e impact of other links on the network. It is assumed that site A is being interfered by the signal from site D. The interference level at site D is a difference between the (interference) signal level at site A and the combined discrimination at both ends. The received interference signal is calculated by using the normal link budget calculation process described previously. The discrimination aspect can be calculated from the radiation patterns of the antennas at both site A and site D. S/I ratio is the ratio of the carrier signal to the interfering signal in the unfaded state. In the faded state, the carrier signal level is the sum of the threshold degradation and the carrier signal level. Hence, the threshold degradation can be calculated as Degradation = 2.3026[1.3-0.1(S/D)ascal Based on these discussions, certain basic rules should be followed when performing frequency plan- ning. A microwave link uses two frequencies, one for the TX and the other for the RX directions. The separation of two frequencies depends upon the duplex separation of the filters. This frequency separation should be provided separately in the frequency plans. Intermodulation is another aspect to be considered during frequency planning. Intermodulation is present when more than one frequency is present. It should be avoided as it creates problems for the third receiver. Consider the following example. There are five frequency spots F1, F2, F3, F4 and F5 and five sites that need to be connected in a loop. Frequency allocation should be such that the same or adjacent channels are not next to each other or hops are placed in a way where over-reach interference possibilities exist. The polarisation also changes with every hop, with switching between vertical (V) and horizontal (H) polarisations, with the frequency Transmission Network Planning and Optimisation Figure 3.65 Example of the frequency plan spots at each site being either high or low. When doing frequency planning, both high and low spots at the same site should be avoided. An e