Photonics

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13 pages, 4548 KiB

Open AccessArticle

Deep Learning-Assisted High-Pass-Filter-Based Fixed-Threshold Decision for Free-Space Optical Communications

by Yan Gao, Qian-Wen Jing, Min-Fang Liu, Wen-Hao Zong and Yan-Qing Hong

Photonics 2024, 11(7), 599; https://doi.org/10.3390/photonics11070599 - 26 Jun 2024

Viewed by 719

This paper proposes a deep learning (DL)-assisted high-pass-filter (HPF)-based fixed-threshold decision (FTD) for free-space optical (FSO) communication. HPF is applied to reduce the scintillation effect by filtering out the low-frequency components of the received signal. However, the performance is limited owing to the [...] Read more.

This paper proposes a deep learning (DL)-assisted high-pass-filter (HPF)-based fixed-threshold decision (FTD) for free-space optical (FSO) communication. HPF is applied to reduce the scintillation effect by filtering out the low-frequency components of the received signal. However, the performance is limited owing to the signal distortion from HPF and remnant scintillation effect due to insufficient filtering. Therefore, the DL model is adopted to improve the performance of HPF-based scintillation effect compensation. The multilayer perceptron (MLP) model is used to adaptively select the peak frequency component of the received signal as the optimized cutoff frequency of HPF. Furthermore, recurrent neural network (RNN) and long short-term memory (LSTM) models are cascaded after HPF to compensate for the remnant scintillation effect and recover the signal distortion without the optimization of HPF cutoff frequency. The simulation was conducted under different turbulence channels and data rates. Simulation results showed that MLP-assisted adaptive optimized cutoff frequency and cascaded LSTM and HPF methods were close to the adaptive-threshold decision with precise channel state information under various turbulence channel degrees. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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18 pages, 5745 KiB

Open AccessArticle

Impact of Crystal Orientation on Modulation Bandwidth: Towards GaN LED-Based High-Speed Visible Light Communication

by Md Jahid Faruki, Krishnendu Bera and Nemai Karmakar

Photonics 2024, 11(6), 542; https://doi.org/10.3390/photonics11060542 - 5 Jun 2024

Viewed by 509

Abstract

Light-emitting diodes (LEDs) with high modulation bandwidth are required for high-speed visible light communication applications. Crystal orientation in the GaN LED structure plays a key factor in its modulation bandwidth as the recombination lifetime is highly dependent on crystal orientation owing to the [...] Read more.

Light-emitting diodes (LEDs) with high modulation bandwidth are required for high-speed visible light communication applications. Crystal orientation in the GaN LED structure plays a key factor in its modulation bandwidth as the recombination lifetime is highly dependent on crystal orientation owing to the Quantum-Confined Stark Effect (QCSE). In this study, six different crystal orientation multi-quantum well (MQW) GaN LEDs are simulated to understand the impact of heterostructure orientation on modulation bandwidth, radiative recombination rates, and emission intensity. The results of this study demonstrate that semi-polar

10 \bar{1} \bar{3}

MQW LEDs provide the highest bandwidth in the current density range of 9–20 kA/cm² compared to the other five orientations. For instance, the semi-polar

10 \bar{1} \bar{3}

-based LED offers a modulation bandwidth of 912.7 MHz at 20 kA/cm² current density. These results suggest that the semi-polar

10 \bar{1} \bar{3}

orientation-based LED has the potential to support a high-speed visible light communication system. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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17 pages, 8171 KiB

Open AccessArticle

Integrated Mobile Visible Light Communication and Positioning Systems Based on Decision Feedback Channel Estimation

by Ruoxuan Wang, Yuzhe Sun, Zhongxu Liu, Mingyi Gao and Xiaodi You

Photonics 2024, 11(6), 537; https://doi.org/10.3390/photonics11060537 - 4 Jun 2024

Viewed by 384

Abstract

Visible light communication (VLC) and visible light positioning (VLP) systems are usually designed separately to prevent mutual interference, while terminal mobility often introduces challenges that can degrade their performance. In this paper, we propose an integrated visible light communication and positioning (VLCP) scheme [...] Read more.

Visible light communication (VLC) and visible light positioning (VLP) systems are usually designed separately to prevent mutual interference, while terminal mobility often introduces challenges that can degrade their performance. In this paper, we propose an integrated visible light communication and positioning (VLCP) scheme designed for mobile scenarios, encompassing both multiple-input–single-output (MISO) and multiple-input–multiple-output (MIMO) configurations. This scheme integrates both functionalities into a unified system. Utilizing decision feedback channel estimation (DFCE), we effectively estimate the dynamic channel state information (CSI) for both VLC and VLP, thereby potentially enhancing both communication and positioning performance. Simulation results across various routes verify the effectiveness of the proposed scheme. It is observed that when the terminal moves at 1 m/s, the VLCP scheme with DFCE can maintain reliable transmission quality, ensuring bit error rates (BERs) consistently below 1.3 × 10⁻². Additionally, the mean positioning errors remain within the centimeter range in different routes, not exceeding 4.3 cm and 15.5 cm in the MISO and MIMO scenarios, respectively. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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13 pages, 5958 KiB

Open AccessArticle

Single-Source VLCP System Based on Solar Cell Array Receiver and Right-Angled Tetrahedron Trilateration VLP (RATT-VLP) Algorithm

by Dawei Xie, Zhongxu Liu and Changyuan Yu

Photonics 2024, 11(6), 536; https://doi.org/10.3390/photonics11060536 - 4 Jun 2024

Viewed by 263

Abstract

A significant deployment limitation for visible light communication and positioning (VLCP) systems in energy- and light-source-restricted scenarios is the reliance of photodetectors (PDs) on external power supplies, compromising sustainability and complicating receiver charging. Solar cells (SCs), capable of harvesting and converting environmental light [...] Read more.

A significant deployment limitation for visible light communication and positioning (VLCP) systems in energy- and light-source-restricted scenarios is the reliance of photodetectors (PDs) on external power supplies, compromising sustainability and complicating receiver charging. Solar cells (SCs), capable of harvesting and converting environmental light into electrical energy, offer a promising alternative. Consequently, we first propose an indoor VLCP system that utilizes an SC array as the receiver, alongside a right-angled tetrahedron trilateration visible light positioning (RATT-VLP) algorithm based on a single light source and multiple receivers. The proposed system uses an SC array in place of PDs, utilizing binary phase shift keying (BPSK) signals for simultaneous communication and positioning. In experiments, we verified the system’s error-free communication rate of 1.21 kbps and average positioning error of 3.40 cm in a 30 cm × 30 cm area, indicating that the system can simultaneously satisfy low-speed communication and accurate positioning applications. This provides a viable foundation for further research on SC-based VLCP systems, facilitating potential applications in environments like underwater wireless communication, positioning, and storage tank inspection. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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22 pages, 1619 KiB

Open AccessEditor’s ChoiceArticle

Optimisation of the Transmitter Layout in a VLP System Using an Aperture-Based Receiver

by José Miguel Menéndez and Heidi Steendam

Photonics 2024, 11(6), 517; https://doi.org/10.3390/photonics11060517 - 28 May 2024

Viewed by 424

Abstract

In this paper, we consider a visible light positioning (VLP) system, where an array of photo diodes combined with apertures is used as a directional receiver and a set of inexpensive and energy-efficient light-emitting diodes (LEDs) is used as transmitters. The paper focuses [...] Read more.

In this paper, we consider a visible light positioning (VLP) system, where an array of photo diodes combined with apertures is used as a directional receiver and a set of inexpensive and energy-efficient light-emitting diodes (LEDs) is used as transmitters. The paper focuses on the optimisation of the layout of the transmitter, i.e., the number and placement of the LEDs, to meet the wanted position estimation accuracy levels. To this end, we evaluate the Cramer–Rao bound (CRB), which is a lower bound on the mean-squared error (MSE) of the position estimate, to analyse the influence of the LEDs’ placement. In contrast to other works, where only the location of the LEDs was considered and/or the optimisation was carried out through simulations, in this work, the optimisation is carried out analytically and considers all the parameters involved in the VLP system as well as the illumination. Based on our results, we formulate simple rules of thumb with which we can determine the spacing between LEDs and the minimum number of LEDs, as well as their position on the ceiling, while also taking into account the requirements for the illumination. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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14 pages, 18406 KiB

Open AccessArticle

Interference Mitigation in VLC Systems using a Variable Focus Liquid Lens

by Krishnendu Bera and Nemai Karmakar

Photonics 2024, 11(6), 506; https://doi.org/10.3390/photonics11060506 - 26 May 2024

Viewed by 448

Abstract

The field of view (FOV) is an important parameter of a visible light communication (VLC) receiver. A variable FOV can be useful to mitigate interference from neighboring cells in a multi-cell VLC network. The existing works on dynamic FOV VLC receivers have used [...] Read more.

The field of view (FOV) is an important parameter of a visible light communication (VLC) receiver. A variable FOV can be useful to mitigate interference from neighboring cells in a multi-cell VLC network. The existing works on dynamic FOV VLC receivers have used a mechanical iris to control the receiver’s FOV, making the VLC receiver bulky, slow, and power-consuming. In this article, an electronically controlled variable focus liquid lens is used to vary the FOV of the receiver dynamically. A low-cost microcontroller-based feedback control system controls the effective FOV of the receiver to reject signals from unwanted transmitters, thus maximizing the signal-to-interference plus noise ratio (SINR) of the received signal. Experimental results show that the proposed technique effectively improves SINR performance in a multi-cell VLC network. To the best of the authors’ knowledge, this is the first reported work to utilize an electronically controlled compact liquid lens for designing a dynamic FOV VLC receiver. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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14 pages, 1987 KiB

Open AccessArticle

Design of the Light Source Layout Optimization Strategy Based on Region Partition and Pre-Bias Compensation for Indoor Visible Light Communication Systems

by Qiong Zhao, Weilin Zhang, Jiacheng Fan and Lijun Deng

Photonics 2023, 10(12), 1344; https://doi.org/10.3390/photonics10121344 - 6 Dec 2023

Viewed by 1065

Abstract

Optimization of the light source layout is an important issue for indoor visible light communication systems, as it affects the received optical power distribution and user perception. In this paper, we propose a local optimization strategy for the light source layout that balances [...] Read more.

Optimization of the light source layout is an important issue for indoor visible light communication systems, as it affects the received optical power distribution and user perception. In this paper, we propose a local optimization strategy for the light source layout that balances optimization effectiveness with optimization efficiency. First, we divide the optimization region into multiple sub-regions with different sizes and optimization priorities, where the sizes and optimization priorities of the individual sub-regions are determined based on the effect minimization principle among the sub-regions. We then calculate the pre-bias factor based on the equivalent mapping, which can compensate for the effect of the light sources in the latter optimized sub-region on the source layout optimization in the current sub-region. Finally, we search for the coordinate of a single light source for each sub-region using the variance of the squared distance between the projection of the light source on the receiving plane and the received point as a fitness function. Simulation results show that the proposed optimization strategy performs well when the vertical distance between the ceiling and the receiving plane is not less than 2.85 m. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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15 pages, 416 KiB

Open AccessArticle

Tolerance-Aided Interference Degradation for Optical OFDM in Power-Constrained Systems

by Yan Gao and Jie Lian

Photonics 2023, 10(11), 1206; https://doi.org/10.3390/photonics10111206 - 29 Oct 2023

Viewed by 784

Abstract

Optical wireless communications (OWCs) are power-efficient for providing short-range and high-speed data transmission. In this paper, we propose an interference degradation algorithm for power-constrained OWC systems when using orthogonal frequency division multiplexing (OFDM). In this algorithm, we introduce a decision tolerance at the [...] Read more.

Optical wireless communications (OWCs) are power-efficient for providing short-range and high-speed data transmission. In this paper, we propose an interference degradation algorithm for power-constrained OWC systems when using orthogonal frequency division multiplexing (OFDM). In this algorithm, we introduce a decision tolerance at the receiver that can be optimally designed to reduce interference caused by peak power clipping distortion and decrease additive noise collected by the photodetectors. Although the recently proposed clipping-enhanced optical OFDM (CEO-OFDM) can transmit clipped information through extra signal frames, reconstructing the received signal introduces more noise. Using the proposed decision tolerance, we can determine whether the signals in the extra frame are required for data reconstruction. By optimally choosing the decision tolerance, the bit error rate (BER) performance is enhanced compared to those of DCO, ACO, and CEO-OFDM. Additionally, the proposed algorithm offers a wider dynamic modulation index range than DCO, ACO, and CEO-OFDM at the same BER. Using the tested parameters, the proposed algorithm with 64-QAM achieves a similar best BER performance as DCO, ACO, and CEO-OFDM using 32-QAM. Therefore, the proposed algorithm achieves a higher data rate than those of the other compared techniques. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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13 pages, 3531 KiB

Open AccessArticle

Performance Analysis of Soft-Switching FSO/THz-RF Dual-Hop AF-NOMA Link Based on Cognitive Radio

by Rongpeng Liu, Ziyang Wang, Xuerui Wang, Jingwei Lu, Yawei Wang, Yizhou Zhuo, Ruihuan Wu, Zhongchao Wei and Hongzhan Liu

Photonics 2023, 10(10), 1086; https://doi.org/10.3390/photonics10101086 - 27 Sep 2023

Viewed by 971

Abstract

This paper presents a promising solution to address the scarcity of spectrum resources and enhance spectrum efficiency in the context of cognitive radio (CR)-based soft-switching free-space optical (FSO)/terahertz (THz) radio frequency (RF) dual-hop amplify-and-forward (AF)–non-orthogonal multiple access (ROMANO) links. The impact of maximum [...] Read more.

This paper presents a promising solution to address the scarcity of spectrum resources and enhance spectrum efficiency in the context of cognitive radio (CR)-based soft-switching free-space optical (FSO)/terahertz (THz) radio frequency (RF) dual-hop amplify-and-forward (AF)–non-orthogonal multiple access (ROMANO) links. The impact of maximum tolerable interference power in the primary network, transmit power in the secondary transmitter, and maximum relay transmission power on the link are thoroughly studied. The numerical results ultimately validate the effectiveness of this link in improving performance, and a comparative analysis is conducted with the without-CR scheme, highlighting the distinctive characteristics of the proposed link. Full article

(This article belongs to the Special Issue Advanced Technologies in Optical Wireless Communications)

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