Guest Editorial: Selected papers from the 8th Biennial Colloquium & 6th International Workshop on Optical Wireless Communications

Abstract

Since 2011, optical wireless communication (OWC) technologies have gained momentum. OWC has been supported by an active research community, and entrepreneurs who have shown the merits of these technologies by showcasing real-time demonstrators and commercial OWC solutions. These technologies include visible light communications (VLC), underwater VLC, Li-Fi, optical camera communications (OCC), visible light positioning, visible light sensing and free space optics, among others. They have been shown to be an efficient means to satisfy the demanding requirements of backhaul and access network levels, and also to provide a means for accurate indoor positioning and sensing, or enable wireless communication in non-RF friendly environments. As a result, it is envisaged to become an indispensable part of 6G and will complement other communication methods to benefit our daily lives. This Special Issue contains a group of 10 selected papers accepted at the 8th Colloquium & 6th International Workshop on Optical Wireless Communications, co-located with the International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP 2022). This Special Issue has provided the best paper authors' the opportunity to extend their work beyond what was presented at this event, enabling new results and development to further advance the respective fields.

Accepted papers can be clustered into three main categories, namely theoretical, applications and performance oriented. The papers lying in the first category exhibit novelties in modelling of optical wireless channel, non-linear effects and synchronisation issues. The papers in this category are of Combeau et al., Vieira et al., and Rodrigues et al. The second category of papers presents different approaches to using OWC concepts in short and medium range applications. These papers are of Younus et al., Rêgo et al., and Uleru et al. The last category proposes new techniques to evaluate performance in systems employing OWC concepts, such as screen to the camera, light-emitting diode (LED) to multispectral camera, LED matrix arrays or reconfigurable intelligent surface (RIS) based OWC. These papers are of Yokar et al., Moreno et al., Mohammadi et al., and Salehiyan et al. A brief presentation of each of the paper in this Special Issue follows.

In paper 1, Combeau et al. propose a method for the optical characterisation of materials to enable realistic channel simulation based on Monte-Carlo Ray-Tracing algorithms. They conduct some experimental measurements in environments containing the materials sought and then apply an optimisation algorithm which searches for the parameters of the material reflection models. Therefore minimising the difference between the optical measurement and the simulation. Results show that the proposed method produces a correct estimate of the Bidirectional Reflectance Distribution Function albedos, provided that the models correspond well to the reflection behaviour of the materials, and that the materials have a significant influence on the measured optical power.

In Paper 2 Vieira et al. investigate the accuracy of the generalised memory polynomial (GMP) for the non-linear modelling of optical Fast-OFDM links. After model validation using measured data of a 10 km single mode fibre link, the GMP is used for performance investigations of a distortion compensation approach to optical Fast-OFDM, for up to 16PAM modulation formats and different numbers of Fast-OFDM subcarriers. This work also investigates the influence of the zero padding length on the performance of optical Fast-OFDM. The proposed distortion compensation scheme shows significant performance improvements relative to conventional system implementation.

In paper 3, Rodrigues et al. address the issue of carrier synchronisation in a multi-user communication system based on a central digital multiband Carrierless Amplitude and Phase optical transmitter, which broadcasts data on multiple channels to a number of low-cost/low-power devices in Internet-of-Things (IoT) scenarios. A simulation model for the traditional Costas Loop applied in this context is presented along with performance results that show the feasibility of this approach and identify possible issues and viable solutions.

In paper 4, Younus et al., investigate the use of an indoor visible light system to provide a three dimensional (3D) high-accuracy positioning solution. It leverages the use of a single LED at the transmitter and an image sensor at the receiver (Rx). The proposed system can retrieve the 3D coordinate of the Rx using a combination of the angle of arrival and received signal strength (RSS). To mitigate the error induced by the lens at the Rx, a novel method is proposed and experimentally tested, which outperforms previously reported RSS under all circumstances and is immune to varying exposure times.

In Paper 5, Rêgo et al., propose an algorithm for recovering transmitted static identifiers (IDs) in rolling shutter based OCC positioning systems. The goal is to allow the correct decoding when the ID frame is only partially detected in the image. Simulation results show that the maximum distance at which the IDs recovery can be guaranteed is increased by 2.5-fold with the proposed algorithm, for 6-bit, 8-bit and 10-bit code words. An experimental validation algorithm was also proposed, using image processing techniques to extract the bitstream and test the ID recovery process. The proposed algorithm is shown to improve the frame recovery success rate for a given distance, even in the presence of bit errors in the extracted data.

In paper 6, Uleru et al. explore the use of VLC to establish a wireless link between neurons in spiking networks. Instead of the traditional wavelength division multiplexing approach in electro-optical spiking neural networks (SNN), this paper explores the use of non-orthogonal multiple access (NOMA) with a pulse amplitude modulation (PAM) scheme. To evaluate NOMA with PAM, they implement an electro-optical SNN that controls the force of two anthropomorphic fingers actuated by the shape memory alloy-based actuators. An optical reference channel is used to dynamically adjust the optical receiver's gain to improve the receiver's decoding performance. Experimental results demonstrate that the electro-optical SNN can maintain control over the fingers and hold an object under varying channel conditions. Hence, the proposed system offers robustness against dynamic optical channels induced by the relative motion of neurons.

In paper 7, Yokar et al., experimentally demonstrate that smartphone-to-smartphone based visible light communications (S2SVLC) systems can support high-quality multimedia transmissions. Different performance metrics are used in this analysis, such as the mean square error estimation, peak signal-to-noise (PSNR) degradation, bit error rate (BER) and data rate achieved. The analysis was conducted for different typical practical background illuminations (i.e. normal illumination and dark condition), distances, tilting, and rotation angles. The PSNR and BER are determined for different frame sizes in the S2SVLC system to provide an insight of system performance. Presented results indicate how the data transmission under different conditions impacts the system's performance.

In Paper 8, Moreno et al. explore the use of multispectral cameras to take advantage of the LED behaviour and provide new features to OCC links. The idea is to detect differences in LED spectral response curves, when their temperature changes, to obtain more than one communication channel using the same LED device. This new approach also resorts to equalisation techniques in the receiver to improve the extraction of the transmitted signal. For the specific multispectral camera employed in the experiments, up to two distinct channels could be obtained with the same transmitter at different temperatures. However, authors also noticed that obtaining satisfactory results is highly dependent on the variation that temperature causes in the spectral signatures of the LEDs, so further experiments are recommended in future work with different devices.

In paper 9, Mohammadi et al. investigate how to minimise the secrecy outage probability (SOP) at the location of the legitimate user (LU) for a multiple-input single-output-VLC scenario. To mitigate the effect of the eavesdropper, they derive the optimal beamforming coefficients which are used at the transmitter LED array with the aim of maximising the signal-to-noise ratio at the LU. Simulation results show that the proposed SOP-based LED arrangement with optimal beamforming offers up to 3 dB gain in transmit power compared to the classical uniform LED arrangement with sub-optimal zero-forcing beamforming. Moreover, they show that by using the proposed method, the SOP is kept below a desired threshold, over a wider area inside the indoor environment compared to classical methods.

Finally, in paper 10, Salehiyan et al. explore the use of a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS), in a non-orthogonal multiple-access visible light communication system, to improve the performance. They considered two data recovery schemes, single-user detection (SUD) and successive interference cancellation (SIC), and formulate a sum-rate optimisation problem for two operating modes of STAR-RIS. They have also compared energy-splitting and mode-switching cases and showed that these two modes have the same performance. Finally, numerical results for SUD and SIC schemes and two benchmarking schemes and timesharing and max-min fairness are presented, and spectral- and energy-efficiency, number of STAR-RIS elements, the position of users and access point are discussed.

Papers selected for this Special Issue show that fundamental investigation in OWC technologies is a crucial and necessary process to unlock its full potential, address challenges, and ensure its relevance and impact in various applications. The fundamental investigation presented in these papers is expected to help researchers and engineers gain a deep understanding of the underlying principles and mechanisms of a broad range of technologies that use light as the medium for wirelessly transmitting information. These developments have the potential to foster additional innovation, facilitate future problem-solving and optimisation strategies, and ultimately contribute to mature OWC technologies.