**5. Discussion**

On plain areas, tropospheric ducting and turbulence could effect the signals to make an over-the-horizon propagation of 5G communication signals. Due to the dense distribution of 5G base stations, chances of CCI increases between intercity links when ducts form because of unusual changes, such as temperature increases with height occurring in meteorological conditions. Moreover, the tropospheric turbulence related to the distribution of the hybrid duct height had a sub-segment effect on the over-the-horizon propagation. Before 40 km, the tropospheric ducting took the lead in the over-the-horizon transmission; in the middle of the 40–80 km, the tropospheric turbulence boosted the propagation effect

by making the propagation signals multi-reflect on the air masses at the top layer of duct. If the PL in this middle section was reduced to the antenna receiving sensitivity range, the transmitted signal might have been received by local users when it reached the middle and rear sections of the urban link, resulting in remote interference.

If there is no major topographic change, the tropospheric turbulence will still play a role in the rear section of the intercity link, which can reduce the PL. However, because the propagation distance is too long, the signal may exceed the antenna receiving sensitivity at a distance of 70 km and will not be received by users in this section. If there are large topographic changes between cities, topographic fluctuations play a dominant role in over-the-horizon propagation. Figure 13b shows that peaks over 200 m make the PL exceed 160 dB immediately. Obstacles beyond the transmit antenna height could cut off the propagation entirely. When no obvious peaks or valleys exist on intercity links, there is the probability of duct occurrences after rain or at night on waterside plain or coastal cities areas. In our simulations on Shanghai–Wuxi and Jiaxing–Wuxi, signals propagating beyond the horizon with propagation loss did not decrease much in the duct-trapping layer when surface-based duct and tropospheric scattering occurred. Therefore, with the dense deployment of 5G communication base stations, tropospheric ducting and scattering effects may jointly cause CCI on intercity links with undulating terrain in plain areas. Therefore, it is essential to accurately model the over-the-horizon propagation of intercity link signals caused by the combined effects of duct and tropospheric turbulence effects, considering real terrain and LCs. The simulation results could be used in preventive deployment of 5G base stations and targeted deployment of relay base station locations, which could greatly save related costs.

A total of 1300 sets of randomly generated data were used to conduct deep learning training based on the TWPE simulated results. The training parameters with better effects after multiple optimizations were selected. The model was built with fixed radar antenna parameters and atmospheric environment parameters. The topography and landform data were entered to predict the PL distribution at the antenna height layer, and achieved good prediction results. When an intercity link whose terrain and land form are similar to the plain area is obtained, the deep learning model can be used to effectively and quickly predict the PL distribution, so as to evaluate the possibility of remote interference and carry out subsequent base station distribution deployment.
