t frequency channel to the carrier frequency channel but a different polarisation. One hop may have two frequency channels either having same the frequency channels and the same polarisations (or different polarisations) or adjacent channels having the same (or different) polarisations, as shown in Figure 3.63. Sometimes the signals (back lobes) from the antennas transmitted in different directions that are co-located or located too closely may interfere with the signals at the receiving antenna. Also, interfering signals may reach the receiving antenna due to diffraction or the over-reach effect. Over- reach is caused when the signal from a distant antenna (usually in a chain) interferes with the main signal for the initial hops (or vice versa). As shown in Figure 3.64, the signal 'I' acts as an interfering signal by over-reaching the far end antenna in the chain. The parameter used to measure the interference levels is the carrier-to-interference (C/I) ratio. When carrying out frequency planning, the C/I ratio is optimised SO that the minimum fade margin required to meet the objectives is actually available on the interfered hop. The main parameters taken into account when evaluating interference effects are: Modulation type and spectrum shaping RF channel frequency spacing C/I sensitivity Frequency Planning and Interference Figure 3.64 Over-reach Cross-polarisation discrimination Antenna patterns Filtering selectivity Due to interfering signals, the receiver threshold level increases (at a given BER). Due to the increase in the strength of the interfering signal, the signal-to-interfering (S/I) ratio decreases, making a negative impact on the threshold degradation of the receiver. This has a direct impact on the link budget calculations. To counter the impact of this phenomenon, the input power to the receiver should be increased by certain means, such as increasing the transmitted power, etc. Thus, link budget calculations are looked into twice, once when designing individual links and again when assessing th