Non-line-of-sight Path Research Articles

WiFi-Based Indoor Line-of-Sight Identification

Wireless LANs, particularly WiFi, have been pervasively deployed and have fostered myriad wireless communication services and ubiquitous computing applications. A primary concern in designing these applications is to combat harsh indoor propagation environments, particularly Non-Line-Of-Sight (NLOS) propagation. The ability to identify the existence of the Line-Of-Sight (LOS) path acts as a key enabler for adaptive communication, cognitive radios, and robust localization. Enabling such capability on commodity WiFi infrastructure, however, is prohibitive due to the coarse multipath resolution with MAC-layer received signal strength. In this paper, we propose two PHY-layer channel-statistics-based features from both the time and frequency domains. To further break away from the intrinsic bandwidth limit of WiFi, we extend to the spatial domain and harness natural mobility to magnify the randomness of NLOS paths while retaining the deterministic nature of the LOS component. We propose LiFi, a statistical LOS identification scheme with commodity WiFi infrastructure, and evaluate it in typical indoor environments covering an area of 1500 m 2. Experimental results demonstrate that LiFi achieves an overall LOS detection rate of 90.42% with a false alarm rate of 9.34% for the temporal feature and an overall LOS detection rate of 93.09% with a false alarm rate of 7.29% for the spectral feature.

NLOS Path Loss Model for Low-Height Antenna Links in High-Rise Urban Street Grid Environments

This paper presents a NLOS (non-line-of-sight) path loss model for low-height antenna links in rectangular street grids to account for typical D2D (device-to-device) communication link situations in high-rise urban outdoor environments. From wideband propagation channel measurements collected in Seoul City at 3.7 GHz, we observed distinctive power delay profile behaviors between 1-Turn and 2-Turn NLOS links: the 2-Turn NLOS has a wider delay spread. This can be explained by employing the idea that the 2-Turn NLOS has multiple propagation paths along the various street roads from TX to RX, whereas the 1-Turn NLOS has a single dominant propagation path from TX to RX. Considering this, we develop a path loss model encompassing 1-Turn and 2-Turn NLOS links with separate scattering and diffraction parameters for the first and the second corners, based on the Uniform Geometrical Theory of Diffraction. In addition, we consider the effect of building heights on path loss by incorporating an adjustable “waveguide effect” parameter; that is, higher building alleys provide better propagation environments. When compared with field measurements, the predictions are in agreement.

NLOS Error Mitigation for TOA-Based Localization via Convex Relaxation

In this paper, we address the time-of-arrival (TOA) based localization problem in an adverse environment, where line-of-sight (LOS) signal propagation between the source and the sensor is not readily available, in which case we have to resort to non-line-of-sight (NLOS) signals. Two convex relaxation methods, i.e., the semidefinite relaxation (SDR) and the second-order cone relaxation (SOCR) methods, are proposed to mitigate the effect of NLOS errors on the localization performance. We consider two separate cases in which the information of the NLOS status is totally unknown and perfectly known, respectively. The proposed methods can be applied without knowing the distribution of NLOS errors. Moreover, we propose a NLOS error mitigation method that is robust to detection errors, which are generated in the process of detecting NLOS paths. Simulation results show that the proposed convex relaxation methods outperform some existing state-of-the-art methods.

Ultraviolet scattering propagation modeling: analysis of path loss versus range

Modeling of the complex atmospheric propagation of deep-ultraviolet (UV) radiation is important for applications such as non-line-of-sight (NLOS) UV communications. Building upon prior work in which it was observed that short-range, singly scattered NLOS path loss varies linearly with range, we formalize this relationship, generalizing it to consider any order of scattering and more-general system characteristics. In particular, we derive the approximate relationship PL[proportionality]r(2-n) between path loss PL and range r for nth-order scattered radiation, and investigate the region of validity of this approximation. Insight arising from the analysis can be invaluable in the development and study of UV systems, as demonstrated by numerical results that illustrate implications of the analysis.

A Hybrid RSS/TOA Method for 3D Positioning in an Indoor Environment

This paper investigates 3D positioning in an indoor line of sight (LOS) and nonline of sight (NLOS) combined environment. It is a known fact that time-of-arrival-(TOA-) based positioning outperforms other techniques in LOS environments; however, multipath in an indoor environment, especially NLOS multipath, significantly decreases the accuracy of TOA positioning. On the other hand, received-signal-strength-(RSS-) based positioning is not affected so much by NLOS multipath as long as the propagation attenuation can be correctly estimated and the multipath effects have been compensated for. Based on this fact, a hybrid weighted least square (HWLS) RSS/TOA method is proposed for target positioning in an indoor LOS/NLOS environment. The identification of LOS/NLOS path is implemented by using Nakagami distribution. An experiment is conducted in the iRadio lab, in the ICT building at the University of Calgary, in order to (i) demonstrate the availability of Nakagami distribution for the identification of LOS and NLOS path, (ii) estimate the pass loss exponent for RSS technique, and (iii) verify our proposed scheme.

Modeling of non-line-of-sight ultraviolet scattering channels for communication

A stochastic non-line-of-sight (NLOS) ultraviolet (UV) communication channel model is developed using a Monte Carlo simulation method based on photon tracing. The expected channel impulse response is obtained by computing photon arrival probabilities and associated propagation delay at the receiver. This method captures the multiple scattering effects of UV signal propagation in the atmosphere, and relaxes the assumptions of single scattering theory. The proposed model has a clear advantage in reliable prediction of NLOS path loss, as validated by outdoor experiments at small to medium elevation angles. A Gamma function is shown to agree well with the predicted impulse response, and this provides a simple means to determine the channel bandwidth. The developed model is employed to study the characteristics of NLOS UV scattering channels, including path loss and channel bandwidth, for a variety of scattering conditions, source wavelength, transmitter and receiver optical pointing geometries, and range.

On the Effect of Shadow Fading on Wireless Geolocation in Mixed LoS/NLoS Environments

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper considers the wireless non-line-of-sight (NLoS) geolocation in mixed LoS/NLoS environments by using the information of time-of-arrival. We derive the CramÉr–Rao bound (CRB) for a deterministic shadowing, the asymptotic CRB (ACRB) based on the statistical average of a random shadowing, a generalization of the modified CRB (MCRB) called a simplified Bayesian CRB (SBCRB), and the Bayesian CRB (BCRB) when the <emphasis emphasistype="italic">a priori</emphasis> knowledge of the shadowing probability density function is available. In the deterministic case, numerical examples show that for the effective bandwidth in the order of kHz, the CRB almost does not change with the additional length of the NLoS path except for a small interval of the length, in which the CRB changes dramatically. For the effective bandwidth in the order of MHz, the CRB decreases monotonously with the additional length of the NLoS path and finally converges to a constant as the additional length of the NLoS path approaches the infinity. In the random shadowing scenario, the shadowing exponent is modeled by <formula formulatype="inline"><tex Notation="TeX">$\varsigma =u\sigma $</tex></formula>, where <formula formulatype="inline"><tex Notation="TeX">$u$</tex> </formula> is a Gaussian random variable with zero mean and unit variance and <formula formulatype="inline"><tex Notation="TeX">$\sigma $</tex></formula> is another Gaussian random variable with mean <formula formulatype="inline"> <tex Notation="TeX">$\mu _{\sigma }$</tex></formula> and standard deviation <formula formulatype="inline"><tex Notation="TeX">$\sigma _{\sigma }$</tex></formula>. When <formula formulatype="inline"><tex Notation="TeX">$\mu _{\sigma }$</tex> </formula> is large, the ACRB considerably increases with <formula formulatype="inline"> <tex Notation="TeX">$\sigma _{\sigma }$</tex></formula>, whereas the SBCRB gradually decreases with <formula formulatype="inline"><tex Notation="TeX">$\sigma _{\sigma }$</tex></formula>. In addition, the SBCRB can well approximate the BCRB. </para>

A Modified SV-Model Suitable for Line-of-Sight Desktop Usage of Millimeter-Wave WPAN Systems

A modified Saleh-Valenzuela (SV) model, which is useful in designing a millimeter-wave WPAN (mmW WPAN) system for line-of-sight (LoS) desktop environments, is proposed. It is demonstrated that a well-known two-path model component is dominantly observed in a typical LoS desktop channel along with the components of the other non-line-of-sight (NLoS) paths. Then, the deterministic two-path model is extended to a statistical two-path model by introducing random variables for the antenna position and merging with the conventional SV-model which is suitable to express NLoS path components. The proposed channel model parameters are extracted in a typical LoS desktop environment, and then the cumulative distribution of total path power gain is discussed using a simulation technique that assumes a specific antenna location scenario. It is shown that the cumulative distribution of total path power gain obtained using the proposed model is useful to estimate the outage probability of the mmW WPAN link.

Short-Range Ultra-Broadband Terahertz Communications: Concepts and Perspectives

We propose the concept of ultra-broadband terahertz communication, based on directed non-line-of-sight (NLOS) transmissions. Potential applications of such a system supporting multi-gigabit data rates are given, and put into the context of currently emerging WLANs/WPANs. The technology and propagation constraints serve as boundary conditions for the determination of the required antenna gain to support ultra-broadband communication. Resulting high-gain antenna requirements will necessitate highly directed transmissions. We propose the use of omni-directional dielectric mirrors to support directed NLOS paths. Their performance is investigated with ray-tracing simulations of a terahertz propagation channel in a dynamic office environment, which is calibrated with measured building-material and mirror parameters. We demonstrate that a directed NLOS path scheme will make a terahertz communication system robust to shadowing. Furthermore, we show that dielectric mirrors covering only parts of the walls will significantly enhance the signal coverage in a typical indoor scenario.

Positioning for NLOS Propagation: Algorithm Derivations and Cramer–Rao Bounds

Mobile positioning has drawn significant attention in recent years. Nonline-of-sight (NLOS) propagation error is the dominant error source in mobile positioning. Most previous research in this area has focused on NLOS identification and mitigation. In this paper, we investigate new positioning algorithms to take advantage of the NLOS propagation paths rather than canceling them. Based on a prior information about the NLOS path, a geometrical approach is proposed to estimate mobile location by using two NLOS paths. On top of this, the least-squares (LS)-based position estimation algorithm is developed to take multiple NLOS paths into account, and its performance in terms of root mean-square error (RMSE) is analyzed. A general LS algorithm considering both LOS and NLOS paths is also derived, and the maximum likelihood-based algorithm is presented to jointly estimate the mobile's and scatterers' positions. The Cramer-Rao lower bound on the RMSE is derived for the benchmark of the performance comparison. The performance of the proposed algorithms is evaluated analytically and is done via computer simulations. Numerical results demonstrate that the derived analytical results closely match the simulated results.

UWB Delay Profile Models for Residential and Commercial Indoor Environments

We present models for the ultrawideband (UWB) channel delay profile in indoor environments, based on the processing of two large sets of measured data. Both measurement sets are for a center frequency of 5 GHz, but the bandwidths are very different-1.25 GHz and 6 GHz. We model both line-of-sight (LOS) and nonline-of-sight (NLOS) paths, and do so for both single-family homes and commercial buildings. Also, we consider both the profile at a receiver point, which we call the multipath intensity profile (MIP), and the locally spatially averaged profile, which we call the power delay profile (PDP). For both cases, we find that the profile for NLOS paths can be modeled as a decaying exponential times a noise-like variation with lognormal statistics and that, for LOS paths, the profile has the same form plus a strong component at the minimum delay. The model consists of statistical descriptions of the parameters of these functions, including the effects of transmit-receive (T--R) distance and of variations from building to building. We show simulation results for a few cases to demonstrate that the model accurately predicts key properties of the measured channels, such as the distribution of rms delay spread.