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.     

Modeling Ka-band scintillation as a fractal process

We propose a model that describes the signal fading process due to scintillation in the presence of rain. We analyzed a data set of uplink (30 GHz) and downlink (20 GHz) attenuation values averaged over 1 s intervals. The data are samples relative to ten significant events, for a total of 180 000 s recorded at the Spino d'Adda (North of Italy) station using the Olympus satellite. Our analysis is based on the fact that the plot of attenuation versus time recalls the behavior of a self-similar process. We then make various considerations, and propose, a fractional Brownian motion model for the scintillation process. We describe the model in detail, with pictures showing the apparent self-similarity of the measured data. We then show that the Hurst parameter of the process is a simple function of the rain fade. We describe a method for producing random data that interpolate the measured samples, while preserving some of their interesting statistical properties. This method can be used for simulating fade countermeasure systems. As a possible application of the model, we show how to optimize fade measurement times for fade countermeasure systems     

Ka频段卫星信道的衰减特性

在Ka及以上高频段卫星通信系统中,其链路信号的衰减很严重,主要是大气、降雨、闪烁对卫星信道的影响。本文主要研究了雨衰和对流层闪烁产生衰减的机理与特性,提出雨衰采用DAH模型,闪烁采用Van de Kamp模型。然后根据实测气象数据,计算我国各参考站点的雨衰和闪烁值。为抗衰减对策的提出提供理论依据,为工程设计提供具体的参考数据。     

Total CNIDR degradation during rain fade conditions of a dual polarized satellite link suffering from double adjacent satellite interference

Atmospheric impairments have a significant influence on the performance of modern satellite communication networks, working at Ku, Ka and potentially Q/V frequency bands. Both differential rain attenuation from an adjacent satellite system, operating at the same frequency range, and cross‐polarization phenomena on dual polarized satellite systems due to rain and ice crystals, induce a further aggravation on the already limited signal‐to‐noise‐plus‐total‐interference ratio (CNIDR), due to the frequency and orbital congestion of today's global communication satellite constellation. In the present paper, a new statistical methodology, towards the modelling of CNIDR under rain fade conditions, is proposed to include interference effects by two adjacent satellites, incorporating the impact of correlated fading channels (spatial rainfall inhomogeneity) of multiple slant paths. The method is based on a convective raincell model and the lognormal assumption for both the point rain‐rate statistics and the slant‐path rain attenuation. The obtained numerical results indicate the significant impact of the second interfering satellite on the aggravation of total interference effects. Some simple mathematical formulas for the prediction of the CNIDR, based on the derived theoretical results, and demonstrating the influence of various link parameters on the total link availability statistics, are also presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Optimum utilization of the channel capacity of a satellite link inthe presence of amplitude scintillations and rain attenuation

By considering the global fading process on the link caused by rain attenuation and amplitude scintillations, particularly at K_a band, it is possible to derive a long-term statistical model of the satellite channel capacity. The four-parameter distribution, which combines amplitude scintillations and rain fade within an up/down link system, is presented. Also presented are the degradation (and improvement) of bit error rate (BER) in the presence of amplitude scintillations, thus complementing the flat fade effect due to rain only. By implementation of adaptive communication systems, a more efficient channel capacity utilization is possible. The concepts and the use of novel analytical expressions combining a log-normal model of rain fade with a Moulsley-Vilar distribution for scintillations are illustrated. These are then applied to a very-small-aperture terminal (VSAT) example of a 29/19-GHz digital communications link through the Olympus satellite using M-ary phase shift keying (PSK) modulation schemes     

Performance of synthetic storm technique in estimating fade dynamics in equatorial Malaysia

In this paper, the synthetic storm technique (SST) is applied to 1‐minute rainfall rate collected from a rain gauge to predict the fade dynamics of the signal in an earth‐to‐satellite link in an equatorial climate location without the needs of satellite beacon measurements. The obtained results are compared with the statistics calculated from measured rain attenuation and with the ITU recommendation model. As for fade duration, a good agreement with measurements has been found together with a significant improvement in terms of prediction errors with respect to the ITU‐R model. Synthetic storm technique is also able to predict fade slope statistics fairly well and has equivalent performance of ITU‐R model.     

Diurnal variations in rain attenuation on Ku band earth–space paths

This paper presents analytical results of the diurnal variations in Ku band rain attenuation along earth–space paths at four locations in Southeast Asia and proposes a new model that can predict rain fade in a short period of every 2 h daily. Data from four radiometers and four rain gauges over a 3 year period were analysed to obtain the characteristics of diurnal variations in rain attenuation and rainfall as well as cumulative attenuation distributions in every 2 h interval. The results of this analysis are applied to develop an intensive prediction model using the knowledge of rainfall and attenuation statistics. This model is tested with the measured data and is found to be useful for the design of a more efficient Ku band satellite system especially between 99 per cent and 99·9 per cent link availability in an area of heavy rainfall. © 1998 John Wiley & Sons, Ltd.     

Empirical models for rain fade time on Ku- and Ka-band satellitelinks

Rain attenuation data from the OLYMPUS satellite beacon measurements are used to investigate fade time in the Ku and Ka bands. Using statistical procedures, an empirical model is developed which predicts fade time as a function of attenuation level, frequency of operation, and fade duration interval. Total annual fade times are predicted over a frequency span of 12-30 GHz for attenuation levels in the 3-18 dB range and for fade duration intervals of 30-60 s, 60-120 s, 2-5 min, and 5-20 min. The predicted fade times are in good agreement with the measured values. An alternate model, described by two simple relationships in two different ranges of attenuation level, is also presented by simplifying the original single-equation model. The simplified model accounts for fades associated with stratiform rain and thunderstorms separately     

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 $beta _{s}$ 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.      

Results from the Virginia Tech propagation experiment using theOlympus satellite 12, 20 and 30 GHz beacons

A comprehensive set of propagation experiments was performed using the Olympus satellite 12, 20, and 30 GHz beacons. This set of experiments is unique in North America because of simultaneous reception of signals spanning the Ku- and Ka-bands from the same orbital slot, which permits direct inference of the frequency behavior of signal variations. The elevation angle from the receiving site in Blacksburg, VA, to the satellite was 14 degrees. Beacon, radiometric, and weather data for one year were analyzed. The statistical results for rain rate, beacon attenuation, attenuation ratios, radiometrically derived attenuation, fade duration and fade slope are presented. They are important to the design of Ku- and Ka-band satellite communication systems. The beacon attenuation results include cumulative statistics for attenuation with respect to free space and with respect to clear air. Attenuation ratio data are presented using attenuation with respect to clear air to focus on rain effects. Instantaneous attenuation ratios computed from instantaneous beacon levels were found to be nearly identical to statistical attenuation ratios obtained from cumulative attenuation statistics at each frequency     

Statistical analysis of rain fade slope

An analysis is made of the measured distributions of the fade slope of rain attenuation, conditional for attenuation values, measured at Eindhoven University of Technology from the satellite Olympus. It is found that the distribution is similar for positive and negative fade slopes and independent of frequency in the range from 12 to 30 GHz. A distribution model for the conditional distribution is found. The only parameter of the distribution is the standard deviation, which is found to be proportional to attenuation level and dependent on rain type, on the low-pass filter bandwidth and on the time interval used in the slope calculation. The observed relation between the standard deviation and attenuation is compared with results from other measurement sites. From this comparison it is found that the fade slope standard deviation is likely to depend on elevation angle and on climate, through its dependence on rain type.     

Real-time prediction of attenuation for applications to fade countermeasures in satellite communications

The author has simulated the behaviour of a fade countermeasure control system that predicts the attenuation level on the satellite link on the basis of previous attenuation samples, using the experimental rain attenuation data measured at 11.6 GHz in three Italian stations with the Sirio satellite. Some dynamic characteristics of fading are investigated and the performances of prediction algorithms based on linear regression are evaluated.<>     

Detection of errors recovered by decoders for signal qualityestimation on rain-faded AWGN satellite channels

We describe a method for estimating satellite link quality for use with rain fade countermeasures. The method applies to convolutionally encoded bit streams. It is based on the measurement of the uncoded data bit-error rate (BER) which is estimated by monitoring the errors recovered by a Viterbi decoder. Using a previously developed attenuation model, the measurement time intervals are optimized for a simple application, and the performance of the estimator is shown     

Frequency and attenuation dependent fade slope statistics

Results obtained from slant path propagation experiments carried out with the OLYMPUS satellite are reported. Concurrent attenuation measurements at 12.5, 20 and 30 GHz have been analyzed with regard to the rate of change of rain attenuation (fade slope). The results indicate that pronounced fade slopes occur mostly in the high attenuation range.<>     

Interference Studies Between Adjacent Satellite Communications Systems operating Above 10 GHz and Using Power Control as Fade Mitigation Technique

Interference problems are arising from the spectral coexistence between satellite communication networks that operate at frequencies above 10 GHz and particularly when they employ power control as fade mitigation technique. This situation is aggravated due to tropospheric propagation phenomena. In this frequency range, rain attenuation is considered to be the dominant tropospheric fading mechanism. The conditional acceptable intersystem interference probability of the Carrier-to-Interference Ratio of a satellite terminal interfered by an adjacent satellite network is defined as a figure of merit and analytically calculated taking into account a physical-mathematical model for the rainfall medium. The correlated propagation fading phenomena over multiple slant paths are accurately incorporated. The proposed model is flexible and can be applied on a global scale since it incorporates the local climatic conditions concerning the point rainfall rate and the spatial rainfall inhomogeneity. Useful numerical results of the proposed model are obtained and the impact of various crucial operational and geometrical parameters of satellite networks’ coexistence is examined. The numerical results have been also verified through simulations using a multi-dimensional rain attenuation synthesizer. Finally, simple and easy-calculated formulas for the satellite communication designers for back of the envelope computations are given.     

The Use of Resource Sharing and Coding to Increase the Capacity of Digital Satellites

Resource sharing is a technique which can improve the circuit availability of digital satellites operating at frequencies above 10 GHz, without requiring large fade margins. In this paper, the resource sharing concept is generalized by fully exploiting the available clear-air carrier-to-noise ratio of the satellite link to achieve very high transmission capacity while maintaining low rain outage. During clear-air conditions, convolutional codes with a large channel signaling alphabet are employed to permit a high rate of information transfer. When the fade depth exceeds the built-in fade margin, the signaling alphabet is reduced and enough time slots are borrowed from a resource sharing reserved pool to maintain the data rate at the fade site. It is shown that this approach greatly diminishes the impact of rain attenuation on satellite communications. Effective utilized capacities exceeding 85 percent of that possible if it never rains are feasible, and the increase in capacity compared to a system not using resourcesharing protection is typically a factor of 3-10.     

On the rain attenuation dynamics: spatial‐temporal analysis of rainfall rate and fade duration statistics

Knowledge about the dynamic characteristics of rain attenuation is of utmost importance for many applications in terrestrial and satellite communication systems operating at frequencies above 10 GHz. Long‐term rain rate statistics and rain rate duration statistics are usually available from meteorological data. In this paper, a spatial–temporal analysis is employed in order to evaluate the rain attenuation power spectrum of a terrestrial/satellite path. The predicted power spectrum is compared with experimental data. Based on the spectral analysis of rainfall rate a method for converting rain rate duration statistics to link fade duration statistics is also proposed. Fade duration statistics are presented for terrestrial and satellite links and compared with available experimental data. The agreement between the predicted results and the experimental data has been found to be quite encouraging. Finally, numerical results are presented for various climatic zones, elevation angles and frequencies. Some very useful conclusions concerning the dynamic properties of rain attenuation for a microwave path are deduced. Copyright © 2003 John Wiley & Sons, Ltd.     

长春和新乡雨衰时间序列的马尔科夫链模拟

基于马尔科夫理论建立的N阶马尔科夫链模型,模拟了长春和新乡地区的降雨衰减时间序列,比较了长春和新乡地区单个模拟和实测雨衰时间序列的概率分布;分别统计了长春和新乡地区50组模拟雨衰时间序列的百分概率分布,并与国际电信联盟无线电通信研究组(ITU-R)提供的卫星轨道位置为92°E、频率为12.5 GHz在线极化情况下长春和新乡雨区不同降雨衰减值下的时间百分概率进行了比较,一致性很好,从而验证了N阶马尔可夫链模型在中国部分地区的可用性。模拟结果对我国在Ku及以上频段通信卫星的抗衰落技术的发展具有重要的应用价值。     

Rain Rate Statistics and Fade Distribution of Millimeter Waves in Indian Continents

For any communication service operating in the Microwave/ Millimeter wave region, statistical information characterising the attenuation due to rain along satellite slant path would be required for the design of satellite communication links and for the broadcasting network above 20 Ghz. It is necessary to have a prior knowledge of the probability of exceeding different levels of rain attenuation in order to design appropriate fade margins into systems and establishing estimates of the year to year variability of rain fade margin for particular geographic regions of India so that the communication system reflects the extremes of these variabilities. Direct measurement of beacon signals from geostationary satellites have been a mean to determine the above information and experiments can be pursued with satellite such as INSAT. [1] Attenuatiuon of Millimeter Waves by rainfall restricts the path length of a communication system. A knowledge of the rain attenuation at such frequencies is therefore desirable in designing a reliable communication system. Signal level fading over line-of-sight links strongly depends on the hop length, frequency and climate. For short hops, the probability of occurance of deep fades becomes diminishingly small. However, since an extended hop length is possible for regions with little rain activity, clear weather fading can affect the link reliability in a similar way ti a rain.[2]     

Large‐scale site diversity for satellite communication networks

The utilization of high frequencies, such as Ka‐band and beyond, necessary to avoid the highly congested lower satellite frequencies and to get larger bandwidth availability is considered for many developing satellite systems. The new satellite low‐margin systems in Ka‐band will need to be designed using fade countermeasures to counteract rain attenuation. One of these techniques foresees the possibility of switching the communication link among different Earth stations spread on a very large territory to reduce the system outage time to the joint outage time of all the stations. The design of such systems depends on the probability that the Earth stations simultaneously exceed their margins. In this paper, a well‐assessed model is utilized for the prediction of joint statistics of rain attenuation in multiple locations, using Monte Carlo simulation. The model is based on a pair of multi‐variate normal processes whose parameters are related to those characterizing the single‐location statistics and whose covariance matrices are assumed to depend only on the distances between locations. The main results concerning both the probability and margin improvement will be presented and discussed. Copyright © 2002 John Wiley & Sons, Ltd.