On the heels of orthogonal time frequency space (OTFS) modulation, the recently discovered affine frequency division multiplexing (AFDM) is a promising waveform for the sixth-generation wireless network. In this paper, we study the widely-used embedded pilot-aided (EPA) channel estimation in multiple-input multiple-output AFDM (MIMO-AFDM) system with fractional Doppler shifts. We first formulate the vectorized input-output relationship of MIMO-AFDM, and theoretically prove that MIMO-AFDM can achieve full diversity in doubly selective channels. Then we illustrate the implementation of EPA channel estimation in MIMO-AFDM and unveil that serious inter-Doppler interference (IDoI) occurs if we try to estimate the channel gain, delay shift, and Doppler shift of each propagation path. To address this issue, the diagonal reconstructability of AFDM subchannel matrix is studied and a low-complexity embedded pilot-aided diagonal reconstruction (EPA-DR) channel estimation scheme is proposed. The EPA-DR scheme calculates the AFDM effective channel matrix directly without estimating the three channel parameters, eliminating the severe IDoI inherently. Since the effective channel matrix is necessary for MIMO-AFDM receive processing, we believe this is an important step to bring AFDM towards practical communication systems. Simulation results validate the effectiveness of the proposed EPA-DR scheme.

In this letter, a simple parasitic structure consisting of a microstrip L-section array and a microstrip U-section array is proposed for mutual coupling reduction of dual-polarization (DP) patch antenna array. The additional coupling paths are introduced by the structure in the co-horizontal polarization (CHP) and co-vertical polarization (CVP) directions of the array respectively to offset the original coupling signals. A prototype array with the center to center distance of 0.42 $\lambda_0$ is fabricated and measured for verification. The isolation between DP array with decoupling structure reaches to 37.5dB and 33.6dB on CHP and CVP at 4.3GHz and envelope correlation coefficient is less than 0.004. The radiation gain of the antenna is almost not distorted while the total efficiency is increased from 84% to 91%. In comparison with other related works, our scheme has high isolation and high total efficiency with smaller space and lower profile.

A co-aperture dual circularly polarized antenna for simultaneous transmit (Tx) and receive (Rx) sensing systems is proposed in this letter. The antenna consists of a radiating patch, a dielectric substrate, and a ground plane. Embedding a rectangular slot in the center of the patch implements left-handed circular polarization for Tx and right-handed circular polarization for Rx. A pair of U-shaped slots (USS) with a 90° phase shift is placed into the ground plane for decoupling. When the USS works, the mutual coupling in the x - and y - direction cancels each other at the Rx port. As a verification, a prototype with the size of $0.43{\lambda _0} \times 0.43{\lambda _0} \times 0.01{\lambda _0}$ is fabricated and measured. The operating bandwidth, in which $| {{S_{11}}} |$ and axial ratio are respectively less than –10 dB and 3 dB, is 4.32–4.42 GHz. Within this band, the measured peak realized gain is 5.4 dBi, and the isolation is better than 23 dB with a maximum of 34 dB. In comparison with other works, the proposed antenna is competitive in terms of antenna size, aperture efficiency, and total efficiency.

Aperture-level simultaneous transmit and receive (ALSTAR) attempts to utilize adaptive digital transmit and receive beamforming and digital self-interference cancellation methods to establish isolation between the transmit and receive apertures of the single-phase array. However, the existing methods only discuss the isolation of ALSTAR and ignore the radiation efficiency of the transmitter and the sensitivity of the receiver. The ALSTAR array design lacks perfect theoretical support and simplified engineering implementation. This paper proposes an adaptive random group quantum brainstorming optimization (ARGQBSO) algorithm to simplify the array design and improve the overall performance. ARGQBSO is derived from BSO and has been ameliorated in four aspects of the ALSTAR array, including random grouping, initial value presets, dynamic probability functions, and quantum computing. The transmit and receive beamforming carried out by ARGQBSO is robust to all elevation angles, which reduces complexity and is conducive to engineering applications. The simulated results indicate that the ARGQBSO algorithm has an excellent performance, and achieves 166.8 dB of peak EII, 47.1 dBW of peak EIRP, and −94.6 dBm of peak EIS with 1000 W of transmit power in the scenario of an 8-element array.

Interrupted sampling repeater jamming (ISRJ) is a classic active coherent jamming. Due to its structural limitations, it has inherent defects such as a discontinuous time–frequency (TF) distribution, strong distribution regularity of pulse compression results, limited jamming amplitude, and strong false targets lagging behind the real target. These defects have not been fully resolved yet due to the limitation of the theoretical analysis system. Based on the analysis of the influence factors of ISRJ on the interference performance for linear-frequency-modulated (LFM) and phase-coded signals, this paper proposes an improved ISRJ method based on the joint subsection frequency shift and two-phase modulation. The coherent superposition of jamming signals at different positions for LFM signals is achieved by controlling the frequency shift matrix and the phase modulation parameters to form a strong pre-lead false target or multiple positions and ranges of blanket jamming areas. For the phase-coded signal, the pre-lead false targets are generated through code prediction and the two-phase modulation of the code sequence, resulting in similar noise interference. The simulation results show that this method can overcome the inherent defects of ISRJ.