Light Communication System Research Articles

Deterministic Fabrication and Quantum-Well Modulation of Phase-Pure 2D Perovskite Heterostructures for Encrypted Light Communication.

Deterministic integration of phase-pure Ruddlesden-Popper (RP) perovskites has great significance for realizing functional optoelectronic devices. However, precise fabrications of artificial perovskite heterostructures with pristine interfaces and rational design over electronic structure configurations remain a challenge. Here, the controllable synthesis of large-area ultrathin single-crystalline RP perovskite nanosheets and the deterministic fabrication of arbitrary 2D vertical perovskite heterostructures are reported. The 2D heterostructures exhibit intriguing dual-peak emission phenomenon and dual-band photoresponse characteristic. Importantly, the interlayer energy transfer behaviors from wide-bandgap component to narrow-bandgap component modulated by comprising quantum wells are thoroughly revealed. Functional nanoscale photodetectors are further constructed based on the 2D heterostructures. Moreover, by combining the modulated dual-band photoresponse characteristic with double-beam irradiation modes, and introducing an encryption algorithm mechanism, a light communication system with high security and reliability is achieved. This work can greatly promote the development of heterogeneous integration technologies of 2D perovskites, and could provide a competitive candidate for advanced integrated optoelectronics.

Back-contact perovskite light-emitting diodes

Light-emitting diodes utilizing halide perovskites have experienced rapid advancements in recent years, demonstrating notable external quantum efficiencies. Despite these strides, the practical implementation of such devices remains constrained. In this contribution, we are dedicated to developing perovskite light-emitting diodes with a back-contact architecture using the MAPbBr3 active layer and SnO2 and Ni/NiOx back electrodes. The quantum efficiency of the fabricated devices stands at 0.015%. The operational voltage of the light-emitting diodes is characterized by its pronounced low values, attaining a maximum luminance of 70 cd/m2 at a mere 3.2 V. These results demonstrate the considerable promise of the developed back-contact perovskite light-emitting diodes for prospective applications in advanced display technologies and light communication systems.

Monolithic III-nitride photonic circuit on a single chip

Inserting multiple quantum wells (MQWs) into a p–n junction, III-nitride MQW diodes can separately function as a light transmitter, modulator, and receiver under different bias conditions. Owing to the spectral overlap between the emission and responsivity spectra, the emitted light from the transmitter is able to be modulated and detected by the modulator and receiver, which have identical MQW structures. Here, we develop a compatible fabrication process to monolithically integrate an III-nitride light transmitter, waveguides, Y-splitter, modulators, Y-combiner, and receiver into a tiny chip. An on-chip 405 nm light communication system is established and exhibits a transmission rate of 260 Mbps in the non-return-to-zero on-off keying scheme. The results pave a feasible route to develop sophisticated monolithic photonic circuit on an III-nitride-on-silicon platform.

Boosting photodetection performance of Cs2AgBiBr6 through A-site Rb substitution and interfacial engineering

Bismuth-based double perovskite Cs2AgBiBr6 shows promise as a photodetection material. However, its detection performance and application are limited by high-exciton binding energy and poor carrier mobility. In this study, we address these limitations by delicately designing a solution-based method for incorporating A-site Rubidium (Rb) substitution into Cs2AgBiBr6 double perovskite films. The introduction of Rb resulted in a significant decrease in trap defect density and an improvement in film quality. The trap-filled limit voltage (VTFL) of pure and Rb-doped CABB film is determined to be 1.71 V and 0.48 V, respectively. Subsequently, by introducing an ultrathin atomic-layer-deposited (ALD) TiO2 films, the fabricated CABB photodetectors exhibit significantly improved photoresponse performance. The response speed and −3dB bandwidth are boosted from ∼93 ms to ∼350 μs and broadened from 1.4 kHz to 17 kHz, respectively. Density Functional Theory (DFT) calculations indicate Rb-substitution shortens the bond length and weaken exciton binding energy. Furthermore, we demonstrate a wireless near ultraviolet (UV) light communication system using CABB photodetectors as light receivers. Our findings provide an efficient approach to utilize A-site cation substitution as a tuning parameter for photodetection in high-exciton binding energy perovskite materials, thereby extending the potential applications of other functional perovskites.

2D Ruddlesden-Popper Sn-Based Perovskite Weak Light Detector for Image Transmission and Reflection Imaging.

2D Ruddlesden-Popper Sn-based perovskite has excellent optoelectronic properties and weak halide ion migration characteristics, making it an ideal candidate for weak light detection, which has great potential in light communication, and medical applications. Although Sn-based perovskite photodetectors are developed, weak light detection is not demonstrated yet. Herein, a high-performance self-powered photodetector with the capability to detect ultra-weak light signals is designed based on vertical PEA2 SnI4 /Si nanowires heterojunction. Due to the low dark current and high light absorption efficiency, the devices present a remarkable responsivity of 42.4mA W-1 , a high detectivity of 8 × 1011 Jones, and an ultralow noise current of 2.47 × 10-13 A Hz-1/2 . Especially, the device exhibits a high on-off current ratio of 18.6 at light signals as low as 4.60 nW cm-2 , revealing the capacity to detect ultra-weak light. The device is applied as a signal receiver and realized image transmission in light communication system. Moreover, high-resolution reflection imaging and multispectral imaging are obtained using the device as the sensor in the imaging system. These results reveal that 2D PEA2 SnI4 -based self-powered photodetectors with low-noise current possess enormous potential in future weak light detection.

Data Transfer Using Li-fi

Light-based wireless communication, abbreviated as Li-Fi or Li-Fi, uses light rather than radio waves to send data from one device to another. This technology relies on visible light communication (VLC) to transmit information through light-emitting diodes (LEDs). By modulating the intensity of the light, Li-Fi enables high-speed data transfer and can be used in various applications such as indoor positioning systems and internet connectivity in areas with high radio frequency interference. This concept was first presented by Harald Haas at a TED Global event in Edinburgh in 2011. Li-Fi, short for light-fidelity, is a light communication system that can send and receive data rapidly across the visible, ultraviolet, and infrared light spectrums. Only light-emitting diode (LED) bulbs are currently capable of transmitting data in the visible spectrum. Similar to Wi-Fi from the perspective of the end user, the technology differs primarily in that Wi-Fi transmits data by inducing a voltage in an antenna through radio waves, whereas Li-Fi does so through the modulation of light intensity. Li-Fi can operate in environments where other wireless technologies would be disrupted by electromagnetic noise (e.g.aircraft cabins, hospitals, or the military).

DH-PIM İletim Yöntemi için Bir Hata Düzeltme Yapısı

Yapılan çalışmada optik haberleşme sistemleri için önerilen DH-PIM yöntemi için hata düzeltme yapılarının performans analizi incelenmiştir. Silme ve Yanlış Alarm hataları için uygulanabilir bir sistem modeli Görünür Işık Haberleşme (Visible Light Communication: VLC) sistemlerine entegre edebilmek için elde edilmiştir. Yapılan çalışmada model için terorik bir analiz verilmiştir. Silme hatası yapısı için bir FPGA (Field Programmable Gate Arrays: Alanda Programlanabilir Kapı Dizileri) derleyicisi Quartus programında yeni bir sayısal devre tabanlı mimari geliştirilmiştir. Sonuçlar Modelsim-Altera Programında zamana göre incelenmiştir. Alınan sonuçlara göre, önerilen sayısal devre tabanlı hata düzeltme yapısının başarılı bir şekilde hatalı slotları düzeltebildiği görülmüştür. Ayrıca teorik ifadesi elde edilen hem silme hatası hem de yanlış alarm hatası için slot hata oranı performansları incelenmiştir. Elde edilen sonuçlara göre geleneksel DH-PIM (G-DHPIM) sistemine göre sadece silme hatası uygulanan sistem (S-DHPIM) daha iyi bir slot hata performansı vermiştir. Ayrıca, hem silme hatası hem de yanlış alarm hatası uygulanan DH-PIM sisteminin (SY-DHPIM) de S-DHPIM göre daha iyi bir slot hata performansı verdiği izlenmiştir.

Orthogonal Subblock Division Multiple Access for OFDM-IM-Based Multi-User VLC Systems

In this paper, we propose and experimentally demonstrate an orthogonal subblock division multiple access (OSDMA) scheme for orthogonal frequency division multiplexing with index modulation (OFDM-IM)-based multi-user visible light communication (MU-VLC) systems, where both single-mode index modulation (SM-IM) and dual-mode index modulation (DM-IM) are considered. In order to overcome the low-pass frequency response and the light-emitting diodes (LED) nonlinearity issues of practical MU-VLC systems, OSDMA is employed together with discrete Fourier transform spreading (DFT-S) and interleaving. The feasibility and superiority of the proposed scheme have been successfully verified via both simulations and hardware experiments. More specifically, we evaluate and compare the peak-to-average power ratio (PAPR) performance and the bit error rate (BER) performance of OFDM-SM-IM, DFT-S-OFDM-SM-IM, OFDM-DM-IM and DFT-S-OFDM-DM-IM without and with interleaving. Experimental results show that remarkable distance extensions can be achieved by employing DFT spreading and interleaving for both SM-IM and DM-IM in a two-user OSDMA-VLC system.

Predistortion Approaches Using Coefficient Approximation and Bidirectional LSTM for Nonlinearity Compensation in Visible Light Communication

A Light-Emitting Diode (LED) has a nonlinear characteristic, and it contains fundamental limitations for the performance of Visible Light Communication (VLC) systems in indoor environments when using intensity modulation with Orthogonal Frequency Division Multiplexing (OFDM). In this paper, we investigate this nonlinear characteristic with analysis and proposal. At first, we identified the LED nonlinear characteristics in terms of bit-error performances. After analysis, we propose initial predistortion schemes to mitigate the nonlinearity matters. In the predistortion schemes, the nonlinear distortion compensation model contains predistortion features with the LED inverse characteristics. Considering a Direct-Current-biased Optical OFDM (DCO-OFDM) system, we compared the Bit-Error Rate (BER) performances with and without compensation via simulations. The performance on the LED with the compensation showed LED nonlinearity could significantly improve the bit-error performance. In addition, with consideration that the predistortion model is insufficient to represent LED distortion, we investigated possible opportunities of distortion correction using Bidirectional Long Short-Term Memory (BLSTM), one of the leading deep learning approaches. Its result showed promising improvement of the distortion compensation as well.

Transfer Learning–Based Artificial Neural Networks Post-Equalizers for Underwater Visible Light Communication

In this article, we demonstrate two transfer learning–based dual-branch multilayer perceptron post-equalizers (TL-DBMLPs) in carrierless amplitude and phase (CAP) modulation-based underwater visible light communication (UVLC) system. The transfer learning algorithm could reduce the dependence of artificial neural networks (ANN)–based post-equalizer on big data and extended training cycles. Compared with DBMLP, the TL-DBMLP is more robust to the jitter of the bias current (Ibias) of light-emitting diode (LED), which indicates that TL-DBMLP does not require further training in Ibias varying UVLC system. In terms of voltage peak-to-peak (Vpp) varying VLC system, DBMLP requires a training set with a size of more than 105 and 50 training epochs, which quantitatively prove the effectiveness of DBMLP in reducing reliance on large amount of training epochs. On the counterpart, the TL-DBMLP only requires a training set with a size of less than 2×104 and 10 training epochs, which quantitatively prove the effectiveness of DBMLP in reducing reliance on big data. Finally, we experimentally demonstrate that transfer learning can effectively reduce ANN dependence on extensive size training data and large amount of training epochs, whether in VLC systems with varying Ibias and varying Vpp.

Deep Learning-Assisted Index Estimator for Generalized LED Index Modulation OFDM in Visible Light Communication

In this letter, we present the first attempt of active light-emitting diode (LED) indexes estimating for the generalized LED index modulation optical orthogonal frequency-division multiplexing (GLIM-OFDM) in visible light communication (VLC) system by using deep learning (DL). Instead of directly estimating the transmitted binary bit sequence with DL, the active LEDs at the transmitter are estimated to maintain acceptable complexity and improve the performance gain compared with those of previously proposed receivers. Particularly, a novel DL-based estimator termed index estimator-based deep neural network (IE-DNN) is proposed, which can employ three different DNN structures with fully connected layers (FCL) or convolution layers (CL) to recover the indexes of active LEDs in a GLIM-OFDM system. By using the received signal dataset generated in simulations, the IE-DNN is first trained offline to minimize the index error rate (IER); subsequently, the trained model is deployed for the active LED index estimation and signal demodulation of the GLIM-OFDM system. The simulation results show that the IE-DNN significantly improves the IER and bit error rate (BER) compared with those of conventional detectors with acceptable run time.

Recent Advances in 2D Materials for Photodetectors

AbstractPhotodetection technology has been systematically studied due to wide practical applications in temperature monitoring, thermal image technology, and light communication systems. To date, photodetectors based on multitudes of 2D materials have been reported because of their excellent performance. On account of their novel physical properties with ultrathin thickness, cost‐effective preparation with mechanical transfer process, natural passivated surface without dangling bonds, various bandgaps corresponding with a wide photoresponse, and so on, new 2D materials emerge to play significant roles in the field of photodetection. In this regard, a great advance has been achieved in terms of preparation and device application, especially in the last decade. However, there are still some challenges to obtain high‐performance photodetectors, such as growing high‐quality 2D materials, achieving higher quantum efficiency, effectively separating the photogenerated electron–hole pairs, and so on. In this review, the recent development of the state‐of‐the‐art photodetection composed of 2D materials is summarized. Moreover, the key parameters and mechanisms in photodetectors are highlighted, and an overview on 2D materials and their heterostructures is provided. Finally, the strategies for improving the performance of photodetectors are also highlighted. The review will provide a guide to further practical applications in photodetection devices.

Comparison of PD-NOMA for RF and VLC Based Vehicular Communication Under Various Weather Conditions

In order to provide vehicles with reliable, ubiquitous, and massive connectivity, an appropriate multiple access (MA) scheme should be adopted. An appealing MA scheme referred to as non-orthogonal multiple access (NOMA) has been gaining significant attention in vehicular networks among academia and industry. This work aims to show the technical feasibility of uplink vehicular-VLC system (V-VLC) using OPD-NOMA, which is a promising technique for future intelligent transportation system which has not been explored before. We aim to present a comprehensive qualitative and quantitative analysis on the performance of Optical Power Domain-NOMA (OPD-NOMA) based Vehicular Visible Light Communication (V-VLC) systems. In particular, we aim for designing V-VLC systems with robust information exchange over various weather conditions such as rain, light fog, dense fog, dry snow, etc. We make use of various analytical tools of stochastic geometry to derive theoretical expression for outage probability and throughput by modelling the location of the vehicle as a spatial Poisson point process. We compare the performance of OPD-NOMA based V-VLC with Vehicular Radio Frequency (V-RF) communication for three different traffic scenarios (i.e., sparse, medium, and dense traffic) in terms of outage probability and throughput. Our numerical results reveal that instead of viewing V-VLC as competing technology, it can be visualized as complementary technology to V-RF technology under different environmental conditions for future Intelligent Transportation System (ITS) to meet diverse requirements of 5G networks and beyond.

Spectral Efficient Asymmetrically Clipped Hybrid FBMC for Visible Light Communication

Filter bank multicarrier (FBMC) modulation has shown sufficient potential for wireless communication. A hybrid optical FBMC technique is proposed to improve the spectral efficiency of a visible light communication (VLC) system. In this technique, a hybrid asymmetrically clipped optical offset quadrature amplitude modulation FBMC (HACO-OQAM-FBMC) modulation technique is used. Asymmetrically clipped optical FBMC (ACO-FBMC) is used for odd subcarriers, and pulse amplitude modulation-discrete multitone (PAM-DMT) is used for the even subcarriers. The proposed hybrid scheme uses an intensity modulation/direct detection (IM/DD) channel. It is shown that there is no interference on odd subcarriers using the proposed method and receiver demodulation is similar to that of ACO-FBMC receiver. However, clipping noise of ACO-FBMC falls on PAM-DMT subcarriers, which can be cancelled at receiver processing after estimation. The analytical performance of the proposed technique is compared using parameters, namely, bit error rate (BER), spectral efficiency, computational complexity, and peak to average power ratio (PAPR). It is found that HACO-OQAM-FBMC is more spectral efficient than ACO-FBMC and other OFDM-based techniques.

HFR Projector Camera Based Visible Light Communication System for Real-Time Video Streaming.

This study develops a projector–camera-based visible light communication (VLC) system for real-time broadband video streaming, in which a high frame rate (HFR) projector can encode and project a color input video sequence into binary image patterns modulated at thousands of frames per second and an HFR vision system can capture and decode these binary patterns into the input color video sequence with real-time video processing. For maximum utilization of the high-throughput transmission ability of the HFR projector, we introduce a projector–camera VLC protocol, wherein a multi-level color video sequence is binary-modulated with a gray code for encoding and decoding instead of pure-code-based binary modulation. Gray code encoding is introduced to address the ambiguity with mismatched pixel alignments along the gradients between the projector and vision system. Our proposed VLC system consists of an HFR projector, which can project 590 × 1060 binary images at 1041 fps via HDMI streaming and a monochrome HFR camera system, which can capture and process 12-bit 512 × 512 images in real time at 3125 fps; it can simultaneously decode and reconstruct 24-bit RGB video sequences at 31 fps, including an error correction process. The effectiveness of the proposed VLC system was verified via several experiments by streaming offline and live video sequences.

Spherical Quasi-Physical Model-Based Color-Shift Keying for Visible Light Communication

In this paper, the new family of spherical color-shift keying (SCSK) constellations is proposed for visible light communication (VLC) systems. Specifically, the constant-intensity constraint (CIC) for conventional color-shift keying (CSK) schemes, which originates from the requirement of non-flickering illumination, confines the constellation symbols on a two-dimensional (2D) plane of the three-dimensional (3D) intensity space. To fully utilize the potential degree of freedom offered by the 3D intensity space, we propose to expand the envelope of CSK constellations by replacing the constant intensity with a new spherical intensity constraint, aiming to maximize the minimum Euclidean distance (MED). More explicitly, we design the robust SCSK constellations with and without white color balance (CB), respectively. Particularly, we transform the constellation design problem to the so-called spherical circle packing (SCP) problem, which is resolved based on the spherical quasi-physical (SQP) model. Then, we propose the so-called spherical heuristic quasi-physical algorithm (SHQPA) integrating the binary search algorithm (BSA) and the configuration update algorithms (CUA) to obtain the optimal SCSK constellations. Furthermore, we also study the corresponding bit-to-symbol mapping rules for the generated SCSK constellations. Simulation results show that the new family of SCSK constellations outperforms the conventional CSK counterparts.

HyperSight: A Precise Decoding Algorithm for VLC With Mobile-Phone Camera

In this paper, we propose and experimentally demonstrate HyperSight, which is a precise image decoding algorithm for the mobile-phone camera based visible light communication (VLC) system. The conventional algorithms have to select an appropriate column matrix, and then decode the image according to the fitting curve. The selection of the column matrix and the fitting of the function are not only time-consuming but also error-prone, leading to low decoding efficiency and high bit error rate (BER). While HyperSight calculates characteristic grayscale matrix to avoid column matrix selection and uses critical grayscale to replace the function fitting to decode the image. As expected, our algorithm not only reduces the BER, but also improves the processing performance. The experimental results show that, the BER can be reduced by two orders of magnitude on average compared with the conventional scheme. For the $4.8\,$ kbps data rate, our algorithm can achieve the transmission distance of $50\,$ cm under the $7\%$ forward error correction (FEC) threshold with the low illuminance of $702\,$ lux.

A speed-optimized, low-noise APD with 0.18 μm CMOS technology for the VLC applications

A new avalanche photodiode device applied to a visible light communication (VLC) system is designed using a standard 0.18 [Formula: see text]m complementary metal oxide semiconductor process. Compared to regular CMOS APD devices, the proposed device adds a [Formula: see text]-well layer above the deep [Formula: see text]-well/[Formula: see text]-substrate structure, and an [Formula: see text]/[Formula: see text] layer is deposited upon it. The [Formula: see text]/[Formula: see text] layer acts as an avalanche breakdown layer of the device, and an STI structure is used to prevent the edge break prematurely. The simulation results shows that the avalanche breakdown voltage is as low as 9.9 V, dark current is below [Formula: see text] A under −9.5 V bias voltage, and the 3 dB bandwidth is of 5.9 GHz. It is due to the use of the 0.18 [Formula: see text]m CMOS process-specific STI protection ring and short-circuits the connection of the deep [Formula: see text]-well/[Formula: see text]-substrate, and the dark current is reduced to be lower than two orders of magnitude compared to regular CMOS APD. Gain and noise characteristics are accurately calculated from Hayat dead-space model applied to this CMOS APD. So, this device’s gain and excess noise factor are 20 and 2.5, respectively.

A Novel Non-DC Biased Intensity Modulated Indoor MIMO-VLC System Using Walsh Precoder

In this paper, we propose an indoor visible light communication (VLC) system which makes use of Walsh precoding and multiple light-emitting diodes (LEDs) at the transmitter for multiplexing the data. The proposed VLC system is based on intensity modulation and uses the notion of spatial modulation for communicating the positive and negative values of the synthesized transmit signal using signal processing technique at the transmitter. We also illustrate the flexibility, ability, and easiness of the system to configure well in a multiuser environment. We have also developed a near-maximum likelihood (ML) detection technique for efficiently decoding the data bits at the receiver. The proposed near-ML reduces the search space involved in ML technique by using the initial ZF solution. The results of the simulation studies illustrate that the proposed technique offers a higher data rate and good bit error rate (BER) performance in indoor VLC environments.

Pre-Distorted Enhanced ADO-OFDM for Hybrid VLC Networks: A Mutual-Interference-Free Approach

Mutual interference is a crucial issue for the visible light communication (VLC) systems to be addressed, including both co-channel interference (CCI) and inter-carrier interference (ICI). In this paper, to solve the CCI problem, a novel optical attocell network is introduced, which can be regarded as a hybrid network based on both single frequency network and multi-frequency network. Meanwhile, to eliminate the ICI, a pre-distorted enhanced asymmetrically clipped DC biased optical orthogonal frequency division multiplexing (PEADO-OFDM) approach is designed. In addition, the corresponding properties of PEADO-OFDM are analyzed, and different modulation schemes are employed for downlink and uplink in the same ADO-OFDM modulation framework to reduce the complexity of user processing. Simulation results indicate that the proposed PEADO-OFDM scheme outperforms the conventional ADO-OFDM approach in terms of bit error rate, while the processing latency and computational complexity are greatly reduced at the receiver.