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Ka-Band Propagation Modeling for Fixed Satellite Applications

Propagation impairments produced by the troposphere are a limiting factor for the effective use of the 20/30 GHz frequency band. Use of smaller earth terminals, while very attractive for consumer and transportable applications, make it difficult to provide sufficient link margin for propagation related outages. In this context, reliable prediction of propagation impairments for low- availability systems becomes important. This paper addresses the issues related to predicting different types of propagation impairments as well as combining them together to determine the overall impact on satellite links over a wide range of outage probabilities. I. INTRODUCTION Limitations imposed by the propagation environment makes it difficult to provide Ka-band satellite services at availability levels comparable to those at lower frequency bands. The majority of the planned Ka-band systems are expected to provide services using smaller earth terminals that will neither have sufficient gain nor output power to compensate for more severe propagation effects. Propagation impairments affecting Ka-band satellite links include gaseous absorption, cloud attenuation, rain attenuation, and tropospheric scintillation. Rain attenuation is considered the dominant impairment. A number of prediction models are available for the estimation of individual components. However, methodologies that attempt to combine them in a cohesive manner are not widely available. For small earth terminals, rain effects may only form a relatively small part of the total propagation link margin. It is therefore necessary to identify and predict the overall impact of every significant attenuating effect along any given path. In the absence of detailed knowledge of the occurrence probabilities of different impairments, empirical approaches are taken in estimating their combined effects. Since the propagation impairments originate in the troposphere, the input parameters to the models include meteorological data such as rain rate, cloud cover, temperature, and humidity. To a large extent, the accuracy of the predictions depends on the availability of reliable meteorological data. When designing a satellite network, it is important to have the meteorological data available with a sufficiently high spatial resolution across the satellite coverage area. Some of the propagation models available for predicting different impairments are discussed in this paper. The discussion is supplemented by results of the recently concluded Ka-band propagation measurement campaign with the Advanced Communication Technology Satellite (ACTS).