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Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes

Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue... hv photonics Communication Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes 1 1 1 1 , 2 , Yuan Zhang , Zixian Wei , Zhaoming Wang and H. Y. Fu * Tsinghua Shenzhen International Graduate School, Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China; zhangyua20@mails.tsinghua.edu.cn (Y.Z.); weizx17@tsinghua.org.cn (Z.W.); wangzm19@mails.tsinghua.edu.cn (Z.W.) Peng Cheng Laboratory (PCL), Shenzhen 518055, China * Correspondence: hyfu@sz.tsinghua.edu.cn Abstract: A significant challenge of visible-light communication systems (VLC) is to overcome their limited converge area in non-line-of-sight (NLOS) transmission. To tackle this problem, for the first time, a real-time high-speed dual-hop VLC system based on blue micro-light-emitting diodes (micro-LED) is proposed and experimentally demonstrated. Benefiting from the advantage of high electrical-to-optical (E-O) bandwidth of the micro-LED, the frequency-response measurements show that the 3-dB modulation bandwidth of 2 m free-space single-hop link is 880 MHz, and the dual-hop system can reach to 715 MHz over a 4 m communication distance. We then investigated the communication performance of our proposed single-hop and dual-hop systems. The real-time waveforms are analyzed at different positions of the dual-hop link and eye diagrams at the receiving terminal are captured for evaluation. Furthermore, the bit error rate (BER) at the target node is measured. The results demonstrate that a 1.1 Gbps on-off keying (OOK) signal with a BER less than the forward-error-correction (FEC) limit could be achieved over a 4 m NLOS free-space link. This work shows that the proposed dual-hop system based on a micro-LED can meet the requirements for most indoor NLOS-transmission scenarios. Citation: Zhang, Y.; Wei, Z.; Wang, Z.; Keywords: visible-light communication (VLC); micro-LED; dual-hop transmission; non-line-of-sight Fu, H.Y. Real-Time Receive-Forward (NLOS); receive-forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes. Photonics 2022, 9, 211. https://doi.org/10.3390/ 1. Introduction photonics9040211 With the rapid deployment of the Internet of things (IoT) for smart cities and smart Received: 2 March 2022 homes, the next generation of wireless communication systems are expected to have the po- Accepted: 21 March 2022 tential to connect and support more devices, including computers, mobile phones, tablets, Published: 23 March 2022 high-definition (HD) cameras, virtual reality (VR), etc. Communications between a large Publisher’s Note: MDPI stays neutral number of devices through traditional radio-frequency (RF) technology will cause serious with regard to jurisdictional claims in congestion and interference problems. Visible-light communication (VLC) is a promising published maps and institutional affil- candidate to address the spectrum-scarcity issue, utilizing visible light as an information iations. carrier with high security and anti-electromagnetic-interference (EMI) characteristics, si- multaneously [1–3]. With the advantages of rich bandwidth resources and low cost, indoor VLC systems have great potential in solving the shortage of spectra and meeting the high data-rate requirements of systems other than 5G. However, due to the inescapable prop- Copyright: © 2022 by the authors. erties of lightwave transmission, line-of-sight (LOS) VLC cannot penetrate objects. VLC Licensee MDPI, Basel, Switzerland. suffers from non-line-of-sight (NLOS) transmission constraints and a shadowing effect, This article is an open access article which cannot guarantee stable and continuous communication, especially for indoor sce- distributed under the terms and narios with random moving obstacles. Therefore, many methods are proposed to tackle conditions of the Creative Commons this problem when the LOS link is not available. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Evolving from point-to-point VLC configuration, in order to improve the inherent 4.0/). issues of limited transmission distance or the link barrier, the multihop framework in VLC Photonics 2022, 9, 211. https://doi.org/10.3390/photonics9040211 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 211 2 of 11 systems has aroused intense research interest recently. In relay systems, the introduction of relay nodes provides more possibilities for optimizing the reliability of source-to-cell links. The concepts of relaying communication that could use certain light-emitting diode (LED) lamps as relays nodes were proposed and different indoor light sources can be deployed as relay nodes, such as ceilings, desks, and floor lights [4–7]. Relays between multiple VLC terminal nodes can effectively extend the transmission distance and reduce the de- pendence of LOS links, which have been experimentally demonstrated with improved performance [8]. The channel response of the VLC system with relay nodes based on an amplification and forwarding (AF) strategy was analyzed, and the relay auxiliary strategy in improving system reliability was simulated [9]. In addition, an experimental comparison of single-channel and relay-based VLC-link performance proved that the relayed VLC link can provide a higher data rate than the direct VLC link [10]. Until now, two types of transmission protocols have been proposed in relay communication, including decode- and-forward (DF) mode and AF mode [11]. Hyeong-ji Kim et al. proposed a multihop VLC system for offshore applications to overcome the limited coverage distance [12]. Omer Narmanlioglu et al. studied the performance of a full-duplex relay-auxiliary VLC system and proved that the full-duplex relay is superior to the half-duplex relay [13]. Alice Faisal et al. adopted a transmission-diversity scheme and showed that it can improve the signal- to-noise ratio (SNR) and reduce outage probability [14]. Kuan Ye et al. investigated the performance of dual-hop underwater optical wireless communication (UOWC) systems with simultaneous lightwave information and power transmission, and numerical results showed that using relay nodes can improve performance [15]. Overall, the relay schemes can be divided into two types: passive relay and active relay. Passive relays currently have some proposed solutions such as intelligent reflecting surfaces (IRS), different reflecting materials, and concave mirrors. These works are mainly for beam forming and beam steering of the emitting ray. Using a concave mirror cannot change the direction of the beam very well, which is usually used to increase the intensity of the beam in point-to-point channels. Another way, the active relay, we will mention below. The above-mentioned works are all based on simulations without experimental re- sults. In recent years, some experiments have been also carried out based on multihop VLC. For example, the demonstration of a relay-assisted VLC system based on Multiband carrierless amplitude and phase modulation (M-CAP) was carried out with a data rate of 10 Mbps [10]. A bidirectional multihop VLC was used to monitor large-area indoor fine particles. The distance between two nodes can reach 13.5 m but the data rate is only 115.2 Kbps with on-off keying (OOK) format, which is unsuitable in high-speed scenar- ios [16]. Elizabeth Eso et al. proposed a relay-assisted vehicle-mounted VLC network based on experimental measurements and presented the eye diagrams from 250 Kbps to 500 Kbps [17]. In addition, the performance of a multihop VLC system can be also tested in a real outdoor environment; however, the work only gave the relationship between the bit-error rate (BER) and average transmission power, which inadequately represents real-time communication [18]. Nonetheless, relatively high-speed multihop experiments are rarely reported. For example, a channel-aware adaptive physical-layer network-coding scheme based on adaptive-loading orthogonal frequency-division multiplexing (OFDM) was proposed, which can double the throughput of a relay-assisted VLC network [19]. A VLC system integrated with vertical-cavity surface-emitting laser (VCSEL) and LED was demonstrated indoors using a DF relay scheme with a data rate of 650 Mbps over a 4 m link [20]. Overall, most reported works on multihop free-space VLC systems are usually based on numerical simulations or only for low-speed scenarios. The data rate of the multi- hop system was limited significantly by the electrical-to-optical (E-O) bandwidth of the transmitter. Using micro-light-emitting diodes (micro-LEDs) as a transmitter has great potential for high-speed multihop VLC implementation due to their high E-O bandwidth characteristics [21,22]. In Ref. [23], a high-bandwidth micro-LED with a self-assembled nanostructure InGaN wetting layer was designed and fabricated, which shows great po- Photonics 2022, 9, 211 3 of 11 tential in point-to-point VLC applications. In addition, the maximum achievable data rate of 4 Gbps has been experimentally demonstrated by using OFDM. In order to overcome the limited communication distance and increase the data rate in the real-time NLOS VLC systems, a multihop structure combined with high-bandwidth micro-LED was adopted in this work. Incidentally, micro-LEDs also have great potential in the field of ultraviolet communication (UVC). Both NLOS and relatively low-speed problems can also be solved in ultraviolet (UV) micro-LED-based UVC systems. Recently, an AlGaN-based deep UV micro-LED emitting at 275 nm was proposed with the 3-dB E-O bandwidth of 380 MHz [24]. In addition, high-speed UVC based on a 276.8 nm UV micro-LED with a 3-dB E-O band- width of 452.53 MHz was experimentally achieved. A UVC link over 3 m with a data rate of 0.82 Gbps was presented [25]. However, ultraviolet radiation is not suitable for indoor scenarios resulting from its characteristics, which can be harmful and cause different degrees of damage to the human body. Moreover, our proposed dual-hop VLC system based on a blue micro-LED showed a higher communication-data rate compared with UVC. Therefore, UVC is not suitable for solving NLOS problems for indoor applications, and our dual-hop VLC system is a promising solution to tackle high-speed NLOS problems indoors. In this work, in order to explore the potential of the multihop structure, a dual-hop AF relay-based VLC system with a wider coverage range and a higher data rate is proposed. We focus on the application of VLC in an indoor environment, by fully routing data between VLC-based nodes and providing a Gbps communication rate. Two 488 nm blue micro- LEDs with 880 MHz E-O bandwidth were designed and then used as the transmitter in two nodes. The modulation characteristics of the micro-LED are the key reason to enable the high-speed system than other works. After a total 4 m free-space transmission, the qualitative analysis of VLC links was carried out by frequency response and real-time BER measurements with a simple OOK-modulation scheme. After passing through the relay node, the NLOS problem could be solved and the communication distance was then extended; the consequent inevitable cost was a reduction in modulation bandwidth from 880 to 715 MHz. This is mainly due to the noise introduced by AF and the long-distance attenuation. On the whole, the communication system also remained high data rates and a slightly reduced modulation bandwidth, while the cost of the dual-hop VLC system was acceptable. In addition, we also studied the influence of distance on the dual-hop system. To the best of our knowledge, this is the first real-time dual-hop link based on micro-LEDs. The results show that our proposed dual-hop VLC system with relay infrastructure can support future high-speed optical wireless networks by extending the coverage area for indoor NLOS-transmission applications. The rest of this paper is organized as follows: Section 2 describes the concept and channel-model dual-hop VLC system based on micro-LEDs. Section 3 shows the experi- mental setup for the proposed dual-hop VLC system. System communication performance including the modulation bandwidth and the BER results with real-time is also given in this section. Finally, we conclude the investigation in Section 4. 2. Concept, System Setups, and Methods In Figure 1, a relaying VLC system based on multiple micro-LEDs is investigated, which consists of the source node, the relay node, and the end-user. The corresponding experimental setup of dual-hop system is shown in Figure 2. Experimental parameters based on Hop 1 and Hop 2 are summarized in Table 1. The proposed framework can be applied in indoor VLC scenarios, which means many high-speed applications including virtual reality, real-time HD communication, second-level download, and real-time cloud storage can be realized. High-speed data transmission not only improves the ultimate sensory experience for users but also creates greater value for operators. However, the transmission system may be limited by NLOS due to the characteristics of VLC. Therefore, we consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure 1a, the link between the Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 sory experience for sory exper users b iu en t ce for also cre user ates b s gre ut a also cre ter value for oper ates greater v ator alue for oper s. However, ator the tr s. Howeve ans- r, the trans- mission system may be limited by NLOS due to the characteristics of VLC. Therefore, we mission system may be limited by NLOS due to the characteristics of VLC. Therefore, we Photonics 2022, 9, 211 4 of 11 consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure 1a, the link between the first transmitter (Tx.1) and the second receiver (Rx.2) suffers 1a, the link between the first transmitter (Tx.1) and the second receiver (Rx.2) suffers first transmitter (Tx.1) and the second receiver (Rx.2) suffers NLOS transmission constraints NLOS transmission constraints due to the obstacle. The use of a relay node composed of NLOS transmission constraints due to the obstacle. The use of a relay node composed of due to the obstacle. The use of a relay node composed of Rx.1 and Tx.2 is an alternative Rx.1 and Tx.2 is an alternative method to solve this NLOS problem. We used a micro-LED Rx.1 and Tx.2 is an alternative method to solve this NLOS problem. We used a micro-LED method to solve this NLOS problem. We used a micro-LED as the light source for the as the light source for the dual-hop VLC system; therefore, the transmitter can be consid- as the light source for the dual-hop VLC system; therefore, the transmitter can be consid- dual-hop VLC system; therefore, the transmitter can be considered as a monochromatic- ered as a monochromatic-point light source with Lambertian radiation pattern. The VLC ered as a monochromatic-point light source with Lambertian radiation pattern. The VLC point light source with Lambertian radiation pattern. The VLC system is based on the AF system is based on the AF relay method. Among various relay strategies based on VLC, system is based on the AF relay method. Among various relay strategies based on VLC, relay method. Among various relay strategies based on VLC, we chose AF relay as our we chose AF relay as our relay node, which is very simple but will inevitably synchro- we chose AF relay as our relay node, which is very simple but will inevitably synchro- relay node, which is very simple but will inevitably synchronously cause amplification of nously cause amplification of noise. In our dual-hop VLC system, channel-intensity mod- nously cause amplification of noise. In our dual-hop VLC system, channel-intensity mod- noise. In our dual-hop VLC system, channel-intensity modulation with direct detection ulation with direct detection (IM/DD) is the preferred choice, which can be modeled as a ulation with direct detection (IM/DD) is the preferred choice, which can be modeled as a (IM/DD) is the preferred choice, which can be modeled as a base-band linear system. At base-band linear system. At the end-user receiver, the noise is composed of larger shot base-band linear system. At the end-user receiver, the noise is composed of larger shot the end-user receiver, the noise is composed of larger shot noise and thermal noise due to noise and thermal noise due to the introduction of additional relay node. In a multi-relay noise and thermal noise due to the introduction of additional relay node. In a multi-relay the introduction of additional relay node. In a multi-relay VLC system that implements VLC system that implements communication, there is a positive correlation between the VLC system that implements communication, there is a positive correlation between the communication, there is a positive correlation between the communication-performance communication-performance deterioration of the end-user receiver and the number of in- communication-performance deterioration of the end-user receiver and the number of in- deterioration of the end-user receiver and the number of introduced nodes. troduced nodes. troduced nodes. Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link setup. (c) The cascade structure of Amp.2. with detailed order and type. (Tx.: Transmitter; Rx.: Re- setup. (c) The cascade structure of Amp.2 with detailed order and type. (Tx.: Transmitter; Rx.: setup. (c) The cascade structure of Amp.2. with detailed order and type. (Tx.: Transmitter; Rx.: Re- ceiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current.). Receiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current). ceiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current.). Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2.) Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2.) Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2). The setup for the dual-hop VLC system is depicted in Figure 1b. A long pseudo-ran- The setup for the dual-hop VLC system is depicted in Figure 1b. A long pseudo-random The setup for dom binar the dual-hop y sequen VLC system is depic ce with a length of ted 2 in −1 is Figmod ure 1b. ulated A long pseudo-r by OOK format and ge an- nerated binary sequence w from bit-erro ith a length ro -f rate 2 tester 1 is m (B o ERT, M dulated Pb 2100B, An y OOK foritsu), the system also used rmat and generated from bit to measure B - ER dom binary sequence with a length of 2 −1 is modulated by OOK format and generated error-rate tester (BERT, MP2100B, Anritsu), the system also used to measure BER performance. performance. Before implementing the intensity modulation of micro-LED, the signal is from bit-error-rate tester (BERT, MP2100B, Anritsu), the system also used to measure BER Before implemen atm inpl g if th ied fi e intrst en sa it n y d su moperimposed wi dulation of mic th ro direct -LED, c tu he rren sigtn (DC) by al is am p a b lifi ias-tee. A ed first fter 2 m free- performance. Before implementing the intensity modulation of micro-LED, the signal is and superimposed with direct current (DC) by a bias-tee. After 2 m free-space transmission, amplified first and superimposed with direct current (DC) by a bias-tee. After 2 m free- the optical signal is received by Rx.1. In this experiment, Rx.1 and Rx.2 are two avalanche photodiode (APD, APD210, Thorlabs) modules. Following transmission over the first optical link, the AF relay scheme is used. At the relay node, Rx.1 is used to convert the received optical signals into electrical signals and provide high gain. In order to ensure the signal quality and Photonics 2022, 9, x FOR PEER REVIEW 5 of 11 space transmission, the optical signal is received by Rx.1. In this experiment, Rx.1 and Rx.2 are two avalanche photodiode (APD, APD210, Thorlabs) modules. Following transmis- sion over the first optical link, the AF relay scheme is used. At the relay node, Rx.1 is used Photonics 2022, 9, 211 5 of 11 to convert the received optical signals into electrical signals and provide high gain. In or- der to ensure the signal quality and adjust the voltage dynamically, Amp.2 is composed of two amplifiers (SHF, Mini-circuits, and DR-AN-20-MO, iXblue), as shown in Figure 2c. adjust the voltage dynamically, Amp.2 is composed of two amplifiers (SHF, Mini-circuits, To obtain the optimum voltage and modulation depth for relay signals, we can traverse and DR-AN-20-MO, iXblue), as shown in Figure 2. To obtain the optimum voltage and different values to select the appropriate voltage corresponding to the lowest BER, specific modulation depth for relay signals, we can traverse different values to select the appropriate voltage values can be seen in Table 1. Similarly, Rx.2 can detect the optical signal trans- voltage corresponding to the lowest BER, specific voltage values can be seen in Table 1. mitted from the Tx.2 after a 2 m distance in another direction. Finally, to evaluate commu- Similarly, Rx.2 can detect the optical signal transmitted from the Tx.2 after a 2 m distance in nication performance, a real-time oscilloscope (RTO, DPO75902SX, Tektronix) is used to another direction. Finally, to evaluate communication performance, a real-time oscilloscope record the signal after dual-hop structure, and then analyze the signal quality and capture (RTO, DPO75902SX, Tektronix) is used to record the signal after dual-hop structure, and then the eye diagrams. The modulation bandwidth of the VLC system is obtained by a vector analyze the signal quality and capture the eye diagrams. The modulation bandwidth of the network analyzer (N5227A, Keysight) which connects with the transmitter and the re- VLC system is obtained by a vector network analyzer (N5227A, Keysight) which connects ceiver side. BERT is used to transmit and receive the signals in order to measure BER at with the transmitter and the receiver side. BERT is used to transmit and receive the signals in different data rates. order to measure BER at different data rates. Table 1. Experimental parameters based on Hop 1 and Hop 2. Table 1. Experimental parameters based on Hop 1 and Hop 2. Parameter Hop1 Hop 2 Parameter Hop 1 Hop 2 Transmitter 100 μm micro-LED 100 μm micro-LED Transmitter 100 m micro-LED 100 m micro-LED Output optical power 67 μW 63 μW Output optical power 67 W 63 W Receiver APD210 (1 GHz) APD210 (1 GHz) Receiver APD210 (1 GHz) APD210 (1 GHz) 31 31 The length of PRBS 2 31−1 2 − 31 1 The length of PRBS 2 1 2 1 Modulation Modulation format format OOK OOK OOK OOK Bias voltage 4.48 V 4.49 V Bias voltage 4.48 V 4.49 V Modulation depth (Vpp) 5.3 V 5.2 V Modulation depth (Vpp) 5.3 V 5.2 V Amplifier SHF SHF iXblue Amplifier SHF SHF iXblue Distance 2 m 4 m Distance 2 m 4 m 3. Experimental Results and Discussions 3. Experimental Results and Discussions 3.1. Device Characteristics 3.1. Device Characteristics Detailed electrical and optical properties of the packaged 100-m blue micro-LED are Detailed electrical and optical properties of the packaged 100-μm blue micro-LED are measured, as shown in Figures 3 and 4. measured, as shown in Figures 2 and 3. Figure 3. Current density vs. voltage (J-V) and current density vs. optical power density (JL) curves Figure 3. Current density vs. voltage (J-V) and current density vs. optical power density (JL) of the package 100-m micro-LED. curves of the package 100-μm micro-LED. Figure 3 shows the current density and voltage (J-V) and current density and optical power density (J-L) of the package 100-m micro-LED. J-V characteristics were measured by a DC supply instrument under different current densities (RIGOL, DP800). JL characteristics were measured using a calibrated silicon photodiode detector (Thorlabs, PM100D). As shown in Figure 3, the linear region of the micro-LED is small. However, the nonlinear part in the subsequent experiment can be used to improve the SNR and the communication performance. The optimal value of DC and modulation depth of the system can be found by traversing. Photonics 2022, 9, x FOR PEER REVIEW 6 of 11 Figure 3 shows the current density and voltage (J-V) and current density and optical power density (J-L) of the package 100-μm micro-LED. J-V characteristics were measured by a DC supply instrument under different current densities (RIGOL, DP800). JL charac- teristics were measured using a calibrated silicon photodiode detector (Thorlabs, PM100D). As shown in Figure 3, the linear region of the micro-LED is small. However, Photonics 2022, 9, 211 6 of 11 the nonlinear part in the subsequent experiment can be used to improve the SNR and the communication performance. The optimal value of DC and modulation depth of the sys- tem can be found by traversing. As shown in Figure 4, the optical spectrum of the micro-LED is measured at various As shown in Figure 4, the optical spectrum of the micro-LED is measured at various driving current densities, which can observe that the emission peak is around 488 nm. As driving current densities, which can observe that the emission peak is around 488 nm. As 2 2 2 2 the current density changes from 509.28 A/cm to 891.24 A/cm , it can be seen that there is the current density changes from 509.28 A/cm to 891.24 A/cm , it can be seen that there is no obvious red/blue shift. Meanwhile, the intensity peaks will all significantly rise as the no obvious red/blue shift. Meanwhile, the intensity peaks will all significantly rise as the current density increases. current density increases. Figure 4. The spectrum of the package 100-m micro-LED. Figure 4. The spectrum of the package 100-μm micro-LED. 3.2. Analysis of Signal Waveform Quality 3.2. Analysis of Signal Waveform Quality To qualitatively analyze the signal distortion in this dual-hop data-transmission sys- To qualitatively analyze the signal distortion in this dual-hop data-transmission sys- tem, a BERT is used to send a 2 1 pseudo-random binary sequence (PRBS) at 1 Gbps tem, a BERT is used to send a 2 − 1 pseudo-random binary sequence (PRBS) at 1 Gbps and a real-time oscilloscope is used to capture the waveform, simultaneously. Figure 5 and a real-time oscilloscope is used to capture the waveform, simultaneously. Figure 5 shows the signals at the test points before Tx.1, after Rx.1, Amp.2, and Rx.2, respectively. shows the signals at the test points before Tx.1, after Rx.1, Amp.2, and Rx.2, respectively. Furthermore, the peak-to-peak-voltage (Vpp) is also noted. Figure 5a,b shows that the OOK Furthermore, the peak-to-peak-voltage (Vpp) is also noted. Figure 5a,b shows that the signal suffers distortion and overlaps introduced noise as it passes through the 2 m link. OOK signal suffers distortion and overlaps introduced noise as it passes through the 2 m To obtain an ideal signal strength to drive Tx.2, the electrical signal generated by Rx.1 is link. To obtain an ideal signal strength to drive Tx.2, the electrical signal generated by Rx.1 amplified. The amplifier (DR-AN-20-MO, iXblue, BesanCon, France) provides a tunable is amplified. The amplifier (DR-AN-20-MO, iXblue, BesanCon, France) provides a tunable gain of 22 dB, which amplifies the electrical signal from 137.6 mV to 4.8 V. This voltage gain of 22 dB, which amplifies the electrical signal from 137.6 mV to 4.8 V. This voltage serves as the optimum modulation depth. The waveform is closer to a square wave after serves as the optimum modulation depth. The waveform is closer to a square wave after the amplifier, mainly because of the influence of oscilloscope noise superimposed on the the amplifier, mainly because of the influence of oscilloscope noise superimposed on the final waveform instead of the improvement of the signal-to-noise ratio (SNR), as shown final waveform instead of the improvement of the signal-to-noise ratio (SNR), as shown in Figure 5b,c. Then, the signal passes through the second 2-m link. Figure 5c,d shows Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 in Figure 5b,c. Then, the signal passes through the second 2-m link. Figure 5c,d shows that that the OOK signal suffers similar distortion and overlaps introduced noise as it passes the OOK signal suffers similar distortion and overlaps introduced noise as it passes through the second 2-m link. through the second 2-m link. Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. 3.3. Communication-Performance Evaluations In this section, an evaluation of the communication performance of the dual-hop sys- tem is given. In order to better understand the influence of the communication perfor- mance of the dual-hop system, a comparison is made with the single-hop system. The frequency response and BER performance were experimentally tested. The normalized frequency response of the micro-LED-based VLC system at different links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics un- der different situations. Without any hardware-equalization technique, it can reach 880 MHz in the single-hop link, which presents this blue micro-LED has great potential as high-speed communication application. Moreover, the values of 3-dB modulation band- width decrease to 715 MHz with the relay node. It is easy to see that modulation band- width only slightly decreases after the relay nodes and longer distance transmission. Figure 6. The comparison of measured normalized frequency response from Hop 1 and Hop 2. Figures 7 and 8 indicate the results of measured BER performance, as well as the obtained insert of it, which represents the corresponding eye diagrams when the data rate is varied. The PRBS pattern length of the OOK signal is set as 2 −1. As illustrated in Figure 6(a), the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The max- −3 imum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8 × 10 ), which can fully demonstrate the great potential of the micro-LED used in the high-speed VLC communication system. As a result, the maximum data of the first blue micro-LED can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 Photonics 2022, 9, 211 7 of 11 Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. 3.3. 3.3. Co Communication-Performance mmunication-Performance E Evaluations valuations In this section, an evaluation of the communication performance of the dual-hop sys- In this section, an evaluation of the communication performance of the dual-hop sys- tem is given. In order to better understand the influence of the communication perfor- tem is given. In order to better understand the influence of the communication performance of mance o the dual-hop f the dual-hop system, sy a comparison stem, a comp is made arisonwith is made w the single-hop ith the single system. -hop The syfr stem. The equency r frequenc esponse y re andsponse an BER performance d BER per wer formanc e experimentally e were experimentally tested. tested. The normalized frequency response of the micro-LED-based VLC system at different The normalized frequency response of the micro-LED-based VLC system at different links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics un- under different situations. Without any hardware-equalization technique, it can reach der different situations. Without any hardware-equalization technique, it can reach 880 880 MHz in the single-hop link, which presents this blue micro-LED has great potential MHz in the single-hop link, which presents this blue micro-LED has great potential as as high-speed communication application. Moreover, the values of 3-dB modulation high-speed communication application. Moreover, the values of 3-dB modulation band- bandwidth decrease to 715 MHz with the relay node. It is easy to see that modulation width decrease to 715 MHz with the relay node. It is easy to see that modulation band- bandwidth only slightly decreases after the relay nodes and longer distance transmission. width only slightly decreases after the relay nodes and longer distance transmission. Figure Figure 6. 6. The The comparison of comparison of measur measured normalized fr ed normalized frequency equency response fr response from om Hop 1 and Hop 1 and Hop Hop 2. 2. Figures 7 and 8 indicate the results of measured BER performance, as well as the Figures 7 and 8 indicate the results of measured BER performance, as well as the obtained insert of it, which represents the corresponding eye diagrams when the data rate obtained insert of it, which represents the corresponding eye diagrams when the data rate is varied. The PRBS pattern length of the OOK signal is set as 2 1. As illustrated in is varied. The PRBS pattern length of the OOK signal is set as 2 −1. As illustrated in Figure Figure 6, the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The 6(a), the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The max- maximum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8  10 ), −3 imum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8 × 10 ), which can fully demonstrate the great potential of the micro-LED used in the high-speed which can fully demonstrate the great potential of the micro-LED used in the high-speed VLC communication system. As a result, the maximum data of the first blue micro-LED VLC communication system. As a result, the maximum data of the first blue micro-LED can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the same 2 m free-space channel and lens, the values of the received optical power are also measured, which remain relatively the same: 67 W in Figure 7 while 63 W in Figure 8. Then, the maximum modulated data of Tx.2 is likewise performed. In the experiment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, which is smaller than the single-hop link. After the AF method, the transmitted optical power of the relay node is almost the same. The change of Vpp also confirms this, which is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and 1.1 Gbps are achievable within the FEC limit, respectively. Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 Photonics 2022, 9, 211 8 of 11 same 2 m free-space channel and lens, the values of the received optical power are also same 2 m free-space channel and lens, the values of the received optical power are also measured, which remain relatively the same: 67 μW in Figure 7 while 63 μW in Figure 8. measured, which remain relatively the same: 67 μW in Figure 7 while 63 μW in Figure 8. Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding eye Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding diagrams at the data rates of 1.0 Gbps and 1.5 Gbps). eye diagrams at the data rates of 1.0 Gbps and 1.5 Gbps.) eye diagrams at the data rates of 1.0 Gbps and 1.5 Gbps.) Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding eye Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps). eye diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps.) eye diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps.) In order to be more intuitive, Table 2 summarizes some measured indicators related to Then, the maximum modulated data of Tx.2 is likewise performed. In the experi- Then, the maximum modulated data of Tx.2 is likewise performed. In the experi- communication performance. Under the premise of the extended distance of the dual-hop ment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second ment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second VLC system which increases from 2 m to 4 m, it is noticeable that some of the communi- blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, cation performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 mV, which is smaller than the single-hop link. After the AF method, the transmitted optical which is smaller than the single-hop link. After the AF method, the transmitted optical which is relatively the same due to the AF method and the same bias current. However, power of the relay node is almost the same. The change of Vpp also confirms this, which power of the relay node is almost the same. The change of Vpp also confirms this, which the AF method will inevitably introduce synchronous amplification of noise; the noise is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and and long-distance attenuation represent a greater impact in the dual-hop link following an 1.1 Gbps are achievable within the FEC limit, respectively. 1.1 Gbps are achievable within the FEC limit, respectively. inevitable reduction in the modulation bandwidth and the maximum data rate. There is an In order to be more intuitive, Table 2 summarizes some measured indicators related In order to be more intuitive, Table 2 summarizes some measured indicators related admissible decrease in modulation bandwidth between 880 MHz and 715 MHz. Moreover, to communication performance. Under the premise of the extended distance of the dual- to communication performance. Under the premise of the extended distance of the dual- the data rate stabilizes at the order of Gbps and the relay node in the real-time dual-hop hop VLC system which increases from 2 m to 4 m, it is noticeable that some of the com- hop VLC system which increases from 2 m to 4 m, it is noticeable that some of the com- VLC system expands the scope of the limited area, tackling the NLOS problem effectively. munication performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 munication performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 Therefore, the cost of the relay node is acceptable. To the best of our knowledge, this is mV, which is relatively the same due to the AF method and the same bias current. How- mV, which is relatively the same due to the AF method and the same bias current. How- the first experimental demonstration of a real-time dual-hop link based on a micro-LED ever, the AF method will inevitably introduce synchronous amplification of noise; the ever, the AF method will inevitably introduce synchronous amplification of noise; the that could provide a potential solution to tackle the NLOS problem of indoor VLC. In this noise and long-distance attenuation represent a greater impact in the dual-hop link fol- noise and long-distance attenuation represent a greater impact in the dual-hop link fol- work, the modulation signal with an OOK format is used to complete the experimental lowing an inevitable reduction in the modulation bandwidth and the maximum data rate. lowing an inevitable reduction in the modulation bandwidth and the maximum data rate. verification of our proposed dual-hop VLC system based on blue micro-LEDs. In future There is an admissible decrease in modulation bandwidth between 880 MHz and 715 There is an admissible decrease in modulation bandwidth between 880 MHz and 715 work, higher-order modulation formats can be considered to obtain higher data rates. Photonics 2022, 9, x FOR PEER REVIEW 9 of 11 MHz. Moreover, the data rate stabilizes at the order of Gbps and the relay node in the real-time dual-hop VLC system expands the scope of the limited area, tackling the NLOS problem effectively. Therefore, the cost of the relay node is acceptable. To the best of our knowledge, this is the first experimental demonstration of a real-time dual-hop link based on a micro-LED that could provide a potential solution to tackle the NLOS problem of indoor VLC. In this work, the modulation signal with an OOK format is used to complete the experimental verification of our proposed dual-hop VLC system based on blue micro- Photonics 2022, 9, 211 9 of 11 LEDs. In future work, higher-order modulation formats can be considered to obtain higher data rates. Table 2. Communication-performance comparison from Hop 1 and Hop 2. Table 2. Communication-performance comparison from Hop 1 and Hop 2. Link Distance Vpp Bandwidth Data rate (FEC) Link Distance Vpp Bandwidth Data Rate (FEC) Hop 1 2.0 m 137.6 mV 880 MHz 2.0 Gbps Hop 1 2.0 m 137.6 mV 880 MHz 2.0 Gbps Hop 2 4.0 m 112.0 mV 715 MHz 1.1 Gbps Hop 2 4.0 m 112.0 mV 715 MHz 1.1 Gbps 3.4. The Impact of Transmission Distance 3.4. The Impact of Transmission Distance To further understand the impact of transmission distance and prove that our pro- To further understand the impact of transmission distance and prove that our proposed posed system can be applied to more scenarios, the measurement results at different dis- system can be applied to more scenarios, the measurement results at different distances tances are discussed in this section. Except for the change of communication distance, the are discussed in this section. Except for the change of communication distance, the system system parameters are consistent with Table 1. parameters are consistent with Table 1. First, to have a better understanding of the distance impact to the received signal First, to have a better understanding of the distance impact to the received signal based on the micro-LED, we can measure the corresponding received optical power and based on the micro-LED, we can measure the corresponding received optical power and Vpp in the free-space transmission of 2 to 4 m. As we can see from Figure 9, the received Vpp in the free-space transmission of 2 to 4 m. As we can see from Figure 9, the received optical power is 16, 10, 8 μW at 2, 3, 4 m. The Vpp of the received signal is proportional to optical power is 16, 10, 8 W at 2, 3, 4 m. The Vpp of the received signal is proportional to the received optical power. At the data rate of 1.5 Gbps, the eye diagrams of 2 m and 4 m the received optical power. At the data rate of 1.5 Gbps, the eye diagrams of 2 m and 4 m are given, respectively. It can be seen that the communication performance of a single-hop are given, respectively. It can be seen that the communication performance of a single-hop system will deteriorate with the increase in transmission distance. system will deteriorate with the increase in transmission distance. Figure 9. Measured received power and Vpp under the different free-space transmission lengths of Figure 9. Measured received power and Vpp under the different free-space transmission lengths of 2 m to 4 m (Inset: the eye diagram at the data rate of 1.5 Gbps at 2 m and 4 m). 2 m to 4 m (Inset: the eye diagram at the data rate of 1.5 Gbps at 2 m and 4 m). Then, the relationship between distance and data rate is analyzed. The comparison Then, the relationship between distance and data rate is analyzed. The comparison between Hop 1 and Hop 2 at different communication distances is given, which is shown between Hop 1 and Hop 2 at different communication distances is given, which is shown in Figure 10. In the single-hop system, the data rates within FEC are 2 Gbps in the cases of in Figure 10. In the single-hop system, the data rates within FEC are 2 Gbps in the cases 2 m and 3 m transmission distance, and the BER performance of 3 m is slightly increased of 2 m and 3 m transmission distance, and the BER performance of 3 m is slightly increased with the increase in distance. When the distance increases to 4 m, the maximum data rate of with the increase in distance. When the distance increases to 4 m, the maximum data rate the single-hop system decreases to 1.6 Gbps. In the dual-hop system, the data rates within of the single-hop system decreases to 1.6 Gbps. In the dual-hop system, the data rates FEC are 1.1 Gbps, 1.1 Gbps, and 1 Gbps at the length of 2 m to 4 m. It can be seen that within FEC are 1.1 Gbps, 1.1 Gbps, and 1 Gbps at the length of 2 m to 4 m. It can be seen when the single-hop communication performance is not limited by distance, the dual-hop communication performance also remains relatively stable with the increase in distance, which can further prove the potential application indoors. Photonics 2022, 9, x FOR PEER REVIEW 10 of 11 that when the single-hop communication performance is not limited by distance, the dual- Photonics 2022, 9, 211 10 of 11 hop communication performance also remains relatively stable with the increase in dis- tance, which can further prove the potential application indoors. Figure 10. Measurement of maximum data rate within FEC limit at different lengths (The blue font Figure 10. Measurement of maximum data rate within FEC limit at different lengths (The blue font on the histogram represents the BER for the corresponding case). on the histogram represents the BER for the corresponding case). 4. Conclusions 4. Conclusions For the indoor environment, the LOS VLC system could not always have been guaran- For the indoor environment, the LOS VLC system could not always have been guar- teed due to many effects, but it can greatly influence the communication quality. To address anteed due to many effects, but it can greatly influence the communication quality. To this unavoidable problem and have high-speed communication simultaneously, a dual-hop address this unavoidable problem and have high-speed communication simultaneously, VLC system based on a blue micro-LED was proposed and experimentally demonstrated. a dual-hop VLC system based on a blue micro-LED was proposed and experimentally The deployment of relay nodes can ensure high-speed communication indoors and solve demonstrated. The deployment of relay nodes can ensure high-speed communication in- the NLOS problem. To the best of our knowledge, this was the first time that a dual-hop doors and solve the NLOS problem. To the best of our knowledge, this was the first time VLC link was based on a micro-LED. In addition, in order to better analyze the cost and that a dual-hop VLC link was based on a micro-LED. In addition, in order to better analyze benefit of the relay node, a comparison between the single-hop and the dual-hop link was the cost and benefit of the relay node, a comparison between the single-hop and the dual- presented. On the premise of increasing the communication distance from 2 m to 4 m, there hop link was presented. On the premise of increasing the communication distance from 2 was an admissible decrease in modulation bandwidth between 880 MHz and 715 MHz. m to 4 m, there was an admissible decrease in modulation bandwidth between 880 MHz Meanwhile, the data rate corresponding to the FEC limit remained in the order of Gbps. and 715 MHz. Meanwhile, the data rate corresponding to the FEC limit remained in the The result demonstrated a dual-hop VLC system based on blue micro-LEDs with a data order of Gbps. The result demonstrated a dual-hop VLC system based on blue micro- rate up to 1.1 Gbps through simple OOK modulation over a 4 m NLOS free-space link, with LEDs with a data rate up to 1.1 Gbps through simple OOK modulation over a 4 m NLOS the help of a low-cost AF relay. In addition, when the overall link distance increases from free-space link, with the help of a low-cost AF relay. In addition, when the overall link 4 m to 8 m, the dual-hop system is still capable of high-speed communication. The data rate distance increases from 4 m to 8 m, the dual-hop system is still capable of high-speed within the FEC limit at 8 m is 1 Gbps. The available 8 m transmission distance exceeded communication. The data rate within the FEC limit at 8 m is 1 Gbps. The available 8 m the LOS distance for the indoor LOS link, which can be applied to more indoor scenarios. transmission distance exceeded the LOS distance for the indoor LOS link, which can be Experimental results confirmed that the dual-hop VLC system based on micro-LEDs can applied to more indoor scenarios. Experimental results confirmed that the dual-hop VLC tackle critical NLOS problems in VLC systems and is sufficient for high-speed indoor system based on micro-LEDs can tackle critical NLOS problems in VLC systems and is application in the future. sufficient for high-speed indoor application in the future. Author Contributions: Conceptualization, Z.W. (Zixian Wei) and Y.Z.; methodology, Z.W. (Zixian Author Contributions: Conceptualization, Z.W. (Zixian Wei) and Y.Z.; methodology, Z.W. (Zixian Wei); validation, Z.W. (Zixian Wei) and Y.Z.; formal analysis, Y.Z.; investigation, Y.Z.; resources, Z.W. Wei); validation, Z.W. (Zixian Wei) and Y.Z.; formal analysis, Y.Z.; investigation, Y.Z.; resources, (Zixian Wei) and Zhang, Y; data curation, Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing— Z.W. (Zixian Wei) and Zhang, Y; data curation, Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); original draft preparation, Y.Z., Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing—review writing—original draft preparation, Y.Z., Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing— and editing, H.Y.F.; visualization, Y.Z.; supervision, H.Y.F.; project administration, H.Y.F.; funding review and editing, H.Y.F.; visualization, Y.Z.; supervision, H.Y.F.; project administration, H.Y.F.; acquisition, H.Y.F. All authors have read and agreed to the published version of the manuscript. funding acquisition, H.Y.F. All authors have read and agreed to the published version of the man- Funding: uscript. This work is supported by Shenzhen Technology and Innovation Council (WDZC202008 20160650001). Funding: This work is supported by Shenzhen Technology and Innovation Council Institutional (WDZC20200820160650001). Review Board Statement: Not applicable. Acknowledgments: Institutional Review Board Statement: The authors thank their Not applicable colleagues. Lei Wang, and Lai Wang from Department of Electronic Engineering, Tsinghua University, for their kind help toward research work. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2022, 9, 211 11 of 11 References 1. Jovicic, A.; Li, J.; Richardson, T. Visible light communication: Opportunities, challenges and the path to market. IEEE Commun. Mag. 2013, 51, 26–32. [CrossRef] 2. Grobe, L.; Paraskevopoulos, A.; Hilt, J.; Schulz, D.; Lassak, F.; Hartlieb, F.; Kottke, C.; Jungnickel, V.; Langer, K.D. High-speed visible light communication systems. IEEE Commun. Mag. 2013, 51, 60–66. [CrossRef] 3. Chi, N.; Zhou, Y.; Wei, Y.; Hu, F. Visible light communication in 6G: Advances, challenges, and prospects. 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Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes

Photonics , Volume 9 (4) – Mar 23, 2022

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Abstract

hv photonics Communication Real-Time Receive-Forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes 1 1 1 1 , 2 , Yuan Zhang , Zixian Wei , Zhaoming Wang and H. Y. Fu * Tsinghua Shenzhen International Graduate School, Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China; zhangyua20@mails.tsinghua.edu.cn (Y.Z.); weizx17@tsinghua.org.cn (Z.W.); wangzm19@mails.tsinghua.edu.cn (Z.W.) Peng Cheng Laboratory (PCL), Shenzhen 518055, China * Correspondence: hyfu@sz.tsinghua.edu.cn Abstract: A significant challenge of visible-light communication systems (VLC) is to overcome their limited converge area in non-line-of-sight (NLOS) transmission. To tackle this problem, for the first time, a real-time high-speed dual-hop VLC system based on blue micro-light-emitting diodes (micro-LED) is proposed and experimentally demonstrated. Benefiting from the advantage of high electrical-to-optical (E-O) bandwidth of the micro-LED, the frequency-response measurements show that the 3-dB modulation bandwidth of 2 m free-space single-hop link is 880 MHz, and the dual-hop system can reach to 715 MHz over a 4 m communication distance. We then investigated the communication performance of our proposed single-hop and dual-hop systems. The real-time waveforms are analyzed at different positions of the dual-hop link and eye diagrams at the receiving terminal are captured for evaluation. Furthermore, the bit error rate (BER) at the target node is measured. The results demonstrate that a 1.1 Gbps on-off keying (OOK) signal with a BER less than the forward-error-correction (FEC) limit could be achieved over a 4 m NLOS free-space link. This work shows that the proposed dual-hop system based on a micro-LED can meet the requirements for most indoor NLOS-transmission scenarios. Citation: Zhang, Y.; Wei, Z.; Wang, Z.; Keywords: visible-light communication (VLC); micro-LED; dual-hop transmission; non-line-of-sight Fu, H.Y. Real-Time Receive-Forward (NLOS); receive-forward NLOS Visible Light Communication System Based on Multiple Blue Micro-LED Nodes. Photonics 2022, 9, 211. https://doi.org/10.3390/ 1. Introduction photonics9040211 With the rapid deployment of the Internet of things (IoT) for smart cities and smart Received: 2 March 2022 homes, the next generation of wireless communication systems are expected to have the po- Accepted: 21 March 2022 tential to connect and support more devices, including computers, mobile phones, tablets, Published: 23 March 2022 high-definition (HD) cameras, virtual reality (VR), etc. Communications between a large Publisher’s Note: MDPI stays neutral number of devices through traditional radio-frequency (RF) technology will cause serious with regard to jurisdictional claims in congestion and interference problems. Visible-light communication (VLC) is a promising published maps and institutional affil- candidate to address the spectrum-scarcity issue, utilizing visible light as an information iations. carrier with high security and anti-electromagnetic-interference (EMI) characteristics, si- multaneously [1–3]. With the advantages of rich bandwidth resources and low cost, indoor VLC systems have great potential in solving the shortage of spectra and meeting the high data-rate requirements of systems other than 5G. However, due to the inescapable prop- Copyright: © 2022 by the authors. erties of lightwave transmission, line-of-sight (LOS) VLC cannot penetrate objects. VLC Licensee MDPI, Basel, Switzerland. suffers from non-line-of-sight (NLOS) transmission constraints and a shadowing effect, This article is an open access article which cannot guarantee stable and continuous communication, especially for indoor sce- distributed under the terms and narios with random moving obstacles. Therefore, many methods are proposed to tackle conditions of the Creative Commons this problem when the LOS link is not available. Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ Evolving from point-to-point VLC configuration, in order to improve the inherent 4.0/). issues of limited transmission distance or the link barrier, the multihop framework in VLC Photonics 2022, 9, 211. https://doi.org/10.3390/photonics9040211 https://www.mdpi.com/journal/photonics Photonics 2022, 9, 211 2 of 11 systems has aroused intense research interest recently. In relay systems, the introduction of relay nodes provides more possibilities for optimizing the reliability of source-to-cell links. The concepts of relaying communication that could use certain light-emitting diode (LED) lamps as relays nodes were proposed and different indoor light sources can be deployed as relay nodes, such as ceilings, desks, and floor lights [4–7]. Relays between multiple VLC terminal nodes can effectively extend the transmission distance and reduce the de- pendence of LOS links, which have been experimentally demonstrated with improved performance [8]. The channel response of the VLC system with relay nodes based on an amplification and forwarding (AF) strategy was analyzed, and the relay auxiliary strategy in improving system reliability was simulated [9]. In addition, an experimental comparison of single-channel and relay-based VLC-link performance proved that the relayed VLC link can provide a higher data rate than the direct VLC link [10]. Until now, two types of transmission protocols have been proposed in relay communication, including decode- and-forward (DF) mode and AF mode [11]. Hyeong-ji Kim et al. proposed a multihop VLC system for offshore applications to overcome the limited coverage distance [12]. Omer Narmanlioglu et al. studied the performance of a full-duplex relay-auxiliary VLC system and proved that the full-duplex relay is superior to the half-duplex relay [13]. Alice Faisal et al. adopted a transmission-diversity scheme and showed that it can improve the signal- to-noise ratio (SNR) and reduce outage probability [14]. Kuan Ye et al. investigated the performance of dual-hop underwater optical wireless communication (UOWC) systems with simultaneous lightwave information and power transmission, and numerical results showed that using relay nodes can improve performance [15]. Overall, the relay schemes can be divided into two types: passive relay and active relay. Passive relays currently have some proposed solutions such as intelligent reflecting surfaces (IRS), different reflecting materials, and concave mirrors. These works are mainly for beam forming and beam steering of the emitting ray. Using a concave mirror cannot change the direction of the beam very well, which is usually used to increase the intensity of the beam in point-to-point channels. Another way, the active relay, we will mention below. The above-mentioned works are all based on simulations without experimental re- sults. In recent years, some experiments have been also carried out based on multihop VLC. For example, the demonstration of a relay-assisted VLC system based on Multiband carrierless amplitude and phase modulation (M-CAP) was carried out with a data rate of 10 Mbps [10]. A bidirectional multihop VLC was used to monitor large-area indoor fine particles. The distance between two nodes can reach 13.5 m but the data rate is only 115.2 Kbps with on-off keying (OOK) format, which is unsuitable in high-speed scenar- ios [16]. Elizabeth Eso et al. proposed a relay-assisted vehicle-mounted VLC network based on experimental measurements and presented the eye diagrams from 250 Kbps to 500 Kbps [17]. In addition, the performance of a multihop VLC system can be also tested in a real outdoor environment; however, the work only gave the relationship between the bit-error rate (BER) and average transmission power, which inadequately represents real-time communication [18]. Nonetheless, relatively high-speed multihop experiments are rarely reported. For example, a channel-aware adaptive physical-layer network-coding scheme based on adaptive-loading orthogonal frequency-division multiplexing (OFDM) was proposed, which can double the throughput of a relay-assisted VLC network [19]. A VLC system integrated with vertical-cavity surface-emitting laser (VCSEL) and LED was demonstrated indoors using a DF relay scheme with a data rate of 650 Mbps over a 4 m link [20]. Overall, most reported works on multihop free-space VLC systems are usually based on numerical simulations or only for low-speed scenarios. The data rate of the multi- hop system was limited significantly by the electrical-to-optical (E-O) bandwidth of the transmitter. Using micro-light-emitting diodes (micro-LEDs) as a transmitter has great potential for high-speed multihop VLC implementation due to their high E-O bandwidth characteristics [21,22]. In Ref. [23], a high-bandwidth micro-LED with a self-assembled nanostructure InGaN wetting layer was designed and fabricated, which shows great po- Photonics 2022, 9, 211 3 of 11 tential in point-to-point VLC applications. In addition, the maximum achievable data rate of 4 Gbps has been experimentally demonstrated by using OFDM. In order to overcome the limited communication distance and increase the data rate in the real-time NLOS VLC systems, a multihop structure combined with high-bandwidth micro-LED was adopted in this work. Incidentally, micro-LEDs also have great potential in the field of ultraviolet communication (UVC). Both NLOS and relatively low-speed problems can also be solved in ultraviolet (UV) micro-LED-based UVC systems. Recently, an AlGaN-based deep UV micro-LED emitting at 275 nm was proposed with the 3-dB E-O bandwidth of 380 MHz [24]. In addition, high-speed UVC based on a 276.8 nm UV micro-LED with a 3-dB E-O band- width of 452.53 MHz was experimentally achieved. A UVC link over 3 m with a data rate of 0.82 Gbps was presented [25]. However, ultraviolet radiation is not suitable for indoor scenarios resulting from its characteristics, which can be harmful and cause different degrees of damage to the human body. Moreover, our proposed dual-hop VLC system based on a blue micro-LED showed a higher communication-data rate compared with UVC. Therefore, UVC is not suitable for solving NLOS problems for indoor applications, and our dual-hop VLC system is a promising solution to tackle high-speed NLOS problems indoors. In this work, in order to explore the potential of the multihop structure, a dual-hop AF relay-based VLC system with a wider coverage range and a higher data rate is proposed. We focus on the application of VLC in an indoor environment, by fully routing data between VLC-based nodes and providing a Gbps communication rate. Two 488 nm blue micro- LEDs with 880 MHz E-O bandwidth were designed and then used as the transmitter in two nodes. The modulation characteristics of the micro-LED are the key reason to enable the high-speed system than other works. After a total 4 m free-space transmission, the qualitative analysis of VLC links was carried out by frequency response and real-time BER measurements with a simple OOK-modulation scheme. After passing through the relay node, the NLOS problem could be solved and the communication distance was then extended; the consequent inevitable cost was a reduction in modulation bandwidth from 880 to 715 MHz. This is mainly due to the noise introduced by AF and the long-distance attenuation. On the whole, the communication system also remained high data rates and a slightly reduced modulation bandwidth, while the cost of the dual-hop VLC system was acceptable. In addition, we also studied the influence of distance on the dual-hop system. To the best of our knowledge, this is the first real-time dual-hop link based on micro-LEDs. The results show that our proposed dual-hop VLC system with relay infrastructure can support future high-speed optical wireless networks by extending the coverage area for indoor NLOS-transmission applications. The rest of this paper is organized as follows: Section 2 describes the concept and channel-model dual-hop VLC system based on micro-LEDs. Section 3 shows the experi- mental setup for the proposed dual-hop VLC system. System communication performance including the modulation bandwidth and the BER results with real-time is also given in this section. Finally, we conclude the investigation in Section 4. 2. Concept, System Setups, and Methods In Figure 1, a relaying VLC system based on multiple micro-LEDs is investigated, which consists of the source node, the relay node, and the end-user. The corresponding experimental setup of dual-hop system is shown in Figure 2. Experimental parameters based on Hop 1 and Hop 2 are summarized in Table 1. The proposed framework can be applied in indoor VLC scenarios, which means many high-speed applications including virtual reality, real-time HD communication, second-level download, and real-time cloud storage can be realized. High-speed data transmission not only improves the ultimate sensory experience for users but also creates greater value for operators. However, the transmission system may be limited by NLOS due to the characteristics of VLC. Therefore, we consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure 1a, the link between the Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 Photonics 2022, 9, x FOR PEER REVIEW 4 of 11 sory experience for sory exper users b iu en t ce for also cre user ates b s gre ut a also cre ter value for oper ates greater v ator alue for oper s. However, ator the tr s. Howeve ans- r, the trans- mission system may be limited by NLOS due to the characteristics of VLC. Therefore, we mission system may be limited by NLOS due to the characteristics of VLC. Therefore, we Photonics 2022, 9, 211 4 of 11 consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs consider the downlink transmission of a dual-hop system, consisting of two micro-LEDs and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure and a single end-user. The system concept is shown in Figure 1a. As illustrated in Figure 1a, the link between the first transmitter (Tx.1) and the second receiver (Rx.2) suffers 1a, the link between the first transmitter (Tx.1) and the second receiver (Rx.2) suffers first transmitter (Tx.1) and the second receiver (Rx.2) suffers NLOS transmission constraints NLOS transmission constraints due to the obstacle. The use of a relay node composed of NLOS transmission constraints due to the obstacle. The use of a relay node composed of due to the obstacle. The use of a relay node composed of Rx.1 and Tx.2 is an alternative Rx.1 and Tx.2 is an alternative method to solve this NLOS problem. We used a micro-LED Rx.1 and Tx.2 is an alternative method to solve this NLOS problem. We used a micro-LED method to solve this NLOS problem. We used a micro-LED as the light source for the as the light source for the dual-hop VLC system; therefore, the transmitter can be consid- as the light source for the dual-hop VLC system; therefore, the transmitter can be consid- dual-hop VLC system; therefore, the transmitter can be considered as a monochromatic- ered as a monochromatic-point light source with Lambertian radiation pattern. The VLC ered as a monochromatic-point light source with Lambertian radiation pattern. The VLC point light source with Lambertian radiation pattern. The VLC system is based on the AF system is based on the AF relay method. Among various relay strategies based on VLC, system is based on the AF relay method. Among various relay strategies based on VLC, relay method. Among various relay strategies based on VLC, we chose AF relay as our we chose AF relay as our relay node, which is very simple but will inevitably synchro- we chose AF relay as our relay node, which is very simple but will inevitably synchro- relay node, which is very simple but will inevitably synchronously cause amplification of nously cause amplification of noise. In our dual-hop VLC system, channel-intensity mod- nously cause amplification of noise. In our dual-hop VLC system, channel-intensity mod- noise. In our dual-hop VLC system, channel-intensity modulation with direct detection ulation with direct detection (IM/DD) is the preferred choice, which can be modeled as a ulation with direct detection (IM/DD) is the preferred choice, which can be modeled as a (IM/DD) is the preferred choice, which can be modeled as a base-band linear system. At base-band linear system. At the end-user receiver, the noise is composed of larger shot base-band linear system. At the end-user receiver, the noise is composed of larger shot the end-user receiver, the noise is composed of larger shot noise and thermal noise due to noise and thermal noise due to the introduction of additional relay node. In a multi-relay noise and thermal noise due to the introduction of additional relay node. In a multi-relay the introduction of additional relay node. In a multi-relay VLC system that implements VLC system that implements communication, there is a positive correlation between the VLC system that implements communication, there is a positive correlation between the communication, there is a positive correlation between the communication-performance communication-performance deterioration of the end-user receiver and the number of in- communication-performance deterioration of the end-user receiver and the number of in- deterioration of the end-user receiver and the number of introduced nodes. troduced nodes. troduced nodes. Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link Figure 1. (a) Schematic diagram of dual-hop VLC-link setup. (b) Diagram of dual-hop VLC-link setup. (c) The cascade structure of Amp.2. with detailed order and type. (Tx.: Transmitter; Rx.: Re- setup. (c) The cascade structure of Amp.2 with detailed order and type. (Tx.: Transmitter; Rx.: setup. (c) The cascade structure of Amp.2. with detailed order and type. (Tx.: Transmitter; Rx.: Re- ceiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current.). Receiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current). ceiver; Amp.: Amplifier; BERT: Bit-error-rate tester; DC: Direct current.). Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2.) Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2.) Figure 2. Experimental setup of dual-hop VLC link. (Inset: Cascade Amp.2). The setup for the dual-hop VLC system is depicted in Figure 1b. A long pseudo-ran- The setup for the dual-hop VLC system is depicted in Figure 1b. A long pseudo-random The setup for dom binar the dual-hop y sequen VLC system is depic ce with a length of ted 2 in −1 is Figmod ure 1b. ulated A long pseudo-r by OOK format and ge an- nerated binary sequence w from bit-erro ith a length ro -f rate 2 tester 1 is m (B o ERT, M dulated Pb 2100B, An y OOK foritsu), the system also used rmat and generated from bit to measure B - ER dom binary sequence with a length of 2 −1 is modulated by OOK format and generated error-rate tester (BERT, MP2100B, Anritsu), the system also used to measure BER performance. performance. Before implementing the intensity modulation of micro-LED, the signal is from bit-error-rate tester (BERT, MP2100B, Anritsu), the system also used to measure BER Before implemen atm inpl g if th ied fi e intrst en sa it n y d su moperimposed wi dulation of mic th ro direct -LED, c tu he rren sigtn (DC) by al is am p a b lifi ias-tee. A ed first fter 2 m free- performance. Before implementing the intensity modulation of micro-LED, the signal is and superimposed with direct current (DC) by a bias-tee. After 2 m free-space transmission, amplified first and superimposed with direct current (DC) by a bias-tee. After 2 m free- the optical signal is received by Rx.1. In this experiment, Rx.1 and Rx.2 are two avalanche photodiode (APD, APD210, Thorlabs) modules. Following transmission over the first optical link, the AF relay scheme is used. At the relay node, Rx.1 is used to convert the received optical signals into electrical signals and provide high gain. In order to ensure the signal quality and Photonics 2022, 9, x FOR PEER REVIEW 5 of 11 space transmission, the optical signal is received by Rx.1. In this experiment, Rx.1 and Rx.2 are two avalanche photodiode (APD, APD210, Thorlabs) modules. Following transmis- sion over the first optical link, the AF relay scheme is used. At the relay node, Rx.1 is used Photonics 2022, 9, 211 5 of 11 to convert the received optical signals into electrical signals and provide high gain. In or- der to ensure the signal quality and adjust the voltage dynamically, Amp.2 is composed of two amplifiers (SHF, Mini-circuits, and DR-AN-20-MO, iXblue), as shown in Figure 2c. adjust the voltage dynamically, Amp.2 is composed of two amplifiers (SHF, Mini-circuits, To obtain the optimum voltage and modulation depth for relay signals, we can traverse and DR-AN-20-MO, iXblue), as shown in Figure 2. To obtain the optimum voltage and different values to select the appropriate voltage corresponding to the lowest BER, specific modulation depth for relay signals, we can traverse different values to select the appropriate voltage values can be seen in Table 1. Similarly, Rx.2 can detect the optical signal trans- voltage corresponding to the lowest BER, specific voltage values can be seen in Table 1. mitted from the Tx.2 after a 2 m distance in another direction. Finally, to evaluate commu- Similarly, Rx.2 can detect the optical signal transmitted from the Tx.2 after a 2 m distance in nication performance, a real-time oscilloscope (RTO, DPO75902SX, Tektronix) is used to another direction. Finally, to evaluate communication performance, a real-time oscilloscope record the signal after dual-hop structure, and then analyze the signal quality and capture (RTO, DPO75902SX, Tektronix) is used to record the signal after dual-hop structure, and then the eye diagrams. The modulation bandwidth of the VLC system is obtained by a vector analyze the signal quality and capture the eye diagrams. The modulation bandwidth of the network analyzer (N5227A, Keysight) which connects with the transmitter and the re- VLC system is obtained by a vector network analyzer (N5227A, Keysight) which connects ceiver side. BERT is used to transmit and receive the signals in order to measure BER at with the transmitter and the receiver side. BERT is used to transmit and receive the signals in different data rates. order to measure BER at different data rates. Table 1. Experimental parameters based on Hop 1 and Hop 2. Table 1. Experimental parameters based on Hop 1 and Hop 2. Parameter Hop1 Hop 2 Parameter Hop 1 Hop 2 Transmitter 100 μm micro-LED 100 μm micro-LED Transmitter 100 m micro-LED 100 m micro-LED Output optical power 67 μW 63 μW Output optical power 67 W 63 W Receiver APD210 (1 GHz) APD210 (1 GHz) Receiver APD210 (1 GHz) APD210 (1 GHz) 31 31 The length of PRBS 2 31−1 2 − 31 1 The length of PRBS 2 1 2 1 Modulation Modulation format format OOK OOK OOK OOK Bias voltage 4.48 V 4.49 V Bias voltage 4.48 V 4.49 V Modulation depth (Vpp) 5.3 V 5.2 V Modulation depth (Vpp) 5.3 V 5.2 V Amplifier SHF SHF iXblue Amplifier SHF SHF iXblue Distance 2 m 4 m Distance 2 m 4 m 3. Experimental Results and Discussions 3. Experimental Results and Discussions 3.1. Device Characteristics 3.1. Device Characteristics Detailed electrical and optical properties of the packaged 100-m blue micro-LED are Detailed electrical and optical properties of the packaged 100-μm blue micro-LED are measured, as shown in Figures 3 and 4. measured, as shown in Figures 2 and 3. Figure 3. Current density vs. voltage (J-V) and current density vs. optical power density (JL) curves Figure 3. Current density vs. voltage (J-V) and current density vs. optical power density (JL) of the package 100-m micro-LED. curves of the package 100-μm micro-LED. Figure 3 shows the current density and voltage (J-V) and current density and optical power density (J-L) of the package 100-m micro-LED. J-V characteristics were measured by a DC supply instrument under different current densities (RIGOL, DP800). JL characteristics were measured using a calibrated silicon photodiode detector (Thorlabs, PM100D). As shown in Figure 3, the linear region of the micro-LED is small. However, the nonlinear part in the subsequent experiment can be used to improve the SNR and the communication performance. The optimal value of DC and modulation depth of the system can be found by traversing. Photonics 2022, 9, x FOR PEER REVIEW 6 of 11 Figure 3 shows the current density and voltage (J-V) and current density and optical power density (J-L) of the package 100-μm micro-LED. J-V characteristics were measured by a DC supply instrument under different current densities (RIGOL, DP800). JL charac- teristics were measured using a calibrated silicon photodiode detector (Thorlabs, PM100D). As shown in Figure 3, the linear region of the micro-LED is small. However, Photonics 2022, 9, 211 6 of 11 the nonlinear part in the subsequent experiment can be used to improve the SNR and the communication performance. The optimal value of DC and modulation depth of the sys- tem can be found by traversing. As shown in Figure 4, the optical spectrum of the micro-LED is measured at various As shown in Figure 4, the optical spectrum of the micro-LED is measured at various driving current densities, which can observe that the emission peak is around 488 nm. As driving current densities, which can observe that the emission peak is around 488 nm. As 2 2 2 2 the current density changes from 509.28 A/cm to 891.24 A/cm , it can be seen that there is the current density changes from 509.28 A/cm to 891.24 A/cm , it can be seen that there is no obvious red/blue shift. Meanwhile, the intensity peaks will all significantly rise as the no obvious red/blue shift. Meanwhile, the intensity peaks will all significantly rise as the current density increases. current density increases. Figure 4. The spectrum of the package 100-m micro-LED. Figure 4. The spectrum of the package 100-μm micro-LED. 3.2. Analysis of Signal Waveform Quality 3.2. Analysis of Signal Waveform Quality To qualitatively analyze the signal distortion in this dual-hop data-transmission sys- To qualitatively analyze the signal distortion in this dual-hop data-transmission sys- tem, a BERT is used to send a 2 1 pseudo-random binary sequence (PRBS) at 1 Gbps tem, a BERT is used to send a 2 − 1 pseudo-random binary sequence (PRBS) at 1 Gbps and a real-time oscilloscope is used to capture the waveform, simultaneously. Figure 5 and a real-time oscilloscope is used to capture the waveform, simultaneously. Figure 5 shows the signals at the test points before Tx.1, after Rx.1, Amp.2, and Rx.2, respectively. shows the signals at the test points before Tx.1, after Rx.1, Amp.2, and Rx.2, respectively. Furthermore, the peak-to-peak-voltage (Vpp) is also noted. Figure 5a,b shows that the OOK Furthermore, the peak-to-peak-voltage (Vpp) is also noted. Figure 5a,b shows that the signal suffers distortion and overlaps introduced noise as it passes through the 2 m link. OOK signal suffers distortion and overlaps introduced noise as it passes through the 2 m To obtain an ideal signal strength to drive Tx.2, the electrical signal generated by Rx.1 is link. To obtain an ideal signal strength to drive Tx.2, the electrical signal generated by Rx.1 amplified. The amplifier (DR-AN-20-MO, iXblue, BesanCon, France) provides a tunable is amplified. The amplifier (DR-AN-20-MO, iXblue, BesanCon, France) provides a tunable gain of 22 dB, which amplifies the electrical signal from 137.6 mV to 4.8 V. This voltage gain of 22 dB, which amplifies the electrical signal from 137.6 mV to 4.8 V. This voltage serves as the optimum modulation depth. The waveform is closer to a square wave after serves as the optimum modulation depth. The waveform is closer to a square wave after the amplifier, mainly because of the influence of oscilloscope noise superimposed on the the amplifier, mainly because of the influence of oscilloscope noise superimposed on the final waveform instead of the improvement of the signal-to-noise ratio (SNR), as shown final waveform instead of the improvement of the signal-to-noise ratio (SNR), as shown in Figure 5b,c. Then, the signal passes through the second 2-m link. Figure 5c,d shows Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 in Figure 5b,c. Then, the signal passes through the second 2-m link. Figure 5c,d shows that that the OOK signal suffers similar distortion and overlaps introduced noise as it passes the OOK signal suffers similar distortion and overlaps introduced noise as it passes through the second 2-m link. through the second 2-m link. Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. 3.3. Communication-Performance Evaluations In this section, an evaluation of the communication performance of the dual-hop sys- tem is given. In order to better understand the influence of the communication perfor- mance of the dual-hop system, a comparison is made with the single-hop system. The frequency response and BER performance were experimentally tested. The normalized frequency response of the micro-LED-based VLC system at different links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics un- der different situations. Without any hardware-equalization technique, it can reach 880 MHz in the single-hop link, which presents this blue micro-LED has great potential as high-speed communication application. Moreover, the values of 3-dB modulation band- width decrease to 715 MHz with the relay node. It is easy to see that modulation band- width only slightly decreases after the relay nodes and longer distance transmission. Figure 6. The comparison of measured normalized frequency response from Hop 1 and Hop 2. Figures 7 and 8 indicate the results of measured BER performance, as well as the obtained insert of it, which represents the corresponding eye diagrams when the data rate is varied. The PRBS pattern length of the OOK signal is set as 2 −1. As illustrated in Figure 6(a), the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The max- −3 imum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8 × 10 ), which can fully demonstrate the great potential of the micro-LED used in the high-speed VLC communication system. As a result, the maximum data of the first blue micro-LED can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the Photonics 2022, 9, x FOR PEER REVIEW 7 of 11 Photonics 2022, 9, 211 7 of 11 Figure 5. Signal observed on oscilloscope after (a) Tx.1, (b) Rx.1, (c) amplifier, (d) Rx.2. 3.3. 3.3. Co Communication-Performance mmunication-Performance E Evaluations valuations In this section, an evaluation of the communication performance of the dual-hop sys- In this section, an evaluation of the communication performance of the dual-hop sys- tem is given. In order to better understand the influence of the communication perfor- tem is given. In order to better understand the influence of the communication performance of mance o the dual-hop f the dual-hop system, sy a comparison stem, a comp is made arisonwith is made w the single-hop ith the single system. -hop The syfr stem. The equency r frequenc esponse y re andsponse an BER performance d BER per wer formanc e experimentally e were experimentally tested. tested. The normalized frequency response of the micro-LED-based VLC system at different The normalized frequency response of the micro-LED-based VLC system at different links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics links is shown in Figure 6, which illustrates the modulation-bandwidth characteristics un- under different situations. Without any hardware-equalization technique, it can reach der different situations. Without any hardware-equalization technique, it can reach 880 880 MHz in the single-hop link, which presents this blue micro-LED has great potential MHz in the single-hop link, which presents this blue micro-LED has great potential as as high-speed communication application. Moreover, the values of 3-dB modulation high-speed communication application. Moreover, the values of 3-dB modulation band- bandwidth decrease to 715 MHz with the relay node. It is easy to see that modulation width decrease to 715 MHz with the relay node. It is easy to see that modulation band- bandwidth only slightly decreases after the relay nodes and longer distance transmission. width only slightly decreases after the relay nodes and longer distance transmission. Figure Figure 6. 6. The The comparison of comparison of measur measured normalized fr ed normalized frequency equency response fr response from om Hop 1 and Hop 1 and Hop Hop 2. 2. Figures 7 and 8 indicate the results of measured BER performance, as well as the Figures 7 and 8 indicate the results of measured BER performance, as well as the obtained insert of it, which represents the corresponding eye diagrams when the data rate obtained insert of it, which represents the corresponding eye diagrams when the data rate is varied. The PRBS pattern length of the OOK signal is set as 2 1. As illustrated in is varied. The PRBS pattern length of the OOK signal is set as 2 −1. As illustrated in Figure Figure 6, the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The 6(a), the different OOK modulations of 1.0 Gbps to 2.1 Gbps are applied on Tx.1. The max- maximum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8  10 ), −3 imum data rate is 2.0 Gbps within the forward error correction (FEC) limit (3.8 × 10 ), which can fully demonstrate the great potential of the micro-LED used in the high-speed which can fully demonstrate the great potential of the micro-LED used in the high-speed VLC communication system. As a result, the maximum data of the first blue micro-LED VLC communication system. As a result, the maximum data of the first blue micro-LED can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the can reach 2 Gbps in 2 m free-space transmission at the FEC limit. Meanwhile, through the same 2 m free-space channel and lens, the values of the received optical power are also measured, which remain relatively the same: 67 W in Figure 7 while 63 W in Figure 8. Then, the maximum modulated data of Tx.2 is likewise performed. In the experiment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, which is smaller than the single-hop link. After the AF method, the transmitted optical power of the relay node is almost the same. The change of Vpp also confirms this, which is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and 1.1 Gbps are achievable within the FEC limit, respectively. Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 Photonics 2022, 9, x FOR PEER REVIEW 8 of 11 Photonics 2022, 9, 211 8 of 11 same 2 m free-space channel and lens, the values of the received optical power are also same 2 m free-space channel and lens, the values of the received optical power are also measured, which remain relatively the same: 67 μW in Figure 7 while 63 μW in Figure 8. measured, which remain relatively the same: 67 μW in Figure 7 while 63 μW in Figure 8. Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding eye Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding Figure 7. Measurement of real-time BER with OOK format from Hop 1. (Inset: the corresponding diagrams at the data rates of 1.0 Gbps and 1.5 Gbps). eye diagrams at the data rates of 1.0 Gbps and 1.5 Gbps.) eye diagrams at the data rates of 1.0 Gbps and 1.5 Gbps.) Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding eye Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding Figure 8. Measurement of real-time BER with OOK format from Hop 2. (Inset: the corresponding diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps). eye diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps.) eye diagrams at the data rates of 500 Mbps, 1.0 Gbps, and 1.2 Gbps.) In order to be more intuitive, Table 2 summarizes some measured indicators related to Then, the maximum modulated data of Tx.2 is likewise performed. In the experi- Then, the maximum modulated data of Tx.2 is likewise performed. In the experi- communication performance. Under the premise of the extended distance of the dual-hop ment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second ment, the different OOK modulations of 500 Mbps to 1.3 Gbps are applied on the second VLC system which increases from 2 m to 4 m, it is noticeable that some of the communi- blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, blue micro-LED. The maximum data rate corresponding to the FEC limit is 1.1 Gbps, cation performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 mV, which is smaller than the single-hop link. After the AF method, the transmitted optical which is smaller than the single-hop link. After the AF method, the transmitted optical which is relatively the same due to the AF method and the same bias current. However, power of the relay node is almost the same. The change of Vpp also confirms this, which power of the relay node is almost the same. The change of Vpp also confirms this, which the AF method will inevitably introduce synchronous amplification of noise; the noise is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and is from 137.6 mV to 112.0 mV. Therefore, for single-hop and dual-hop links, 2.0 Gbps and and long-distance attenuation represent a greater impact in the dual-hop link following an 1.1 Gbps are achievable within the FEC limit, respectively. 1.1 Gbps are achievable within the FEC limit, respectively. inevitable reduction in the modulation bandwidth and the maximum data rate. There is an In order to be more intuitive, Table 2 summarizes some measured indicators related In order to be more intuitive, Table 2 summarizes some measured indicators related admissible decrease in modulation bandwidth between 880 MHz and 715 MHz. Moreover, to communication performance. Under the premise of the extended distance of the dual- to communication performance. Under the premise of the extended distance of the dual- the data rate stabilizes at the order of Gbps and the relay node in the real-time dual-hop hop VLC system which increases from 2 m to 4 m, it is noticeable that some of the com- hop VLC system which increases from 2 m to 4 m, it is noticeable that some of the com- VLC system expands the scope of the limited area, tackling the NLOS problem effectively. munication performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 munication performance is changed. The value of Vpp goes down from 137.6 mV to 112.0 Therefore, the cost of the relay node is acceptable. To the best of our knowledge, this is mV, which is relatively the same due to the AF method and the same bias current. How- mV, which is relatively the same due to the AF method and the same bias current. How- the first experimental demonstration of a real-time dual-hop link based on a micro-LED ever, the AF method will inevitably introduce synchronous amplification of noise; the ever, the AF method will inevitably introduce synchronous amplification of noise; the that could provide a potential solution to tackle the NLOS problem of indoor VLC. In this noise and long-distance attenuation represent a greater impact in the dual-hop link fol- noise and long-distance attenuation represent a greater impact in the dual-hop link fol- work, the modulation signal with an OOK format is used to complete the experimental lowing an inevitable reduction in the modulation bandwidth and the maximum data rate. lowing an inevitable reduction in the modulation bandwidth and the maximum data rate. verification of our proposed dual-hop VLC system based on blue micro-LEDs. In future There is an admissible decrease in modulation bandwidth between 880 MHz and 715 There is an admissible decrease in modulation bandwidth between 880 MHz and 715 work, higher-order modulation formats can be considered to obtain higher data rates. Photonics 2022, 9, x FOR PEER REVIEW 9 of 11 MHz. Moreover, the data rate stabilizes at the order of Gbps and the relay node in the real-time dual-hop VLC system expands the scope of the limited area, tackling the NLOS problem effectively. Therefore, the cost of the relay node is acceptable. To the best of our knowledge, this is the first experimental demonstration of a real-time dual-hop link based on a micro-LED that could provide a potential solution to tackle the NLOS problem of indoor VLC. In this work, the modulation signal with an OOK format is used to complete the experimental verification of our proposed dual-hop VLC system based on blue micro- Photonics 2022, 9, 211 9 of 11 LEDs. In future work, higher-order modulation formats can be considered to obtain higher data rates. Table 2. Communication-performance comparison from Hop 1 and Hop 2. Table 2. Communication-performance comparison from Hop 1 and Hop 2. Link Distance Vpp Bandwidth Data rate (FEC) Link Distance Vpp Bandwidth Data Rate (FEC) Hop 1 2.0 m 137.6 mV 880 MHz 2.0 Gbps Hop 1 2.0 m 137.6 mV 880 MHz 2.0 Gbps Hop 2 4.0 m 112.0 mV 715 MHz 1.1 Gbps Hop 2 4.0 m 112.0 mV 715 MHz 1.1 Gbps 3.4. The Impact of Transmission Distance 3.4. The Impact of Transmission Distance To further understand the impact of transmission distance and prove that our pro- To further understand the impact of transmission distance and prove that our proposed posed system can be applied to more scenarios, the measurement results at different dis- system can be applied to more scenarios, the measurement results at different distances tances are discussed in this section. Except for the change of communication distance, the are discussed in this section. Except for the change of communication distance, the system system parameters are consistent with Table 1. parameters are consistent with Table 1. First, to have a better understanding of the distance impact to the received signal First, to have a better understanding of the distance impact to the received signal based on the micro-LED, we can measure the corresponding received optical power and based on the micro-LED, we can measure the corresponding received optical power and Vpp in the free-space transmission of 2 to 4 m. As we can see from Figure 9, the received Vpp in the free-space transmission of 2 to 4 m. As we can see from Figure 9, the received optical power is 16, 10, 8 μW at 2, 3, 4 m. The Vpp of the received signal is proportional to optical power is 16, 10, 8 W at 2, 3, 4 m. The Vpp of the received signal is proportional to the received optical power. At the data rate of 1.5 Gbps, the eye diagrams of 2 m and 4 m the received optical power. At the data rate of 1.5 Gbps, the eye diagrams of 2 m and 4 m are given, respectively. It can be seen that the communication performance of a single-hop are given, respectively. It can be seen that the communication performance of a single-hop system will deteriorate with the increase in transmission distance. system will deteriorate with the increase in transmission distance. Figure 9. Measured received power and Vpp under the different free-space transmission lengths of Figure 9. Measured received power and Vpp under the different free-space transmission lengths of 2 m to 4 m (Inset: the eye diagram at the data rate of 1.5 Gbps at 2 m and 4 m). 2 m to 4 m (Inset: the eye diagram at the data rate of 1.5 Gbps at 2 m and 4 m). Then, the relationship between distance and data rate is analyzed. The comparison Then, the relationship between distance and data rate is analyzed. The comparison between Hop 1 and Hop 2 at different communication distances is given, which is shown between Hop 1 and Hop 2 at different communication distances is given, which is shown in Figure 10. In the single-hop system, the data rates within FEC are 2 Gbps in the cases of in Figure 10. In the single-hop system, the data rates within FEC are 2 Gbps in the cases 2 m and 3 m transmission distance, and the BER performance of 3 m is slightly increased of 2 m and 3 m transmission distance, and the BER performance of 3 m is slightly increased with the increase in distance. When the distance increases to 4 m, the maximum data rate of with the increase in distance. When the distance increases to 4 m, the maximum data rate the single-hop system decreases to 1.6 Gbps. In the dual-hop system, the data rates within of the single-hop system decreases to 1.6 Gbps. In the dual-hop system, the data rates FEC are 1.1 Gbps, 1.1 Gbps, and 1 Gbps at the length of 2 m to 4 m. It can be seen that within FEC are 1.1 Gbps, 1.1 Gbps, and 1 Gbps at the length of 2 m to 4 m. It can be seen when the single-hop communication performance is not limited by distance, the dual-hop communication performance also remains relatively stable with the increase in distance, which can further prove the potential application indoors. Photonics 2022, 9, x FOR PEER REVIEW 10 of 11 that when the single-hop communication performance is not limited by distance, the dual- Photonics 2022, 9, 211 10 of 11 hop communication performance also remains relatively stable with the increase in dis- tance, which can further prove the potential application indoors. Figure 10. Measurement of maximum data rate within FEC limit at different lengths (The blue font Figure 10. Measurement of maximum data rate within FEC limit at different lengths (The blue font on the histogram represents the BER for the corresponding case). on the histogram represents the BER for the corresponding case). 4. Conclusions 4. Conclusions For the indoor environment, the LOS VLC system could not always have been guaran- For the indoor environment, the LOS VLC system could not always have been guar- teed due to many effects, but it can greatly influence the communication quality. To address anteed due to many effects, but it can greatly influence the communication quality. To this unavoidable problem and have high-speed communication simultaneously, a dual-hop address this unavoidable problem and have high-speed communication simultaneously, VLC system based on a blue micro-LED was proposed and experimentally demonstrated. a dual-hop VLC system based on a blue micro-LED was proposed and experimentally The deployment of relay nodes can ensure high-speed communication indoors and solve demonstrated. The deployment of relay nodes can ensure high-speed communication in- the NLOS problem. To the best of our knowledge, this was the first time that a dual-hop doors and solve the NLOS problem. To the best of our knowledge, this was the first time VLC link was based on a micro-LED. In addition, in order to better analyze the cost and that a dual-hop VLC link was based on a micro-LED. In addition, in order to better analyze benefit of the relay node, a comparison between the single-hop and the dual-hop link was the cost and benefit of the relay node, a comparison between the single-hop and the dual- presented. On the premise of increasing the communication distance from 2 m to 4 m, there hop link was presented. On the premise of increasing the communication distance from 2 was an admissible decrease in modulation bandwidth between 880 MHz and 715 MHz. m to 4 m, there was an admissible decrease in modulation bandwidth between 880 MHz Meanwhile, the data rate corresponding to the FEC limit remained in the order of Gbps. and 715 MHz. Meanwhile, the data rate corresponding to the FEC limit remained in the The result demonstrated a dual-hop VLC system based on blue micro-LEDs with a data order of Gbps. The result demonstrated a dual-hop VLC system based on blue micro- rate up to 1.1 Gbps through simple OOK modulation over a 4 m NLOS free-space link, with LEDs with a data rate up to 1.1 Gbps through simple OOK modulation over a 4 m NLOS the help of a low-cost AF relay. In addition, when the overall link distance increases from free-space link, with the help of a low-cost AF relay. In addition, when the overall link 4 m to 8 m, the dual-hop system is still capable of high-speed communication. The data rate distance increases from 4 m to 8 m, the dual-hop system is still capable of high-speed within the FEC limit at 8 m is 1 Gbps. The available 8 m transmission distance exceeded communication. The data rate within the FEC limit at 8 m is 1 Gbps. The available 8 m the LOS distance for the indoor LOS link, which can be applied to more indoor scenarios. transmission distance exceeded the LOS distance for the indoor LOS link, which can be Experimental results confirmed that the dual-hop VLC system based on micro-LEDs can applied to more indoor scenarios. Experimental results confirmed that the dual-hop VLC tackle critical NLOS problems in VLC systems and is sufficient for high-speed indoor system based on micro-LEDs can tackle critical NLOS problems in VLC systems and is application in the future. sufficient for high-speed indoor application in the future. Author Contributions: Conceptualization, Z.W. (Zixian Wei) and Y.Z.; methodology, Z.W. (Zixian Author Contributions: Conceptualization, Z.W. (Zixian Wei) and Y.Z.; methodology, Z.W. (Zixian Wei); validation, Z.W. (Zixian Wei) and Y.Z.; formal analysis, Y.Z.; investigation, Y.Z.; resources, Z.W. Wei); validation, Z.W. (Zixian Wei) and Y.Z.; formal analysis, Y.Z.; investigation, Y.Z.; resources, (Zixian Wei) and Zhang, Y; data curation, Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing— Z.W. (Zixian Wei) and Zhang, Y; data curation, Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); original draft preparation, Y.Z., Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing—review writing—original draft preparation, Y.Z., Z.W. (Zixian Wei) and Z.W. (Zhaoming Wang); writing— and editing, H.Y.F.; visualization, Y.Z.; supervision, H.Y.F.; project administration, H.Y.F.; funding review and editing, H.Y.F.; visualization, Y.Z.; supervision, H.Y.F.; project administration, H.Y.F.; acquisition, H.Y.F. All authors have read and agreed to the published version of the manuscript. funding acquisition, H.Y.F. All authors have read and agreed to the published version of the man- Funding: uscript. This work is supported by Shenzhen Technology and Innovation Council (WDZC202008 20160650001). Funding: This work is supported by Shenzhen Technology and Innovation Council Institutional (WDZC20200820160650001). Review Board Statement: Not applicable. Acknowledgments: Institutional Review Board Statement: The authors thank their Not applicable colleagues. Lei Wang, and Lai Wang from Department of Electronic Engineering, Tsinghua University, for their kind help toward research work. Conflicts of Interest: The authors declare no conflict of interest. Photonics 2022, 9, 211 11 of 11 References 1. Jovicic, A.; Li, J.; Richardson, T. Visible light communication: Opportunities, challenges and the path to market. IEEE Commun. Mag. 2013, 51, 26–32. [CrossRef] 2. Grobe, L.; Paraskevopoulos, A.; Hilt, J.; Schulz, D.; Lassak, F.; Hartlieb, F.; Kottke, C.; Jungnickel, V.; Langer, K.D. High-speed visible light communication systems. IEEE Commun. Mag. 2013, 51, 60–66. [CrossRef] 3. Chi, N.; Zhou, Y.; Wei, Y.; Hu, F. Visible light communication in 6G: Advances, challenges, and prospects. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Mar 23, 2022

Keywords: visible-light communication (VLC); micro-LED; dual-hop transmission; non-line-of-sight (NLOS); receive-forward

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