Short‐scale diversity in a dynamic rain fade environment

This article is concerned with using time‐varying uniformly spaced rainfall data to investigate the concept and values of instantaneous diversity gain. This has been obtained on a minute‐by‐minute basis in order to illustrate the behaviour of a VSAT communication system which switches to the most reliable link every minute. The simulations have been carried out using a whole year of rain data obtained from the recordings of a dense rain gauge network located in Barcelona. After illustrating the concept of instantaneous diversity gain, this paper shows the results obtained for various site separations and 20 GHz. These results have been plotted together with the values given by some long‐term‐based models. The comparison shows that such models are not appropriate for dynamic site diversity. This was found to increase the reliability of a VSAT system, especially under severe rain conditions in which the rainfall rates are spatially arranged in rain cells and the attenuation experienced by the two sites is strongly decorrelated. The correlation coefficient between the attenuation series calculated for several separation distances has also been obtained to illustrate this concept. Finally, the paper gives the attenuation distributions calculated for a VSAT system with and without site diversity and for several site separations. Copyright © 2005 John Wiley & Sons, Ltd.     

Design and operational issues of VSAT application in ISDN-typenetworks

Advances in satellite technology in the area of very small aperture terminals (VSATs) offer economical data and voice services for corporate networks and rural applications. The authors describe VSAT application and technology with emphasis on analyzing the issues of quality after the sale. The quality and maintenance requirements of the VSAT networks in operation are defined. The reliability, maintainability, and availability calculations for VSAT networks with different topologies are evaluated. Results show that both star and point-to-point VSAT networks are capable of meeting higher availability standards     

Packet switch architectures and user protocol interfaces for VSATnetworks

The architecture of VSAT (very-small-aperture terminal) satellite networks is discussed. The use of VSAT networks in existing applications is examined. Internal VSAT architectures, most of which are designed as packet-switching networks in a star configuration with a central packet switch performing routing and relay functions, are discussed. Three types of interface-synchronous-multipoint-protocol, asynchronous, and X.25-are then considered     

Guidelines for a preliminary dimensioning of a transaction-oriented vsat network

Very small aperture terminal (VSAT) satellite networks have recently emerged as an attractive and cost-effective alternative to traditional terrestrial circuit or packet switching networks for transaction enquiry-response data applications. VSAT star networks are particularly well adapted to this kind of application, offering high link reliability, better flexibility, wide service area coverage and several economic and operational advantages. However, one major drawback associated with satellite links is the relatively high propagation delay, which is about 0·27 s for one hop (ground–satellite-ground). This requires special design and dimensioning optimization in order to keep the response time of the network within acceptable limits. This paper provides guidelines for the dimensioning of such enquiry-response star VSAT networks. Dimensioning refers to determining the minimum transmission bit rates required, on the different terrestrial and satellite segments of the network, for a given network configuration and mean delay requirement. The approach presented in this paper allows network designers to determine, in a simple way, the required modem speeds of the network segments.     

VSAT网络实时拓扑结构与通联分析模型

以通信体制为TDM/MF-TDMA/DAMA的星形VSAT网络为研究对象,探讨了实时拓扑结构与通联分析在此类VSAT网络中的应用与实现原理,提出了基于分组的双线程分层分类参数提取方法,并采用UML面向对象的建模方法,建立了此类VSAT网络实时拓扑结构与通联分析的通用模型。     

Ku-Band satellite data networks using very small aperture terminals—part II: System design

In this part of the paper on Ku-band VSAT networks, the issue of overall system design is considered. After discussing possible system architectures, primary attention is given to the star network configuration in which customer-premise VSATs communicate with a central hub station, co-located with, or terrestrially connected to a host computer. The component elements of a Ku-band star network for interactive data applications are discussed and the critical items from the point-of-view of performance, capacity and cost are identified. An analytical model for the network, which combines delay-throughput characterization of the multi-access inbound and TDM outbound channels with satellite link analysis is described. The analytical model is exercised over a range of typical system component, traffic model and performance objective scenarios to provide a set of general design guidelines. These guidelines are useful for evaluating the trade-offs between key system elements (channel access unit, modem, VSAT antenna and satellite), VSAT traffic message length parameters and the performance objectives (average and peak network response time and availability). The results are used to address the issue of multi-access protocol selection over a range of scenarios typical of 1.2 or 1.8m VSAT-based interactive networks using current commercial Ku-band satellites. The results show that interactive star networks are often limited by satellite power rather than bandwidth, so that the use of simple contention access for VSATs may not imply a significant system capacity penalty.     

Service oriented statistics of interruption time due to rainfall in Earth-space communication systems

The paper reports and discusses simulated statistics obtained by the synthetic storm technique, in the Po Valley, Northern Italy, on the interruptions (outages), due to rainfall, observed in contiguous (clock) periods of the day of duration T, with 1 min /spl les/T/spl les/ 24 h. The results refer to a 32/spl deg/ slant path at 11.6 GHz, although the main conclusions are independent of carrier frequency and of site, and are based on a large experimental rain rate database (10.6 years of observation). The results can be used to assess the quality and unavailability of services of duration T during a day in earth-space communication systems affected by rain attenuation. A distinction is made and discussed between channel unavailability and service unavailability. The numerical results and the best fit and extrapolation formulas derived might provide a rough approximation to the same statistics at different elevation angles, clock intervals and carrier frequencies, for sites with the same climate of the Po Valley.     

Performance analysis of Ku-band VSAT networks

A critical parameter of a very small aperture terminal (VSAT) satellite network, namely the availability of its links, is defined as the percentage of time that information can be transmitted and received with acceptable quality. The information quality is quantified, and the factors that affect it are described. Link unavailability (outage) due to equipment problems and rainfall is discussed. The use of spread-spectrum techniques is considered. A performance analysis example is provided and some conclusions are drawn     

VSAT communications networks-an overview

The very-small-aperture-terminal (VSAT) fixed satellite communication network is a star network in which many dispersed micro terminals attempt to send data in a packet form through a random access/time-division multiple-access (RA/TDMA) satellite channel with transmission delay. The basic concept of the VSAT and its service potential are discussed. Two classes of traffic are addressed, namely business-oriented low-rate-data traffic and bulk data traffic of corporate networks. Satellite access, throughput, and delay are considered. The size of the network population that can be served in an RA/TDMA environment is calculated. User protocols are examined. A typical VSAT business scenario is described     

Simulation models for performance evaluation of satellite multipleaccess protocols

Simulation models are described for the performance evaluation of satellite multiple-access protocols used in VSAT (very small aperture terminal) data networks. Since the design of VSAT-based wide-area networks for interactive applications is critically dependent on the delay-throughput characteristics of the channel access protocol used, detailed simulation models have been developed to augment analytical performance-evaluation methods. Simulation models for several candidate VSAT access protocols, including random-access techniques such as Aloha, selective reject Aloha and slotted Aloha and demand-assignment multiple-access based techniques, are discussed in terms of traffic-source models, access algorithms, and performance-evaluation criteria. Numerical results for a realistic interactive data VSAT network environment are presented     

Inter‐annual variability,risk and confidence intervals associated with propagation statistics. Part I: theory of estimation

This set of two companion papers aims at providing a statistical framework to quantify the inter‐annual variability observed on the statistics of rain attenuation or rainfall rate derived from Earth‐space propagation measurements. This part I is more specifically devoted to the theoretical study of the variance of estimation of empirical complementary cumulative distribution functions (ECCDFs) derived from Earth‐space rain attenuation or rainfall rate time series. To focus the analysis on the statistical variability but without loss of generality, synthetic rain attenuation time series are considered. A large variability on the ECCDFs, which depends on the duration of the synthetic data, is first put into evidence. The variance of estimation is then derived from the properties of the statistical estimator. The formulation is validated numerically, by comparison with the ECCDF variances derived from the synthetic data. The variance of the fluctuations around the CCDF is then shown to be dependent on the average of the correlation function of the time series, on the probability level and on the measurement duration. This variance of estimation is needed as a prerequisite in conjunction with the knowledge of the climatic variability to characterize the yearly fluctuations of propagation statistics computed from experimental time series. The extensions from simulations to experiments as well as the application to system planning are detailed in part II. Copyright © 2013 John Wiley & Sons, Ltd.     

走向宽带化的VSAT卫星通信

受数据音频视频广播、计算机卫星宽带交互接入和音视频会议等业务的推动,VSAT卫星通信网向宽带业务发展巳成为必然趋势。分别对应以上业务的VSAT卫星通信网也日益趋于融合成一个统一的宽带VSAT通信网。发展宽带VSAT通信网的关键技术涉及到宽带数据广播、宽带多址接入、卫星通信规程、网络综合管理和宽带虚拟子网等。     

VSAT networks in the intelsat system

This paper describes how VSAT networks currently operate in the INTELSAT system. Four classes of VSAT networks (data transaction; circuit-switched; data distribution; microterminals) are identified, and it is verified that all of them can operate with INTELSAT satellites. Most VSAT networks in operation on INTELSAT today operate in fractional transponder leases. Fractional transponder capacity estimates are presented for a wide range of scenarios and different INTELSAT satellite series. These estimates clearly show increasing bandwidth utilization efficiencies for newer generations of INTELSAT satellites. Provided that VSAT and hub sizes are appropriately selected, efficiencies are already significant with existing satellites. Two possible ways of increasing the utilization of satellite resources are examined in the paper: demand assignment multiple access (DAMA) and multiple channel-per-carrier (MCPC) techniques. The impact of using DAMA in circuit-switched VSAT networks is quantified.     

Ku-Band satellite data networks using very small aperture terminals—part I: Multi-access protocols

This two-part paper presents a comprehensive overview on the technology of very small aperture terminal (VSAT)-based satellite data networks, which have received increasing attention in recent years because of a combination of technical and economic factors. Ku-band networks with customer-premise earth-stations can overcome the inherent local access bottleneck in wide-area terrestrial data networks by suitably exploiting the unique multi-access and broadcast features of satellite communication. In this part of the paper, the issue of multiple access, which is an important critical path technology in the ongoing evolution of VSAT data networks, is considered in detail. Satellite multi-access protocols are classified in terms of channel synchronization (slotted and unslotted) and the qualitative nature of message access (fixed assigned, contention and reservation). In addition to well known techniques such as ALOHA, slotted ALOHA, tree CRA, packet CDMA and demand assigned (DAMA) TDMA, a number of new approaches to efficient unslotted access, including selective reject (SREJ) ALOHA, time-of-arrival CRA and locally synchronous reservation, are discussed. The general review is followed by a detailed performance comparison of a number of candidate first-generation VSAT protocols, namely ALOHA, slotted ALOHA, SREJ-ALOHA, DAMA with TDMA reservation access and DAMA with slotted ALOHA reservation access. The performance evaluation, based on detailed simulations with VSAT equipment and transaction traffic parameters, is summarized by curves of average delay and throughput vs. number of VSATs per channel for each of the protocols considered. Higher-order differences in delay performance are illustrated with appropriate delay distribution results and the sensitivity of performance with respect to key equipment and traffic parameters is investigated. For the transaction traffic model under consideration, the results suggest the use of SREJ-ALOHA among contention techniques or DAMA with slotted ALOHA reservation among controlled access alternatives. The trade-off between contention and reservation access is shown to be more complex, depending on the combination of average delay, peak delay, transmission cost and equipment complexity suited to the particular application. In Part II

1 To published in the next issue

of this paper on Ku-band VSAT networks, the issue of overall system design is considered.     

Wet antenna effect on VSAT rain margin

The contribution of wet antennas to the antenna signal path losses in a VSAT environment is treated theoretically. The current commercial VSAT systems operating in either C-band or Ku-band generally have their remote terminal antenna reflectors and the antenna feed horn radomes coated with hydrophobic materials. The aim is to prevent the antenna and radome surfaces from becoming wet during a rainfall. This precaution relieves the burden of added rain margin necessary on the link budget. The magnitude of the propagation loss when the antenna reflector and the antenna feed horn radome surfaces are wet is determined. The results can indicate whether the expense of applying and maintaining the hydrophobic materials on the VSAT remote antennas and radomes is justified under specific loss conditions     

Advanced Communications Technology Satellite (ACTS) fadecompensation protocol impact on very small-aperture terminal bit errorrate performance

The Advanced Communications Technology Satellite (ACTS) communications system operates at Ka band. ACTS uses an adaptive rain fade compensation protocol to reduce the impact of signal attenuation resulting from propagation effects. The purpose of this paper is to present the results of an analysis characterizing the improvement in VSAT performance provided by this protocol. The metric for performance is VSAT bit error rate (BER) availability. The acceptable availability defined by communication system design specifications is 99.5% for a BER of 5E-7 or better. VSAT BER availabilities with and without rain fade compensation are presented. A comparison shows the improvement in BER availability realized with rain fade compensation. Results are presented for an eight-month period and for 24 months spread over a three-year period. The two time periods represent two different configurations of the fade compensation protocol     

Prediction of triple‐orbital diversity performance in Earth‐space communication

Orbital diversity is considered to be an effective technique to overcome large fade margins in satellite communication links. This paper discusses triple‐orbital diversity, which uses three satellites and an Earth receiving site. A method for calculating the outage probability of a triple‐orbital diversity protection scheme is proposed. It is based on a model for convective raincells and the lognormal assumption for point rainfall rate statistics. Numerical results are compared with an available set of experimental data taken from a VSAT Earth‐station located in Japan. The agreement was found to be quite encouraging. Some useful conclusions, concerning the relative advantage of using triple‐ against the double‐orbital diversity scheme are also deduced. Copyright © 2002 John Wiley & Sons, Ltd.     

Comparison of the simulated performance of a vsat satellite link with measurements

This paper combines a number of features. First the simulation of the transmission performance of a K_a-band VSAT system (CODE) is presented. The results are used to verify the systems design that was performed for CODE and to demonstrate that the implementation margin due to intermodulation noise, adjacent channel interference, filter ISI, amplifier non-linearities and phase noise was sufficient. In the design of VSAT systems of this type phase noise was thought to be particularly important, and a new and detailed simulation of its effects is presented in the paper. For the first time, experimental measurements of system degradations have been taken on a VSAT system which have been used to verify simulation results. This was particularly important for the new phase noise simulation results and good agreement was obtained. An overall loop-back experiment using the Ka-band VSAT terminal with the Olympus satellite shows good agreement with the simulation results. Finally, the availability of experimental results has enabled us to compare the accuracy of TOPSIM III and BOSS simulations for the VSAT system. The paper indicates some shortcomings in the TOPSIM III simulation of phase noise and highlights the generally good agreement obtained with experimental results by the BOSS simulations.     

Personal computer communications via VSAT networks

The very small aperture terminal (VSAT) satellite network, a transport medium that is becoming important for long-distance personal computer (PC) communications, is described and analyzed. The mature and readily available VSAT technology offers many benefits and advantages over conventional terrestrial networks, including lower operating costs, ease of installation and maintenance, support for multiservices, bringing locations where the cost of leased lines is exorbitant into the communications loop, and integrating a large number of VSATs into a single network to provide a cost-effective expansion from small to large systems. The following aspects of VSAT networks are discussed: network configurations, networking and multiaccess, error-control protocols, and end-to-end performance analysis