Antenna Location
Combating Multipath Fading Space Diversity
Antenna Spacings Polarisation Diversity Antenna Configurations Air Combining System Antenna Specifications Downtilt
Example 16 Downtilt Notch
Objectives :
The objectives of this section are to enable the student to:
Explain the factors to be considered regarding antenna selection Describe the advantages and disadvantages of certain antenna locations Explain antenna diversity
Describe different antenna configurations
Explain the various specifications given for antennas Explain downtilt
Antenna Considerations
The primary objective for a proper antenna location and choice of an appropriate diversity scheme is to provide a uniform coverage within the cell area and minimum interference to and from other BTS antennae. Choice of antenna location ( cell site ) is based on proper containment of coverage and alignment of the sites in to a specific hexagonal pattern. The choice may be limited due to availability of space, links to BSC etc.
Containment of Coverage in Urban/Suburban areas: In Urban areas, the following conditions usually exist:
Several Sites may be needed Frequency re use is unavoidable In-building penetration is a must
Large coverage obtained by keeping an antenna at a height may not satisfy in-building coverage requirements.
In fact, one can rely on the buildings to serve as radio path shields, limiting the coverage area. Also the reflections from the buildings provide coverage to areas which would not have been possible in the normal LOS mode. (Street Canyons). These additional paths consequently increase in-building penetration also.
Antenna Considerations
Uniform Coverage in the cell Alignment with hexagonal pattern Space availability
Connectivity to BSC/MSC
• Several Sites may be needed • Frequency re use is unavoidable • In-building penetration is a must.
• Buildings act as RF shield and contain coverage.
• Buildings reflect signals and provide coverage to areas where • LOS would have failed.
• Such additional paths improve in-building penetration.
• Antenna at a very high point may not meet In-building coverage requirements. Antenna Location
The location of an antenna needs to be chosen not only for coverage needs, but also to ensure that the minimum interference to and from other sites is acheived.
Choice of location is driven by proper containment of coverage and site alignment within the confines of the specified pattern.
In urban areas, there are certain conditions which prevail: Several sites will be required
The re-use of frequencies is common In-building penetration needs to be provided
Merely placing an antenna at the highest point is not the answer to providing best coverage.
As well as giving a source of interference ot other sites in the coverage area, in-building coverage will not be fully acheived as succesfully as more specific solutions.
Figure 5-1 illustrates the possibilities of antenna location within a built-up area. In the first case, whilst a large area is covered by the high mounted antenna, interference control is difficult and in-building coverage is limited.
In the second instance, the buildings act as a natural containment for the propagation, and can also give coverage in areas that would otherwise be considered ’ dead spots’ . In-building coverage is also improved in this scenario. In Figure 5-2, location of the antenna at a high point within a suburban environment will be more beneficial than in a city environment and cause less of an interference problem.
Antenna Location
Combating Multipath Fading
We have the following techniques by which the effects of Multi path fading can be minimized: In the Time Domain: Interleaving
In the Frequency Domain: Frequency Hopping In the Spatial Domain: Space Diversity
In the Polarisation Domain: Polarisation Diversity.
Of the 4 different schemes listed above, the last two techniques are related to antenna systems.
In general, a diversity antenna system provides a number of receive paths ( normally 2). The diverse output from each path is combined by the receiver to give a signal of sufficient S/N.
Thus a Diversity antenna System essentially has: Two or More antennae
A combiner circuitry.
Another major requirement of Diversity antenna systems is that the signals arriving at the different receive paths/ports should have very low correlation. This is because if a signal is fading at one port, the chances of it happening in the other port should be LOW. This is the basis of Diversity.
Antenna Diversity
A Diversity antenna System essentially has: Two or More antennae
A combiner circuitry
Space Diversity
There are 3 ways in which Space Diversity could be realized: Horizontal Separation
Vertical Separation Composite Separation
Antenna Spacings:
The separation between antenna is a function of the correlation coefficient. To achieve a desired correlation coefficient, say <0.7, different configurations need different spacings. Figures given in Table 5-1 are for minimum required separation. If space is not a constraint, larger separation is always recommended. Horizontal separation is preferred because it provides low correlation values.
However, horizontal separation suffers from angular dependence (as in Figure 5-4). Vertical separation does not suffer much from angular dependence. It also requires minimum supporting fixtures and does not occupy a lot of space. But, as the distance increases, the correlation between the RF signals at the antenna points increases rapidly, thereby negating the very advantage of space diversity.