In order to enable the simultaneous transmission and reception of wireless signals on the same frequency, a full-duplex (FD) radio must be capable of suppressing the powerful self-interference (SI) signal emitted from the transmitter and picked up by the receiver. Critically, a major bottleneck in wideband FD deployments is the need for adaptive SI cancellation (SIC) that would allow the FD wireless system to achieve strong cancellation across different settings with distinct electromagnetic environments. In this work, we evaluate the performance of an adaptive wideband FD radio in three different locations and demonstrate that it achieves strong SIC in every location across different bandwidths.

The relentless demand for data in our society has driven the continuous evolution of wireless technologies to enhance network capacity. While current deployments of 5G have made strides in this direction using massive multiple-input-multiple-output (MIMO) and millimeter-wave (mmWave) bands, all existing wireless systems operate in a half-duplex (HD) mode. Full-duplex (FD) wireless communication, on the other hand, enables simultaneous transmission and reception (STAR) of signals at the same frequency, offering advantages such as enhanced spectrum efficiency, improved data rates, and reduced latency. This article presents a comprehensive review of FD wireless systems, with a focus on hardware design, implementation, cross-layered considerations, and applications. The major bottleneck in achieving FD communication is the presence of self-interference (SI) signals from the transmitter (TX) to the receiver, and achieving SI cancellation (SIC) with real-time adaption is critical for FD deployment. The review starts by establishing a system-level understanding of FD wireless systems, followed by a review of the architectures of antenna interfaces and integrated RF and baseband (BB) SI cancellers, which show promise in enabling low-cost, small-form-factor, portable FD systems. We then discuss digital cancellation techniques, including digital signal processing (DSP)- and learning-based algorithms. The challenges presented by FD phased-array and MIMO systems are discussed, followed by system-level aspects, including optimization algorithms, opportunities in the higher layers of the networking protocol stack, and testbed integration. Finally, the relevance of FD systems in applications such as next-generation (xG) wireless, mmWave repeaters, radars, and noncommunication domains is highlighted. Overall, this comprehensive review provides valuable insights into the design, implementation, and applications of FD wireless systems while opening up new directions for future research.

$N$-path switched-$RC$circuits have been extensively investigated as a promising solution for realizing various novel functionalities, including on-chip tunable high-$Q$filters, true time delays operating beyond the delay-bandwidth product, and non-reciprocal components. The clock frequency typically sets important parameters of the functionality, such as the frequency of the filter, the amount of achievable delay etc. The ability to tune the clock frequency through a wideband synthesizer enhances the reconfigurability of$N$-path switched-$RC$circuits, but the presence of multiple independent$N$-path circuits on a chip results in the need for multiple independent wideband synthesizers. Here, we introduce a novel concept of rotary clocking in$N$-path circuits which enables us to passively frequency shift any$N$-path filter by either rotating clockwise or anti-clockwise the clocks exciting the switches of each of the paths. The effect of rotary clocking is analyzed using linear periodically-time-variant (LPTV) circuit theory, and has been verified through simulations and measurements. The effects of quantization due to finite clock phases, and the resultant spurs produced by this method, are also analyzed and compared with measurement results. Measurement results are presented for a two-port$N$-path filter implemented in a 65-nm CMOS 0.1-1 GHz highly-reconfigurable self-interference canceling receiver. The two-port filter achieves a maximum frequency shift of$f_{s}/8$in the steps of$f_{s}/160$and also has a phase control covering all of 360° in steps of 45°.

We present a set of experiments utilizing wideband real-time adaptive full-duplex (FD) radios, demonstrating simultaneous transmission and reception on the same frequency channel. Each FD radio consists of a circulator-based antenna interface, a switched-capacitor delay-line-based configurable Radio-Frequency Integrated Circuit (RFIC) that implements Self-Interference Cancellation (SIC), an FPGA that optimizes the RFIC configuration in under 1.1 sec and can adapt to environmental changes in under 0.3 sec, and a Software-Defined Radio (SDR) transmitting OFDM-like packets. We demonstrate a real-time adaptive FD radio that achieves the SIC necessary to reach the noise floor across a wide bandwidth of 50 MHz. Then, we use two FD radios to create a wireless link and showcase the superior FD throughput.