IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012 901

Increasing Phased Arrays Resilience via PhotonicSensor Network FeedbackPietro Vinetti, Michele D’Urso, and Massimiliano Dispenza

Abstract—Photonic sensor networks are exploited to increase re-silience of large phased arrays for radar and space applications.The network is embedded in the phased array front end to contin-uously enable easy and real-time control of the system components.The photonic sensors are able to measure amplitude and phase ofthe actual electromagnetic field distribution on the array aperture.The photonic network provides an estimation of failure/calibra-tion errors and allows the minimization of the mismatch betweenthe actual pattern and the reference one. The feedback system dy-namically acts on the weights of the beamforming network (BFN),thanks to accurate and effective synthesis methods. Preliminaryresults concerning the overall system design, the possibility to inte-grate the photonic sensor inside the active radiating elements, andthe capability to accurately correct undesired failure are reported.

Index Terms—Convex optimizations, dynamic array beam-forming, nonuniform fast Fourier transform (NUFFT), phasedradar arrays, photonic sensors.

I. INTRODUCTION AND MOTIVATIONS

P HASED-ARRAY antennas [1], [2] and, particularly,active antenna front ends adopted for multifunctional

radars [2], [3] are high-cost, very complex, and electricallylarge systems. The overall performances are affected by theactual radiated beam, and a mismatch between the expectedand the actual aperture field distribution can lead to significantperformance degradation [4], [5]. When performance degrada-tion becomes unacceptable, system maintenance is mandatory.Obviously, leaving aside the related costs, maintenance impliestemporary interruption of normal service, of course undesiredin military surveillance and/or for air traffic control systems.Similar considerations apply for space application, where main-tenance operations can be unaffordable and/or impracticable.This letter proposes an innovative solution able to improveresiliency and robustness of array systems, enabling the capa-bility to dynamically adapt the system to possible failures. Theproposed method exploits an embedded noninvasive photonicsensors network, properly integrated in the radiating antennafront end, and an adaptive beamforming algorithm, embeddedin the array control unit.The sensor network is made of electrically small probes,

aimed at continuously measuring the actual field on the arrayaperture, thus providing an estimation of failure or calibration

Manuscript received June 07, 2012; revised July 03, 2012; accepted July 05,2012. Date of publication July 11, 2012; date of current versionAugust 16, 2012.The authors are with Selex Sistemi Integrati, Giugliano 80014, Italy (e-mail:

mdurso@selex-si.com).Color versions of one or more of the figures in this letter are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2012.2208092

Fig. 1. Proposed system architecture.

errors. Sensor data are properly processed and allow to deter-mine the actual status of the array front end (beam diagnosisphase). A second phase aimed at dynamically minimizing themismatch between the actual pattern and the reference one byacting on the weights of the beamforming network (BFN) isalso implemented, if required (beam correction phase).It worth noting that microwave photonics is a rapidly growing

interdisciplinary field (see [6] and references within) linkingmi-crowave and optical technologies. Currently, microwave pho-tonics is a vivid research area with practical applications, alsoto the cases of array antennas [6]–[10].Different from previous contributions, the photonic devices

proposed in this letter are organized into a sensor network, ableto detect the array failures, and integrated into a control systemable to mitigate pattern degradations. To the best of our knowl-edge, no previous contributions on the same topic and approachcan be found.This letter is organized as follows. In Section II, the system

architecture is introduced and discussed. Section III concernsthe experimental validation of the proposed systems and tools.The conclusion follows.

II. PROPOSED ARCHITECTURE

The system architecture is reported in Fig. 1. It is composedof two main subsystems: the photonic sensors network and BFNFeedback Control Unit. In the following, the mentioned subsys-tems will be introduced and discussed.

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902 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

A. BFN Feedback Control Unit

The BFN feedback control is the system component aimed atanalyzing the signals coming from the photonic sensor networkand at applying possible corrections to the BFN in order to mit-igate the effect of undesired failures and/or calibration errors.The feedback system can be tuned according to the consideredsystem, taking into account desired performances, robustness,resiliency, and, moreover, the overall costs.The Feedback Control Unit is made of two main blocks. The

first one, the beam diagnosis block, generates the optical signalsto interrogate the photonic sensors network embedded in arrayfront end and to estimate calibration mismatches or componentsfailures. The beam diagnosis block is the system block devotedto determine the status of the antenna front end from the interro-gation of the photonic sensors network. In particular, it polls theintegrated photonic network with optical carriers and analyzesthe reflected amplitude signals to retrieve the field distributionover the aperture. According to required correction strategies(see in the following), different levels of complexity can char-acterize the logic of the block. Diagnosis logics can concern di-rect comparisons between estimated field distribution with theexpected one (through binary marks as OK/KO). More sophis-ticated logics can be adopted, based on specific test sequencesaimed at retrieving information about the type/features of theanomalies, such as calibration errors, hardware component fail-ures, impedance mismatch, etc. The beam diagnosis block pro-vides a synoptic picture of the status of the phased array ele-ments, which becomes an input for the second important block,the beam correction block.The beam correction block (see Fig. 1) addresses the mit-

igation of the retrieved failures/calibration errors. It containseffective algorithms able to synthesize the required patternsby properly mitigating the undesired effects of failures, asderived from the previous block. The beam correction blockaccomplishes a constrained BFN synthesis. Constraints arerepresented by a binary mask that excludes element failuresfrom the synthesis, or places bounds on the complex weights.This problem can be solved by adopting Convex Programmingalgorithms [11], [12]. For any fixed-array geometry, the weightdetermination for given upper bounds of the radiation (power)pattern away from the main beam direction can be solvedwithout global optimizations, also with arbitrary (but convex)constraints [13]–[15]. Note that, thanks to their convex nature,null constraints on the weight corresponding to failures elementcan be easily inserted in the adopted synthesis algorithm,particularly fast and effective thanks to a joint use of convexprogramming algorithms and nonuniform fast Fourier trans-forms (NUFFTs), the latter able to efficiently evaluate the arrayfactor in case of arbitrary lattices.

B. Photonic Sensors Networks

The photonic sensors network of the beam diagnosis blockis based on electrooptic techniques for field sensing [16]–[18].These techniques have been adopted in several applica-tions [19]–[23]. Interesting results are given in [23], whereinan integrated electrooptic modulator has been designed and ex-perimentally used for very near-field antenna characterization.

Fig. 2. Electrooptical sensing of electromagentic field values. (a) Referencescheme. (b) Electooptical amplitude modulation. (c) Schematic of the photonicsensor with proposed real-time antenna beamforming by photonic feedbacksensor network.

The electrooptic (EO) sensing scheme adopted in this letteris shown in Fig. 2(a), while Fig. 2(b) shows the amplitude mod-ulation of the optical carrier. The photonic network is based onan electrically small, dielectric electromagnetic field probe, em-bedded into the array element. The probe is made of a LiNbOcrystal substrate, wherein an optical circuit has been properlyintegrated to take advantage from the crystal’s EO effect inorder to modulate an optical carrier travelling though it [16].A Mach–Zehnder interferometer [16], [23] has been realized inthe sensor optical circuit, according to [16] and [23].Since such a device has to be integrated into the antenna ele-

ment, its design cannot be generalized to any case and shall beoptimized taking into account the specific features of the arrayelement. Indeed, the actual electromagnetic field distribution in-side the element, its physical properties and, possibly, the manu-facturing procedure of the radiating element impose limitationsto size, positioning, and installation approach of the photonicdevice. Therefore, the final photonic sensor layout and its op-timal placement into the radiating element come from a com-prehensive analysis, considering both the sensor and the arrayradiating element.A numerical analysis aimed to estimate the invasiveness and

the sensitivity of the sensor has been performed. In particular,field distribution inside the radiating element, perturbation in-troduced by the dielectric sensor, and the electromagnetic (EM)couplingwith the active region of the sensor have been analyzed.Numerical results have been exploited as input to an analyticalanalysis aimed at estimating the sensitivity of the sensor in orderto tune the sensor layout and to determine the optimal positioninginside the radiating elements. The physical dimensions of the de-signed and realized probe are 1 mm (width) 3 mm (length),which make it electrically small when compared to free-spacewavelength at - -, and -band.As a last consideration to the system architecture of Fig. 1,

note that a wavelength division multiplexing has been alsoadded. As each sensor needs a linking optical fiber to be in-terrogated, the integration of the sensor network can requirean equivalent number (very large for active phased arrays)of optical fibers. This can be a challenging task, especiallywhen dealing with a high number of sensors. To simplify theintegration, a wavelength division multiplexing scheme isproposed, as shown in Fig. 1: This solution allows a significant

VINETTI et al.: INCREASING PHASED ARRAYS RESILIENCE VIA PHOTONIC SENSOR NETWORK FEEDBACK 903

Fig. 3. Active module with the integrated photonic sensor under the metallicridge. (a) Overall view, with the arrow indicating the region where the sensorhas been deployed. (b) Detail picture of the deployment region, with the arrowindicating sensor.

reduction of the complexity of the integration. In this case, thenumber of fibers before the concentration stage (multiplexing)can be significantly reduced. Obviously, wavelength divisionmultiplexing imposes that each photonic sensor is tuned on aspecific wavelength.

III. EXPERIMENTAL VALIDATION

The diagnostic/correction proposed system has been val-idating in two steps. The first one is aimed at validating theintegration of the photonic network into the active radiatingelement of the considered radar system. Note that in the consid-ered system, the radiating element is integrated in the radiatingelement of a transmit/receive module (TRM) of a commercial-band radar system, the active radiating element in the fol-

lowing. The photonic sensor has been attached longitudinallyunder the metallic central ridge of the active radiating element,together with its (white) connecting fiber. Fig. 3 shows picturesof the integration of the photonic devices in the active radiatingmodule: Fig. 3(a) gives an overall view of the integration of thesensor (arrow), while Fig. 3(b) magnify the sensor deploymentregion (arrow).The active element, with the embedded photonic devices, has

been tested with the standard procedure, required to qualify thefabricated TRM module before the integration into the radarsystem. The test procedure involves several tests aimed at de-termining, for all the frequencies, the overall behavior of themodule from the circuital/radiating/system point of view. Thisletter only shows the most significant and representative resultsof the experimental validation. In particular, Fig. 4 shows thecomparison of VSWRs between the standard module and themodified one, i.e., with the embedded photonic sensor. We havereported the VSWR behavior, measured at the RF feeding portof the TRMmodule, when it has been set for receiving (Rx) andfor transmitting (Tx), respectively. Indeed, according to the spe-

Fig. 4. (a) Comparison of VSWR of TRM in Rx mode versus frequency: Eachcolored curve corresponds to a specific configuration of the receiving TRM.(b) Comparison of VSWR of TRM in Tx mode versus frequency: Each coloredcurve corresponds to a specific configuration of the receiving TRM.

Fig. 5. Radiated field with and without the photonic sensor: (top) horizontalcut (upper plot) and (bottom) vertical cut.

cific operating mode (Tx/Rx), the internal switches of the TRMselect a different RF chain. Each curve in the figures representsthe measured (output) VSWR against frequency, according tothe possible configurations of the TRM (phase shift and ampli-fication/attenuation).In Fig. 5, the experimental characterization of the radiating

behavior of the standard and modified TRM is provided.

904 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 6. (color online) Pattern comparison in U-cut: reference (blue), uncor-rected (black), and corrected pattern (cyan). Mask specification (red dash-dot).

Fig. 7. (color online) Pattern comparison (V-cut): reference (blue), uncor-rected (black), and corrected pattern (cyan line). Mask specification (reddash-dot).

As can be seen, the presence of the phonic device does notaffect at all the active module behavior, thus confirming thepossibility to adopt the proposed correction/diagnosis system inreal-world applications.The second step is aimed at evaluating the effectiveness of

the proposed system in mitigating the effect of failures. To thisaim, a failure has been introduced into the radar system. In par-ticular, the failure has been simulated by switching off all theactive modules belonging to a specific array row. From the prac-tical point of view, a number nearly equal to 3.2% of the overallnumber of elements has been assumed to be in failure. The arraydiagram in presence of failures is shown in Figs. 6 and 7, wherethe compensated one, i.e., the array factor determined by ap-plying the synthesis procedure described above, is also reported.Figs. 6 and 7 also show the pattern mask specifications.As it can be seen, the array failures significantly affect the

radiating behavior of the system. The proposed system allowscorrecting the performance degradation, thus recovering a radi-ated pattern matching with the specification mask.

IV. CONCLUSION

An innovative system based on a proper designed photonicsensors network embedded in the antenna front end has been

proposed for on-site diagnosis and correction strategies. Thesystem has been effectively integrated into TRMs of a real-world radar manufactured by SELEX Sistemi Integrati S.p.A.

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