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10-W power light co-transmission with optically carried 5G NR signal over standard single-mode fiber
The radio-over-fiber (RoF) technique, which transmits optically carried radio-frequency (RF) signals between the central station (CS) and remote antenna unit (RAU), is a promising candidate to fulfill the urgent need of mobile traffic growth [ 1 ]. The cell size of the RAU in the 5G wireless communication systems is reduced to achieve a higher data rate, which inevitably results in a greatly growing number of RAUs [ 2 , 3 ]. Therefore, the electrical power supply for numerous RAUs becomes a headache for 5G network deployment. If copper wires are still used to supply power for each RAU, the implementation cost and complexity of the 5G fronthaul link are huge. Therefore, it is critically important to explore a simple and efficient scheme to solve the power supply of electronic devices in the RAU. Power-over-fiber (PWoF) is an attractive approach to provide a power supply for remote sensing and communication devices that are inconvenient in obtaining electrical supplies [ 4 – 9 ]. Through simultaneous transmission of the optically carried RF signal and power light over standard single-mode fiber (SSMF), the RAU with PWoF feeding is able to function independently, leading to cost-saving and convenient maintenance. Although the power delivery efficiency of PWoF is lower than that of traditional copper wires, PWoF is able to dynamically adjust the power delivery capability according to the requirement of each RAU, which can efficiently reduce the overall power consumption of the 5G network [ 10 – 12 ]. In addition, RAUs can avoid lightning strikes owing to the excellent insulation of the PWoF. Over the past years, optically powered RAUs based on double-clad fiber (DCF), multi-mode fiber (MMF), and multicore fiber (MCF) have been demonstrated [ 13 – 20 ]. For the scheme based on DCF, a 150-W power light at 808?nm is delivered by the inner cladding with a large mode area, and the optically carried 5.2-GHz RF signal at 1550?nm based on a 64-level quadrature amplitude modulation orthogonal frequency division multiplexing (64QAM-OFDM) format propagates at the single-mode core to achieve a data rate of 54?Mbit/s over 1-km DCF [ 15 ]. Nevertheless, the power transmission efficiency of 150?W is only 19.46%, because it is challenging to efficiently extract the optical power from the inner cladding. Moreover, DCF has limited compatibility with the current SSMF-based network infrastructure. The 2-km MMF can also be used to deliver 9.7-W power light at 1550?nm [ 18 ]. However, the data rate of 2.45-GHz 64QAM-OFDM signal at 1310?nm is limited to 54?Mbit/s, due to the large modal dispersion arising in MMF. Furthermore, there exists serious cross talk between the power light and signal light, leading to a performance penalty of RF signal transmission [ 8 ]. For the schemes based on MCF, the power light and signal light at the same wavelength of 1549.3?nm are propagated with different dedicated cores, whereas the total optical power arising in the seven-core fiber is generally smaller than 1?W, due to cross talk and nonlinearity [ 9 , 10 ]. In addition, the RF signal in the above-mentioned schemes is not consistent with the 5G new radio (NR) standard, where the data peak rate, channel bandwidth, subcarrier spacing, and spectral efficiency are better than that of 4G long term evolution (LTE). Recently, an optically powered RoF system based on SSMF has been demonstrated [ 21 ], which is valuable for the 5G wireless communication system, because the SSMF has been widely deployed in the current network. A 2-W power light at 1480?nm co-transmits with the 5G NR signal at 1550?nm over 10-km SSMF [ 21 ]. Since the optical-to-electrical conversion efficiency of the photovoltaic power converter is inversely proportional to the operation wavelength, power light at a short wavelength is favorable to obtain high conversion efficiency. The optical-to-electrical conversion efficiency at 1480?nm is only 26% [ 21 ]. Moreover, the wavelength spacing between the power light and signal light is so small that the energy of the power light will be transferred to that of the signal light through stimulated Raman scattering (SRS), leading to a relatively small optical power transmission efficiency (OPTE). Consequently, the delivered optical power over SSMF is below 2?W. In the current submission, a cost-effective approach to realizing the co-transmission of high-power power light and optically carried 5G NR signal is experimentally verified. Specifically, the linewidth of the power light is intentionally broadened to significantly suppress the stimulated Brillouin scattering (SBS) effect. In addition, the wavelength spacing between the power light and signal light is intentionally designed to be more than 40?THz to avoid nonlinear power transfer and improve the optical power conversion efficiency at the remote antenna unit (RAU). The operation wavelength of the power light is set in the normal dispersion region of the SSMF to avoid generation of a pulse train due to the modulation instability effect [ 22 ]. When 10-W power light is transmitted over SSMF with a core diameter of 8.2??m, the calculated value of optical power density is about ${19.9}\;{\rm{MW/c}}{{\rm{m}}^2}$ , which is smaller than the damage threshold of SSMF [ 23 ]. As a result, the 10-W power light and optically-carried 5G NR signal are co-transmitted over an SSMF length of 1?km, where the collected power of the power light is 7.18?W, and the EVM deterioration of the 64QAM-OFDM signal is less than 0.02%, in comparison with that of back-to-back (B2B) transmission. ? Fig. 1. Experimental setup for the co-transmission of power light and optically carried 5G NR signal over the SSMF. Download Full Size PPT Slide PDF Figure? 1 shows the experimental setup of co-transmission over SSMF for both high-power power light and optically carried 5G NR signal. A distributed-feedback laser diode (DFB-LD) at 1550?nm is employed to generate a continuous-wave (CW) optical carrier with a power of 16?dBm. A 1.5-Gbit/s 5G NR 64QAM-OFDM signal centered at 3.5?GHz with a bandwidth of 100?MHz is generated by a vector signal generator (VSG, R&S SMBV100B). Then, the 5G NR signal is modulated onto the optical carrier by using a 20-Gbit/s ${\rm{LiNb}}{{\rm{O}}_3}$ -based electro-optic Mach–Zehnder modulator (MZM, EOSpace AX-0MSS-20-LV). A single-mode high-power laser diode (SM-HPLD) centered at 1064.8?nm and with an optimized linewidth of 0.134?nm is used to generate the high-power power light, which is favorable to mitigate the SBS effect and realize high-power transmission over the SSMF. The optically carried 5G NR signal and power light are combined via a 1064-nm/1550-nm single-mode wavelength division multiplexer (SM-WDM), which is specifically fabricated with a high-power damage threshold. Then, the combined light is introduced to an SSMF spool (Corning SMF-28e) with a length of 1?km and core diameter of 8.2??m. Please note that, although the core diameter of the SM-HPLD pigtail (5.8??m) is different from that of the SM-WDM (8.2??m), a successful fiber fusion splicing with an insertion loss less than 0.1?dB is achieved, as shown in the inset of Fig.? 1 . At the SSMF output, the optically carried 5G NR signal and high-power power light are separated by using another 1064-nm/1550-nm SM-WDM with an isolation of 38?dB for the 1550-nm port. The power of power light is characterized by using an optical power meter (OPM, Laser Point A-40-D25-HPB), and the optically carried 5G NR signal is converted to an electrical signal by using a photodetector. Then, the error-vector magnitude (EVM) and constellation are evaluated by a vector spectrum analyzer (VSA, R&S FSW-K70) with a digital demodulation module. In addition, a variable optical attenuator (OATT) is used to adjust the signal power for the ease of characterizing receiver sensitivity. Figure? 2 presents the collected power of the power light after co-propagation with the 5G NR optical signal over 1-km SSMF and the corresponding OPTE, which is defined as the ratio of collected optical power to the injection power of power light. The maximum collected optical power is 7.18?W, when 10-W power light is introduced to 1-km SSMF, and the corresponding OPTE is 71.8%. This value is higher than that based on DCF and MCF [ 13 , 20 ], which is mainly attributed to the low insertion loss of the SM-WDM ( ${\sim}{0.3}\;{\rm{dB}}$ ) and the relatively low transmission loss ( ${\sim}{0.7}\;{\rm{dB}}$ ) of SSMF at 1064.8?nm. If an InGaAsP-based multi-junction photovoltaic power converter with an optical-to-electrical conversion efficiency of 35% [ 24 ] is adopted, an electrical power of 2.51?W can be obtained, which is sufficient for the power supply of the current RAU [ 25 ]. In addition, the collected optical power linearly increases with the injection power, indicating both SBS and SRS have been effectively suppressed. Please note that the OPTE slightly decreases as the launched power of the power light increases, due to the temperature variation of the SM-WDM. Figure? 3 shows the spectra of the input power light and backscattered light under the injection power of 10?W. It can be seen that there is no significant redshift in the backscattered light, indicating the SBS of the power light has indeed been suppressed through linewidth broadening. The slight spectrum broadening of the backscattered light is attributed to Rayleigh-wing scattering [ 26 ]. Next, we evaluate the stability of the high-power power light transmission, where the injection power is 10?W. Figure? 4 shows the collected power of the power light over 6?h, with a measurement resolution of 10?min. The variation of the collected optical power is smaller than 0.2%, indicating that the proposed power-over-SSMF system is capable of providing stable optical power feeding to the RAU over a long term. ? Fig. 2. Collected power of power light and corresponding OPTE under different injection powers. Download Full Size PPT Slide PDF ? Fig. 3. Spectra of input power light and back-scattered power light under injection power of 10?W. Download Full Size PPT Slide PDF Figure? 5 exhibits the measured EVMs of the 5G NR 64QAM-OFDM signal under different received electrical powers (REP) with and without co-transmission of the 10-W power light over 1-km SSMF. As a comparison, the EVM of B2B transmission is also presented in Fig.? 5 . We can conclude that EVMs with 10-W power light co-transmission are almost identical to those without power light co-transmission, indicating that the fiber nonlinearity introduced by the power light, such as cross-phase modulation and four-wave mixing, has a negligible influence on the signal light. In addition, the EVM values after 1-km SSMF transmission are identical to the results under the scenario of B2B transmission. Thus, the proposed power-over-SSMF system is favorable for realizing a high-quality 5G NR signal transmission. When the injection power of the power light varies under the condition of fixed REP of ${-}{{25}}\;{\rm{dBm}}$ by adjusting the OATT before the photodetector (PD), Fig.? 6 presents the measured EVM penalty, which is defined as the EVM difference between co-propagation with power light and the case of B2B transmission. As shown in Fig.? 6 , the EVM penalty with power light co-transmission over 1-km SSMF is less than 0.02%, which is similar to the result in the scheme based on DCF [ 13 – 16 ], but much better than the result in the scheme based on MMF [ 17 , 18 ]. In addition, the data rate of the 5G NR signal in our scheme is much higher than that of DCF and MMF [ 13 – 18 ]. Two insets in Fig.? 6 show the constellations of the received 5G NR 64QAM-OFDM signal after B2B transmission and the co-transmission of a 10-W power light over 1-km SSMF, where the EVM values are measured to be 0.46% and 0.48%, respectively. Although the signal light and high-power power light co-propagate over the 1-km SSMF, the transmission performance of the 5G NR 64QAM-OFDM signal has not been degraded, which indicates that the nonlinearity induced by the power light has a negligible impact on the signal light. Finally, the long-term stability of the 5G NR 64QAM-OFDM signal transmission is examined under 10-W power light co-transmission over 1-km SSMF. Figure? 7 presents the measured EVM variation with a duration of 30?min and a measurement resolution of 5?min. ? Fig. 4. Collected power of power light versus time under injection power of 10?W. Download Full Size PPT Slide PDF ? Fig. 5. Measured EVMs under different received electrical powers with (w/) and without (w/o) 10-W power light co-transmission over 1-km SSMF. Download Full Size PPT Slide PDF As shown in Fig.? 7 , the temporal EVM variation is smaller than 0.01%, indicating high-quality transmission of 5G NR signal even under 10-W power light co-transmission. In conclusion, we have demonstrated the co-transmission of 10-W power light and optically carried 5G NR 64QAM-OFDM signal over 1-km SSMF. Through the linewidth manipulation of the SM-HPLD, the insertion loss management of the fiber link, and the wavelength spacing optimization between the power light and signal light, a record OPTE arising in the SSMF link has been obtained. In addition, the power fluctuation of the collected power light is smaller than 0.2% over 6?h monitoring, and the EVM variation of the 5G NR 64QAM-OFDM signal is less than 0.01%. To enhance the SSMF reach of power light delivery, the linewidth of the power light can be further optimized to suppress the SBS. For the co-transmission of a high-speed 5G millimeter wave (MMW) signal in the future, the operation wavelength of the optically carried signal can be shifted to the O-band, for the purpose of mitigating the chromatic dispersion arising in SSMF. Given that the SSMF has been widely deployed in the current 5G fronthaul link, the SSMF-based optically powered 5G NR signal transmission scheme reported in this work is promising to revolutionize the next-generation power supply for the RAU. ? Fig. 6. EVM penalty under various injection powers of power light (insets are constellations of the 5G NR 64QAM-OFDM signal after B2B transmission and the co-transmission with 10-W power light over 1-km SSMF). Download Full Size PPT Slide PDF ? Fig. 7. EVM variation under 10-W power light co-transmission over 1-km SSMF. Download Full Size PPT Slide PDF Funding . National Key Research and Development Program of China (2018YFB1801001); National Natural Science Foundation of China (62175038); Guangdong Introducing Innovative and Entrepreneurial Teams of “The Pearl River Talent Recruitment Program” (2019ZT08X340); Research and Development Plan in Key Areas of Guangdong Province (2018B010114002). Disclosures . The authors declare no conflicts of interest. Data Availability . Data underlying the results presented in this Letter are not publicly available at this time but may be obtained from the authors upon reasonable request. REFERENCES . 1. M. Sauer, A. Kobyakov, and J. George, J. Lightwave Technol. 25 , 3301 (2007). [ CrossRef ] ? 2. N. Takehiro, N. Satoshi, B. Anass, K. Yoshihisa, H. Tang, X. D. Shen, and N. Li, IEEE Commun. Mag. 51 (2), 98 (2013). [ CrossRef ] ? 3. C. X. Wang, F. Haider, X. Q. Gao, X. H. You, Y. Yang, D. F. Yuan, H. M. Aggoune, H. Haas, S. Fletcher, and E. Hepsaydir, IEEE Commun. Mag. 52 (2), 122 (2014). [ CrossRef ] ? 4. B. C. Deloach, R. C. Miller, and S. Kaufman, Bell Syst. Tech. J. 57 , 3309 (1978). [ CrossRef ] ? 5. R. C. Miller and R. B. Lawry, Bell Syst. Tech. J. 58 , 1735 (1979). [ CrossRef ] ? 6. F. V. B. de Nazare and M. M. Werneck, IEEE Sens. J. 12 , 1193 (2012). [ CrossRef ] ? 7. G. Bottger, M. Dreschmann, C. Klamouris, M. Hubner, M. Roger, A. W. Bett, T. 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