A Ku-band electronic 2-dimensional (2-D) beam-scanning transmitarray employing a new reconfigurable dual-layer Huygens element is developed in this article. The Huygens element consists of two metallic crosses printed on two layers of a dielectric substrate, which enables a near nonreflection Huygens resonance. A 1 bit phase compensation with low transmission loss is realized by controlling two p-i-n diodes on the element. Compared with many other reconfigurable transmitarray elements using multilayer structures with metallic vias, the proposed reconfigurable Huygens element has a much simpler configuration with a simpler biasing network, and it is not affected by multilayer alignment errors. This particularly facilitates large aperture array development at higher frequencies. To validate the design concept, an electronically reconfigurable transmitarray with the proposed element is fabricated at 13 GHz. Good agreement between the measured and simulated results is found, showing 2-D scanning beams within ±50° in the E-plane and ±40° in the H-plane with a maximum realized gain of 18.4 dBi.

In-band full-duplex (IBFD) technology has been extensively researched over the last decade, and numerous techniques have been proposed to cancel self-interference (SI) caused by the simultaneously transmitted local in-band signal. Although some prototypes for SI cancellation (SIC) have been published, there are limited works demonstrating the communication performance of the IBFD system. The noise enhancement caused by analog and/or digital SIC is mostly ignored in the literature. This article presents a prototype for an IBFD system with solutions in three domains of SI mitigation. A practical structure for joint analog and digital SIC is proposed and implemented using off-the-shelf components. The transmit data frame is designed to align with the track/hold control signaling so that the irreducible noise caused by the analog SIC can be avoided. By combining a full-duplex antenna, an analog SIC filter, and a field-programmable gate array (FPGA) platform, this prototype successfully demonstrates the performance of the desired IBFD communications. Experiments on the prototype with 50-MHz bandwidth signaling show that under a certain hardware condition, the proposed SIC solution can sufficiently mitigate the SI with 75 dB of cancellation, and a low bit error rate (BER $< $ $10^{-4}$ ) is demonstrated to allow practical applications.

Wideband millimeter wave and terahertz systems face severe nonlinearity and other practical impairments such as transmitter and receiver in-phase/quadrature imbalances (IQIs), carrier frequency offset, and phase noise. Based on a simplified yet effective received signal model including an expanded memory polynomial (EMP) and a noisy receiver filter, this paper proposes a low-complexity digital post-cancellation (DPC) framework for transmitter IQI and overall system nonlinearity mitigation. The nonlinearity parameters with reduced nonlinearity order are firstly estimated with low-complexity using a novel transmission protocol incorporating both frame rotation and preamble power scaling. Through widely linear system equalization and interference cancellation, the signal distortion caused by frequency-dependent IQI and nonlinearity is then mitigated with significant performance improvement. The mean-squared-error measurement of the EMP-modelled signals also provides a practical means for the nonlinear system identification and characterization. Both simulation and experiment results obtained from a millimeter wave system with 2.125 GHz bandwidth and 73.5 GHz carrier frequency are presented to verify the theoretical analyses and demonstrate the effectiveness of the DPC technology.

For wideband transceivers operating at millimeter wave and terahertz frequencies, the implementation of conventional digital predistortion for nonlinearity mitigation faces significant challenges due to the limited availability and/or complexity of high-speed digital signal processing. In this paper, a simple received signal model is proposed for wideband system with nonlinearity and other practical impairments, such as transmitter (Tx) and receiver (Rx) I/Q imbalances (IQIs), carrier frequency offset (CFO), and phase noise, to enable low-complexity impairment mitigation. An expanded memory polynomial (EMP) model is firstly proposed to capture Tx IQI and the nonlinearity over the entire transceiver chain. Exploiting the CFO and a novel transmission protocol, a blind Rx IQI estimation is also proposed. The noise enhancement after Rx IQI and CFO compensation is then evaluated as a noise factor related to the mean-square-error of the Rx IQI estimation. As a result, the received signal of the wideband system is finally modelled as an EMP plus additive noises followed by a band-limited noisy receiver filter. Simulation results using a millimeter wave system with 2.5 GHz bandwidth and 73.5 GHz carrier frequency are presented to verify the accuracy of the EMP modelling and validate the theoretical analyses.

This paper proposes an orthogonal expanded memory polynomial (OEMP) model for circular complex Gaussian processes. The basis functions of the OEMP model are constructed from those of the expanded memory polynomial (EMP) model recently derived by expanding the conventional memory polynomial (MP) model for I/Q imbalanced signals, and proved orthogonal to each other thanks to the derived closed-form bivariate distribution moments. In addition to the perfect duality with its power spectral density expression, the OEMP model also demonstrates unique minimum residual power property. It is believed that the OEMP model will be highly beneficial to nonlinearity mitigation for wideband wireless communication systems. Simulation and experimental results obtained from a terahertz system with 4.25 GHz bandwidth and 252 GHz carrier frequency confirm that significant performance improvement can be achieved for digital post-cancellation with low-complexity after applying the proposed OEMP model.

This paper presents a 245 GHz wireless communications system with a data rate of 30 Giga bits per second (Gbps) at a 1.2 m distance, which proves the potential for future high-speed communications beyond 5G technology. The system consists of low-complexity and real-time base-band modules to provide the high-speed wideband signal processing capability. Multi-channel base-band signals are combined and converted to 15.65 ± 6.25 GHz wideband intermediate frequency (IF) signals. A novel 245 GHz waveguide bandpass filter (BPF) with low loss and high selectivity is designed and applied to a terahertz (THz) front-end for image rejection and noise suppression. Configuration of the base-band, IF, and THz front-end modules is also given in detail. The 245 GHz wireless communication link is demonstrated over a distance of 1.2 m.