25-01-2012, 02:51 PM
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.
