing Jiaotong University, Beijing 100044, China
2School of Electrical and Computer Engineering, Georgia Institute
of Technology, Atlanta, GA 30332 USA
3Department of Photonics and Institute of Electro-Optical Engineering,
National Chiao Tung University, Hsinchu 30010, Taiwan
4Institute of Photonics Technologies, National Tsing Hua University, Hsinchu 30013, Taiwan
DOI: 10.1109/JPHOT.2015.2508426
1943-0655Ó 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 November 17, 2015; revised December 6, 2015; accepted December 8, 2015. Date of publication December 17, 2015; date of current version January 29, 2016. This work was supported in part by the Georgia Institute of Technology and in part by the National Natural Science Foundation of China under Grant 61275091 and Grant 61327006. The work of B. Wu was supported by the China Scholarship Council. Corresponding author: F. Yan (e-mail: fpyan@bjtu.edu.cn).
Abstract:To reduce the complexity and cost of remote access units (RAUs), colorless transport architecture is preferred in a wavelength-division multiplexing (WDM) system. The wavelength-reuse technique is commonly adopted to realize colorless RAUs. How-ever, in traditional approaches, RAUs employ crystal-based Mach–Zehnder modulators (MZMs) to modulate the distributed carriers. Hence, polarization tracking systems are necessary to align the polarization state of the distributed carriers with the MZM principal axis. This increases the cost of the RAUs. In this paper, a full-duplex millimeter-wave (MMW) radio-over-fiber (RoF) access architecture employing polarization-insensitive RAUs is proposed. In our proposed RAUs, the upstream signals are generated using polarization-orthogonal lights, and the performance of the upstream signals is independent of the polarization state of the MZM input lights. The proposed system is mathematically formulated and experimentally demonstrated. The 1.2-Gb/s quadrature phase-shift key-ing (QPSK) orthogonal frequency-division multiplexkey-ing (OFDM) downstream signal at the 60-GHz band and 5-Gb/s on–off keying (OOK) polarization-insensitive upstream signal over a 15-km standard single-mode fiber (SSMF) are demonstrated. In our results, a polarization-insensitive RAU is successfully obtained. The power penalty resulting from the interference of downstream signal is about 0.4 and 0.6 dB with a bit error rate (BER) of 109in back-to-back and 15-km fiber transmission, respectively.
Index Terms:Radio-over-fiber (RoF), remote access units, polarization-insensitive.
1. Introduction
In view of massive proliferation of smart mobile devices, system capacity requirement for wire-less applications is rapidly increasing. Optical fiber for mobile fronthaul is widely recognized as the future-proof solution in order to maintain the coverage of a central station (CS), with links
between the CS and the remote access units (RAUs). To solve the current spectrum crunch for the frequency bands, wireless transmission using millimeter wave (MMW) is extensively investi-gated [1]–[3]. Besides, radio-over-fiber (RoF) architecture is also widely investiinvesti-gated to further reduce the complexity and cost of the RAUs [4]–[7].
Data distributed from a CS to the RAUs are commonly provided using wavelength-division multiplexing (WDM) technique. The transmission wavelengths used by a specific RAU should be accurately controlled. In order to reduce the maintenance cost of RAUs, colorless RAUs are preferable. A colorless RAU can be easily achieved using wavelength-reuse techniques [8]–[13]. While wavelength-reuse techniques are adopted, the optical carriers used for upstream transmis-sion are distributed from the CS and modulated by other devices installed in the RAUs. Mach–Zehnder modulators (MZMs) using LiNbO3 are commonly used in the RAUs to modulate
the distributed carriers. However, some polarization manipulation approaches should be used to align the distributed carriers with the principal axis (PA) of the LiNbO3 crystal in the MZM. The
complexity and cost of RAUs will thus increase. Electro-absorption modulator (EAM) was pro-posed for polarization-insensitive modulation [14]. However, due to its possible chirp issues, it is still questionable if EAM will be an ultimate choice [15].
In this paper, a bidirectional MMW-RoF mobile fronthaul architecture with colorless and polarization-insensitive RAUs is proposed. In the CS, an optical signal composed of two polarization-orthogonal un-modulated optical tones and one modulated signal is generated. In the RAU, the MMW is generated by coherent beating between the modulated signal and the un-modulated carrier. The unun-modulated polarization-orthogonal optical tones are further used to carry the upstream data through a crystal based MZM. Since the overall power aligned on the PA of the LiNbO3crystal in the MZM is an invariant, the modulation depth of the output signal is
independent to the relative polarization states of the input lights. The polarization-independent feature of the proposed scheme is analyzed mathematically and proved experimentally. 1.2 Gb/s quadrature-phase-shift-keying (QPSK) orthogonal frequency-division multiplexing (OFDM) down-stream signals (DS) at 60 GHz band and 5 Gb/s on-off keying (OOK) polarization-insensitive upstream signal (US) over 15 km standard single-mode fiber (SSMF) are demonstrated. Perfor-mance comparisons between the proposed scheme and the conventional scheme (using single-wavelength light (SL) for the upstream modulation) are also provided.
2. Operation Principles
Fig. 1(a) is the architecture of the conventional wavelength-reuse scheme (using SL for modula-tion). In the RAU, the polarization state of the input carrier is tracked and aligned with the PA of the MZM crystal (shown as the dashed line in Fig. 1). While the polarization state of the input light and the PA of the MZM crystal are not well aligned, the modulation depth will decrease and degrade the signal performance. Recently, automatic polarization trackers have been proposed and demonstrated in practical systems [16]–[19]. The adaptive loop-based polarization tracking
Fig. 1. Schematic diagrams of (a) the conventional modulation scheme and (b) the proposed polarization-insensitive modulation scheme. SL: single-wavelength light; PLs: polarization-orthogonal lights; PA: principal axis.
0 E0 ej!2tþ’2
where!1 and!2 are, respectively, the center frequencies of E1 and E2, and’1 and’2 are their
initial phase constants. To avoid confusion, the nonlinear behavior of the MZM is not considered here. The MZM is modeled as an ideal modulator which transforms the electrical signal into the optical field without any distortion. If we assume the PA of the MZM crystal is aligned to the x -axis with an arbitrarily angle, the Jones matrix of the MZM can be expressed as
Q¼ cosðÞ sinðÞ sinðÞ cosðÞ mðtÞ 0 0 1 cosðÞ sinðÞ sinðÞ cosðÞ ¼1 2 ðm 1Þcosð2Þ þ m þ 1 ðm 1Þsinð2Þ ðm 1Þsinð2Þ ðm þ 1Þ ðm 1Þcosð2Þ (2)
where m¼ mðtÞ is the modulation signal. After the modulation process, the received photocurrent after the photo-diode (PD) is
iPD / jQE1j2þ jQE2j2 ¼E02 4 ½ðm 1Þcosð2Þ þ m þ 1 2þ2 ðm 1Þsinð2Þ½ 2þ ðm þ 1Þ ðm 1Þcosð2Þ½ 2 n o ¼ E2 0ðm2þ 1Þ: (3)
The cross-beating terms between E1 and E2do not appear since E1and E2 are orthogonally
po-larized. From (3), we can see that the generated photocurrent is independent to the value of . Hence, it is not necessary to align the polarization state of the optical signal with the PA of the MZM crystal in the RAU. The RAU is polarization insensitive.
3. Experiment Setup and Results
Fig. 2 illustrates the experimental setup of the proposed architecture. A single-wavelength opti-cal carrier was output from a laser diode (LD). The center wavelength of the optiopti-cal carrier was 1553.69 nm. The optical carrier was split into two branches by a 50/50 optical coupler. In the up-per branch, the optical carrier was modulated by a 20 GHz local oscillator (LO1) through MZM1. MZM1 was biased at its null point to suppress the zero-order carrier. Thus, two optical tones with 40 GHz frequency spaces generated after MZM1. Inset (a) of Fig. 2 was the conceptual spectrum of the optical two tones. The optical two tones were then input into MZM2 which was biased at its quadrature point. MZM2 was driven by an intermediate frequency (IF) signal. The IF signal was generated by mixing the 1.2 Gb/s QPSK-OFDM signal with a 20 GHz sine-wave source (generated by LO2). The QPSK-OFDM signal was generated by an arbitrary waveform generator (AWG) with 12 GSa/s sampling rate. The conceptual spectrum of the output signal from MZM2 was shown in inset (c) of Fig. 2. After MZM2, a 33/66 optical interleaver (IL) was used to filter the trivial wavelengths. The optical single-sideband (OSSB) RoF signal was gener-ated after the IL as depicted in inset (d) of Fig. 2. The experimentally measured optical spectra before and after the IL were respectively shown in Fig. 3(a) and (b). The RoF signal was com-bined with the un-modulated optical carrier in the lower branch through a polarization beam combiner (PBC). The PLs were obtained after the PBC as shown in inset (e) of Fig. 2. A
polarization controller (PC) and a variable optical attenuator (VOA) were used in the lower branch to make sure the power of the two orthogonal tones (carrier and +1st sideband) was equal. After the PBC, the resultant optical signal was transmitted over a 15 km SSMF to the RAU.
At the RAU, the optical signal was divided into two signals by an optical coupler. The first sig-nal was received by a high speed PD (PD1). An erbium-doped fiber amplifier (EDFA) was used in front of PD1 to compensate the power loss in the CS and the fiber. The desired MMW signal
Fig. 3. Optical spectra measured at different positions of Fig. 2. (a) Output of MZM2 (blue line) and the 33/66 IL transmission response (purple line). (b) Output of the 33/66 IL. (c) Input of the OBF (blue line) and the OBF transmission response (purple line). (d) Output of the OBF.
Fig. 2. Experimental setup of the proposed architecture. (Insets) (a)–(f) Conceptual optical spectra at location a–f. LD: laser diode; LO: local oscillator; CS: central station; RAU: remote access unit; PC: polarization controller; PD: photo-diode; IL: interleaver; PBC: polarization beam combiner; OBF: optical band-pass filter; ED: envelope detector; VOA: variable optical attenuator; DC: direct current.
was obtained after PD1. A pair of 60-GHz horn antennas with 2-ft separation was used for wiless communication. After the receiving antenna, an envelope detector (ED) was used to re-trieve the envelope of the signal. The envelope of the signal was recorded by a real-time oscilloscope with 40 GSa/s sampling rate and the QPSK-OFDM signal was decoded offline. At the other output port of the optical coupler, the polarization-orthogonal unmodulated tones were selected by the optical band-pass filter (OBF) as illustrated in inset (f) of Fig. 2. The experimen-tally measured optical spectra before and after the OBF were respectively shown in Fig. 3(c) and (d). Another EDFA (EDFA2) was used to increase the input power to MZM3 to maintain the optical signal-to-noise ratio (OSNR) of the signal. Then, as discussed in Section 2, the resultant PLs were input into MZM3. The PLs were modulated by the 5 Gb/s OOK signal through MZM3. Then another 15 km SSMF was used to transmit the US. In the CS, the US was received by PD2 and analyzed by a bit error rate (BER) tester. Here the downstream data rate is lower than that of the upstream signal due to the bandwidth limitation of our wireless components. In order to show the high speed capability of our system, we are not decreasing our upstream data rate to make it lower than the downstream signal, which is the conventional case. Since we can achieve higher data rate without the wireless transmission, it is believed that our downstream data rate may increase while devices with wider bandwidth are used.
To characterize the signal quality, the error vector magnitude (EVM) of DS is calculated ac-cording to our decoded signal. Then the signal BER is estimated from the EVM acac-cording to the equation provided by [20]. Fig. 4(a) shows the BER performance of the DS versus the RAU re-ceiving optical power. The insets are respectively the resultant constellations for back-to-back (BTB) and 15 km fiber transmission cases while the RAU receiving power is−12 dBm. We can observe that signal performance with BER about 109 can be roughly obtained; hence forward error correction (FEC) is not necessary. This decreases the RAU system complexity. Fig. 4(b) shows the BER performance of the upstream OOK signal while the DS is, respectively, transmit-ted or closed at both BTB and 15 km fiber transmission. BER lower than 109 can be obtained for the US. A power penalty of 0.4 dB and 0.6 dB at BER¼ 109 can be observed while the DS is transmitted simultaneously in BTB and 15 km fiber transmission, respectively. This may come from the additional interference, as shown in Fig. 3(d) (the DS is not completely filtered). About 0.8 dB and 1 dB power penalty at BER¼ 109 are shown due to fiber transmission for without DS and with DS case, respectively. Insets of Fig. 4(b) are the clear eye diagrams for the BTB case and 15 km fiber transmission.
The polarization sensitivity of the conventional (using SL) and the proposed (using PLs) schemes for the upstream transmission is studied. In the experiment setup for the conventional scheme, the lower branch of the CS in Fig. 2 was removed. Hence the optical spectrum of the signal transmitted to the RAU is similar with the spectrum in Fig. 3(b). In the RAU, after the OBF, only SL (+1st sideband) was reused for the upstream transmission. For both the
Fig. 4. Signal performance. (a) BER versus received optical power for the DS. (b) BER versus re-ceived optical power for the US.
conventional and our proposed upstream schemes, a polarization scrambler was used before MZM3. Fig. 5 shows the resultant BER time trace for different schemes at BTB transmission. For the conventional scheme, the BER value fluctuates and quickly accumulates to the order of 103. Since the scenario that the polarization state of the input light is orthogonal with the PA of MZM crystal occurs frequently, the overall BER will never recover back to the order of 109. Compared to the conventional scheme, our proposed scheme has better BER performance with a more flat time trace.
Another experiment was further performed to analyze how different polarization states will contribute to the overall signal performance. The experiment setup of the CS is identical to Fig. 2. A modified setup was set in the RAU as shown in Fig. 6. A PC and a 90/10 optical coupler were further used before MZM3 in the RAU. For our proposed scheme, the PLs were selected by the OBF [see Fig. 2(f)] and were divided into two streams by a 90/10 coupler after a PC. The PC was used to change the polarization state of the light waves. The 10% PLs were input into a polarizer (Pol) and monitored by an optical spectrum analyzer (OSA). The relative polarization states of the PLs are estimated according to the optical spectrum after the Pol. Three arbitrary polarization states (P1, P2 and P3) were chosen whose optical spectra were shown as insets of Fig. 6. For the conventional scheme, the PC was also used before MZM3 to change the polarization state of the input light wave. Three polarization states were chosen according to the measured eye dia-grams (see the insets of Fig. 7): (1) parallel to the PA of MZM3 crystal (P4), (2) deviated from the PA of MZM3 crystal (P5), and (3) completely orthogonal to the PA of MZM3 crystal (P6). The re-sultant BER performance for different schemes is shown in Fig. 7. For the conventional scheme, only P4 can reach BER 109 level. While the FEC threshold of BER 3:8 103 is considered,
Fig. 5. Comparison of the accumulated BER time trace between the conventional modulation scheme and the proposed polarization-insensitive scheme.
Fig. 6. Experimental setup for comparing the performance difference among different schemes. (In-set) Optical spectra to determine the relative polarization states of the PLs for the proposed wave-length reusing scheme. OBF: optical band-pass filter; PC: polarization controller; Pol: polarizer; OSA: optical spectrum analyzer. BERT: bit error rate tester.
about 5 dB power penalty was observed for P5 compared to the P4 case. For P6 case, the BER cannot be measured by the BER tester since the upstream data is not modulated onto the optical carrier [21]. Compared to the conventional scheme, P1-P3 cases show nearly identical performance. BERG109can all be obtained for P1-P3 cases. This again shows the polarization-independent fea-ture of the proposed scheme.
4. Conclusion
In summary, a bidirectional radio-over-fiber mobile fronthaul architecture with polarization-insensitive RAUs is proposed and studied. Compared with conventional wavelength-reusing scheme, the performance of our proposed scheme is less sensitive to the polarization state of the modulated carriers in the RAU. In our scheme, the polarization tracking devices and align-ment processes are not necessary. This highly reduces the complexity and cost of the RAU. Hence it is an attractive candidate for the next-generation integrated fiber wireless access net-works. A proof-of-concept experiment is performed to show the feasibility of the proposed scheme. In our results, 1.2 Gb/s QPSK-OFDM DS at 60 GHz band and 5 Gb/s OOK polarization-insensitive US over 15 km SSMF are demonstrated.
Acknowledgment
The authors would like to thank Dr. M. Zhu and Dr. J. Zhang for their valuable advices and fruit-ful discussions.
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