This is the accepted version of the journal article: Parrón Granados, Josep; Cabrera-Hernández, Edith Annette; Tennant, Alan; [et al.]. «Multiport compact stacked patch antenna with 360° beam steering for generating dynamic directional modulation». IEEE Transactions on Anten- nas and Propagation, Vol. 69, Issue 2 (February 2021), p. 1162-1167. DOI 10.1109/tap.2020.3008065 This version is available at https://ddd.uab.cat/record/273565 under the terms of the license IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION 1 Multiport Compact Stacked Patch Antenna with 360º beam steering for Generating Dynamic Directional Modulation Josep Parron, Edith A. Cabrera-Hernandez, Alan Tennant and Pedro de Paco Abstract— In this contribution, we propose a multiport compact through the combination of the radiation patterns of a monopole and antenna that supports four radiating modes operating at the same a circular patch with two orthogonal TM21 modes excited frequency thus making it a suitable candidate to substitute a linear array of four antennas. It will be also shown as this antenna produces dynamic simultaneously in quadrature [17]. This configuration allows for the directional modulation (DDM) with low bit error rate (BER) in a unique, introduction of phase variation without the need for spacing between unambiguous (secure) angular region that can be chosen freely in the XY array elements. However, the combination of these two radiation plane. Trade-offs between the BER beam width of the secure angular patterns produces a wide beam width and a reduced bit error rate region, the BER at side lobes and the extra power required in the (BER) not only in the desired secure direction, but also in the generation of DDM are assessed through measurements with real-time data transmission. opposing hemisphere due to the presence of a grating lobe. Therefore, eavesdroppers located away from the intended secure Index Terms— Directional modulation (DM), multiport compact direction can still access freely to the transmitted information. In antenna, secure communication. addition, the profile of this antenna is not low due to height of the monopole. I. INTRODUCTION In this contribution, we reduce the physical profile of the antenna In recent years, directional modulation (DM) has been introduced proposed in [16] by eliminating the monopole. We also remove the as a transmission technique that can improve physical layer security quadrature condition for exciting the orthogonal TM21 modes of wireless communications [1]-[2]. DM uses different antenna suggested in [17] so they can be excited independently. We also add radiation patterns to transmit a given constellation of symbols another patch that supports two orthogonal TM31 modes operating at towards a desired secure observation angle while simultaneously the same frequency as the TM21 modes. As a result, this new design transmitting distorted versions of the constellation to other produces DM with low BER in a unique, unambiguous direction in observation angles [2]. the XY plane if the four available radiation patterns are combined in Implementations of DM are often based on antenna arrays a prescribed manner. (dipoles, patches, etc.) excited with feeding networks that enable the This document is structured as follows. Section II presents the configuration of different radiation patterns. Several DM antenna design and the measurements of reflection coefficient and beamforming techniques have been reported in the open literature isolation between antenna ports. In section III, we show the including: switches [3]-[6], a dual-beam method [7], reconfigurable measured radiation patterns for each mode and describe the attenuators and/or phase shifters [8]-[10], vector modulators [11]- procedure to generate dynamic DM with the proposed antenna. In [14] and the use of a Fourier Rotman lens [15]. section IV, we evaluate experimentally the BER in the XY plane to When the number of array elements antennas in a DM system is demonstrate the existence of a single low BER (secure) direction. less than four, the spacing between elements is typically larger than Finally, in section V, we summarize the conclusions of this work. λ (free space wavelength) [7], [11] and tends to λ/2 as the number of elements in the array increases [5], [6], [8]-[10], [12]-[15]. In either case, the size of the resulting array is larger than several λ, which (a) x φ limits the use of DM to applications where the antenna size is not a constraint. Another limitation of the many reported DM systems is the y z Dh Dv Dv restricted angular range where the secure transmission can be achieved. For example, in a patch (directive antenna) array, the array Ru β Ru factor is shaped by the patch radiation pattern, thus limiting the Rl α Rl available angular range of secure transmit directions [5]-[9], [13], [15]. In contrast, if dipoles (omnidirectional antennas) are used [10]- [12], [14], the symmetry of the array generates a symmetric H plane radiation pattern resulting in two angular directions transmitting the same information in both the forward and rear hemispheres. Recently, a multiport compact antenna with maximum dimensions of the order of λ/2 was presented for generating DM with 360º beam steering [16]. The full circular angular coverage was achieved Manuscript received May 2020. This work was supported by the (b) Universitat Autònoma de Barcelona, FEDER funds and the Spanish Ministry z Dgp of Science, Innovation and Universities (RTI2018-096019-B-C33). θ J. Parron, E. Cabrera-Hernandez, Pedro de Paco are with the Department of Dl Telecommunications & Systems Engineering, Universitat Autònoma de Du x y Barcelona, Catalunya, Spain. Josep.Parron@uab.cat h A. Tennant is with the Department of Electronic and Electrical Engineering h at The University of Sheffield, U.K. 2 4 3 1 Color versions of one or more of the figures in this communication are Fig. 1. Scheme of the proposed antenna. Dimensions (in mm) are: Dgp = 85, available online at http://ieeexplore.ieee.org. Du = 56.4, Dl = 74, h = 3, Rl = 20, Ru = 14, Dh = 2, Dv = 1.3. Angles (in Digital Object Identifier 10.1109/TAP.2016.xxx degrees) are α = 45, β = 90. (a) Upper view. (b) Side view. 2 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION (a) (b) Fig. 3. Fabricated antenna. (a) Bottom view. (b) Top view (a) (b) |S 11 | |S 31 | |S 13 | |S 33 | |S | |S | |S | |S | 21 41 23 43 0 0 -10 -10 -20 -20 -30 -30 (c) (d) -40 -40 2.3 2.4 2.5 2.6 2.3 2.4 2.5 2.6 Fig. 2. Simulated current distribution (dBA/m) at 2.435 GHz when one f (GHz) f (GHz) port is fed and the others are loaded with 50 Ω. (a) Port 1, mode TM31 in Fig. 4. Measured S parameters. The geometry is symmetric so the S lower patch. (b) Port 2, mode TM31 (orthogonal) in lower patch. (c) Port 3, parameters for ports 2 and 4, not shown here for sake of briefness, are almost mode TM21 in upper patch. (d) Port 4, mode TM21 (orthogonal) in upper identical to ports 1 and 3, respectively. patch. (1) II. ANTENNA DESIGN The proposed multiport antenna consists of two stacked circular where AEPn(φ) is the active element pattern of antenna n [19] and microstrip patches that support four radiating modes at the frequency Bmn is the weight of antenna n to transmit symbol m. of 2.435 GHz (Fig. 1). The substrate used for both patches is Fig. 5 shows the measured magnitude (θ component) of the four TACONIC TRF 45 (εr = 4.5, tanδ = 0.0037) with thickness 3 mm. AEPs that can be obtained with the proposed antenna design. It must The lower patch diameter (Dl) was chosen to generate the TM31 be remarked that orthogonal current distributions generate mode [18]. This patch is fed through two vias of diameter Dv that orthogonal radiation patterns. The measured AEP in the XY plane form an angle β = 90º with respect to the disc center in order to (magnitude and phase) are plotted in Fig. 6. excite two orthogonal modes (Fig. 2a and 2b). Theoretically, β = 30º To illustrate the process of DM generation, we follow the is also an option but, in that case, SMA connectors overlap. The procedure reported in [12]. First, we calculate the weights Bmn_conv upper patch diameter (Du) was adjusted to obtain the TM21 mode for conventional modulation: amplitudes are set constant whereas [18] at the same frequency as the TM31 mode. The two orthogonal phases are tuned to transmit maximum radiation at the secure modes of the upper patch (Fig. 2c and 2d) are fed using two vias of direction φs [12]. Fig. 7 shows as the maximum of radiation can be diameter Dv that pass through the lower patch and form an angle α set arbitrarily in the XY plane with a front to back ratio higher than =45º with respect to the disc center. A hole of diameter Dh was 10 dB. This is an important improvement with respect to [16] where milled in the lower patch to provide isolation between these vias and the front to back ratio was 0 dB. However, the proposed compact the lower patch. The relative angular position between the vias that antenna also produces two side lobes at approximately φs±72º, which feed each patch was chosen in order to obtain a symmetric design are only a few dB below the maximum value. High side lobes could that maximizes isolation among ports whereas the distances Rl and compromise the security of a transmission since the signal to noise Ru were optimized to match the ports to 50 Ω. ratio detected by a potential eavesdropper at those observation angles The antenna was fabricated (Fig. 3) using the dimensions that can would be of a similar level to that at the desired recipient. However, be found in Fig. 1 and the measured S parameters are presented in DM can help overcome this limitation of the design as described in Fig. 4. All the ports show optimum matching around 2.435 GHz, the following section. however, the measured bandwidth (referred to -10 dB) is lower for The weights Bmn_DM that generate DM are computed using the modes TM31 (12 MHz) than for modes TM21 (46 MHz). We did not orthogonal vector approach described in [12]. Here, without loss of put a particular emphasis in improving this bandwidth since it can be generality, we consider that the desired secure observation angle is φs achieved by increasing the thickness of the dielectric substrate [16]. = 180º and the modulation to transmit is QPSK (Fig. 8a). In DM, we Measured isolation between modes of the same patch is always increase the total transmitted power by adding a certain amount of better than 25 dB and the isolation between modes of different orthogonal noise Wmn to the former weights (Bmn_DM = Bmn_conv + patches is better than 15 dB. Wmn) so that the transmitted constellation at φs remains invariant whereas it is scrambled in the other observation angles [12]. The III. PRINCIPLES FOR DM OPERATION orthogonal noise can be constant for each symbol (static DM) or it can be updated dynamically at the symbol rate (dynamic DM) [20]. If we consider an array of N antennas along the x-axis, the radiation The level of security achieved in the communication is related to the pattern used for the transmitted symbol m in a given direction φ of extra orthogonal noise power added through the parameter power the XY plane (θ =90º) can be written as efficiency [12] IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION 3 (2) z z where M is the number of radiation patterns used to generate DM. For a conventional modulation (Fig. 8a), PEDM is 100% and the improvement of security is related to a decrease in PEDM. Indeed, Fig.8b depicts a set of M = 64 radiation patterns (16 per x x y y each QPSK symbol) that can be produced with our antenna to transmit dynamic DM with PEDM = 75%. For most observation (a) (b) angles, it is hard to distinguish the shape of the transmitted constellation, however, we still recognize, with a naked eye, noisy QPSK constellations around 108º and 252º. In order to overcome this situation, we further increase the power of orthogonal noise and z z reduce PEDM to 50% thus obtaining the radiation patterns of Fig. 8c. Now, we can only recognize, by visual inspection, a QPSK constellation at 180º. Therefore, for a low enough PEDM, we should be able to obscure the transmitted signal even in the direction of the x y x y high side lobes of the conventional radiation pattern and achieve a (c) (d) single secure transmission direction in the XY plane. Note that, in this procedure, radiation patterns are generated randomly without Fig. 5. Measured magnitude (dB) of the AEP (θ component) of each mode considering the resulting direction of maximum radiation. The excited in the antenna at 2.435 GHz. (a) Port 1, mode TM31 in lower patch. performance in terms of PEDM can be improved if an optimization (b) Port 2, mode TM31 (orthogonal) in lower patch. (c) Port 3, mode TM21 in algorithm is used to select suitable radiation patterns [21]. upper patch. (d) Port 4, mode TM21 (orthogonal) in upper patch. IV. DM PERFORMANCE IN TERMS OF BER In order to assess the performance of our antenna when it is transmitting dynamic DM in the XY plane, we conducted several Magnitude (dB) Phase (deg) measurements with real-time data transmission in a 5 x 3 x 3 m3 anechoic chamber. The scheme and photographs of the measurement setup are shown in Fig. 9; distance between transmitting and receiving antennas is 2.5 m. The selected operating frequency was 2.435 GHz and the bit rate 6 Mbps. Magnitude (dB) Phase (deg) (a) Fig. 6. Measured AEP (θ component) in the XY plane Magnitude (dB) Phase (deg) 90 s =60º 0 =180º 120 60 s s =300º -5 150 30 -10 (b) -15 180 -20 0 210 330 240 300 (c) 270 Fig. 8. Magnitude and phase of the generated radiation patterns for Fig. 7. Normalized radiation patterns (dB) for transmitting a conventional transmitting QPSK at φs = 180º with DM. (a) PEDM =100% (conventional). modulation in different secure observation angles φs. (b) PEDM =75%. (c) PEDM =50% 4 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION assumptions, which are similar to those in [12]: 2.5 m • QPSK modulation with Gray coding. x AUT • The receiver does not assume a priori any modulation x φ function. Instead, there is a training stage where the Logperiodic antenna receiver computes reference symbols for every observation y z TX1 TX4 direction by averaging all the received training symbols. In (H-pol) TX2 TX3 the transmission stage, the receiver decides which of these RX1 FMCOMMS5-EBZ reference symbols is received using the minimum + Euclidean distance. ZynQ-7000-SoC Ethernet • Uniform additive white Gaussian noise (AWGN) in all PC with Matlab switch Roll observation directions. Signal to noise ratio (SNR) will be controller set to 12 dB for the receiver at the secure direction. Thus, the SNR will change accordingly in the other observation directions. Since the system intrinsic AWGN is low enough to be ignored, artificial AWGN is added onto the received raw data in Matlab. • Data stream with 106 transmitted symbols. In a first experiment, we considered that the desired secure observation angle was φs = 180º. Fig. 10 displays the simulated and measured BER for different values of PEDM, both results are in good agreement, in particular around φs. In the case of PEDM = 100%, BER follows the trend of the conventional radiation pattern of Fig 8a. and, consequently, there is only a single angular region in the XY plane with BER < 10-3. When dynamic DM was introduced (decreasing PEDM), we verified that there was a reduction of the beamwidth with BER < 10-3 and the BER in the side lobes decreased, approximately, one order of magnitude. BER 10 0 10 -1 Fig.9. Experimental setup for the measurement of BER in the XY plane when the AUT is transmitting dynamic DM. The weights Bmn that feed our antenna were previously computed 10 -2 BER using Matlab and generated in the evaluation board FMCOMMS5- EBZ [22], a FPGA Mezzanine Card (FMC) that is mounted on the 10 -3 ZynQ-7000 SoC ZC706 evaluation kit of Xilink [23]. The board FMCOMMS5-EBZ contains two AD9361 transceivers, each one of them with two transmitters and two receivers. In our setup (Fig.9), PE =100% 10 -4 DM each one of the transmitters was connected to a port of our compact PE DM =50% PE DM=25% antenna (AUT) whereas one of the board receivers was connected to (b) a log-periodic antenna. Control of transmitted and received signals, 10 -5 as well as the rotation of the AUT, was carried out via Ethernet using 0 60 120 180 240 300 360 Matlab. Fig.10. BER angular distribution for different values of PEDM. AWGN is The usual metric for evaluating the performance of a DM system set to obtain at 180º a SNR of 12 dB. (a) Simulated. (b) Measured. is bit-error-rate (BER) [20]; we computed it using the following IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION 5 PE DM=100% PE DM=50% PE DM=25% 0 0 10 10 10 -1 10 -1 10 -2 10 -2 10 -3 10 -3 10 -4 10 -4 (a) (b) 10 -5 10 -5 0 60 120 180 240 300 360 0 60 120 180 240 300 360 Fig.13. Measured BER angular distribution for different values of PEDM. AWGN is set to obtain a SNR of 12 dB in the secure direction. (a) φs = 60º. (b) φs = 300º. TABLE I Fig.11. Received constellations at 180º before adding AWGN. BER MEASUREMENTS FOR DIFFERENT PEDM VALUES, SNR @ φS = 12 DB. SUMMARY PEDM = 100% ∆φ BER BER BER φs BER<10-3 @ φs @ φs -72º @ φs +72º 60º 34º 1.5·10-5 7.0·10-4 2.5·10-3 180º 38º 1.9·10-5 2.0·10-2 1.2·10-2 300º 36º 1.1·10-5 5.7·10-3 5.0·10-3 PEDM = 50% ∆φ BER BER BER φs BER<10-3 @ φs @ φs -72º @ φs +72º 60º 18º 1.3·10-5 5.7·10-2 6.1·10-2 180º 20º 2.0·10-5 1.5·10-1 1.0·10-1 300º 19º 1.1·10-5 1.0·10-1 1.0·10-1 PEDM = 25% ∆φ BER BER BER φs BER<10-3 @ φs @ φs -72º @ φs +72º 60º 10º 9.0·10-5 1.8·10-1 2.0·10-1 180º 11º 7.1·10-5 2.7·10-1 2.1·10-1 Fig.12. Measured BER angular distribution for different values of PEDM. 300º 11º 7.7·10-5 2.2·10-1 2.2·10-1 AWGN is set to obtain at 180º a SNR of 18 dB. In Fig. 10b we can also observe that, at φs = 180º, there is a slight region and there is not a noticeable change in the case of PEDM = increase of the measured BER as PEDM decreases although the added 25%. Therefore, we can achieve, with our antenna, a secure AWGN is the same for the three cases. Fig. 11 depicts the measured transmission in a single angular region of the XY plane no matter constellations in the observation angle before adding AWGN where how good the eavesdropper SNR is away of this region. it can be deduced that the received signal becomes noisier as PEDM Out last experiment was devoted to verify that a single secure decreases. This aspect was already addressed with simulations in angular region could be set freely in the XY plane (0-360º). Fig. 13 [24], showing that this noise could be eliminated through an accurate shows the measured BER for different values of PEDM and φs. calibration (magnitude and phase) of the transmitters return loss. In For sake of comparison, Table I summarizes the main results our case, the board specifications only mention return losses higher extracted from Fig. 10 and Fig. 13. Firstly, we can see that, for a than 10 dB and, although they are good enough for a standard given PEDM, the same beamwidth with BER < 10-3 is achieved application, they can produce an increasingly noisy constellation at around the desired observation angle φs. Secondly, we noticed that, φs as the PEDM is reduced in order to achieve a narrower secure beam for PEDM = 50%, the BER at the side lobes is, at least, one order of width. magnitude higher than the conventional modulation (PEDM = 100%). As potential eavesdroppers could have higher signal level than the Finally, we want to point out that, again, for PEDM = 25%, the BER desired receiver (e.g. because they are closer to the transmit at the side lobes is of the order of 0.2 and it does not change with the antenna), we carried out a second experiment where the performance secure observation angle. of the system was evaluated under an arbitrary improvement of 6 dB in SNR (Fig. 12). If we recall that eavesdroppers also follow the V. CONCLUSIONS training stage before demodulating, this scenario is very unfavorable In this contribution, we have presented the design of a multiport from the point of view of security. If we compare Fig. 12 to Fig. 10b, compact stacked patch antenna (diameter 0.7λ) that radiates four we see, as expected, an important decrease of BER around φs for all different modes at the same frequency. Measured AEPs show measurements and a slight increase of the beamwidth with BER < variations in magnitude and phase that make this antenna a suitable 10-3. Regarding the side lobes, we find four additional angular candidate for generating dynamic direction dependent antenna regions with BER < 10-3 for the conventional modulation (PEDM = modulation. The compact stacked patch antenna can provide 100%). However, in the case of dynamic DM with PEDM = 50% equivalent performance to a conventional 4 element array with a there only exists a slight improvement of BER in the main side lobes spacing of λ/2, but with significantly reduced size. 6 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION Our design improves the previous work in [16] in two main [7] T. Hong, M.-Z. Song, and Y. Liu, “Dual-beam directional modulation aspects. Firstly, the removal of the monopole results in a low profile technique for physical-layer secure communication,” IEEE Antennas Wireless Propag. Lett., vol. 10, pp. 1417–1420, 2011. antenna (thickness 0.05λ). Secondly, the combination of the four [8] M. P. Daly and J. T. Bernhard, “Directional modulation technique for radiating modes (instead of two) allows the generation of a radiation phased arrays,” IEEE Trans. Antennas Propag., vol. 57, no. 9, pp. pattern with a front to back ratio of at least 10 dB and 360º steering. 2633–2640, 2009. 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