Polarization-spatial modulation (PSM) has been recently proposed to improve the system performance and energy efficiency of the conventional polarization modulation (PM) by activating only a single dual-polarized (DP) antenna for signal transmission. However, the spectral efficiency (SE) of PSM is significantly degraded due to the single DP antenna transmission. To tackle this problem, we propose a generalized PSM (GPSM) scheme to enhance the SE of PSM through transmission using multiple DP antennas. In the GPSM scheme, the information bits are mapped to antenna activation patterns (AAPs), polarization matrix activation patterns (PAPs), and modulated symbols. To further enhance the SE of GPSM, we propose a more practical scheme termed multi-mode GPSM (MM-GPSM), which transmits information bits not only through the AAPs, PAPs, and modulated symbols but also via the constellation activation patterns (CAPs). A low-complexity maximum likelihood (ML) detector and a log-likelihood ratio detector for both GPSM and MM-GPSM are proposed to relieve the high computational complexity of the optimal ML detector at the cost of a negligible performance loss. An upper bound on the bit error rate (BER) is derived in closed-form to evaluate the performance of both GPSM and MM-GPSM. Simulation results show that GPSM and MM-GPSM outperform the conventional PM and PSM schemes, particularly in the high signal-to-noise ratio (SNR) region, and verify the accuracy of the theoretical analysis of the upper-bounded BER.

Unmanned aerial vehicles (UAVs) serve as flexible aerial platforms, enriching air-ground communication networks in various ways. To support massive connectivity within limited time-frequency blocks, non-orthogonal multiple access (NOMA) is proposed to be integrated into UAV networks. However, a common issue associated with almost all NOMA schemes is the susceptibility to inter-user interference (IUI). Therefore, in this paper, we propose a multi-UAV cooperative system aided by NOMA with index modulation (IM), termed MCU-NOMA-IM, to improve the performance of air-ground networks by mitigating IUI and also avoiding the successive interference cancellation (SIC) decoding method that is prone to error floors. With MCU-NOMA-IM, the information bits pertaining to multiple UAVs are mapped into multiple dimensions, including the modulated symbols, subcarrier indices, and energy allocation patterns. To fully investigate the performance of MCU-NOMA-IM on air-ground networks, we consider scenarios in the presence of three and four UAVs and derive upper-bounds for the bit error rates (BERs). In addition, we propose a multi-clustered-UAV cooperative system aided by NOMA with IM (MCCU-NOMA-IM), which groups closely located UAVs into several clusters to reduce the requirement for time resources. Simulation results demonstrate that both MCU-NOMA-IM and MCCU-NOMA-IM greatly outperform cooperative NOMA and non-cooperative NOMA-IM schemes, especially for distant UAVs when the signal-to-noise ratio is sufficiently high. Also, we show that the derived BER upper bounds are asymptotically tight.

As a promising technology, simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) is proposed to improve the transmission quality and coverage, which is considered to be widely applied to intelligent automated transportation (IAT) systems. However, due to the open environment and 360° omnidirectional transmission, security issues have always been a major difficulty hindering the implementation of STAR-RIS aided IAT systems. To tackle this challenge, we propose an enhanced security index modulation (ESIM) scheme in this paper, which combines well-known IM and higher-order linear decoding quasi-orthogonal space-time block coding (LD-QO-STBC) techniques to improve the system security performance. Specifically, the information bits and transmit antennas are organized into four distinct sets, with each subset of antennas activated by individual IM. Subsequently, the STAR-RIS is employed to meticulously craft the LD-QO-STBC scheme, which involves determining the phase of the unmodulated carrier. In a strategic move to thwart eavesdropping attempts, the eavesdroppers are subjected to continuously varying and disruptive continuous artificial noise (CAN) and is thus unable to reliably decode the transmitted information, which collectively achieves secure communications amidst potential eavesdropping threats. In addition, the theoretical analysis of both the bit error rate (BER) and secrecy capacity are derived to explore the potential of ESIM-LD-QO-STBC. Numerical results reveal that our proposed method excels in achieving a substantial diversity gain while maintaining low computational complexity. Furthermore, our proposed scheme enhances secrecy capacity by at least 50%, offering a marked improvement over conventional physical layer security (PLS) schemes.

In this paper, we investigate a full-duplex (FD) cell-free massive multiple-input multiple-output (mMIMO) system. To cope with the cross-link interference, especially the interaccess point (inter-AP) interference, we formulate a sum spectral efficiency maximization problem by jointly optimizing the digital and analog beamforming at FD AP and digital precoder at the uplink user equipment. To solve it, we present a fully digital design first by utilizing successive convex approximations and majorization-minimization to obtain a closed-form solution in each iteration. After that, we exploit the Riemannian manifold to get a hybrid design, which is shown to approximate the fully digital one well. Simulation results show that the proposed design enhances the sum spectral efficiency as well as uplink performance effectively when compared to other benchmarks.

Involving proper boundaries is an important aspect of molecular communication, especially when considering it as a potential candidate for intra-body communication. Inspired by the massive cellular environments, this paper proposes a novel approach to model the channel for a diffusive molecular communication (DMC) system in a bounded spherical environment. Specifically, this spherical boundary is made up of a reflecting and absorbing portion, whose property depends on whether it is covered by absorbing receptors (ARs). To obtain an accurate channel impulse response (CIR), we employ the homogenization technique to convert the mixed Dirichlet-Neumann boundary into a Robin boundary. Assuming these ARs with the same size and uniform distribution, we derive a valid CIR expression for the bounded DMC system with arbitrary sizes and numbers of ARs. Numerical results based on the particle-based simulation validate our analysis.

In this article, we study a previously overlooked work of designing the constellation for concentration shift keying (CSK) in diffusion-based molecular communication (MC) systems, considering the adverse effects of signal-dependent noise and intersymbol interference (ISI). Against this background, the conventionally used uniform constellation strategy is no longer the optimal choice for CSK. Considering the discrete nature of messenger molecules, the constellation design of CSK is essentially a nonlinear integer programming problem. However, due to the nonconvexity of the goal function, general convex optimization methods cannot be easily employed. To solve the aforementioned problem, a novel CSK strategy with a brute-force search algorithm is proposed to obtain the optimal constellation numerically. Moreover, a low-complexity search algorithm that provides new insight into reducing the complexity of MC systems is further presented. Specifically, we theoretically derive the optimal threshold and employ a maximum likelihood detector that effectively minimizes the average symbol error rate (SER) by leveraging the knowledge of ISI at the receiver. Finally, we can obtain a fitted equation for the nearly optimal constellation points under specific channel parameters, attaining the SER performance similar to the optimal one, while the search complexity is much lowered.

In this paper, we redesign the concentration shift keying (CSK) constellation design for diffusion-based molecular communication systems, where both signal-dependent noise and inter-symbol interference (ISI) are considered. The conventionally used uniform constellation strategy is no longer the optimal choice when signal-dependent noise exists. Against this background, a novel CSK constellation strategy with the maximum likelihood detector is presented by minimizing the symbol error rate (SER) with statistical ISI knowledge. The well-matched analytical and simulated results in terms of SER demonstrate the effectiveness of our proposed CSK constellation design.

Overcoming inter-symbol interference (ISI) is one of the key challenges in the design of molecular communication systems. In this paper, we propose a scheme for optimizing the detection interval to minimize the impact of ISI while ensuring the acquisition of effective information. Our detection interval optimization applies to both the absorbing and passive receivers. For analysis, we consider as the performance metrics signal-to-interference difference (SID) and signal-to-interference and noise amplitude ratio (SINAR) proposed in the literature rather than the intractable bit error rate (BER). Accordingly, we derive the optimal detection interval in closedform. Finally, simulation results in terms of BER verify the theoretical analysis and also show the promising advantages of the proposed scheme in signal detection.

In this paper, we propose a two-layer heterogeneous network to realize the remote monitoring and advanced warning for infectious diseases spread by airborne pathogens, whose detection can be considered as a binary detection problem. To intuitively study the detection process, we abstract it as a molecular communication via diffusion (MCvD) model and uncover that one of the key factors to impact the detection performance is inter-symbol interference (ISI), i.e., the viral aerosol from other biological entities. Therefore, overcoming ISI is imperative to ensure reliable detection. In the abstracted MCvD model, the detection performance can be described by the bit error rate (BER) performance. Following this assumption, we propose to optimize the detection interval to minimize the impact of ISI while ensuring the accurate detection of the transmitted information symbol, which is suitable for both the absorbing and passive receivers. For tractability, based on the signal-to-interference difference (SID) and signal-to-interference-and-noise amplitude ratio (SINAR), we design a modified-SINAR (mSINAR) to measure BER performance for the MCvD system with a variable detection interval. Besides, we derive the optimal detection interval in closed-form. Using simulation results, we show that in terms of BER, our proposed mSINAR scheme is superior to the competitive schemes, and performs similarly to the scheme with optimal intervals determined by the exhaustive search.

Molecular communication (MC) is an emerging communication paradigm, where the information is carried via the patterns of molecules that are mainly governed by the diffusion process. Current MC literature concentrates on the time-domain analysis, while the signal analysis in other domains may facilitate the MC research. To this end, this paper performs the frequency-domain analysis by deriving the frequency response of the diffusion-based MC channels, manifesting an explicitly low-compass characteristic. The energy of the channel impulse response in the diffusion-based MC is also derived, and the corresponding bandwidth definition is proposed, which determines the sampling frequency for the one-shot diffusive channel impulse response in MC. The results in this work lay the foundation for the frequency-domain signal processing in diffusion-based MC channels.

Communication between nanomachines is still an important topic in the construction of the Internet of Bio-Nano Things (IoBNT). Currently, molecular communication (MC) is expected to be a promising technology to realize IoBNT. To effectively serve the IoBNT composed of multiple nanomachine clusters, it is imperative to study multiple-access MC. In this article, based on the molecular division multiple access technology, we propose a novel multiuser MC system, where information molecules with different diffusion coefficients are first employed. Aiming at the user fairness in the considered system, we investigate the optimization of molecular resource allocation, including the assignment of the types of molecules and the number of molecules of a type. Specifically, three performance metrics are considered, namely, min-max fairness for error probability, max-min fairness for achievable rate, and weighted sum-rate maximization. Moreover, we propose two assignment strategies for types of molecules, i.e., best-to-best (BTB) and best-to-worst (BTW). Subsequently, for a two-user scenario, we analytically derive the optimal allocation for the number of molecules when types of molecules are fixed for all users. In contrast, for a three-user scenario, we prove that the BTB and BTW schemes with the optimal allocation for the number of molecules can provide the lower and upper bounds on system performance, respectively. Finally, numerical results show that the combination of BTW and the optimal allocation for the number of molecules yields better performance than the benchmarks.

In this paper, we present the space shift keying based molecular communication (SSK-MC), where the space symbol is of interest. For micro-scale SSK-MC, the information is embedded into the index of a single activated transmitter nanomachine, while the index of the sensor is considered for the macro-scale SSK-MC. The cancellation of the strongest inter-link interference in multiple-input multiple-output (MIMO) based MC is available with the implementation of SSK-MC, in which only one transmitter is activated during each symbol duration. We derive the symbol error rate (SER) of SSK-MC, which shows a perfect match with its numerical simulations. Apart from the theoretical analysis, we further design an underwater prototype under the 4×4 MIMO-MC configuration for SSK-MC, demonstrating its SER performance with different detection algorithms.

There is a recent surge of research interest in the study of performance-enhancing techniques for orthogonal frequency division multiplexing (OFDM)-based relay systems. Among those, subcarrier mapping has been verified to be an effective one for boosting the system capacity and improving the error performance. However, it has to be performed at the relay, which subsequently conveys the subcarrier permutation information to the destination. The existing signaling scheme occupies a portion of subcarriers to this end, leading to a loss of spectral efficiency. In this paper, we propose a novel signaling scheme to eliminate this overhead by transferring the subcarrier permutation to the mode permutation that can be implicitly conveyed without consuming additional spectrum resources. We adopt phase rotation for mode design considering both non-adaptive and adaptive modulation, and illustrate the proposed scheme by taking the dual-hop OFDM relaying with semi-blind amplify-and-forward protocol as an example. An asymptotically tight upper bound on the bit error rate (BER) of the proposed scheme is derived in closed-form over Rayleigh fading channels. BER simulation results validate the analysis and show that the proposed scheme asymptotically approaches the ideal case that assumes perfect knowledge of subcarrier permutation information at the destination and significantly outperforms the existing scheme in the asymptotic signal-to-noise ratio region at the same spectral efficiency.