Vectorial optical wireless communications: bridging optical physics and engineering for 6G and beyond

Guest Editorial

The metasurface based beam-steering devices with the advantages of large steering angles, arbitrary channels and ultra-compactness have played an important role for data allocation and exchange in the optical wireless communication. However, the current metasurface based optical wireless communication systems are mainly on intensity modulation and direct detection (IM/DD), which shows a relative lower transmission capacity, lower received optical signal-to-noise ratio (OSNR) and complexity of system. In this study, a bidirectional multichannel optical wireless system enabled by a polarization independent metasurface with coherent modulation and reception is designed and experimentally demonstrated, which exhibits exclusive 100 Gbps coherent optical signals to multiusers with their own wavelengths, 2 m free space distance and field of view of 20° × 20°. In addition, the proposed system can support optical broadcasting system with capacity of 900 Gbps. The demonstrated metasurface assisted optical wireless communication system merges the optical coherent communication techniques and emerging concept of metasurface, which reduces the complexity and cost of the system while contributing a high transmission capacity, opening a new avenue for high performance optical wireless communications.

With the escalation of heterogeneous data traffic, the research on optical wireless communication (OWC) has attracted much attention, owing to its advantages such as wide spectrum, low power consumption and high security. Ubiquitous optical devices, e.g. light-emitting diodes (LEDs) and cameras, are employed to support optical wireless links. Since the distribution of these optical devices is usually dense, multiple-input-multiple-output (MIMO) can be naturally adopted to attain spatial diversity gain or spatial multiplexing gain. As the scale of OWC networks enlarges, optical MIMO can also collaborate with network-level operations, like user/AP grouping, to enhance the network throughput. Since OWC is preferred for short-range communications and is sensitive to the directions/rotations of transceivers, optical MIMO links vary frequently and sharply in outdoor scenarios when considering the mobility of optical devices, raising new challenges to network design. In this work, we present an overview of optical MIMO techniques, as well as the cooperation of MIMO and user/AP grouping in OWC networks. In consideration of the challenges for outdoor OWC, key technologies are then proposed to facilitate the adoption of optical MIMO in outdoor scenarios, especially in vehicular ad hoc networks. Lastly, future applications of MIMO in OWC networks are discussed. This article is part of the theme issue 'Optical wireless communication'.

Free space, dynamic, optical wireless communications will require topology control for optimization of network performance. Such networks may need to be configured for bi- or multiple-connectedness, reliability and quality-of-service. Topology control involves the introduction of new links and/or nodes into the network to achieve such performance objectives through autonomous reconfiguration as well as precise pointing, acquisition, tracking, and steering of laser beams. Reconfiguration may be required because of link degradation resulting from obscuration or node loss. As a result, the optical transceivers may need to be re-directed to new or existing nodes within the network and tracked on moving nodes. The redirection of transceivers may require operation over a whole sphere, so that small-angle beam steering techniques cannot be applied. In this context, we are studying the performance of optical wireless links using lightweight, bi-static transceivers mounted on high-performance stepping motor driven stages. These motors provide an angular resolution of 0.00072 degree at up to 80,000 steps per second. This paper focuses on the performance characteristics of these agile transceivers for pointing, acquisition, and tracking (PAT), including the influence of acceleration/deceleration time, motor angular speed, and angular re-adjustment, on latency and packet loss in small free space optical (FSO) wireless test networks.

This paper presents hybrid RFID – OWC with an adaptive priority transmission scheme. In this model, we apply optical wireless communication (OWC) for transmitting confidential information to a radio frequency identification (RFID) reader. Since OWC provides high security, this improves the security of our system. The priority of information is assigned according to the level of confidentiality and significance of the information. The more confidential information is transmitted by OWC with adaptive data rate, and the rest of the information (if there is any) is transmitted by RFID. The energy harvesting of RFID is used to modulate the reflected optical signal by Corner Cube Retroreflector (CCR). Bit error rate (BER) is analyzed regarding the half-power angle of the transmitter, optical transmit power, data rate, and the angle between tag and reader. The analytical results indicate that by employing the adaptive priority transmission scheme, the required BER of OWC transmission is achieved.

Research in the field of optical wireless communications (OWC) has increased significantly over the past decade. As new researchers begin to explore this field, there are various hurdles related to developing a research program in OWC. This is particularly noticeable when comparing the breadth of available RF toolkits to the limited number of openly available OWC tools. Furthermore, many of the available OWC resources are standalone with limited capabilities for higher layer integration. In this work, we present gr-owc, an open source out-of-tree module for the widely used GNURadio signal processing toolkit. Within gr-owc, we have developed signal processing blocks for OWC channel simulation and common OWC modulation/demodulation techniques. We demonstrate use of this module to simulate a multi-cell/multi-user OWC network. We also describe how gr-owc signal processing blocks can be deployed in a physical implementation using Software Defined Radio (SDR) hardware, namely the universal software radio peripheral (USRP). Furthermore, methods for contribution to the project are described since the project is designed for continued development internally and with the goal of engaging the broader OWC research community.

Although many researchers have achieved wireless optical ranging and communication based on Avalanche Photon Diode (APD) detectors and PIN detectors. However, due to the low sensitivity of APD and PIN detectors, their working distances are limited. In order to achieve long-distance underwater wireless optical communication, a Single-Photon Avalanche Diode (SPAD) with photon-limited sensitivity is used to detect optical signals. In addition, most literatures do not specially design the data frames of ranging and communication, which makes it difficult to perform ranging and communication at the same time. In view of the fact that there is currently no systematic solution for simultaneous communication and high-precision ranging under underwater photon counting, this paper sorts out the relevant technical details, proposes an integrated ranging and communication scheme applied to photon counting, designs a special data frame that integrates ranging and communication, and proposes an extraction and recovery scheme for ranging and communication signals. Simulation shows that this scheme can achieve underwater photon counting wireless optical communication with a communication distance of 160m and an SER of 9.75×10-6, and the ranging accuracy is better than 1.59cm.

Recently, a novel direct current-biased optical filter bank multi-carrier (DCO-FBMC) scheme has been proposed for optical wireless communication (OWC) in intensity modulation and direct detection channel. Although the DCO-FBMC scheme supports fully asynchronized multi-user uplink transmission and has been proven to have higher spectral and power efficiency than direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM), its achievable data rates, the fundamental performance metric from information theory perspective, has not been fully unveiled. In this paper, we study the uplink achievable rate and region of the DCO-FBMC system for OWC. Because of the low cost implementation of the realistic OWC system, we consider using low-resolution analog-to-digital converters (ADCs) instead of full-resolution ADCs at the receiver, and study its impact on the achievable rate of the DCO-FBMC system. Simulation results indicate that although using low-resolution ADCs causes loss of system achievable rate, the performance of the 4-bit quantized DCO-FBMC system is very close to that of the full-resolution DCO-FBMC system. In addition, compared with DCO-OFBM, the maximum achievable rate of each user in DCO-FBMC system is larger. Moreover, the DCO-FBMC system with 4-bit quantization gets larger achievable rate region than the full-resolution DCO-OFDM system, especially in the case of oversampling. Therefore, our paper show that from the achievable rate perspective, using the DCO-FBMC system with low-resolution ADCs is a good candidate for optical wireless communication.

The relatively unsatisfactory performance of optical wireless communication (OWC) with respect to WiFi and millimeter-wave communications has formed a key issue preventing its commercialization. We experimentally demonstrate an OWC technology using a combination of positive real-valued orthogonal frequency-division multiplexing (OFDM) and optical beamforming (OB). Due to the intensity-modulation and direct-detection aspects of OWC systems, a positive real-valued OFDM signal can be suitably utilized to maximize the OWC data rate. Further, the OB technique, which can focus laser light on a desired target, can be utilized to increase the OWC data rate and transmission distance. Our experimental results show that the received optical signal power and electrical signal increase by up to 42 and 25 dB, respectively. Further, the data rate increases by a factor of 200 with OB over the conventional approach.

Light detection and ranging (LiDAR) and optical wireless communication (OWC) are in high demand and rapidly developing owing to the explosive growth of smart systems that require automotive and mobile devices. Optical phased arrays (OPA) have become a key technology in LiDAR and OWC owing to their nonmechanical beam steering capabilities. However, using separate LiDAR and OWC platforms in one system creates problems, such as spectrum congestion, resource consumption, and high complexity. We propose a dual-function OPA that enables LiDAR and OWC to function on a single platform based on the simultaneous amplitude and phase modulation of optically injection-locked semiconductor lasers. We numerically demonstrated that the primary LiDAR and secondary OWC function simultaneously by independent control of the main and side lobes in the OPA signal. The variation in side lobe levels is controlled at 20 or 25 dB to realize low- and high-level data for OWC function as well as maintaining the main beam LiDAR function. We successfully achieved wide-opening eye patterns of 10 Gbps data transmission of the OWC operation.

This presentation describes recent activities on ultra-high speed Optical Wireless Communications (OWC) using Gallium-Nitride micro-LEDs designed and fabricated at CEA-Leti. Micro-LEDs are one of the most promising OWC optical sources due to their high illumination efficiency and their large modulation bandwidths. Pre-liminary work focused on the implementation of a 10-μm single blue micro-LED on sapphire wafer within an experimental OWC setup, mixing software generation of direct-current optical orthogonal frequency division multiplexing (DCO-OFDM) patterns and hardware optical components for light collection, high speed photo-detection and digital acquisitions. Intensity modulation conveys DCO-OFDM waveform and direct detection is used at reception. A high current density of 25.5 kA/cm² provided a modulation bandwidth of 1.8 GHz. Associated to bit and power loading with up to a 256-QAM subcarrier modulation, it enabled a new data rate of 7.7 Gb/s, compared to the previous record of 5.37 Gb/s reached with a blue 21-μm microLED in 2016. Towards a better understanding of the micro-LED design impact on OWC performance, next investigations will study the electrical modelling of such micro-LEDs in the high frequency regime. Future works will cover the use of large arrays of more than 10 thousands micro-LEDs. The first objective is to open the way to new digital-to-optical modulations by independently driving each pixel, to remove digital-to-analogue converter and target highly integrated system-on-chips for ultra-high speed OWC transmitters. Secondly, higher emitted optical power is expected to open such technology to indoor multiple access applications where light collection and emitter-receiver alignment may not be possible anymore.

This thesis presents two original ontributions on orthogonal frequency division multiplexing (OFDM). The first is a thorough investigation of the error performance and channel capacity of asymmetrically clipped optical OFDM (ACO-OFDM). ACO-OFDM is a modulation scheme designed specifically for optical communications. The second concerns the use of multiple-input multiple-output (MIMO) OFDM in radio frequency (RF) systems. A new scheme which uses polynomial cancellation coding (PCC) OFDM with time offsets is proposed. This new scheme is robust to both carrier frequency offset (CFO) and channel estimation errors. OFDM has been used extensively to reduce the effect of dispersion in wired and wireless RF systems. Although dispersion is also a problem in optical wireless systems, OFDM has not until recently been considered for use in these. This is because optical wireless systems use intensity modulation (IM) and so the transmitted signals must be nonnegative and as a result conventional OFDM signals which are bipolar cannot be used directly. ACO-OFDM and DC biased optical OFDM (DCO-OFDM) are two non-negative forms of OFDM developed for optical systems. Both are robust to ISI but ACO-OFDM is in most cases much more power efficient. In this thesis results are presented for the performance of ACO-OFDM, DCO-OFDM and conventional optical modulation schemes such as on-off keying (OOK) and pulse position modulation (PPM) for both dispersive and non-dispersive channels. These show that ACO-OFDM is more tolerant to dispersion and requires less optical power than the other modulation schemes. To investigate the theoretical limits of an ACO-OFDM system, closed form analytical expressions for the channel capacity of ACO-OFDM are derived both for a flat channel and for representative dispersive optical wireless channels. The capacity of ACO-OFDM cannot be compared with the general case of an intensity modulated signal, because no general solution for this has yet been found. Instead the mutual information of an ACO-OFDM signal is calculated and compared with two other signal distributions: an exponential distribution and a clipped-Gaussian distribution. The combination of OFDM and MIMO has been used in several radio standards. However, MIMO OFDM systems like single-input single-output (SISO) OFDM systems are very sensitive to CFO. An improved MIMO PCC OFDM system is proposed in this thesis. In this system, the signals transmitted from different antennae have different time offsets. At the receiver, OFDM symbols are demodulated by using each of these time offsets in a matched detection window. This system shows better performance than a MIMO PCC OFDM system without time offsets in two cases: when there are channel estimation errors and in MIMO systems with more transmit antennae than receive antennae.

Underwater wireless optical communication (UWOC) has emerged as a key technology for next-generation underwater wireless data transmission. Semantic communication is regarded as a promising communication paradigm to improve the efficiency and robustness of UWOC systems. However, existing underwater wireless optical semantic communication (UWOSC) system lacks the capability to adapt the code rate to different channel states, leading to intelligence inadequacy and coding inefficiency. In this paper, we introduce environment semantics into the UWOSC system for the first time, and propose an environment semantics aided underwater wireless optical semantic communication (ESA-UWOSC) framework. Based on this framework, we develop a system leveraging computer vision and deep learning for adaptive rate transmission. The ESA-UWOSC system is realized by a channel state perception mechanism (CSPM) for extracting environment semantics from the channel environment image to obtain the channel gain, a link network (LinkNet) for enabling variable code rate transmission of source semantic information, and a prediction network (PreNet) for predicting the optimal code length that satisfies the transmission task requirement. The combined operation of these three modules enables the ESA-UWOSC system to actively perceive channel state and perform adaptive rate transmission accordingly. In a series of experiments conducted on an emulated UWOC experimental platform, the results show that the ESA-UWOSC system outperforms the baseline scheme, achieving superior performance with lower bandwidth consumption, especially in harsh channel environments. Moreover, the results also demonstrate that the ESA-UWOSC system can achieve adaptive rate transmission based on the perceived channel state to meet transmission task requirement.

The development of optical communications systems from long fibers to compact wireless networks has led to the possibility of linking two satellites using a wireless optical link. The optical communication systems have been upgraded to use of wireless systems. The two satellites can be connected using optical wireless link. The optical wireless communication allows the exchange of information between two satellites at a higher speed. To act as a source of light, laser can be used and as an optical link, and a high-frequency light signal can be used. In this paper, we are analyzing and optimizing the different aspects of an optical communication link by selecting different modulation scheme. The IS-OWC link has been analyzed using PSK, DPSK, and OQPSK. The bit error rate (BER) and Q-factor have been used as the main determinant to analyze the performance of optical link between two satellites.

SummaryThe emergence of 5G and 6G technologies has resulted in a faster, more automated, and more intelligent data flow in the digital world. A wide spectrum of optical wireless communication is made available to support 5G and 6G technologies, and it can meet the demands of high‐speed optical wireless communication. Trade‐offs involving latency, power consumption, deployment costs, and hardware/computational overhead have always been a part of 5G/6G technologies. The decline in the trade‐offs indicated above has major implications for networking capacity. The proposed work modulated a new grid‐based communication protocol (GCP) with elephant swarm optimization (ESO) to surpass those issues with optical wireless communication devices. This protocol employs an integrated strategy to split the network and choose the cluster head effectively. Different GCP‐ESO‐related algorithms have been explored. Various components of the OSNs, including network energy usage, cluster, and non‐cluster header nodes, are also calculated using mathematical analysis. The results have been simulated using MATLAB software. The simulation precisely demonstrates that the proposed GCP‐ESO outperforms the emerging MCH‐EOR and RDSAOA‐EECP methods concerning energy consumption (0.46 J), packet loss ratio (PLR) (0.60%), end‐to‐end latency (11.0 s), packet delivery rate (PDR) (99.41%), routing (0.112), the lifetime of the network (3718 s), and throughput (0.989 Mbps).