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Antennas for WLAN (WiFi) Applications 255

to enhance the capacity by having an adaptive system that selectively
connects a subset of available antennas to the electronic modules. 17−20
Furthermore, in a rich multipath environment, six uncorrelated signals
at the receiver can be achieved from sensing the three Cartesian vector
components of the electric and magnetic fields. A large multipath angle
spread is able to offer a higher number (maximum of six) independent
communication channels. 21−23 However, constructing half wavelength
dipoles or full wavelength loops to achieve this is difficult due to mutual
coupling, and nonideal radiation pattern characteristics, which reduces
the number of effective (independent) channels. Typically, when two
polarizations are used, scattering leads to a 4−10-dB higher co-polarized
24
signal as compared to the cross-polarized signal. As a result, the trans-
fer matrix exhibits low correlation coupled with weak channel gain
between the two orthogonally polarized channels. 25−28 A capacity gain
of around 10−20% is achieved for dual-polarization over single polarized
25
spatially separated elements in an indoor environment. Regardless of
the environment, with the use of dual polarization, at least two chan-
nels will be enabled. Mutual coupling is also one of the key issues in
MIMO systems. 29−30 Studies have shown that when the power collection
capability is enhanced through mutual coupling, the capacity when two
dipoles are closely coupled to each other through proper termination can
be increased. 31−32 For a fixed length array, a strong mutual coupling will
lead to an upper bound in the capacity performance, especially when
the spacing between the elements is less than l/2. 33
Therefore, in the antenna design for MIMO systems, the mutual cou-
pling between the elements has a critical impact on system performance
although the overall system performance is influenced by many factors,
in particular, the channels through which the RF signals propagate. The
optimized design of antennas with low mutual coupling will be able
to exploit the full benefits of diversity in MIMO systems. As a result,
antennas with low mutual coupling will be conducive to the performance
of MIMO systems. Furthermore, MIMO systems will simultaneously
benefit from all types of possible diversity, for instance, spatial, pattern,
and polarization diversity.

7.3 State-of-the-Art Designs
There are many types of commercial antennas that meet the demands of
the fast growing market (Table 7.2). A series of practical antenna designs
that are commercially available are discussed in the following sections.

7.3.1 Outdoor Point-to-Point Antennas
34
Outdoor antennas have to be weatherproof (IEC IP66), i.e., operate
under direct and indirect sunlight, resistant to wind, rain, snow, and pro-
vide lightning surge protection. Common outdoor P2P antennas include