Page 270 - Antennas for Base Stations in Wireless Communications

Antennas for WLAN (WiFi) Applications 243

Antennas play an important role in the design of WLANs. The anten-
nas for the client devices have critical constraints in terms of cost and
6
size, which severely limits the antenna performance. The antennas for
base stations in point-to-point (P2P) and/or point-to-multipoint (P2MP)
systems face different challenges such as the performance, cost, size,
and integration of multiple functions into one antenna design, as well
as the integration of antennas into radios.

7.1.2 MIMO in WLANs
Over the past decade, a wide range of applications has spurred the need
to provide a reliable high-speed wireless communication link. This is
especially challenging in an indoor wireless environment, where trans-
mitted signals are received through multiple paths, which may add up
destructively at the receiver, resulting in serious degradation in the over-
all system performance. This phenomenon is known as multipath fading.
Multipath is the arrival of the transmitted signal at an intended receiver
through different angles and/or different time delays and/or different
frequency shifts (i.e., Doppler effect). Consequently, the received signal
fluctuation can be larger than 10dB within one wavelength due to angle
spread and/or frequency due to delay spread and/or time due to Doppler
spread through the random superposition of the impinging multipath
components. The scarcity of available bandwidth, transmit power con-
straints, hardware complexity, and signal interference are some of the
other challenges that high-speed wireless communication faces.
With the steady increase in the number of new wireless applications
and the expansion of existing ones, the limited frequency spectrum prob-
lem can be alleviated effectively by using diversity techniques, which
provide the receiver with independently faded copies of the transmitted
signal. This increases the probability of reception at the receiver. There
are many diversity techniques based on frequency, time, angle, space,
polarization, and spatial diversity. Usually, the MIMO system uses mul-
tiple (two or more) antennas in both its transmitter and receiver sides. A
significant advantage of MIMO technology is that it provides a substan-
tial increase in channel capacity, resulting in higher data throughputs
with a low bit error rate, i.e., enhanced data transmission reliability.
The wireless channel’s impairments can also be overcome by channel
coding. Examples include convolution and block coded modulation, trellis
coded modulation, bit-interleaved coded modulation, and turbo and low-
density parity check codes. The combination of various diversity tech-
niques, such as space-time coding in MIMO systems, offers high data
rates with increased reliability. To maximize the transmission reliability,
transmit diversity schemes should be adopted. In this case, the data rate
is the same as for a single-input-single-output system since all the degrees
of freedom in the MIMO channel are used to improve transmission