 B, level of demand, might be more convenient to manage each sub-band would be sub-divided into channels, each wide would not necessarily be less efficient in spectrum use. enough for a single uni-directional link. See for example Figure There will be circumstances in which the locations of stations are 26.1. One channel from each sub-band would be assigned to in that will need assignments in a frequency band in the future each hi-directional link, as in Figure 26.2. The plan shown known in advance. In such cases a frequency assignment utilisation, plan, Figure 26.1 is suitable, for example, for multiplexed telephony PCM links carrying 24 channel FDM with FM or 30 channel optimising might be prepared at the outset, assignments being the geographical dimension of spectrum made in with 4 phase PSK. Such a plan can facilitate the selection band of accordance with the plan as each station starts operating. frequencies for assignment. Where the planned frequency charac- is to be used for a large number of stations with similar Channel An teristics, a regular radio channelling plan may give other advan- to tages also; for example, the use of frequency synthesisers and re- determine the operating frequencies of transmitters utilisation Channel Bn ceivers may be facilitated and efficient spectrum strict be more readily achieved through the imposition of may approval specifications for transmitters and receivers. type The radio channelling plan concept might be extended to become a national frequency allotment plan. A wide geographi- into cal area, perhaps nation-wide in extent, would be divided cells, and a list of channels would be allotted for assignment for within each cell. The same channel would be allotted assignment in other cells, with enough geographical separation Figure 26.2 Channels An and Bn from the plan in Figure to make the probable level of interference acceptably low. The 26.1 are assigned to a bi-directional link plan might, in the simplest case, consist of a regular tessellation Frequency assignment 2