1.ABSTRACT Smart antennas are a promising technology to increase the capacity of cellular systems. However, under severe channel conditions, the capacity gain may be small. This work presents a general methodology for the analysis of the effect of the power angular dispersion on the capacity gain when using smart antennas. The omni-directional case becomes particular case of the presented methodology. Finally, the maximum number of users is determined for a smart antenna system and for general antenna beam pattern. As the growing demand for mobile communications is constantly increasing, the need for better coverage, improved capacity, and higher transmission quality rises. Thus, a more efficient use of the radio spectrum is required. Smart antenna systems are capable of efficiently utilizing the radio spectrum and are a promise for an effective solution to the present wireless systems’ problems while achieving reliable and robust, high-speed, high-data-rate transmission. The purpose of this topic is to provide a broad view of the system aspects of smart antennas. In fact, smart
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1. ABSTRACT
Smart antennas are a promising technology to increase the capacity of
cellular systems. However, under severe channel conditions, the capacity gain may
be small. This work presents a general methodology for the analysis of the effect of
the power angular dispersion on the capacity gain when using smart antennas. The
omni-directional case becomes particular case of the presented methodology.
Finally, the maximum number of users is determined for a smart antenna system
and for general antenna beam pattern.
As the growing demand for mobile communications is constantly increasing,
the need for better coverage, improved capacity, and higher transmission quality
rises. Thus, a more efficient use of the radio spectrum is required. Smart antenna
systems are capable of efficiently utilizing the radio spectrum and are a promise for
an effective solution to the present wireless systems’ problems while achieving
reliable and robust, high-speed, high-data-rate transmission. The purpose of this
topic is to provide a broad view of the system aspects of smart antennas. In fact,
smart antenna systems comprise several critical areas such as individual antenna
array design, signal processing algorithms, space-time processing, wireless channel
modeling and coding, and network performance. In this topic an overview of smart
antenna concepts is included.
Aim of this contribution is to illustrate the state of the art of smart antenna
research from several perspectives. The bow is drawn from transmitter issues via
channel measurements and modeling, receiver signal processing, network aspects,
technological challenges towards first smart antenna applications and current status
of standardization. Moreover, some future prospects of different disciplines in
smart antenna research are given.
2. INTRODUCTION
Throughout the world, including the United States, there is significant
research and development on smart antennas for wireless systems. This is because
smart antennas have tremendous potential to enhance the performance of future
generation wireless systems as evidenced by the antennas’ recent deployment in
many systems.
In mobile communication systems, capacity and performance are usually
limited by two major impairments. They are multipath and co-channel interference.
Multipath is a condition which arises when a transmitted signal undergoes
reflection from various obstacles in the propagation environment. This gives rise to
multiple signals arriving from different directions. Since the multipath signals
follow different paths, they have different phases when they are arrive at the
receiver. The result is degradation in signal quality when they are combined at the
receiver due to the phase mismatch. Co-channel interference is the interference
between two signals that operate at the same frequency. In cellular communication
the interference is usually caused by a signal from a different cell occupying the
same frequency band.
Smart antenna is one of the most promising technologies that will enable a
higher capacity in wireless networks by effectively reducing multipath and co-
channel interference. This is achieved by focusing the radiation only in the desired
direction and adjusting itself to changing traffic conditions or signal environments.
Smart antennas employ a set of radiating elements arranged in the form of an array.
Smart antenna systems consist of multiple antenna elements at the transmitting and/or receiving side of the communication link, whose signals are processed adaptively in order to exploit the spatial dimension of the mobile radio channel. Depending on whether the processing is performed at the transmitter, receiver, or both ends of the communication link, the smart antenna technique is defined as
multiple-input single-output (MISO), single-input multiple-output (SIMO), or multiple-input multiple-output (MIMO).
The signals from these elements are combined to form a movable or
switchable beam pattern that follows the desired user. In a Smart antenna system
the arrays by themselves are not smart, it is the digital signal processing that makes
them smart. The process of combining the signals and then focusing the radiation
in a particular direction is often referred to as digital beam-forming.
The early smart antenna systems were designed for use in military
applications to suppress interfering or jamming signals from the enemy. Since
interference suppression was a feature in this system, this technology was
borrowed to apply to personal wireless communications where interference was
limiting the number of users that a network could handle. It is a major challenge to
apply smart antenna technology to personal wireless communications since the
traffic is denser. Also, the time available for complex computations is limited.
However, the advent of powerful, low-cost, digital processing components and the
development of software-based techniques have made smart antenna systems a
practical reality for cellular communications systems.
3. WHAT IS A SMART ANTENNA?Smart antennas are the antenna arrays with smart signal processing
algorithms used to identify spatial signature such as direction of arrival (DOA) of
the signal, and use it to calculate the beam-forming vectors, to track and locate the
antenna beam on the target (mobile-phone). In the context of smart antennas, the
term "antenna" has an extended meaning. The block diagram of Smart Antenna
System is shown below:
Figure 1: Block Diagram of Smart Antenna System It consists of a number of radiating elements, a combining/dividing network
and a control unit. The control unit can be called the smart antenna’s intelligence, normally realized using a DSP component. The processor controls feeder parameters of the antenna, based on several inputs, in order to optimize the communications link. This shows that smart antennas are more than just the antenna,” but rather a complete transceiver concept. One may wonder why it is necessary to invest time and money into such an idea, what was wrong with the current use of the cellular antennas?
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. In other words, such a system can automatically change
the directionality of its radiation patterns in response to its signal environment. One
should say that their smarts reside in their digital signal processing facilities. Smart
antenna not only combats multipath fading, but also suppresses interference
signals. It employs Diversity and Adaptive combining schemes. Smart Antenna
techniques have been considered mostly for the base stations so far because of high
system complexity and high power consumption. Recently, smart antenna
techniques have been applied to mobile stations or handsets.
The following are distinctions between the two major categories of smart antennas
regarding the choices in transmit strategy:
1. Switched beam-A finite number of fixed, predefined patterns or combining
strategies (sectors).
2. Adaptive array-An infinite number of patterns (scenario-based) that are
adjusted in real time.
3.1 Beam forming
Beam forming is the method used to create the radiation pattern
of the antenna array by adding constructively the phases of the signals
in the direction of the targets (mobile-phones) desired, and nulling the
pattern of the targets (mobile-phones) that are undesired/interfering
targets. This can be done with a simple FIR tapped delay line filter.
The weights of the FIR filter may also be changed adaptively, and
used to provide optimal beam forming, in the sense that it reduces the
MMSE between the desired and actual beam pattern formed. Typical
algorithms are the steepest descent, and LMS algorithms.