The Propagation of radio signals has a number of inherent problems of electromagnetic waves, of which LTE or 4G is not exempt. Aspects such as absorption, refraction, diffraction, fading, and other properties can cause serious problems of coverage, interference, and general degradation of the transmitted signal. Some of these parameters are largely unpredictable, mainly those arising from meteorological phenomena or the ground, and therefore, fewer steps may be taken in such cases.
Moreover, the free space loss (FSL) is directly related to the system operating frequency and the distance between transmitter and receiver. The case of frequency is a parameter already defined for this type of technology by the International Telecommunication Union , also depends on the frequency assignments of each country, and therefore it is not something that operators can easily handle to increase the coverage of their mobile systems.
The case of multipath is a problem that is associated with the spread of radio beam by several pathways, supported by the phenomena of reflection, diffraction and scattering. This type of phenomenon produces a difference in the distance that a beam that travels with sight line over another who has suffered the effect of reflection (to name one example) and consequently there is a difference between the arrival times of the signals and the receiver, which may cause the overlap of the signals at the receiving antenna causing inter-symbol interference (ISI), or too low power to be demodulated, it is also possible that even a given beam incident on the receiving antenna as shown in the following Figure # 1.
Figure # 1: Multipath example.