Aerospace and Electronic Systems Magazine March 2017 - 40
Feature Article:
DOI. No. 10.1109/MAES.2017.160113
Complementary Pair Radar Waveforms-Evaluating and
Mitigating Some Drawbacks
Nadav Levanon, Itzik Cohen, Pavel Itkin, Tel Aviv University, Tel Aviv, Israel
INTRODUCTION
Complementary pulse pair is a radar waveform that achieves the
ultimate range sidelobe reduction zero sidelobe. It is an early and
simple embodiment of radar waveform diversity (WD), presently
a popular topic. However, the use of complementary pulse waveforms is not widely spread because of several drawbacks. The
main problem is the sensitivity to Doppler shift. Usually the two
complementary coded pulses are separated in time. Doppler shift
causes a phase ramp as function of time. That ramp causes two
problems: (a) the two pulses in a pair are centered on different average phases; (b) there is a phase ramp during each pulse. Problem
(a) also known as slow-time mismatch, is handled by the pulse-topulse conventional Doppler processing, which provides slow-time
phase compensation. Problem (b) requires fast-time compensation,
not provided by a simple linear Doppler processor. It causes loss of
the ideal delay-sidelobe cancellation resulting in near range-sidelobes. Those near sidelobes increase with longer codes and with
higher Doppler shifts. At the same time a complementary pulse
pair also causes a difficulty at low Doppler shifts.
Moving target indication (MTI) is a radar processing approach
designed to help stationary pulse-Doppler radars to separate weak
reflections of moving targets from strong returns of stationary clutter. This task becomes more difficult at low Doppler (slow targets).
A classical MTI processor is constructed from a pulse canceller
followed by discrete Fourier transform (DFT). A pulse canceller
subtracts returns from consecutive pulses, assuming stationary
clutter returns are identical and will cancel out. This concept fails
if consecutive pulses are differently coded.
A very early version of MTI was used in the FPS-18 radar [1].
The receiver included a 3-pulse canceller followed by 8-pulse DFT.
The interpulse weighting was a raised cosine. Special measures
were added to circumvent the excessive attenuation of returns from
very low-Doppler targets, caused by the three-pulse canceller.
Progress in devices and signal processing [2,3] allows considerable improvements in: (a) Doppler resolution (e.g., by increasing
the coherent processing interval (CPI) by increasing the number of
Authors' current address: Dept. of Electrical Engineering-
Systems, Tel Aviv University, Tel Aviv 6997801, Israel. E-mail:
(nadav@eng.tau.ac.il).
Manuscript received May 11, 2016, revised August 8, 2016, and
ready for publication September 18, 2016.
Review handled by D. O'Hagan.
0885/8985/17/$26.00 © 2017 IEEE
40
pulses in the CPI, while maintaining the pulse repetition interval
(PRI); (b) range resolution (e.g., by pulse compression); (c) Doppler sidelobe reduction (e.g., by improved weighting windows); and
(d) range sidelobe reduction (e.g., by using mismatched filters).
Since complementary pairs are phase-coded they suffer from
high- and slow-decaying spectral sidelobes. There are several measures [4, sec. 6.8] to improve spectral efficiency of phase-coded
waveforms. When applied to complementary pairs they raise the
question of how well the zero range-sidelobes property is preserved.
This article considers the above issues, suggests mitigating
measures, and evaluates performances. The specific complementary Golay binary pair in this demonstration is the longest (L = 26
element) known binary sequence pair [5] that is not constructed
from shorter sequences. The phases of the pair are given by
φ1 = π 00011000101101010110010000
φ2 = π 00001001101000001011100111
The autocorrelation functions (ACF) of each coded pulse by
itself are shown in subplots (a) and (b) of Figure 1. Note the equal
magnitudes but opposite polarities at each delay, except at the origin. That fact is responsible for the sidelobes cancellation when the
sidelobes of the two correlations are added. Such addition happens
when a train of repeated complementary pulse pairs {s1 s2 s1 s2 s1 s2
s1 s2 ...} is cross-correlated with at least one reference pair. The resulting periodic cross-correlation, with a reference containing one
complementary pulse pair {s1 s2}, is shown in subplot (c) of Figure
1. Selected duty cycle of d = 0.2 resulted in a PRI five times longer
than the pulse duration, namely
Tr = t p d = Ltb d = 26tb 0.2 = 130tb
where tb is the duration of a code element (bit), L is the code length,
tp is the pulse duration, and Tr is the PRI.
The main property of a complementary pair is demonstrated in
Figure 1(c) by the zero near-sidelobes at 1 ≤ |τ/tb| ≤ L = 26. When
the delay equals the PRI, signal and reference pulses overlap again
but now the overlapping pulses are not matched. Signal pulse 1 is
aligned with reference pulse 2 and signal pulse 2 overlaps reference pulse 1. This results in the recurrent delay lobes at the delay
spans
(T
r
IEEE A&E SYSTEMS MAGAZINE
− tp
)
(
tb = (130 − 26 ) < τ tb < (130 + 26 ) = Tr + t p
)
tb
MARCH 2017
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