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Abstract
We built a full-duplex high-speed optical wireless communication (OWC) system based on high-bandwidth micro-size devices, for which micro-LED and VCSEL arrays are implemented to establish downlink and uplink, respectively. The high-capacity downlink based on a single-pixel quantum dot (QD) micro-LED can reach a data rate of 2.74 Gbps with adaptive orthogonal frequency division multiplexing (OFDM). VCSEL-based line-of-sight (LOS) and non-line-of-sight (NLOS) uplinks are designed with lens-free receiving functions for a 2.2-m communication distance. Experimental results have been demonstrated and confirmed that both downlink and uplinks are capable of providing sufficient bandwidth for a multi-gigabit OWC. Besides, the lens-free uplink receiver can alleviate requirements for aligning and improve the mobility of the transmitter. The VCSELs implemented for both systems work with low driving currents of 140-mA and 190-mA under consideration of the human eye safety. For non-return-to-zero on-off keying (NRZ-OOK), both uplinks can achieve 2.125 Gbps with bit-error-rate (BER) lower than the forward error correction (FEC) threshold of 3.8×10−3 for Ethernet access.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Visible light communication (VLC), one of the typical optical wireless communication (OWC) technologies, loads digital baseband signals into illumination light emitting diodes (LEDs) for data communication through high-speed intensity modulation [1]. Owing to the advantages of high-capacity and confidentiality, VLC does not require any new spectrum license nor introduce electromagnetic interference (EMI) to other devices that are unfavorable in applications [2–3]. Indoor lighting LEDs are generally fixed at specific locations on the ceiling and then broadcast data to each terminal in the room through visible light of the downlink. While most of current reports present unidirectional VLC links, a complete duplex communication system must include a cooperative and data rate-matched uplink [4–5]. Generally, in a typical indoor environment, the downlink needs to be high-capacity to support larger download traffic and multiple users which requires the downlink to use high electrical-to-optical (E-O) bandwidth illumination device such as micro-size LED (micro-LED) and complex multi-carrier modulation techniques represented by quadrature amplitude modulation-orthogonal frequency division multiplexing (QAM-OFDM). Micro-LEDs are promising candidates for future high-speed VLC and have been applied in VLCs and underwater OWCs. Chen et al. developed a green micro-LEDs with 340 MHz E-O bandwidth and the data rate can reach 2.16 Gbps with OFDM [6]. Multiple color micro-LED and polarization-multiplexing can deliver data rate more than 2 Gbps at extremely low illumination [7]. The data rate of VLC has reached 7.91 Gbps by using a gallium nitride (GaN) violet micro-LED with E-O bandwidth of 655 MHz, but the system is limited by a very short communication distance of 27.5 cm [8]. Up to now, over 10 Gbps data rates have been achieved by using a GaN-based series-biased micro-LED array and bit-loading OFDM [9]. For underwater, the data rate reached 2 Gbps employing quantum dot (QD) blue micro-LED with simple non-return-to-zero on-off keying (NRZ-OOK) [10].
In order to match the transmission rate of VLC downlink, the uplink needs to have a similar order of magnitude transmission rate for Gbps Ethernet access. In addition, as the VLC system is generally targeting for a short-range wireless mobile access, the uplink also needs to be in wireless and mobile manner with a relatively larger coverage [11–12]. Therefore, compared with the downlink, conversely, the uplink often requires higher mobility, better compatibility, and more convenient receivers. For the uplink for Gbps Ethernet access, we have employed a series of methods, including using a specific standard data rate, adopting the simple NRZ-OOK modulation format, and removing the lens of the receiver. The simple modulation format reduces the complexity of the transmitter and receiver which also ensures a low delay for the uplink. Using radio frequency (RF) communication methods such as wireless fidelity (Wi-Fi) as an uplink is unable to form an all-optical link because of the introduction of EMI. While adopting the visible light uplink as similar as downlink causes significant visual interference, especially when the visible light source transmitter is embedded on consumer electronic terminals [13–14]. Compared with RF and visible light, near-infrared light can provide high-speed access while avoiding visual interference. Near-infrared vertical cavity surface emitting lasers (VCSELs) are increasingly implemented on consumer electronics devices due to their sensing and biometric capabilities such as Face-ID and 3D-Sensing. In addition, using light sources with different wavelengths also can avoid the interference between uplink and downlink. For VCSEL-based OWC, Ali et al. provided a 10-Gbps OWC system for data center networks but 1310-nm VCSELs are not common in consumer electronics [15]. Wei et al. used a single 850-nm VCSEL to set up an OWC system with 1 GHz modulation bandwidth limited by the weak optical power [16]. Yoshikawa et al. presented a high-power 940-nm VCSEL-based OWC system using an integrated VCSEL array [17–18]. Moreover, compared with the wavelength of 850 nm which is closer to the visible light band, the wavelength of 940 nm is not dangerous for eyes. Red VCSEL OWC system is presented by Lu et al. which cannot be used in uplink due to the visual interference [19].
In this work, for the first time, we propose a full-duplex high-speed low-consumption indoor OWC system design using visible QD micro-LED and near-infrared VCSEL array which shows good flexibility, mobility and compatibility for existing facilities. A homemade GaN-based single layer structure QD micro-LED is designed, fabricated and packaged for this application. For downlinks, we employed the QD micro-LED with modulation bandwidth of 811 MHz to achieve a data rate of 2.74 Gbps with BER of 2.3×10−3 by adaptive bit-loading and power-loading OFDM. For uplinks, we firstly implemented a widely-used and commercially available 940-nm near-infrared VCSEL array with 1 GHz modulation bandwidth to construct line-of-sight (LOS) and non-line-of-sight (NLOS) uplinks. Moreover, the uplink operates at a low driving current which protects human eyes, and the receiving lens-free design enhances the mobility of the transmitter. Then, the optical power and link loss of LOS and NLOS uplink are modeled and analyzed and both uplinks configurations use a simple NRZ-OOK modulation scheme which can provide a data rate more than 2 Gbps for free-space Gbps Ethernet access.
2. Devices and experimental setup
2.1 QD micro-LED and VCSEL array
In this work, an epitaxial structure of blue InGaN-based QD micro-LED array were grown on standard c-plane sapphire substrate by metal-organic-chemical-vapor-deposition (MOCVD, AIXTRON, 2000HT) equipment. The substrate growth method is reported in our previous works [20], and the fabrication processes of QD micro-LED with different patterns are similar to those presented in Ref. [21]. Compared with the quantum well micro-LED, our fabricated QD micro-LED has a single layer structure of InGaN-based QD between n-doped GaN with 60-nm thickness and undoped GaN with 20-nm thickness. The QD micro-LED array is cut into individual pixels and encapsulated with resin as a TO-CAN (Transistor-Outline window-can) packaged shape. This packaging format can reduce the modulation bandwidth degradation of the devices from chip on wafer to practical form, and the impedance mismatch between packaged device and transmission line on printed circuit board (PCB).
The VCSEL array (LD0940-B500-0120CB-7060, QLINK) is a typical multimode oxide isolation laser diode with a relatively small emission area and divergence angle of 20$^\circ $ that can be easily collimated by convex lens. Moreover, its modulation bandwidth is more than 1 GHz, hence, it can be used to construct high-bandwidth OWC uplink as well as broadband access networks. This VCSEL-based uplink can be matched with a downlink in transmission rate compared with the other uplink methods [22]. In addition, the VCSEL array has a wide temperature tolerance (−40°C to + 80°C operating temperature) and humidity tolerance (10% to 90% relative humidity). When being applied to the indoor OWC uplink, the VCSEL with low current driving will not cause any damage to human eyes or skin relatively which will be analyzed in Section 3.
2.2 Experimental setup and measurements
As shown in Fig. 2(a), we propose a full-duplex high-speed indoor OWC system in a typical indoor environment with 5 m × 5 m × 3 m size and implement the measurement for the downlink and uplink, respectively. For QD micro-LED-based downlink transmitter and receiver, the schematic is further illustrated in Fig. 1(b) including the transmitter, free-space channel and receiver. The downlink consists of two parts, real-time and off-line, where the off-line digital signal processing (DSP) is carried out in MATLAB. Compared with the conventional optical OFDM, signal-to-noise ratio (SNR) estimation is firstly conducted to calculate the SNR of every sub-carrier of OFDM and the modulation order of QAM is controlled by bit-loading and power-loading. Series data streams are divided to generate parallel data streams which are transformed as Hermitian Symmetry format. Before concatenating back series data streams format, each data stream branch will undergo inverse fast Fourier transform (IFFT) and cyclic prefix (CP) is added. The synchronizing prefix is inserted to the serial data frame, then up-sampled and sent to an arbitrary waveform generator (AWG, AWG7000A, Tektronix). The RF signal generated by the AWG is amplified by a pre-amplifier (ZX60-43+, Mini-circuit) and sent to a bias-tee (ZFBT-6GW, Mini-Circuits) which directly drives the QD micro-LED to emit signal light. Intensity modulation / direct detection (IM/DD) is adopted in the uplink and downlink of the OWC network. After 2.2 m free-space link and lens convergence, a 1-GHz high-speed avalanche photodiode (APD, APD210, Menlo Systems) with 1 GHz bandwidth and 0.5 mm diameter of photosensitive area converts the optical signal to electrical signal. After being amplified by a post-amplifier, a real-time mixed signal oscilloscope (OSC, MSO73304DX, Tektronix) records the collected signal. Afterward, down-sampling, synchronizing, series-parallel-converting (S/P), removing CP, and fast Fourier transform (FFT) are implemented, sequentially. Before parallel-series-converting (P/S), channel estimation and equalization are employed which is feedback to SNR estimation in the transmitter. Finally, the series data streams are demodulated and output.
Fig. 2. (a) Schematic and experimental setup of the indoor full-duplex high-speed OWC network and (b) schematic of QD micro-LED-based downlink with adaptive bit-loading OFDM.
For the indoor infrared uplink to facilitate Gbps Ethernet access as illustrated in Fig. 2(a), a near-infrared VCSEL array integrated into a consumer electronic devices or mobile terminals is applied to emit rays with different directions which reach the ceiling receivers and then access the Ethernet, at a typical height of 0.8 meter. LOS and NLOS are the two typical link geometries for indoor uplinks, so we construct LOS and NLOS uplinks and measure their received optical power distributions, modulation bandwidths and communication performances. The distance of LOS between transmitter and receiver is set to be 2.2 m which is the same with the downlink. In addition, the NLOS uplink is another typical scenario to be considered for indoor uplink where a near-infrared reflective film is included as a reflector. For more expanded information, the setup and measurement of VCSEL-based NLOS diffuse OWC system can be referred to our previous work in Ref. [23]. The VCSEL and the APD are 1 m and 1.2 m away from the reflecting surface, respectively. The incident angle and the angle between the path of incident and reflected light are 45° and 90° correspondingly as indicated in Fig. 2(a). It is worth noting that no lens is utilized at the receiver for both the LOS and NLOS links to fully guarantee the mobility of the uplinks. A digital baseband signal generated by a 3.125 Gbps serial BER tester (BERT, N5980A, Agilent) is superimposed on the bias voltage provided by the DC power supply (DP832, RIGOL) and delivered to the VCSEL via a front-end bias driver (ZFBT-6GW, Mini-Circuits). The near-infrared light emits and diverges into free space after converging through the plano-convex lens and then is collected at the receiver side without a receiving lens.
At the uplink receiver, a same APD with the downlink converts the near-infrared optical signal into electrical format. And then the electrical signal is delivered to the digital storage oscilloscope (DS6004A, Keysight) after passing an additional amplifier (ZX60-43+, Mini-circuit). With the aforementioned VCSEL-based LOS and NLOS uplinks, the maximum data rates are measured under NRZ-OOK modulation by the BERT. A pseudo-random binary sequence (PRBS) of length 231-1 is generated by the BERT at six various selected data rates from 155.52 Mbps to 2.488 Gbps with NRZ-OOK format. For Gbps Ethernet access, the selected patterns satisfy different communication standards including OC-3, OC-12, 1 × FC, 1 × Gigabit Ethernet, 2 × FC and OC-48 with the input peak-to-peak voltage is 800 mV. For the uplink, assuming VCSEL-based uplink integrated into commercial electronic devices, the performances of lower cost, simpler architecture, better compatibility and mobility are the more pressing issues rather than increasing the data rate and decreasing the inter-symbol interference (ISI) problem. Therefore, for the consideration of actual deployment, we adopted the existing short-distance access standard of the NRZ-OOK modulation scheme instead of the OFDM with stronger anti-interference capability. This modulation approach also reduces the complexity of the transmitter and receiver and it is compatible with current existing Internet facilities. Finally, a vector network analyzer (VNA, N5227A, Agilent) is used to obtain 3-dB modulation bandwidths of the downlink and uplink, respectively.
3. Results and discussions
3.1 Devices characteristics of QD micro-LED and VCSEL
In this section, the basic optical-electrical characteristics of the micro-size devices for our proposed OWC systems are investigated. For the QD micro-LED for downlink, the L-I-V and the spectral characteristics are measured after packaging and shown in Figs. 3(a) and 3(b), respectively. Parasitic capacitance and inductance will be introduced after the TO-CAN package, and the electrical performance will be slightly reduced compared with the on-chip test results [22]. In Fig. 3(a), as the current changes from 0 mA to 40 mA, the voltage varies from 0 V to 6.5 V, which corresponds to an increase in the output optical power from 0 mW to 0.36 mW. The spectrum of the QD micro-LED has a peak at 463 nm and the entire profile includes a wetting layer around 420 nm and a main peak around 460 nm as exhibited in Fig. 3(b).
Fig. 3. The L-I-V characteristics and spectrum of (a), (b) the packaged QD micro-LED and (c), (d) the packaged near-infrared VCSEL array. (Inset in (c): The far field and near field of the VCSEL array.)
For the near-infrared VCSEL arrays with a divergence angle of 20°, in order to fulfill the human eye safety requirement with a low optical power, the driving current is limited at 140 mA and 190 mA for LOS and NLOS scenarios. The L-I-V characteristics and the light intensity distribution profile of the VCSEL under different injection currents are illustrated in Fig. 3(c). The VCSEL spectral characteristics at driving currents from 120 mA to 240 mA are provided in Fig. 3(d). When the driving current exceeds 200 mA, the VCSEL outputs optical power is close to its typical value of 500 mW. A charge coupled device (CCD, 960H, Sony) is utilized as beam quality analyzer (BQA) with wavelength ranges from 190 nm to 1700nm to measure the light intensity distribution characteristics of the VCSEL. A plano-convex lens with a focal length of 40 mm and a diameter of 40 mm is used to concentrate the light emitted from VCSEL to the BQA of which the photosensitive area is only 6 mm × 5 mm. Figure 3(c) shows the profile of light intensity measured using a 40-mm focal length lens with an object distance of 40 mm, but with different imaging distances of (a) 250 mm, (b) 270 mm, (c) 285 mm and (d) 360 mm from VCSEL emitting area, respectively. It is obvious that the cross section of the light beam approximates a Gaussian distribution. When the imaging distance increases to 360 mm, the near field is clearly captured by the BQA. The driving voltage of 2.35 V corresponds to a current of 190 mA where the peak luminescence spectrum occurs at 922.39 nm as shown in Fig. 3(d). The peak wavelength is only shifted by 0.65 nm when the driving current changes from 120 mA to 240 mA, which further proves the wavelength stability of the VCSEL. The VCSEL array is a typical multimode laser diode with relatively large linewidth and its spectra consist of overlapping peaks of patterns at multiple wavelengths.
3.2 Communication performance of the full-duplex system
Figure 4(a) shows the frequency responses under different driving currents of the QD micro-LED and the highest modulation bandwidth of the downlink is 811 MHz at a driving current of 25 mA. This result has a deviation compared to our previous work [22,24]. The reason for this phenomenon is that the characteristics of QD micro-LED grown on the same uneven epitaxial wafer would not be exactly the same and the contact of packaging also introduced some further influences. Figure 4(b) is the SNR distribution with every subcarrier, and Fig. 4(c) shows the optimal bit allocation and power allocation according to the SNR. In order to reduce the signal distortion, a 6-dB attenuation is included in the pre-stage of the amplifier. The purpose of this process is to achieve a constant SNR for all subcarriers with the same constellation size. Figure 4(d) shows the BER versus different data rate of the micro-LED-based downlink at the optimal driving current of 25 mA. For the highest data rate of 2.74 Gbps with a BER of 2.3×10−3 under the forward error correction (FEC) limit of 3.8×10−3, the constellation diagrams are presented in Fig. 5. In this downlink experiment, the modulation orders and constellation diagrams of different subchannels could be different and we set the signal length of 20000 points to make the constellation diagrams look focused as shown in Fig. 5. In addition, the frequency spectrograms of the QD micro-LED-based VLC system at the transmitter and the receiver are further presented as shown in Fig. 6, including the binary phase shift keying (BPSK) and bit-loading OFDM at the transmitter and the receiver, respectively. The out-of-band noise level at receiver is lower than at transmitter because the out-of-band noise is filtered by the low-pass channel. However, the main factor that affects the signal quality is the in-band noise so the SNR at the receiver is still lower than the transmitter. In addition, it is obvious that the trend of the spectra fit well with the frequency responses shown in Fig. 4(a) and the spectra of bit-loading OFDM shown in Figs. 6(c) and 6(d) are stepped because of the power allocation shown in Fig. 4(c).
Fig. 4. (a) The frequency responses under different driving currents, (b) SNR, (c) allocated bits and power and (d) BER versus data rate of micro-LED-based downlink over 2.2 m free-space link.
Fig. 6. Spectrograms of (a) BPSK at the transmitter, (b) BPSK at the receiver, (c) bit-loading OFDM at the transmitter and (d) bit-loading OFDM at the receiver of micro-LED-based downlink.
The normalized frequency response of the LOS uplink with different driving currents is shown in Fig. 7(a). The extracted 3-dB bandwidth exhibits a profile increasing at first and then slightly decrease afterwards which reaches a maximum of 1.025 GHz with a bias current of 190 mA as shown in Fig. 7(b). Figure 7(b) also presents the received optical power at the receiver of the LOS link. The output power intensity of the VCSEL demonstrates a linear behavior when the driving current ranges from 120 mA to 240 mA. The modulated signal superimposed with a high bias results in a strong optical intensity which might exceed the linear region of the APD-based receiver, and then results in a slight bandwidth decrease at high driving currents. Figure 7(c) provides the normalized frequency response of the VCSEL-based NLOS uplink at different driving currents. The extracted 3-dB modulation bandwidth together with the received optical power are further given in Fig. 7(d). The obtained modulation bandwidth follows the same variation tendency as the 2.2-m LOS uplink, which stabilizes at around 1 GHz as the driving current increases. Consistent with the LOS uplink, the maximum bandwidth is achieved at a driving current of 190 mA for this NLOS uplink. The corresponding maximum value of 1.05-GHz modulation bandwidth is slightly larger than that of the LOS link which results from the linearity and responsivity of the APD-based receiver. Since no lens is utilized at the receiver to concentrate the light, the spot with all power is not fully projected on the effective area of the APD, which results in the increase of the mobility. Compar wedith NLOS uplink, a much higher optical power is received in the LOS uplink than that in the reflection-based NLOS uplink. Consequently, it is reasonable to infer that the VCSEL-based LOS uplink can achieve longer communication distance in a typical indoor environment.
Fig. 7. The normalized frequency responses, extracted 3-dB modulation bandwidth and receiving optical power of the LOS uplink ((a), (b)) and NLOS uplink ((c), (d)).
The measured BERs of the LOS uplink at various data rates under different driving currents are shown in Fig. 8(a). With bias currents varied from 140 mA to 180 mA, a maximum data rate of 2.125 Gbps can be achieved, where the lowest BER of 8.6 × 10−4 is obtained at 140 mA. When the data rate reaches up to 2.488 Gbps, the BER of LOS link exceeds the FEC limit of 3.8×10−3. The eye diagrams for the 622.08 Mbps, 1.062 Gbps, 1.250 Gbps and 2.125 Gbps data rates at a bias current of 190 mA are provided in the inset of Fig. 8(a). The eye diagrams are clear when the data rate is below 1.250 Gbps, however, it becomes very blurred at 2.125 Gbps owing to the higher BER over the FEC threshold. For the VCSEL-based NLOS uplink, the data rate of 2.125 Gbps at 2.2 m communication distance is also obtained as shown in Fig. 8(b). However, the feasible range of driving currents for 2.125 Gbps rate communication shrinks to 130 mA to 150 mA and the minimum BER of 1.8 × 10−3, obtained at the same value of 140 mA, is much higher than that of the LOS link. The introduced reflective surface further deteriorates the BER performance of the OWC system. When the driving current exceeds 220 mA, the BER under different data rates increases drastically. Figure 8(b) also shows eye diagrams for different data rates at 622.08 Mbps, 1062.5 Mbps, 1250 Mbps, and 2.125 Gbps with a bias current of 190 mA. At a same 2.2 m communication distance, the comparison of the system communication performances between LOS and NLOS links under different driving currents is given in Tab. 1. With a driving current of 140 mA, both the LOS uplink and NLOS uplink are capable of achieving a data rate of 2.125 Gbps below the FEC threshold. However, when the driving current rises to 190 mA, both the uplinks cannot achieve the same 2.125 Gbps data rate although the modulation bandwidth is similar with the former scenario. We believe the changes in the received optical power together with the linearity and the response sensitivity of the APD receiver are the main reasons behind.
Fig. 8. The BER versus data rate of the (a) LOS uplink and (b) NLOS uplink. (Inset: eye diagrams of the various data rate at the current of 190 mA.)
Table 1. Comparison performances of VCSEL-based LOS and NLOS uplink at 140 mA and 190 mA.
3.3 Optical power measurement and analysis
In addition, we further investigate the optical power for eye safety. Optical power measurement system of LOS link is shown in Fig. 9. Our VCSEL array emits a Gaussian beam with waist radius of ${w_0}$ and goes through a Tx lens with focal length f. The distance between the VCSEL array and Tx lens is l. The Gaussian beam is shaped by a Tx lens and then travels a distance, denoted by L. To measure the optical power at this place, we put a measuring lens at the end so the beam can be converged to the optical power meter (PM100D, Thorlabs).
The irradiance distribution of Gaussian beam is given by [25]
Table 2. Parameters of device and link geometry of VCSEL-based uplink.
The calculated power loss at $L + l = 1\textrm{m}$ and $L + l = 2.2\textrm{m}$ are 2.66 dB and 8.13 dB, respectively. The loss through the reflector is 0.63 dB.
The measurement results by optical power meter were also recorded. For the LOS uplink, at a bias current of 140 mA of the VCSEL array, the received optical power of 40 mW at 2.2 m link is about 1/4 of the VCSEL emitting light power of 150 mW, which indicates a relatively small path loss with respect to the free-space LOS link. Controlling the drive current to be smaller is also in line with the low-consumption characteristics of the uplink, which is easier to integrate and will not cause harm to the human eyes. Moreover, for the NLOS uplink, the optical power measured at different positions along the light transmission path are also measured under different driving current as shown in Fig. 10. The difference between the curves before and after the reflector, representing the power loss, indicates that the power loss caused by reflection is small if compared to the free-space path loss. At the driving current of 140 mA for the VCSEL array, the power loss from the transmitter to the reflector, through the reflector and from the reflector to the receiver are 3.34 dB, 0.54 dB and 8.02 dB, respectively. While at the driving current of 190 mA, the power loss from the transmitter to the reflector, through the reflector and from the reflector to the receiver are 2.73 dB, 0.76 dB and 9.40 dB, respectively. We can find the gaps between measurement results and calculating results are not very large implying the correction of the optical path analysis.
4. Conclusions
A safe, reliable, low-consumption and mobilizable optical uplink has always been a problem in the research of VLC which restricts the construction of further full-duplex all-optical OWC networks in a typical indoor environment. In this work, for the first time to our best knowledge, we proposed a full-duplex high-speed micro-size devices-based indoor OWC system by combining a blue QD micro-LED-based downlink and a near-infrared VCSEL array-based lens-free uplink. For the downlink, a QD micro-LED was fabricated and packaged to achieve an 811-MHz bandwidth. By using bit-loading and power-loading adaptive QAM-OFDM, the data rate of 2.74 GHz was demonstrated with a BER of 2.3×10−3. For the sake of experimental maneuverability, compatibility and human eye safety, power-limited VCSEL array-based LOS and NLOS uplinks are both experimentally realized with a maximum data rate of 2.125 Gbps at 140 mA driving current with the BER of 8.6 × 10−4 and 1.08 × 10−3 satisfying the FEC limit over a distance of 2.2 m. In a typical indoor environment, our proposed high-speed full-duplex OWC system can provide users with both high-capacity download data stream and a stable and matched uplinked access to gigabit Ethernet.
Funding
National Key Research and Development Program of China (2016YFB0401803); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20180507183815699); Tsinghua-Berkeley Shenzhen institute(TBSI) Faculty Start-up Fund; Shenzhen Fundamental Research Program (JCYJ20170817161720819); Overseas Research Cooperation Fund of Tsinghua Shenzhen International Graduate School (HW2018003).
Disclosures
The authors declare no conflicts of interest.
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