Improved algorithm
for
the deinterleaving
of
radar pulses
D.J.
Milojevic
B.M.
Popovik
lndexing
terms:
Algorithms, Radar and radio navigation, Signal processing
Abstract:
The paper presents an improved
method for the deinterleaving of radar signals,
based on a time of arrival analysis and the use of
the sequential difference histogram (SDIF) for
determining the pulse repetition interval (PRI).
The optimal detection threshold in the SDIF his-
togram is derived, which greatly contributes to the
efficiency of the algorithm. The algorithm is
applied to classic, frequency-agile and staggered
PRI radar signals. It is shown that the new
method is very successful in high-pulse-density
radar environments and for complex signal types.
Special attention is given to an application of this
method to the multiple-parameter deinterleaving
algorithm.
1
Introduction
Deinterleaving of radar pulses, as an important part of a
radar reconnaissance system, is a process of detection
and recognition of different, simultaneously active, radar
emitters. It assumes that received pulses have to
be
sorted, depending on their specific radar emitters.
Deinterleaving algorithms are based on the analysis of
various parameters of the received radar pulses, such as
time of arrival, angle of arrival, pulse amplitude, pulse
width and carrier frequency. The algorithm for deinter-
leaving presented in this paper belongs to the family of
so-called ‘interval-only’ algorithms. It uses only the
information about the time of arrival (TOA) for de-
interleaving.
The original version of this algorithm
[l]
is based on
the so-called CDIF (cumulative difference) histogram of
TOA pulses and is described in the following Section.
A
modified and improved version of this algorithm, based
on the new sequential difference histogram technique
(SDIF), is described here.
2
Basic algorithm description
The basic algorithm
[l]
leans on the CDIF histogram
analysis. The CDIF histogram is formed by using the
TOA differences; the calculated TOA difference between
adjacent pulses (distance
1)
is called the first difference.
The TOA difference between each pulse and the next
pulse but one (distance
2)
is the second difference, and
so
Paper 8413F
(E5,
ElS),
first received 2nd November 1990 and in revised
form
28th May 1991
The authors are with the Institute
of
Microwave Techniques and
Elec-
tronics, Bulevar Lenjina 165b, 11071 Novi Beogad, Yugoslavia
98
on. The CDIF histogram assumes the accumulation of
histogram values for each difference level. Potential
values of the pulse repetition interval (PRI) will corre-
spond to the histogram peaks.
For every TOA difference of level
c,
the CDIF histo-
gram is formed and the new threshold is defined (the
choice of the threshold will be discussed later). Each his-
togram value, and double that value, is compared to the
threshold, and if none of these couples exceeds the
threshold, the next TOA difference (of level
c
+
1)
and
the new cumulative histogram will be calculated.
If
the potential PRI is identified, the algorithm per-
forms the
sequence
search
[l],
i.e. it looks for a group of
pulses that form a periodical pulse train, with periods
equal to PRI. Such a group of pulses is known as a PRI
sequence. If the search is successful, the PRI sequence will
be
extracted from the input buffer and the new CDIF
histogram will be formed (starting from the first
difference). This process is repeated as long as there are
enough pulses in the input buffer to form any PRI
sequence. If there is no detection (i.e. pulses do not form
the PRI sequence), the next difference will be calculated.
If none of the histogram values exceeds the threshold, the
next difference will be calculated as well.
If
more than
one histogram value exceeds the threshold, the sequence
search is performed for every potential
PRI
value, start-
ing from the lowest. The technique presented in Refer-
ence
1
is less sensitive to interference and missing pulses
than conventional published techniques, such as the delta
z-histogram, which is formed of
all
TOA differences
[2].
Fig.
1
shows the four successive CDIF TOA histo-
grams of a signal which can represent either two inter-
leaved stable sequences of pulses with equal PRI, or a
two-level staggered signal with stagger frame rate equal
to the PRI. By this algorithm, a staggered signal of level
N
will be identified as
N
signals with a constant PRI
equal to the stagger frame rate
[l].
To extract the true
PRI value, the difference level must be increased to four.
The most significant drawback of the original algo-
rithm, in our opinion, concerns the high number of differ-
ence levels required even in very simple cases, such as
that in Fig.
1.
By requiring the second harmonics to be
present, the PRI calculation is limited to cases where the
sequences of three events occur, rather than only the
pairs. However, the same thing can be achieved in the
second part of the algorithm, in the sequence search,
using the appropriate conditions for the detection and
extraction of PRI sequences. Therefore, if we discard the
condition that the second harmonics have to be present,
the accumulation in the difference histograms is no
longer necessary. This was our main reason for using the
sequential difference histogram, which does not include
accumulation in the successive difference levels.
IEE
PROCEEDINGS-F,
Vol.
139, No. 1, FEBRUARY 1992