Aerospace and Electronic Systems Magazine May 2017 - 46

Feature Article:

DOI. No. 10.1109/MAES.2017.150259

Potential Transmit Beamforming Schemes for Active
LPI Radars
Wen-Qin Wang, University of Electronic Science and Technology of China, Chengdu,
China

INTRODUCTION
Active radar using phased array has the ability to steer a high-gain
beam toward any desired direction, while nulling in undesired directions to cancel jammers or interferences. However, the highpower transmission signals are often highly visible to intercept
receivers; consequently, the radar may be detected and destroyed
[1]. It is, thus, necessary to develop low probability of intercept
(LPI) techniques to increase the system surveillance [2]. Note that
although LPI technology has been defined for nearly half a century
and used operationally for more than 20 years [3], which provides
the user a significant advantage over his adversary by making it
more difficult for a foe to detect an opponent, current LPI technologies concentrate on low observability against radars by reducing
the radar cross section (RCS). Different from these existing RCS
reduction methods against radar detection, this article discusses
potential transmit beamforming techniques for active LPI radars
to increase their survivability against undesired interceptors. However, active radar is not LPI by its nature, because the radar cannot
avoid illustrating the interceptor. The interceptor is passive, and
its location is probably unknown. The interceptor can have a good
antenna with significant receiving aperture and a low-noise amplifier. All of these factors can be more or less equal for the radar and
the interceptor. What is not equal is the large reduction of the radar
signal on the return path from the target [4].
As radar has the advantage of knowing the transmitted signal,
near optimum signal processing and a large signal-to-noise ratio
(SNR) can be achieved at the radar receiver. Although the interceptor
can also process the signal and improve the SNR, the lack of knowledge of the signal imposes an unavoidable loss. Frequency hopping,
orthogonal frequency division multiplexing, and random waveforms
have been proposed to reduce radar visibility and enhance the LPI
performance by decreasing the probability of detection and parameter identification at the interceptor [5]-[8]. We can classify them
Author's current address: University of Electronic Science and
Technology of China, School of Communication and Information Engineering, No. 2006, XiYuan DaDao, Hi-Tech District,
Chengdu, Sichuan 611731 China, E-mail: (wqwang@uestc.
edu.cn).
Manuscript received November 21, 2015, revised April 1, 2016,
June 14, 2016, and ready for publication June 14, 2016.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 IEEE
46

into three kinds [9]: 1) larger bandwidth waveforms with spread energy in the frequency domain; 2) higher duty cycle waveforms with
spread energy in the time domain; 3) broader antenna beam pattern
with spread energy in the spatial domain. Although the probability of
being detected in the sidelobe region can be significantly reduced by
a specific beamforming technique, the high-gain beam is still easily
detectable for high-sensitivity receivers [10], [11].
It is, thus, necessary to reduce the instantaneous transmit
peak power to possible interceptors. In fact, in conjunction with
waveform design, transmit antenna modifications (transmit beamforming) can also improve LPI performance. For instance, an antenna-hopping approach is proposed in [12] to improve the LPI
performance by using irregular scan patterns to reduce the susceptibility to unintended receivers to achieve LPI in the sidelobe
region with a specific beamforming technique, but the high-gain
scanning main beam is likely still detected. It is, thus, necessary to
reduce the instantaneous transmit peak power to undetectable for
possible interceptors. A broad transmit beam is adopted in [13] to
reduce the peak radiated power, but it needs simultaneous receive
beams, which for large arrays, significantly increase hardware
complexity.
This article introduces several potential transmit beamforming
(antenna modification) schemes, including spoiled transmit beamforming weights, range-dependent beamforming using a frequency
diverse array (FDA) antenna, and directional modulation (DM)
beamforming for active LPI radars. Note that the most important
difference of the FDA, as opposed to a phased array, is that the
former uses a small frequency increment across the elements. This
stepped frequency difference results in the beam-focusing direction changing as a function of the range, angle, time, and even the
frequency increments, which contrasts with the range-independent
transmit beam pattern in a phased array. The FDA was overviewed
in [14] as a range-dependent beam with potential applications for
LPI radars. Additionally, the DM technique used for secure communications [15] is also suggested for LPI applications. The potential transmit beamforming schemes for active LPI radars are introduced from a top-level system description, with a need for more
publications and research on this meaningful topic.

LPI RADAR USING SPOILED TRANSMIT BEAMFORMING
A novel LPI transmit beamforming scheme was proposed in [9], in
which the transmitter sends pulse signals by using a set of spoiled

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MAY 2017

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