The Development of Smart Antennas
Randy L. Haupt
Utah State Universitv
Electrical and Computer En ’ineering
4120 Old Main Hilf
Logan, UT 84322-4120
haupt8ieee.org
~
Introduction
A smart or adaptive antenna is an antenna that modifies it’s receive or transmit
characteristics in order to enhance the antenna’s performance. The performance
modification results from purposely modifying the signal cancellations that occur.
Smart antennas are important for reducing the deleterious effects of intentional
jamming signals, unintentional co-channel interference, and multipath. These
antennas were first known as sidelobe cancelers then adaptive antennas and more
recently smart antennas.
A smart antenna is composed of two or more antennas. The amplitude andor time
delay (phase) of the signals received by all the antennas are modified then
combined in such a manner as to improve reception of the desired signal. This
paper presents the development of smart antennas from sidelobe cancelers in the
1950’s until today.
Sidelobe Cancelers
The first type of smart antenna was the sidelobe canceler developed at GE in the
late 1950’s [1]. A sidelobe canceler has a high gain antenna for receiving the
desired signal accompanied by one or more small low gain, broad beam antennas
for sidelobe cancellation (Figure 1).
low gain
$TIi
feedback
output
Figure 1. Single Howells-Applebaumloop for a sidelobe canceler.
0-7803-7070-8/01/$10.00 02001 IEEE
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The outputs of the antennas are weighted and combined until the jamming signal
is eliminated. The general idea is that the low gain antenna amplifies the jamming
signal about the same as the desired signal and close to the amplification of the
sidelobe that the jammer is incident upon. Thus, when the low gain antenna signal
is subtracted from the high gain antenna signal, the desired signal remains close to
the same while the jamming signal is cancelled. The feedback process where the
weights were calculated was developed by Applebaum [2]. Combining several of
these loops together in an array antenna created the adaptive array.
Adaptive Antennas
The first adaptive antennas were arrays employing the sidelobe cancellation
technique. An error results when the received signal differs from the desired
signal
En = d,
- W'S,
(1.1)
where is &n the difference between the desired signal (d) and the weighted (w)
receive signal (s).The mean square error is given by
E [ E.'] = E [ d,'] - w'Sw
-2
4 E [w's]
(1.2)
where the signal covariance matrix is
s,sz
"'
S,SN
1
(1.3)
The optimal adaptive weights are found by taking the gradient of (1.2) with
respect to the weights and setting it to zero. The resulting least mean square
(LMS) optimal weights is given by the Weiner solution
w,,
= S-'s
Inverting the signal covariance matrix is not always easy. Thus, a myriad of
algorithms has been developed to avoid the matrix inversion. One technique is the
least mean square algorithm [3], and it is based on using the steepest descent
method to find the minimum of (1.2). The weights are updated iteratively and are
given by
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Adaptive Nulling Based on the Output Power
Adaptive nulling described in the previous section has a receiver at every element
in the array. This approach is a very expensive proposition and requires a method
that maintains calibration of all the receivers. A much simpler approach makes
use of conventional phased array architecture and varies the phase shifters and
attenuators and phase shifters to minimize the total output power of the array.
Phase only adaptive nulling has the least amount of hardware requirements of any
adaptive nulling approach [4].
antenna V
'
elements
BV
V
phase
shifters
amplitudf
weights
Figure 2. Diagram of an adaptive array using the total output power as the
feedback mechanism.
Digital Beamforming
The signal covariance matrix is easily formed when every element in the array has
a receiver. Ideally, placing an AD converter at each element in the array feeds a
digital signal to the computer where all the beamforming and beam steering is
done. Multiple beams are important in spatial discrimination of signals in a multisignal environment. Adaptively switching beams as well as placing nulls in
sidelobe becomes relatively easy with a digital beamformer. Unfortunately,
calibrating the hardware and developing the hardware necessary to do the
processing is difficult and expensive.
antenna
elements
AD converters
I
I
computer
Figure 3. Digital beamforming array.
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Biological Beamforming
Living things constantly adapt to their environments. Sometimes the time scale is
short, such as chameleons changing color. Other times the adaptation is over
along time scale, such as evolution. Several recent efforts have used neural
networks [SI and genetic algorithms [6] to perform adaptive beamforming. This
area of research is relatively new and shows some promise.
Smart Antennas
All this historical development leads to smart antennas. It's a new buzzword for
adaptive antennas. Since commercial communications technology is driving the
need for this type of antennas, there is a different niche from the military drive for
adaptive antennas. In any event, the explosion of wireless communications
applications necessitates the need for antennas that optimally adjust their
performance based upon the signal environment. References [7] through [9] are
good references to learn more about smart antennas.
Bibliography
[l] P.W. Howells, "Explorations in fixed and adaptive resolution at GE and
SURC," IEEE AP-S Trans., Vol. 24, No. 5, Sep 76,575-584.
[2] S. P. Applebaum, "Adaptive arrays,'' IEEE AP-S Trans., Vol. 24, No. 5, Sep
76,585-598.
[3] B. Widrow, P.E. Mantey, L.J. Griffiths, and B.B. Goode, "Adaptive antenna
systems," Proceedings of the IEEE, vol. 55, no. 12, Dec 67, 2143.
[4] C.A. Baird and G.G. Rassweiler, "Adaptive nulling using digitally controlled
phase-shifters," IEEE AP-S Trans., Vol. 24, No. 5, Sep 76, pp. 638-649.
[5] Southall, H.L., Simmers, J.A., and ODonnell, T.H., "Direction finding in
phased arrays with a neural network beamformer," IEEE AP-S Trans., Vol. 43,
No. 12, Dec 97, pp. 1369-1374.
[6] R. L. Haupt, "Phase-only adaptive nulling with genetic algorithms," IEEE
AP-S Trans., vol. 45, May 97.
[7] R. A. Monzingo and T. W. Miller, Introduction to Adaptive Antennas, New
York: Wiley, 1980.
[8] R.T. Compton, Jr., Adaptive Antennas Concepts and Performance, Englewood
Cliffs, NJ: Prentice Hall, 1988.
[9] T.S. Rappaport, Smart Antennas :Adaptive Arrays, Algorithms, & Wireless
Position Location, New York: Prentice-Hall, 1998.
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