Page 119 - Antennas for Base Stations in Wireless Communications

92 Chapter Two

likely devise new ways in which the capabilities of these “semi-smart”
techniques can be extended to create additional operational flexibility.
The licensing of mobile radio operations on additional frequency bands
is likely to create new requirements for multiband and /or wideband
antennas. This is an area in which the changing policies of regulators
may have a significant impact on antenna design and manufacturing. The
very large number of combinations of frequencies, polarizations, beam-
widths, and other parameters has, for some time, resulted in antenna
manufacturers requiring a very wide range of products, a trend that
has persisted for 20 years. The high cost of antenna development and
the economics of large volume manufacturing have had a great impact
on the structure of the antenna industry worldwide and are some of the
drivers that have encouraged the advent of wideband RET antennas as
well as the rationalization of the industry in recent years.
Moves to provide higher user data rates are stimulating intensive
research on multiple antenna techniques. Planning constraints as well
as hardware costs and the public’s mistrust of large antenna installa-
tions will limit the possibility of simply adding more hardware to exist-
ing installations, but operators can respond to these new requirements
by sharing their antenna estate, pooling their resources (tower space,
planning consents, and other hardware), and using them in a more
technically advantageous manner.
It seems unlikely that what many would regard as true “smart” anten-
nas will be introduced in any quantity into networks using the existing
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GSM/UMTS air interfaces, but practice in East Asia shows that they
may play a wider and more significant role if future air interfaces are
designed to exploit their advantages while accommodating their limi-
tations. Technical developments such as the introduction of transmit
diversity and MIMO raise the performance bar of standard antennas
combined with semi-smart capabilities. In both cases what is being done
is effectively to combine intelligent signal processing with conventional
antenna elements, and the fact that there are many different ways in
which some near-optimum solution may be found by combining these
techniques in various ways comes as no surprise.
There is currently much interest in possible applications for anten-
nas using artificial materials such as electromagnetic bandgap (EBG)
structures and frequency selective surfaces (FSSs), and there may be
some possibilities for their use in base station antenna design. However,
requirements for wide bandwidth and dual-polarized operation, together
with constraints on cost and efficiency, may limit their possible uses for
mainstream applications.
The use of optical fiber to carry RF signals is well-known, and this
technique could be extended to the development of antennas in which
beamforming is performed in fiber, the signals being converted to RF