Impact of Synchronization on the Ambiguity Function shape for PBR based on DVB-T signals
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Impact of Synchronization on the Ambiguity
Function shape for PBR based on DVB-T signals
Diego Langellotti – (1st year doctoral student)
Abstract—In this paper the use of Digital Video Broadcasting false alarms. The level of these peaks cannot be reduced by
Terrestrial (DVB-T) signals for Passive Bistatic Radar (PBR) is increasing the integration time [9].
addressed. The impact of synchronization on the Ambiguity
Function (AF) shape is analyzed in term of Peak to Side Lobe Different techniques have been proposed in order to remove
Ratio (PSLR) specifically timing synchronization, frequency these unwanted peaks in the DVB-T signal CAF.
offset synchronization and frequency sampling offset In [9], a guard interval modification (blanking) in the
synchronization respectively. The performance, obtained with
reference signal is proposed to cope with guard interval peaks,
different approaches for the improvement of DVB-T AF, are
evaluated against a real data set collected by a PBR prototype
together with two complementary strategies to mitigate the
developed and fielded at the INFOCOM Dept. of the University peaks due to the pilots: the power equalization of pilot carriers
of Rome “La Sapienza”. in the reference signal, and the suppression of pilot carrier
components in the reference signal prior to correlation.
Keywords-component; DVB-T Syncrhonization, Passive Radar. In [10], firstly the guard interval blanking is applied to
remove guard interval peaks; then the pilots are modified
I. INTRODUCTION directly on the pilot carriers, in order to remove the pilot peaks
In recent years there has been a renewed interest in Passive in the CAF. This approach has two advantages: (i)
Bistatic Radar (PBR), using existing transmitters as computational load reduction and (ii) convenient realization
illuminators of opportunity to perform target detection and compared with the conventional filters and equalizers.
localization [1]. In particular, broadcast transmitters represent In [11], this is achieved by using a linear filter based on the
some of the most attractive choices for long range surveillance knowledge of the expected value of the DVB-T signal
application due to their excellent coverage. The most common Ambiguity Function (AF). This approach appears not to require
signals for PBR in use today are FM radio and UHF television both time and frequency synchronization while in the
broadcasts [2]-[4] as well as digital transmissions such as approaches proposed in [9] and [10] frame synchronization is
Digital Audio Broadcasting (DAB) and Digital Video intrinsically required as a necessary step in order to remove the
Broadcasting-Terrestrial (DVB-T) [5]-[7]. Currently, digital undesired peaks in the DVB-T signal CAF. In fact, in the
broadcasting are proliferating and rapidly replacing the approaches presented in [9]-[10], the result can be obtained
analogue counterparts. Specifically, with reference to television only after a suitable time and frequency synchronization to
broadcast, a number of countries have already switched or eliminate all the periodic structures of the DVB-T signal.
planned to switch to the DVB-T standard. These signals show
both excellent coverage and wider frequency bandwidth which In this paper we present a comparison between the
results in increased range resolution achievable. Following previously mentioned different techniques with respect to
these considerations, in this paper we focus our attention on synchronization errors, with particular reference to the Peak to
DVB-T signals and on the problems arising from their use as Side Lobe Ratio (PSLR) achievable on the ambiguity function.
opportunity waveform in PBR systems. The paper is organized as follows. Section II briefly
As well known, passive radar performs target detection in describes the DVB-T signal and the corresponding CAF. The
the time delay/Doppler frequency plane by evaluating the description of the different approaches for CAF control is
Cross-Ambiguity Function (CAF) between the reference signal presented in Section III. The technique for time and frequency
and the surveillance signal [2]. In addition to the desired synchronization is described in Section IV with focus on its
reflected target echo, several interferences might corrupt this effect on CAF PSLR. A performance comparison of the three
system: due to the uncontrollable nature of the exploited techniques for CAF improvement against real data is presented
waveform, the direct path interference (DPI) and the multipath in section V. Finally, our conclusions are drawn in Section VI.
reflections can mask the desired target signal, even in the
presence of a large delay/Doppler separation. II. DVB-T SIGNAL
Basically, the presence of specific features of the DVB-T The DVB-T signal is organized in frames [8]. Each frame
signal, as guard intervals and pilots [8], introduces a number of consists of 68 orthogonal frequency division multiplexing
undesired deterministic peaks in the CAF, which might yield (OFDM) symbols; four consecutive transmission frames
significant masking effect of the target signal and/or produce constitute a super frame. Each symbol is composed by a set of
1705 carriers in the 2k mode and 6817 carriers in the 8k mode.In addition to useful data, each symbol contains the pilots 0
intra-symbol inter-symbol
(divided into continual and scattered pilots) and the -5 peaks peaks
transmission parameter signalling (TPS) for receiver
- 10
synchronization and transmission parameter estimation. Pilots guard interval peak
and TPS are transmitted at given carriers inside each symbol, - 15
as shown in Fig. 1. The modulation of data and TPS is - 20
normalized, while the pilots (continual and scattered) are
| χ(τ ,0)|, (dB)
transmitted at boosted power (the average power EP=16/9). A - 25
cyclic prefix copying the last part the OFDM symbol (Guard - 30
Interval (GI)) is inserted to prevent possible inter-symbol - 35
interference (ISI) in OFDM. Main parameters of 2k and 8k
mode DVB-T signals for 8MHz channels are shown in Table I. - 40
- 45
TABLE I
MAIN PARAMETERS OF 2K AND 8K MODE DVB-T SIGNALS - 50
Parameter 2k mode 8k mode - 55
-0.2 0 0.2 0.4 0.6 0.8 1
Number of carriers 1705 6817 τ , (ms)
Duration TU 224 µs 896 µs
Guard interval duration TG (1/32) 7 µs 28 µs Figure II.2 - DVB-T signal autocorrelation function
Guard interval duration TG (1/16) 14 µs 56 µs
Guard interval duration TG (1/8) 28 µs 112 µs III. TECHNIQUES FOR CAF IMPROVEMENT
Guard interval duration TG (1/4) 56 µs 224 µs
Total bandwidth 7.61 MHz 7.61 MHz
As briefly described in the introduction of this paper,
different approaches have been proposed to improve the DVB-
T signal CAF by removing these undesired peaks. The
techniques proposed in [9] and [10] are summarized in the
block diagram sketched in Figure III.1.
REFERENCE TARGET
CHANNEL CHANNEL
DPI SUPPRESSION
Removing TIME SYNCHRONIZATION
Guard Interval
Peak GUARD INTERVAL BLANKING
Figure II.1 - DVB-T signal frame structure
Removing PILOTS PILOTS
The Ambiguity Function (AF) is defined as [12]: Pilot Peaks BLANKING EQUALIZING
2
+∞
χ (τ , f d ) = ∫ u(t )u (t + τ ) exp( j 2πf t )dt
2 * CAF1 CAF2
d (1)
−∞
CAF3
where u(t) is the DVB-T complex baseband signal, τ is the
time delay and fd is the Doppler frequency. Figure III.1 - Block diagram of the DVB-T signal CAF
As highlighted in [9] and [10], the AF of the DVB-T signal improvement technique proposed in [9] and [10]
shows the presence of one main peak and many side peaks. These approaches only exploit the knowledge of pilot
These (unwanted) peaks are generated by the introduction, in carrier positions inside the OFDM symbols for the removal of
the OFDM symbol, of the guard interval and pilot carriers. the unwanted deterministic peaks in the DVB-T signal. In
Specifically, in 2k mode, the peak generated by guard interval particular, continual pilots have fixed positions from symbol to
occurs at τ = 224 µs (896µs in 8k mode), while the peaks due symbol, while scattered pilot carriers are inserted into the same
to pilot carriers can be divided into two categories (Figure positions every four OFDM symbols (named as super-symbol).
II.2): (i) intra-symbol peaks (0 ≤ τ ≤ Ts=TU+TG), and (ii) inter- Therefore, time synchronization is firstly required as a
symbol peaks (τ > Ts). These peaks can mask the signal necessary operation.
reflected from targets and/or introduce false alarms. After the frame synchronization of the reference signal, the
guard interval blanking removes the guard interval peak. Then,
the intra-symbols and the inter-symbols peaks are removedthrough the pilot equalization and the pilot blanking,
respectively. However, these approaches intrinsically require
frame synchronization and two different procedures performed
in two parallel processing stages, while with the approach
proposed in [11] the unwanted peaks removal is performed by
processing the reference signal with a linear filter based on the
knowledge of the expected value of the DVB-T signal AF so
that time and frequency synchronization is not required. A
simplified block diagram of the approach proposed in [11] is Figure IV.1- OFDM receiver structure
sketched in Figure III.2
• PRE-FFT synchronization: in this case, since the guard
interval is the repetition of a section of useful data, a
REFERENCE TARGET
coarse estimate can be obtained by detecting this
CHANNEL CHANNEL repetition. However, a proper selection of the useful data
section has a significant impact on the performance of
all POST-FFT synchronization algorithms. It is therefore
DPI SUPPRESSION
highly desirable to achieve a good timing
synchronization.
AF-BASED FILTER
• POST-FFT synchronization: in this case, continual and /or
CAF scattered pilots in the DVB-T signal can be used for a
fine synchronization (POST-FFT training).
Moreover, symbol synchronization is closely related to frame
Figure III.2 - Block diagram of the DVB-T signal CAF synchronization that is implicitly available if time
improvement technique proposed in [11] synchronization has been performed.
In [11], after DPI suppression, the unwanted peaks removal
is performed by processing the reference signal with a linear A. PRE-FFT synchronization
filter based on the knowledge of the expected value of the In [13], the joint maximum likelihood (ML) symbol-time
DVB-T signal AF. We noticed that using an integration time (θ) and carrier-frequency (ε) estimator for OFDM system is
equal to an integer number of super-symbols, the expected presented. Assuming an observation window equal to 2TU+TG,
value of the AF has a periodic behaviour. Based on this as sketched in Figure IV.2, the position of the symbol within
property, an appropriate filter can be designed to remove all the observed window is unknown because the channel delay is
sidelobes of the AF. unknown to the receiver.
IV. TECHNIQUES FOR THE SYNCHRONIZATION
A well-known problem of OFDM is its vulnerability to
synchronization errors (see for example [13], [14] and [15]).
An OFDM receiver can extract the information needed for
synchronization in two ways as follows: Figure IV.2 - DVB-T OFDM symbol structure
• Before demodulation of the subcarriers, either from The symbol-time (θ) and carrier-frequency (ε) estimations
explicit training data or from the structure of the can be achieved by the maximization of the log-likelihood
OFDM signal. Since in DVB-T standard no function that can be written as
additional training data is foreseen, the use of the
guard interval for synchronization is of particular Λ(θ , ε ) = γ (θ ) cos(2πε + ∠γ (θ )) − ρ ⋅ Φ(θ ) (2)
interest. Unfortunately, the GI may naturally be with
subject to severe ISI so the performance of such
scheme depends on channel characteristics. 1 θi +NG −1
Φ(θi ) = ∑ s(k ) + s(k + NU )
2 2
(3)
• After demodulation of the subcarriers, the 2 k=θi
synchronization information can be obtained from θ i + N G −1
training symbols embedded in the regular data
format. This approach can significantly increase
γ (θ i ) = ∑θ s(k )s (k + N )
*
U (4)
k=
system acquisition time. i
(in (3) and (4) s(k) represents the considered DVB-T signal
and θi indicates a generic time sample in the observation
In [14]-[15], DVB-T synchronization can be achieved through window) and
the following steps, as sketched in Figure IV.1:ρ=
{
E s(k )s * (k + NU ) }
{ }{ }
(5)
E s (k ) E s(k + NU )
2 2
Equation (5) is the magnitude of the correlation coefficient
between s(k) and s(k+Nu). The first term in (2) is the weighted
magnitude of γ(θ); the generic element of γ(θ), (see (4)) is the
sum of NG consecutive products between pairs of samples
spaced of Nu samples. The weighting factor depends on the Figure IV.4 - OFDM symbol window drift
frequency offset ε. The Φ(θ) term is an energy term and does The OFDM symbol window drift can be considered a long-
not depend on the frequency offset. time effect so, in this case, it can be neglected ([14]). The
carrier and sampling frequency detectors are also based on
B. POST-FFT synchronization post-FFT temporal correlation. In essence, we can divide the
In this section, we consider the residual frequency offset set of continual pilots in two parts: the first one contains the
(∆f’) as the sum of two contributions: an integer carrier indices of continual pilots of the left half of the OFDM
frequency offset nI / TU that is a multiple of the subcarrier spectrum while, in the second one, there are the indices of the
spacing 1/TU, and a fractional carrier frequency offset ∆f’F / TU right half. A simple algorithm, in [14], takes into account the
being responsible for subcarrier misalignment and thus Inter- correlation of the two parts of the OFDM symbol separately
Channel Interference (ICI). and then evaluates the sampling error and carrier frequency
offset as follows :
∆f ' = ∆f ⋅ Tu = n I + ∆f ' F (6)
Notice that, due to PRE-FFT stage, the fractional carrier ~ (ϕ 2 ,l + ϕ1,l )
∆f R =
frequency offset (∆f’F) will not contain one or more multiples N (8)
of the sub-carrier spacing 1/TU. 2 ⋅ 2π 1 + G
NU
1) Integer carrier frequency offset ~ (ϕ 2,l − ϕ1,l ) 1
The value of the spectral shift nI can be found by exploiting ζ = ⋅
the continual pilots that are transmitted both boosted in power NG K (9)
2 ⋅ 2π 1 +
and modulated by time-invariant symbols. Specifically, similar N U 2
to the ML algorithm, by correlating FFT output samples of two with
consecutive OFDM symbols the maximum absolute value of
the correlation in (7) yields the integer carrier frequency
estimate n̂ I .
ϕ1,l = arg ∑
k∈C
s (k ) , ϕ 2,l = arg ∑ s ( k )
k∈C (10)
1, l 2 ,1
where subscript indices 1 and 2 denote left and right half
nˆ I = arg max ∑ s l (k )s l +1 (k + m ) (7) respectively. The estimates in (8) and (9) are then post-
m∈M
k ∈I processed by their own proportional integral tracking loops as
where l indicates the OFDM symbol, I indicates the set of in [14].
the continual pilots and M indicates the set of the considered
values for nI estimation. C. Effects of synchronization errors on the CAF
2) Fractional carrier frequency offset In this sub-section we evaluate the effects of
This error is due to sampling error, which results in two synchronization errors on the CAFs obtained with the
different effects; the former is the OFDM symbol window drift techniques described in [9], [10] and [11] in term of PSLR,
while the latter is the subcarrier symbol rotation (Figure IV.3 defined as the ratio between the amplitude of the main beam
and Figure IV.4 respectively). peak and the amplitude of the highest side lobe. PSLR against
time synchronization error, carrier frequency offset and
sampling frequency offset is drawn in Figure IV.5, Figure IV.6
and Figure IV.7, respectively. All the curves refer to simulated
DVB-T signals obtained according to 8k transmissions mode .
Same performance can be obtained with 2k transmission mode.
In Figure IV.5 notice that, as expected, the PSLR of the
CAF obtained with the approach described in [11] (linear filter)
is independent of time synchronization error. In contrast, the
time synchronization error causes a strong performance
degradation in the PSLR of CAF obtained with the approaches
described in [9] and [10]. Specifically, the performance
degradation starts for time synchronization error lower than
Figure IV.3 - Subcarrier symbol rotation TG, for each TG value. However, in the specific case of interest,ML estimator guarantees an error so that the PSLR is always
higher than 40 dB.
Referring to Figure IV.6, PSLR of the CAF obtained with
the approaches described in [9] and [10] rapidly decreases also
for low frequency offset values. Moreover, if frequency offset
is equal or higher than 1/TU (sub-carrier spacing), i.e. integer
carrier frequency offset nI is not zero, PSLR decreases to its
lower bound, represented by the PSLR of the DVB-T ACF. So
the ML estimator does not guarantee by itself good
synchronization performance and a POST-FFT synchronization
is hence required. With respect to sampling offset (see Figure
IV.7), CAF obtained with linear filter ([11]) shows a significant
performance degradation in term of PSLR. However, this
degradation can be neglected because, in the practical case of
interest, sampling error values are lower or equal to ± 1ppm
(ζ=10-6).
Figure IV.7 SLR against the sampling frequency error for the
different approaches of the CAF improvement
V. REAL DATA PERFORMANCE COMPARISON
Data used in this paper have been collected with a PBR
prototype developed and fielded at the INFOCOM Dept. of the
University of Rome “La Sapienza”, [16]-[17]. Direct signal
data has been used to evaluate the ACF, with sampling
frequency of 64/7 MHz. Signals from different television
channels have been collected.
In the following, we will consider the results obtained with
DVB-T signal collected on 25th June 2009. The carrier
frequency of acquired channel is 714 MHz and the
transmission mode is 8k, with guard interval duration equal to
Figure IV.5 - PSLR against time error for different approaches 28µs and useful part duration equal to 896µs.
for CAF improvement Figure V.1(a) and Figure V.1(b) show the DVB-T signal
AF obtained by combining equalization and blanking (see
[9]and [10]), and after the Linear Filter approach ([11])
respectively.
(a) (b)
Figure V.1 DVB-T signal AF after combining equalization and
blanking (a), and with Linear Filter (b)
As it is apparent the linear filter has the same performance
without splitting the processing in two parallel stages and
Figure IV.6 - PSLR against carrier frequency error for different without the synchronization steps, while Figure V.1(a) has
approaches for CAF improvement been obtained after symbol synchronization and TPS decodingfor frame synchronization. The results of the synchronization [3] Griffiths, H.D., Baker, C.J., “Passive coherent location radar systems.
algorithms are shown in Table II. Part 1: performance prediction” IEE Proc. on RSN, Vol. 152, No. 3, pp.
153-159, June 2005.
[4] Griffiths, H.D., and Long, B.A., “Television based bistatic radar”, IEE
TABLE II
Proc. F, Commun. Radar Signal Process, Vol. 133, No. 7, pp. 649 – 657,
SYNCHRONIZATION RESULTS
Dec. 1986
Value [5] D. Poullin, “Passive detection using digital broadcasters (DAB, DVB)
PRE-FFT delay estimate 0.6016 ms with COFM modulation”, IEE Proc. on RSN, Vol. 152, No. 3, pp. 143 –
152, June 2005
PRE-FFT frequency offset estimate 24 Hz
POST-FFT Integer frequency offset estimate 2.23 KHz [6] Coleman, C.; Yardley, H.; “Passive bistatic radar based on target
illuminations by digital audio broadcasting”, IET Radar, Sonar &
POST-FFT Sampling frequency offset estimate 2.15 10-6 Navigation, Volume 2, Issue 5, October 2008.
POST-FFT Fractional frequency offset estimate 0.05 Hz [7] P.E. Howland, “Target tracking using television-based bistatic radar”,
IEE Proc. on RSN, Vol. 146, No. 3, pp. 166 – 174, June 1999.
Notice that this signal has a strong frequency offset, as it is [8] European Telecommunications Standard Institute, “Digital video
broadcasting (DVB); framing structure, channel coding and modulation
apparent from integer frequency offset estimate. for digital terrestrial television”, EN 300 744, V1.1.2, 1997.
[9] R. Saini and M. Cherniakov, “DTV signal ambiguity function analysis
VI. CONCLUSIONS for radar application”, IEE Proc. on RSN, Vol. 152, No. 3, pp. 133 –
142, June 2005
[10] Z. Gao, R. Tao, Y. Ma, and T. Shao, “DVB-T Signal Cross-Ambiguity
In this paper different DVB-T signal CAF improvement Functions Improvement for Passive Radar”, Radar, 2006. CIE '06.
techniques have been compared in terms of synchronization International Conference on, pp.1-4, 16-19 Oct. 2006.
errors. In particular, the PSLR curves against different [11] C. Bongioanni, F. Colone, D. Langellotti, P. Lombardo and T.
synchronization errors have been evaluated. Bucciarelli, “A new Approach for DVB-T Cross-Ambiguity Fucntion
Evaluation,” EuRAD proceedings, 30-02 october 2009 Rome, Italy.
The considered algorithms have been tested against real [12] M.H. Skolnik, “Introduction to radar systems, third edition”, Boston,
data sets collected by a PBR prototype developed and fielded at MA: McGraw-Hill, 2001
the INFOCOM Dept. of the University of Rome “La [13] van de Beek, J.J.; Sandell, M.; Borjesson, P.O., "ML estimation of time
Sapienza”. and frequency offset in OFDM systems," Signal Processing, IEEE
Transactions on , vol.45, no.7, pp.1800-1805, Jul 1997.
The different DVB-T signal CAF improvement techniques [14] Speth, M.; Fechtel, S.; Fock, G.; Meyr, H., "Optimum receiver design
show the same performance but the Linear Filter approach is for OFDM-based broadband transmission .II. A case study,"
not sensitive to different synchronization errors therefore it Communications, IEEE Transactions on , vol.49, no.4, pp.571-578, Apr
represents the effective choice to obtain the CAF improvement. 2001
[15] Speth, M.; Fechtel, S.A.; Fock, G.; Meyr, H., "Optimum receiver design
for wireless broad-band systems using OFDM. I," Communications,
REFERENCES IEEE Transactions on , vol.47, no.11, pp.1668-1677, Nov 1999
[1] Special Issue on Passive Radar Systems – IEE Proceedings on Radar, [16] C. Bongioanni, F. Colone, S. Bernardini, L. Lelli, A. Stavolo, P.
Sonar and Navigation, June 2005, Vol. 152, Issue 3, pp. 106-223. Lombardo, “Passive radar prototypes for multifrequency target
detection”, Signal Proc. Symp. 2007, Jachranka (PL) 24-26 May 2007.
[2] P.E. Howland, D. Maksimiuk, G. Reitsma, “FM radio based bistatic
radar” IEE Proc. on RSN, Vol. 152, Issue 3, 3 June 2005, pp. 107-115. [17] P. Lombardo, F. Colone, C. Bongioanni, “PBR activity at INFOCOM:
adaptive processing techniques and experimental results”, 2008 IEEE
Radar Conference, Rome (Italy), 26-30 May 2008.You can also read
