4.4 X 6 = 26.4 mi (42.25 km). Lee (Ref. 7) reports that cochannel interference can be reduced by other means such as directional antennas, titled beam antennas, lowered antenna height, and an appropriately selected site. One way we can protect a cell that is using the same frequency family as a nearby cell is by keeping that cell base station below line-of-sight of the nearby cell. In other words, we are making our own shadow conditions. Consider a 26.4-mi path, what is the height of earth curvature midpath? From Section 9.2.3.3, = 0.667(d/2)2/1.33 = 87.3 ft (26.9 m). Providing the cellular base station antennas are kept under 87 ft, the 40 dB/decade rule of Lee holds. It holds SO long as we are below line-of-sight conditions. The total available (one-way) bandwidth is split up into N sets of channel groups. The channels are then allocated to cells, one channel set per cell on a regular pattern, which repeats to fill the number of cells required. As N increases, the distance between channel sets (D) increases, reducing the level of interference. As the number of channel sets (N) increases, the number of channels per cell decreases, reducing the system capacity. Selecting the optimum number of channel sets is a compromise between capacity and quality. Note that only certain values of N lead to regular repeat patterns without gaps. These are N = 3, 4, 7, 9, 12, and then multiples thereof. Figure 18.15 shows a repeating 7 pattern for frequency reuse. This means that N = 7 or there are 7 different frequency sets (or families) for cell assignment. Figure 18.15 A cell layout based on N = 7. CELLULAR AND PCS RADIO SYSTEMS Figure 18.16 Breaking a cell up into three sectors (left) and six sectors (right). Cell splitting will take place, especially in urban areas, in some point in time because the present cell structure cannot support the busy hour traffic load. Cell splitting, in effect, provides more frequency slots for a given area and relieves the congestion problem. Macario (Ref. 10) reports that cells can be split as