This contribution focuses on the optimization of bit error rate (BER) performance in fluid antenna (FA)-aided wireless communication systems, which leverage the new degrees of freedom provided by positional flexibility, thus surpassing the limitations of conventional fixed-position antenna (FPA) systems. In this context, a theoretical analysis identifies key metrics for maximum likelihood (ML) and zero-forcing (ZF) detectors, specifically the minimum singular value and effective rank, as determinants of system performance. Then, for single-input single-output (SISO) channels, the optimization problem is formulated as channel gain maximization constrained by the predefined moving region and then solved using a mixed-integer linear programming (MILP) model. Furthermore, for multiple-input multiple-output (MIMO) channels, an alternating optimization (AO) algorithm incorporating the Frank-Wolfe method is proposed to optimize ML and ZF performances, targeting minimum singular value maximization for ML and singular value balancing for ZF, both subject to moving region and antenna spacing constraints. Finally, numerical results exhibit significant performance gains for the FA system over its conventional FPA alternative. In general, these findings highlight the potential of FA systems for addressing ultra-reliable communication challenges in the sixth generation (6G) wireless communications.

This paper proposes a framework for applying a novel multi-domain modulation scheme, orthogonal chirp division multiplexing with index modulation (OCDM-IM), in integrated sensing and communication (ISAC) systems by combining OCDM and index modulation (IM). To support simple SISO-ISAC applications, a low-complexity fast Fourier transform (FFT)-based sensing algorithm is developed for the sake of the superior performance of OCDM-IM over traditional OCDM. Building on this, the framework is further extended to more attractive MIMO-ISAC systems in order to achieve improved bit error rate (BER) and a lower peak-to-average power ratio (PAPR) compared to the conventional MIMO-OCDM scheme. For sensing, it enables distance, velocity, and angle estimation by formulating the OCDM-IM waveform within a compressed sensing framework, which is then efficiently solved using the proposed orthogonal matching pursuit (OMP) algorithm. Simulation results confirm that the OCDM-IM waveform enhances communication performance through IM while preserving the promising sensing performance.

Directional modulation (DM) is a physical layer security (PLS) technique implemented at the transmitter, leveraging antenna arrays to ensure secure communications. Through a process of spatial precoding between transceivers to transmit signals in specific directions, DM is capable of disrupting communications in unintended directions to prevent eavesdropping. In general, recent progress in the development of multiple-input multiple-output (MIMO) systems, including advanced radio frequency (RF), antenna technologies, along with innovative precoding algorithms, has enhanced the capabilities of DM techniques, leading to a multitude of robust DM variants. Hence, this survey aims to offer a comprehensive overview of DM, covering its fundamentals, promising variants, applications, hardware implementations, and future trends. Initially, the basic principle of DM is outlined in a general manner for subsequent comprehension. Subsequently, the large family of DM techniques is categorized into distinct variants based on the types of transmitting arrays. Next, we give a comprehensive survey of DM in common wireless scenarios, including multi-user (MU), relay, Internet of Things (IoT), and non-orthogonal access (NOMA) networks. Furthermore, we provide an illustration of DM system implementations, encompassing foundational architectures and cost-effective hardware realizations. Finally, concerning the unresolved challenges and current research focal points in DM, we present future research directions that merit further exploration and reference.

This paper addresses the problem of pilot contamination in single-cell networks, especially in dense-user areas where intra-cell pilot reuse is inevitable. We utilize fluid antennas (FAs) at the base station (BS) to improve the uplink (UL) channel estimation accuracy in terms of normalized mean square error (NMSE). To this end, we invoke channel spatial correlation as a key metric to characterize the similarity among the spatial structures of users’ channels. Inspired by the fact that the users with low spatial correlation experience less interference, we propose to mitigate pilot contamination by optimizing the FA positions with an innovative objective of minimizing the channel spatial correlation among the pilot-sharing users. To handle the resulting fractional programming problem, we recast it into a more tractable form via establishing upper and lower bounds for the objective function. Subsequently, the reformulated problem is effectively solved through an alternating optimization (AO) framework, where the position of each FA is iteratively optimized exploiting the successive convex approximation (SCA) method. Finally, simulation results validate the effectiveness of the proposed algorithm and demonstrate its remarkable performance in channel estimation and data transmission. In particular, the proposed 16-FA scheme achieves up to 71.2% NMSE reduction over its fixed-position antenna (FPA) counterpart, translating to an approximate 11 dB SNR gain in symbol detection.

In this paper, a novel design of amplitude phase shift keying-aided space-time block coded differential spatial modu-lation (APSK-STBC-DSM) is proposed. This design conceived not only maintains the diversity benefits of the space-time block coded differential spatial modulation (STBC-DSM) system, but also conveys extra information bits with the aid of amplitude phase shift keying (APSK), so as to enhance the transmission rate. Moreover, an improved low-complexity detector is also proposed, in order to achieve near-optimal detection performance. Ultimately, simulation results exhibit that the proposed APSK-STBC-DSM system is capable of providing significant bit error rate (BER) performance gains over conventional STBC-DSM and other differential spatial modulation (DSM) counterparts.

Polarization modulation (PM) is a recently developed multiple-input multiple-output (MIMO) toward low-cost implementation by exploiting the polarization degree of freedom in an efficient way. In the context of a PM transmitter, an improved expectation propagation (EP) detector is proposed in this contribution, while the bit-error rate (BER) performance and computational complexity are both evaluated and quantified respectively. We demonstrate that the proposed detector outperforms its original EP-based counterpart and traditional linear minimum mean square error (LMMSE) detector as a promising receiver design for PM systems.

In this contribution, the system of quasi-orthogonal space-time block coded spatial modulation (QOSTBC-SM) with minimum decoding complexity (MDC) is proposed toward linear detection complexity by utilizing the corresponding single-symbol decoding algorithm at the receiver. In addition, we compare the performance of the MDC-QOSTBC-SM system with different decoding algorithms in conjunction with comparing this scheme with space-time block coded spatial modulation (STBCSM). Furthermore, we extend it to uplink multi-user multiple-input multiple-output (MIMO) communications and compare the performance with STBC-SM in the context of quasi-static flat Rayleigh fading channels. The simulation results indicate that the proposed scheme achieves a significant reduction in decoding complexity at the cost of a slight performance loss when employing the single-symbol decoding algorithm.

To enable secure and effective downlink multiple-input multiple-output (MIMO) transmission, it has recently been proposed to integrate space-time block code (STBC) with spatial modulation (SM) and artificial noise (AN). However, this technique suffers from high detection complexity at the receiver side. In this contribution, therefore, we present an AN-assisted transmission scheme for the quasi-orthogonal space–time block coded receive spatial modulation (QOSTBC-RSM) system while developing two low-complexity detection algorithms. Our simulation results demonstrate that the proposed scheme and the corresponding detection methods are capable of guaranteeing safe transmission and achieving near-optimal bit error rate (BER) performance while significantly reducing computational overhead.

In this contribution, the concepts of polarization modulation (PM) and spatial modulation (SM) are integrated, to reap their respective advantages toward single-radio frequency (RF) multiple-input multiple-output (MIMO) transmissions. In the so-called polarization and spatial modulation (PSM) system, the information is conveyed by activated antenna indices as well as polarization modulated symbols, while at the receiver, a low-complexity near-optimal detection algorithm based on compressive sensing (CS) is proposed. Furthermore, a closed-form union bound of the average bit-error rate (BER) over fading channels is quantified by theoretical derivation, which is then extended to the case of spatial correlation (SC) as well as channel estimation error (CSE) toward practical use. Finally, our simulation results demonstrate the superiority of the developed PSM system over conventional PM in terms of BER performance over various fading channels.

The introduction of space-time block code (STBC) can efficiently increase the diversity of original spatial modulation (SM). However, the spectral efficiency of STBC will be reduced gradually with the increase of the transmit antennas. In this contribution, we conceive a novel quasi-orthogonal space-time block coded spatial modulation (QOSTBC-SM) design for multiple-input and multiple-out (MIMO) transmission by reaping their respective benefits, while improving the spectral efficiency compared to conventional space-time block coded spatial modulation (STBC-SM). More specifically, in the proposed QOSTBC-SM structure, the information bits are conveyed via the active antenna index, as well as the QOSTBC blocks at the transmitter, while at the receiver, a low-complexity detection scheme is proposed. Furthermore, a closed-form union bound of the bit error rate (BER) is also quantified by theoretical derivation. Finally, our simulation results demonstrate that QOSTBC-SM is capable of outperforming the conventional counterpart as STBC-SM with higher spectral efficiency.