rrain Loss) + antenna system gain at 4 - wave-guide loss at 4 - combined antenna discrimination at both ends. - - additional terrain loss - combined antenna discrimination. The combined antenna discrimination can be derived from radiation pattern details of site 1 and site 4 antennas, depending on the polarisation combination (HH, HV, VH or VV) between sites 1 and 4. Example 5: Frequency Plan Taking a cue from Figure 3.17, an example of a possible frequency plan is shown in Figure 3.19. Interferences 1, 3 and 4 (from Figure 3.17) can be cured by using different frequency bands and polarisations. Interferences from 2, 5 and 6 can be removed by using a polarisation-altering technique (i.e. altering the horizontal and vertical polarisations). DETAILED TRANSMISSION NETWORK PLANNING Figure 3.19 Interference calculation Interference due to signal 7, which is detected only (usually) after the network is live, can be removed by using either different frequency bands or polarisation. As mentioned previously, other techniques such as antenna tilting can also be used; but as this requires the link to be non-functional, other techniques such as power reduction can be used. It is always preferable to have more frequency bands at one's disposal for better frequency planning, but congestion of the frequency bands, especially at lower frequencies, usually makes this difficult. 3.4.2 Time-slot Allocation Planning Understanding of time-slot allocation principles and methodology is crucial for the effi- cient utilisation of the PCM lines available. These principles are also crucial for the traffic distribution at the base station. First let us examine the movement of one PCM or 2 Mbps from the MSC to the MS. One PCM was shown in Figure 3.5. On the A-interface, one 2 Mbps consists of 32 time slots. TSO is used for link management. Each of these time slots is a 64 kbps channel. The next stop for the 2 Mbps signal is the transcoder/sub-multiplexers, which convert these 64 kbps signals into 16 kbps traffic channels. The common channel 