**5. Discussion of results**

The response of propagation models for frequency variation from VHF and UHF shows different value for various models. For the models described above, a detailed comparison is made so as identify which propagation model is best fit for quantifying the available TVWS channels in Ethiopia's TV frequency interest. When the proposed propagation model (ITU-R P.1546–5) is implemented after interpolation, the pathloss must be changed toa new loss with the following formula:

$$\text{L} = \text{139.3-E} + 20 \log(f) \text{ where } \text{E is in } dB\left(\mu \frac{V}{m}\right) \\ \text{where } f \text{ in MHz} \tag{12}$$

After this transformation is made different calculations are performed and compared with the loss difference of other models by using the scenario depicted in **Figure 1**.

As can be seen from **Figure 2**, the comparison between different pathloss models for VHF and UHF TV transmission spectrums are illustrated. The path loss is computed based on the scenario illustrated in **Figure 1**. As seen from the **Figure 1**, the proposed propagation model (ITU-R.P.1546–5) gave a much better result than others. Hence, so as to compute the available TVWS free spectrums at a certain place and location, ITU-R.P.1546–5 is the best propagation model.

Based on the results obtained in **Figure 2**, sensitivity of the propagation models comparison can be best described at different frequency ranges as depicted in **Figure 3**. As seen in **Figure 3**, the Longley Rice model is very sensitive at different frequency and distance ranges. It has also varying value of pathloss differences at different test points. So, taking the average of these values, we have a difference of *18.28.* The other values, as can be seen from the graph, are somewhat with constant value. Comparing these values, the pathloss model, ITU-R P.1546–5 shows the minimum variation for frequency variation of TV signals from VHF to UHF with a value of 9.9736. The other model with the nearest value is Okumura Hata model. But selecting ITU-R P.1546–5 model has better advantage in consideration of terrain data. So, ITU-R P.1546–5 is the best pathloss model to represent TV signal coverage and signal modeling. Using this pathloss model, the free channels found free with assumption that the WSD is placed at the center of each TV transmitter, which is the worst-case scenario for WSD placement, we have found free channels as shown in **Figure 4** below.

As can be seen from **Figure 4**, among the 58 TV transmitter channels, sites which have 1 occupied co-channel and adjacent channel is 28% and 47% respectively.

*Coverage Determination of Incumbent System and Available TV White Space Channels… DOI: http://dx.doi.org/10.5772/intechopen.98784*

**Figure 2.** *Pathloss model results (a) and (b).*

**Figure 3.** *Variations of Pathlosses for VHF and UHF.*

**Figure 4.** *Number of occupied channels and their respective percentages for Co and adjacent channels.*

Likewise, sites which have 2 occupied co-channels and adjacent channels is 21% and 26% respectively and so on up to 7 occupied co-channels and adjacent channels. These values indicated that there are 51 to 57 free unoccupied free channels in Ethiopia

*Coverage Determination of Incumbent System and Available TV White Space Channels… DOI: http://dx.doi.org/10.5772/intechopen.98784*

among the 58 TV transmitter. Exploiting such vacant TV spectrums for affordable wireless broadband, machine to machine communication, vehicle to vehicle communication vision sensors and etc. are very promising. **Figure 4** also depicts that the highest capacity for secondary use is concentrated around one up to three. This implies that the majority of free channels, which can reach 80% of free channels, have a minimum of 51 channels (contiguous bandwidth of 408 MHz) available.
