Modelling of a VSAT link physical layer fade countermeasure controlled at the data link layer

Abstract

A satellite data communication link with fade countermeasures applied at the physical layer, in the form of adaptive modulation and forward error correction coding, will only perform as well as governed by the way in which the inherent system adaptivity is controlled. By considering the controlled quality attribute to be the error-free frame throughput of the link, the place where the control happens has to move from the physical layer to the data link layer. Following earlier research which produced excellent results when the OSI HDLC protocol was simulated, the authors have embarked on the development of a fast executing simulator based on a Markov chain architecture. The basic model has been modified by application of adaptive filtering techniques in the calculation of the state change probability values. This refined the model to perform within 2% of the original simulator in the case of the selective repeat continuous ARQ. The paper presents the development philosophy of the new model and some of the results obtained using both deterministic test fades and propagation fades previously recorded by satellite beacon receivers. The good accuracy of the model, coupled with its computation efficiency, makes it suitable for application in future work where a potentially large number of links would have to be simultaneously considered.