# Radio Channel Modelling for VHF System Operating in the Offshore Wind Farms Propagation Environment
## 3 Results and Discussion
### The Measurement Results
#### 3.1.2 Measurements of the Doppler Power Spectra Density
The second parameter to be considered when modeling the radio channel is the maximum Doppler frequency. The value of the maximum Doppler deviation determined during the measurements was below 50 Hz, while the theoretical value calculated for the maximum rotational speed of the turbine (16 rpm) and blade length of 61.7 m was 111.5 Hz. The determined values should be related to the frequency stability of devices operating in the considered frequency band. In the VHF band, two types of transmission should be distinguished:  
Digital--the AIS system;  
Analog--Voice VHF transmission. According to the AIS system specification [28], the frequency stability of transmitters and receivers should be at least 500 Hz. On the other hand, in accordance with the recommendation ITU-R M.489-2 [29], the required frequency stability is at least 5 ppm for coastal stations and 10 ppm for ship devices. For the considered frequency range, these two values translate into a deviation of 809.1875 Hz and 1618.375 Hz, respectively. The values of the Doppler deviation determined by measurement and calculated theoretically are much lower than the required frequency stability of the systems under consideration (AIS devices ensure correct operation of the system with a frequency deviation of up to 500 Hz). It should also be noted that the channel coherence time determined during the measurements was from 4.778 to 7.984 ms, while the symbol duration in the AIS system is 0.104 ms. The relation (36) is fulfilled; therefore, in the considered channel, the impulse response does not change during the symbol duration, which means that only slow fading occurs in the channel. Radio Channel Model for Systems Operating in the VHF Band in the Propagation Environment of Offshore Wind Farm with the Use of the ITU-R Recommendation Methodology BT.1893-1  
On the basis of the measurement campaign and the obtained results, the authors of this article have defined specific situations in which the appropriate models described in the ITU-R BT.1893-1 recommendation should be used. These guidelines can be very useful in the practical application of the developed channel model in simulation studies for wind farms in the marine environment. 2.1 Dependence on the Location of the Transmitter and Receiver in Relation to the Specific Wind Turbine  
In the Annex 1 model, the primary reflection mechanism is the radio signal reflection from the wind turbine blades. As a result, this model is generally the most reliable for scenarios where the signal reflection from the rotor is as close as possible to the maximum reflection, i.e., when the transmitter is approximately perpendicular to the rotor. In the case of the Annex 2 model, the basic reflection mechanism from the turbine is the reflection from its mast. Of course, when the signal is transmitted towards the rotor directed at an angle of 90\({}^{\circ}\) to the transmitter, the reflecting surface (i.e., the surface of the rotor blades) will be much larger than the surface of the mast--especially in the case of offshore farms. Therefore, it should be stated that for scenarios where the signal reflection is close to the maximum reflection, the Annex 1 model will generally be more pessimistic than the Annex 2 model, and in such situations the Annex 1 should be used. In the case of reflection from other turbines (with more shallow angles), the bias (difference between measured and theoretical value) of the Annex 1 model in many cases is not only greater but, above all, its values are positive (biases ranging from +4 dB to +92 dB), which means that the model becomes overly optimistic and the power of the reflected signal calculated with its use is lower than actually measured. Relating these results to the location of the transmitter, receiver, and turbines (Figure 7), we can see, however, that in this case the situation concerns the reflection of the signal from the turbines located at a large (and often very large) angular distance from the direction of the maximum reflection.