**4. System model and problem formulation**

#### **4.1 Problem formulation**

For efficient utilization of free channels, keeping the interference minimum at the same time, the path loss model used by calculation engine must be efficient, where it should support both VHF and UHF. The path loss model should be less variant in different frequency ranges. If the path loss value has significant difference for frequency values in VHF and UHF, this will cause significance difference in contour coverage of the digital TV (DTV). This will on the other hand cause a variation in channels which are set free or occupied. Free channels for VHF may be occupied on UHF and vice versa. Propagation models selected for comparison must support the working frequency range of TV system. They must also be applicable for rural areas, since TV white space is assumed to be more applicable for rural areas. For our case, the TV frequency ranges from 174 MHz to 846 MHz. So, all the propagation models are defined for this frequency range. After the propagation model is selected, the coverage of the TV transmitter should be analyzed and this is used further to analyze the free channels which will be available as white space channels.

#### **4.2 System Modeling**

For the analysis purpose we have selected the above propagation models. These models are compared by their response for frequency variation. The analysis is done *Coverage Determination of Incumbent System and Available TV White Space Channels… DOI: http://dx.doi.org/10.5772/intechopen.98784*

using Matlab software and SPLAT!. For those which can be simulated using Matlab, we have selected the transmitter to be Furi, which is located south west of Addis Ababa city and five different test points. To guarantee the reason of selection, Furi is working in frequencies under channel 7 and channel 42 [23]. The test points are selected in order to consider different points in geographical location, distance and terrain types. All the points shown in the **Figure 1** below are different in pathloss calculation parameters.

Frequencies of comparison are center frequencies for VHF and UHF frequency ranges of Ethiopian TV transmission band. The center frequency for VHF in Ethiopian case is taken by using the calculation formula:

$$f\_{c(VHF)} = (178 \text{MHz} + 220 \text{MHz})/2 \tag{10}$$

And its value is 199 MHz. For UHF case, the minimum frequency is 474 MHz and the maximum frequency is 786 MHz. The center frequency then is 636 MHz, with equation below.

$$f\_{c(UHF)} = (\text{474MHz} + \text{786MHz})/2 \tag{11}$$

Other parameters taken as common for all models are:

• Transmitter height above ground level = 30 m, which results in HAAT of 639.42 m at Furi.

**Figure 1.** *Test points for pathloss analysis (adapted from Google Earth).*


For the pathloss analysis of Longley rice model the real data is used for different locations. These test points (shown in **Figure 1**) are selected at random. The points selected, although they are random, are selected from different locations to include different propagation path types.
