Feasibility Analysis of Utilizing The 8k Mode' DVB-T Signal in Passive Radar Applications
Feasibility Analysis of Utilizing The 8k Mode' DVB-T Signal in Passive Radar Applications
Feasibility Analysis of Utilizing The 8k Mode' DVB-T Signal in Passive Radar Applications
KEYWORDS Abstract One non-cooperative illuminator recently considered for passive radar applications is the
Passive radar; DVB-T (Digital Video Broadcasting-Terrestrial) signal. The thumbtack ambiguity function of the DVB-
DVB; T signal, in addition to being stationary over time, makes such a signal a good candidate for such
Cross ambiguity function; applications. However, certain ambiguities in its ambiguity function necessitates certain issues to be
Processing gain; carefully considered when the DVB-T signal is to be utilized in these scenarios. Methods have been already
CAF side-peak. proposed to resolve them. In this paper, after studying the origins of these ambiguities, we propose special
processing schemes to reduce the complexity of the parts associated with resolving these ambiguities
efficiently. Then, the DVB-T’s Cross Ambiguity Function (CAF) processing gain is carefully studied, and it
is shown that its noisy nature results in a high processing gain and resolution. Finally, the detection range
of the DVB-T based passive radar is examined, besides simulations, to show the practical feasibility of this
signal for cases of passive coherent location.
© 2012 Sharif University of Technology. Production and hosting by Elsevier B.V.
Open access under CC BY-NC-ND license.
1. Introduction [6–9], satellite systems [10], and GSM [11]. New digital signals,
like the Digital Audio/Video Broadcast (DAB/DVB), are also
The first passive radar was built about 70 years ago after the excellent candidates [6,7]; as they are widely available, they
British radar experiment held at Daventry in February 1935 in can be easily decoded, and employ the Orthogonal Frequency
which Watson–Watt and Wilkins were able to detect a Heyford Division Multiplex (OFDM); a multicarrier transmission scheme
bomber from about 8 miles away by using the radio waves of based on channel equalization in the frequency domain, using
the BBC broadcasting signals [1]. Recently, this kind of radar the Fast Fourier Transform (FFT) [12]. In this paper, the goal is to
has once again attracted much attention due to its advantages investigate, in more detail, how much the DVB-T signal is proper
over active radar. Low-cost passive radar, which requires for passive radar applications and to provide some solutions to
no frequency allocation, is a good solution for increased improve its properties from a radar-application point of view.
surveillance at a lower cost [2]. In addition, its undetectability as In Section 2, we review the principles of passive radar
a covert radar significantly increases its importance in military and, subsequently, in Section 3, the standard DVB-T signal
applications. The feasibility of different kinds of opportunistic is introduced. In Section 4, the ambiguities of this signal
signal for passive radar application has been investigated are studied and methods to remove them are proposed. The
before, such as FM [3], analogue TV [4,5], DTV (Digital TV) processing gain and detection range of such signals are studied
in Sections 5 and 6. Finally, the conclusion is presented in
Section 7.
∗ Corresponding author.
E-mail addresses: radmard@ee.sharif.edu (M. Radmard),
bastanih@sharif.edu (M.H. Bastani), behnia@sharif.edu (F. Behnia),
2. Principles of passive radar
nayebi@sharif.edu (M.M. Nayebi).
Peer review under responsibility of Sharif University of Technology. In traditional radar systems, the target’s range is defined
by comparing the time of the transmitted and received pulses.
However, such information is not directly available in the case
of the passive radar receiver. Instead, two receivers are used:
one for receiving the signal directly from its main source,
1026-3098 © 2012 Sharif University of Technology. Production and hosting by Elsevier B.V. Open access under CC BY-NC-ND license.
doi:10.1016/j.scient.2012.01.012
1764 M. Radmard et al. / Scientia Iranica, Transactions D: Computer Science & Engineering and Electrical Engineering 19 (2012) 1763–1770
Mode ‘8k’-hierarchical
Figure 1: The passive radar structure. Guard interval (∆/TU ) 1/4
Duration of symbol part (TU ) 896 µs
without reflections from targets (reference channel), and the Duration of guard interval (∆) TU /4 (224 µs)
Modulation scheme 64-QAM α = 1
other for collecting reflections from targets in the environment
(target channel). Figure 1 depicts the overall structure of the PCL
(Passive Coherent Location) radar.
Detection is done through computation of the CAF (Cross 4. Ambiguity function
Ambiguity Function), computed according to Eq. (1). It is a
criterion of how much correlation exists between the reference We briefly showed in [8,15] how to remove the ambiguities
and the target signal. A given CAF’s peak in a range-Doppler of the DVB-T signal in order to make it feasible for detection
cell is representative of a target in that range and Doppler in the passive radar. Here, we study this topic with analytical
frequency. details.
The ambiguity function represented in Eq. (1) is directly
2
1 N associated with the relative position of the receiver, transmitter
−j 2Nπ ν n
|χ (τ , ν)| =
2
x(n)r (n − τ )e
∗
, (1) and target. This should be considered in designing, irrespective
N n=1 of the source of the signal used (e.g. FM, GSM, DVB, . . . ). Here,
where x[n] is the signal at the target channel, r [n] is the we are not to consider that, and we confine our analysis to the
reference signal, ν is the Doppler shift, τ is the sample shift and properties of the DVB signal. To do that, we introduce a function
N is the number of samples collected. called the Self Ambiguity Function (SAF):
It should be noted that the direct signal (also known as DPI
1 N
2
2π
(Direct Path Interference)) in the target antenna can cause the
|χ(τ , ν)|2 = r (n)r ∗ (n − τ )e−j N ν n , (2)
weak target echoes to be lost. This DPI can be efficiently emitted N n=1
by adaptive filters used in the passive radars.
where x[n] is replaced by the reference signal r [n].
3. DVB-T: the COFDM-based system for terrestrial television The DVB-T signal is simulated in MATLAB SIMULINK⃝ R
.
Among the different kinds, the specifications of the simulated
In November 1995, the Technical Module (TM) of the DVB-T signal are demonstrated in Table 1.
European DVB Project finalized what is called the ‘common 2k, Its spectrum, after passing through an AWGN channel, is
8k specification’ for such a standard [13]. depicted in Figure 2. The SNR at the output of the channel is
As DVB transmission is based on OFDM (Orthogonal 30 dB.
Frequency Division Multiplexing) signals, in ‘8k mode’, 8192 In simulations, in order to evaluate the SAF for the DVB-T
carriers are equally spaced in the frequency, each carrying signal, the integration interval was chosen equal to the period
either a data sample (6817 carriers for data) or pilots (scattered, of 4 symbols (or 4.48 ms), corresponding to a range of 672
continual and TPS (Transmission Parameter Signaling)) values km. Also, the function is evaluated at an interval of 4 kHz
defined by PRBS (Pseudo Random Binary Sequence) values [13]. Doppler shift, corresponding to a velocity of 800 m/s at a carrier
The scattered pilot frequencies differ from symbol to symbol frequency of 750 MHz. The resulting DVB-T SAF cross section is
while the continual pilot frequencies are constant during the shown in Figure 3. The main peak at zero can be seen with some
transmission. In spite of TPS, both scattered and continual are ambiguities. Also, note the thumbtack and resolvable peak in
transmitted at a boosted level. TPS carriers convey information, this figure. The reason for such behavior is the noisy nature of
such as guard interval length and scattered pilot frequencies the DVB-T signal.
in the current symbol. Figure 2 shows the DVB-T frame Subsequently, causes of these ambiguities will be analyzed
transmission. and means to remove them will be investigated.
As an OFDM transmission system, the multipath problem is
overcomed in DVB-T by introducing a proper guard interval. At 4.1. Ambiguity caused by the guard interval
the beginning of each symbol duration (TU ), a guard interval
(Tg ) is added as a copy of a fraction of the end of TU , so that The strong ambiguity at the delay equal to TU (in Figure 4
carriers will remain orthogonal at the receiver side (cyclic prefix at the 8192’th sample) is removed by setting the signal level at
transmission). the guard interval equal to zero in the reference channel [9].
M. Radmard et al. / Scientia Iranica, Transactions D: Computer Science & Engineering and Electrical Engineering 19 (2012) 1763–1770 1765
Figure 5: The zero delay cross-section of MSAF after guard interval deletion.
Figure 6: The zero Doppler cross-section of MSAF after filtering out continual Figure 8: The zero Doppler cross-section of MSAF for noise DVB-T after
pilots. choosing boosted level for pilot frequencies.
Figure 7: The zero Doppler cross-section of MSAF for noise DVB-T. Figure 9: The zero Doppler cross-section of MSAF for noise DVB-T after
choosing zero level for pilot frequencies.
However, ambiguities are produced when we choose a
boosted level for pilots and then apply them to the IFFT, In addition, it can be observed that although the continual
in the DVB modulator. In fact IFFT transports these samples ambiguities have been significantly reduced, they are not
to the frequency domain, and, as the pilots are at boosted completely removed (e.g. the one at τ = 10240). The reason
level, we have raised the spectrum level in particular (pilots’) is that the continual pilots, which are the cause of continual
frequencies. This disturbs the smoothness of the spectrum and ambiguities, are weakened, but still present. Note that the
proposed equalizer can be partitioned into two equalizers: one
ambiguities are resulted. To clarify this issue, consider the case
with constant taps to equalize continual pilots and one with
of white noise in which samples corresponding to the pilots
varying taps to equalize scattered pilots. But another problem
in the DVB-T signal are multiplied by 4/3, and then applied to
arises due to the fact that as we have filtered out continual
the modulator (including IFFT and guard interval insertion). The
pilots, we cannot equalize them.
resulting zero Doppler cross-section of the MSAF is depicted in
In [9], the use of two channels (SP1, SP2) is proposed to
Figure 7. solve the problem (for ‘2k mode’ DVB-T signal). But, such
Considering the aforementioned issues, we can conclude an approach results in other problems. There, the theoretic
that by filtering out these pilots, the ambiguities will not be discussion that we had, does not exist, and it is mentioned that
eliminated. By doing that, we have, in fact, set the points in the just by doing this, the ambiguities will be removed. But now,
spectrum to zero after they were at a boosted level. To show after we know the theoretic causes behind the problem, we
this, we have applied this change to the earlier white noise can go more in depth, reveal some mistakes in [15] and correct
example. The result is shown in Figure 8. them. Also, we will undertake some remarkable simplifications
Based on earlier discussions, the solution would be to to the receiver’s structure.
equalize the pilots so that their boosted level is compensated. In [9], ambiguities are divided into two categories: intra-
Here, we have equalized the DVB-T signal in scattered and symbol (ambiguities at τ ≤ Ts , where Ts is the symbol duration)
continual pilot frequencies, without filtering out continual and inter-symbol (ambiguities at τ > Ts ). It is then claimed
pilots. The zero Doppler cross-section of the MSAF (without that inter-symbol ambiguities can be removed by filtering out
filtering out continual pilots) is shown in Figure 9. Note that the all pilots (both scattered and continual) – at SP2 – and that intra-
remained ambiguities are removed. symbol ambiguities can be removed by equalizing all pilots – at
M. Radmard et al. / Scientia Iranica, Transactions D: Computer Science & Engineering and Electrical Engineering 19 (2012) 1763–1770 1767
6. Detection range
5. Processing gain
The well-known bistatic radar equation is used for analysis
The CAF’s processing gain can be evaluated from PG = of the detection range of the passive radar:
BTs [16,17]. It is the ratio of the main peak at zero to the mean
Pr Pt Gt 1 Gr λ2 1
level of the other noisy cells. But, increasing Ts (the integration = · σrcs · · · · L, (6)
time) leads to the migration of the target from its cell, causing Pn 4π r12 4π r22 4π kT0 BF
the reduction of PG [3], so that we get no more increase in PG where:
by increasing Ts . The relation of the DVB-T signal’s PG with Ts
(without considering the migration effect) is studied through Pr the received signal power,
simulations. The result is depicted in Figure 12. Pn the receiver noise power,
1768 M. Radmard et al. / Scientia Iranica, Transactions D: Computer Science & Engineering and Electrical Engineering 19 (2012) 1763–1770
Figure 12: Block diagram of the receiver’s part which removes ambiguities.
Parameter Value
Pt Gt 8 kW
σrcs 1 m2
Gr 0 dB
λ 0.4 m
k 1.38 × 10−23
T0 290 K
B 6 MHz
F 20 dB
L −5 dB
guard interval and boosted level pilots in the DVB-T signal. The
solution of the main ambiguity caused by the guard interval is
to set the reference signal level to zero in this interval, before
evaluating CAF. The ambiguities existing after this were divided
into two categories:
1. Continual ambiguities caused by continual pilots, which
were removed by filtering them out.
2. Ambiguities caused by transmitting the pilots at a boosted
level. To remove this kind of ambiguity, the equalization of
pilots (both scattered and continual) was introduced.
But, as the continual pilots have been filtered out at the
previous stage and their number is not considerable, the
complexity has been reduced by only equalizing the scattered
pilots. Another advantage of this signal is its stationary
over time. The coding (including MPEG2, Reed–Salomon) and
interleaving (inner and outer) schemes applied to the original
data, make the signal totally independent of the primary data.
Therefore, unlike some opportunistic signals applied in passive
Figure 15: SNR contours for our PCL setup.
radar (like FM), the DVB-T signal has an ambiguity function
independent of the data transmitted, which is very favorable
around the transmitter and receiver in the form of the well for radar applications.
known Ovals of Cassini (loci corresponding to r1 r2 = constant). Another concern about the use of this signal in PCL is the low
power of the DVB-T stations, which may result in low detection
ranges, a case which has been shown for the GSM signal in
6.1. Simulations previous research. But, after studying this, we could see that
targets at about 9 km can be detected with no serious problems
To study the DVB-T coverage in a more practical example, (although there are some problems, such as DPI cancellation,
we consider the city of Tehran, the capital of Iran. The DVB-T which are not confined to the DVB-T case). Another applicable
transmitting station in Tehran is located in ‘Jamaran’, northern and good idea is to use multiple DVB-T stations of a single
Tehran. We place our PCL receiver at Azadi Tower in western frequency network (SFN) to improve the detection range. But,
Tehran. The locations of the setup can be viewed in Figure 14. simultaneously, other problems will emerge, which is our goal
The distance between the receiver and the transmitter is about in future studies.
17 km.
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University of Technology (2006). in 1979, from Sharif University of Technology, Tehran, Iran, and Dipl.-Ing. and
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systems, part 2: waveform properties’’, IEE Proceedings-Radar, Sonar and Telecommunications (ENST), Paris, France, in 1981 and 1984, respectively.
Navigation, 152, pp. 160–168 (2005). He has been Assistant Professor with the Department of Electrical
[18] Saini, R., Cherniakov, M. and Lenive, V. ‘‘Direct path interference Engineering at Sharif University of Technology since 1984. His research
suppression in bistatic system: DTV based radar’’, Proceedings of the interests are stochastic signal processing, data fusion, and radar design.
International Radar Conference, pp. 309–314 (2003).
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FM radio-based passive radar’’, 8th International Conference on Signal Fereydun Behnia received his B.S., M.S. and Ph.D. degrees in Electrical
Processing, 1 (2006). Engineering and Communication Systems from Sharif University of Technology,
[20] Inggs, M., Paichard, Y. and Lange, G. ‘‘Passive coherent location system Tehran, Iran, where he has been Assistant Professor since 1996. He is engaged
planning tool’’, International Radar Conference-Surveillance for a Safer in the research and development of radar systems including communication
World, pp. 1–5 (2009). systems, signal processing, and communication circuits.
[21] Coleman, C. and Yardley, H. ‘‘Passive bistatic radar based on target
illuminations by digital audio broadcasting’’, IET Radar, Sonar & Navigation,
2(5), pp. 366–375 (2008). Mohammad Mahdi Nayebi was born in Iran in 1967. He received the B.S. and
M.S. degrees in Electrical Engineering (1st class honors) from Sharif University
of Technology, Tehran, Iran, in 1988 and 1990, respectively, and a Ph.D. degree
Mojtaba Radmard received B.S. and M.S. degrees in Electrical Engineering, in Electrical Engineering (1st class honor) from Tarbiat Modarres University,
and Communication Systems from Sharif University of Technology, Tehran, Tehran, Iran, in 1994.
Iran, where he is now pursuing his doctorate studies as a Ph.D. candidate. His He joined Sharif University of Technology in 1994, became Associate
research interests include MIMO communication systems, MIMO radar systems, Professor in 1998, and Professor in 2003. His main research interests are radar
passive coherent location, tracking, signal processing and speech processing. signal processing and detection theory.