Monday, August 15, 2011


What is a Smart Antenna?
The term “smart antenna” generally refers to any antenna array, terminated
in a sophisticated signal processor, which can adjust or adapt
its own beam pattern in order to emphasize signals of interest and to
minimize interfering signals.
Smart antennas generally encompass both switched beam and beamformed
adaptive systems. Switched beam systems have several available
fixed beam patterns. A decision is made as to which beam to
access, at any given point in time, based upon the requirements of
the system. Beamformed adaptive systems allow the antenna to steer
the beam to any direction of interest while simultaneously nulling
interfering signals. The smart antenna concept is opposed to the fixed
beam “dumb antenna,” which does not attempt to adapt its radiation
pattern to an ever-changing electromagnetic environment. In the past,
smart antennas have alternatively been labeled adaptive arrays or digital
beamforming arrays. This new terminology reflects our penchant for
“smart” technologies and more accurately identifies an adaptive array
that is controlled by sophisticated signal processing. Figure 1.1 contrasts
two antenna arrays. The first is a traditional, fixed beam array
where the mainlobe can be steered, by defining the fixed array weights
w¯ . However, this configuration is neither smart nor adaptive.
The second array in the figure is a smart antenna designed to adapt to
a changing signal environment in order to optimize a given algorithm.
An optimizing criterion, or cost function, is normally defined based upon
the requirements at hand. In this example, the cost function is defined
as the magnitude of the error squared, |ε|2, between the desired signal
d and the array output y. The array weights w¯ are adjusted until the
output matches the desired signal and the cost function is minimized.
This results in an optimum radiation pattern.

Why are Smart Antennas
Emerging Now?
The rapid growth in demand for smart antennas is fueled by two major
reasons. First, the technology for high speed analog-to-digital converters
(ADC) and high speed digital signal processing is burgeoning at an
alarming rate. Even though the concept of smart antennas has been
around since the late 50s [1–3], the technology required in order to
make the necessary rapid and computationally intense calculations has
only emerged recently. Early smart antennas, or adaptive arrays, were
limited in their capabilities because adaptive algorithms were usually
implemented in analog hardware. With the growth of ADC and digital
signal processing (DSP); what was once performed in hardware
can now be performed digitally and quickly. ADCs, which have resolutions
that range from 8 to 24 bits, and sampling rates approaching
20 Gigasamples per second (GSa/s), are now a reality . In time,
superconducting data converters will be able to sample data at rates
up to 100 GSa/s [6]. This makes the direct digitization of most radio
frequency (RF) signals possible in many wireless applications. At the
very least, ADC can be applied to IF frequencies in higher RF frequency
applications. This allows most of the signal processing to be
defined in software near the front end of the receiver. In addition, DSP
can be implemented with high speed parallel processing using field
programmable gate arrays (FPGA). Current commercially available
FPGAs have speeds of up to 256 BMACS.1 Thus, the benefits of smart
antenna integration will only flourish, given the exponential growth in
the enabling digital technology continues.
Second, the global demand for all forms of wireless communication
and sensing continues to grow at a rapid rate. Smart antennas are
the practical realization of the subject of adaptive array signal processing
and have a wide range of interesting applications. These applications
include, but are not limited to, the following: mobile wireless
communications, software-defined radio [8, 9], wireless local
area networks (WLAN) , wireless local loops (WLL) , mobile
Internet, wireless metropolitan area networks (WMAN) , satellitebased
personal communications services, radar , ubiquitous radar
, many forms of remote sensing, mobile ad hoc networks (MANET)
, high data rate communications , satellite communications,
multiple-in-multiple-out (MIMO) systems , and waveform diversity
systems .
The rapid growth in telecommunications alone is sufficient to justify
the incorporation of smart antennas to enable higher system capacities
and data rates. It is projected that the United States will spend over
$137 billion on telecommunications in the year 2006. Global expenditures
on telecommunications are rapidly approaching $3 trillion.

 What are the Benefits of Smart Antennas?
Smart antennas have numerous important benefits in wireless applications
as well as in sensors such as radar. In the realm of mobile wireless
applications, smart antennas can provide higher system capacities by
1BMACS: Billion

directing narrow beams toward the users of interest, while nulling other
users not of interest. This allows for higher signal-to-interference ratios,
lower power levels, and permits greater frequency reuse within the
same cell. This concept is called space division multiple access (SDMA).
In the United States, most base stations sectorize each cell into three
120◦ swaths as seen in Fig. 1.2a. This allows the system capacity to
potentially triple within a single cell because users in each of the three
sectors can share the same spectral resources. Most base stations can
be modified to include smart antennas within each sector. Thus the 120◦
sectors can be further subdivided as shown in Fig. 1.2b. This further
subdivision enables the use of lower power levels, and provides for even
higher system capacities and greater bandwidths.
Another benefit of smart antennas is that the deleterious effects of
multipath can be mitigated. As will be discussed in Chap. 8, a constant
modulus algorithm, which controls the smart antenna, can be
implemented in order to null multipath signals. This will dramatically
reduce fading in the received signal. Higher data rates can be realized
because smart antennas can simultaneously reduce both co-channel interference
and multipath fading. Multipath reduction not only benefits
mobile communications but also applies to many applications of radar
Smart antennas can be used to enhance direction-finding (DF) techniques
by more accurately finding angles-of-arrival (AOA) . A vast
array of spectral estimation techniques can be incorporated, which are
able to isolate the AOA with an angular precision that exceeds the resolution
of the array.The accurate estimation of AOA is especially beneficial in radar systems for
imaging objects or accurately tracking moving objects. Smart antenna
DF capabilities also enhance geo-location services enabling a wireless
system to better determine the location of a particular mobile user.
Additionally, smart antennas can direct the array main beam toward

signals of interest even when no reference signal or training sequence
is available. This capability is called blind adaptive beamforming.
Smart antennas also play a role in MIMO communications systems
[18] and in waveform diverse MIMO radar systems [21, 22]. Since
diverse waveforms are transmitted from each element in the transmit
array and are combined at the receive array, smart antennas will play
a role in modifying radiation patterns in order to best capitalize on
the presence of multipath. With MIMO radar, the smart antenna can
exploit the independence between the various signals at each array element
in order to use target scintillation for improved performance, to
increase array resolution, and to mitigate clutter .
. In summary, let us list some of the numerous potential benefits
of smart antennas.
■ Improved system capacities
■ Higher permissible signal bandwidths
■ Space division multiple access (SDMA)
-Higher signal-to-interference ratios
■ Increased frequency reuse
■ Sidelobe canceling or null steering
■ Multipath mitigation
■ Constant modulus restoration to phase modulated signals
■ Blind adaptation
■ Improved angle-of-arrival estimation and direction finding
■ Instantaneous tracking of moving sources
■ Reduced speckle in radar imaging
■ Clutter suppression
■ Increased degrees of freedom
■ Improved array resolution
■ MIMO compatibility in both communications and radar

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