Because of the limited bandwidth allocated in the 800-MHz band for cellular radio communications, frequency reuse is crucial for its successful operation. A certain level of interference has to be tolerated. The major source of interference is co-channel interference from a " nearby'' cell using the same frequency group as the cell of interest. For the 30-kHz bandwidth AMPS system, Ref. 15 suggests that C/I be at least 18 dB. The primary isolation derives from the distance between the two cells with the same frequency group. In Figure 10.2 there is only one cell diameter for protection.
Refer to Figure 10.19 for the definition of the parameters R and D. D is the distance between cell centers of repeating frequency groups and R is the " radius'' of a cell. We let a = D/R
The D/R ratio is a basic frequency reuse planning parameter. If we keep the D/R ratio large enough, co-channel interference can be kept to an acceptable level.
Figure 10.19. Definition of R and D.
Lee (Ref. 9) calls a the co-channel reduction factor and relates path loss from the interference source to R"4.
A typical cell in question has six co-channel interferers, one on each side of the hexagon. So there are six equidistant co-channel interference sources. The goal is C/I G 18 dB or a numeric of 63.1. So
C/I = C/XI = C/61 = R"4/6D"4 = a4/6 G 63.1
This means that D must be 4.4 times the value of R. If R is 6 miles (9.6 km) then D = 4. 4 X 6 = 26.4 miles (42.25 km).
Lee (Ref. 9) reports that co-channel interference can be reduced by other means such as directional antennas, tilted beam antennas, lowered antenna height, and an appropriately selected site.
If we consider a 26.4-mile path, what is the height of earth curvature at midpath? From Chapter 2, h = 0.667(d/2)2/1.33 = 87.3 ft (26.9 m). Providing that the cellular base station antennas are kept under 87 ft, the 40-dB/decade rule of Lee holds. Of course, we are trying to keep below LOS 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, and 12, and then multiples thereof. Figure 10.20 shows a repeating 7 pattern for frequency reuse. This means that N = 7 or there are 7 different frequency sets for cell assignment.
Cell splitting can 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. Marcario (Ref. 15) reports that cells can be split as far down as a 1-km radius.
Co-channel interference tends to increase with cell splitting. Cell sector-ization can cut down the interference level. Figure 10.16 shows a three- and six-sector plan. Sectorization breaks a cell into three or six parts each with a directional antenna. With a standard cell, co-channel interference enters from six directions. A six-sector plan can essentially reduce the interference to just one direction. A separate channel set is allocated to each sector.
The three-sector plan is often used with a seven-cell repeating pattern, resulting in an overall requirement for 21 channel sets. The six-sector plan with its improved co-channel performance and the rejection of secondary interferers allows a four-cell repeat plan (Figure 10.21) to be employed. This results in an overall 24-channel set requirement. Sectorization requires a larger number of channel sets and fewer channels per sector. Outwardly it appears that there is less capacity with this approach; however, the ability to use much smaller cells actually results in a much higher capacity operation (Ref. 15).
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