This invention relates to communication systems and more particularly to systems having a network of fixed base stations with which mobile terminals communicate.
In communication systems in which each base station is allocated a frequency channel for communication, it is necessary to ensure that those stations operating on the same channel are separated by a large enough distance so as to reduce any interference between them to acceptable levels. Interference may also occur between base stations operating on adjacent frequency channels. The physical distribution of base stations is also governed by the degree of coverage of the region required and, in the case of air-to-ground applications, stations must be spaced close enough together to ensure that aircraft at low altitudes are in line-of-sight with at least one base station.
One known air-to-ground communication system operates on seven channels and has a network of ground stations distributed as shown in FIG. 1. Each point reepresents a ground station and the associated reference numeral indicates the channel on which it operates. As can be seen, the ground stations are arranged in a hexagonal pattern in such a way that no ground station is arranged immediately adjacent another ground station operating on the same channel. Typically, the stations are spaced 200 nautical miles apart. Thus, ground stations operating on the same channel are spaced at 529 nautical miles apart. With this arrangement, aircraft at an altitude of less than 46,000 ft. Will not experience interference between ground stations operating on the same channel. At lower altitudes, in this case below 8,700 ft, aircraft may not be within line-of-sight of a ground station because of the curvature of the earth. As the normal operating height for local flights is between 15,000 and 35,000 ft, this spacing of ground stations is regarded as being a reasonable compromise, although special provisions are required near airports.
Each station is surrounded by ground stations which are allocated to the other six channels, so that stations operating on adjacent channels are spaced apart by 200 nautical miles. An aircraft at 26,000 ft. over one station has a line-of-sight of 200 nautical miles and can therefore receive an adjacent ground station operating on a channel adjacent to that of the ground station below. If it is desired to receive the adjacent station, the adjacent channel signal is 33 dB higher. This factor, together with the receiver filtering, sets the minimum spacing between adjacent channels.