Explain the designing of an omnidirectional antenna system in the worst case scenario

3. Explain the designing of an omnidirectional antenna system in the worst case scenario.

Design of an Omnidirectional Antenna System in the Worst Case: The value of q = 4.6 is valid for a normal interference case in a K=7 cell pattern. In this section we would like to prove that a K=7 cell pattern does not provide a sufficient frequency re-use distance separation eyen when an ideal condition of flat terrain is assumed. The worst case is at the location where the weakest signal from its own cell site but strong interference from all interfering cell sites. In the worst case the mobile unit is at the cell boundary R, as shown in Fig. 3. The distances from all six co channel interfering sites are also shown in the figure: two distances of D - R, two distances of D, and two distances of D + R.

Following the mobile radio propagation rule of 40 dB/dec, we obtain

Then the carrier-to-interference ratio is

Cochannel interference (a worst case)

Where q=4.6 is derived from the normal case. Substituting q=4.6 into above eqn. we obtain C/I =54 or 17 dB, which is lower than 18 dB. To be conservative, we may use the shortest distance D – R for all six interferers as a worst case; then we have

In reality, because of the imperfect site locations and the rolling nature of the terrain configuration, the C/I received is always worse than 17 dB and could be 14 dB and lower. Such an instance can easily our in a heavy traffic situation; therefore, the system must be designed around the C/I of the worst case. In that case, a co channel interference reduction factor of q=4.6 is insufficient.

Therefore, in an omnidirectional-cell system, K = 9 or K 12 would be a correct choice. Then the values of q are