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AN OVERVIEW OF SMART ANTENNA TECHNOLOGY FOR MOBILE COMMUNICATIONS SYSTEMS

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WHAT ARE SMART ANTENNAS?
Base station antennas have up till now been omnidirectional
or sectored. This can be regarded as a “waste” of power as
most of it will be radiated in other directions than toward the
user. In addition, the power radiated in other directions will
be experienced as interference by other users. The idea of
smart antennas is to use base station antenna patterns that are
not fixed, but adapt to the current radio conditions. This can
be visualized as the antenna directing a beam toward the communication
partner only. The difference between the fixed
and the smart antenna concept is illustrated in Fig. 1. Smart
antennas will lead to a much more efficient use of the power
and spectrum, increasing the useful received power as well as
reducing interference.


SHARING THE RADIO SPECTRUM
Traditionally, users communicating via the same base station
have been separated by frequency, as in FDMA (frequency
division multiple access); by time, as in TDMA (time division
multiple access); or by code, as in CDMA (code division multiple
access). Smart antennas add a new way of separating
users, namely by space, through SDMA (space
division multiple access). As will be explained later,
SDMA, which means that users in the same cell can
use the same physical communication channel, is the
final step in an evolutionary path toward increasingly
more advanced utilization


BASIC PRINCIPLES
What do we mean by the term “smart antenna?” The theory
behind smart antennas is not new. The technique has for
many years been used in electronic warfare (EWF) as a
countermeasure to electronic jamming. In military radar
systems similar techniques were already used during World
War II. There are in principle a number of ways in which
an adaptively adjustable antenna beam can be generated,
for instance by mechanically steered antennas.

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SMART ANTENNA TECHNOLOGY

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In mobile communication systems, capacity and performance are usually limited by two major impairments. They are multipath and co-channel interference [5]. 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.


Types of Smart Antenna Systems

There are basically two approaches [3], [4], [5], [7], [14], [15] to implement antennas that dynamically change their antenna pattern to mitigate interference and multipath affects while increasing coverage and range. They are
• Switched beam
• Adaptive Arrays
The Switched beam approach is simpler compared to the fully adaptive approach. It provides a considerable increase in network capacity when compared to traditional omnidirectional antenna systems or sector-based systems. In this approach, an antenna array generates overlapping beams that cover the surrounding area as shown in figure 4.1. When an incoming signal is detected, the base station determines the beam that is best aligned in the signal-of-interest direction and then switches to that beam to communicate with the user.


Switched Beam Systems

This type of adaptive technique actually does not steer or scan the beam in the direction of the desired signal. Switched beam employs an antenna array which radiates several overlapping fixed beams covering a designated angular area. It subdivides the sector into many narrow beams. Each beam can be treated as an individual sector serving an individual user or a group of users. Consider a traditional cellular area shown below in figure 4.3 that is divided into three sectors with 120° angular width, with each sector served by six directional narrow beams. The spatially separated directional beams leads to increase in the possible reuse of a frequency channel by reducing potential interference and also increases the range.


Butler Matrix Arrays

In this approach a Butler Matrix [5], [15] is used to provide the necessary phase shift for a linear antenna array. Abutler matrix can producebeams looking in different directions with an N-element array. A butler matrix requires an ( 90° hybrids interconnected by rows of fixed phase shifters to form the beam pattern. When a signal impinges upon the input port of the Butler Matrix, it produces a different inter-element phase shifts between the output ports. The set of different inter-element phase shifts is given by:


Blass Arrays

The Blass matrix uses directional couplers and transmission lines to provide the necessary phase shift for the arrays in order to produce multiple beams. Figure 4.7 shows an 8-element array fed by a Blass Matrix. Each node is the direction coupler to cross-connect the transmission lines. Port 0 provides equal delays to all elements and hence produces a broad side beam, whereas other ports provide progressive time delays between elements and hence produces beams at different angles. Therefore, when you send signal into the different inputs, you will get different steering angles. The Blass Matrix, is simple but has a low performance because its loss is attributed to the resistive terminations.