Page 95 - Antennas for Base Stations in Wireless Communications

68 Chapter Two

prevent troublesome differences in performance between the transmit
and receive bands of a frequency-division duplex (FDD) system.

2.3.5.2 Microstrip, Coaxial, and Hybrid Line Systems Microstrip design
is relatively easy because both the Z 0 and the length of any line sec-
tion can be adjusted at will. Coaxial line dividers must make use of the
characteristic impedances of available cables, although cables may be
connected in parallel where an alternative impedance is required. Many
commercial antenna designs use hybrid techniques to provide design
flexibility at the lowest possible cost, using microstrip power dividers
connected by coaxial cables. The microstrip components often employ
standard microwave PTFE-glass PCB laminates, but alumina and other
substrates are also in common use.
The requirements of low PIM, together with the power levels common
in high-capacity GSM base stations, has led to the adoption of 7/16-DIN
connectors as the almost universal standard connector for base station
antennas.

2.3.6 Practical Cost/Performance Issues
The worldwide success of mobile radio services and the prospect of the
continuing growth of data- and information-based systems have led to
requirements for extremely large numbers of base station antennas.
There are usually many ways in which an effective technical solution
can be found and antenna design is now strongly determined by the
economics of production. Many manufacturers have production facilities
in regions with low labor costs, so the emphasis of design has tended to
be to minimize material costs while accepting significant mechanical
complexity.
There is a significant difference in antenna economics between net-
works in rural areas, where the network optimization emphasis is on
coverage, and networks in urban areas, where network engineering is
dominated by the need to maximize capacity. The cost of establishing
a base station is high and includes not only the equipment and instal-
lation costs but also the costs of planning, backhaul links, site rentals,
power, taxes, and maintenance. If the number of base stations required
to cover the desired service area can be reduced by using high-cost
antennas with the maximum possible gain, this may be a very worth-
while expenditure. In a dense network, the antenna parameter that is
of most importance is clean radiation patterns, and the achievement of
maximum gain is far less significant, so antennas can be designed using
low-cost materials. Optimum network design depends on a wide variety
of economic and technical factors, including the local planning régime,
labor rates, and site access costs, so there is no single best choice.