Scattering in non-stationary mobile-to-mobile communications channels

Abstract

The aim of this thesis is to introduce a non-stationary model for the scattering in mobile-to-mobile channels. Due to the evolution of wireless technology, fixed-to-mobile communications systems are nowadays complemented by mobile-to-mobile communications systems. In the vehicular sector, mobile-to-mobile communications systems are used to enable intelligent transportation systems. Such systems aim to make transportation safer and more efficient by distributing sensor information among the cars. In the aeronautical sector, mobile-to-mobile communications systems will be used, for example, to exchange position, altitude, speed, and heading data between aircraft during flight thus allowing a reduced separation between them. Hence, these systems are necessary to further increase the air traffic density. Stochastic channel models for the fixed-to-mobile channel are based on the assumption that the channel is wide-sense stationary and exhibits uncorrelated scattering behavior both in a stochastic sense. It has been shown that many mobile-to-mobile channels do not adhere to such assumptions, especially vehicle-to-vehicle and air-to-air channels. The need for new models is therefore due to the fact that mobile-to-mobile channels are fundamentally different from fixed-to-mobile channels. To overcome the limitations of current channel models, two measurement campaigns to characterize both the vehicle-to-vehicle and the air-to-air channel as exemplary mobile-to-mobile channels were conducted. The measurements confirm the non-stationary behavior of those channels and subsequently, a theoretical model is created on the basis of the measurement data. The employed model is a geometry-based stochastic channel model, which means it consists of two parts. In the geometric part, new expressions for channel parameters, such as delay and Doppler frequency, are derived. In the stochastic part, those expressions enable us to determine closed-form solutions for the required time-variant probability density functions. The presented model can be seen as a generalization of the wide-sense stationary, uncorrelated scattering models. Since the calculations of the probability density functions are time consuming, the application of a different coordinate system is investigated and we show that computational gains are achieved by using prolate spheroidal coordinates. Additionally, other important channel parameters, such as mean Doppler and Doppler spread, are derived. The presented theoretical channel model is validated by the measurement data that has been recorded for both the vehicle-to-vehicle and the air-to-air channel. In each case, there is a remarkable agreement between measurement data and the channel model. The model also matches with measurement data recorded by other institutions that used different scenarios. This versatility allows the model to be very general and it can be applied to a wide range of scenarios.