Optical Camera Communications in Healthcare: A Wearable LED Transmitter Evaluation during Indoor Physical Exercise

This paper presents an experimental evaluation of a wearable light-emitting diode (LED) transmitter in an optical camera communications (OCC) system. The evaluation is conducted under conditions of controlled user movement during indoor physical exercise, encompassing both mild and intense exercise scenarios. We introduce an image processing algorithm designed to identify a template signal transmitted by the LED and detected within the image. To enhance this process, we utilize the dynamics of controlled exercise-induced motion to limit the tracking process to a smaller region within the image. We demonstrate the feasibility of detecting the transmitting source within the frames, and thus limit the tracking process to a smaller region within the image, achieving an reduction of 87.3% for mild exercise and 79.0% for intense exercise.

Curved OLED-based NLOS optical camera communications links

In this paper, for the first time, to the best of our knowledge, we experimentally demonstrate the use of a curved organic light emitting diode (OLED) as a transmitter (Tx) in the non-line-of-sight (NLOS) optical camera communication (OCC) link for an indoor environment using a camera as a receiver. The proposed NLOS-OCC scheme is evaluated for the signal-to-noise ratio (SNR) and the reception success rates R r s under key photographic and communication parameters, including exposure times t e x p and gain values G v, as well as the transmission frequency f s and the distance L. The SNR analysis is performed using a binary classification procedure based on a Gaussian mixture model for the first time, to the best of our knowledge, for OLED-based NLOS-OCC links. We also derive and demonstrate that the effect of G v on the SNR with respect to L is minimal based on the pixel illumination model. The initial analysis suggests that, for a wall reflector-based NLOS-OCC link that is 2 m long, the SNR and R r s increase by 1 dB and 4% (83-87%) for f s of 600 Hz, with an increase in t e x p of 1000-1500 µs and G v of 25-45 dB.

Deep learning for signal clock and exposure estimation in rolling shutter optical camera communication

In rolling shutter (RS)-based optical camera communication (OCC) links, selecting the appropriate camera's exposure time is critical, as it limits the reception bandwidth. In long exposures, the pixels accumulate over time the incoming irradiance of several consecutive symbols. As a result, a harmful intersymbol interference corrupts the received signal. Consequently, reducing the exposure time is required to increase the reception bandwidth at the cost of producing dark images with impracticable light conditions for human or machine-supervised applications. Alternatively, deep learning (DL) equalizers can be trained to mitigate the exposure-related ISI. These equalizers must be trained considering the transmitter clock and the camera's exposure, which can be exceptionally challenging if those parameters are unknown in advance (e.g., if the camera does not reveal its internal settings). In those cases, the receiver must estimate those parameters directly from the images, which are severely distorted by the exposure time. This work proposes a DL estimator for this purpose, which is trained using synthetic images generated for thousands of representative cases. This estimator enables the receiver operation under multiple possible configurations, regardless of the camera used. The results obtained during the validation, using more than 7000 real images, registered relative errors lower than 1% and 2% when estimating the transmitter clock and the exposure time, respectively. The obtained errors guarantee the optimal performance of the following equalization and decoding receiver stages, keeping bit error rates below the forward error correction limit. This estimator is a central component of any OCC receiver that operates over moderate exposure conditions. It decouples the reception routines from the cameras used, ultimately enabling cloud-based receiver architectures.

Clustering-based data detection for spectral signature multiplexing in multispectral camera communication

Optical camera communication (OCC) is a promising technology to be used in future wireless communication systems. In this work, a cluster-based data detection procedure is applied to enhance the performance of an OCC system. A multispectral camera is employed to capture the spectral variations in light-emitting diodes (LEDs) caused by temperature. This strategy's system performance is compared with a system that uses traditional linear methods, such as zero-forcing (ZF) and minimum mean square error (MMSE) equalizers. The findings of this study indicate that an improvement in the bit error rate (BER) can be achieved by applying a clustering approach.

VLC Network Design for High Mobility Users in Urban Tunnels

Current vehicular systems require real-time information to keep drivers safer and more secure on the road. In addition to the radio frequency (RF) based communication technologies, Visible Light Communication (VLC) has emerged as a complementary way to enable wireless access in intelligent transportation systems (ITS) with a simple design and low-cost deployment. However, integrating VLC in vehicular networks poses some fundamental challenges. In particular, the limited coverage range of the VLC access points and the high speed of vehicles create time-limited links that the existing handover procedures of VLC networks can not be accomplished timely. Therefore, this paper addresses the problem of designing a vehicular VLC network that supports high mobility users. We first modify the traditional VLC network topology to increase uplink reliability. Then, a low-latency handover scheme is proposed to enable mobility in a VLC network. Furthermore, we validate the functionality of the proposed VLC network design method by using system-level simulations of a vehicular tunnel scenario. The analysis and the results show that the proposed method provides a steady connection, where the vehicular node is available more than 99% of the time regardless of the number of vehicular nodes on this network. Additionally, the system is able to achieve a Frame-Error-Rate (FER) performance lower than 10^-3.

Survey on Optical Wireless Communications-Based Services Applied to the Tourism Industry: Potentials and Challenges

In this paper, we explore the potential applications of Optical Wireless Communications in the tourism industry, considering both indoor and outdoor scenarios and different transmission speeds. They range from high-speed atmospheric outdoor links (Free-Space Optics (FSO)) to indoor systems based on high-speed lighting networks (known under the trade name LiFi©) or low-speed services support the Internet of Things networks, using visible light (VLC) or IR emitters, with receivers based on either on classical photodiodes or in image sensors, known as Optical Camera Communications. The avant-garde applications of this technology have been studied focusing on three possible use scenarios: the traveler himself, in what we have called TAN (Tourist Area Network); the tourist facility, which includes not only the hotel but also leisure areas (theme parks, museums, natural protected areas) or services (restaurants, shopping areas, etc.); and the entire destination, which can be both the city or the territory where the tourist is received, within the paradigm of the Smart Tourist Destination (STD). In addition to the classic services based on radio frequency and wired broadband networks, these technologies will make it possible to meet the tourist's challenging needs, the establishment, and the destination. Besides, they cover the services imposed by the new marketing services related to location or context and feed the big data systems used to study tourist behavior.

Convolutional autoencoder for exposure effects equalization and noise mitigation in optical camera communication

In rolling shutter-based optical camera communication (OCC), the camera's exposure time limits the achievable reception bandwidth. In long-exposure settings, the image sensor pixels average the incident received power, producing inter-symbol interference (ISI), which is perceived in the images as a spatial mixture of the symbol bands. Hence, the shortest possible exposure configuration should be selected to alleviate ISI. However, in these conditions, the camera produces dark images with impracticable light conditions for human or machine-supervised applications. In this paper, a novel convolutional autoencoder-based equalizer is proposed to alleviate exposure-related ISI and noise. Furthermore, unlike other systems that use artificial neural networks for equalization and decoding, the training procedure is conducted offline using synthetic images for which no prior information about the deployment scenario is used. Hence the training can be performed for a wide range of cameras and signal-to-noise ratio (SNR) conditions, using a vast number of samples, improving the network fitting and the system decoding robustness. The results obtained in the experimental validation record the highest ISI mitigation potential for Manchester encoded on-off keying signals. The system can mitigate the ISI produced by exposure time windows that are up to seven times longer than the transmission symbol duration, with bit error rates (BER) lower than 10^-5 under optimal SNR conditions. Consequently, the reception bandwidth improves up to 14 times compared to non-equalized systems. In addition, under harsh SNRs conditions, the system achieves BERs below the forward error correction limit for 1dB and 5 dB while operating with exposure times that are 2 and 4 times greater than the symbol time, respectively.

Design and Experimental Characterization of a Discovery and Tracking System for Optical Camera Communications

Visible light communications (VLC) technology is emerging as a candidate to meet the demand for interconnected devices' communications. However, the costs of incorporating specific hardware into end-user devices slow down its market entry. Optical camera communication (OCC) technology paves the way by reusing cameras as receivers. These systems have generally been evaluated under static conditions, in which transmitting sources are recognized using computationally expensive discovery algorithms. In vehicle-to-vehicle networks and wearable devices, tracking algorithms, as proposed in this work, allow one to reduce the time required to locate a moving source and hence the latency of these systems, increasing the data rate by up to 2100%. The proposed receiver architecture combines discovery and tracking algorithms that analyze spatial features of a custom RGB LED transmitter matrix, highlighted in the scene by varying the cameras' exposure time. By using an anchor LED and changing the intensity of the green LED, the receiver can track the light source with a slow temporal deterioration. Moreover, data bits sent over the red and blue channels do not significantly affect detection, hence transmission occurs uninterrupted. Finally, a novel experimental methodology to evaluate the evolution of the detection's performance is proposed. With the analysis of the mean and standard deviation of novel K parameters, it is possible to evaluate the detected region-of-interest scale and centrality against the transmitter source's ideal location.

Wireless Sensor Networks Using Sub-Pixel Optical Camera Communications: Advances in Experimental Channel Evaluation

Optical wireless communications in outdoor scenarios are challenged by uncontrollable atmospheric conditions that impair the channel quality. In this paper, different optical camera communications (OCC) equipment are experimentally studied in the laboratory and the field, and a sub-pixel architecture is raised as a potential solution for outdoor wireless sensor networks (WSN) applications, considering its achievable data throughput, the spatial division of sources, and the ability of cameras to overcome the attenuation caused by different atmospheric conditions such as rain, turbulence and the presence of aerosols. Sub-pixel OCC shows particularly adequate capabilities for some of the WSN applications presented, also in terms of cost-effectiveness and scalability. The novel topology of sub-pixel projection of multiple transmitters over the receiver using small optical devices is presented as a solution using OCC that re-uses camera equipment for communication purposes on top of video-monitoring.

Sandstorm effect on experimental optical camera communication

Sandstorms can severely affect the reliability of outdoor optical wireless communications (OWC) by diminishing large regions' visibility. In this work, the effect of a real sandstorm on optical camera communications (OCC) links is experimentally evaluated. Two link ranges are essayed using a cost-efficient telescope-based camera setup with commercial LEDs. Using on-off keying modulation, a data rate of 1035 and 630 bps with error probabilities of 9.14⋅10^-5 and 4.1⋅10^-3 for 100 m and 200 m, respectively, can be achieved. The signal-to-noise ratio of the links was optimized by tuning the analog amplifier's gain of the camera, increasing it by up to 9 dB. It is shown that scattering due to the sandstorm can even be beneficial for increasing the data rate in OCC (contrary to classical photodetector-based OWC links), thanks to an increment of 33% on the region of interest dimensions compared to the expected clear air link.

Spatial frequency-based angular behavior of a short-range flicker-free MIMO-OCC link

In this paper, we provide a solution based on spatial frequency f_sf to study the angular behavior of a flicker-free, short-range indoor multiple-input multiple-output (MIMO) optical camera communications (OCC) link. We focus on the experimental investigation of OCC's performance for the transmitters (Txs) [i.e., light-emitting diode (LED) based arrays] located at the same and different distances from the receiver (Rx) with the off-axis rotation angle θ. We have used two 8×8 distributed LED arrays and a commercial low-cost complementary metal-oxide-semiconductor (CMOS) Raspberry Pi camera with the rolling-shutter capturing mode as the Tx and Rx, respectively. The image and the respective communications link quality metrics are measured in terms of the peak signal-to-noise ratio (PSNR) and the rate of successfully received bits with respect to f_sf for different camera shutter speeds (SS). A CMOS image sensor noise characterization is carried in terms of the signal-to-noise ratio (SNR) and PSNR. The proposed study provides a 100% success rate in data reception at the optimum θ of 50° at lower captured values of f_sf, which is projected onto the image sensor in the form of pixels. Moreover, the effect of channel saturation over f_sf is studied with respect to θ and SS and we show that, for θ exceeding the optimum value along transmission range, the f_sf area of the Txs reduces to less than ∼50% of the captured Tx units at θ of 0°, where no data can be fully recovered.

Optical Camera Communications for IoT-Rolling-Shutter Based MIMO Scheme with Grouped LED Array Transmitter

In optical camera communications (OCC), the provision of both flicker-free illumination and high data rates are challenging issues, which can be addressed by utilizing the rolling-shutter (RS) property of the image sensors as the receiver (Rx). In this paper, we propose an RS-based multiple-input multiple-output OCC scheme for the Internet of things (IoT) application. A simplified design of multi-channel transmitter (Tx) using a 7.2 × 7.2 cm² small 8 × 8 distributed light emitting diode (LED) array, based on grouping of LEDs, is proposed for flicker-free transmission. We carry out an experimental investigation of the indoor OCC system by employing a Raspberry Pi camera as the Rx, with RS capturing mode. Despite the small area of the display, flicker-free communication links within the range of 20-100 cm are established with data throughput of 960 to 120 bps sufficient for IoT. A method to extend link spans up to 1.8 m and the data throughput to 13.44 kbps using different configurations of multi-channel Tx is provided. The peak signal-to-noise ratio of ~14 and 16 dB and the rate of successfully received bits of 99.4 and 81% are measured for the shutter speeds of 200 and 800 µs for a link span of 1 m, respectively.

Characterization and Performance of a Thermal Camera Communication System

This work presents a novel communications technology named Thermal Camera Communication (TCC), which is analogous to Optical Camera Communication (OCC). Thermographic cameras and Peltier cells are proposed as receiver and transmitter, respectively, changing completely their usual field of application. Furthermore, a comprehensive characterization of the Peltier-Thermal camera pair is carried out, presenting their bandwidth, achievable data rate under On-Off-Keying (OOK) modulation, noise characteristics, and energy efficiency. A comparison against the current state-of-the-art OCC technology is also provided, showing that TCC is a promising technology suitable for sensor networks. The thorough analysis of TCC performed in this work shows that commercial Peltier cells can be re-thought under a communications viewpoint in order to improve their performance. This novel communication technology can be applied in environments such as the access to public transportation or buildings due to the new health emergency situation. The use of thermographic cameras will become massive and dual measurement and communication purposes could be considered for applications such as sensor networks, using a yet unexploited wavelength range.

Experimentally Derived Feasibility of Optical Camera Communications under Turbulence and Fog Conditions

Optical camera communications (OCC) research field has grown recently, aided by ubiquitous digital cameras; however, atmospheric conditions can restrict their feasibility in outdoor scenarios. In this work, we studied an experimental OCC system under environmental phenomena emulated in a laboratory chamber. We found that the heat-induced turbulence does not affect our system significantly, while the attenuation caused by fog does decrease the signal quality. For this reason, a novel strategy is proposed, using the camera's built-in amplifier to overcome the optical power loss and to decrease the quantization noise induced by the analog-digital converter of the camera. The signal quality has been evaluated using the Pearson's correlation coefficient with respect to a reference template signal, along with the signal-to-noise ratio that has been empirically evaluated. The amplification mechanism introduced allows our system to receive the OCC signal under heavy fog by gradually increasing the camera gain up to 16 dB, for meteorological visibility values down to 10 m, with a correlation coefficient of 0.9 with respect to clear conditions.

Hybrid RF/VLC Network Architecture for the Internet of Things

In recent years, there has been a remarkable advance in monitoring technologies in many environments, be they urban or rural. These technologies, included in the Internet of Things (IoT) domain, allow remote control and acquisition of data from sensors for their subsequence analysis. All these systems are based on the interaction between sensors and actuators. To achieve this goal, it is necessary to provide a very high level of connectivity between the devices, especially as far as wireless systems are concerned. In this sense, there is a great variety of standards in the market of communication networks oriented to this end. One of the biggest challenges today is to allow inter-operability between these different technologies in order to homogenize this field. In addition to this, it is intended to introduce new communication techniques that can provide certain additional advantages to those already existing. The main idea is the creation of a cellular network where radiofrequency and optical technologies coexist, and whose link with the rest of the world is through long-range and low-consumption wireless technologies. The center of each cell, that is the lighting system, can be powered using solar panels, as can the existing systems in the market. The objective is that these panels are capable of providing the necessary energy to the rest of the necessary systems.

Effect of moving microalgae on underwater wireless optical links

Underwater wireless optical communications is a promising technique for addressing short-range data networks, as it provides cost, performance, and complexity improvements as compared with other alternatives, such as acoustic communications or radio frequency links. It is a part of the optical wireless communications research area, since for these applications, broad optical sources such as visible LED lamps can be used. Unless those links are designed for short distances (about 1 m, as in data-muling services on internet-of-things submerged systems), they are still severely affected by channel perturbations, such as scattering due to the presence of particles. This effect is particularly important when considering sensing applications for algae or aquaculture farming, which are becoming a crucial economic resource in many maritime areas. In this work, the effects of moving microalgae on underwater short-range optical links are studied so as to estimate a model for this scattering under dynamic conditions. The statistical parameters over experimentally measured received signal level and signal-to-noise ratio (SNR) are calculated, and the experimental setup is described. Results show that in clear water (no-algae scenario), the water-pump-induced movement effect over the mean and variance of the received optical power can be neglected, while when microalgae are present, the average optical power value decreases and the variance increases with all measured wavelengths. Finally, the SNR penalty due to the movement of microalgae is statistically evaluated.

Correlation-based receiver for optical camera communications

In color-multiplexed optical camera communications (OCC) systems, data acquisition is restricted by the image processing algorithm capability for fast source recognition, region-of-interest (ROI) detection and tracking, packet synchronization within ROI, estimation of inter-channel interference and threshold computation. In this work, a novel modulation scheme for a practical RGB-LED-based OCC system is presented. The four above-described tasks are held simultaneously. Using confined spatial correlation of well-defined reference signals within the frame's color channels is possible to obtain a fully operating link with low computational complexity algorithms. Prior channel adaptation also grants a substantial increase in the attainable data rate, making the system more robust to interferences.

Considerations on Visible Light Communication security by applying the Risk Matrix methodology for risk assessment

Visible Light Communications (VLC) is a cutting edge technology for data communication that is being considered to be implemented in a wide range of applications such as Inter-vehicle communication or Local Area Network (LAN) communication. As a novel technology, some aspects of the implementation of VLC have not been deeply considered or tested. Among these aspects, security and its implementation may become an obstacle for VLCs broad usage. In this article, we have used the well-known Risk Matrix methodology to determine the relative risk that several common attacks have in a VLC network. Four examples: a War Driving, a Queensland alike Denial of Service, a Preshared Key Cracking, and an Evil Twin attack, illustrate the utilization of the methodology over a VLC implementation. The used attacks also covered the different areas delimited by the attack taxonomy used in this work. By defining and determining which attacks present a greater risk, the results of this work provide a lead into which areas should be invested to increase the safety of VLC networks.

Hybrid Visible Light and Ultrasound-Based Sensor for Distance Estimation

Distance estimation plays an important role in location-based services, which has become very popular in recent years. In this paper, a new short range cricket sensor-based approach is proposed for indoor location applications. This solution uses Time Difference of Arrival (TDoA) between an optical and an ultrasound signal which are transmitted simultaneously, to estimate the distance from the base station to the mobile receiver. The measurement of the TDoA at the mobile receiver endpoint is proportional to the distance. The use of optical and ultrasound signals instead of the conventional radio wave signal makes the proposed approach suitable for environments with high levels of electromagnetic interference or where the propagation of radio frequencies is entirely restricted. Furthermore, unlike classical cricket systems, a double-way measurement procedure is introduced, allowing both the base station and mobile node to perform distance estimation simultaneously.

Constraints on drivers for visible light communications emitters based on energy efficiency

In this work we analyze the energy efficiency constraints on drivers for Visible light communication (VLC) emitters. This is the main reason why LED is becoming the main source of illumination. We study the effect of the waveform shape and the modulation techniques on the overall energy efficiency of an LED lamp. For a similar level of illumination, we calculate the emitter energy efficiency ratio η (P_LED/P_TOTAL) for different signals. We compare switched and sinusoidal signals and analyze the effect of both OOK and OFDM modulation techniques depending on the power supply adjustment, level of illumination and signal amplitude distortion. Switched and OOK signals present higher energy efficiency behaviors (0.86≤η≤0.95) than sinusoidal and OFDM signals (0.53≤η≤0.79).

Multiuser CSK scheme for indoor visible light communications

Color Shift Keying (CSK) is a new modulation scheme for visible light communication systems using RGB LEDs which has been standardized in the PHY III level of the IEEE 802.15.7. This paper proposes some modifications so as to include multiuser capabilities provided by a time-based multiplexing, with the modulation constellation symbols being adapted to encode data with the luminux powers of the red, green and blue color bands respectively. This is achieved by employing a simple and low-complexity time-based pulse signals structure to separate the users' data symbols, while a three-dimensional signal constellation design is merged to improve data throughput. Numerical simulations are carried out to assess the performance of this novel architecture. The statistical properties of the transmitted RGB signals ensure dimming capabilities and that the illumination function is unaffected by flickering.