Lin Mei

Affine frequency division multiplexing (AFDM) achieves full diversity in doubly selective channels. By exploiting its clear interference structure, decision feedback equalization (DFE) offers a robust, non-iterative receiver, yet suffers from severe error propagation. A permutation-driven DFE (P-DFE) framework is proposed to optimize both single-trajectory reliability and parallel structural diversity. For the single-branch P-DFE, permutation acts as an interference-shaping mechanism. Guided by derived statistical bounds, a low-complexity energy graph (EG) algorithm is developed to leverage the sparse AFDM channel for approximating optimal reliability sorting, reshaping the signal-to-interference-plus-noise ratio (SINR) profile to suppress initial errors. To decouple error propagation in high order constellations, permutation is elevated to a branch-generation tool for a parallel scheme. Specifically, a combination of cyclic shifts and time reversal is applied to the base order, generating a quality-preserving branch family. An optimal permutation-driven quad-branch DFE (P-QDFE) is configured by maximizing the harmonic mean of Kendall-Tau (KT) distances, minimizing inter-branch correlation. Simulations show P-QDFE effectively suppresses error floors in high-order modulations. At practical pre-decoding thresholds, this non-iterative scheme outperforms highly-iterated orthogonal approximate message passing (OAMP) under low-density parity-check (LDPC) coding, while integrating incomplete Cholesky factorization (ICHOL) ensures linear minimum mean square error (LMMSE)-level complexity.

In this letter, we investigate a multi-antenna joint transmission scheme based on affine frequency division multiplexing (AFDM) in the mosaic communication system. Considering the presence of time synchronization errors among different antennas, we propose a non-coherent joint transmission AFDM (NCJT-AFDM) system to improve the reliability of multi-antenna transmission systems. Since multi-antenna transmissions introduce distinct delay-Doppler spreads, which are coupled with the fractional-Doppler-induced interference inherent in AFDM, we develop an embedded single-pilot-aided approximate maximum likelihood successive interference cancellation (ESPA-AML-SIC) method. This method mitigates the adverse impact of insufficient spacing coefficient kv on channel estimation. Simulation results validate the effectiveness of the proposed scheme.

The rapid evolution of reconfigurable intelligent surfaces (RISs) has opened new frontiers in channel impairment mitigation, particularly through passive beamforming. However, when integrated with waveforms under doubly selective channels, a fundamental timescale mismatch exists: the existing RIS designs typically employ a single static phase configuration per frame due to hardware constraints, whereas the channel is time-varying. To address this issue, we propose a novel RIS-aided orthogonal time frequency space (OTFS) system integrated with index modulation (IM). IM serves as a generalized framework encompassing conventional modulation, enhancing the spectral efficiency (SE) and transmission flexibility. By analyzing the structure and energy distribution of time-varying channels in the delay-Doppler (DD) domain, we develop a diagonal phase alignment (DPA) strategy that aligns the RIS-reflected paths with the direct path, maximizing the achievable rate with negligible computational overhead. Unlike existing works, we explicitly consider the impact of imperfect biorthogonality in rectangular pulse-shaping waveforms, which alters the structure of the equivalent channel matrix. This paper provides detailed mathematical system models and RIS deployment scenarios. Furthermore, the closed-form upper bound of bit error rate (BER) is derived for Rician fading channels. Simulation results validate the theoretical analysis and demonstrate promising performance in terms of the achievable rate and BER.

Orthogonal chirp division multiplexing (OCDM) has emerged as a promising multi-carrier scheme for future wireless communications. To further enhance its spectral efficiency (SE), we propose spectrally efficient chirp division multiplexing (SECDM), which introduces chirp-domain compression by reducing the sub-carrier spacing of OCDM. To address the extremely high complexity of maximum likelihood (ML) detector, we develop a preprocessing-aided fixed sphere decoder (FSD) for SECDM that delivers near-ML performance with low complexity. Compared to the representative non-orthogonal waveform, spectrally efficient frequency division multiplexing (SEFDM), the proposed SECDM exhibits stronger resilience to inter-carrier interference (ICI) caused by compression-induced non-orthogonality, owing to its chirp-based nature. Simulation results validate the superiority of SECDM over SEFDM in terms of both bit error rate (BER) and SE.

In this letter, a dual-band, dual-beam shared-aperture antenna combining a variable inclination continuous transverse stub (VICTS) antenna and a patch antenna array (PAA) is proposed for simultaneous connections with two satellites. The VICTS generates a Ku-band scanning beam, whereas the PAA provides a K-band fixed-direction beam. By mounting the PAA on the surface of the radiating layer of the VICTS antenna, a shared-aperture configuration is achieved. The feeding network of the PAA is specially designed, and long slots are etched on both the substrate and the ground plane to effectively mitigate the interference. To verify the proposed design, a prototype was fabricated and measured, showing good agreement between the measured and simulated results.

Carrier mode shift keying (CMSK) is a recently proposed waveform-domain index modulation scheme that improves the spectral efficiency (SE) of hybrid carrier (HC) communication systems. This innovative scheme simultaneously transmits information bits mapped to the traditional constellation symbols as well as information bits mapped to the index of the weighted-type fractional Fourier transform (WFRFT) parameter. In this paper, we propose an enhanced version of CMSK (E-CMSK) that introduces a subblock precoding module at the transmitter side. Compared with CMSK, E-CMSK conveys index information through subblock-level parameter mapping, thereby further improving the SE. Furthermore, E-CMSK provides enhanced system flexibility, enabling trade-offs among the SE, reliability and detection complexity through subblock size adjustment. To address the prohibitive complexity of the maximum likelihood (ML) detector, a low-complexity sequential detector based on linear frequency domain equalization (FDE) is developed, which reduces the computational complexity from exponential to linear. Furthermore, to compensate for the performance loss inherent in linear equalization, an enhanced iterative block decision feedback equalization (IB-DFE) detector is proposed. This iterative scheme effectively suppresses residual interference, enabling the system to approach the global ML performance. An analytical expression for the average overall bit error rate (BER) is derived to provide theoretical performance insights. While this expression reveals the impact of key parameters like Euclidean distance, its complexity makes it intractable for direct optimization. Therefore, we directly analyze the Euclidean distance distribution of the E-CMSK constellation as an intuitive yet effective tool for system design. This analysis guides the design of a Gray-coded bit-parameter mapping scheme and an equally spaced WFRFT parameter selection strategy, which aims to improve the overall BER performance. Simulation results demonstrate that the proposed E-CMSK scheme outperforms the conventional HC system in multi-path fading channels, particularly at high SNRs.

Beampattern synthesis is essential in modern communication, radar, and electronic warfare systems as it enables the design of customized radiation patterns. This letter introduces a method that employs Riemannian manifold optimization to effectively handle both the constant modulus constraint and dynamic range ratio (DRR) control in array beampattern synthesis. A multi-objective optimization function is developed to ensure uniform mainlobe response, ensure smooth beam transitions, concentrate energy at the beam center, and effectively suppress sidelobes. Additionally, log-sum-exp (LSE) smoothing techniques are utilized to convert non-smooth problems into differentiable forms, precise control of the beam direction is achieved through exponential transformations, and a smoothing term based on the Laplacian matrix is introduced to minimize power deviation between beams. In addition, a DRR constraint is constructed using complex circles, ensuring that the constraint remains rigorously maintained throughout the optimization process. This letter demonstrates the effectiveness of the proposed method through examples of array pattern optimization under amplitude constraints. The results indicate that the proposed method not only achieves precise main lobe performance but also provides effective sidelobe suppression, all while strictly adhering to the amplitude constraints.