 however, may belong to the same or a different network, as shown in Figure 3.18. Site 2 Tx: F2 Site 4 Rx: F1 Tx: F1 Rx: F2 Polarization P2 (H or V) Polarization P2(H or V) Site 1 Site 3 Tx: F1 Tx: F1 Rx: F2 Rx: F2 Figure 3.18 Interference calculation The following aspects are considered: Site 4 will receive interference signals from site 1 and vice versa. The interference may cause degradation of the receiver threshold to an extent dependent on the received S/I ratio at the site. Since the antennas at sites 1 and 4 are oriented towards sites 2 and 3 respectively, an antenna discrimination will result, depending on the off-axis angles 0 and . Also contributing to this discrimination could be a difference in polarization of the two antennas at sites 1 and 4. Furthermore, since the antennas at sites 1 and 4 are mounted at heights dependent on the paths between sites 1-2 and sites 3-4 respectively, the path between sites 1 and 4 may cause additional attenuation over and above the free-space loss, because of obstructions. As a first analysis, however, the additional loss may be assumed to be zero. (Note: Along with the above form of interference, other sources may also contribute to interference, as shown in Figure 3.17.) The following factors should be taken into account: antenna discrimination cross-polarisation discrimination diffraction loss due to obstructions in the interfered path. TRANSMISSION NETWORK PLANNING AND OPTIMISATION Table 3.1 System Details SITE 1 SITE 4 Units Transmitter Output Feeder Loss Connector Loss Received Carrier Power Polarization Threshold Degradation Receiver Threshold Antenna Gain Geographic Details SITE 1 SITE 4 Units Distance Off Axis Angle Polarization SITE 1 SITE 4 The inputs required are tabulated in Table 3.1. The interference level at site 4 (I4) is given by: I4 = Received signal level at site 4, receiving from site 1 - combined antenna discrimination at both ends = TX power level of 1 + antenna gain at 1 - wave-guide loss at 1 - Attenuation over path 1 to 4 (FSL + additional Te