Compact Beamforming Antennas for IoT-Enabled Directional Modulation and Localization

Abstract

In recent years, there has been a considerable focus in various research domains directed towards the miniaturization of wireless communication devices. This increasing demand for highly compact systems is a growing trend to facilitate cutting-edge technologies like the Internet of Things (IoT). In this regard, a fundamental component of wireless communication systems is the antenna that transmits or receives the radio waves, enabling information broadcast. With the rapidly expanding number of IoT applications, increasing constraints are placed on the available space for antenna installation while demanding more functionalities like beamforming and multi-band operation. The beamforming technology is critical to enable the control of the shape and direction of the antenna generated radiation pattern. This, in turn, enhances the overall wireless link by providing improved antenna gain, enhanced Signal-to-Noise Ratio (SNR), elimination of undesired interference, and enabling several modern applications like localization in terms of Angle of Arrival (AoA) and emerging physical layer security techniques like Directional Modulation (DM).

Traditionally, antenna arrays have been applied to realize flexible beamforming properties to enable AoA measurements and DM techniques. However, antenna arrays comprise a set of radiators requiring inter-element spacing of typically half of the free space wavelength at the antenna center operating frequency. Such a requirement makes the final structure too bulky for integration into compact IoT devices, especially those operating on the Sub-6 GHz frequency bands. Miniaturization of classical arrays can be accomplished by reducing the inter-element spacing. However, such miniaturization usually results in stronger mutual coupling between the elements, which leads to undesired effects like the radiation of distorted antenna pattern, reduced gain, polarization mismatch, among other issues. This ultimately degrades the beamforming characteristics and the performance of AoA-based localization and DM security approach.

The main goal of this thesis is to address the problem of performing beamforming while using miniaturized antennas. This thesis contributes to this issue by proposing novel beamforming solutions based on the theory of Spherical Modes. The work also highlights the benefits of using Spherical Modes Beamforming (SMB) antennas to enable AoA estimation and DM in small IoT platforms, and provides a framework linking the SMB characteristics with the performance metrics of both AoA estimation and DM techniques. Although the SMB solution provides significant miniaturization compared to classical arrays, non-planar geometries and single-band operation may still be limiting factors for its integration in emerging compact IoT devices, like on-body systems. This thesis addresses those limitations by introducing planar antennas capable of performing beamforming within the upper hemisphere and extending the SMB principle to support multi-band operation.