# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 3 Results and Discussion
### Simulations for Systems Operating in the VHF Band in the Marine Environment of Wind Farms  
Based on the ITU-R BT.1893-1 recommendation and the proposed radio channel model, the simulation analyzes of the offshore wind farms' impact on the VHF systems were carried out. In this section, the assumptions and results of the simulation will be presented. In order to investigate the impact of the offshore radio channel on the VHF systems operating near the wind farm, the channel coherence bandwidth and time were first calculated for selected scenarios. The calculations were made for the following parameters:  
Two variant of the turbines' arrangement--5 \(\times\) 5 (see Figure 10a) and 10 \(\times\) 10 (see Figure 10b), differing in the turbine density (the area is the same): 1 km and 2 km distance between the turbines, respectively;  
Ship locations (various variants)--ships at 3 distances: 500 m, 2 Nm (2 nautical miles), and 10 Nm from the wind farm;  
Frequency--161.8 MHz;  
Average radius of the tower--two variants: 3 m and 4 m;  
Tower height--150 m;  
Blade length--115 m;  
Turbine speed--8 rpm. Additionally, the height of the ship terminal antenna was assumed to be 10 m above sea level. The mutual location of the transmitter, receiver, and wind farm, as well as the distances between them, were different for each scenario. For the purposes of the article, the level of the signal received in the analyzed area was simulated, and the CIR parameter value (i.e., Carrier to Interference Ratio) was analyzed for the VHF transmitter located in three distances (Figure 10):  
500 m south of the border of the offshore wind farm area (point P1);  
2 Nm south of the boundary of the offshore wind farm area (point P2);  
10 Nm south of the boundary of the offshore wind farm area (point P4). The points P3, P5, and P6 mark the locations of the receiver. These values, based on the recommendations published by the MCA (Maritime and Coastguard Agency) of Great Britain, correspond to three risk levels: high, medium, and low [30]. Figure 10: VHF transmitter locations for simulation for 5 \(\times\) 5 (**a**) and 10 \(\times\) 10 (**b**) turbine arrangement configurations. In the context of the following simulations, the term \"useful radio range\" for interference analyzes means a situation in which the following two criteria are simultaneously met at a given point in the area:  
Sensitivity criteria--i.e., the level of the signal received from a given station must be higher than the receiver sensitivity;  
Interference criteria--i.e., the actual value of the useful carrier to interference ratio (CIR) must be higher than the assumed value of the minimum required CIR (for the purposes of these simulations, CIR_minimum = 10 dB was assumed). It should be noted that simulations were carried out for the reference ship's communication system with the following parameters:  
EIRP power = 41 dBm;  
Receiver sensitivity: \(-105\) dBm;  
terminal antenna height: 10 m above sea level. These are typical parameters of VHF transmitters used in ships. The results presented in this section were obtained with the use of the radio planning simulation tool described in the article [12]. The presence of the wind farm has been simulated as a scattered source of interference, where each turbine was modelled as a source of reflected (unwanted) power radiating towards individual stations of the communication systems. The theoretical reflected signal level (UFSR) was determined for each turbine using the ITU-R BT.1893-1 Annex 1 or Annex 2 models [11], separately for each scenario. The cumulative impact of the interferences has been simulated using the k-LNM log-normal model [31; 32]. This method was developed in order to be used in practice when designing radio networks (especially where the levels of signals measured in the radio channel change over time). It provides very accurate results in a relatively short calculation time, possible to be carried out by network designers.