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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 19 — Sep. 23, 2013
  • pp: 22905–22910
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Photonic generation and independent steering of multiple RF signals for software defined radars

Paolo Ghelfi, Francesco Laghezza, Filippo Scotti, Giovanni Serafino, Sergio Pinna, and Antonella Bogoni  »View Author Affiliations


Optics Express, Vol. 21, Issue 19, pp. 22905-22910 (2013)
http://dx.doi.org/10.1364/OE.21.022905


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Abstract

As the improvement of radar systems claims for digital approaches, photonics is becoming a solution for software defined high frequency and high stability signal generation. We report on our recent activities on the photonic generation of flexible wideband RF signals, extending the proposed architecture to the independent optical beamforming of multiple signals. The scheme has been tested generating two wideband signals at 10GHz and 40GHz, and controlling their independent delays at two antenna elements. Thanks to the multiple functionalities, the proposed scheme allows to improve the effectiveness of the photonic approach, reducing its cost and allowing flexibility, extremely wide bandwidth, and high stability.

© 2013 OSA

1. Introduction

The rapid progress of digital processing is improving the functionalities of the electronic systems that permeate and serve our lives. In this run to digital, radar systems are no exception, and several new radar concepts have been recently imagined as the software defined radar (SDR), the multi-function radar, the arbitrary beamforming, or the shared aperture radar: all these novel system paradigms in the radar field would require to flexibly control the generation of one or more radio-frequency (RF) signals, and this would be conveniently managed in a digital fashion. In order to apply the digital trend to RF signals, high-speed digital electronics must be developed to surpass the need for the old (and hardly flexible) analog electronics. But as the carrier frequency increases, the task becomes more and more challenging, and nowadays direct digital synthesizers can only range up to few GHz.

In this paper we report on our recent activities on the photonic generation of flexible wideband RF signals, focused (but not limited) to radar applications, and we extend the proposed architecture to the independent beamforming of multiple signals.

2. Flexible photonics-based generation of wideband RF signals

Coherent radars are requiring RF carriers with lower phase noise for improved sensitivity, and with higher frequency for smaller antennas. A modulation or coding of the phase of the radar pulse carrier is also necessary to implement pulse compression techniques for increased resolution without dangerous transmitted peak power [1

1. M. L. Skolnik, Introduction to Radar Systems, 2nd Ed. (McGraw-Hill, 1980).

]. Moreover, the generation of multiple signals is sought for frequency agility and for multiband multifunctional radars [2

2. V. Ravenni, “Performance evaluations of frequency diversity radar system,” Proceedings of European Microwave Conference 2007, 1715–1718 (2007).

]. For high frequency carriers, the usual frequency multiplication processes reduce the phase stability of the original RF oscillators, and frequency diversity radars are commonly realized by using two or more radar transmitters, with increasing cost and power consumption [3

3. J. J. Zhang and A. Papandreou-Suppappola, “MIMO radar with frequency diversity,” Proceedings of Waveform Diversity and Design Conference 2009, 208–212 (2009). [CrossRef]

].

In the last years, photonics has been suggested to effectively generate low phase-noise RF carriers, in particular at high frequency. Among other techniques [4

4. J. Sun, Y. Dai, X. Chen, Y. Zhang, and S. Xie, “Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation,” IEEE Photon. Technol. Lett. 18(24), 2587–2589 (2006). [CrossRef]

,5

5. L. Goldberg, R. D. Esman, and K. J. Williams, “Generation and control of microwave signals by optical techniques,” IEE Proc.-J. 139(4), 288–295 (1992). [CrossRef]

], the heterodyning of modes from a mode locking laser (MLL) has proven to generate low phase noise RF carriers up to the EHF band (30-300GHz) [6

6. G. Serafino, P. Ghelfi, G. E. Villanueva, J. Palaci, P. Pérez-Millán, J. L. Cruz, and A. Bogoni, “Phase and amplitude stability of EHF-band radar carriers generated from an active mode-locked laser,” J. Lightwave Technol. 29(23), 3551–3559 (2011). [CrossRef]

]. Wideband modulation and coding can also be applied in the photonic domain, avoiding to recur to frequency-specific electronic devices [7

7. J. Chou, Y. Han, and B. Jalali, “Adaptive RF-photonic arbitrary waveform generator,” IEEE Photon. Technol. Lett. 15(4), 581–583 (2003). [CrossRef]

11

11. T. Yilmaz, C. M. DePriest, T. Turpin, J. H. Abeles, and P. J. Delfyett Jr., “Toward a photonic arbitrary waveform generator using a modelocked external cavity semiconductor laser,” IEEE Photon. Technol. Lett. 14(11), 1608–1610 (2002). [CrossRef]

]. But most of these modulation schemes exploit optical interferometric structures which are hardly suitable for coherent radars with demanding frequency agility.

In [12

12. P. Ghelfi, F. Scotti, F. Laghezza, and A. Bogoni, “Phase coding of RF pulses in photonics-aided frequency-agile coherent radar systems,” IEEE J. Quantum Electron. 48(9), 1151–1157 (2012). [CrossRef]

] we have proposed a technique for optically generating multiple phase-coded RF signals with flexible carrier frequencies and with a phase stability suitable for coherent radar systems. The proposed scheme, as sketched in Fig. 1
Fig. 1 Photonic generation of RF signals by modulating a MLL at intermediate frequency fI.
, modulates the spectrum of a MLL at intermediate frequency (IF), so that phase- and amplitude-modulated RF signals at any carrier frequency can be obtained by mixing a modulation sideband and a MLL mode in a photodiode (PD), thus realizing a stable photonic up-conversion. Therefore in principle a precise optical filtering device selecting only one sideband and one MLL mode would be necessary, but the filtering task in this case is easier and more efficient if realized in the RF domain. Thus, after the PD, an RF filter centered at the desired frequency allows to select the RF signal. The use of a precise direct digital synthesizer (DDS) for generating the coding signals at variable IF allows the implementation of a software-defined radar transmitter without losing the original phase stability of the MLL, matching the requirements of demanding surveillance systems. The scheme has been tested generating some of the most common pulse-compression techniques used in radar applications. As an example, Fig. 2
Fig. 2 Frequency and amplitude transients of linearly chirped signals. a) Signal generated at 9.95GHz. b) Signal generated at 39.8GHz.
reports the results obtained by applying a 25MHz linear chirp to RF pulses at around 10GHz and 40GHz.

With the reported scheme, frequency agility can be implemented with a single MLL instead of a series of electronic oscillators. The proposed architecture allows to generate the desired RF signals either simultaneously or alternately, or even to continuously change their frequency by opportunely programming the DDS, provided a tunable RF filtering is available. The modulation as well can be changed meanwhile, thus implementing a waveform diversity technique. Besides its application to coherent radars, the proposed method can be helpful wherever phase- or amplitude-modulated RF signals are needed, as for example in the removal of the range ambiguity in radars, or in radio-over-fiber systems for the generation of complex modulation formats [13

13. P. Ghelfi and A. Bogoni, “Design of flexible photonics-based RF transmitter and receiver for future mobile networks,” Proceedings of CODEC 2012, Kolkata (2012). [CrossRef]

].

3. Extension to flexible optical beamforming

The photonics-based RF generation method described above can be conveniently exploited also to manage the beamforming of multiple RF signals in phased array antennas (PAAs).

PAAs allow to steer the transmitted RF beam without physically moving the antenna, and are used in an increasing number of applications such as multifunctional radars, electronic warfare, and communications. Common PAAs use electronic phase shifters at each antenna element to control the viewing angle of the array, but when steering broadband signals this approach suffers the squint phenomenon which causes different frequencies of the signal spectrum to aim at a different angle. This can be avoided if the phase shifters are substituted by true-time delays (TTDs). The approach based on TTDs is actually implemented in high-performance applications and requires complicated signal processing techniques.

The TTD functionality can be easily implemented exploiting photonics, thanks to its capability of realizing controllable delays with wide bandwidth, and with the additional advantages of electro-magnetic interference (EMI) insensitivity. Optical tunable TTDs have been demonstrated through optical path switching [14

14. A. P. Goutzoulis, D. K. Davies, and J. M. Zomp, “Hybrid electronic fiber optic wavelength-multiplexed system for true time-delay steering of phased array antennas,” Opt. Eng. 31(11), 2312–2322 (1992). [CrossRef]

], wavelength tuning or switching in conjunction with dispersive elements [15

15. J. L. Corral, J. Martì, S. Regidor, J. M. Fuster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microw. Theory 45(8), 1531–1536 (1997). [CrossRef]

,16

16. L. Yaron, R. Rotman, S. Zach, and M. Tur, “Photonic beamformer receiver with multiple beam capabilities,” IEEE Photon. Technol. Lett. 22(23), 1723–1725 (2010). [CrossRef]

], or slow light [17

17. A. Zadok, O. Raz, A. Eyal, and M. Tur, “Optically controlled low-distortion delay of GHz-wide radio-frequency signals using slow light in fibers,” IEEE Photon. Technol. Lett. 19(7), 462–464 (2007). [CrossRef]

]. Experimental results are reported covering up to 16 antenna elements [14

14. A. P. Goutzoulis, D. K. Davies, and J. M. Zomp, “Hybrid electronic fiber optic wavelength-multiplexed system for true time-delay steering of phased array antennas,” Opt. Eng. 31(11), 2312–2322 (1992). [CrossRef]

], with total delays up to 2.5ns [15

15. J. L. Corral, J. Martì, S. Regidor, J. M. Fuster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microw. Theory 45(8), 1531–1536 (1997). [CrossRef]

] and scanning angles up to 90° at millimeter waves [18

18. B. Vidal, M. A. Piqueras, J. Herrera, V. Polo, J. L. Corral, and J. Martì, “Experimental demonstration of a 3-bit phtonic beamformer at the mm-band in transmission and receiving modes,” Proceedings of Microwave Photonics Conference 2004, WD-3 (2004).

]. Few techniques also report the capability for continuous beam steering [15

15. J. L. Corral, J. Martì, S. Regidor, J. M. Fuster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microw. Theory 45(8), 1531–1536 (1997). [CrossRef]

].

3.1 Photonics-based RF signal generation and beamforming

The scheme of principle of the proposed approach is shown in Fig. 3(a)
Fig. 3 a) Scheme of principle of the proposed beamforming technique. b) optical/electrical spectrum content in different position of the scheme.
. To allow the generation of multiple RF signals, the modulator is driven by multiple signals at different IFs so that the modulation sidebands are clearly separated in the optical spectrum. The optical signal is sent to each element of the arrayed antenna through a spool of optical fiber which introduces a wavelength-dependent delay through chromatic dispersion. A special tunable optical filter (tunable multi-pair bandpass filter, TMP-BPF) is then added at each antenna element to opportunely select in principle a pair of optical signals (one MLL mode and one sideband) for each RF signal to be generated. The spectral region where the optical pairs are taken will control the delay of the generated RF signals, according to the chromatic dispersion induced by the remoting fiber. Changing the filter position induces a delay Δt on the optical signal given by Δt = D·Δλ, where D is the value of the accumulated chromatic dispersion and Δλ the wavelength difference of the selected optical pairs. At each array element, after the TMP-BPF the filtered spectrum is sent to a PD which produces the RF signals as the beatings between the input spectral lines. In case the TMP-BPF is not able to select only the signal pair of interest, electrical bandpass filters (BPFs) can be added after the PD, as reported in Fig. 3(a), to isolate the appropriate RF signals to be transmitted through a multi-band (or several single-band) PAA element. From the formula above it is evident that the delay induced on the RF signals changing the filter position is independent from the RF signal frequency and bandwidth. Figure 3(b) sketches the optical and electrical spectra in the case of simultaneously generating two different RF signals.

3.2 Experimental setup and results

To demonstrate the broadband TTD capability of the proposed beamforming scheme, the setup depicted in Fig. 4(a)
Fig. 4 a) Experimental setup used to demonstrate the broadband TTD beamforming. b) Experimental setup used to emulate a multi-element antenna.
has been implemented. The optical path is composed of a fiber MLL with a repetition rate of about 10GHz (namely 9953MHz) and a 3-dB bandwidth of about 0.7nm, and a spool of dispersion compensating fiber (DCF) with a chromatic dispersion of −320ps/nm. The TMP-BPF is emulated here by a liquid-crystal-on-silicon programmable filter (Finisar WaveShaper 4000S, WS) configured to operate as a single 50GHz-bandwidth BPF selecting 5 adjacent lines of the MLL. The optical signal is detected by a 40GHz-bandwidth PD, generating an RF signal made of components at about 10, 20, 30, and 40GHz. The PD output is split into two paths, and two electrical BPFs centered exactly at 9953MHz and 39812MHz isolate the spectral components. The RF signals are then acquired by a dual-channel sampling oscilloscope. First, the WS is centered at 194.165THz, −50GHz offset from the MLL center wavelength, and then moved by 10GHz steps in order to select different groups of modes, up to 194.265THz. The filtered spectra at the two extreme positions of the tuned range are reported in Fig. 5(a)
Fig. 5 a) The spectra filtered from the MLL by the WS at the extreme positions of the tuned range. b) Measured delay for different carrier frequencies versus the filter offset.
. Figure 5(b) reports the measured delays for the 10GHz and 40GHz components, as a function of the optical BPF offset, i.e. the frequency difference from the initial position, and the theoretical delay curve. As can be seen, the results at 10GHz and 40GHz present an identical linear trend, and fit the theory very well. This experiment thus emulates the TTD of a signal spanning over 30GHz and confirms the effectiveness of the scheme. Since the MLL presents a discrete spectrum, the available delays are discrete as well, and the steps are determined by the lines spacing (i.e. the MLL repetition rate) and the amount of chromatic dispersion. In this work the step is 25.2ps.

4. Comments and conclusions

In conclusion, we have proposed and reported a photonics-based scheme integrating the functions of RF signal generation and TTD beamforming. The photonics-based multi-functional block is composed of a MLL and a tunable filtering matrix. The chromatic dispersion, necessary to generate the delays, can be provided by the optical fiber used for feeding the PAA. The scheme has been experimentally validated by photonically generating two wideband signals at about 10GHz and at 40GHz, and controlling their independent delays at two antenna elements, thus emulating the independent TTD beamforming of the generated signals. Thanks to the multiple functionality, the proposed scheme allows to improve the effectiveness of the photonic approach, reducing its cost and allowing flexibility, extremely wide bandwidth, and high stability. These features, added to the EMI immunity, low losses, and potentials for low weight and power consumption, make the proposed photonics-based scheme a promising solution for advanced RF transmitters with beamforming capability.

Acknowledgments

This work has been supported by the ERC projects PHODIR (contract n. 239640) and PREPARE (contract n. 324629), and by the Italian Defense Ministry project SOPHIA (contract n. 20008).

References and links

1.

M. L. Skolnik, Introduction to Radar Systems, 2nd Ed. (McGraw-Hill, 1980).

2.

V. Ravenni, “Performance evaluations of frequency diversity radar system,” Proceedings of European Microwave Conference 2007, 1715–1718 (2007).

3.

J. J. Zhang and A. Papandreou-Suppappola, “MIMO radar with frequency diversity,” Proceedings of Waveform Diversity and Design Conference 2009, 208–212 (2009). [CrossRef]

4.

J. Sun, Y. Dai, X. Chen, Y. Zhang, and S. Xie, “Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation,” IEEE Photon. Technol. Lett. 18(24), 2587–2589 (2006). [CrossRef]

5.

L. Goldberg, R. D. Esman, and K. J. Williams, “Generation and control of microwave signals by optical techniques,” IEE Proc.-J. 139(4), 288–295 (1992). [CrossRef]

6.

G. Serafino, P. Ghelfi, G. E. Villanueva, J. Palaci, P. Pérez-Millán, J. L. Cruz, and A. Bogoni, “Phase and amplitude stability of EHF-band radar carriers generated from an active mode-locked laser,” J. Lightwave Technol. 29(23), 3551–3559 (2011). [CrossRef]

7.

J. Chou, Y. Han, and B. Jalali, “Adaptive RF-photonic arbitrary waveform generator,” IEEE Photon. Technol. Lett. 15(4), 581–583 (2003). [CrossRef]

8.

I. S. Lin, J. D. McKinney, and A. M. Weiner, “Photonic synthesis of broadband microwave arbitrary waveform applicable to ultra-wideband communication,” IEEE Microw. Wirel. Co. 15(4), 226–228 (2005). [CrossRef]

9.

Z. Li, W. Li, H. Chi, X. Zhang, and J. Yao, “Photonic generation of phase-coded microwave signal with large frequency tunability,” IEEE Photon. Technol. Lett. 23(11), 712–714 (2011). [CrossRef]

10.

P. Ghelfi, F. Scotti, F. Laghezza, and A. Bogoni, “Photonic generation of phase-modulated RF signals for pulse compression techniques in coherent radars,” J. Lightwave Technol. 30(11), 1638–1644 (2012). [CrossRef]

11.

T. Yilmaz, C. M. DePriest, T. Turpin, J. H. Abeles, and P. J. Delfyett Jr., “Toward a photonic arbitrary waveform generator using a modelocked external cavity semiconductor laser,” IEEE Photon. Technol. Lett. 14(11), 1608–1610 (2002). [CrossRef]

12.

P. Ghelfi, F. Scotti, F. Laghezza, and A. Bogoni, “Phase coding of RF pulses in photonics-aided frequency-agile coherent radar systems,” IEEE J. Quantum Electron. 48(9), 1151–1157 (2012). [CrossRef]

13.

P. Ghelfi and A. Bogoni, “Design of flexible photonics-based RF transmitter and receiver for future mobile networks,” Proceedings of CODEC 2012, Kolkata (2012). [CrossRef]

14.

A. P. Goutzoulis, D. K. Davies, and J. M. Zomp, “Hybrid electronic fiber optic wavelength-multiplexed system for true time-delay steering of phased array antennas,” Opt. Eng. 31(11), 2312–2322 (1992). [CrossRef]

15.

J. L. Corral, J. Martì, S. Regidor, J. M. Fuster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microw. Theory 45(8), 1531–1536 (1997). [CrossRef]

16.

L. Yaron, R. Rotman, S. Zach, and M. Tur, “Photonic beamformer receiver with multiple beam capabilities,” IEEE Photon. Technol. Lett. 22(23), 1723–1725 (2010). [CrossRef]

17.

A. Zadok, O. Raz, A. Eyal, and M. Tur, “Optically controlled low-distortion delay of GHz-wide radio-frequency signals using slow light in fibers,” IEEE Photon. Technol. Lett. 19(7), 462–464 (2007). [CrossRef]

18.

B. Vidal, M. A. Piqueras, J. Herrera, V. Polo, J. L. Corral, and J. Martì, “Experimental demonstration of a 3-bit phtonic beamformer at the mm-band in transmission and receiving modes,” Proceedings of Microwave Photonics Conference 2004, WD-3 (2004).

OCIS Codes
(280.5600) Remote sensing and sensors : Radar
(060.5625) Fiber optics and optical communications : Radio frequency photonics

ToC Category:
Signal Generation and Processing

History
Original Manuscript: May 31, 2013
Revised Manuscript: August 30, 2013
Manuscript Accepted: September 5, 2013
Published: September 23, 2013

Virtual Issues
Microwave Photonics (2013) Optics Express

Citation
Paolo Ghelfi, Francesco Laghezza, Filippo Scotti, Giovanni Serafino, Sergio Pinna, and Antonella Bogoni, "Photonic generation and independent steering of multiple RF signals for software defined radars," Opt. Express 21, 22905-22910 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-19-22905


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References

  1. M. L. Skolnik, Introduction to Radar Systems, 2nd Ed. (McGraw-Hill, 1980).
  2. V. Ravenni, “Performance evaluations of frequency diversity radar system,” Proceedings of European Microwave Conference 2007, 1715–1718 (2007).
  3. J. J. Zhang and A. Papandreou-Suppappola, “MIMO radar with frequency diversity,” Proceedings of Waveform Diversity and Design Conference 2009, 208–212 (2009). [CrossRef]
  4. J. Sun, Y. Dai, X. Chen, Y. Zhang, and S. Xie, “Stable dual-wavelength DFB fiber laser with separate resonant cavities and its application in tunable microwave generation,” IEEE Photon. Technol. Lett.18(24), 2587–2589 (2006). [CrossRef]
  5. L. Goldberg, R. D. Esman, and K. J. Williams, “Generation and control of microwave signals by optical techniques,” IEE Proc.-J. 139(4), 288–295 (1992). [CrossRef]
  6. G. Serafino, P. Ghelfi, G. E. Villanueva, J. Palaci, P. Pérez-Millán, J. L. Cruz, and A. Bogoni, “Phase and amplitude stability of EHF-band radar carriers generated from an active mode-locked laser,” J. Lightwave Technol.29(23), 3551–3559 (2011). [CrossRef]
  7. J. Chou, Y. Han, and B. Jalali, “Adaptive RF-photonic arbitrary waveform generator,” IEEE Photon. Technol. Lett.15(4), 581–583 (2003). [CrossRef]
  8. I. S. Lin, J. D. McKinney, and A. M. Weiner, “Photonic synthesis of broadband microwave arbitrary waveform applicable to ultra-wideband communication,” IEEE Microw. Wirel. Co.15(4), 226–228 (2005). [CrossRef]
  9. Z. Li, W. Li, H. Chi, X. Zhang, and J. Yao, “Photonic generation of phase-coded microwave signal with large frequency tunability,” IEEE Photon. Technol. Lett.23(11), 712–714 (2011). [CrossRef]
  10. P. Ghelfi, F. Scotti, F. Laghezza, and A. Bogoni, “Photonic generation of phase-modulated RF signals for pulse compression techniques in coherent radars,” J. Lightwave Technol.30(11), 1638–1644 (2012). [CrossRef]
  11. T. Yilmaz, C. M. DePriest, T. Turpin, J. H. Abeles, and P. J. Delfyett., “Toward a photonic arbitrary waveform generator using a modelocked external cavity semiconductor laser,” IEEE Photon. Technol. Lett.14(11), 1608–1610 (2002). [CrossRef]
  12. P. Ghelfi, F. Scotti, F. Laghezza, and A. Bogoni, “Phase coding of RF pulses in photonics-aided frequency-agile coherent radar systems,” IEEE J. Quantum Electron.48(9), 1151–1157 (2012). [CrossRef]
  13. P. Ghelfi and A. Bogoni, “Design of flexible photonics-based RF transmitter and receiver for future mobile networks,” Proceedings of CODEC 2012, Kolkata (2012). [CrossRef]
  14. A. P. Goutzoulis, D. K. Davies, and J. M. Zomp, “Hybrid electronic fiber optic wavelength-multiplexed system for true time-delay steering of phased array antennas,” Opt. Eng.31(11), 2312–2322 (1992). [CrossRef]
  15. J. L. Corral, J. Martì, S. Regidor, J. M. Fuster, R. Laming, and M. J. Cole, “Continuously variable true time-delay optical feeder for phased-array antenna employing chirped fiber gratings,” IEEE Trans. Microw. Theory45(8), 1531–1536 (1997). [CrossRef]
  16. L. Yaron, R. Rotman, S. Zach, and M. Tur, “Photonic beamformer receiver with multiple beam capabilities,” IEEE Photon. Technol. Lett.22(23), 1723–1725 (2010). [CrossRef]
  17. A. Zadok, O. Raz, A. Eyal, and M. Tur, “Optically controlled low-distortion delay of GHz-wide radio-frequency signals using slow light in fibers,” IEEE Photon. Technol. Lett.19(7), 462–464 (2007). [CrossRef]
  18. B. Vidal, M. A. Piqueras, J. Herrera, V. Polo, J. L. Corral, and J. Martì, “Experimental demonstration of a 3-bit phtonic beamformer at the mm-band in transmission and receiving modes,” Proceedings of Microwave Photonics Conference 2004, WD-3 (2004).

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