Photonics-Based RF Phase Shifter for Ultra-Broadband Communications M. S. B. Cunha, R. N. Da Silva, R. M. Borges and Arismar Cerqueira S. Jr. Laboratory WOCA (Wireless and Optical Convergent Access), National Institute of Telecommunications (Inatel), 510 João de Camargo Av., Santa Rita do Sapucaí-MG, Brazil, 37540-000. matheusseda@gee.inatel.br, regivan@mtel.inatel.br, ramonmb@inatel.br and arismar@inatel.br Abstract This paper presents a simple and reconfigurable photonics-based radiofrequency phase shifter (PBPS) for ultrabroadband communications. The proposed approach makes use of a single DC voltage control to manage the RF signal phase. It employs a single-drive Mach-Zehnder modulator, an optical filter and optical phase shifter for performing a continuous (from 0 to 360 ) RF phase shift over an ultra-wide frequency range. Numerical results demonstrate a flatness phase shift of RF signals up to 100 GHz, with low amplitude and phase deviations of 0.002 db and 0.050, respectively. The proposed device can be applied to future wireless networks, including 5G systems, operating in the millimeter-waves. Keywords 5G networks; broadband communications; microwave photonics; radars; RF phase shifter. I. INTRODUCTION Microwave photonics (MP) is defined as the interaction area between microwave and optoelectronics [1-3]. It has continuously led to new solutions and functionalities for wideband wireless access networks, satellite communications, instrumentation, radar and war systems. MP takes advantages of the optoelectronics benefits - such as electromagnetic interference immunity, broadband operation, cost effectiveness, size and consumption reduction - to overcome the complexity and limitations in deployment of electronic-based devices, mainly at high frequencies. Particularly, microwave and millimeter-waves (mmwaves) have been recognized as promising bands of radio spectrum to favor gigabit wireless communication networks, which are required to address the massive demand growth in mobile data and transmission throughput. Regarding the fifth generation (5G) technology, many groups have proposed the use of two bands [4,5]: a lower frequency band, using frequencies below 6 GHz and a higher band using frequencies higher than 20 GHz, including mm-waves. However, moving to higher frequencies establishes new challenges to bring electronic products to the market and, in order to overcome these challenges, photonic technologies are suggested as a prospective solution for mm-waves components design [4,5]. In this context, several devices initially produced entirely with electronic components have been developed based on optoelectronics, such as RF converters, amplifiers, front-ends, radars and phase shifters [6-8]. The last one typically address applications as frequency multiplication [9], reflected power control suitable for radar [10] and analog beamforming network to be used in satellite terminals for moving communications [11]. The replacement of conventional narrowband and electronics-based RF phase shifter by those based on optical technologies, brings the benefits of broadband operation, reconfigurability, low attenuation and phase deviation, electromagnetic interference immunity, as well as simple control and easy integration using optical-wireless networks. Diverse phase-control techniques are present in the literature, which are differentiated by the number of parameters to be controlled, the shift range capability and the operational frequency band [12-14]. Examples of techniques with two or more adjustable parameters are presented in [9], by adjusting the DC bias voltage of an integrated polarization division multiplexing dual-parallel Mach Zehnder Modulator (PDM- DPMZM). Xudong Wang et al have proposed a technique based on amplitude and phase controls and two anti-phase RF modulation sidebands of two phase modulated optical signals, by adjusting the optical carrier wavelengths [12]. There are also techniques with only one control as in [13], in which the wavelength of a RF single-band modulated optical signal is controlled by using a Bragg grating filter. On the other hand, T. Li et al proposed to manage the RF phase by controlling the phase modulator DC voltage that receives optical carrier and sideband of an orthogonally polarized modulated RF signal [14]. In this work, we introduce the concept and numerically demonstrate a simple and ultra-wideband low cost photonicsbased RF phase shifter (PBPS). The proposed approach provides continuous phase shift from 0 to 360 for microwaves and mm-waves without any hardware modification, by integrating a low cost optical phase shifter (OPS), an optical bandpass filter (OBF) and a single-drive Mach-Zehnder Modulator (MZM). A promising application relies on the use of the proposed PBPS for developing reconfigurable broadband antennas for optical-wireless systems, including 5G networks. The manuscript is structured in four Sections. Section II describes the simple and ultra-wideband photonics-based RF phase shifter topology and its operational principle. The PBPS numerical validation as a function of RF phase shift, operating frequency range, amplitude and phase deviation is reported in Section III. Finally, Section IV presents the relevant conclusions and future works.
Fig. 1. The proposed photonics-based RF phase shifter (PBPS): LD Laser diode; OPS Optical phase shifter; MZM Mach-Zenhder modulator; OBF Optical bandpass filter; PD Photodetector. II. PBPS - PHOTONICS-BASED RF PHASE SHIFTER The proposed photonics-based RF phase shifter (PBPS), schematized in Fig. 1, is based on the use of a Mach-Zehnder Modulator, an optical phase shifter and an optical bandpass filter. A numerical proof of concept has been realized using the Optisystem commercial software in order to demonstrate its applicability in microwave and mm-waves, by means of providing a continuous phase shift from 0 to 360 aimed to broadband communications. The main advantages of our approach are simplicity, low cost and ultra-broadband operation. For instance, it also needs a single DC voltage control as is in [14], but at much lower cost, since the strategy described in [14] requires an optical filter with an ultra-sharp edge roll-off of 1500 db/nm (12 db/ghz), which cost around $ 25,000.00 (American Dollars), to largely suppress the right RF modulation sideband. A dual- parallel MZM, which cost around $ 12,000.00, and a phase modulator as an optical phase shifter, which costs around $ 1,500.00. On the other hand, we propose to use a cost-effective optical filter ($ 6,000.00), a single-driver MZM ($ 4,000.00) and a low cost optical phase shifter ($ 200.00) for obtaining the same performance level in mm-waves. Initially, linear polarized light from a CW laser at 1550 nm is divided into two arms by using an optical splitter. At the top arm, a low cost OPS is responsible to shift the optical carrier phase ( ) by simply manipulating its DC voltage (V ). At the bottom arm, a single-driver MZM, set to operate at the minimum transmission point (MITP), modulates the optical carrier with the RF signal. This is ensured by an appropriate adjustment of the MZM bias voltage (, to suppress the optical carrier and transfer only the odd-order sidebands to its output. An optical bandpass filter allows only one sideband reaching at the second optical splitter, with the optical carrier from the top arm, preventing the harmonics and undesirable sidebands to reach the photodetector (PD). As result of the photodetection process, the RF signal phase is shifted by the difference between its original phase and the optical carrier phase, which is shifted by the OPS. The PBPS operational principle is mathematically explained as follows. The linearly polarized light electric field is given by t 2 cos (1) where and are the laser amplitude of electric field and angular frequency, respectively. The OPS output electric field can be obtained by,t cos (2) 2 where, is the OPS insertion loss and is the shifted optical carrier phase. In parallel to optical phase shift in the Fig. 1 upper path, a modulation and filtering process occurs through the MZM and OBF at the bottom. Therefore, the MZM output electric field is described by [15] 2 cos 2 sin cos (3) where is the MZM half-wave voltage, is the MZM bias voltage, and are the RF input signal amplitude and angular frequency respectively. By considering the MZM optical extinction ratio as infinite, we can rewrite (3) in terms of the Bessel functions as [6] 2 coscos 2 2 (4)
2 2 2 cos2 2 sin 2 sin2 1 Expression (4) proves that the electric field at the MZM output depends essentially on and. To simplify this expression, we consider (5) (6) 2 2 Through these simplifications and in order to further facilitate these equations analysis, (4) can be rewritten considering the first three Bessel functions as follows 2 cos2 cos2 2 sin sincos where are the Bessel functions. In addition, considering the MZM operation mode at MITP ( equal to, the Bessel functions products by cos (k) are null. Therefore, the MZM output electric field has only the first order sidebands and, considering the modulator insertion loss as, its expression is given by (7) the optical carrier and the transmitted sideband phases, it is notorious the desired RF phase shift is adjusted by the optical phase shifter, with the remarkable advantage of being ultrabroadband. III. NUMERICAL RESULTS This section presents a proof of concept of the proposed photonics-based RF phase shifter based on numerical simulations. It is worth mentioning about two important considerations in the simulations. Firstly, the OPS available in the Optisystem library has a parameter called phase shift, which is responsible for shifting the optical signal phase present at its input, by directly introducing the desired shift in degrees instead of apply a continuous voltage ( on its terminal, as proposed in in PBPS. Secondly, the following commercial components parameters have been used in the numerical model in order to make it realistic before practice implementation: the FTM7939EK MZM from Fujitsu with 4 and 8 db insertion loss; a narrowband tunable optical filter from Teraxion with a bandwidth of 0.075 nm ( 9.375 GHz). Fig. 2 reports the proposed PBPS phase shift response at the 38 GHz from -180 to 180. We have divided the simulation into eight steps of 45, called iterations, each one corresponding to an optical phase shift induced by the OPS, with the purpose of simulating the DC voltage adjustment. It is important to highlight that the photonics-based phase shifter is continuous, thus any desired phase shift between 0 to 360 might be set..cos 2. cos (8) where is the RF signal phase. Furthermore, the electrical field expression after modulation, optical filtering and combining, named here as, is described by cos 2. c 2.. (9) where is the optical filter insertion loss. Finally, the photocurrent is given by the beating of the electric fields at the PD input, multiplied by its responsivity () [16]. Its expression is given by cos (10) where represents the total insertion loss of all PBPS devices, E is electric field dependent of the linearly polarized light ( and RF signal amplitude (, is the responsivity of the photodetector and is the signal phase shifting. Since the RF signal output phase depends on the difference between Fig. 2. RF signal phase shift as a function of the optical phase. Fig. 3 presents an investigation of the RF amplitude and phase responses as a function of frequency. One can observe there is no either significant amplitude or phase variation from 10 to 100 GHz. The amplitude and phase deviations are only 0.02 db 0.05, respectively. Particularly, for the range from 5 to 10 GHz, there are an amplitude and phase variations of 1 db and 10, respectively. This phenomenon occurs because the optical filter is not narrow enough, allowing the transfer of the sidebands up to 9.375 GHz. Consequently, there is also a beating at the photodetector of these sidebands, causing the
observed distortions. A possible solution for this problem is the implementation of a narrower bandwidth filter to avoid those sidebands. Fig. 4 and Fig. 5 present a study on the influence of the filter width on the RF signal amplitude and phase, respectively. It is clear that the use of a 0.01 nm ( 1.25 GHz) optical filter makes possible to obtain extremely flat amplitude and phase response from MHz up to 100 GHz. (a) 0.075 nm bandwidth (a) Amplitude (b) 0.01 nm bandwidth Fig. 4 Investigation of the PBPS amplitude response from 5 to 10 GHz for two different filter bandwidths. (b) Phase Fig. 3 PBPS characterization up to 100 GHz. (a) 0.075 nm bandwidth.
Reference Center project. Authors also thank the financial support from FINEP, CNPq, CAPES, MCTI and FAPEMIG. REFERENCES (b) 0.01 nm bandwidth. Fig. 5 Investigation of PBPS phase response from 5 to 10 GHz for two different filter bandwidths. IV. CONCLUSIONS This work has introduced the concept and presented a numerical analysis of a simple, ultra-broadband and reconfigurable photonic-based RF phase shifter. The proposed approach applies a combination of external modulation technique and optical phase shift for performing ultrabroadband electrical shift in the RF signal phase. A simple optical phase shifter DC voltage control is required to manage the RF phase shift. Numerical results have demonstrated continuous phase shift from 5 to 100 GHz frequency range, including the 6, 28 and 38 GHz that are potential spectrum bands for the 5G networks. Specifically, by considering the frequency range from 10 to 100 GHz, it has been observed signal amplitude and phase deviation of 0.002 db and 0.05, respectively, which relies on flatness amplitude and negligible phase deviation. The use of a narrower bandwidth filter has confirmed the possibility of developing a PBPS with fully flattened amplitude and phase response for an even larger frequency range, from a few MHz to 100 GHz. Future works regards the implementation of the proposed photonics-based RF phase shifter and its characterization in the mm-waves frequency range, aiming its use in reconfigurable antennas arrays for 5G networks. ACKNOWLEDGMENTS This work was partially supported by Finep/Funttel Grant No. 01.14.0231.00, under the Radio Communications [1] Jianping Yao, Senior Member, IEEE, Member, OSA, Microwave Photonics, Journal of Lightwave Technology, Vol. 27, No. 3, February, 2009. [2] J. Capmany and D. Novak, Microwave photonics combines two worlds, Nature Photon., vol. 1, no. 6, pp. 319-330, June 2007. [3] Thomas R. Clark and Rodney Waterhouse, Photonics for RF Front Ends, IEEE microwave magazine, P. 1527-3342, May 2011. [4] T. S. Rappaport et al., "Millimeter Wave Mobile Communications for 5G Cellular: It Will Work!," in IEEE Access, vol. 1, no., pp. 335-349, 2013. [5] S. Rangan, T. S. Rappaport and E. Erkip, "Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges," in Proceedings of the IEEE, vol. 102, no. 3, pp. 366-385, March 2014. [6] L. M. Muniz, R. M. Borges, Regivan N. Da Silva, D. F. Noque and Arismar Cerqueira S. Jr. Ultra-broadband Photonics-based RF Front- End Toward 5G Networks, Journal of Optical Communications and Networkis, vol. 8(11), 2016. [7] L. M. Muniz, Arismar Cerqueira S. Jr, R. M. Borges, Regivan N. Da Silva, D. F. Noque, A. Bogoni and Masaaki Hirano. Ultra-Wideband Photonics-assisted RF Amplifier for 5G Networks. Microwave and Optical Technology Letters, 2017. [8] Valeria Vercesi1, Daniel Onori1, Arismar Cerqueira S. Jr.2, Antonella Bogoni3, Mirco Scaffardi, Tunable dual-frequency lidar exploiting a mode-locked laser for integrated coherent radar-lidar architectures, OFC - Optical Fiber Communication Conference, 2015 [9] Yamei Zhang and Shilong Pan, Frequency-multiplying microwave photonic phase shifter for independent multichannel phase shifting, Optics Letters, Vol. 41, No. 6, March 2016. [10] Tze-Pin Young, Ti-Tan Chen and Yi-Chyun Chiang, Senior Member, IEEE, Appropriate Reflected Power Control for Vital Signal Radar Adopting Phase Shifting Method, RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-BIO), September 2015. [11] Saverio Alessandro, Maria Concetta De Bilio, Salvatore Coco, Gaspare Bavetta, Ignazio Pomona, Antonio Laudani, Analog Beamforming Network for Ka Band Satellite on the Move Terminal with phase shifting technique based on I/Q mixer, European Microwave Conference EuMA 7-10, Paris, France, September 2015 [12] Xudong Wang, Erwin H. W. Chan, and Robert A. Minasian, Fellow, IEEE, Fellow, OSA, All-Optical Photonic Microwave Phase Shifter Based on an Optical Filter With a Nonlinear Phase Response, IEEE, 2013 [13] Xudong Wang, Erwin H. W. Chan and Robert A. Minasian, Optical-to- RF phase shift conversion-based microwave photonic phase shifter using a fiber Bragg grating, Optical Society of America, 2014 [14] T. Li, E. H. W. Chan, X. Wang, X. Feng. and B. Guan, All-Optical Photonic Microwave Phase Shifter Requiring Only a Single DC Voltage Control, IEEE, 2016 [15] J. J. O'Reilly, P. M. Lane, R. Heidemann and R. Hofstetter, Optical generation of very narrow linewidth wave signals, Electronics Letters, Vol. 28, Issue 25, Pages 22309 2311, 1992. [16] Jianping Yao, A Tutorial on Microwave Photonics, IEEE Photonics Society Newsletter, April 2012.