With the uninterrupted revolution of communications technologies and the great-leap-forward development of emerging applications, the ubiquitous deployment of Internet of Things (IoT) is imperative to accommodate constantly growing user demands and market scales. Communication security is critically important for the operations of IoT. Among the communication security provisioning techniques, physical layer security (PLS), which can provide unbreakable, provable, and quantifiable secrecy from an information-theoretical point of view, has drawn considerable attention from both the academia and the industries. However, the unique features of IoT, such as low-cost, wide-range coverage, massive connection, and diversified services, impose great challenges for the PLS protocol design in IoT. In this article, we present a comprehensive review of the PLS techniques toward IoT applications. The basic principle of PLS is first briefly introduced, followed by the survey of the existing PLS techniques. Afterwards, the characteristics of IoT are identified, based on which the challenges faced by PLS protocol design are summarized. Then, three newly-proposed PLS solutions are highlighted, which match the features of IoT well and are expected to be applied in the near future. Finally, we conclude the paper and point out some further research directions.

A single square patch antenna for pattern diversity is investigated. The proposed antenna consists of a double-feed square patch antenna with a rat-race network. By switching the feeding ports, the different radiation patterns for two modes (TM 10 mode and the capacitive-loaded monopole radiating mode) operating over an overlapped frequency band from 1.88 to 2.34 GHz are achieved. TM 10 mode reveals good broadside radiation patterns and the capacitive-loaded monopole radiating mode shows conical radiation patterns. The antenna is fabricated and tested. The measured gains across the common frequency band are 8.9-9.9 dBi and 2.8-3.8 dBi for TM 10 mode and the capacitive-loaded monopole radiating mode, respectively. Besides, the measured bandwidths of -10 dB reflection coefficient are 750 MHz (1.59-2.34 GHz, 38.2%) for TM 10 mode and 880 MHz (1.88-2.76 GHz, 37.9%) for the capacitive-loaded monopole radiating mode. High isolation (<-20dB) between the two feeding ports for the common impedance matching band is achieved. These results make the single dual-port patch antenna an attractive solution for 3G/4G pattern diversity applications such as in gym scenarios.

Millimeter-wave (mmWave) and orbital angular momentum (OAM) multiplexing are two key technologies for modern wireless communications, where significant efforts have been devoted to combining these two technologies for extremely high channel capacities. Recently, programmable metasurfaces have been extensively studied for stimulating dynamic multi-mode OAM beams, owing to their ability of subtle dynamic modulation over electromagnetic waves in a digital manner. However, programmable metasurfaces for mmWave OAM stimulation are rarely mentioned, due to the requirement of extremely high processing precision for mmWave applications. In this paper, a programmable metasurface is presented to stimulate dynamic multi-mode mmWave vortex beams. The proposed metasurface is composed of electronically reconfigurable units, which is obtained through configuration integration of a PIN diode within each radiation patch for modulating the unit resonant property. Both low reflection losses and stabilized inverse phase states are obtained for the binary unit coding states within the operation band. Through modulating the real-time coding distribution on the metasurface by programmable bias circuit, the generation of mmWave OAM beams with mode numbers l = 0, l = +1, l = +2, and l = +3 are numerically designed and experimentally verified. Our study paves a new perspective for the cross amalgamation of both mmWave and multi-mode OAM technologies.

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