Page 193 - Antennas for Base Stations in Wireless Communications

166 Chapter Four

Multibeam antennas at these radio base stations mitigate the interfer-
ence situation considerably. The interference reduction is used to tighten
the frequency plan in the macro layer either by adding more transceivers
at selected sites or by using fewer frequencies or a combination of these
alternatives. The frequencies so released can be allocated in added cells
or for new services.
An interference-limited GSM macro cell network consisting of three-
sector sites with mixed sizes and a heterogeneous traffic distribution
43
covering a downtown area was analyzed. In this example, capacity
is gained by introducing multibeam array antennas in a step-by-step
migration of radio base stations in the network. Increasing the number
of transceivers and improving the quality in the network in both the
target cells equipped with multibeam antennas and surrounding
cells using sector antennas are possible. The simulations show that
a large increase in capacity is achieved by replacing only a limited
number of existing installations with multibeam antennas, as shown
in Figure 4.25. Replacing only 12% of the cells in a real network with
multibeam antennas improved capacity 40%. In this way, additional
new site locations can be avoided even though a significant capacity
increase is achieved. Two reference cases, cell split from three- to six-
sector sites and the introduction of multibeam antennas, show linear
increases in capacity in idealized homogeneous networks. This shows
the particular advantage of selective use of multibeam antennas in
heterogeneous scenarios.

80
70
Fixed multibeam antennas
in a real network
Capacity increase (%) 50 Fixed multibeam antennas
60
40
in a homogeneous network
30
20
via cell split
10 Standard sites
0
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Fixed multibeam antenna cells introduced
in network (% of total number of cells)
Figure 4.25 Capacity gain in a GSM network as a function of relative number of cells
migrated to use four-column array antenna compared to six-sector migration