How interference can be reduced by using the directional antennas at the cell site?

4. How interference can be reduced by using the directional antennas at the cell site?

For Interference Reduction Use: Directional Antennas

When the frequency reuse scheme must be used in AMPS, co channel interference will occur.The co channel interference reduction factor q = D/R = 4.6 is based on the assumption that the terrain is flat. Because actual terrain is seldom flat, we must either increase q or use directional antennas.

Directional Antennas: A 120◦-corner reflector or 120◦-plane reflector can be used in a 120◦ - sector cell. A 60◦-corner reflector can be used in a 60◦-sector cell. A typical pattern for a directional antenna of 120◦ beamwidth is shown in Fig.4.1.

Normal Antenna (Mature System) Configuration:

1. K = 7 cell pattern (120◦sectors). In a K = 7 cell pattern for frequency reuse, if 333 channels are used, each cell would have about 45 radios. Each 120◦ sector would have one transmitting antenna and two receiving antennas and would serve 16 radios. The two receiving antennas are used for diversity (see Fig. 4.2a).

2. K = 4 cell pattern (60◦sectors). We do not use K = 4 in an omnicell system because the cochannel reuse distance is not adequate. Therefore, in a K = 4 cell pattern, 60◦ sectors are used. There are 24 sectors. In this K = 4 cell-pattern system, two approaches are used.

a. Transmitting-receiving 60◦sectors. Each sector has a transmitting antenna carrying its own set of frequency radios and hands off frequencies to other neighboring sectors or other cells. This is a full K = 4 cell-pattern system. If 333 channels are used, with 13 radios per sector, there will be one transmitting antenna and one receiving antenna in each sector. At the receiving end, two of six receiving antennas are selected for angle diversity for each radio channel (see Fig.4.2b).

b. Receiving 60◦sectors. Only 60◦-sector receiving antennas are used to locate mobile units and handoff to a proper neighboring cell with a high degree of accuracy. All the transmitting antennas are omnidirectional within each cell. At the receiving end, the angle diversity for each radio channel is also used in this case.

Abnormal Antenna Configuration: If the call traffic is gradually increasing, there is an economic advantage in using the existing cell systems rather than the new splitting cell system (splitting into smaller cells). In the former, each site is capable of adding more radios. In a K = 7 cell pattern with 120◦ sectors, two transmitting antennas at each sector are used (Fig.4.2c). Each antenna serves 16 radios if a 16-channel combiner is used. One observation from Fig. 4.2c

should be mentioned here. The two transmitting antennas in each sector are placed relatively closer to the receiving antennas than in the single transmitting antenna case. This may cause some degree of desensitization in the receivers. The technology cited can combine 32 channels in a combiner; therefore, only one transmitting antenna is needed in each sector. However, this one transmitting antenna must be capable of withstanding a high degree of transmitted power. If each channel transmits 100 W, the total power that the antenna terminal could withstand is3.2 kW.

The 32-channel combiner has a power limitation which would be specified by different manufacturers. Two receiving antennas in each 120◦ sector remain the same for space diversity use.