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ABSTRACT
The adoption of smart / adaptive antenna techniques in future wireless systems is expected to have a significant
impact on the efficient use of the spectrum, the minimization of the cost of establishing new wireless networks,
the optimization of service quality and realization of transparent operation across multi technology wireless
networks . This paper presents brief account on smart antenna (SA) system. SAs can place nulls in the direction
of interferers via adaptive updating of weights linked to each antenna element. SAs thus cancel out most of the
co-channel interference resulting in better quality of reception and lower dropped calls. SAs can also track the
user within a cell via direction of arrival algorithms . This paper explains the architecture, evolution and how the
smart / adaptive antenna differs from the basic format of antenna. The paper further explains about the radiation
pattern of the antenna and why it is highly preferred in its relative field. The capabilities of smart / adaptive
antenna are easily employable to Cognitive Radio and OFDMA system.
INTRODUCTION
In view of explosive growth in the number of digital cellular subscribers, service providers are becoming
increasingly concerned with the limited capacities of their existing networks. This concern has led to the
deployment of smart antenna systems throughout major metropolitan cellular markets. These smart
antenna systems have typically employed multibeam technologies, which have been shown, through
extensive analysis, simulation, and experimentation, to provide substantial performance improvements in
FDMA, TDMA and CDMA networks. Multibeam architectures for FDMA and TDMA systems provide the
straight-forward ability of the smart antenna to be implemented as a non-invasive add-on or appliqué to an
existing cell site, without major modifications or special interfaces.
This paper mainly concentrates on use of smart antennas in mobile communications that enhances the
capabilities of the mobile and cellular system such as faster bit rate, multi use interference, space division
multiplexing (SDMA), adaptive SDMA , increase in range, multipath mitigation, reduction of errors due to
multipath fading, best suitability of multi-carrier modulations such as OFDMA. The best application of SAs
is its suitability for demand based frequency allocation in hierarchical system approach (flexible antenna
pattern are achieved electronically and no physical movement of receiving antennas is necessary). The
advantage of SAs application in cellular systems are decreased inter symbol interference, decreased cochannel
interference & adjacent channel interference, improved bit error rate (due to decreased amount of
multipath and ISI), increase in receiver sensitivity, reduction in power consumption & RF pollution. Smart
antennas are most appropriate for use of cognitive radio (software radio technology provides flexibility) and
the greatest advantage of smart antenna is a very high security.
The main impediments to high-performance wireless communications are interference from other users (cochannel
interference), the inter-symbol interference (ISI) and signal fading caused by multipath. Co-channel
interference limits the system capacity, defined as the number of users which can be serviced by the system.
However, since the desired signal and co-channel interference typically arrive at the receiver from different
directions, smart antennas can exploit these differences to reduce co-channel interference, thereby increasing
system capacity. The reflected multipath components of the transmitted signal also arrive at the receiver from
different directions, and spatial processing can use these differences to attenuate the multipath, thereby reducing
ISI and fading. Since data rate and BER are degraded by these multipath effects, reduction in multipath through
spatial processing can lead to higher data rates and better BER performance.
In a cellular system, omni-directional antennas have traditionally been used at base stations to enhance the
coverage area of the base stations but it also leads a gross wastage of power that in-fact is the main cause
of co-channel interference at neighboring base stations. The sectoring concept with diversity system
exploits space diversity and results in improve reception by counteracting with negative effects of multipath
fading. Adaptive / smart antenna technology represents the most advanced smart antenna approach to
date. Using a variety of new signal-processing algorithms, the adaptive system takes advantage of its
ability to effectively locate and track various types of signals to dynamically minimize interference and
maximize intended signal reception. Both adaptive / smart systems attempt to increase gain according to
the location of the user; however; only the adaptive system provides optimal gain while simultaneously
identifying, tracking, and minimizing interfering signals.
2. SMART ANTENNA
2.1. Smart
The concept of using multiple antennas and innovative signal processing to serve cells more intelligently
has existed for many years. In fact, varying degrees of relatively costly smart antenna systems have
already been applied in defense systems. Until recent years, cost barriers have prevented their use in
commercial systems.The advent of powerful low-cost digital signal processors (DSPs), general-purpose
processors (and ASICs), as well as innovative software-based signal-processing techniques (algorithms)
have made intelligent antennas practical for cellular communications system.
2.2. What Is a Smart Antenna System?
In truth, antennas are not smart antenna systems are smart. Generally co-located with a base station,
a smart antenna system combines an antenna array with a digital signal-processing capability to
transmit and receive in an adaptive, spatially sensitive manner. Such a configuration dramatically
enhances the capacity of a wireless link through a combination of diversity gain, array gain and
interference suppression. Increased capacity translates to higher data rates for a given number of
users or more users for a given data rate per user. In other words, such a system can automatically
change the directionality of its radiation patterns in response to its signal environment. This can
dramatically increase the performance characteristics (such as capacity) of a wireless system.
Multipath of propagation are created by reflections and scattering. Also, interference signals such as
that produced by the microwave oven in the picture fig (1) are superimposed on the desired signals.
Measurements suggest that each path is really a bundle or cluster of paths, resulting from surface
roughness or irregularities. The random gain of the bundle is called multipath fading.
2.3. How Many Types of Smart Antenna Systems Are There?
Terms commonly heard today that embrace various aspects of a smart antenna system technology
include intelligent antennas, phased array, SDMA, spatial processing, digital beam forming, adaptive
antenna systems, and others. Smart antenna systems are customarily categorized, however, as
either switched beam or adaptive array systems.
The following are distinctions between the two major categories of smart antennas regarding the
choices in transmit strategy:
• Switched Beam—a finite number of fixed, predefined patterns or combining strategies (sectors)
• Adaptive Array—an infinite number of patterns (scenario-based) that are adjusted in real time
2.3.1. What Are Switched Beam Antennas?
Switched beam antenna systems form multiple fixed beams with heightened sensitivity in particular
directions. These antenna systems detect signal strength, choose from one of several predetermined,
fixed beams, and switch from one beam to another as the mobile moves throughout the sector.
Instead of shaping the directional antenna pattern with the metallic properties and physical design of a
single element (like a sectorized antenna), switched beam systems combine the outputs of multiple
antennas in such a way as to form finely sectorized (directional) beams with more spatial selectivity
than can be achieved with conventional, single-element approaches fig (2).
2.3.2. What Are Adaptive Array Antennas?
Adaptive antenna technology represents the most advanced smart antenna approach to date. Using a variety of
new signal-processing algorithms, the adaptive system takes advantage of its ability to effectively locate and
track various types of signals to dynamically minimize interference and maximize intended signal reception.
Both systems attempt to increase gain according to the location of the user; however, only the adaptive
system provides optimal gain while simultaneously identifying, tracking, and minimizing interfering signals.
Adaptive Array Coverage: A representative depiction of a main lobe extending toward a tser with a
null directed toward a co-channel interferer as shown in fig (3).
2.3.3. What Do They Look Like?
Omni-directional antennas are obviously distinguished from their intelligent counterparts by the number of
antennas (or antenna elements) employed. Switched beam and adaptive array systems, however, share
many hardware characteristics and are distinguished primarily by their adaptive intelligence.
To process information that is directionally sensitive requires an array of antenna elements (typically 4
to 12), the inputs from which are combined to control signal transmission adaptively. Antenna
elements can be arranged in linear, circular, or planar configurations and are most often installed at
the base station, although they may also be used in mobile phones or laptop computers.
2.3.4. What Makes Them So Smart?
A simple antenna works for a simple RF environment. Smart antenna solutions are required as the number
of users, interference, and propagation complexity grow. Their smarts reside in their digital signalprocessing
facilities. Like most modern advances in electronics today, the digital format for manipulating
the RF data offers numerous advantages in terms of accuracy and flexibility of operation. Speech starts
and ends as analog information. Along the way, however, smart antenna systems capture, convert, and
modulate analog signals for transmission as digital signals and reconvert them to analog information on the
other end. In adaptive antenna systems, this fundamental signal-processing capability is augmented by
advanced techniques (algorithms) that are applied to control operation in the presence of complicated
combinations of operating conditions. The benefit of maintaining a more focused and efficient use of the
system's power and spectrum allocation can be significant.