Railway satellite channel at Ku band and above: Composite dynamic modeling for the design of fade mitigation techniques

Past studies on the railway satellite channel (RSC) at Ku band and above consider exclusively the attenuation coming from the metal power arches (PAs) along the railway route, producing significant though deterministic periodical fast fading. Nevertheless, limited attention has been given to model tropospheric effects on the RSC. The present paper takes a more comprehensive view of the RSC by introducing a novel stochastic dynamic model of rain fading in mobile satellite systems on top of the diffraction because of PAs. The proposed approach builds upon well‐established research on rain attenuation time series synthesizers employing stochastic differential equations. It is shown that this propagation tool may provide significant aid, in general, in mobile satellite system simulations and in the design of fade mitigation techniques (FMTs), particularly aiming at the railway scenario. The tool enables the generation of fade events, fade duration statistics, rain attenuation power spectrum and predicting the necessary FMT control loop margin. This is particularly useful for the RSC because most of the proposed FMTs focusing on PAs are not appropriate for compensating atmospheric fading. Copyright © 2011 John Wiley & Sons, Ltd.     

Experimental assessment of slant‐path rain attenuation variability in the Ka‐band

This study is based on the results of a slant‐path Ka‐band propagation experiment carried out in Madrid, Spain, regarding rain attenuation, which is the main propagation impairment in this frequency band. The experimental and statistical results correspond to seven complete years of measurements, a period large enough to accomplish a comprehensive analysis in order to characterize the variability of rain rate and attenuation. It is shown that year‐to‐year variability is significant in temperate climates as Madrid's. The aforementioned significance is also apparent with regards to seasonal, monthly, worst‐month and hourly variability concerning rain attenuation, which are also discussed and related when possible to the variability of the rain phenomena, either represented by the total amounts of rainfall in the different periods or by rain rate statistics. Copyright © 2015 John Wiley & Sons, Ltd.     

Time series predictor of Ku band rain attenuation over an Earth–space path at a tropical location

A channel model is proposed to predict the time series of Ku band rain attenuation during a rain event at a tropical location. The model is based on considering the Gaussian distribution of the conditional occurrence of rain attenuation with a particular value of the attenuation occurring before. The mean (μ) and standard deviation (σ) for the distribution are modeled with the experimental data. The measured attenuation at a particular time instant is used to obtain μ and σ and to predict the attenuation after certain interval. The channel model has tested well giving the predicted attenuation that agrees with the measured value with a mean error within 15% higher than 1 dB. Validity of the model is also tested with the first‐order and second‐order statistics of attenuation occurrence, on a long‐term basis. The method can also be applied even if attenuation measurements are missing for certain period of time with increased error. Copyright © 2011 John Wiley & Sons, Ltd.     

A mathematical theory of de‐integrating long‐time integrated rainfall and its application for predicting 1‐min rain rate statistics

To date, the methods devised for converting long‐term experimental probability distribution (pd), , of the rain rate ρ_T integrated in T min (T > > 1 min) to 1‐min pd, P_R(R), of the instantaneous rain rate R, are based on flawed T‐min data and, as such, are not based on fully reliable first principles. is not only an upward translated version of P_R(R) but also rotated clockwise and distorted. The current methods do not correct these errors. We propose and discuss a mathematical theory, which corrects these errors and thus de‐integrates T‐min experimental pds into the corresponding 1‐min pd, the input required by all rain attenuation prediction methods. The theory is based on simple first principles whose parameters are calibrated by means of a large and reliable rain‐rate data bank recorded in Spino d'Adda, a site held as an experimental laboratory and used for exploratory data analysis. We show that P_R(R) is modelled by four distinct functions in four disjoint ranges, and that this modelling is physically meaningful. We have tested the theory up to integration times of 12 h, with a large experimental data bank of 1‐min rain‐rate time series recorded in Gera Lario, Fucino, Rome, Prague, and Montreal, besides Spino d'Adda. Defined the fraction of rainy time in an average year, P_o (%), we have found that: (a) the modelling is very good up to 6 h; (b) in the range from about P_o to 0.001%, the error values are constant, with average error set at about ? 3% and RMS error less than 8% for T ≤ 120 min, less than about 9% for 120 < T ≤ 360 min. We have also applied the theory to rain‐rate time series provided by meteorological agencies with integration time T = 60 min (blind test) with excellent result. Copyright © 2011 John Wiley & Sons, Ltd.     

Three‐year fade and inter‐fade duration statistics from the Q‐band Alphasat propagation experiment in Madrid

Propagation campaigns are carried out at different frequencies and geographical areas to characterize the slant‐path propagation channel. One of the objectives of the Alphasat Propagation Experiment is to evaluate the performance of satellite links that operate in the Q/V band. Since March 2014, the copolar level of the Alphasat Q‐band beacon signal has been measured at Universidad Politécnica de Madrid, Spain. The fade dynamics—fade and inter‐fade durations—results for three complete years (March 2014 to February 2017) of measurements are presented in this paper. Moreover, the experimental setup and receiver characteristics are described in detail. The collected data (with a mean availability of 97%) can be used to evaluate the atmospheric propagation impairments with a very good degree of accuracy. The probability of occurrence and the fraction of time of fade duration for an average‐year have been compared with the ITU‐R and CRC models with moderate agreement. For this reason, a modeling effort has been made leading to the conclusion that there is room for improvement in the models.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

Second‐order statistics of system with N‐branch microdiversity and L‐branch macrodiversity operating over gamma shadowed Nakagami‐m fading channels

The problem concerning short‐term fading and long‐term fading (shadowing) and their deleterious effects on wireless systems performance has been in focus for a long time. In this paper, motivated by the results of propagation measurements in land‐mobile and indoor‐mobile systems, and by the fact that gamma distribution can describe shadowing reliably, Nakagami‐m distribution is used to model the signal envelope and gamma distribution is used to model the average signal power. Receive diversity with maximal‐ratio combining and selection combining is implemented at the microlevel and macrolevel, respectively. The general case is explored, which assumes that microdiversity and macrodiversity are provided through arbitrary number of channels. Because shadowing has larger correlation distance than short‐term fading, correlated macrodiversity channels are studied. This paper investigates the dynamics of the received signal. A novel rapidly converging infinite‐series expression for average level crossing rate and average fade duration are obtained. Numerical results are graphically presented to examine the impact of fading severity, shadowing severity, number of diversity branches at the microlevel, number of base stations and correlation between base stations to the system's performance. Computer simulations are also performed to verify the validity and the accuracy of proposed theoretical analysis. Copyright © 2012 John Wiley & Sons, Ltd.     

Linear time‐invariant/time‐varying equalization of MIMO channels

In this paper, the problem of equalization of linear multi‐input multi‐output (MIMO) channels is addressed. Using an algebraic approach, we propose a unifying formalism for the linear time‐invariant/time‐varying channel equalization problem. A new definition called algebraic equalization is presented. This allows us to derive the necessary and sufficient condition on the existence of equalizer. We describe the received signals in terms of Kalman model. This then provides us with a test of channel equalization, which may be performed formally in terms of ranks of some related matrices. Copyright © 2007 John Wiley & Sons, Ltd.     

Implementation and analysis of spectral subtraction in deterministic wide‐band anti‐jamming scenarios

In this paper, we propose an approach for mitigating deterministic and partially deterministic jamming signals from the received signal space, thus yielding recoverable signal vectors at a target receiver. Aimed at friendly or self‐jamming environments, where enemy communication jamming is paramount. The proposed approach employs a concept called spectral subtraction, where one or more known signals can be removed from the received signal space via the subtraction of their spectral characteristics from the received composite signal. Although spectral subtraction is used in a variety of speech communication scenarios, it has not been extensively employed in wireless applications because of several practical challenges, such as achieving both sufficient time alignment and accurate waveform characterization of the signal to be removed. In order to assess these challenges, as well as analyze the potential benefits of the proposed approach, validation was performed using actual over‐the‐air experimentation using software‐defined radio technology. The feasibility study of the proposed approach for achieving sufficient signal removal was examined for a constrained operating scenario, and experimental results show that spectral subtraction can be achieved in a physical transmission environment. Although physical simulations were limited, they provide baseline results for a previously untested method of jammer suppression. Copyright © 2016 John Wiley & Sons, Ltd.     

Simulation of the performance of a Ka‐band VSAT videoconferencing system with uplink power control and data rate reduction to mitigate atmospheric propagation effects

The purpose of this paper is to evaluate the impact of propagation effects at 20/30 GHz on the performances of the radio interface of a VSAT videoconferencing system and to demonstrate the interest to implement Fade Mitigation Techniques in order to improve the link availability. Specifications and system requirements of the VSAT network are briefly described. System performances of the physical layer are simulated using time‐series data measured with the ESA's OLYMPUS satellite. The simulations enable to implement and optimize Up‐Link Power Control and Data Rate Reduction, and to assess the related link performance improvement. Moreover, the introduction of propagation data sets allows to have a better idea of some system parameters which directly impact on the quality of service such as the switching rate or the link availability that such a system should have when deployed in operational conditions. Copyright © 2002 John Wiley & Sons, Ltd.     

BER‐modified regenerative protocols for OFDM‐based two‐way wireless relay channel

It is well known that symbol‐level regenerative relay protocols suffer the error propagation problem because receiver decodes blindly and overlooks the probability of relay forwarding wrong bits. In a two‐way relay networks, the problem still exists in both network coding (decode‐and‐forward) and physical network coding (denoise‐and‐forward) protocols. For today's widely adopted wide band Orthogonal frequency‐division multiplexing (OFDM) systems, error propagation will dramatically restrict the system's end‐to‐end performance especially when frequency selective fading exists. In this paper, we propose a bit error rate (BER) modified decoding algorithm for these OFDM‐based two‐way symbol‐level regenerative relay strategies. By confining the confidence level of demodulated soft information according to the likelihood of relay having made an error on each bit, this proposed algorithm significantly boosts the end‐to‐end BER performance of the system. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis,Characterization, Two‐Photon Absorption,and Optical Limiting Properties of Ladder‐Type Oligo‐p‐phenylene‐Cored Chromophores

This paper reports on the two‐photon absorption (TPA) and related up‐converted emission properties of a novel series of chromophores containing ladder‐type oligo‐p‐phenylenes with various π‐conjugation lengths. The design and synthesis of these ladder‐type two‐photon chromophores are first discussed. An increase in the π‐conjugated length of the ladder‐type oligo‐p‐phenylene for these chromophores leads to an increase in TPA cross‐section together with an increased fluorescence quantum yield. These chromophores exhibit high fluorescence quantum yields because of the rigid planar structure of the ladder‐type oligomers. The chromophore with an enhanced TPA cross‐section together with an increased fluorescence quantum yield would provide significant benefits for two‐photon excited fluorescence based applications. An improved optical limiting behavior was also demonstrated using the ladder‐type pentaphenylene cored chromophore.     

Sizing and optimization of high throughput radio‐frequency data down link of earth observation satellites

This paper analyses the sizing of networks of radio‐frequency ground stations operating at Ka band or above for the reception of data acquired by low earth orbit observation satellites. In particular, the way to account for the use of adaptive transmission schemes, to cope with the high propagation impairments in the troposphere is detailed. A solution to size optimally the ground stations and satellite memory, relying on propagation channel simulations, is also proposed. The resulting performances are assessed in a latter stage using simulated propagation time series. Copyright © 2015 John Wiley & Sons, Ltd.     

Impact of TH‐UWB interference on MB‐OFDM UWB systems: interference modeling and performance analysis

In this paper, the coexistence issue between multiband‐orthogonal frequency‐division multiplexing (MB‐OFDM) and time‐hopping ultra‐wideband (TH‐UWB) networks is widely analyzed. For this purpose, to study and model the TH‐UWB interference, an analytical framework which describes key features of the interference distribution is provided. The interference distribution is studied in the context of TH‐UWB's signaling parameters. Our results reveal that the interference distribution highly depends on its time‐hopping parameters. Therefore, choosing proper time‐hopping parameters leads to less destructive interferences. The Generalized Gaussian and the Symmetric‐α‐Stable (SαS) distributions are used to model the interference‐plus‐noise signal. The maximum likelihood and a characteristic function‐based regression‐type methods are adopted to estimate parameters of Generalized Gaussian and SαS distributions, respectively. Moreover, the interference channel effects on the impulsive behavior of the TH‐UWB signal is studied. It is shown that impulsive behavior of the faded interference signals highly depends on the channel time‐dispersiveness. Furthermore, an exact performance of a multiband‐orthogonal frequency‐division multiplexing system impaired by a TH‐UWB system is derived. The comparison of the analytical performance, the empirical simulation, and the approximation results show that both approximation methods are valid for low interference‐to‐noise‐ratio, while SαS provides a more accurate approximation for high interference‐to‐noise‐ratio. Copyright © 2015 John Wiley & Sons, Ltd.     

On the mobile‐to‐mobile linear time‐variant shallow‐water acoustic channel response

We expose some concepts concerning the channel impulse response (CIR) of linear time‐varying (LTV) channels to give a proper characterization of the mobile‐to‐mobile underwater channel. We find different connections between the linear time‐invariant (LTI) CIR of the static channel and 2 definitions of LTV CIRs of the dynamic mobile‐to‐mobile channel. These connections are useful to design a dynamic channel simulator from the static channel models available in the literature. Such feature is particularly interesting for overspread channels, which are hard to characterize by a measuring campaign. Specifically, the shallow water acoustic (SWA) channel is potentially overspread because of the signal low velocity of propagation, which prompts long delay spread responses and great Doppler effect. Furthermore, from these connections between the LTI static CIRs and the LTV dynamic CIRs, we find that the SWA mobile‐to‐mobile CIR does not only depend on the relative speed between transceivers, but also on the absolute speed of each of them referred to the velocity of propagation. Nevertheless, publications about this topic do not consider it and formulate their equations in terms of the relative speed between transceivers. We illustrate our find using 2 couples of examples where, even though the relative speed between the mobiles is the same, their CIRs are not.     

Analysis of transmit antenna selection/orthogonal space‐time block coding with receive selection and combining in the presence of feedback errors

Examining the effect of imperfect transmit antenna selection (TAS) caused by the feedback link errors on the performance of hybrid TAS/orthogonal space‐time block coding (OSTBC) with single receive antenna selection (i.e., joint transmit and receive antenna selection (JTRAS)/OSTBC) and TAS/OSTBC (with receive maximal‐ratio combining‐like combining structure) over slow and frequency‐flat Nakagami‐m fading channels is the main objective of this paper. Under ideal channel estimation and delay‐free feedback assumptions, statistical expressions and several performance metrics related to the post‐processing signal‐to‐noise ratio are derived by defining a unified system model concerning both JTRAS/OSTBC and TAS/OSTBC schemes. Exact analytical expressions for outage probability (OP) and bit/symbol error rates of M‐ary modulations are presented in order to provide a detailed examination on the OP and error performances of the unified system that experiences feedback errors. Also, the asymptotic diversity order analysis, which shows that the diversity order of the investigated schemes is equal to the diversity order provided by OSTBC transmission itself, is included in the paper. Moreover, we have validated the theoretical results via Monte Carlo simulations. Copyright © 2014 John Wiley & Sons, Ltd.     

Series resistance characterization of industrial silicon solar cells with screen‐printed contacts using hotmelt paste

This work presents the results of a detailed series resistance characterization of silicon solar cells with screen‐printed front contacts using hotmelt silver paste. Applying the hotmelt technology energy conversion efficiencies up to 18·0% on monocrystalline wafers with a size of 12·5 cm × 12·5 cm have been achieved, an increase of 0·3% absolute compared to cells with conventional screen‐printed contacts. This is mainly due to the reduction in the finger resistance to values as low as 14 Ω/m, which reduces the series resistance of the solar cell significantly. To retrieve the lumped series resistance as accurately as possible under the operating condition, different determination methods have been analyzed. Methods under consideration were fitting of the two‐diode equation function to a dark IV‐curve, integration of the area A under an IV‐curve, comparison of a j_sc–V_oc with a one‐sun IV‐curve, comparison of the j_sc and V_oc points of a shaded curve with the one‐sun IV‐curve as well as comparison of a dark IV‐curve with a one‐sun IV‐curve, and comparison of IV‐curves measured at different light intensities. The performed investigations have shown that the latter four methods all resulted in reliable series resistance values. Copyright © 2007 John Wiley & Sons, Ltd.     

In‐band bootstrapping in database‐driven multi‐hop cognitive radio networks

Database‐driven approach has emerged recently as an alternative or supplement for spectrum sensing for cognitive radio networks (CRNs). Within database‐driven CRNs, master devices obtain spectrum information by direct connection to a spectrum database, while slave devices can only access spectrum information indirectly via masters. The in‐band approach completely based on primary spectrum channels can be used, which eliminates the need for out‐of‐band connections and eases the adoption of the database‐driven spectrum sharing. In this paper, we study the in‐band bootstrapping process for database‐driven multi‐hop CRNs, where master/slave devices form a multi‐hop networks, and slaves need multi‐hop communications to obtain spectrum information from the master during bootstrapping. We start with the basic design of in‐band bootstrap protocol, whose performance is unsatisfactory of protocol overhead and bootstrap time. Then we propose 2 enhancements: first, we incorporates the recursive fractional spectrum information query scheme to reduce protocol overhead; then we propose the prefetch scheme to reduce the bootstrap time. According to the analysis and simulation results, our proposed protocols can greatly improve the performance: the recursive fractional spectrum information query enhancement reduces up to 40% of the overhead, the prefetch enhancement reduces more than 20% of the bootstrap time.     

Characterization and modelling of time diversity statistics for satellite communications from 12 to 50 GHz

To compensate propagation impairments on Earth–space communication links, a specific fade mitigation technique to make up for rain propagation impairments is studied in this paper: the time diversity. This process consists in sending the information when the propagation channel allows to get it through. Here the time diversity technique is applied to different experimental attenuation time series collected in Europe: Spino d'Adda (Italy), Louvain‐la‐Neuve and Lessive (Belgium). These propagation measurements have been collected from 12 to 50 GHz and the performance of time diversity technique is assessed from the generation of time diversity statistics conditioned to the time delay. A prediction method of these statistics is also described. The proposed model relies on the time correlation of the attenuation time series. The model is validated and its accuracy analysed in terms of prediction error calculated using the different databases. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance Investigation of Space‐Time Block Coded Multicarrier DS‐CDMA in Time‐Varying Channels

In this letter, we evaluate the system performance of a space‐time block coded (STBC) multicarrier (MC) DS‐CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS‐CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS‐CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true.