Indoor Optical Wireless Communication Research Articles

Fast-beam-switching optical phased array for moving objects in wireless optical communication networks.

For optical wireless communication systems, mechanical beam steering struggles to timely switch between multiple users or search for moving users. Here we demonstrate a fast-beam-switching optical phased array (OPA) for agile wireless communication networks. For point-to-multi-point (P2MP) scenarios, a setup of OPA-based fast beam switching between two aligned receivers was developed. A loss-free image transmission experiment was used to demonstrate the stability of switching. Furthermore, we have developed an approach to using the fast-switching OPA to follow the trajectory of moving objects so as to help enable agile random-access switching between moving objects. These results could help offer fast switching and reconfiguration for indoor wireless optical communications.

Coverage extended MMF-based indoor OWC using overfilled launch and diversity reception

Speckle patterns generated as coherent optical beams are reflected by scattering elements. Multimode fibers (MMFs) can modify the transverse intensity distribution of speckle patterns with macro perturbations, i.e., pressures, providing a simple and low-cost way to achieve equivalent beam-steering for indoor optical wireless communications (OWCs) with divergent optical beams. However, the received optical power (ROP) variance severely limits the mobility of user terminals. In this paper, the issue is alleviated by using the overfilled launch of MMFs and the diversity gain of multi-receivers. By adjusting the axial spatial coupling distance between the MMF and the single mode fiber (SMF) emitting coherent laser, the number of excited modes of MMF can be significantly increased at 1550 nm with negligible coupling and bending losses. In addition, the signal-to-noise ratio (SNR) enhancement obtained by applying two receivers is theoretically analyzed for the case when either thermal noise or shot noise is dominant. The experimental results demonstrate that the proposed scheme can efficiently compensate for the ROP inhomogeneity, and at the same time it can extend the achievable full steering angle up to 12° at a 1.5-m free-space distance for bit error rate (BER) values of less than 3.8 × 10−3.

Efficient utilization of adaptive LMS channel estimation with activity detection guidance and tap decoupling in indoor optical wireless communication systems

Efficient utilization of adaptive LMS channel estimation with activity detection guidance and tap decoupling in indoor optical wireless communication systems

Joint Beam-and-Probabilistic Shaping Scheme Based on Orbital Angular Momentum Mode for Indoor Optical Wireless Communications

In this paper, we propose a novel beam-and-probabilistic shaping technique based on orbital angular momentum (OAM) modes for indoor optical wireless communications (OWC) to improve the system effective coverage. Our results show that both Huffman coding (HC) and sign inversion coding (SIC) based probabilistic shaping schemes can improve the effective coverage for 16QAM modulation format while only SIC based probabilistic shaping can improve the effective coverage for 64QAM modulation under different beam shaping schemes with different power ratios of different modes. Furthermore, it can be seen that more significant performance improvement can be obtained by employing both of the probabilistic shaping schemes by decreasing the power ratio of the fundamental mode to the higher-order OAM mode. Therefore, further increment of the effective coverage can be predicted with the probabilistic shaping schemes by decreasing the power ratio.

HCM‐based indoor optical wireless communications: Timing and data recovery

AbstractLight‐fidelity (Li‐Fi) is a promising solution to provide high‐rate, secure, and green communications to be used in the next generation of wireless networks. Since visible or infrared (IR) light‐emitting diodes (LEDs) are used as the optical source and have a non‐linear transfer function, the transmitted modulated signal can be distorted if the signal has a high peak‐to‐average power ratio (PAPR). Recently, a new modulation scheme called Hadamard‐coded modulation (HCM) is proposed which has no PAPR concern since it produces symbols with discrete levels. In the HCM technique, the symbol levels can be created using multiple LEDs. Therefore, each LED operates in its linear region and just switches on or off. A low‐complexity transceiver architecture for the HCM‐based communication links is proposed and a complete synchronisation procedure based on the spread‐spectrum techniques is presented. Finally, the bit error rate of the system is evaluated by Monte Carlo simulations, and effects of system parameters such as preamble length, fall and rise time of optical devices, and timing jitter on the bit error rate of the link are discussed.

Doing More with Ambient Light: Harvesting Indoor Energy and Data Using Emerging Solar Cells

On one side, the capacity of the world’s photovoltaic (PV) systems is experiencing unprecedented growth; on the other side, the number of connected devices is rapidly increasing due to the development of advanced communication technologies. These fields are not completely independent, and recent studies show that indoor energy harvesting is a great candidate for answering the energy challenges of future generations of telecommunications, namely 5G and 6G, ideal for internet-of-things (IoT) scenarios, i.e., smart homes, smart cities, and smart factories. The emerging PV technologies have shown amazing capabilities for indoor energy harvesting, displaying high power conversion efficiency, good flexibility, and champion-specific powers. Recently, the excellent dynamic performance of PV devices enabled them to be used as data receivers in optical wireless communication (OWC) scenarios, calling forth an innovative system able to simultaneously harvest energy and receive communication data with a single PV device. This article reviews the recent literature devoted to the exploitation of photovoltaic technologies for simultaneous indoor energy harvesting and OWC data reception. This contribution highlights the strong potential of the approach toward the next generation of Green IoT systems and the current challenges that need to be addressed with regard to the physics of solar cells, from laboratory to large-scale applications.

Digital-filter-aided crosstalk-mitigation for a high spatial resolution AWGR-based 2D IR beam-steered indoor optical wireless communication system.

The growing demand for wireless connectivity is attracting interest in optical wireless communication (OWC) technique. In this paper, a filter-aided crosstalk mitigation scheme, employing digital Nyquist filters, is proposed to eliminate the trade-off between the spatial resolution and the channel capacity for the AWGR-based 2D infrared beam-steered indoor OWC system. By shaping the transmitted signal for narrow spectral occupancy, the inter-channel crosstalk resulting from the imperfect AWGR filtering can be avoided, which enables a denser AWGR grid. In addition, the spectral-efficient signal reduces the bandwidth requirement of the AWGR, which allows a low-complexity AWGR design. Thirdly, the proposed method is not sensitive to the wavelength misalignment between AWGRs and lasers, which relaxes the design of high wavelength stability lasers. Moreover, the proposed method is cost-efficient as we can make use of the mature DSP technique without additional optical components. The 20-Gbit/s data rate OWC capacity using PAM4 format has been experimentally demonstrated over a 6-GHz bandwidth-limited AWGR-based 1.1-m free-space link. The experimental results show the feasibility and effectiveness of the proposed method. By combining our proposed method with the polarization orthogonality technique, a promising capacity per beam of 40 Gbit/s is potentially attainable.

Fairness analysis of indoor multi‐user communications through steerable IR‐beam

AbstractDownlink data transmission of multi‐user indoor optical wireless communications and its corresponding fairness analysis is studied by the authors. It is assumed that access points (APs) may use infrared beam (IR‐beam) laser or light emitting diodes (LEDs) for data transmissions. For the case of APs with IR‐beams, since the bandwidth of the transmitted optical signal can be very large than that of the LED’s, a typical time‐division multiple access (TDMA) scheme is utilised to serve multiple users and the respective achievable rates are derived. Then, for a general fairness‐scheduling utility function with the capability of adjusting the degree of fairness known as α‐proportional fairness, optimal time‐sharing coefficients are analytically derived in closed‐forms. To compare the performance of the IR‐based communications for the LED‐based APs, two non‐cooperative and cooperative data transmission schemes are also studied and the corresponding achievable rates are presented based on the classical superposition coding. Finally, to compare the performance of the two types of APs, per user data rate of the proposed systems are compared through numerical results for different scenarios to highlight advantages and drawbacks of the two types of APs. Also, by considering the Jain's index as a fairness metric, the impact of the system parameters on the fairness is discussed through numerical results.

Dual-carrier multiplexed laser-based hybrid transmitter for high data-rate indoor optical wireless communication

In this paper we experimentally demonstrate dual-carrier modulation scheme to achieve spectrally efficient OFDM transmission in an optical wireless communication link using a hybrid laser-LED transmitter. The hybrid transmitter used in this work consists of two near infrared laser diodes at 785 nm and 808 nm used to separately modulate the real and imaginary components of the complex OFDM waveforms, collinearly combined and subsequently mixed with white LED array at a diffuser surface for indoor illumination. At the receiver end, two separate high-speed photodetectors with spectral filters are used to detect the real and imaginary components and subsequently combined to generate the complex OFDM waveforms. The optical system performance is characterized to quantify the beam spread, spot-size and spectral content of the laser beams and white LED array. The communication performance is also characterized using 16, 32, and 64-QAM/OFDM with 1, 1.25 and 1.5 Giga-samples/second sampling rates. The best communication performance with a net data-rate of 4.2 Gbps for 64-QAM/OFDM at an optimum sampling rate of 1.5 G-Samples/second is demonstrated. The dual-carrier modulation scheme studied in this work helps achieve improved spectral efficiency for OFDM transmission in an OWC link using dual transmitter-receiver pair, while at the same time requiring simplified electronic front-ends and encoding/decoding algorithms.

Investigation on Orbital Angular Momentum Mode-Based Beam Shaping for Indoor Optical Wireless Communications

In this paper, we report a novel indoor optical wireless communications system employing orbital angular momentum (OAM) mode-based beam shaping. Different shapes of transmitted beams with various power distribution can be obtained by superposing different OAM modes incoherently with different power ratios. The bit-error-rate (BER) performance of the proposed system has been numerically and experimentally investigated. The results show that the effective coverage (BER<3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> ) can be expanded significantly by employing OAM mode-based beam shaping scheme at certain power ratios between OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode and higher-order OAM mode, compared to conventional OWC systems without OAM mode-based beam shaping scheme and the optimal performance for symbol rate of 10 Gbaud can be obtained when the power ratio of OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> /OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is 0.3 and 0.33, respectively. Furthermore, the BER performance distribution in different directions within the optical cell has also been evaluated. It has been shown that less variation in the performance can be achieved in the system employing OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> with OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based-beam shaping compared with that in the system employing OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> with OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes.

Capacity enhancement of WDM visible light communication system employing 3-SOPs/channel/LD color

Abstract Nowadays, polarization division multiplexing (PDM) employing different states of polarizations (SOP) is a prominent candidate to offer high capacity and improved performance. However, the incorporation of PDM in indoor wireless optical communication (WOC) systems is limited to horizontal and vertical SOPs only. Therefore, in this research article, a cost-effective and high capacity 90 Gbps non-return to zero (NRZ) line coding based 18 channels VLC system is presented by incorporating red–green–blue laser diodes (RGB LDs). Performance investigations of different SOPs such as SOP 0°, SOP 45°, and SOP 90° are carried out at diverse VLC link distances in terms of log BER and Q factor. Results revealed that the proposed VLC system can successfully cover the 6 m distance and red LD together with SOP0 is found to be the highest performing.

Demonstration of Spatial Modulation Using a Novel Active Transmitter Detection Scheme with Signal Space Diversity in Optical Wireless Communications.

Line-of-sight (LOS) indoor optical wireless communications (OWC) enable a high data rate transmission while potentially suffering from optical channel obstructions. Additional LOS links using diversity techniques can tackle the received signal performance degradation, where channel gains often differ in multiple LOS channels. In this paper, a novel active transmitter detection scheme in spatial modulation (SM) is proposed to be incorporated with signal space diversity (SSD) technique to enable an increased OWC system throughput with an improved bit-error-rate (BER). This transmitter detection scheme is composed of a signal pre-distortion technique at the transmitter and a power-based statistical detection method at the receiver, which can address the problem of power-based transmitter detection in SM using carrierless amplitude and phase modulation waveforms with numerous signal levels. Experimental results show that, with the proposed transmitter detection scheme, SSD can be effectively provided with ~0.61 dB signal-to-noise-ratio (SNR) improvement. Additionally, an improved data rate ~7.5 Gbit/s is expected due to effective transmitter detection in SM. The SSD performances at different constellation rotation angles and under different channel gain distributions are also investigated, respectively. The proposed scheme provides a practical solution to implement power-based SM and thus aids the SSD realization for improving system performance.

Safety Analysis for Laser-Based Optical Wireless Communications: A Tutorial

Light amplification by stimulated emission of radiation (laser) sources has many advantages for use in high-data-rate optical wireless communications (OWCs). In particular, the low-cost and high-bandwidth properties of laser sources, such as vertical-cavity surface-emitting lasers (VCSELs), make them attractive for future indoor OWCs. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the International Electrotechnical Commission (IEC) standards for laser safety. In this article, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which and the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 Standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher mode beams. It is shown that the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$M$ </tex-math></inline-formula> -squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits; however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements, such as lens and diffuser, as well as for the VCSEL array, have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this article. We have studied the impacts of various parameters, such as wavelength, exposure duration, and the divergence angle of laser sources on the safety analysis by presenting insightful results.

Experimental demonstration of an indoor optical wireless communication system with a waveform index modulated uplink.

The optical wireless communication (OWC) technology has been widely studied to provide high-speed wireless connections in indoor environments. Compared with the downlink, the uplink faces more restrictions, such as the stringent power, complexity, and cost requirements. In this Letter, we propose a waveform index modulated OWC uplink with each waveform seen as a virtual transmitter. Extra data is transmitted via the index of selectively activated virtual transmitter, and the OWC uplink speed can be improved whilst achieving similar bit-error-rate (BER) performance. The waveform index modulated uplink is experimentally demonstrated in a near-infrared OWC system. Results show that up to 15 Gb/s data rate is achieved with the proposed waveform index modulated uplink, which increases the data rate significantly with similar power and BER.

Comparison of Adaptive Equalization Methods for Improving Indoor Optical Wireless Communications Employing Few-Mode Based Uniform Beam Shaping

Few-mode based uniform beam shaping exploiting the use of different spatial modes has been recently shown to improve the system resistance against pointing errors and link outage in indoor optical wireless communications. However, the inter-symbol interference arising from transmission of different spatial modes following different paths and mode crosstalk during the transmission becomes another important challenge. In this paper, we theoretically investigate and experimentally compare the performance of two popular adaptive equalization methods, namely least mean square and recursive least-square in both multi-equalizer and single-equalizer configurations, to improve the performance of indoor optical wireless communication system employing few-mode based uniform beam shaping. For both equalization methods, the multi-equalizer scheme achieves better equalization performance and higher convergence speed than the single-equalizer scheme for a similar cost of required computational complexity for each iteration. Moreover, recursive least-square method has a higher convergence speed than least mean square equalization while least mean square method has a lower computational complexity than recursive least-square method under the same desired equalization performance. More importantly, for a fixed reference BER threshold (KP4 FEC limit), the least mean square based single-equalizer scheme with a step size of 0.0035 and a tap number of 61 has the lowest computational complexity among the four types of equalization schemes. This kind of equalizer with relatively low computational complexity can provide guidelines for selecting an applicable adaptive equalization scheme in optical wireless communications with few-mode based beam shaping based on the comprehensive requirements of BER performance and computational complexity.

Hybrid Laser-LED Transmitter With Closed-Loop Beam-Steering Control for Indoor Optical Wireless Communication

In this paper, we demonstrate a hybrid Laser-LED transmitter module for indoor optical wireless communication with closed-loop, non-mechanical beam steering capability. The hybrid transmitter module consists of a near infrared laser diode for data communication and white LED array for illumination, combined on a diffuser surface. Dual-axis non-mechanical beam steering of the laser beam is implemented using two off-centered liquid lenses. The diffused laser beam directed towards the receiver is steered over an angular range of −7.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> to 7.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> (−1.7 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> to 2.6 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> ) along the horizontal (vertical) axes spanning −200 to 200 mm (−44 to 67 mm) at the receiver placed 1.5-meter from the transmitter. M-QAM/OFDM in combination with adaptive bit-and power-loading is utilized to achieve a total data throughput of 5.15 Gbps for the diffused laser beam with steering. Laser intensity levels as measured at the receiver plane are kept below the maximum permissible exposure limit for indoor usage across the entire beam steering range. Closed-loop beam steering is also demonstrated by scanning the transmitted laser beam horizontally, measuring the signal strength using a low bandwidth photodetector and locking the laser beam to the receiver position for data-communication. Such hybrid transmitters offer the benefit of decoupling the data communication and illumination requirements of the indoor optical link, thereby tailoring the individual light emitter's performance to specific use-case.

Investigation on repetition-coding and space-time-block-coding for indoor optical wireless communications employing beam shaping based on orbital angular momentum modes.

In this paper, we propose a novel beam shaping technique based on orbital angular momentum (OAM) modes for indoor optical wireless communications (OWC). Furthermore, we investigate two spatial diversity techniques, namely repetition-coding (RC) and Alamouti-type orthogonal space-time-block-coding (STBC) for indoor OWC employing the new beam shaping technique. The performance of both diversity schemes is systematically analyzed and compared under different beam shaping techniques using different OAM modes with different power ratios of the modes. It is shown that both RC and STBC can improve the system performance and effective coverage and RC outperforms STBC in all the beam shaping techniques regardless of the power ratios of the different modes. In addition, to further understand the performance of RC and STBC schemes against the signal delays induced during OAM mode conversion, the system tolerance of the two schemes to the delay interval is investigated with different OAM mode-based beam shaping techniques. Numerical results show that higher resistance to the delay interval can be achieved in STBC scheme. The advantage is more obvious when employing OAM0 and OAM1 based beam shaping technique.

Remote-Powered Infrared Indoor Optical Wireless Communication Systems

Near-infrared optical wireless communications (OWCs) have been widely studied in indoor environments, and the beam steering has been widely used to achieve both high-speed and wide coverage. Compared with active steering, the passive method eliminates the need for local power supply at OWC access points. However, current passive beam steering is based on wavelength tuning, which requires precisely controlled and costly tunable lasers. In this letter, we propose a beam-steered indoor OWC system without the need for local power through remote powering, providing the benefits of low power consumption, passive OWC access points, and low-cost. The proposed remote-powered indoor OWC system is experimentally demonstrated with 10 Gb/s OWC provided to users. An indirect remote powering scheme is also proposed and realized to support large-scale OWC access points.

12 Gbit/s indoor optical wireless communication system with ultrafast beam-steering using tunable VCSEL.

Optical wireless communication (OWC) using line-of-sight connections has great application potential for indoor scenes due to its advantages of high data transmission speed and privacy. In our proposed system, we use infrared tunable vertical-cavity surface-emitting laser (VCSEL) as light source, array waveguide gratings (AWG) combined with 6 × 6 integrated optical fiber arrays as a router to realize ultrafast beam-steering and high capacity point-to-point data transmission, which makes the indoor OWC system compact, low-cost, and easy to install. The high tuning rate of VCSEL enables the channel switching to be completed within 1.7 µs. Based on the modulation format of non-return-to-zero on-off keying (NRZ-OOK), a data rate of 12 Gbit/s per channel can be realized with high sensitivity through 3.1 m free space when the detected optical power is low. The system is proved to be a flexible link with high-speed communication for mobile terminals in a limited space.

Hybrid NOMA-based ACO-FBMC/OQAM for next-generation indoor optical wireless communications using LiFi technology

Light fidelity (LiFi) has successfully achieved high data transfer rates, high security, great availability, and low interference. In this paper, we propose a LiFi system consisting of a combination of non-orthogonal multi-access (NOMA), asymmetrically-clipped optical (ACO), and filter bank multicarrier (FBMC) techniques combined with offset quadrature amplitude modulation (OQAM). The paper also applies a mu -law companding approach for a high peak to average power ratio (PAPR) reduction of the FBMC/OQAM scheme. The combination of NOMA, ACO-FBMC/OQAM, and mu -law companding allows a significant increase in throughput and a significant reduction in unserved users. Considering two scenarios, an appropriate algorithm is developed to maximize the throughput and minimize the number of blocked (unserved) users. The results show that the overall system throughput could be increased by 1.8 compared to FBMC, OFDM, and OFDM-NOMA. Furthermore, the proposed system reduces the number of unserved users below 10%, while the system can provide 30 or 60% in case only OFDM-NOMA, FBMC, or OFDM is applied.