Design and Evaluation of an Electromagnetic Band Gap Structure for Self-Interference Reduction in mmWave Full-Duplex Systems

Full-duplex (FD) wireless is a new technology that allows a device to transmit and receive at the same time and on the same frequency band. It has the potential to double the capacity and spectral efficiency of a wireless link compared to the family of conventional half-duplex wireless systems. The key challenge in implementing FD wireless communication is self-interference (SI): a node's transmitting signal generates significant interference to its receiver. Several previous studies have demonstrated the potential to reduce SI and develop FD radios; however, these studies are mostly limited to sub-6 GHz systems. Previous and on-going research explored various techniques, including analog self-interference cancellation (SIC), digital self-interference cancellation, passive self-interference cancellation, and combinations of these approaches. However, at millimeter wave (mmWave) frequencies, the sizes of components decrease significantly in relation to their wavelength, leading to a more compact form factor for mmWave designs. This miniaturization allows for the possibility of innovative FD design approaches; however, it also presents challenges, such as reduced spacing between components. As a result, implementing analog or digital SIC techniques using robust component devices for mmWave radio and wireless applications demand careful consideration and adaptation. Furthermore, the transceiver architecture developed for FD solutions at sub 6 GHz frequencies requires modifications to effectively transition to mmWave applications, highlighting the need for architectural advancements in both the digital and analog SIC sections of the transceiver. By addressing these challenges, we can improve the performance and functionality of FD mmWave systems. This dissertation investigates the potential of a passive solution for SIC at the mmWave frequency of 28 GHz. The research study approach involves the design and integration of a novel electromagnetic band gap (EBG) structure within a substrate design of transmit (Tx) and receive (Rx) antenna arrays. The research investigates novel EBG structures aimed at mitigating planar and surface wave coupling between Tx and Rx antennas within the reactive near-field region of the antenna system. By attenuating these undesired coupling mechanisms, the EBG structure effectively reduces SI to levels that permit reliable reception of the desired signal in a FD operation. The Tx and Rx antenna system considered for this research comprises unit antenna (single Tx and Rx) elements and Multiple Input Multiple output (MIMO) antenna arrays (1x4 and 4x4). The designs were also fabricated for testing, measurement, and comparison to simulation data. The simulation and manufacturing of prototype antenna and novel EBG designs took into account manufacturing process variations, and substrate material permittivity changes due to temperature and humidity effects. Furthermore, the design of the unit and MIMO antenna array was validated through passive measurement in an anechoic chamber and using a Vector Network Analyzer (VNA) to determine and compare measured antenna performance metrics with simulated data. The novelty of our EBG designs lies in the substantial reduction of effective capacitance compared to conventional mushroom-type EBG structures operating at 28 GHz. This reduction is achieved through miniaturization of the unit cell, which allows a greater number of EBG elements to be integrated within a fixed footprint. The miniaturization and effective capacitance change of the novel EBG results in enhanced suppression of surface and leaky wave modes. Consequently, the structure provides wideband isolation between the Tx and Rx antenna arrays, significantly improving FD performance for mmWave applications. Ultimately, this dissertation aims to advance mmWave FD wireless communication by substantially mitigating SI in the passive RF domain. By addressing SI at the physical layer, the approach has the potential to eliminate reliance on complex and bulky analog cancellation circuits, thereby enabling the development of more compact, energy-efficient, and cost-effective mmWave full-duplex radio architectures.