Energy-Efficient Physical Layer Security for Wearable IoT Devices Abel Zandamela * , Nicola Marchetti * , Adam Narbudowicz * * CONNECT Centre, Trinity College Dublin, The University of Dublin, Dublin, Ireland. {zandamea, nicola.marchetti, narbudoa}@tcd.ie Abstract—This work proposes an energy-efficient Directional Modulation (DM) scheme for on-body Internet of Things (IoT) devices. DM performance is tested using a 5-port stacked-patch MIMO antenna under two scenarios: free space case and using a four-layer human forearm phantom to simulate the user’s wrist. It is demonstrated that the scheme achieves steerable secure transmissions across the entire horizontal plane. With low Bit Error Rate (BER) of 1.5 × 10 -5 at the desired directions, eavesdroppers experience a high error rate of up to 0.498. Furthermore, this work investigates the DM performance using a subset of the stacked patches in the MIMO antenna, revealing that some combinations achieve a low BER performance using a lower antenna profile, albeit high side-lobes of BER< 10 -2 seen outside the desired region. Overall, the solution is proposed as a good candidate to enable secure wireless communications in emerging wearable IoT devices that are subject to size and energy-constraints. Index Terms—Physical layer security (PLS), directional modu- lation (DM), MIMO antennas, wearable IoT devices, compact IoT devices, on-body IoT systems, pattern reconfigurable antennas. I. I NTRODUCTION The Internet of Things (IoT) technology is increasingly shaping our daily-life, making IoT devices key enablers of cutting-edge technologies like smart cities and healthcare [1], [2]. On-body IoT devices such as wrist-worn devices are becoming indispensable for many modern applications, e.g. patient monitoring, in-hospital localization and tracking, sports fitness and tracking, smart building monitoring [2], [3]. However, miniaturization, multi-functionality and low energy- consumption capabilities are key design challenges in wearable IoT devices; such characteristics limit implementation of more traditional security methods for wearable IoT scenarios [3]. Physical Layer Security (PLS) is an emerging technique proposed to enhance wireless transmissions by exploiting the physical characteristics of the wireless channel. An example of PLS solution is the Directional Modulation (DM) method, which transmits intelligible modulation only in the direction of the intended legitimate receiver [5], [6]. In state-of-the-art it is executed using antenna arrays [7]–[9], which due to the inter-element spacing, are too large for wearable IoT devices at sub-6 GHz bands. DM implementations using small antennas are discussed in [10]–[12]. However, those implementations require simultaneous port activation to realize DM, which is still challenging for wearable IoT systems, as the necessary number of active RF transceivers increases the cost and exac- erbates the tight energy constraint of wearable technologies. Single port DM implementations using time-modulated arrays are proposed in [13], [14]; however, these methods are still demanding for wearable IoT implementation due to switch synchronization (which requires more computational power), and larger space for antenna packaging. In our previous work [15], we propose a single-port DM method relying on switches but without the need for any specific sequence, i.e. the switching can be controlled by a fully random mechanism, independent from the transceiver. However, the method was demonstrated using a 5-element circular array which still has too large a profile and size for wearable devices. In this work, we propose a single-port DM scheme for on-body IoT devices. The system is tested using a four- layer human forearm phantom and a 5-port stacked patch antenna capable of unidirectional beamsteering around the entire horizontal plane [12]. It is demonstrated that a low Bit Error Rate (BER) of 1.5 × 10 −5 is experienced by the desired receiver, while a high error rate of up to 0.498 is observed outside the desired regions. In addition, to allow for advanced DM properties like multi-target transmissions, subsets comprising different combinations of stacked-patches of the investigated antenna are used for feasibility of multiport DM with reduced number of RF transceivers. It is demon- strated that DM transmissions can be realized using a lower profile device, reduced number of ports and higher efficiency, albeit for such a case the compromise requires side-lobes in some directions. II. SIMULATION SETUP A. Antenna Configuration To analyze the performance of the energy-efficient direc- tional modulation scheme, a simulation setup comprising a multimodal beamsteering stacked-patch antenna proposed in [12] and a forearm phantom are used. For completeness, the design is shown in Fig. 1a; it comprises three stacked dielectric-loaded patches designed to operate at the center fre- quency of 5 GHz and fed using 5-ports to excite five different orthogonal modes. The top patch has diameter D 1 = 30.4 mm and is fed using P4 and P5 rotated by α 1 = 45°; such arrangement excites two orthogonal TM 21 modes. The middle patch has diameter D 2 = 34.6 mm and is fed using P2 and P3 rotated by 30° to excite two orthogonal TM 31 modes. Finally, the bottom patch has the largest diameter, D 3 = 78 mm, and is fed using P1 to generate a monopole-like TM 02 mode. The ports of the top and middle patches are fed with ±90° phase arXiv:2301.06174v1 [eess.SP] 15 Jan 2023