Non-line-of-sight Links Research Articles

NLOS visible light positioning and communication based on LoRa modulation

With the rapid development of the Internet of Things, location-based services are becoming increasingly important, especially in indoor environments. Visible light positioning (VLP) has garnered widespread attention due to its high accuracy, low cost, and immunity to the radio frequency electromagnetic interference. However, traditional VLP relies on line-of-sight paths, making it impractical in complex and dynamic indoor environments. In this paper, we propose a non-line-of-sight (NLOS) visible light positioning and communication system based on LoRa modulation to address the issue of link obstruction. LoRa is employed to recover position information transmitted from light emitting diodes (LEDs) over NLOS links, enabling reliable communication under weak lighting conditions. We establish a geometric relationship between the LED and the virtual image of the photodetector (PD). Leveraging the NLOS channel model, we derive the relationship between the received signal strength and the distance from LEDs to the virtual image of the PD. Through this relationship, the trilateration method is applied to calculate the position of the receiver. Based on experimental results, the proposed system achieves 90th percentile localization accuracy of less than 25 and 35 cm for the PD heights of 60 and 80 cm, respectively.

NLOS Identification and Mitigation for Time-based Indoor Localization Systems: Survey and Future Research Directions

One hurdle to accurate indoor localization using time-based networks is the presence of Non-Line-Of-Sight (NLOS) and multipath signals, affecting the accuracy of ranging in indoor environments. NLOS identification and mitigation have been studied over the years and applied to different time-based networks, with most works considering NLOS links with WiFi and UWB channels. In this paper, we discuss the effects and challenges of NLOS conditions on indoor localization and present current state-of-the-art approaches to NLOS identification and mitigation in literature. We survey these approaches and classify them under different categories together with their merits and demerits. We further categorize approaches to tackle NLOS effects into single and hybrid measurement-based approaches in this work. Lessons learnt from the survey with future directions are also presented in this paper.

Exploiting Anchor Links for NLOS Combating in UWB Localization

UWB (Ultra-wideband) has been shown to be a promising technology to provide accurate positioning for the Internet of Things. However, its performance significantly degrades in practice due to Non-Line-Of-Sight (NLOS) issues. Various approaches have implicitly or explicitly explored the problem. In this article, we propose RefLoc , which leverages the unique benefits of UWB to address the NLOS problem. While we find that NLOS links can vary significantly in the same environment, LOS links possess similar features that can be captured by the high bandwidth of UWB. Specifically, the high-level idea of RefLoc is to first identify links among anchors with known positions and leverage those links as references for tag link identification. To achieve this, we address the practical challenges of deriving anchor link status, extracting qualified link features, and inferring tag links with anchor links. We implement RefLoc on commercial hardware and conduct extensive experiments in different environments. The evaluation results show that RefLoc achieves an average NLOS identification accuracy of 96% in various environments, improving the state-of-the-art by 10%, and reduces 80% localization error with little overhead.

Enhanced performance of an indoor non-line-of-sight VLC system utilizing a multi-pixel photon counter and interleaved single-carrier FDM scheme

A non-line-of-sight (NLOS) visible light communication (VLC) system, utilizing a multi-pixel photon counter (MPPC) as the detector and interleaved single-carrier frequency division multiplexing (I-SC-FDM) scheme, was first proposed and experimentally investigated in this work. Specifically, the I-SC-FDM scheme exhibits a significantly lower peak-to-average power ratio (PAPR) than the original orthogonal frequency division multiplexing (OFDM) and widely used discrete Fourier transform spread (DFTs)-OFDM schemes, thereby necessitating a much lower power gain, which is greatly beneficial for indoor power-sensitive VLC systems. Meanwhile, the implementation of an MPPC with high sensitivity enables the detection of faintly scattered light and thus facilitates the possibility of omnidirectional optical transmission based on indoor NLOS links. Following this, an MPPC-based omnidirectional VLC system was developed and successfully achieved a net data rate up to 18 Mbps in a 3.6 m × 3 m × 3 m room, with a joint deployment of the I-SC-FDM scheme and hybrid time-frequency domain equalization (TFD-EQ) strategy. Such results can be viewed as an expansion of indoor VLC systems beyond line-of-sight (LOS) links and serve as a reference for the practical application of Light-Fidelity (LiFi) in future optical wireless access.

Joint Localization and Environment Sensing by Harnessing NLOS Components in RIS-Aided mmWave Communication Systems

This study explores the use of non-line-of-sight (NLOS) components in millimeter-wave (mmWave) communication systems for joint localization and environment sensing. The radar cross section (RCS) of a reconfigurable intelligent surface (RIS) is calculated to develop a general path gain model for RISs and traditional scatterers. The results show that RISs have a greater potential to assist in localization due to their ability to maintain high RCSs and create strong NLOS links. A one-stage linear weighted least squares estimator is proposed to simultaneously determine user equipment (UE) locations, velocities, and scatterer (or RIS) locations using line-of-sight (LOS) and NLOS paths. The estimator supports environment sensing and UE localization even using only NLOS paths. A second-stage estimator is also introduced to improve environment sensing accuracy by considering the nonlinear relationship between UE and scatterer locations. Simulation results demonstrate the effectiveness of the proposed estimators in rich scattering environments and the benefits of using NLOS paths for improving UE location accuracy and assisting in environment sensing. The effects of RIS number, size, and deployment on localization performance are also analyzed.

Ultraviolet communication with a large scattering angle via artificial agglomerate fog.

Considering strong signal attenuation of the large-angle non-line-of-sight (NLOS) link achieved due to the ultraviolet (UV) scattering properties, we propose to increase the UV communication link gain under a large scattering angle via generating agglomerate fog within a certain range as a secondary light source. In this study, a channel model with locally strong scatterers from agglomerate fog is proposed based on Monte Carlo ray-tracing approaches. Mie theory is adopted to calculate the atmospheric channel parameters, to further evaluate the link gain of a channel under non-uniform atmosphere. The performance of scattering system in the presence of fog conditions depends on the relative positions of the light source to the fog and to the receiver. The link gain results reveal the transmission capabilities for different scattering channel geometries, and give the optimal spray point location within a 10 m communication range. We further establish a foggy NLOS system using an agglomerate fog generator to verify our research in the real environment. The results show that the received signal strength of the NLOS link can be effectively enhanced by constructing scatterers in the atmospheric channel, which significantly decreases the bit-error rate (BER).

Indoor Visible-Light 3D Positioning System Based on GRU Neural Network

With the continuous development of artificial intelligence technology, visible-light positioning (VLP) based on machine learning and deep learning algorithms has become a research hotspot for indoor positioning technology. To improve the accuracy of robot positioning, we established a three-dimensional (3D) positioning system of visible-light consisting of two LED lights and three photodetectors. In this system, three photodetectors are located on the robot’s head. We considered the impact of line-of-sight (LOS) and non-line-of-sight (NLOS) links on the received signals and used gated recurrent unit (GRU) neural networks to deal with nonlinearity in the system. To address the problem of poor stability during GRU network training, we used a learning rate attenuation strategy to improve the performance of the GRU network. The simulation results showed that the average positioning error of the system was 2.69 cm in a space of 4 m × 4 m × 3 m when only LOS links were considered and 2.66 cm when both LOS and NLOS links were considered with 95% of the positioning errors within 7.88 cm. For two-dimensional (2D) positioning with a fixed positioning height, 80% of the positioning error was within 9.87 cm. This showed that the system had a high anti-interference ability, could achieve centimeter-level positioning accuracy, and met the requirements of robot indoor positioning.

Three-Dimensional NLOS VLP Based on a Luminance Distribution Model for Image Sensor

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 paper proposes a three-dimensional (3D) 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 image sensor 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 3D positioning. To the best of author’s knowledge, this is the first work to demonstrate 3D 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 a NLOS link.

Scattering and Eavesdropping in Terahertz Wireless Link by Wavy Surfaces

Wireless communication at terahertz (THz) frequencies is viewed as one of the potentials for future 6G wireless systems. However, the path of secure data transmission by THz links is facing many complex challenges. Of that, scattering-induced eavesdropping threat remains one of the most critical but less explored. With the updating of carrier frequency into the THz range, the wavelength becomes comparable to the surface roughness of typical objects in our daily life. Surface scattering, which is negligible in current wireless networks, becomes serious and would lead to desperate signal eavesdropping. For a non-line-of-sight (NLOS) link achieved by surface reflection, how does the scattering affect link security at the physical layer? Such direct information is crucial for the inspection of security threats and proposing of possible solutions. Using metallic wavy surfaces, we characterize possibilities of signal eavesdropping caused by bistatic scattering, by several unmodulated channels at different frequencies. We also demonstrate a real-time data transmission for this eavesdropping configuration, with a data rate of 4 gigabits/s by 16 quadrature amplitude modulation. We observe that successful signal interception from an NLOS link happens, even when transceiver designs incorporating backscatter measures are considered. This is likely to be a common security threat for any THz communication systems in which NLOS channels are employed to achieve wider user coverage.

Vehicular Visible Light Communication System: Modeling and Visualizing Critical Outdoor Propagation Characteristics

We propose a practical vehicle-to-vehicle (V2V) visible light communications (VLC) propagation model by considering the impact of the outdoor propagation characteristics. Contrary to the existing literature, we emphasize a dynamic V2V-VLC model considering several propagation characteristics. Both line-of-sight (LOS) and non-line-of-sight (NLOS) V2V-VLC propagation models are proposed. To this end, we comprehensively demonstrate the impact of the considered propagation characteristics on the proposed LOS and NLOS V2V-VLC propagation models. To analyze the proposed propagation models, we also derive the closed-form expressions for the average path loss and discrete-input continuous-output (DCMC) capacity incorporating the considered propagation characteristics. With the help of derived closed-form expressions and simulations, we demonstrate comparative insights for the proposed LOS and NLOS V2V-VLC propagation models. Considering our key findings, it is observed that there is a path loss penalty of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\approx$</tex-math></inline-formula> 6 dB compared to the conventional path loss model for a worst-case weather scenario. Moreover, it is observed that the considered propagation characteristics have a significant impact on the V2V-VLC performance. Further, we observe that the ergodic DCMC capacity corresponding to the NLOS-road surface link indicates a dominant behavior compared to an NLOS link arising from an interfering vehicle; but up to a certain range of the link length.

A measurement data-based investigation of fading modeling for indoor THz wireless systems

The terahertz (THz) band offers a vast amount of still unallocated bandwidth, which makes it a promising enabler for future sixth generation wireless systems. The high frequencies of the THz band lead to a significantly reduced multipath richness of the propagating THz signals. However, there are still paths that can carry a significant amount of power. As a result, the THz band small-scale fading characterization is of particular interest and the appropriate stochastic distributions that best fit the empirical channels need to be identified. This work investigates the suitability of the 'alpha-mu', Rice and Nakagami-m distributions to adequately model the small-scale fading statistics of the channel gain measurements of various Line-of-Sight (LoS) and Non-Line-of-Sight (NLoS) indoor THz wireless links. The fitting accuracy of the examined analytical distributions is validated by means of the Kolmogorov-Smirnov test, the Kullback-Leibler divergence, the logarithmic Kolmogorov-Smirnov test and the root-mean-square-error. Also, the ergodic capacity based on the channel gain measurements as well as on the 'alpha-mu', Rice and Nakagami-m distributions is presented. Based on the fitting accuracy metrics the Rice and 'alpha-mu' yield the best fit for the LoS and NLoS links, respectively. The Nakagami-m does not fit the empirical distributions for any of the presented links. Furthermore, insights are provided for the ranges of the extracted values of the analytical distributions in LoS and NLoS transmission conditions.

Propagation Measurements for IQRF Network in an Urban Environment.

Recently, IQRF has emerged as a promising technology for the Internet of Things (IoT), owing to its ability to support short- and medium-range low-power communications. However, real world deployment of IQRF-based wireless sensor networks (WSNs) requires accurate path loss modelling to estimate network coverage and other performances. In the existing literature, extensive research on propagation modelling for IQRF network deployment in urban environments has not been provided yet. Therefore, this study proposes an empirical path loss model for the deployment of IQRF networks in a peer-to-peer configured system where the IQRF sensor nodes operate in the 868 MHz band. For this purpose, extensive measurement campaigns are conducted outdoor in an urban environment for Line-of-Sight (LoS) and Non-Line-of-Sight (NLoS) links. Furthermore, in order to evaluate the prediction accuracy of well-known empirical path loss models for urban environments, the measurements are compared with the predicted path loss values. The results show that the COST-231 Walfisch–Ikegami model has higher prediction accuracy and can be used for IQRF network planning in LoS links, while the COST-231 Hata model has better accuracy in NLoS links. On the other hand, the effects of antennas on the performance of IQRF transceivers (TRs) for LoS and NLoS links are also scrutinized. The use of IQRF TRs with a Straight-Line Dipole Antenna (SLDA) antenna is found to offer more stable results when compared to IQRF (TRs) with Meander Line Antenna (MLA) antenna. Therefore, it is believed that the findings presented in this article could offer useful insights for researchers interested in the development of IoT-based smart city applications.

Non-line-of-sight WDM-MIMO optical camera communications with the DBPWR algorithm

Non-line-of-sight (NLOS) optical camera communications (OCC) is a potential wireless communication technology, which offers robustness to blocking and blooming effect, higher data throughput over longer transmission distances compared with line-of-sight OCC. In this work, we propose a wavelength division multiplexing (WDM)-multiple inputs multiple outputs (MIMO) NLOS OCC system with multi-level pulse width modulation (MPWM) and difference-based pulse width recognition (DBPWR) schemes. Signals on different color channels are separated through the RGB filter of the CMOS camera, while MPWM signals on the same color channel are separated using the DBPWR algorithm. Compared with the conventional threshold-based demodulation scheme, the proposed DBPWR scheme has lower complexity, higher reliability, and improved sampling frequency offset tolerance. A total data rate of 10.8 kb/s is experimentally demonstrated over more than 2 m NLOS link using the proposed WDM-MIMO OCC system, which is sufficient in many Internet of things applications.

V-Band Channel Modeling, Throughput Measurements, and Coverage Prediction for Indoor Residential Environments

With the increased resolution and frame rates of video recordings, in combination with the current evolution towards video-on-demand streaming services and the user expecting ubiquitous wireless connectivity, it is necessary to design wireless communication systems that allow high-rate data transfer. The large bandwidths that are available in the mmWave frequency band allow such high data rates. In this paper, we provide an experimental and simulated indoor residential radio channel model at V-band frequencies and perform packet error rate and throughput measurements at 60 GHz using IEEE 802.11ad transceivers. We compare the path loss and throughput measurements to simulations using a network performance prediction tool. The path loss measurement results using an omnidirectional transmit antenna correspond well to generic indoor mmWave channel models. Double-directional path loss measurements show that generic models underestimate path loss of non-Line-of-Sight (NLOS) links. A ray-launching algorithm is designed and validated, and used for IEEE 802.11ad throughput estimation based on link budget calculations. The link budget underestimates the achieved throughput, when comparing to adaptive-rate MCS selection in a commercial transceiver, based on the measured signal-to-noise ratio. Packet error rate measurements confirm that, even for NLOS links, throughputs exceeding 1 Gbps are possible.

Vehicle Localization Based on Hypothesis Test in NLOS Scenarios

Recently, vehicle-to-vehicle (V2V) based localization has attracted much attention due to its potential to achieve high accuracy. However, the error caused by non-line-of-sight (NLOS) propagation significantly affects the localization performance. In this paper, a novel method combining V2V communication and NLOS identification is proposed to improve accuracy of vehicle localization in NLOS scenarios. First, the algorithm identifies NLOS links with statistical methods. In this step, identification is conducted by decision theory or z-test, which depends on whether priori NLOS information is known or not. After that, NLOS links are discarded. Using the information including length of remaining links and GPS positions of surrounding vehicles, the estimated positions of target vehicles can be obtained by multilateration. It is shown by simulations that the proposed algorithm outperforms several classical methods in accuracy. Specifically, when priori NLOS information is available, 80% of vehicles have a localization error less than 5 m. For the case where NLOS information cannot be obtained, the algorithm still have good performance, and the ratio of vehicles having error within 5 m reaches 70%.

Transmitter Half Distribution Angle Optimization in Non-Line of Sight Visible Light Communication

Visible light communication is a subset of wireless optical communication that uses white light-emitting diode-based illumination infrastructure for data transmission. Visible light communication (VLC) for indoor communication has recently received much attention to meet the ever-increasing data demands. In VLC, line-of-sight (LOS) connections can provide massive bandwidth while ensuring low path loss if there is an unobstructed path between the transmitter and the receiver, but they are prone to blocking. Non-line of sight (NLOS) links can provide significant blocking resilience and ease of usage by relying on light reflections from the walls, ceiling, and other diffusely reflecting surfaces. Non-line-of-sight optimization is required in VLC to improve receiver optical power and, as a result, system performance. Various studies have been conducted to optimize receiver setups to improve the optical power at the receiver end. In a non-line of sight (NLOS) scenario, however, transmitter configurations significantly impact the received optical power at the detector. The received optical power is spatially non-uniform owing to the scattered trait of white LEDs. This reduces the NLOS link performance. While employing LEDs with a larger half distribution angle minimizes these spatial power fluctuations, it also intensifies reflections from the walls. The more the reflections the more the signal attenuation. In consequence, to improve the receiver optical power in NLOS, this research provides a Transmitter half distribution angle optimization technique, in which the spatial power heterogeneity and the receiver’s mean power gain are recursively estimated for different reflections. The optimization results in a higher mean power gain and reduced spatial power heterogeneity, improving VLC link performance when transmitter and receiver are not in-line.

A Convex Optimization Approach For NLOS Error Mitigation in TOA-Based Localization

This paper addresses the target localization problem using time-of-arrival (TOA)-based technique under the non-line-of-sight (NLOS) environment. To alleviate the adverse effect of the NLOS error on localization, a total least square framework integrated with a regularization term (RTLS) is utilized, and with which the localization problem can get rid of the ill-posed issue. However, it is challenging to figure out the exact solution for the considered localization problem. In this case, we convert the RTLS problem into a semidefinite program (SDP), and then obtain the solution of the original problem by solving a generalized trust region subproblem (GTRS). The proposed method has a relatively good robustness in localization even under the circumstance that the prior knowledge of the NLOS links or its distribution does not know. The outperformance of the proposed method is demonstrated in the simulations compared with other state-of-the-art techniques.

The Impact of Vertical Salinity Gradient on Non-Line-of-Sight Underwater Optical Wireless Communication

The non-line-of-sight (NLOS) underwater communication can offer a viable route in signal propagation and coverage, thus mitigating the pointing, acquisition, and tracking difficulties in line-of-sight optical communication. However, implementing the NLOS link is non-trivial. While the NLOS technique relies on light scattering, i.e., channel turbulence can facilitate NLOS communication, the associated path-loss (PL) can be significant. Signal fading can degrade link robustness, which arises due to ocean water temperature and salinity fluctuation and gradients. To evaluate the robustness of NLOS in natural waters, we systematically measure the link metrics, such as the bit error ratio, PL, and signal-to-noise ratio (SNR), of water bodies of uniform and nonuniform vertical salinity ranging from 30–40‰ (part-per-thousand). We found that salinity-induced turbulence can establish NLOS communication with PL reduction of 2.35 dB/m and SNR increase by 32.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> for dynamic water. Furthermore, a strong correlation was obtained between the strength of signal fluctuations and the received SNR. Finally, we obtained a Gaussian distribution of the statistical scintillation behavior. These results demonstrated the benefit of using the NLOS regime for underwater wireless sensor networks for aiding designers and engineers.

On-Body UWB Channel Classification and Characterization for Various Physical Exercises

This paper presents classification, characterization and modelling of ultra-wideband (UWB) on-body channel links while performing various physical exercises. The physical exercises mainly focus on range-of-motion exercises in which the upper and lower limb movement is dominant. Such exercises are suitable for various applications in medical domain such as rehabilitation, physiotherapy, in sports for training and general-well-being. Wrist and ankle regions have been chosen for the wearable antenna placement and three different antenna orientations have been considered with respect to the location on the limb, inner, outer, and front region. The channel for various activities has been classified into line of sight (LOS) and non-line of sight (NLOS) links based on computation of Kurtosis. On-body statistical analysis and distance-dependent study is performed on two important channel parameters, path loss magnitude and rms delay spread considering the LOS and NLOS links. The channel variation is attributed to the location/orientation of the antenna, Tx-Rx distance, obstruction caused by the human body and different categories of activities.

Performance Analysis of Hybrid Beamforming Precoders for Multiuser Millimeter Wave NOMA Systems

This paper proposes a low-complexity and energy efficient sub-connected structure (SCS) hybrid beamforming (HBF) and fully-connected structure (FCS) HBF for non-orthogonal multiple access (NOMA) systems operating under line of sight (LOS) and non-line of sight (NLOS) millimeter wave (mmW) environments. The mathematical model to maximize sum-rate and energy efficiency (EE) of these two HBF-NOMA configurations is formulated to corroborate our simulation results. We aim to maximize both the sum-rates and EEs of the SCS-HBF-NOMA and FCS-HBF-NOMA systems via successive interference cancellation-zero forcing (SIC-ZF) and phased-zero forcing (P-ZF) schemes, respectively. Associated performance analysis under New York University (NYU) mmW channel model is investigated for both two and four users per cluster. It is found that our proposed SCS-HBF-NOMA generally yields higher EE than the FCS-HBF-NOMA in both LOS and NLOS links having high signal to noise ratio (SNR) regime. Moreover, the proposed multistream HBF-NOMA leverages spatial multiplexing to attain higher sum-rates than the single stream counterpart. The employment of $\text{3}\,\text{bit}$ quantization is capable of optimizing the sum-rate for energy efficient finite resolution HBF-NOMA systems. Results also manifest that SCS-HBF structure with reduced antennas in its sub-array beamforms a wider beam capable of mitigating against the performance degradation on the account of channel imperfection in NOMA system. Furthermore, it is found that the number of users per cluster should be carefully selected for optimal performance of HBF-NOMA system. Lastly, space complexity of the HBF-NOMA scheme is found to be the lowest in P-ZF FCS-HBF-NOMA.