Publikace

Publikace našeho týmu

On Horizontal Distribution of Vertical Gradient of Atmospheric Refractivity

  • DOI: 10.1002/asl.755
  • Odkaz: https://doi.org/10.1002/asl.755
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    The horizontal distribution of the vertical gradient of atmospheric refractivity is important for the assessment of the propagation of electromagnetic waves on nearly horizontal paths. A 5-year set of meteorological data, obtained fromthe ERA-Interim numerical weather product, has been used to analyze this distribution statistically. Vertical gradient maps of the Central European region have been processed to derive empirical probability distributions of the difference between local point gradient values at two locations. The difference of the effective (path-averaged) gradient along the path and the local gradient is shown to be statistically distributed so that the quantiles increase linearly with distance in the interval from 100 to 1000 km. Finally, a spatial correlation function is obtained and described by an exponential model with correlation distances determined in the range of 400–700 km.

A New Propagation Channel Synthesizer for UAVs in the Presence of Tree Canopies

  • DOI: 10.3390/rs9020151
  • Odkaz: https://doi.org/10.3390/rs9020151
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Following the increasing popularity of unmanned aerial vehicles (UAVs) for remote sensing applications, the reliable operation under a number of various radio wave propagation conditions is required. Assuming common outdoor scenarios, the presence of trees in the vicinity of a UAV or its ground terminal is highly probable. However, such a scenario is very difficult to address from a radio wave propagation point of view. Recently, an approach based on physical optics (PO) and the multiple scattering theory (MST) has been proposed by the authors, which enables fast and straightforward predictions of tree-scattered fields at microwave frequencies. In this paper, this approach is developed further into a generative model capable of providing both the narrowband and wideband synthetic time series of received/transmitted signals which are needed for both UAV communications and remote sensing applications in the presence of scattering from tree canopies. The proposed channel synthesizer is validated using both an artificially-generated scenario and actual experimental dataset.

Open area 2 x 2 MIMO channel model for 2 GHz low-elevation links with diversity and capacity applications

  • Autoři: Zelený, J., Perez-Fountan, F., prof. Ing. Pavel Pechač, Ph.D., Marion-Espineira, P.
  • Publikace: Radio Science. 2017, 55(5), 616-629. ISSN 0048-6604.
  • Rok: 2017
  • DOI: 10.1002/2016RS006199
  • Odkaz: https://doi.org/10.1002/2016RS006199
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In civil surveillance applications, unmanned aerial vehicles (UAV) are being increasingly used in floods, fires, and law enforcement scenarios. In order to transfer large amounts of information from UAV-mounted cameras, relays, or sensors, large bandwidths are needed in comparison to those required for remotely commanding the UAV. This demands the use of higher-frequency bands, in all probability in the vicinity of 2 or 5 GHz. Novel hardware developments need propagation channel models for the ample range of operational scenarios envisaged, including multiple-input, multiple-output (MIMO) deployments. These configurations may enable a more robust transmission by increasing either the carrier-to-noise ratio statistics or the achievable capacity. In this paper, a 2 x 2 MIMO propagation channel model for an open-field environment capable of synthesizing a narrowband time series at 2 GHz is described. Maximal ratio combining diversity and capacity improvements are also evaluated through synthetic series and compared with measurement results. A simple flat, open scenario was evaluated based on which other, more complex environments can be modeled.

A New Model for Scattering From Tree Canopies Based on Physical Optics and Multiple Scattering Theory

  • Autoři: Kvičera, M., Peréz Fontán, F., Israel, J., prof. Ing. Pavel Pechač, Ph.D.,
  • Publikace: IEEE Transactions on Antennas and Propagation. 2017, 65(4), 1925-1933. ISSN 0018-926X.
  • Rok: 2017
  • DOI: 10.1109/TAP.2017.2669980
  • Odkaz: https://doi.org/10.1109/TAP.2017.2669980
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    This paper presents a new model for scattering from tree canopies based on a modified physical optics approach. In this way, propagation through a canopy is accounted for by respecting the complex propagation constant, which can be obtained either by the multiple scattering theory (MST) or approximated based on the canopy specific attenuation. Unlike the case when MST is applied directly, the proposed approach offers significant benefits, including a straightforward software implementation, feasible computation times for high frequencies and electrically large canopies, and, most importantly, near-field calculations in regions close to a canopy. The new model is first tested against MST using two artificial single-tree scenarios at 2 and 10 GHz. Then, experimental data at 2 GHz obtained with the use of a remote controlled airship for an actual single-tree scenario are utilized as well. In this way, the model is thoroughly validated and its advantages over MST are presented in detail.

A Model for the Detailed Analysis of Radio Links Involving Tree Canopies

  • Autoři: Perez-Fontan, F., Kvičera, M., prof. Ing. Pavel Pechač, Ph.D., Israel, J.
  • Publikace: Radioengineering. 2016, 25(4), 637-643. ISSN 1210-2512.
  • Rok: 2016
  • DOI: 10.13164/re.2016.0637
  • Odkaz: https://doi.org/10.13164/re.2016.0637
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Detailed analysis of tree canopy interaction with incident radiowaves has mainly been limited to remote sensing for the purpose of forest classification among many other applications. This represents a monostatic configuration, unlike the case of communication links, which are bistatic. In general, link analyses have been limited to the application of simple, empirical formulas based on the use of specific attenuation values in dB/m and the traversed vegetated mass as, e.g., the model in Recommendation ITU-R P.833-8 [1]. In remote sensing, two main techniques are used: Multiple Scattering Theory (MST) [25] and Radiative Transfer Theory (RT), [5], [6]. We have paid attention in the past to MST [710]. It was shown that a full application of MST leads to very long computation times which are unacceptable in the case where we have to analyze a scenario with several trees. Extensive work using MST has been also presented by others in [1116] showing the interest in this technique. We have proposed a simplified model for scattering from tree canopies based on a hybridization of MST and a modified physical optics (PO) approach [16]. We assume that propagation through a canopy is accounted for by using the complex valued propagation constant obtained by MST. Unlike the case when the full MST is applied, the proposed approach offers significant benefits including a direct software implementation and acceptable computation times even for high frequencies and electrically large canopies. The proposed model thus replaces the coherent component in MST, significant in the forward direction, but keeps the incoherent or diffuse scattering component present in all directions. The incoherent component can be calculated within reasonable times. Here, we present tests of the proposed model against MST using an artificial single-tree scenario at 2 GHz and 10 GHz.

Sensitivity Analysis of Multiple Scattering Theory Applied to Tree Canopies at Microwave Frequencies

  • Autoři: Kvičera, M., Israel, Jonathan, Perez-Fontan, F., prof. Ing. Pavel Pechač, Ph.D.,
  • Publikace: IEEE Antennas and Wireless Propagation Letters. 2016, 15 1175-1178. ISSN 1536-1225.
  • Rok: 2016
  • DOI: 10.1109/LAWP.2015.2498645
  • Odkaz: https://doi.org/10.1109/LAWP.2015.2498645
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    A sensitivity analysis of the multiple scattering theory applied to propagation in tree canopies is presented. This analysis aims at addressing the influence of input parameters on the canopy’s equivalent scattering amplitude as well as the overall coherent and incoherent scattered fields. The main input parameters of interest were identified as frequency, number densities and dielectric parameters of branches and leaves, polarization and a canopy shape. A number of graphical results are provided enabling to identify the key parameters of corresponding radiowave propagation models based on the multiple scattering theory.

Inverse Gaussian-based composite channel model and time series generator for land mobile satellite systems under tree shadowing

  • Autoři: Kourogiorgas, C., Kvičera, M., Panagopoulos, A. D., prof. Ing. Pavel Pechač, Ph.D.,
  • Publikace: IET Microwaves, Antennas & Propagation. 2016, 10(6), 612-616. ISSN 1751-8725.
  • Rok: 2016
  • DOI: 10.1049/iet-map.2015.0725
  • Odkaz: https://doi.org/10.1049/iet-map.2015.0725
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    In this study, a new composite channel model for land mobile satellite (LMS) systems under tree shadowing is proposed utilising the inverse Gaussian (IG) distribution to model the shadowing component. Using the experimental data of received power under tree shadowing at L-band, which were carried out at Stromovka Park in Prague, the Czech Republic, the assumption of the IG distribution for the description of the first-order statistics of large-scale effects is investigated and validated. A new LMS channel model is developed as a sum of a signal experiencing IG shadowing and a signal under Rayleigh multipath effects. Moreover, a methodology to generate accurate time series of the received signal envelope using the IG distribution is presented. Such methodology and generally time series generators can be used for the evaluation of system’s performance and for end-to-end simulations. Finally, it is shown that the proposed time series generator reproduces the first-order and second-order statistics of the LMS channel model with a very good accuracy.

Clutter Loss Modelling for Low Elevation Link Scenarios

  • Autoři: Valtr, P., Zelený, J., prof. Ing. Pavel Pechač, Ph.D., Grábner, M.
  • Publikace: International Journal of Antennas and Propagation. 2016, 2016 ISSN 1687-5869.
  • Rok: 2016
  • DOI: 10.1155/2016/1478471
  • Odkaz: https://doi.org/10.1155/2016/1478471
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    An analysis of the accuracy of several deterministic and semideterministic propagation models related to calculating clutter loss for low elevation links is presented. Predictions are compared with measurements carried out at 2GHz over three locations. By comparing prediction and measurement techniques, improved predictive capabilities of deterministic models are demonstrated and quantified in cases where a description of the propagation environment exists and an air-to-ground scenario is considered.

Generalized Propagation Channel Model for 2GHz Low Elevation Links Using a Ray-tracing Method

  • DOI: 10.13164/re.2015.1044
  • Odkaz: https://doi.org/10.13164/re.2015.1044
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Unmanned Aerial Vehicles (UAV) will increasingly be used for responding to emergencies or for law enforcement in civil surveillance applications. Transferring the enormous amounts of information from UAV-mounted cameras or sensors will require large bandwidths, unlike the information required for remotely controlling a UAV, thus necessitating higher frequency bands typically in the vicinity of 2 or 5 GHz. Novel hardware developments will need to rely on a versatile propagation channel model for the envisaged scenarios ranging from deep shadow urban areas to open fields. This paper studies more complex intermediate scenarios, which fall between the aforementioned ones, and which are more difficult to model. A semideterministic model, first developed for open, flat areas, has been generalized to accommodate any possible operational scenario and was tested in built-up areas. The model involves a stochastic part and a determinist which is a raytracing based part used to compute the long term mean (LTM) of the signal's coherent component.

A Mode-Matching Technique for Analysis of Scattering by Periodic Comb Surfaces

  • Autoři: Valtr, P., Davenport, Ch.J., prof. Ing. Pavel Pechač, Ph.D., Rigelsford, J.M.
  • Publikace: IEEE Transactions on Antennas and Propagation. 2015, 63(9), 4016-4023. ISSN 0018-926X.
  • Rok: 2015
  • DOI: 10.1109/TAP.2015.2452945
  • Odkaz: https://doi.org/10.1109/TAP.2015.2452945
  • Pracoviště: Katedra elektromagnetického pole
  • Anotace:
    Numerical techniques for calculating electromagnetic fields within three-dimensional surfaces are computationally intensive. Therefore, this paper presents the application of a modematching technique developed for analyzing electromagnetic scattering from periodic comb surfaces illuminated by a plane wave. A set of linear equations has been developed to calculate mode coefficients of the field distribution for both E- and H-polarized incident waves. Analysis is performed for two cases where the comb thickness is either infinitely thin or of a finite thickness. The technique is shown to accurately predict both field intensities within the near-field of the periodic surface and far-field scattering patterns. Results are compared to those obtained using the finite integration techniques (FIT) implemented in CST Microwave Studio. Furthermore, numerical results are compared to measurements of an aluminum prototype. Additional far-field scattering measurements using a bi-static system provide additional confidence in CST simulations and the mode-matching methods presented here.

Za stránku zodpovídá: RNDr. Patrik Mottl, Ph.D.