In this paper, we experimentally demonstrate and analyze the performance of software-defined radio (SDR)-based hybrid optical wireless communication (OWC) links for indoor Internet-of-Things (IoT) networks. We propose all-optical hybrid visible light communication (VLC) and optical camera communication (OCC) using a single light emitting diode (LED) as a transmitter and photodiode (PD) and image sensor (IS) based receivers for simultaneous high-and low-speed data transmission. We present a testbed using an SDR platform, i.e., the ADALM-PLUTO, to transmit and receive high-and low-speed signals in the very-high frequency band and a media converter as part of an OWC link. We evaluate the performance of the proposed hybrid system employing a chip LED modulated with quadrature phase shift keying signal in terms of the bit error rate and the reception success rate$\boldsymbol{R}_{\mathbf{rs}}$. We show that the proposed scheme can provide an independent link performance, irrespective of the significant differences between the operating data rates of VLC and OCC links. The results show that for a link span of 1 m, we achieve (i) error-free detection for high-speed PD-based VLC; and (ii) 100 %$\boldsymbol{R}_{\mathbf{rs}}$at varying modulation frequencies for low-speed IS-based OCC.

Semantic communication subverts classical communication techniques by changing the focus on the preservation of meaning rather than on correctly transmitting logical bits. This is achieved by implementing a semantic encoder/decoder pair that operates at a higher level of abstraction and takes into account the nature of the source and destination, as well as their shared knowledge. One approach for the implementation of semantic encoder/decoder pairs is the use of artificial neural networks, and in particular, variational autoencoders. In this work, we propose the use of hierarchical quadrature amplitude modulation to improve the perceived quality of data in a semantic communication link using a variational autoencoder. We show that the use of the hierarchical modulation consistently improves the performance of the semantic communication link in terms of the SSIM under noisy channel scenarios, even achieving the overcoming of noise for signal-to-noise ratio values above 10 dB.

Visible light positioning (VLP) has become one of the most important technologies for providing highly accurate location-based services. Despite this, line-of-sight (LOS) VLP systems have severe limitations when operating in complex and dynamic indoor environments. We propose a non-LOS VLP system based on an image sensor (IS) and pixel coordinate fingerprinting operating under shadowing and blocking. The system does not require communication between the transmitter and the receiver eliminating the need for a complex modulation process. For the first time, we utilize the highlighted center pixel coordinates on an IS as the fingerprint features and extract highlight center coordinates using the deep learning model Yolov8. Furthermore, we propose an Elastic Net regression with Weighted K-Nearest Neighbor Residual Correction algorithm to improve the positioning performance, which employs an Elastic Net for global prediction and a weight k-nearest neighbor algorithm to correct the global prediction residuals by leveraging the residual information from weighted neighboring samples. The experimental results show that the average positioning error and 90th percentile errors of the proposed system are 4.77 and 6.67 cm, respectively, with only 16 training points. In addition, the stability of the system is verified by arbitrary and diagonal sets.

Visible light positioning (VLP) is a low-cost, highly accurate alternative localization technology for indoor applications that makes use of existing light emitting diode (LED)-based lights, which is highly accurate and low costs. It is, however, a major challenge for the existing VLP systems to achieve line-of-sight (LOS) positioning in complex and variable indoor environments. We propose a non-LOS (NLOS) fingerprint-based VLP system based on a multipixel photon counter (MPPC) to address the problem of obstructed LOS paths. Using MPPC, very faint light can be detected with a very high sensitivity and excellent photon counting capability, which enhances the ability to recognize and detect signals in an NLOS environment. We propose a novel method of generating fingerprint database using the NLOS channel model, which construct the relationship between the received signal strength and the distance from MPPC to the virtual image of LED interpolated by only knowing the distance between the LED and the interpolation position. Furthermore, we propose an optimal parameter weighted K-nearest neighbor algorithm, which utilizes the mean absolute error (MAE) as the evaluation metric. In this algorithm, a grid search method is employed to determine the optimal number of neighbors and the distance metric for each test point, thereby enhancing the positioning accuracy. Using only 25 offline measurements, the measured average positioning error (PE) and 90th percentile error are 4.02 and 9.98 cm, respectively, when the MPPC height is 70 cm.

As part of Internet of Things networks, visible light communication (VLC) technology has excelled in recent years. Compared to radio frequency (RF) technologies, VLC offers unlicensed spectrum, low interference, high security, and energy efficiency, making it ideal for short-range communication. However, its performance is severely limited under the non-line-of-sight (NLOS) links. Intelligent reflecting surface (IRS), as an innovative technology, can dynamically and passively adjust the wireless environment. In this article, we propose the use of optical IRSs (OIRSs) to enhance VLC performance under NLOS scenarios. Specifically, we develop ZEMAX-based nonsequential ray tracing to accurately model light propagation in OIRS-assisted indoor VLC environments. Then, we establish a system model and formulate an optimization problem aimed at maximizing the received optical power. To solve this, we propose a genetic-enhanced sine–cosine algorithm with elite strategy and dynamic opposition learning (GESCA-ED), which optimizes the rotational degrees of freedom of each OIRS element. Furthermore, we design a hardware-based experimental platform featuring a configurable OIRS control system, which is integrated with GESCA-ED for real-time optimization in an actual NLOS environment. Experimental results demonstrate significant improvements in received optical power, bit error rate (BER), and transmission distance, achieving a BER of$1\times 10^{-6}$over a 3.8-m NLOS link. These results confirm that the proposed system effectively extends signal coverage, enabling communication between mobile users regardless of their spatial positions.

In this paper, two techniques including uplink positioning and multiple-input multiple output (MIMO) singular value decomposition (SVD) beamforming are proposed and utilized to provide a novel physical layer security (PLS) scheme for visible light communication (VLC) systems, which much enhance the integrated location-sensing and data security strength. The paper first develops a closed-form mathematical model for ubiquitous uplink positioning using a single light-emitting diode with the trilateration algorithm, considering the field-of-view constraints due to the user’s optical transmitter and access point receivers. SVD in MIMO VLC is subsequently introduced to exploit the acquired user’s location to form a pair of public and private keys for secure data transfer over the physical medium. The proposed system performance is evaluated in terms of the root mean square positioning error$\varepsilon _{\mathbb {X}}$and the achieved bit error rate (BER) for the uplink positioning and the downlink data transmission, respectively. Results show that the effective area of the photodiode significantly impacts$\varepsilon _{\mathbb {X}}$, with smaller areas resulting in higher$\varepsilon _{\mathbb {X}}$. It is also observed that the$\varepsilon _{\mathbb {X}}$directly relates to the strength of ambient light inducing noise in the system. However, the proposed uplink positioning is less prone to natural ambient light as compared to the traditional downlink visible light positioning. The BER analysis of the downlink data transmission shows that the proposed SVD-based PLS enhances the system’s confidentiality by significantly narrowing the communication zone area by over 90% in comparison to the zero-forcing beamforming scheme and achieves reasonable secrecy capacity.

Visible light positioning (VLP) is one of the effective solutions for low-cost and high-precision indoor positioning due to its advantages of low cost, high energy efficiency, and immunity from radio frequency induced electromagnetic interference. However, severe performance limitations in occluded environments present a major challenge for the development of VLP. We propose a non-line-of-sight (NLOS) VLP system based on a multipixel photon counter (MPPC) to solve the problem of obstructed line-of-sight (LOS) paths. MPPC consists of multiple avalanche photodiodes operating in Geiger mode, which has very high sensitivity, excellent photon counting capability and can perform detection of very weak light at photon counting levels. It can effectively improve the recognition and detection of signals in NLOS environments. Using the NLOS channel model, we innovatively propose a NLOS-MPPC-VLP positioning theory, which utilizes the NLOS optical signals received by the MPPC to construct the relationship between the received signal strength and the distance from the MPPC to the virtual images of the light emitting diode (LED). Through this theory, the trilateration algorithm is used to calculate the position of the receiver. The experimental results show that the average positioning errors for the proposed system are 8.77 and 10.15 cm for 70 and 100-cm MPPC heights, respectively.

Positioning and sensing have long been an important area of research. Recently, this field has attracted more attention due to the rapid deployment of emerging applications and next-generation communication networks. On the one hand, emerging applications like extended reality (XR) and autonomous vehicle systems need to precisely “see” the physical world, thus greatly increasing the demands on positioning and sensing technologies. Moreover, these applications also require data rate communication links, and thus technologies like cellular networks and WiFi are excellent for supporting these applications. On the other hand, with the evolution of wireless networks, positioning, and sensing have also been considered important functions of future wireless networks that can further enhance communication performance. Although existing wireless communication has achieved significant success in the past several decades, achieving satisfying positioning and sensing performance for these emerging applications remains a challenge due to the complexity of the wireless environment and the stringent performance requirements.

Positioning has recently received considerable attention as a key enabler in emerging applications such as extended reality, unmanned aerial vehicles, and smart environments. These applications require both data communication and high-precision positioning, and thus they are particularly well-suited to be offered in wireless networks (WNs). The purpose of this paper is to provide a comprehensive overview of existing works and new trends in the field of positioning techniques from both academic and standard perspectives. The paper provides a comprehensive overview of indoor positioning in WNs, covering the background, applications, measurements, state-of-the-art technologies, and future challenges. The paper outlines the applications of positioning from the perspectives of public facilities, enterprises, and individual users. We investigate the key performance indicators and measurements of positioning systems, followed by the review of the key enabler techniques such as artificial intelligence/large models and adaptive systems. Next, we discuss a number of typical wireless positioning technologies. We extend our overview beyond the academic progress, to include the standardization efforts, and finally, we provide insight into the challenges that remain. The comprehensive overview of existing efforts and new trends in the field of indoor positioning from both academic and standardization perspectives would be a useful reference to researchers in the field.

This is Part II of the double-part Special Issue (SI) on Positioning and Sensing Over Wireless Networks. The two-part SI aims to bring cutting-edge and novel contributions on positioning and sensing over wireless networks for future and emerging applications. The accepted 51 papers are arranged into eight groups: 1) fundamental performance analysis and optimization; 2) positioning and sensing with cellular networks; 3) positioning and sensing with WiFi networks; 4) positioning and sensing with emerging communication technologies; 5) positioning and sensing applications; 6) cooperative positioning and sensing; 7) reconfigurable intelligent surfaces (RIS)-assisted positioning and sensing; and 8) privacy and security. The contributions made by the papers in Part II are summarized as follows, which correspond to the last four paper groups.

The use of quantum key distribution (QKD) to improve security may impact the quality of service (Qos) of high rate classical data links. The key generation process with an empty buffer can be modeled as a queuing system, and data throughput limits can be determined using queueing theory to assess the impact on the QoS. The proposed key generation model is based on an on-demand paradigm. Based on the results of this study, a discrete variable (DV)-QKD-secured link could be designed using the data rate bounds to guarantee a given QoS. Furthermore, we validate the derived bounds by comparing the theoretical derivations with simulations. We show that in a one-time-pad system, the achievable throughput approaches 0.59 times the secret key rate as the key length increases. In the case of a key scheme with key reuse, the throughput is asymptotically linear with the number of key reuses, and the slope of the curve corresponds to the secret key rate multiplied by a function of the protocol and channel characteristics. The analysis is carried out on optical fiber and free-space links.

This paper proposes handover-aware scheduling solutions for multi-cell small-and large-scale visible-light communication networks, enabling soft handover. For this, we coordinate the transmissions at the access points (APs), serving the users in different time slots based on their locations with respect to the AP coverage areas for an efficient utilization of the resources. For scenarios where coverage in large areas is needed, we additionally propose clustering solutions to decrease the handover rate. Compared with non-coordinated schemes, the proposed soft handover techniques offers improved performance in terms of user achievable throughput and link reliability.

Visible light positioning (VLP) is playing a critical role in delivering better location-based services. However, traditional VLP systems rely on line-of-sight (LOS) paths and have a requirement for large numbers of light-emitting diodes (LEDs) and sensors, making them unprepared for various scenarios. This article proposes a 3-D non-line-of-sight (NLOS) VLP system using a single LED and an image sensor (IS) to address the problem of obstructed LOS paths. On the one hand, an optical camera communication (OCC) subsystem is designed to receive the transmitter’s position based on NLOS links. In contrast, two highlights are visible in the picture when the IS captures the reflected light from the floor. Using the proposed luminance distribution model (LDM), it can be demonstrated that the two highlights can be regarded as the projections formed by two virtual LEDs. Based on the projection equations of the two virtual LEDs, an image sensing algorithm can estimate the receiver’s position. Experimental results show that the proposed system can achieve a 90th percentile accuracy of < 22 cm for 3-D positioning. To the best of author’s knowledge, this is the first work to demonstrate 3-D NLOS VLP using just a single LED and an IS. Additionally, the proposed LDM is the first physically based model for the first reflected light to estimate the channel gain of an NLOS link.

A low-complexity and high-performance clipping-free multilayer optical orthogonal frequency division multiplexing scheme is proposed for optical wireless communication (OWC). At the transmitter, the symbols of multi-layer quadrature amplitude modulation (QAM) are first arranged according to a specific rule in the frequency domain to generate a bipolar time-domain signal using a single inverse fast Fourier transform (IFFT). Then the bipolar signal is superimposed with an adaptive periodic DC bias to meet the real and positive requirements in intensity modulation / direct detection (IM/DD) systems. At the receiver, a zero-value regression (Re0) -based demodulation is proposed to optimize the bit error rate (BER). As the signal generation and demodulation are realized by single IFFT and fast Fourier transform (FFT) respectively, the proposed scheme has nearly the same system complexity as the single-layer modulations and similar spectral efficiency with those advanced multilayer modulations. In addition, the proposed periodic DC improves the power efficiency and avoids the interlayer interference. Numerical simulation results show that, compared with LACO-OFDM with 3 layers, the proposed scheme can attain a PAPR gain of 2.4 dB and BER gain of 1.4 dB and 3 dB for the linear and nonlinear channel, respectively.

Visible-light communication (VLC) is an emerging technology utilizing light-emitting diodes and photodetectors (PDs) and cameras [i.e., image sensors (ISs)] as the transmitter and receivers (Rx), respectively, for simultaneous data communications, illumination, localization, and sensing in the indoor optical Internet of Things. We propose a single-input–multiple-output (SIMO) hybrid VLC system using PD-based and IS-based Rxs for simultaneous high-speed and low-speed (i.e.,$R_{b-{\mathrm{ High}}}$and$R_{b-{\mathrm{ Low}}}$) data transmission. In addition, we propose amplitude overlapping${A} _{\mathrm{ OL}}$in the modulation format to increase$R_{b}$and reduce the attenuation due to the high-pass filter effect of the bias-T. We experimentally evaluate the performance of the proposed scheme considering the interlink impacts. Results show that at${A} _{\mathrm{ OL}}$of 0–0.2: 1) the measured bit error rate for the PD-based VLC is below the forward error correction limit of$3.8 \times 10^{-3}$for$R_{b-{\mathrm{ Low}}}$of 2.5 and 5 kb/s at$R_{b-{\mathrm{ High}}}$of 35 and 60 Mb/s, respectively, and 2) for the IS-based VLC link with the camera gain of 4 dB and$R_{b-{\mathrm{ High}}}$of up to 70 Mb/s, the reception success rates are >96 and 90% at$R_{b-{\mathrm{ Low}}}$of 2.5 and 5 kb/s, respectively.

In this work, a vehicle-to-vehicle (V2V) visible light communications (VLC) model for twopractical scenarios, is proposed. Inscenario 1, the random lateral shift of vehicles and the deterministic longitudinal separation between two communicating vehicles are considered, whereas inscenario 2, longitudinal separation between two vehicles is considered to be random, and lateral shift of vehicles is considered to be deterministic. To this end, we emphasizecomprehensive modelingof thepractical characteristicsof the considered V2V-VLC system, such asrandom path lossdue to therandom mobilityof the vehicle,random lateral shiftandrandom longitudinal separationof the vehicle. Moreover, we analyze the performance of the proposed V2V-VLC model in terms of different metrics under the consideration of anovel channel model. Considering ourfindings, it is observed that the random lateral shift of the vehicle and the random longitudinal separation between two vehicles have a significant impact on the V2V-VLC system performance. Further, at a distance of 40 m, for example, the path losspenaltiesfor moderate and dense fog weather scenarios are 2 and 3 dB, respectively, compared with the clear weather. Furthermore, thecombined impactof path loss and atmospheric turbulence affects the V2V-VLC performance significantly.

In this work, a new indoor visible light positioning algorithm is proposed based on support vector machines (SVM) and polynomial regression. Two different multipath environments of an empty room and a furnished room are considered. The algorithm starts by addressing the received power distance relation, considering polynomial regression models fitted to the specific areas of the room. In the second stage, an SVM is used to classify the best-fitted polynomial, which is used with nonlinear least squares to estimate the position of the receiver. The results show that, in an empty room, the positioning accuracy improvement for the positioning error, εpof 2.5 cm are 36.1, 58.3, and 72.2 % for three different scenarios according to the regions’ distribution in the room. For the furnished room, a positioning relative accuracy improvement of 214, 170, and 100 % is observed for εpof 0.1, 0.2, and 0.3 m, respectively.

One of the major challenges in indoor vehicular visible light communications (VVLC) links is associated with the mobility leading to frequent handovers, consequently resulting in increased latency and processing time. Therefore, to address this challenge, an acceptable light coverage within the indoor channel, i.e., cell planning, is of paramount importance. In this paper we investigate the performance of a VLC link for an autonomous indoor vehicle and provide insights on the impact of the distance between light access points, and the light client’s (LC’s) angular field of view (AFOV) employing an imaging optical concentrator with a photodiode. We derive a mathematical expression to determine the optimum AFOV of the LC as a function of other system parameters such as the vertical link distance and incidence angles. Results show step responses in the bit-error-rate (BER) performance as the incidence angle increases outside the AFOV of the LC with the BER degrading sharply from 3.8 × 10−3to 4.4 × 10−1with a small increment in the incidence angle over 7.35 °.

In this paper, we study the novel idea of fiber optic lighting - optical camera communications. We carry out analyses of the proposed scheme employing a light emitting diode (LED) illuminated plastic optical fiber (POF) and a rolling-shutter-based camera as the transmitter and the receiver (Rx), respectively in an indoor static environment. We also provide optical and electrical characterization of the LED and LED coupled POF illumination sources. The experiment results demonstrate that, despite the small diameter of the illuminating POF, flicker-free wireless communications at modulation frequencies of 300 and 600 Hz over the transmission distances of 50 and 75 cm can be achieved. Consequently, we show that a 100 % reception success rate is achieved for the camera-based optical Rx with the exposure time and the gain of 400 µs and 25 dB, respectively.

This paper addresses the effects of blocking and shadowing due to peoples’ movement on the performance of visible light positioning system based on received signal strength. We consider both line-of-sight and non-line-of-sight paths and using a non-sequential ray-tracing model, the channel is characterised with and without blocking and shadowing. A linear least square with a polynomial regression model is proposed for the position estimation. We show that, the VLP accuracy degrade by ~90 and ~67% due to blocking and shadowing, respectively. Besides, at a confidence interval of 90%, the positioning error εpof 1.47 m is recorded with human blockage as compared with 0.15 m for the non-blocking cases while shadowing introduces εpof 0.82 m compared with 0.27 m for the non-blocking case.

Layered or hybrid optical orthogonal frequency division multiplexing (OFDM) has been proposed for use in optical communications due to its excellent spectral and power efficiencies, especially in visible light communications (VLC). However, most of the current works concentrate on transmitter and receiver design as well as the quality of service in communication networks. In this paper, we propose a spectrum-efficient dimmable triple-layer hybrid optical OFDM (DTH-OFDM) scheme to tackle the illumination requirements, considering different practical indoor VLC scenarios from low illumination to high illumination intensities. In the proposed DTH-OFDM scheme, the required dimming level is achieved by jointly adjusting the dimming factors and direct current bias. We investigate the comprehensive performance analysis of the proposed DTH-OFDM in detail, including probability density function, bit error rate (BER), spectral and energy efficiencies. In addition, a joint dimming control and optimal power allocation problem for DTH-OFDM is formulated and solved using convex optimization under the constraints of light emitting diode (LED) nonlinearity, dimming target and communications reliability. Numerical results show that, the proposed DTH-OFDM can offer continuous and arbitrary dimming target with higher spectral efficiency and lower BER compared with its counterparts, as well as an enhanced tolerance to the LED nonlinearity.

Visible light communications (VLC) has been introduced as a complementary wireless technology that can be widely used in industrial indoor environments where automated guided vehicles aim to ease and accelerate logistics. Despite its advantages, there is one significant drawback of using an indoor vehicular VLC (V-VLC) network that is there is a high handover outage duration. In line-of-sight VLC links, such handovers are frequently due to mobility, shadowing, and obstacles. In this paper, we propose a frequency diversity and link aggregation solution, which is a novel technique in Data link layer to tackle handover challenge in indoor V-VLC networks. We have developed a small-scale prototype and experimentally evaluated its performance for a variety of scenarios and compared the results with other handover techniques. We also assessed the configuration options in more detail, in particular focusing on different network traffic types and various address resolution protocol intervals. The measurement results demonstrate the advantages of our approach for low-outage duration handovers in V-VLC. The proposed idea is able to decrease the handover outage duration in a two-dimensional network to about 0.2 s, which is considerably lower compared to previous solutions.

The authors propose an integrated Mach–Zehnder interferometer and diversity combining receiver to mitigate the atmospheric turbulence‐induced fading in a millimetre‐wave (mmW) radio‐over‐free‐space optics (RoFSO) link. They use a carrier frequency of 28 GHz as recommended for the fifth‐generation wireless access networks and consider two optical mmW signal generation schemes, namely double‐sideband (DSB) and single‐sideband (SSB). In direct detection (DD)‐based RoFSO, the link performance is limited by atmospheric turbulence. They show that the proposed Rx can overcome this detrimental effect, which is verified by investigation of a 10 Gb/s 16‐quadrature amplitude modulation orthogonal frequency‐division‐multiplexing signal at 28 GHz over a 1 km free‐space optics link under weak and strong turbulence regimes. For the DSB scenario, the proposed Rx offers improved error vector magnitudes of about 0.8 and 5.7%, and modulation error ratios of 1.3 and 4.9 dB under weak and strong turbulence regimes, respectively, compared with the DD receiver (DD Rx). For the SSB scenario under weak turbulence, the proposed Rx achieves a 4 dB improvement in the receiver sensitivity and four orders of magnitude enhancement in the bit error rate over the DD Rx. The proposed Rx can be integrated on a single chip for further cost reduction.

In this paper, we study the impact of correlation on the bit error rate (BER) and the channel capacity of a free-space optical (FSO) multiple-input-multiple-output (MIMO) system employing optical space shift keying (OSSK) over a fading channel. In order to study a practical correlated channel, we consider the effect of channel correlation due to both small-and large-scale eddies and show that the use of OSSK over correlated FSO channel can lead to an improved system performance with increasing correlation level of upto 0.9. In this work, we first develop an analytical framework for different performance metrics of the OSSK multiple-input single-output system with correlation and then extend our investigation by proposing an asymptotically accurate mathematical framework for MIMO. We also validate all the analytical results using MATLAB simulations. Finally, we develop an experimental setup of FSO with two correlated links to study the throughput and latency of the links at different turbulence levels.

In this paper, various link characteristics and bit error rate (BER) performance of a free-space optical (FSO) multiple-input-single-output (MISO) system with repetition coding (RC) is investigated both experimentally and theoretically. The study is performed in the presence of atmospheric turbulence (AT) over a correlated channel with an erbium- doped fiber amplifier employed at the receiver (Rx). It is shown experimentally that the measured AT levels of two parallel links are different and the links experience non-identical fading influence. The analytical BER expression for an optically pre-amplified correlated FSOMISO system with non-identical AT channel is derived. The BER results of the considered system with pre- amplifier is compared with the BER results of an FSO system without pre-amplifier, and it is shown analytically that a 2 X 1 FSO system with pre-amplifier offers better BER performance in terms of coding gain of /spl approx/ 6 dB over the system without pre-amplifier. Moreover, it is also shown that the FSO system with RC gives better performance with identical links as compared to non- identical links. Further, it is also revealed that the diversity order of the considered system is independent of correlation and use of pre- amplifier. All the analytical results are validated by comparing them with Matlab simulations.

Visible light communication systems often suffer from high frequency attenuation when transmitting out-of-band. This effect has been ameliorated by multi-band modulations such as multi-band carrier-less amplitude and phase (m-CAP), which minimises the effect of decreased high frequency magnitude and maximises signal-to-noise ratio-per-sub-band. On the other hand, in the pass-band region, super-Nyquist CAP (SCAP) can offer throughput improvements with no additional complexity at the receiver, at the cost of bit error rate. We propose, for the first time, a new hybrid SCAP modulation format that takes advantageous of both SCAP (i.e. overlapped sub-bands within the modulation bandwidth) and conventional m-CAP (orthogonally spaced bands outside the modulation bandwidth) while maintaining isolation between noise sources. We show higher baud rates within the passband region whilst supporting out-of-band transmission at lower error vector magnitudes.

We experimentally demonstrate a multicarrier interleave division multiple access (IDMA) scheme for bidirectional visible light communications using orthogonal frequency division multiplexing (OFDM), which has the advantages of good tolerance against multi‐path, high spectral efficiency and excellent transmission performance. The generated IDMA signals are modulated on OFDM subcarriers and the different users are separated by the interleavers. When compared with orthogonal frequency domain multiple access, IDMA‐OFDM offers increased transmission spans of 30 and 50 cm at a bit error rate (BER) of 10 −3 in the cases of two and four users, respectively.

Accurate orientation information is the key in many applications, ranging from map reconstruction with crowdsourcing data, location data analytics, to accurate indoor localization. Many existing solutions rely on noisy magnetic and inertial sensor data, leading to limited accuracy, while others leverage multiple, dense anchor points to improve the accuracy, requiring significant deployment efforts. This paper presents LiCompass, the first system that enables a commodity camera to accurately estimate the object orientation using just a single optical anchor. Our key idea is to allow a camera to observe varying intensity level of polarized light when it is in different orientations and, hence, perform estimation directly from image pixel intensity. As the estimation relies only on pixel intensity, instead of the location of the anchor in an image, the system performs reliably at long distance, with low resolution images, and with large perspective distortion. LiCompass' core designs include an elaborate optical anchor design and a series of signal processing techniques based on trigonometric properties, which extend the range of orientation estimation to full 360 degrees. Our prototype evaluation shows that LiCompass produces very accurate estimates with median errors of merely 2.5 degrees at 5 meters and 7.4 degrees at 2.5 meters with an irradiance angle of 55 degrees.

In visible light communications (VLC) system, the linearity of the light emitting diode is an issue when being intensity modulated with an analogue signal. In optical orthogonal frequency division multiplexing (O-OFDM) based VLC systems, the source nonlinearity imposes a tight restriction on the achievable peak-to-average-power-ratio. To address this issue, we propose a hybrid OFDM-PTM (pulse time modulation) scheme, where a bipolar O-OFDM signal is converted into digital PTM formats for switching `on' and `off' the light source. The simulation results demonstrate that the proposed scheme offers an improved bit error rate performance compared to the traditional asymmetrically clipped O-OFDM (ACO-OFDM).

The performance of a free space optical (FSO) communication system is significantly affected by atmospheric turbulence and pointing errors (PEs) apart from the additive noise which is assumed to be Gaussian. The Málaga or M-distribution encompasses various proposed statistical models for atmospheric turbulence in FSO systems. An optical pre-amplifier is an essential component of FSO systems for improving the receiver (Rx) sensitivity. However, optical pre-amplification results in amplified spontaneous emissions (ASE), which dominate the Rx thermal and shot noises. The square law photodetection process at the Rx in a FSO system necessitates the consideration of Chi-square statistics for the decision variable contrary to the Gaussian approximation that is widely used in the literature. In this paper, we evaluate the bit error rate (BER) performance of a FSO system assuming non-return-to-zero on-off keying modulation in the presence of ASE noise under M-turbulence and PEs. We also derive asymptotic BER expressions for the considered FSO system for large values of signal-to-noise ratio in terms of simple elementary functions. A further insight into the system is provided by performing the diversity analysis.

The authors present efficient frequency‐domain channel estimation methods based on the intra‐symbol frequency‐domain averaging (ISFA), minimum mean squared error (MMSE) and weighted inter‐frame averaging (WIFA) schemes for the orthogonal frequency division multiplexing (OFDM) visible light communications (VLC) system. OFDM‐VLC with quadrature phase shift keying, 16‐ and 64‐quadrature amplitude modulation mapping is experimentally demonstrated. Compared with the conventional least square channel estimation method, ISFA, MMSE and WIFA offer improved performance with MMSE offering the best performance in terms of the error vector magnitude but at the cost of high complexity. The authors show that the WIFA can improve the estimation accuracy of time‐varying VLC optical channel.

In this paper we present experimental results from extended measurement campaign for an analog dual polarization (DP) radio over fiber and radio over free space optics (FSO) link as part of the cloud mobile network architecture. System utilization with long-term evolution (LTE) test models is described and different link types are evaluated in terms of the error vector magnitude (EVM). Results imply that combined radio over fiber and radio over FSO schemes in combination with Erbium-doped fiber amplifier (EDFA) can provide almost the same performance for a fiber subpart length of between 10 km to 50 km. Moreover, both sub-systems can operate with a high dynamic range and meet the required EVM limit of 9% for 64-quadrature amplitude modulation (QAM) with the minimum optical signal to noise ratio (OSNR) of 24 dB for a 50 km fiber link and a LTE bandwidth 20 MHz. In addition, the combination of 10 km fiber and FSO link is tested under turbulence FSO channel condition showing EVM values below the threshold of 9 % even under a strong turbulence regime.

We propose a quantized feedback-based differential signaling scheme for 2 × 1 free space optical (FSO) multiple-input single-output communication systems. The analytical bit error rate (BER) for the proposed differential signaling scheme is derived and compared with the existing differential signaling scheme for FSO systems. The asymptotic BER of the proposed scheme is also determined and it is shown by using the asymptotic BER expression that the proposed scheme achieves full spatial diversity of two. Moreover, the effect of the feedback error on the performance of the proposed scheme is studied and it is demonstrated by simulation-based and analytical results that the proposed scheme significantly outperforms a previously proposed differential signaling scheme in terms of turbulence fades reduction for tolerable feedback errors.

In this paper, we study the equal gain combining (EGC) scheme in free space optical (FSO) multiple-input multiple-output (MIMO) communication systems over the Gamma-Gamma fading channel with pointing errors. The probability density function (PDF) of the Gamma-Gamma fading with pointing errors contains the Meijer-G function, which makes the error analysis of the considered system very difficult (almost undoable). We propose a new power series based representation for the PDF of the Gamma-Gamma fading with pointing errors. The average bit error rate (BER) for the EGC scheme is derived by using the proposed series based expression of the PDF. We also derive the asymptotic BER for the EGC scheme. The effect of pointing errors over the performance of the scheme is analyzed under different scenarios and it is observed that the pointing errors significantly degrade the diversity of the FSO MIMO system; but under strong atmospheric turbulence pointing errors can be neglected.

In this study, the authors present the measurements performed on a free space optics (FSO) communications link using an indoor atmospheric chamber. In particular, the authors have generated several different optical turbulence conditions, demonstrating how even the weak turbulence regime can strongly affect the FSO link performance. The authors have carried out an in‐depth analysis of the data collected during the measurements, and calculated the turbulence strength (i.e. scintillation index and Rytov variance) and the important performance metrics (i.e. the Q ‐factor and bit error rate) to evaluate the FSO link quality. Moreover, the authors have tested, for the first time, an appositely developed temporally‐correlated gamma–gamma channel model to generate the temporal irradiance fluctuations observed at the receiver. This has been accomplished by using a complete analysis tool that enables the authors to fully simulate the experimental FSO link. Finally, the authors compare the generated time‐series with the collected experimental data, showing a good agreement and thus proving the effectiveness of the model.

In free space optical (FSO) communications, several factors can strongly affect the link quality. Among them, one of the most important impairments that can degrade the FSO link quality and its reliability – even under the clear sky conditions – consists of optical turbulence. In this work, the authors investigate the generation of both weak and moderate turbulence regimes in an indoor environment to assess the FSO link quality. In particular, they show that, due to the presence of the turbulence, the link experiences both erasure errors and packet losses during transmission, and also compare the experimental statistical distribution of samples with the predicted Gamma–Gamma model. Furthermore, the authors demonstrate that the application of the RaptorQ codes noticeably improves the link quality decreasing the packet error rate (PER) by about an order of magnitude, also offering – in certain cases – an error‐free transmission with a PER of ∼10 −2 at Rytov variance value of 0.5. The results show that the recovery rate increases with the redundancy, the packet length and the number of source packets, and it decreases with increasing data rates.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications.

We propose an analytical approach to evaluate the performance of space‐diversity free‐space optical (FSO) systems over correlated Gamma–Gamma (ΓΓ) fading channels. To do this, we firstly derive an infinite series representation for the moment generating function (MGF) of the sum of arbitrarily correlated ΓΓ random variables (RVs). Then, a closed‐form approximate expression for the probability density function (PDF) of the sum of ΓΓ RVs is obtained from the MGF with the aid of Padé approximation. The performance of the FSO system is then evaluated based on this PDF. Through some numerical results we illustrate the accuracy of the proposed method and further discuss its limitations.

This paper presents the first ever experimental demonstration of a multiple-input multiple-output (MIMO) visible light communications system employing four silicon (Si) light emitting diodes (LEDs) and four organic photodetectors (OPDs) as transmitters and receivers, respectively. The proposed link is relatively low cost and it employs the on-off keying (OOK) modulation format offering a data rate of 200 kb/s without the need for equalization, which is a significant increase compared with previous non-equalized systems. In order to speed up date rates further, we implement an artificial neural network (ANN) to classify the signal and correct the error induced by the matrix inversion at the receiver, allowing a gross bit rate of 1.8 Mb/s in the best case.

This paper reveals that nonlinear propagation is a critical problem for the long term evolution (LTE) technology based on the radio-over-fibre (RoF) system. Therefore, a direct modulation based frequency dithering (DMFD) method is proposed for nonlinear propagation compensation, namely the self phase modulation (SPM) and stimulated brillouin scattering (SBS). We found that DMFD method operates substantially different in RoF systems. The major difference is that DMFD signal frequency fdhas to be much smaller than the RoF carrier frequency fRF; thus the condition of {fdRF} has to be stringently obeyed. Analysis of the optical launch power (OLP) with DMFD method reveals that the SBS threshold is above ~6 dBm for LTE-RoF system. In addition, we also unfold that DMFD method does not induce an additional distortion for the linear and optimum OLP regions, which are frequency chirp dependent. Hence the proposed method improves the LTE-RoF system without any shortcoming. Finally, at OLP values of 8 dBm and 10 dBm, LTE-RoF system exhibits an average error vector magnitude (EVM) improvement of ~4.32% and ~6.18%, respectively, for the 50 km transmission span.

In this paper, we present the experimental results free space optics (FSO) communication systems operating at visible and near infrared wavelengths (from 0.6 μm to 1.6 μm) under fog conditions. Different empirical fog models are typically used to characterize the fog attenuation of optical beam in FSO systems. A number of empirical fog models are evaluated in order to verify them experimentally and to fill several unmeasured gaps within the entire spectrum from the visible - near infrared (NIR) range for light to dense fog conditions. The experimental results in the controlled laboratory fog environment are compared with the selected empirical fog models in order to practically validate their performance over wide range of wavelengths. The results indicate wavelength dependency of the fog attenuation for visibilities higher than 15 m, whereas the cases with shorter visibilities contradict the Kim model.

This paper introduces a Bayesian model to predict and classify the mobility of a node in Mobile Ad-hoc Networks (MANETs). The proposed model does not use the additional information from Global Positioning System (GPS) for its prediction as some existing models did. Instead, it relies on the “average encounter rate” and “node degree” calculated at each node. However, the outcome is still recorded at high accuracy, i.e. prediction error is fewer than 10% at high speed level (above 15m/s). The aim of this model is to help a routing protocol in MANETs avoid broadcasting request messages from a high mobility node/region relied on the outcome of the prediction. Through simulation experiments, route error rate observed reduced significantly compared to normal broadcast scheme of the Ad-hoc On-demand Distance Vector (AODV) protocol. The packet delivery ratio improved up to 46.32% at the maximum velocity of 30m/s (equal to 108km/h) in the density of 200nodes/km2.

Relay nodes (RN) are used as an important structure to extend the coverage of the Third Generation Partnership Program's Long Term Evolution (3GPP-LTE). The promising technology as the interface between eNodeB (eNB) and RN is radio-over-fibre (RoF), due to its longer span transmission capability. In this paper, we propose an alternative technique to 2×2 multiple-input and multiple-output (MIMO) in LTE structure for transmission over 60 km directly modulated RoF link by introducing frequency division multiplexing (FDM) for orthogonal FDM (OFDM). The system is demonstrated theoretically and experimentally. In the baseband, quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (QAM) and 64-QAM are considered as the single carrier modulations (SCM) according to the LTE standard. The system degradation pattern is identical between the theoretical and experimental system, thus proving the accuracy of the theoretical system design. The real time QPSK, 16-QAM and 64-QAM system achieved an average EVM of 5.84%, 5.90% and 5.97%, respectively for 2GHz and 2.6 GHz bands. These resultant EVMs are below the 8% 3GPP-LTE EVM requirement.

Binary polarisation shift keying (BPOLSK) has been proposed to mitigate the atmospheric turbulence-induced fading in free space optical (FSO) communication systems. In this study, the Q-factors obtained for the BPOLSK-FSO system are verified in conjunction with theoretical results to confirm the validity of the proposed scheme. The analytical bit error rate (BER) for the BPOLSK and non-return-to-zero on–off keying (NRZ-OOK) schemes are presented. The authors show that the BPOLSK scheme with direct detection offers improved BER performances compared to NRZ-OOK in the presence of weak turbulence, which is inferred from the experimental Q-factor and theoretical BER. For a turbulence variance σ21 of 0.003 and the transmitted optical power of −16.8 dBm, values for Q-factor are ∼11 and ∼8.5 for BPOLSK and NRZ-OOK schemes, respectively. The authors show that the predicted signal-to-noise ratio (SNR) for BPOLSK and NRZ-OOK schemes are ∼13.5 and ∼15 dB, respectively, for a BER of 10−6 and σ21 of 0.01. When σ21 increases to 0.1, ∼8 dB lower values of SNR is required for BPOLSK compared with NRZ-OOK.

In this study, we propose applying non-uniform biasing current technique to improve the gain uniformity of the semiconductor optical amplifier (SOA) for high-speed optical routers. The output pulses will have high-gain deviation because of the high-speed input pulses and the slow SOA gain recovery, thus resulting in a high system power penalty. The theoretical SOA operation principle is demonstrated using a segmentation model that employs the rate and propagation equations with third-order gain coefficients. The impact of the bias current on the SOA gain responses owing to the input packet of 1 mW input Gaussian pulses at a wavelength of 1550 nm is analysed in order to optimise the proposed non-uniform bias current shape. The uniform and the optimised non-uniform bias current techniques are investigated in terms of the SOA gain uniformity and the average output power for high-speed data rates from 10 to 160 Gb/s. The impact of the average bias current applied to the SOA on the non-uniform shape is also investigated at all input data rates. Results obtained show a significant improvement in the gain standard deviation of 4.6, 6.3, 8.7, 10.1 and 10.2 dB for the data rates of 10, 20, 40, 80 and 160 Gb/s, respectively, when applying 150 mA average non-uniform biasing and these values reaches 15.2, 16.3, 17.7, 18 and 17.4 dB at 200 mA. The proposed technique also offers an increase in the average output power for the input pulses compared with uniform biasing.

In this work, the performance of binary phase shift keying (BPSK) subcarrier intensity-modulated terrestrial free-space optical (FSO) communication link is analysed in the presence of noise and atmospheric turbulence. To ameliorate channel fading caused by the atmospheric turbulence, we propose a subcarrier time delay diversity (STDD) scheme in which the delayed versions of the original data are re-transmitted on different subcarriers. In short range links, a single re-transmission scheme results in an estimated gain of up to 4.5 dB in received irradiance. With sufficient link margins, short range FSO links might be able to cope without the use of any diversity techniques but for links spanning over 1 km, turbulence induced fading effect must be compensated for. In this study, the authors show that the subcarrier STDD is a viable scheme for this purpose.

The cursive and ligature nature of the Arabic script make the segmentation of words into individual characters a difficult task. Despite attempts to apply methods for cursive Latin and other scripts to Arabic script, it is generally insufficient to segment the Arabic text. This paper proposes a new segmentation algorithm for the handwritten Arabic text and the main idea consists of segmenting the word into sub-words and then computing the baseline of each sub-word. Using the descenders of sub-words and the baseline, candidate points are then calculated using a vertical projection. The algorithm has been tested using 800 handwritten Arabic words taken from the IFN/ENIT database and a comparison made against some existing methods and promising results have been obtained.

The number of sub-carriers, cyclic prefix (CP), and the characteristics of the analogue to digital converter (ADC), are significant parameters influencing the transmission capacity and distance of an optical orthogonal frequency division multiplexing (OOFDM) modem. Numerical simulations of coherent OOFDM modems are undertaken to investigate the effect of the number of sub-carriers, the CP length and ADC associated parameters such as the sampling speed, clipping ratio and quantisation bit on the system performance over single mode fibre (SMF) links for data rates up to 80 Gb/s. The use of a large number of sub-carriers is more effective in combating fibre dispersion when compared with employing a long CP. Moreover in the presence of fibre non-linearities identifying the optimum number of sub-carriers is crucial factor for the modem performance. For various signal data rates up to 40 Gb/s, a set of data rate and transmission distance-dependent optimum ADC parameters are identified. These parameters give rise to negligible clipping and quantisation noise, moreover ADC sampling speed can increase dispersion tolerance while transmitting over SMF links.

In future high-speed photonic packet switching networks, it is highly desirable to carry out robust all-optical header recognition to provide high-throughput routing. The authors present a pulse-position modulation header processing (PPM-HP) scheme, offering significantly reduced routing table size and employing a single bit-wise AND operation to carry out correlation of the packet header with the entire routing table entries. The downsized routing table also offers multiple transmission modes (unicast, multicast and broadcast) in the optical layer and improves core network scalability where the number of core/edge nodes could be altered without the need for changing the number of routing table entries. The authors present modelling and simulation of the packet switching router based on PPM-HP. Noise propagation and crosstalk incurred in a multiple-hop routing scenario are investigated. The simulation results are presented and compared with the theoretical calculations.

Pilot insertion for channel estimation in orthogonal frequency division multiplexing (OFDM) is a promising technique for broadband/high data rate wireless communications. In indoor diffuse optical wireless links, channel impulse response depends on the location of the receiver. For links to offer mobility tracking is essential which can be complex to implement. In this paper we propose channel estimation to allow receiver mobility. We have examined channel estimation with different interpolation methods for OFDM system and have compared the symbol error rates (SER) performance.

In this paper we present the modeling and simulation of multiple-hop routing in ultrafast all-optical packet switching based routers employing multiple pulse position modulation (PPM) formatted routing table. In multiple PPM routing tables (PPRTs) with reduced entry length, only a subset of the header address is converted into a PPM format in order to reduce the packet header recognition time. Packet header address correlation is carried out using only a single optical AND gate to improve the processing time. We show that the proposed scheme offers unicast/multi-cast/broadcast transmitting capabilities. The simulation results for the optical signal-to-noise ratio (OSNR) at each hop are presented and compared with the theoretical calculations.

In indoor optical wireless communication (OWC) systems, ambient lights in particular fluorescent lamps introduce a periodic interference signal, which has the potential to severely degrade link performance. One of the simplest techniques often used to mitigate the interference from artificial sources of ambient light is electrical high-pass filtering (HPF). However, HPF introduces baseline wander (BLW), which is more severe for modulation techniques that contain a significant amount of power located at DC and low frequencies. In this paper the effects of BLW on systems employing digital pulse interval modulation (DPIM) are extensively investigated. Also investigated are the effects of cut-on frequency of the HPF on the optical power requirement and power penalty for different bit rates, where results are compared with the more established techniques of on–off keying (OOK) with a non-return to zero signalling and pulse position modulation (PPM). The optimum HPF cut-on frequency, which minimises the overall power penalty, is estimated. It is shown that at high data bit rates (100 Mbps), DPIM power requirement is considerably lower (∼5 dB) than OOK and similar to that of PPM.

This paper presents a high-speed multiple-channel optical time division multiplexed (OTDM) demultiplexer based on a chained symmetric Mach-Zehnder (CSMZ) switch. In CSMZ switch semiconductor optical amplifier (SOA) are shared between two chained arms in contrast to the MZ based multiple-channel demultiplexers where there is an SOA in each arm. The analysis for bit error rate (BER) and the average received power penalty is presented and the results are compared with the simulated data. It is shown that the power penalty incurred is highly dependent on the value of coupling ratio and the channel number.

Optical fiber is capable of providing the solution to the need of high bandwidth communication. The main degradation to the optical signal integrity in single mode fibers (SMFs) is chromatic dispersion (CD). The effect of CD is pulse spreading which in turn leads to inter-symbol interference (ISI), thus increasing the bit error rate (BER) of the system. This paper presents a CD compensation technique that utilizes a parallel optical all pass filter (p-OAPF), where the design is based on the inverse phase response of the SMF. The p-OAPF is based on a class of all-pass filter. Simulation results of the proposed technique show an increase in the repeater-less length of a point-to-point optical communication system by 3 times with Gaussian pulses at 10 Gb/s and BER of 10-9. A passive infinite impulse response filter structure is proposed for the implementation of the p-OAPF.

In this paper we present modeling and simulation of multiple-hop all-optical packet routing based on the pulse- position modulation header processing (PPM-HP) scheme in ultrafast optical packet switching network. Optical core node employing PPM-HP requires a dedicated routing table based on PPM scheme that will result in significantly reduced number of routing table entries. In this scheme, the header address correlation, using optical AND gates, is carried out by employing only a single bit-wise for each entry rather than a multiple-bit stream as in the existing systems thus resulting in reduced header processing time and avoiding the slow gain-recovery of AND gates. The reduced routing table also offers embedding multiple transmitting modes (unicast, multicast, broadcast) in optical layer and improves core network scalability where the number of core/edge nodes could be altered without the need for changing the number of routing table entries. The simulation results for the optical signal-to-noise ratio (OSNR) of packets at each hop are presented and compared with the theoretical calculations.