# 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
K-LNM is an estimation method used for statistical calculations of the sum of distributions of several log-normally distributed variables (interfering signals from each turbine). To increase the accuracy of the LNM method in regions where high land coverage is required (high probability of coverage), a correction factor \(k\) is introduced. This method is based on the assumption that the sum of distributions of desirable and undesirable fields is also log-normally distributed. In summary, it is a commonly recognized method often applied for the purpose of designing radio networks and also used in the planning tool implemented by the authors of the article. The ITU-R BT.1893-1 model also describes the standardized equivalent directional characteristic of a wind turbine, which was used in the planning tool to model each turbine (i.e., each interference source) as a hypothetical \"antenna\" featuring a specific radiation pattern (see Figure 11). In case of using ITU-R BT.1893-1 Annex 1 (reflections from the turbine rotors), the \"antenna\" representing the interference source was set on turbine mast height, and in case of ITU-R BT.1893-1 Annex 2 (reflection from the turbine mast), the \"antenna\" representing the interference source was set at a half of the turbine mast height. The simulations have been conducted using the newest, general-purpose ITU-R P.2001-4 propagation model [33], which by definition is a very universal model for general purpose terrestrial radio systems operating in the frequency range from 30 MHz to 50 GHz. It has a wide range of applicability in frequency, distance, and percentage time with no discontinuities in its output. In particular, it predicts both fading and enhancements of signal level, for signals which may be either wanted or potentially interfering. Due to the fact that the channel coherence time depends only on the wind turbine parameters and the operating frequency of the system, its value is the same for all considered scenarios and equals to 4.07 ms. Therefore, the relationship (36) is satisfied, as the symbol duration in the AIS system is 0.104 ms. Thus, in the considered channel, the impulse response does not change during the symbol duration, and only slow fading occurs in the channel. The coherence bandwidth determined during the tests of various simulation scenarios is presented in Table 14. The case 6 is given as an example for a wide coherence bandwidth--such situation occurs when the distance between the transmitter and receiver is less than their distances to the wind turbines. The coherence bandwidth determined during the tests of various simulation scenarios ranged from 73.748 kHz (for cases of high spatial separation of the transmitter and receiver) to 262.733 kHz (when the transmitter and receiver were close to each other and close to the offshore wind farm). The bandwidth of the AIS system operating in the VHF frequency range is 12.5 kHz and for this system, mainly flat fading will occur. The situation will be different in case of the novel VHF system--VDES [6], which will feature channels with a width up to 100 kHz. For this system, frequency-selective fading will also occur, as the channel coherence bandwidth will in some cases be smaller than the channel bandwidth. However, it should be mentioned here that the VDES system, developed and standardized in the last few years at the IALA and ITU, is a much more advanced system, employing channel equalizers. Moreover, the VDES system allows us to use the narrower transmission