Fade Mitigation Techniques Research Articles

Fade Mitigation for Air-to-Ground Communication in Aeronautical Telemetry

Telemetry links were comprised of a transmit antenna mounted on the underside of fuselage and a fixed ground station equipped with a tracking antenna. Air-to- ground channel for this telemetry link can be estimated as line of sight propagation. But there is link loss observed at ground control station for certain instant of time. Possible causes of link loss in this case are due to aircraft maneuvering and fading. So analysis of various fade mitigation techniques as well as bit error rate (BER) and signal to noise (SNR) ratio performance comparison between selection diversity, equal gain combining and maximal ratio combining scheme is done. Finally bit error rate performance comparison curve between maximal ratio scheme (MRC) and Alamouti dual antenna transmission scheme is plotted, in order to find out the best scheme for fade mitigation, among all the above mentioned schemes.

Subset Optimization of ACM Schemes Based on DVB-S2 Standard

A rain fading mitigation technique, called Adaptive Coding and Modulation (ACM), is supported by the DVB-S2 standard. But the number of Coding and Modulation Schemes is over dimensioned. It is shown that by a careful selection of the ACM schemes, with one fourth of the total number of schemes, it is possible to get performance close to the optimum, but with a much lower terminal complexity.

COMPARATIVE ANALYSIS OF MODULATION SCHEMES USING ADAPTIVE EQUALIZATION AS FADING MITIGATION TECHNIQUE

Inter-symbol interference (ISI) due to multipath fading is a vital problem in high-speed wireless communication which restricts communication quality and capacity. Therefore, in addition to choosing a fading mitigation technique, it is also important to strategically select a modulation scheme for effective data transmission. Recent literature review on wireless standards, such as 3G and 4G indicates that QAM and QPSK are suitable choices for data transmission. In this paper, a comparative analysis on selected modulation schemes is performed in a fading environment. The mitigation of fading is done using adaptive equalization technique. Also, we show that the signal to noise ratio (SNR) is an important parameter to choose. It is observed that, even when an adaptive equalizer is used at the receiver, a very low SNR gives very high symbol error rate (SER). We derive some important conclusions from our simulation result: QPSK shows minimum SER, whereas 256-PSK and 256-PAM perform worse. Given its spectral efficiency and a low SER, the best choice is 256- QAM.

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

SUMMARYPast 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.

Attenuation prediction for fade mitigation using neural network with in situ learning algorithm

Increasing demand of bandwidth in communication satellites has forced satellite links to be designed in Ku bands and above. But at these frequencies, rain and other tropospheric elements result in large attenuation. To mitigate the tropospheric attenuation of microwave satellite signals above 10 GHz using any standard Fade Mitigation Technique (FMT), it is essential to have a priori knowledge about the level of attenuation. Hence, short-term rain attenuation prediction models play a key role in maintaining the link in which necessary compensation can be applied depending on the early information of attenuation. This paper presents a method of attenuation prediction using Adaptive Artificial Neural Network. Here In situ Learning Algorithm (ILA) has been used to enable the system to track the non-stationary nature of the attenuation. To validate this, Ku Band data, collected at three different sites in India have been used for the purpose of prediction. The performance of the algorithm is determined through the estimation of prediction accuracy by comparing the predicted values with the measured data. Results obtained using the mentioned technique shows considerably good accuracy even up to 20 s of prediction interval with acceptable ratio between the under and over predictions. The prediction performance is evaluated for different prediction intervals. Furthermore the present model is also compared with the persistence model and the relative performance is quantified.

Dynamic Power Allocation for Broadband Multi-Beam Satellite Communication Networks

Broadband satellite communication networks, operating at Ka band and above, play a significant role to worldwide telecommunication infrastructure, providing backhaul and direct-to-user satellite services. Rain attenuation at these frequencies is the most dominant impairment factor that degrades the performance of the system. The dynamic reconfiguration of multi-beam satellite antennas has recently been proposed as a fading mitigation technique. In this paper, the problem of finding the power allocation among the beams of such an antenna, aiming that the number of subscribers not receiving the desired quality of service is minimized is addressed for a satellite communications system with a GEO satellite serving fixed earth terminals. An algorithm for the dynamic power allocation using a physical-mathematical model for rain attenuation prediction is proposed. Extended simulation results highlight the improvement of the dynamic scheme over the static allocation based on long term rain attenuation characteristics.

Rain induced attenuation studies for V-band satellite communication in tropical region

Satellite communications operating at 10 GHz and above in the tropics suffer severe signal degradation due to rain. Attenuation due to rain at 38 GHz had been measured for a period of 20 months in Malaysia. Analyses carried out include seasonal variations, diurnal effects and the annual cumulative distributions. Obtained results were compared with several established prediction models including the ITU-R. The rain fade characteristics were also investigated in determining the levels of signal loss and fading. In addition, the studies highlight several potential fade mitigation techniques that can be embarked. These fundamental aprehensions are very critical for future earth space communication link design and can be exploited as preliminary groundwork plan for the researchers as well as engineers.

A new ‘event‐on‐demand’ synthesizer of rain attenuation time series at Ku‐, Ka‐ and Q/V‐bands

AbstractTo develop and test real‐time fade mitigation techniques control algorithms, propagation time series are needed. An alternative to using real data collected from propagation experiments is to generate typical fading time‐series making use of climatological characteristics as well as geometrical and radio‐electrical parameters of the link. The objective of this paper is to present a rain attenuation time‐series synthesizer and able to generate events on demand. The model is based on an enhanced version of the Maseng–Bakken stochastic model. In the first part of this paper, the basic principles of the enhanced Maseng–Bakken model are recalled and the parameterization of this channel model is discussed for temperate European climates. Then, the theoretical bases of the Lacoste–Carrie ‘event‐on‐demand’ model and its validation constitute the second part of this paper. The enhanced Maseng–Bakken model is fully stochastic, whereas the Lacoste‐Carrie ‘event‐on‐demand’ one offers the possibility to command the maximum attenuation level and the duration of the synthesized event. Copyright © 2009 John Wiley & Sons, Ltd.

Combined Dynamic Channel Simulator for High Capacity Broadband Fixed Wireless Access Systems

We present a time dynamic channel simulator for broadband fixed wireless access systems which properly combines earlier reported models for single degradation effects. The wideband model combines the effect of rain, scintillation, vegetation and multipath propagation. The objective is to specify the relationship between propagation effects and combine them to construct a comprehensive channel model suitable for designing fade mitigation techniques and evaluating system performance. The dynamics of rain attenuation is done using the Maseng-Bakken model. The multipath propagation is modeled using the geometrical based single bounce elliptical channel model. The spatial correlation between multipath taps during rain is incorporated using a space-time rain attenuation model. A lowpass filter which models the power spectral density of scintillation is used to describe the dynamic characteristics of scintillation. The signal fading due to swaying vegetation is described by utilizing a multiple mass-spring system to represent a tree and a turbulent wind model. In addition, numerical results under different combinations of propagation impairments are presented.

Simplified Channel Model for Simulation of Free-Space Optical Communications

Free-space optical (or optical wireless) communications represent an attractive technology for the realization of high-bandwidth wireless communications. However, for propagation through the atmosphere the characteristics of the optical signal are different from those of the signals from, e.g., fiber channels or radio-frequency wireless channels, and therefore the error characteristics on these links are also different. For evaluating fading mitigation techniques for optical wireless communications a channel model is needed that can be easily used by protocol designers. Existing channel models for optical wireless communications are based on atmospheric propagation theory and therefore require a deep physical understanding of the optical propagation through the atmosphere. The goal of this publication is to develop a simplified approach for modeling the received power dynamics of the atmospheric free-space optical channel. The proposed model consists of a random number generator and a low-pass filter and is therefore easy to implement and use. This approach is only valid for systems utilizing intensity modulation with direct detection, but this limitation is acceptable since most commercially available systems use this modulation format. The channel model is developed based on the statistics of received power measurements from a maritime-mobile link, a land-mobile link, and a satellite downlink.

Micro rain cell measurements in tropical India for site diversity fade mitigation estimation

[1] Intersite separation (ISS) between two terminals is one of the important parameters required for implementation of site diversity (SD), as a fade mitigation technique (FMT), in moderate to heavy rain conditions usually encountered in tropical climates. This paper presents the findings of an experiment during the monsoon periods of years 2007 and 2008. It establishes the existence of micro rain cell (MRC) at a tropical site, Ahmedabad (23.06°, 72.62°), India, using measurements from tipping bucket rain gauge network. Spatial extent of MRCs and derived attenuation cells is examined on the basis of their maximum occurrence for initial to mid and last spells of years 2007 and 2008. The analysis also presents ISS statistics of monsoon periods for both the years. Microscale SD as FMT has been discussed on the basis of the calculated diversity gain at Ka band frequency in view of the proposed GSAT-4 satellite. These are the very first findings of their kind over the region of study.

Assessment of rain fade mitigation techniques in the EHF band on a Syracuse 3 20/44-GHz low elevation link

An Earth-to-satellite propagation experiment in the EHF band has been carried out within the framework of the Syracuse 3 program, which is a new generation French military SATCOM system. The originality of this experiment resides in the link's frequencies (20 GHz downlink and 44 GHz uplink) and its low elevation angle (17°). The first part of the article presents a statistical analysis of attenuation data providing the long-term statistics, frequency scaling ratios and fade durations. These results are compared to standard ITU models. The second part of the article is dedicated to the short-term forecasting of rain fade, useful for the implementation of Fade Mitigation Techniques (FMT). Firstly, the downlink attenuation is predicted based on a non-linear ARIMA-GARCH model. The prediction result is then separated into several physical components (gases, clouds and rain) that are scaled to the uplink frequency using specific frequency scaling factors. The performance of the model is assessed based on Syracuse 3 20/44-GHz data collected during a period of 1 year.

Joint Statistics of Rain Attenuation at 20 GHz in Madrid and Aveiro

Rain attenuation is the most severe propagation impairment affecting radiowave signals in the Ka-band. The use of these frequencies for satellite communication systems is facilitated by the implementation of fade mitigation techniques (FMTs). Spatial correlation of rain rate and rain attenuation for distances of several hundred kilometers has been rarely characterized in experiments, however it is relevant for site diversity studies, with distances in this range, and for the design and operation of multibeam satellite systems with shared on-board radio resources. In this letter, experimental results regarding joint statistics of rain and rain attenuation are presented for Madrid, Spain, and Aveiro, Portugal, located 420 km apart. These results have been achieved from two years of simultaneous measurements of the Eutelsat HB-6 Ka-band beacon at 19.7 GHz. It is shown that high-attenuation events are independent, whereas there is some correlation for low-attenuation levels.

Fade Slope Analysis of Ka-Band Earth-LEO Satellite Links Using a Synthetic Rain Field Model

Because the motion of a low Earth orbit (LEO) satellite across the sky causes the Earth-space path to very quickly pass through any rain cells in the vicinity, the degree of rain fading on such paths changes more rapidly and leads to steeper fade slopes than in the geostationary case. Because comprehensive measurement data have not yet been compiled for fading on LEO links in the Ka-band, we have used simulations based on Goldhirsh's method for determining the key parameters of the well-known EXCELL model of a horizontal rain structure from long-term global rain statistics to obtain plausible estimates of the fade slope distributions for selected scenarios. The results that we obtained for geostationary satellites closely match those observed at selected sites during the Advanced Communications Technology Satellite program. The results that we obtained for LEO satellites show how fade slopes will steepen as 1) the altitude of the satellite decreases; 2) the frequency band of operation increases; and 3) the average rain rate increases. Furthermore, they suggest that, at a given probability level, the fade slopes could be between two and ten times greater than those for geostationary satellites and that mobile terminals with a clear view of the sky will experience fade slopes that are similar to those encountered by fixed or transportable terminals. These results have important implications for the design of power control algorithms and other fade-mitigation techniques.

On the Space-Time Variations of Rain Attenuation

Rain attenuation shows a considerable temporal and spatial variability. To simulate fade mitigation techniques such as route diversity, a space-time channel model which accounts for the spatial and temporal variation of rain attenuation is needed. In this paper we investigate the space-time correlation of rain attenuation utilizing 42 GHz star-like network measurements. By combining the spatial and temporal correlation properties of rain attenuation, a simulation model for generating multiple correlated rain attenuation time series based on the Maseng-Bakken model is developed. The model is validated by comparing the statistical and angular diversity properties of the model with those of measurements and theoretical diversity gain models. Furthermore, parameters for the Maseng-Bakken dynamic rain attenuation model were extracted from the star-like network measurements. In addition, using a systematic multivariable technique a model for the parameter betas which controls the dynamics of rain attenuation in the Maseng-Bakken model was developed. Moreover, using available rain attenuation measurements the advantage of route diversity with selection combining is investigated.

Dynamic Model of Signal Fading due to Swaying Vegetation

In this contribution, we use fading measurements at 2.45, 5.25, 29, and 60 GHz, and wind speed data, to study the dynamic effects of vegetation on propagating radiowaves. A new simulation model for generating signal fading due to a swaying tree has been developed by utilizing a multiple mass-spring system to represent a tree and a turbulent wind model. The model is validated in terms of the cumulative distribution function (CDF), autocorrelation function (ACF), level crossing rate (LCR), and average fade duration (AFD) using measurements. The agreements found between the measured and simulated first- and second-order statistics of the received signals through vegetation are satisfactory. In addition, Ricean K-factors for different wind speeds are estimated from measurements. Generally, the new model has similar dynamical and statistical characteristics as those observed in measurements and can thus be used for synthesizing signal fading due to a swaying tree. The synthesized fading can be used for simulating different capacity enhancing techniques such as adaptive coding and modulation and other fade mitigation techniques.

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

AbstractTo 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.

Integration of Fade Mitigation within centrally managed MF-TDMA/DVB-RCS networks

This paper addresses the integration of a Fade Mitigation Technique FMT within a centrally managed MF-TDMA satellite network in the context of guaranteed QoS delivery. Burst Length Control BLC is introduced as a suitable FMT and the methodology of its design and implementation within the existing DVB-RCS standard is described and simulated. The proposed technique provides compensation for rain attenuation at the expense of capacity utilisation. Within any fixed partition of the MF-TDMA space, by considering the space-time properties of rain attenuation, suitable geographical multiplexing of rain-attenuated and non-attenuated links allows the optimisation of the channel capacity utilisation while still exploiting the statistical multiplexing of the sources on the return channels.

A study on SNR estimation algorithms for channel state estimation in Communication Satellite Systems employing Fade Mitigation Techniques

Channel state estimation is one of the key processes carried out at the physical layer of Satellite Systems employing Fade Mitigation Techniques such as Adaptive Coding and Modulation ACM. An accurate and reliable channel estimate is needed to fully realize the capacity gains accrued by using ACM. In this paper the SNR estimation algorithms, which perform the role of channel state estimators, are analysed from a system point of view to get a quantitative idea on the number of received symbols needed to get a reliable estimate, and the impact of interference noise on it. A DVB type satellite system has been investigated for the study. An improvement over the Maximum Likelihood ML estimator using Bayesian principles is suggested and illustrated.

Performance assessment of Fade Mitigation Techniques for the GEOCAST IST project with transparent and OBP architectures

Performance assessment of Fade Mitigation Techniques for the GEOCAST IST project with transparent and OBP architectures