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.

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.

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.

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 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 °.

Optical camera communication (OCC) is an innovative set of optical wireless communication that exploits the ever-increasing availability of cameras and solid-state lighting equipment to establish low data rate optical links. This paper evaluates the feasibility of implementing OCC links between sensor nodes in a field outdoors and a camera connected to the cloud in an emulated farming environment. The power efficiency and low maintenance constraints of wireless sensor networks (WSN) are considered in the hardware design of the nodes, which include an energy harvesting system. The system is capable of transmitting 7.5 bps per node at distances of about 100 m. The proposed system is considered a cost-effective and scalable solution for WSN in precision farming.

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.

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.

We present the experimental demonstration of photonic doubling to achieve 25 GHz millimeter wave signal generation, following the 5th generation network frequency band recommendations. A hybrid optical network combining up to 50 km long radio over fiber and 40 m long radio over free space optics systems plus 40 m long radio frequency (RF) 25 GHz wireless channel is demonstrated. The proposed system employs double sideband suppressed carrier modulation with 27 dB optical carrier suppression ratio and RF carrier phase noise below - 107 dBc/Hz with 10 kHz offset. The proposed system is optimized in terms of RF input power and the system quality is tested in terms of error vector magnitude for different modulation formats and variable fiber section lengths. Furthermore, the effect of optical atmospheric turbulence on longer FSO spans is investigated.

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.

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).

Future mobile networks are supposed to serve high data rates to users. To accommodate the high data rates, a direct communication between nearby mobile terminals (MTs) can be exploited. This type of communication in mobile networks is known as Device-to-device (D2D). Furthermore, a communication in high frequency bands, such as, visible light communication (VLC), is also foreseen as an enabler for the high data rates. In a conventional D2D communication, pairs of the communicating MTs should reuse the same frequencies to maximize spectral efficiency of the system. However, this implies either interference among the D2D pairs or a need for complex resource allocation algorithms. In this paper, we introduce a new concept for D2D communication combining VLC and RF technologies in order to maximize capacity of the system. The objective of this paper is to analyze operational limits of the proposed concept and to assess potential capacity gains to give motivation for future research in this area. Thus, we also discuss several practical issues related to the proposed RF-VLC D2D concept and outline major research challenges. The performance analysis carried out in this paper shows that the RF-VLC D2D is able to improve the capacity in an indoor scenario by a factor of 4.1 and 1.5 when compared to standalone RF D2D and VLC D2D, respectively.

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 experimentally demonstrate a multiband carrierless amplitude and phase modulation format for the first time in VLC. We split a conventional carrierless amplitude and phase modulated signal into m subcarriers in order to protect from the attenuation experienced at high frequencies in low-pass VLC systems. We investigate the relationship between throughput/spectral efficiency and m, where m = {10, 8, 6, 4, 2, 1} subcarriers over a fixed total signal bandwidth of 6.5 MHz. We show that transmission speeds (spectral efficiencies) of 31.53 (4.85), 30.88 (4.75), 25.40 (3.90), 23.65 (3.60), 15.78 (2.40), and 9.04 (1.40) Mb/s (b/s/Hz) can be achieved for the listed values of m, respectively.

In this letter, the detectability of fire by a microwave radiometer is investigated. In order to understand the fire from a radiometric point of view, the parameters of fire emissivity had to be determined. Therefore, representative fire types were analyzed, and fire spot emissivity values have been derived from measured data. Four typical burning fuels were tested, i.e., gasoline, wood, flood trash, and straw, respectively. In addition, the parameters of a scenario in which the fire temperature was changed was set and analyzed in order to investigate detectable radiometric contrasts. As a result, specific values of the fire spot emissivity are presented. The obtained results can provide indispensable inputs for designers of microwave fire detection instruments.

In the paper the detectability of a fire by a microwave radiometer is investigated. Simulations of several scenarios and together with outdoor measurement were performed in order to confirm detectability of the fire of specific temperature distributions and dimensions. Various scenarios for fire sensing were numerically modeled and simulated. Real parameters of antenna radiation pattern and radiometer sensitivity were taken into account. The concept of fire detectability verification was tested on real measurement data and then applied in a large scale on analyses of scaled airborne sensing of an environment. The results clearly indicate the possibilities and limits of detection with currently available radiometers.

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.

A development of millimeter waveband indoor sensors has brought new research challenges especially with respect to the sensing within complex environments. To investigate the propagation aspects for these systems the quasioptical approaches including Gaussian beams of basic and higher orders can be utilized. Difference between conventional and Gaussian beam based models can be significant especially for remote sensing systems being deployed within indoor scenarios. The paper discusses the advantages and drawback of these simulation approaches, based on measurement and analyses of electromagnetic wave propagation.