The use of in-band full-duplex (FD) enables nodes to simultaneously transmit and receive on the same frequency band, which challenges the traditional assumption in wireless network design. The full-duplex capability enhances spectral efficiency and decreases latency, which are two key drivers pushing the performance expectations of next-generation mobile networks. In less than ten years, in-band FD has advanced from being demonstrated in research labs to being implemented in standards, presenting new opportunities to utilize its foundational concepts. Some of the most significant opportunities include using FD to enable wireless networks to sense the physical environment, integrate sensing and communication applications, develop integrated access and backhaul solutions, and work with smart signal propagation environments powered by reconfigurable intelligent surfaces. However, these new opportunities also come with new challenges for large-scale commercial deployment of FD technology, such as managing self-interference, combating cross-link interference in multi-cell networks, and coexistence of dynamic time division duplex, subband FD and FD networks.

We propose a new analog self-interference cancellation (SIC) technique for in-band full-duplex transmission in single-antenna systems. We use an RF circulator to separate transmitted (Tx) and received (Rx) signals. Instead of estimating the SI signals and subtracting them from the Rx signals, we use the inherent secondary SI signals at the circulator, reflected by the antenna, to cancel the primary SI signals leaked from the Tx port to the Rx port. We modified the frequency response of the secondary SI signals using a reconfigurable impedance mismatched terminal (IMT) circuit, which consists of two varactor diodes at the antenna port. We can also adjust the frequency band and the bandwidth by controlling the varactor diodes bias voltages. The IMT adjustability makes it robust to antenna input impedance variations and fabrication errors. We analyze and fabricate a prototype of the proposed technique at 2.45 GHz. We achieved more than 40-dB cancellation over 65 MHz of bandwidth. Our technique is independent of the RF circulator and antenna type and it can be applied to any frequency band. It is also very relevant to small mobile devices because it provides a simple and low-power and low-cost adjustable analog SIC technique.

In this article, we review the key concepts and the progress in the design of physical-layer aspects of in-band full-duplex (IBFD) communications. One of the fundamental challenges in realizing IBFD is self-interference that can be up to 100 dB stronger than signals of interest. Thus, we start by reviewing state-of-the-art research in self-interference cancellation, addressing both model-based and emerging machine learning-based methods. Then, we turn our attention to new wireless systems with many degrees of freedom for which the traditional IBFD designs do not gracefully scale and, hence, require many innovations to enable IBFD. We provide an extensive review of basic concepts and state of the art in massive multiple-input–multiple-output IBFD. Then, we consider the mmWave band IBFD and review advanced physical-layer architectures. The above review provides the proper context to discuss IBFD innovations and new challenges for sixth-generation networks and beyond, where wireless networks are envisioned to be multifunctional, combining communications with functions such as sensing, cognitive radios, physical-layer security, and wireless power transfer. We conclude this article with a status update on the adoption of IBFD in communication standards.

The increasingly demanding objectives for next-generation wireless communications have spurred recent research activities on multiantenna transceiver hardware architectures and relevant intelligent communication schemes. Among them belong full-duplex (FD) multiple-input, multiple-output (MIMO) architectures, which offer the potential for simultaneous uplink (UL) and downlink (DL) operations in the entire frequency band. However, as the number of antenna elements increases, the interference signal leaking from the transmitter (Tx) of the FD radio to its receiver (Rx) becomes more severe. In this article, we present a unified FD massive MIMO architecture comprising analog and digital transmit and receive beamforming (BF) as well as analog and digital self-interference (SI) cancellation, which can be jointly optimized for various performance objectives and complexity requirements. The performance evaluation results for applications of the proposed architecture to fully digital and hybrid analog and digital-BF operations using recent algorithmic designs as well as simultaneous communication of data and control (SCDC) signals are presented. It is shown that the proposed architecture, for both small and large numbers of antennas, enables improved spectral-efficient FD communications with fewer analog-cancellation elements compared to various benchmark schemes. The article concludes with a list of open challenges and research directions for future FD massive MIMO communication systems and their promising applications.

We designed a reconfigurable dual-band reflection amplifier with operation frequency bands over 1.8 GHz and/or 2.4 GHz. This one-port amplifier boosts the reflected signal over either of these two frequency bands or both as a dual-band reflection amplifier. The amplifier circuit consists of an FET transistor and two p-i-n diodes, which act as switches to form a reconfigurable system. The measured reflection gains at single 1.8 GHz and 2.4 GHz frequency bands are 17.6 dB and 16.75 dB, respectively. The dual-band operation frequency shows 11.2 dB and 15 dB gain at 1.8 GHz and 2.4 GHz, respectively. Then, we realized a bidirectional amplifier using a dual-band -3 dB 90° branch line coupler integrated with two of the proposed reconfigurable reflection amplifiers. This bidirectional amplifier is a two-port bilateral amplifier. The measured reflection gains show at least 10 dB transmission gains (S ₂₁ and S ₁₂ ) at operation frequency bands. The proposed reconfigurable bidirectional amplifier is then used in a dual-band Van Atta array. The proposed active retrodirective system outperformed passive Van Atta array by 5 dB gain with only half the number of antenna elements. Each component is investigated analytically and analyzed by advanced design system. Fabricated circuits are measured and show good agreement with simulations.

A printed dipole antenna is designed with two parasitic elements loaded by embedded resonators to provide reconfigurable patterns. The embedded resonators in the parasitic elements are loaded by varactor diodes, which change the phase and the transmission coefficient responses of the parasitic elements. This allows the parasitic elements to act as either a director, a reflector, or a neutral element. The proposed printed dipole antenna with two loaded parasitic elements acts as a printed Yagi-Uda antenna. Because the parasitic elements can act as either a director or a reflector, the directed gain can be increased, and the front-to-back ratio (FBR) may be improved up to 42 dB. It is also possible to achieve different target FBR if broadcasting is needed in two directions of the boresight. This pattern reshaping can be obtained as quickly as the diodes can change their capacitance. The antenna is designed to operate at 2.4 GHz, but this design approach can be applied to other frequency bands. The simulation and measurement results are in good agreement.