Results of Airborne PCL under CCI conditions usingDVB-T Illuminators of Opportunity

Philipp Wojaczek∗‡, Ashley Summers†, Diego Cristallini∗, Ingo Walterscheid∗, Pierfrancesco Lombardo‡,Fabiola Colone‡,

∗Fraunhofer Institute FHR, Wachtberg, Germany†Defence Science and Technology Group, Australia

‡University of Rome ”La Sapienza”, Italy

Abstract—Passive radar, or Passive Coherent Location (PCL),has reached a stage of maturity for stationary PCL receiversexploiting stationary transmitters of opportunity. The next logicalstep is to place PCL receivers on moving platforms. MountingPCL receivers on airborne platforms poses a challenge to thesystem designer. One issue that has received little attention in theliterature is that airborne PCL receivers can be more susceptibleto Cochannel Interference (CCI) than ground based PCL systemsdue to the receiver operating at higher altitudes (no terrainmasking), therefore potentially having a line-of-sight (LOS) tothe interfering transmitter, and atmospheric propagation.For efficient spectrum usage the carrier frequency for a cluster ofDigital Video Broadcasting–Terrestrial (DVB-T) transmitters inone Single-Frequency-Network (SFN) will be reused in a secondmore distant SFN. The distance between the two SFNs is designedsuch that the signals from each SFN will not self-interferefor ground based receivers. However for airborne receivers thecombination of higher operating altitude, propagation effects, andintentional suppression of the Direct Path Interference (DPI)can result in the broadcast signal from the more distant SFNinterfering with the signal from the intended SFN transmitter ofopportunity. The interfering signal can be regarded as widebandnoise jamming and if unaccounted for it can impede PCLperformance quite severely. To investigate the problem of CCI inthe case of airborne PCL, we conducted an airborne experimentto utilize DVB-T signals in a SFN.

I. INTRODUCTION

In recent years more effort was put in research on PassiveCoherent Location (PCL), thus improving its capabilities. Theapplication on stationary receiving platforms provides high-fidelity results such that in the future PCL could supportactive radar [1], [2]. This stage of maturity enables radardesigners now with the possibility of mounting PCL systemson moving receiving platforms. Some contributions were madeproving the capability of SAR-imaging using Digital VideoBroadcasting–Terrestrial (DVB-T) signals [3], [4], [5]. Alsofirst results of Ground Moving Target Indication (GMTI) fromairborne platforms were presented using FM radio as theIlluminator of Opportunity (IO) [6], [7]. In [8] a maritimereceiver was installed on a boat exploiting DVB-T signalsfor GMTI using Displaced Phase Center Antenna (DPCA) toremove the clutter Doppler spread. Overall one can say that theresearch of PCL on moving platforms is still at the beginning.The exploited signals for PCL are generally broadcast signals,e.g. DVB-T, DAB, or FM, or communication signals like WiFi,GSM or LTE. One of the widely used waveforms for PCL isDVB-T, due to its widespread use, its high transmit powerlevel, and bandwidth of 8 MHz, which provides acceptable

range resolutions of in between 20 m and 30 m depending onthe bistatic angle. Another advantage of DVB-T refers to thetransmission format: it is an Orthogonal Frequency-DivisionMultiplex (OFDM) waveform, and due to the digital modula-tion the transmitted signal can be recovered from the recordedsignal, if the Signal-to-Noise Ratio (SNR) is sufficiently high.Therefore there is no requirement for a dedicated referenceantenna to collect the transmitted signal. Furthermore there isthe advantage of using sophisticated approaches to suppresscharacteristics of the digital communication waveforms [8][9].These advantages make DVB-T a favourable waveform to beexploited by PCL receivers.A cluster of transmitters, e.g. DVB-T, spatially distributed ina closed region is usually referred to as Single-Frequency-Network (SFN). Each transmitter (Tx) from the SFN is trans-mitting the same content on the same frequency. Directly ad-jacent SFNs transmit on different frequencies, but availabilityof frequency bandwidth is limited, so a used frequency will bereused again in the next but one SFN. The reception of morethan one signal in the same frequency band from differentSFNs may dramatically impede the recovery of the waveformand can therefore be regarded as wideband noise jamming orCochannel Interference (CCI).Dependent on the received power of the CCI and the receivedpower of the signal from the IO, reference signal reconstructioncan be made impossible or at least impeded severely, thushindering target detection. It also degrades the quality ofSynthetic Aperture Radar (SAR) images based on PCL, whichis demonstrated in [10].In contrast to a ground-based PCL receiver, an airborne PCLreceiver will be affected more by CCI due to the higheroperating altitude. The higher operating altitude results in line-of-sight (LOS) to the interfering source, and - as the wavetravels in free space - there are no masking and shading effectsbecause of the terrain. The problem of CCI for airborne PCLusing Advanced Television Systems Committee (ATSC) wave-forms has previously been shown theoretically in simulations[11].This paper presents real-data results from a measurement cam-paign with a DVB-T–PCL receiver on an airborne platform.We justify the already published simulations on CCI [11] andpresent a preliminary approach to suppress the CCI.The document is structured as follows: In Sec. II we describethe test site of the measurement campaign and the receivingequipment. The results of the airborne measurements arepresented in Sec. III. In Sec. IV we show the preliminaryapproach of CCI suppression and its results. Finally in Sec. V

we draw our conclusions.

II. SCENARIO

A. Location of the measurement campaign

A measurement campaign site in the countryside of westernGermany was chosen. A number of SFNs covers the westernpart of Germany as well as France, Netherlands, Luxem-bourg, and Belgium. Regarding the digital television status,Germany is in the progress of soft-switching from DVB-Tto Digital Video Broadcasting–Terrestrial 2 (DVB-T2), whichmeans, DVB-T stations in densely populated and metropolitanareas were switched to DVB-T2 first, while stations servingthe countryside will follow later. Fig. 1 shows a map of themeasurement campaign site and the surrounding region withannotations to mark the exploited transmitter of opportunityand some surrounding SFNs. The SFNs are labeled with SFNA1, A2, and B1, where the letter indicates SFNs using commonfrequencies, and the cypher indicates a running number inorder to differentiate between distributed SFNs. In fact thereare more SFNs transmitting at the same or at other frequenciesaround the considered illuminator of opportunity, but in orderto show the principle and to keep Fig. 1 clear, only two otherSFNs are depicted in Fig. 1.Close to the center of Fig. 1 one Tx is marked with SFNA1 – IO, which is regarded as the transmitter-of-opportunitytransmitting DVB-T signals at carrier frequencies fC ={674, 690}MHz using as Quadrature Amplitude Modulation(QAM) the constellation of 16-QAM. This transmitter is calledEifel.A zoom into the region around Tx Eifel is provided inthe upper right corner of Fig. 1. The white arrows definetrajectories the airborne receiver was flying.A single DVB-T channel occupies a bandwidth of 8 MHz ofwhich it uses 7.61 MHz to transmit on 6817 subcarriers in the8k mode [12]. The SFN in the northern part of the map labeledwith SFN A2 is an SFN transmitting at the same frequenciesas SFN A1. It is located in and around the densely populatedarea of the city Dusseldorf.The SFN around the city Dusseldorf is transmitting a DVB-T2signal in either the extended 16k or 32k mode, which meansit occupies 13921 or 27841 carriers in the bandwidth of8 MHz [13]. By doing so it extends the occupied bandwidthby 0.16 MHz compared to the 8k mode of DVB-T.As the DVB-T2 SFNs transmit in extended mode we have thechance to differentiate between the DVB-T and the DVB-T2signals in frequency domain and therefore it gives us thepossibility to indicate the strength of CCI in UHF as longas the DVB-T2 is weaker than the DVB-T signal. But as themeasurement’s location is closer to the DVB-T Tx Eifel, theDVB-T signal should be the stronger one.

B. Description of the receiving system

The receiving system used in the airborne measurementconsisted of a front-end and a back-end. The back-end isa two channel Software-Defined-Radio (SDR) radar receiver,which is able to sample two receiving channels at a samplingfrequency fS = 64MHz with an effective bandwidth of32 MHz.The front-end consisted of two antennas. Each antenna was

Fig. 1. Map of the region with some annotated SFNs. The IO is markedwith SFN A1 - IO. A zoom into the region around the IO is shown in theupper right corner where the trajectories of the airborne receiver are labeledwith T1, T2, and T3. Image source: Google Earth.

connected to one of the two receive channels of the receivinghardware. For the antennas “discones” or so-called “spider el-ements” were chosen, which have an omnidirectional azimuthpattern. Furthermore an Inertial Measurement Unit (IMU)with Global Navigation Satellite System (GNSS) antennas wasincluded providing accurate platform position truth data. Thefront-end was mounted in a “pod”, which can be mountedbeneath the wing of an ultralight aircraft (ULA). The usageof an ULA limits the amount of additional weight that can becarried. And – as the pod is mounted outside of the ULA – thepod’s dimensions are very restricted as it’s shape influencesthe ULA’s aerodynamics and flight characteristics. The podtogether with equipment mounted inside of it is shown inFig. 2. It shows both antennas at the bottom, the IMU inthe center of the pod and Radar Absorbing Material (RAM)mounted on the opposite side, which was used to achieve asignal attenuation towards the IO in order to prevent damagingof the hardware equipment due to an expected strong directsignal from the IO. A strong direct signal was expected due tothe close range to the Tx; and the exact strength of the directsignal could not be determined due to the unknown tiltingof the transmit antennas. The RAM was not matched to theUHF wavelengths, but due to the restricted size of the pod,the space for mounting the equipment was limited. The limitedspace inside the pod is as well the reason for choosing disconeantennas: generally it is preferred to use directive antennaswith a higher gain, such as Yagi-Uda antennas. But theseantennas have usually greater dimensions than compared tothe discone antennas and would therefore be more difficult tofit into the pod. The equipment all together is mounted on aplate consisting of plastic material. The plate itself is mountedin the interior of the pod. In between the ground plate andthe pod a gap of approximately 5 cm was left in order to havespace for two GNSS antennas needed for the IMU. Fig. 2shows the pod upside-down, which means the pod was flippedaround the longitudinal axis to be mounted on the bottom ofthe airplane’s wing.The ULA with the pod mounted below the wing is shown inFig. 3.The receiver was flying on a circular-like path around the Txand recorded data when flying on trajectories marked by the

white arrows which are labeled with T1, T2, and T3 in Fig. 1.

Fig. 2. Image of the measurement equipment: The pod, two spider elements(at the bottom), IMU, and RAM to prevent hardware damaging from a strongdirect signal.

Fig. 3. The ULA with the pod mounted below the right wing.

III. RESULTS OF THE MEASUREMENTS

For the results shown in this section, the receiver wasflying along trajectory T2 (see Fig. 1) with an averagevelocity of 45 m/s at an average altitude of 1220 m abovesea level. In Fig. 4 the spectrum of one receiving channelfor this trajectory is shown. The DVB-T channels at carrierfrequencies fC = 674MHz and fC = 690MHz are thoseused by the IO Eifel.One particular detail is the appearance of other DVB-Tchannels at fC = 666MHz and fC = 698MHz. These aretransmitted from a neighbouring SFN.Fig. 5 shows an 8 MHz spectrum centered at the DVB-TfC = 690MHz, where the DVB-T channel occupies abandwidth of 7.61 MHz. This spectrum provides an exampleof CCI, which can be seen by a zoom into the frequenciesaround 686.2 MHz, see Fig. 6. Together with the DVB-Tchannel a DVB-T2 channel was recorded on the same carrierfrequency fC . The red line indicates the start of a DVB-Tspectrum, which is supposed to be at fS = fC − 1

2K−1TU

.Here in the analyzed case fS ≈ 686.2MHz for fC = 690MHz, K equals the number of subcarriers K = 6817, and TUrepresents the duration of the useful part of a DVB-T symbolTU = 896µs. It can be seen from the figure that the spectrum

Fig. 4. The 32 MHz wide recorded spectrum of trajectory T2.

Fig. 5. The spectrum at the carrier frequency fC = 690MHz.

is extended over the bandwidth of the DVB-T spectrum byapproximately 0.08 MHz down to 686.12 MHz, indicated bythe black line.The bandwidth extension of BWext = 0.08MHz can resulteither from the 16k extended carrier mode or from the 32kextended carrier mode of DVB-T2 [13]. This result shows aclear example of CCI received by an airborne passive radar.The DVB-T2 signal was most likely transmitted by the SFNlocated at the region of Dusseldorf, see Sec. II and Fig. 1,where it is labeled as SFN A2.The overlap of DVB-T and DVB-T2 can also be observed forthe spectra at fC = {666, 674, 682, 698}MHz.

IV. CO-CHANNEL INTERFERENCE SUPPRESSION

To demonstrate the impact of CCI on reference signalestimation, an initial attempt at estimating the reference signalwithout any prior CCI suppression was performed. The processof estimating the transmitted signal is described in detail in

Fig. 6. Zoom into the left border of the DVB-T channel at fC = 690MHz.The red line and the black line indicate the left end of the occupied spectrumby the DVB-T channel and the DVB-T2 channel respectively.

[14][15].The first estimation is done with the received signal Y (1)(f)from antenna 1. During the estimation of a transmitted symbolso-called constellation maps can be produced. In Fig. 7 theconstellation maps for four DVB-T symbols are shown. TxEifel uses 16-QAM, but this constellation format can not berecognized in Fig. 7. A range-Doppler map for a CoherentProcessing Interval (CPI) of 512 DVB-T symbols is shown inFig. 8. One can see the returns of strong clutter sources, butoverall the range-Doppler map is covered with a high noise.There are two possible reasons for this disturbance: one reasonis the already mentioned CCI. The other reason is the velocityof the receiver, which leads to Inter-Carrier Interference (ICI)[16]. Due to ICI the subcarriers in one OFDM symbol arenot orthogonal anymore, but the energy of one subcarrierspreads into the neighbouring subcarriers [17], which meansthe channel becomes fast fading. In [18] the performance ofDVB-T for mobile reception is addressed. It is described howthe required SNR increases with increasing Doppler. Here inthe measurements the SNR is not limited by the thermal noise,but by the CCI, which means the CCI needs to be suppressedfirst, before one can address the implications of the ICI. Moresophisticated approaches are necessary to circumvent the fastfading [19], but these are beyond the scope of this paper.To improve the reference signal estimation we show a first

approach to suppress the CCI. The received signal Y (n)(f) onantenna n = (1, 2) can be described in frequency domain as:

Y (n)(f) =H(n)(f)X(f) +H(n)I (f)XI(f) +N (n)(f) (1)

where X(f) defines the transmitted DVB-T signal from TxEifel and XI(f) defines the CCI source, i.e. the transmittedDVB-T2 signal. H(n)(f) and H

(n)I (f) define the channel

transfer function for the DVB-T signal and the interferingDVB-T2 signal respectively at antenna n. N (n)(f) definesAWGN.The transmission of DVB-T2 in extended bandwidth givesus the opportunity to suppress the DVB-T2 signal, usingthe subcarriers on frequencies occupied only by the DVB-T2

Fig. 7. Constellation map of four OFDM symbols before CCI suppression.The black crosses indicate the positions of the ideal constellation points.

Fig. 8. Range-Doppler map before CCI suppression.

signal. By formulating a minimization problem one can usethese subcarriers and both antennas to estimate suitable valuesto suppress the CCI.The domains and codomains of X(f) and XI(f) are definedin (2) and (3):

X(f) =

{6= 0, for f ∈ FE

= 0, for f /∈ FE(2)

XI(f) =

{6= 0, for f ∈ FI

= 0, for f /∈ FI(3)

where:

FBW =

[− 7168

2TUD

,7168

2TUD

], FE =

[−K − 1

2TUD

,K − 1

2TUD

](4)

FI =

[−KI − 1

2TUI

,KI − 1

2TUI

], FE ( FI ( FBW

K = 6817 and TUD= 896 · 10−6 s define the num-

ber of carriers and the duration of the useful symbolpart for a DVB-T symbol. KI = {13921, 27841} andTUI

= {1792, 3584} · 10−6 s define the number of carriersand the duration of the useful symbol part of the extendedbandwidth of a DVB-T2 symbol transmitted using the 16k and32k mode respectively. Equation (4) defines frequency sets:FBW defines the set of frequencies in the complete 8 MHzbandwidth. FE defines the set of frequencies in the bandwidthoccupied by the subcarriers from DVB-T. FI defines the set offrequencies occupied by DVB-T2, where FE and FI overlap atthe DVB-T subcarriers. The set of frequencies occupied fromDVB-T2 only is defined by: FT = FI \ FE .The minimization problem is then:

minimizez ∈C

∑fT∈FT

|Y (1)(fT )− zY (2)(fT )|2 (5)

where z is a complex number: z = r exp(jϕ). z is then appliedon Y (2)(fE) on the subcarriers at the frequencies fE ∈ FE

in order to suppress the interfering signal by calculating thedifference of Y (1)(fE) and zY (2)(fE):

Ym(fE) = Y (1)(fE)− zY (2)(fE) (6)

The described process was done for each OFDM symbolindividually.Figs. 9 and 10 show the spectrum at the left edge and right edgeof the DVB-T signal after the CCI suppression. The DVB-T2signal is reduced by approximately 10 dB when compared toFig. 6.Assuming that by using (5) and (6), the CCI in Ym(fE) issufficiently suppressed while the signal of opportunity X(fE)is preserved, an estimation of the transmitted DVB-T signalX(fE) can be achieved. Constellation maps of four estimatedDVB-T symbols acquired during the process of referencesignal estimation are shown in Fig. 11. Each symbol is stillnoisy, but the structure of the 16-QAM is now recognizable.A range-Doppler map of the data with suppressed CCI isshown in Fig. 12. The estimated reference signal was usedfor range-compression.Comparing Fig. 12 with Fig. 8 the improvement is visible dueto the overall reduced noise floor. A reduction of noise Nredin the exo-clutter region was calculated using:

Nred = 10 log 10

∑

fD∈FD

∑r∈Rg

|R(a)(fD, r)|2∑fD∈FD

∑r∈Rg

|R(b)(fD, r)|2

(7)

where FD are Doppler frequencies FD = [120, 195]Hz andRg is a set of bistatic ranges Rg = [750, 7060]m. R(a) andR(b) define the range-Doppler maps after and before CCIsuppression. The calculated value of Nred is approximately-9.6 dB.Suppressing the CCI allows for an improved estimate of thereference signal. The better the estimate of the referencesignal is the better are ambiguities and sidelobes of theexploited waveform removed [8]. This effect in combinationwith the suppression of the CCI – which is comparable toa wideband noise jamming – leads to the reduction in noisefloor and in conclusion to a better target detection performance.

Fig. 9. Left side of the spectrum of the DVB-T channel after CCI suppression.

Fig. 10. Right side of the spectrum of the DVB-T channel after CCIsuppression.

V. CONCLUSION

This paper has presented the results of a measurementcampaign for airborne PCL. We have shown first results andjustified the simulations on CCI and the assumptions of itssevereness made in [11]. The CCI means a severe impact onreference channel estimation in a way that the reference signalreconstruction is made impossible thus drastically reducing thepassive radar performance. We demonstrated this issue with anexample of the constellation map and a range-Doppler map. Asthe DVB-T signal – the IO – and the DVB-T2 signal – the CCI– did not overlap on some frequencies, we had the possibilityto suppress in a preliminary approach the interfering signal anddemonstrated the enhancement with an improved constellationmap and a range-Doppler map with decreased noise floor.Due to the limited space and weight provided by the use of anULA, it was not possible for us to use more than two antennasduring the measurements. This limitation also decreases theperformance of algorithms to suppress CCI, e.g. null-steeringtowards the direction of the interfering source.

Fig. 11. Constellation map of four OFDM symbols after CCI suppression.The black crosses indicate the positions of the ideal constellation points.

Fig. 12. Range-Doppler map after CCI suppression.

Nevertheless the next steps will analyze the CCI and its impactand focus on its mitigation.

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