channel parameters based persion, is extremely difficult to model. Thus, part of on actual measurements. Subsection 4.7.3.5 deals with recent investigations into The approach used for electromagnetic propagation this topic, in an effort to incorporate the scattering modelling depends on the zone under study. For instance, phenomena into the propagation modelling tools. if only the immediate vicinity of the antenna is to be Modelling as well as experiments providing comple- considered, an integral method can be employed, while mentary information about the channel, channel mea- the coverage area of a transmitting antenna is usually surement techniques and parameter estimation are determined via a ray method. Section 4.7.2 describes detailed in Section 4.7.4. Since the development of several recently proposed improvements to various MIMO systems implies obtaining complete information existing techniques: integral methods, differential equa- about the field characteristics in the vicinity of both the tion methods, parabolic equations, diffraction theory and receiving and transmitting antennas, double-directional Gaussian beams. If, as in the example mentioned above, channel sounding is usually needed. New channel sounder the ray theory is applied to determine the coverage area architectures, including antennas, have been proposed to of a BS, a large database is needed in order to accurately provide precise angular information on the rays' Direction model the environment's electromagnetic and geometric of Departure (DoD) and Direction of Arrival (DoA) and characteristics. Section 4.7.3 discusses such input data- to ensure a better compromise between measurement base issues, with an emphasis on the complexity that is accuracy, acquisition time and the number of array ele- often cited as the principal obstacle to the widespread ments. To increase the accuracy of the channel transfer adoption of deterministic propagation models. Since matrix's spatial and temporal structure, high-resolution Communications Engineering De