The emerging technology of reconfigurable intelligent surface-based access points (RIS-APs) shows excellent potential as it effectively replaces conventional RF front ends with passive RISs. This paper introduces RIS-based dual-mode receive spatial modulation (RDM-RSM), a low-complexity scheme designed to improve the spectral efficiency and error performance of RIS-APs. Both versions of the proposed RDM-RSM scheme configure the RIS to simultaneously enhance the RF signals at two different receive antennas while synthesizing distinct modulation symbols from two separate constellations at each antenna, thereby enabling efficient transmission of two parallel, conventionally-modulated data-streams. Additionally, by selecting two antennas and assigning each a distinct constellation mode, more bits can be encoded per index-modulated symbol. The first version, RDM-RSM-Independent (RDM-RSMI), operates by maximizing the energy of the synthesized signalsindependentlyfor each antenna. In contrast, RDM-RSM-Joint (RDM-RSMJ) optimizes the reflection phase of each RIS element to maximize an objective function that considers the signals at both antennasjointly, resulting in stronger received signals compared to RDM-RSMI, where only half of the elements can serve each antenna. A low-complexity detector is proposed, with its theoretical performance rigorously analyzed by modeling the conditional reflection phases of the RIS elements as von-Mises distributed, providing a tractable and insightful framework.

This paper introduces the in-phase and quadrature (IQ) extension to the permutation matrix modulation (PMM) technique and proposes the quadrature PMM (QPMM) scheme for higher spectral efficiency without any expense in RF chains. Assuming a single-user MIMO (SU-MIMO) system model with all antennas activated, the proposed QPMM scheme utilizes the permutation matrices as the spatial indexing unit for both the IQ components of the complex symbol vector. Furthermore, a low-complex detector, conditional maximum likelihood detector (C-MLD), that achieves the same bit error rate (BER) performance as the optimal joint MLD is presented. In addition to the low-complex C-MLD detector, a deep learning (DL) based detector, called FusionDet, is also proposed for the QPMM scheme. This DL-based detector provides a trade-off between complexity and BER performance. The BER performance of the QPMM scheme and the complexities of the detectors are examined. The provided computer simulations reveal that the proposed QPMM scheme outperforms the conventional PMM method for different MIMO setups and modulation levels. In addition, the complexity analysis shows that C-MLD attains the same BER performance as the optimal joint MLD with significantly lower complexity. Finally, FusionDet is observed to further decrease detector complexity in exchange for sub-optimal detection performance.

Beamforming plays a crucial role in millimeter wave (mmWave) communication systems to mitigate the severe attenuation inherent to this spectrum. However, the use of large active antenna arrays in conventional architectures often results in high implementation costs and excessive power consumption, limiting their practicality. As an alternative, deploying large arrays at transceivers using passive devices, such as reconfigurable intelligent surfaces (RISs), offers a more cost-effective and energy-efficient solution. In this paper, we investigate a promising base station (BS) architecture that integrates a beyond diagonal RIS (BD-RIS) within the BS to enable passive beamforming. By utilizing Takagi’s decomposition and leveraging the effective beamforming vector, the RIS profile can be designed to enable passive beamforming directed toward the target. Through the beamforming analysis, we reveal that BD-RIS provides robust beamforming performance across various system configurations, whereas the traditional diagonal RIS (D-RIS) exhibits instability with increasing RIS size and decreasing BS-RIS separation-two critical factors in optimizing RIS-assisted systems. Comprehensive computer simulation results across various aspects validate the superiority of the proposed BS-integrated BD-RIS over conventional D-RIS architectures, showcasing performance comparable to active analog beamforming antenna arrays.

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.

This paper introduces a novel approach to efficient localization in next-generation communication systems through a base station (BS)-enabled passive beamforming utilizing beyond diagonal reconfigurable intelligent surfaces (BD-RISs). Unlike conventional diagonal RISs (D-RISs), which suffer from limited beamforming capability, a BD-RIS provides enhanced control over both phase and amplitude, significantly improving localization accuracy. By conducting a comprehensive Cramér-Rao lower bound (CRLB) analysis across various system parameters in both near-field and far-field scenarios, we establish the BD-RIS structure as a competitive alternative to traditional active antenna arrays. Our results reveal that BD-RISs achieve near active antenna arrays performance in localization precision, overcoming the limitations of D-RISs and underscoring its potential for high-accuracy positioning in future communication networks. This work envisions the use of BD-RIS for enabling passive beamforming-based localization, setting the stage for more efficient and scalable localization strategies in sixth-generation networks and beyond.

Sub-band full duplex (SBFD) operation, which enables simultaneous uplink (UL) and downlink (DL) transmissions over adjacent subbands, has been introduced as a promising duplexing technique for 6G and beyond. However, SBFD introduces cross-link interference (CLI), particularly between neighboring base stations (gNB-to-gNB), which can severely degrade uplink performance. Uniform UL resource muting algorithms have been proposed in the literature to make accurate CLI measurements for the effectiveness of gNB-to-gNB interference mitigation mechanisms. In this paper, we propose an adaptive and non-uniform UL resource muting scheme that dynamically adjusts the number and position of muted resource elements (REs) based on the estimated large-scale interference power distribution. Furthermore, we introduce a frequency-shaped adjacent channel leakage ratio (ACLR) model to reflect realistic leakage behavior in SBFD networks. Simulation results show that non-uniform muting pattern (NuMP) outperforms uniform muting pattern (UMP) in terms of estimation performance, while also requiring fewer muted resource elements, as demonstrated by lower normalized mean square error (NMSE) values and reduced muting overhead.

This paper proposes the orthogonal time frequency space-based code index modulation (OTFS-CIM) scheme, a novel wireless communication system that combines OTFS modulation, which enhances error performance in high-mobility Rayleigh channels, with CIM technique, which improves spectral and energy efficiency, within a single-input multiple-output (SIMO) architecture. The proposed system is evaluated through Monte Carlo simulations for various system parameters. Results show that increasing the modulation order degrades performance, while more receive antennas enhance it. Comparative analyses of error performance, throughput, spectral efficiency, and energy saving demonstrate that OTFS-CIM outperforms traditional OTFS and OTFS-based spatial modulation (OTFS-SM) systems. Also, the proposed OTFS-CIM system outperforms benchmark systems in many performance metrics under high-mobility scenarios, making it a strong candidate for sixth generation (6G) and beyond.

This paper proposes a new orthogonal time frequency space (OTFS)-based index modulation system called OTFS-aided media-based modulation (MBM) scheme (OTFS-MBM), which is a promising technique for high-mobility wireless communication systems. The OTFS technique transforms information into the delay-Doppler domain, providing robustness against channel variations, while the MBM system utilizes controllable radio frequency (RF) mirrors to enhance spectral efficiency. The combination of these two techniques offers improved bit error rate (BER) performance compared to conventional OTFS and OTFS-based spatial modulation (OTFS-SM) systems. The proposed system is evaluated through Monte Carlo simulations over high-mobility Rayleigh channels for various system parameters. Comparative throughput, spectral efficiency, and energy efficiency analyses are presented, and it is shown that OTFS-MBM outperforms traditional OTFS and OTFS-SM techniques. The proposed OTFS-MBM scheme stands out as a viable solution for sixth generation (6G) and next-generation wireless networks, enabling reliable communication in dynamic wireless environments.

The coexistence of radar and communications in wireless systems marks a paradigm shift for the sixth-generation (6G) networks. As 6G systems are expected to operate at higher frequencies and employ larger antenna arrays than fifth-generation (5G) systems, they can also enable more accurate sensing capabilities. To this end, the integrated sensing and communication (ISAC) paradigm aims to unify the physical and radio frequency (RF) domains by introducing the sensing functionality into the communication network. However, the clutter poses a challenge, as it can significantly degrade the sensing accuracy in ISAC systems. This paper presents a novel two-dimensional root multiple signal classification (2D-rootMUSIC)-based algorithm for static background clutter suppression. Computer simulation results indicate that the proposed method effectively mitigates the strong background clutter, yields accurate parameter estimation performance, and offers a notable improvement in the signal-to-clutter-and-noise ratio (SCNR), while outperforming the prior-art benchmark methods.

Accurate and cost-effective indoor localization re-mains a critical challenge for smart environments. Traditional fingerprint-based methods require measurements from multiple access points (APs), which increases deployment costs and system complexity. Reconfigurable intelligent surfaces (RISs) have recently been explored to enrich the spatial diversity of fingerprint data, allowing localization with fewer APs. However, many existing RIS-aided approaches rely on RIS phase shift optimization, limiting their practicality. In this paper, we propose a novel RIS-assisted fingerprint localization method that eliminates the need for multiple APs and RIS phase shift optimization. In the proposed method, multiple RISs reflect the unmodulated signal transmitted by a radio frequency (RF) source using their unique phase shift reflection patterns (PSRPs). These modulated reflections allow the user equipment (UE) to extract fingerprint features by correlating the received signal with the known PSRPs of the RISs. Computer simulation results demonstrate that the proposed method achieves submeter-level accuracy under moderate transmit power conditions.

Affine frequency division multiplexing (AFDM) is a novel modulation technique based on chirp signals that has been recently proposed as an effective solution for highly reliable communications in high-mobility scenarios. In this paper, we focus on the design of robust index modulation (IM) schemes under the multiple-antenna AFDM transmission framework. To this end, the cyclic delay diversity (CDD) technique is employed to harvest the transmit diversity gain. As a result, we propose two novel AFDM-IM schemes with transmit diversity, termed as CDD-AFDM-IM-I and CDD-AFDM-IM-II. We analyze the full diversity conditions and parameter settings of the proposed CDD-AFDM-IM schemes for both integer and fractional Doppler cases over linear time-varying (LTV) channels. Moreover, we prove that IM enables AFDM to have stronger diversity protection when the full diversity condition is not satisfied. Asymptotically tight upper bounds on the average bit error rates (BERs) of the proposed schemes with maximum-likelihood (ML) detection are derived in closed-form. Furthermore, we propose a low-complexity double-layer message passing (DLMP) algorithm for practical large-dimensional signal detection in the proposed CDD-AFDM-IM systems. Comparison with existing detections shows that the proposed DLMP algorithm achieves a better tradeoff between the BER performance and the computational complexity. Finally, BER simulation results confirm that our proposed CDD-AFDM-IM schemes with both the ML and DLMP detections outperform the benchmark schemes over the LTV channels.

The information-theoretically (unconditionally) secure Kirchhoff-law-Johnson-noise (KLJN) bit exchange protocol uses two identical resistor pairs with high (H) and low (L) resistance values, driven by Gaussian noise generators emulating Johnson noise with a high common temperature. The resulting mean-square noise voltage on the wire connecting Alice and Bob has three levels: low (L/L), intermediate (H/LorL/H), and high (H/H), and secure key sharing is achieved at the intermediate level (L/HorH/L). This paper introduces the Flip-KLJN scheme, where a pre-agreed intermediate level, such asH/L, triggers a flip of the bit map value during the bit exchange period. For Eve, the bit map flips appear random. Thus, the formerly discardedH/HandL/Lsituations can also have a pre-agreed bit value mapping, which flips together with the original bit mapping. Thus, Flip-KLJN doubles the key rate and ensures that all three levels on the wire are indistinguishable for Eve. Bit error probabilities are addressed through analytic calculations and computer simulations.

Integrated sensing and communication (ISAC) is recognized as one of the key enabling technologies for sixth-generation (6G) wireless communication networks, facilitating diverse emerging applications and services in an energy and cost-efficient manner. This paper proposes a multi-target, multi-user ISAC system to enable full-space coverage for communication and sensing tasks. The proposed system employs a hybrid simultaneous transmission and reflection reconfigurable intelligent surface (STAR-RIS) comprising active transmissive and passive reflective elements. In the proposed scheme, the passive reflective elements support communication and sensing links for local communication users and sensing targets situated within the same physical region as the base station (BS), while low-power active transmissive elements are deployed to improve sensing performance and overcome high path attenuation due to multi-hop transmission for distant communication users and sensing targets situated far from of the coverage area of the BS. Moreover, to optimize the transmissive/reflective coefficients of the hybrid STAR-RIS, a semi-definite relaxation (SDR)-based algorithm is proposed. Furthermore, to evaluate communication and sensing performance, signal-to-interference-noise ratio (SINR) and Cramer-Rao bound (CRB) metrics have been derived and investigated via conducting extensive computer simulations.

In recent years, reconfigurable intelligent surfaces (RISs) have attracted significant attention from both academia and industry due to their ability to manipulate electromagnetic waves in wireless communication. In particular, the characteristics of wireless channels incorporating RISs offer substantial research value. Several previous studies have investigated the path loss characteristics of RIS-assisted wireless channels in practical scenarios. However, these pioneering studies ignore the effect of RIS array gain on path loss. This paper proposes a floating-intercept (FI) path loss model that includes the array gain of the RIS, the distance between the transmitter and the RIS, and the distance between the receiver and the RIS. In this RIS-FI model, the RIS array gain component is related to the reflection coefficients of all the unit cells of RIS. The proposed model can effectively characterize path loss in both the far-field and near-field regions of the RIS. A fabricated 512-element RIS and a vector network analyzer (VNA) are utilized to set up the channel measurement system. The proposed RIS-FI path loss model is validated by using measurement data collected from multiple scenarios, including outdoor square, indoor corridor, and classroom. Compared to other RIS path loss models, the experimental results show that the proposed model offers better accuracy in describing path loss in the near-field region of the RIS. The RIS-FI path loss model presented in this paper may pave the way for further research on RIS-assisted wireless channels.

Non-coherent chaotic communication has gained increasing attention as it provides an efficient solution for reliable communications without requiring channel state information. In this paper, we propose a non-coherent reconfigurable intelligent surface (RIS)-empowered differential chaos shift keying scheme with two-level nested index modulation (RIS-DCSK-TLNIM). In the proposed RIS-DCSK-TLNIM scheme, the reference index and information index are nested to form two-level nested index modulation. This design enhances both the spectral efficiency and bit error rate (BER) performance of RIS-DCSK-TLNIM, albeit at the expense of slightly increased complexity. Furthermore, we propose a joint detection algorithm to recover the information bits transmitted via the reference index, information index, and two distinct-mode signals. We then extend RIS-DCSK-TLNIM into an enhanced system to achieve higher spectral efficiency. Subsequently, we analyze the BER performance, spectral efficiency, and system complexity of RIS-DCSK-TLNIM, and compare these metrics with those of benchmark systems. Comparison results demonstrate that the proposed RIS-DCSK-TLNIM, when configured with a small number of time slots, can achieve more than twice the spectral efficiency and at least a 6 dB gain in BER performance compared to benchmark systems.

As a superior multicarrier technique utilizing chirp signals for high-mobility communications, affine frequency division multiplexing (AFDM) is envisioned to be a promising candidate for sixth-generation (6G) wireless networks. AFDM is based on the discrete affine Fourier transform (DAFT) with two adjustable parameters of the chirp signals, termed the pre-chirp and post-chirp parameters, respectively. Whilst the post-chirp parameter complies with stringent constraints to combat the time-frequency doubly selective channel fading, we show that the pre-chirp counterpart can be flexibly manipulated for an additional degree of freedom. Therefore, this paper proposes a novel AFDM scheme with the pre-chirp index modulation (PIM) philosophy (AFDM-PIM), which can implicitly convey extra information bits through dynamic pre-chirp parameter assignment, thus enhancing both spectral and energy efficiency. Specifically, we first demonstrate that the subcarrier orthogonality is still maintained by applying distinct pre-chirp parameters to various subcarriers in the AFDM modulation process. Inspired by this property, we allow each AFDM subcarrier to carry a unique pre-chirp signal according to the incoming bits. By such an arrangement, extra bits can be embedded into the index patterns of pre-chirp parameter assignment without additional energy consumption. We derive asymptotically tight upper bounds on the average bit error probability (BEP) of the proposed schemes with the maximum-likelihood detection, and validate that the proposed AFDM-PIM can achieve full diversity under doubly dispersive channels. Based on the derived result, we further propose an optimal pre-chirp alphabet design to enhance the bit error rate (BER) performance via intelligent optimization algorithms. Simulation results demonstrate that the proposed AFDM-PIM outperforms the classical benchmarks.

Reconfigurable intelligent surface (RIS)-assisted communications have received widespread attention recently, whose channel modeling is the cornerstone for its future practical application. Nevertheless, the RIS-related channel modeling stays at the level of either the theoretical free-space derivation or the end-to-end channel model, yet its general segment channel model applicable to real environment is still rarely reported. In this paper, we formulate a temporal segment channel model for RIS-assisted non-line-of-sight (NLOS) communications, which separates the global channel into three parts, i.e., transmitter (Tx)-RIS, RIS-receiver (Rx), and Tx-Rx. Then we bridge the relationship between the proposed model and the existing models. In addition, we illustrate the Ricean K–factor characteristics of the global channel and the sub-channel in the proposed model. The lower boundary for the Ricean K–factor of sub-channel is also deduced. Two low-complexity estimation algorithms for the Ricean K–factor of sub-channel are proposed, whose accuracy is verified by the measured channel data.

Today's wireless communication networks have many requirements such as high data rate, high reliability, low latency, low error data transmission, and high energy efficiency. High-performance index modulation (IM) techniques and reconfigurable intelligent surface (RIS) technology, which has recently attracted the attention of researchers, are strong candidates to meet these requirements. This paper introduces a novel RIS-supported code IM-based receive spatial modulation (RIS-CIM-RSM) system. The proposed RIS-CIM-RSM system uses quadrature amplitude modulation (QAM) symbols, receive antenna indices, and spreading code indices for wireless data transmission. In the proposed system, an RIS applies a phase rotation that maximizes signal-to-noise ratio (SNR) to the signals coming to the reflecting elements and directs them to the selected receive antenna. Performance analyses of the proposed RIS-CIM-RSM system such as data rate, throughput, and energy saving are obtained. The results obtained show that the proposed RIS-CIM-RSM system is superior to the counterpart RIS-based IM systems in the literature in terms of data rate, throughput, energy saving, and error performance.

Intelligent surfaces (ISs) have emerged as a key technology to empower a wide range of appealing applications for wireless networks, due to their low cost, high energy efficiency, flexibility of deployment, and capability of constructing favorable wireless channels/radio environments. Moreover, the recent advent of several new IS architectures further expanded their electromagnetic functionalities from passive reflection to active amplification, simultaneous reflection, and refraction, as well as holographic beamforming. However, the research on ISs is still in rapid progress and there have been recent technological advances in ISs and their emerging applications that are worthy of a timely review. Thus, in this article, we provide a comprehensive survey on the recent development and advances of ISs-aided wireless networks. Specifically, we start with an overview on the anticipated use cases of ISs in future wireless networks such as 6G, followed by a summary of the recent standardization activities related to ISs. Then, the main design issues of the commonly adopted reflection-based IS and their state-of-the-art solutions are presented in detail, including reflection optimization, deployment, signal modulation, wireless sensing, and integrated sensing and communications. Finally, recent progress and new challenges in advanced IS architectures are discussed to inspire future research.

Besides steering the incoming signals toward a receiver, a reconfigurable intelligent surface (RIS) can simultaneously serve as a backscatter node to communicate additional data by modulating the same signal. Quadrature reflection modulation (QRM) is a promising contender among index modulation (IM)-based techniques for implementing RIS-based backscatter communications (RIS-BackCom). In this paper, we demonstrate that the error performance of RIS-QRM heavily relies on the employed indexing scheme, which configures each RIS element into one of the two orthogonally-reflecting modes of QRM to encode the transmitted data onto the reflection pattern of RIS elements. In this context, we present an efficient guideline for designing indexing schemes that ensure the resulting channel-dependent QRM symbols are evenly spaced with maximum separation. Following this guideline, we propose two specific indexing techniques: the incremental and uniform indexing schemes, both of which substantially enhance the error performance of RIS-QRM without increasing detection complexity. We derive a closed-form expression for the bit error probability (BEP) of RIS-QRM with the incremental indexing scheme. Furthermore, we propose a low-complexity detector for RIS-BackCom systems employing RIS-QRM. Extensive computer simulations illustrate the effectiveness of our proposed schemes in improving the error performance of RIS-QRM-based BackCom systems.

Orthogonal time frequency space (OTFS) is a promising waveform with outstanding performance in doubly-selective channels. Spreading information symbols across the entire delay-Doppler plane enables OTFS to exploit full diversity. Nevertheless, there is a need for enhanced reliability in OTFS systems to satisfy the rigorous requirements of forthcoming communication systems. To tackle this concern, we propose a scheme called joint delay-Doppler index modulation OTFS (JDDIM-OTFS). This scheme activates delay or Doppler resource elements in a subframe, and in order to obtain higher spectral efficiency, we use different constellations for each active resource block. Furthermore, we utilize a low-complex detector with the help of a greedy approach. Moreover, a theoretical upper bound is derived for the bit error rate (BER). Simulation results validate that JDDIM-OTFS outmatches classical OTFS and other benchmark schemes in terms of BER under perfect and imperfect channel conditions.

Reconfigurable intelligent surface (RIS) provides a new electromagnetic response control solution, which can proactively reshape the characteristics of wireless channel environments. In RIS-assisted communication systems, the acquisition of channel state information (CSI) and the optimization of reflecting coefficients constitute major design challenges. To address these issues, codebook-based solutions have been developed recently, which, however, are mostly environment-agnostic. In this paper, a novel environment-aware codebook protocol is proposed, which can significantly reduce both pilot overhead and computational complexity, while maintaining expected communication performance. Specifically, first of all, a channel training framework is introduced to divide the training phase into several blocks. In each block, we directly estimate the composite end-to-end channel and focus only on the transmit beamforming. Second, we propose an environment-aware codebook generation scheme, which first generates a group of channels based on statistical CSI, and then obtains their corresponding RIS configuration by utilizing the alternating optimization (AO) method offline. In each online training block, the RIS is configured based on the corresponding codeword in the environment-aware codebook, and the optimal codeword resulting in the highest sum rate is adopted for assisting in the downlink data transmission. Third, we analyze the theoretical performance of the environment-aware codebook-based protocol taking into account the channel estimation errors. Finally, numerical simulations are provided to verify our theoretical analysis and the performance of the proposed scheme. In particular, the simulation results demonstrate that our protocol is more competitive than conventional environment-agnostic codebooks.

Reconfigurable intelligent surfaces (RISs) bring great potential to the advancement of 6G and beyond wireless communication technologies. RISs introduce a great degree of flexibility, allowing some sort of virtual control over the wireless channel. Exploiting the flexibility introduced by RISs, we propose a novel RIS-enabled downlink (DL) non-orthogonal multiple access (NOMA) scheme where NOMA is enabled over-the-air rather than at the base station (BS) or the receiver (Rx). Here, the RIS is partitioned into distinctive groups where each part of the RIS serves a different user equipment (UE) to perform multiple accessing. The BS transmits an unmodulated signal to the RIS, and each partition modulates the impinging signal over-the-air by introducing a phase shift according to the incoming information bits to serve the corresponding UE. First, the end-to-end system model for the proposed system is presented. Furthermore, outage probability calculations, theoretical error probability analysis, and bit error rate (BER) derivations are discussed and reinforced with comprehensive computer simulation results.

This paper presents a novel index modulation (IM) technique named hybrid reconfigurable intelligent surface (RIS) enabled enhanced reflection modulation (Hyb-ERM), which capitalizes on the hybrid RIS architecture comprising both active and passive elements. In Hyb-ERM, a hybrid RIS is partitioned into groups, with information bits transmitted through both conventional M-ary phase shift keying (PSK) modulated symbols and the number of active RIS groups. This approach, particularly the manipulation of transmitted symbols by the hybrid RIS based on the number of active groups, is designed to improve the minimum Euclidean distance, thereby significantly enhancing error performance. We have derived a theoretical upper bound for the bit error probability (BEP) and proposed two low-complexity detectors as alternatives to the maximum likelihood (ML) detector. Extensive simulation results validate the effectiveness of Hyb-ERM, showing its superior error performance compared to benchmark schemes under both ideal and non-ideal channel conditions. This study underscores the potential of Hyb-ERM in advancing RIS-assisted communications, paving the way for more efficient and robust 6G wireless networks.

In this paper, a novel amplitude phase shift keying (APSK) modulation scheme for cooperative backscatter communications aided by a reconfigurable intelligent surface (RIS-CBC) is presented, according to which a passive or an active RIS is configured to modulate backscatter information onto unmodulated or PSK-modulated signals impinging on its panel via APSK. In passive RIS-CBC-APSK, the backscatter information is conveyed through the number of RIS reflecting elements being in the ON state and their phase shift values, whereas, in active RIS-CBC-APSK, this information is embedded through the number of RIS elements being in the active mode as well as the phase shift values of all elements. By using the optimal APSK constellation to ensure that reflected signals from the RIS undergo APSK modulation, a bit-mapping mechanism is developed. Assuming maximum-likelihood detection, we also present closed-form upper bounds for the symbol error rate (SER) performance for both proposed passive and active RIS-CBC-APSK schemes over Rician fading channels. In addition, we devise a low-complexity detector that can achieve flexible trade-offs between performance and complexity. Finally, we extend RIS-CBC-APSK to multiple-input single-output scenarios and present an alternating optimization approach for the joint design of transmit beamforming and RIS reflection. Our extensive simulation results on the SER performance of the proposed RIS-CBC-APSK framework corroborate our conducted performance analysis and showcase the superiority of both designed modulation schemes over the state-of-the-art RIS-CBC benchmarks.

As wireless information transmission (WIT) progresses into its sixth generation (6G), a challenge arises in sustaining terminal operations with limited batteries for Internet-of-Things (IoT) platforms. To address this, wireless power transfer (WPT) emerges as a solution, empowering battery-less infrastructures and enabling nodes to harvest energy for sustainable operations. Thus, the eclectic integration of WPT with WIT mechanisms becomes crucial to mitigate the need for battery replacements while providing secure and reliable communication. A novel protocol that amalgamates WIT and WPT calledInformation Harvesting (IH)has recently been proposed to effectively handle challenges in wireless information and power transfer (WIPT) by employing index modulation (IM) techniques for data communication atop the existing far-field WPT mechanism. This paper presents a unified framework for IM-based IH mechanisms and evaluates their energy harvesting capability, bit error rate (BER), and ergodic secrecy rate (ESR) performance for diverse IM schemes. The findings indicate the significant potential of the IM-based IH mechanism in facilitating reliable data communication within existing far-field WPT systems while underscoring promising refinements in green and secure communication paradigms for next-generation IoT wireless networks.

In this paper, we propose a transmission mechanism for a reconfigurable intelligent surface (RIS)-assisted millimeter wave (mmWave) system based on cluster index modulation (CIM), named best-gain optimized cluster selection CIM (BGCS-CIM). The proposed BGCS-CIM scheme considers effective cluster power gain and spatial diversity gain obtained by the additional paths within the indexed cluster to construct an efficient codebook. We also integrate the proposed scheme into a practical system model to create a virtual path between transmitter and receiver where the direct link has been blocked. Thanks to the designed whitening filter, a closed-form expression for the upper bound on the average bit error rate (ABER) is derived and used to validate the simulation results. It has been shown that the proposed BGCS-CIM scheme outperforms the existing benchmarks thanks to its higher effective cluster gain, spatial diversity of indexed clusters, and lower inter-cluster interference.

This paper presents a promising design concept for reconfigurable intelligent surfaces (RISs), named plug-in RIS, wherein the RIS is plugged into an appropriate position in the environment, adjusted once according to the location of both base station and blocked region, and operates with fixed beams to enhance the system performance. The plug-in RIS is a novel system design, streamlining RIS-assisted millimeter-wave (mmWave) communication without requiring decoupling two parts of the end-to-end channel, traditional control signal transmission, and online RIS configuration. In plug-in RIS-aided transmission, the transmitter efficiently activates specific regions of the divided large RIS by employing hybrid beamforming techniques, each with predetermined phase adjustments tailored to reflect signals to desired user locations. This user-centric approach enhances connectivity and overall user experience by dynamically illuminating the targeted user based on location. By introducing plug-in RIS’s theoretical framework, design principles, and performance evaluation, we demonstrate its potential to revolutionize mmWave communications for the limited channel state information (CSI) scenarios. Simulation results illustrate that plug-in RIS provides power/cost-efficient solutions to overcome blockage in the mmWave communication system and a striking convergence in average bit error rate and achievable rate performance with traditional full CSI-enabled RIS solutions.

Reconfigurable intelligent surface (RIS)-empowered communication, has been considered widely as one of the revolutionary technologies for next generation networks. However, due to the novel propagation characteristics of RISs, underlying RIS channel modeling and measurement research is still in its infancy and not fully investigated. In this paper, we conduct multi-scenario broadband channel measurements and modeling for RIS-assisted communications at the sub-6 GHz band. The measurements are carried out in three scenarios covering outdoor, indoor, and outdoor-to-indoor (O2I) environments, which suffer from non-line-of-sight (NLOS) propagation inherently. Three propagation modes including intelligent reflection with RIS, specular reflection with RIS and the mode without RIS, are taken into account in each scenario. In addition, considering the cascaded characteristics of RIS-assisted channel by nature, two modified empirical models including floating-intercept (FI) and close-in (CI) are proposed, which cover distance and angle domains. The measurement results rooted in 2096 channel acquisitions verify the prediction accuracy of these proposed models. Moreover, the propagation characteristics for RIS-assisted channels, including path loss (PL) gain, PL exponent, spatial consistency, time dispersion, frequency stationarity, etc., are compared and analyzed comprehensively. These channel measurement and modeling results may lay the groundwork for future applications of RIS-assisted communication systems in practice.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM), which transmits information bits through ordinary constellation symbols and indices of active subcarriers, is a promising multicarrier transmission scheme and has attracted the attention of researchers due to numerous benefits such as flexibility and simplicity. Nonetheless, OFDM-IM cannot satisfy the needs of future wireless communication services such as superior reliability, high data rates, and low complexity. In this article, we propose a novel OFDM-IM scheme named coordinate interleaved OFDM with repeated in-phase/quadrature IM (CI-OFDM-RIQIM), which provides superior error performance and enhanced spectral efficiency due to its diversity order of two and clever subcarrier activation pattern (SAP) detection mechanism, respectively. In addition, CI-OFDM-RIQIM is further extended to coordinate interleaved OFDM with in-phase/quadrature IM (CI-OFDM-IQIM) by doubling information bits transmitted by IM. Furthermore, log-likelihood ratio (LLR) based low-complexity detectors are designed for both proposed schemes. Theoretical analyses are performed and an upper bound on the bit error probability is derived. Comprehensive computer simulations under perfect and imperfect channel state information (CSI), are conducted to compare the proposed and reference schemes. It is shown that CI-OFDM-RIQIM and CI-OFDM-IQIM show superior results and can be considered promising candidates for next-generation wireless communication systems.

The integration of orthogonal time frequency space (OTFS) and index modulation (IM) is an attractive innovation, which can meet both high throughput and low energy consumption in highly mobile scenarios. This paper presents a novel dual frequency index modulation (DFIM) scheme for OTFS to improve spectral efficiency (SE). Exploring the internal relationship between indices, the proposed scheme, termed OTFS-DFIM, turns the classic index mapping process into a cascaded procedure involving partial index mapping and index updating. The differential information between grid points in the delay-Doppler domain and the time-frequency information are utilized to jointly determine the active grid points, which enables the OTFS frame to remain sparse and results in promising bit error rate (BER) performance. We also propose three distribution modes of IM subblocks in the two-dimensional grid and analyze their impact on receiver structure and system performance. The tight closed-form expressions of BER are derived. Additionally, we evaluate existing linear/non-linear equalization algorithms in OTFS and adapt them to the OTFS-IM/OTFS-DFIM system, offering a comprehensive insight on the tradeoffs between complexity and BER performance. Numerical simulations validate the superior performance of the OTFS-DFIM scheme in both SE and BER.

In this paper, we employ a reconfigurable intelligent surface (RIS) to boost the single-carrier frequency-domain equalization (SC-FDE) transmission for both single- and multi-antenna broadband wireless communications. Different from the existing RIS-SC-FDE schemes that use an RIS to harvest cyclic delay diversity with a limited beamforming gain, our advanced schemes unlock the potential of the RIS in passive beamforming to maximize the receive signal-to-noise ratio. Concerning the high complexity issue, a low-complexity minorization-maximization (MM) method, which leads to a closed-form solution in each iteration, is developed for the RIS beamforming optimization problem. Finally, simulation results show that the proposed RIS-SC-FDE with the MM-based beamforming significantly outperforms the existing counterparts in terms of bit error rate.

Reconfigurable intelligent surface (RIS)-empowered communications represent exciting prospects as one of the promising technologies capable of meeting the requirements of the sixth generation networks such as low-latency, reliability, and dense connectivity. However, validation of test cases and real-world experiments of RISs are imperative to their practical viability. To this end, this paper presents a physical demonstration of an RIS-assisted communication system in an indoor environment in order to enhance the coverage by increasing the received signal power. We first analyze the performance of the RIS-assisted system for a set of different locations of the receiver and observe around 10 dB improvement in the received signal power by careful RIS phase adjustments. Then, we employ an efficient codebook design for RIS configurations to adjust the RIS states on the move without feedback channels. We also investigate the impact of an efficient grouping of RIS elements, whose objective is to reduce the training time needed to find the optimal RIS configuration. In our extensive experimental measurements, we demonstrate that with the proposed grouping scheme, training time is reduced from one-half to one-eighth by sacrificing only a few dBs in received signal power.

This paper introduces a reconfigurable intelligent surface (RIS)-assisted grant-free non-orthogonal multiple access (GF-NOMA) scheme. To ensure the power reception disparity required by the power domain NOMA (PD-NOMA), we propose a joint user clustering and RIS assignment/alignment approach that maximizes the network sum rate by judiciously pairing user equipments (UEs) with distinct channel gains, assigning RISs to proper clusters, and aligning RIS phase shifts to the cluster members yielding the highest cluster sum rate. Once UEs are acknowledged with the cluster index, they are allowed to access their resource blocks (RBs) at any time requiring neither further grant acquisitions from the base station (BS) nor power control as all UEs are requested to transmit at the same power. In this way, the proposed approach performs an implicit over-the-air power control with minimal control signaling between the BS and UEs, which has shown to deliver up to 20% higher network sum rate than benchmark GF-NOMA and grant-based optimal (OPT) PD-NOMA schemes depending on the network parameters. The given numerical results also investigate the impact of UE density, RIS deployment, and RIS hardware specifications on the overall performance of the proposed RIS-aided GF-NOMA scheme.

There have been recently many studies demonstrating that the performance of wireless communication systems can be significantly improved by a reconfigurable intelligent surface (RIS), which is an attractive technology due to its low power requirement and low complexity. This paper presents a measurement-based characterization of RISs for providing physical layer security, where the transmitter (Alice), the intended user (Bob), and the eavesdropper (Eve) are deployed in an indoor environment. Each user is equipped with a software-defined radio connected to a horn antenna. The phase shifts of reflecting elements are software controlled to collaboratively determine the amount of received signal power at the locations of Bob and Eve in such a way that the secrecy capacity is aimed to be maximized. An iterative method is utilized to configure a Greenerwave RIS prototype consisting of 76 passive reflecting elements. Computer simulation and measurement results demonstrate that an RIS can be an effective tool to significantly increase the secrecy capacity between Bob and Eve.

Reconfigurable intelligent surface (RIS)-empowered communication has recently drawn significant attention due to its superior capability in manipulating the wireless propagation environment. However, the channel modeling and measurement of RIS-assisted wireless communication systems in real environment has not been adequately studied. In this paper, we construct a channel measurement system using vector network analyzer (VNA) is used to investigate RIS-assisted wireless communication channel in outdoor scenarios at 2.6 GHz. New path loss (PL) models including angle domain information are proposed by refining the traditional close-in (CI) and floating-intercept (FI) models. In the proposed models, both influences of the distance from transmitter (TX) to RIS and the distance from receiver (RX) to RIS on the PL, are taken into account. In addition, the value of root mean square (RMS) delay spread of RIS-assisted wireless communication is found to be much smaller than that of the traditional non line-of-sight (NLOS) case, implying that RIS provides a virtual line-of-sight (LOS) link.

Reconfigurable intelligent surfaces (RISs) are capable of enhancing the wireless propagation environment of the future Internet of Things (IoT). Recently, they have also been configured as a transmitter to realize information modulation at low hardware complexity. In this article, we conceive a transmitter relying on a single radio frequency (RF) chain for low-complexity RIS-aided multiuser downlink communication. More explicitly, in the proposed architecture, the multiuser information is transmit precoded and modulated at the RIS by appropriately configuring the phase shift and amplitude of each RIS element. We assume that the distribution of multiple users obeys on a Poisson point process (PPP), where we jointly optimize the total power reflected from the RIS and the power allocation fraction assigned to each user, under the practical constraint of a realistic amplitude limitation of each RIS element. Additionally, we theoretically analyze the ergodic rate, symbol error probability, outage probability, and coverage range of the proposed RIS-aided single-RF downlink and confirm the accuracy of our analysis by simulations. Finally, we compare its performance to that of the conventional multiple-input-multiple-output (MIMO) systems employing multiple RF-chains.

In this paper, the paradigm of thermal noise communication (TherCom) is put forward for future wired/wireless networks with extremely low power consumption. Taking backscatter communication (BackCom) and reconfigurable intelligent surface (RIS)-based radio frequency chain-free transmitters one step further, a thermal noise-driven transmitter might enable zero-signal-power transmission by simply indexing resistors or other noise sources according to information bits. This preliminary paper aims to shed light on the theoretical foundations, transceiver designs, and error performance derivations as well as optimizations of two emerging TherCom solutions: Kirchhoff-law-Johnson-noise (KLJN) secure bit exchange and wireless thermal noise modulation (TherMod) schemes. Our theoretical and computer simulation findings reveal that noise variance detection, supported by sample variance estimation with carefully optimized decision thresholds, is a reliable way of extracting the embedded information from noise modulated signals, even with limited number of noise samples.

In this paper, we propose a three-dimensional (3D) physics-based double reconfigurable intelligent surface (RIS) cooperatively assisted multiple-input multiple-output (MIMO) stochastic channel model for unmanned aerial vehicle (UAV)-to-ground communication scenarios. The double-RIS is distributed on the surface of buildings can assist the UAV transmitter to reflect its own signals to the ground receiver (GR), and simultaneously enhance the propagation by passive beamforming on the RISs. In the proposed channel model, we derive thepath power gainsfor different propagation links between the UAV and GR, thus enabling the proposed channel model to effectively characterize both large- and small-scale fading characteristics of realistic UAV-to-ground communication systems. Furthermore, we derive the critical propagation properties of the proposed channel model, including the spatial cross-correlation functions (CCFs), temporal auto-correlation functions (ACFs), and frequency correlation functions (FCFs), with respect to different RIS reflection phase configurations as well as different RIS orientation angles. Numerical simulation results demonstrate the propagation characteristics of the proposed double-RIS assisted UAV-to-ground channel model behave better than those of traditional channel models with single-RIS link or LoS link, thereby validating the necessity of introducing double-RIS into UAV-to-ground communication.

In reconfigurable intelligent surface (RIS)-aided coherent communications, channel state information (CSI) is often assumed to be perfectly estimated at the receiver. However, perfect CSI cannot be available in practice. Furthermore, the complex and ever-changing channel makes the acquisition of accurate CSI often unaffordable because of the large overhead in transmitting pilot signals. Motivated by these considerations, a novel non-coherent RIS-aided joint index keying$M$-ary differential chaos shift keying (RIS-JIK-MDCSK) system is proposed in this paper, where the receiver can retrieve information bits by performing non-coherent correlation demodulation without requiring CSI, thereby reducing the system complexity. In RIS-JIK-MDCSK, the states of the reference signal, RIS elements, and information-bearing subcarriers are jointly optimized to devise a joint index keying mechanism, where additional information bits are implicitly transmitted by these state indices, thus increasing the throughput and spectral efficiency. Furthermore, an effective joint index keying detection algorithm is proposed to recover the information bits. The analytical bit error rate (BER) of RIS-JIK-MDCSK is derived over a Rayleigh fading channel. Other evaluation metrics, including the throughput, spectral efficiency, and system complexity are also analyzed and compared against benchmark systems. Numerical simulations are performed to evaluate the superiority of RIS-JIK-MDCSK compared to existing systems.

Reconfigurable intelligent surface (RIS)-assisted communications has been a hot topic due to its promising advantages for future wireless networks. Existing works on RIS-assisted channel modeling have mainly focused on far-field propagation condition with planar wavefront assumption. In essence, the far-field condition does not always hold because the RIS array dimension may be comparable to the terminal distance, especially in RIS-assisted mobile networks. To this end, we propose a hybrid far- and near-field stochastic channel model for characterizing a RIS-assisted vehicle-to-vehicle (V2V) propagation environment, which takes into account both far-field and near-field propagation conditions. To achieve the balance between the modeling accuracy and complexity for the investigation of the RIS-assisted V2V propagation characteristics, we develop a sub-array partitioning scheme to dynamically divide the entire RIS array into several smaller sub-arrays, which makes planar wavefront assumption applicable for the sub-arrays. Important channel statistical properties, including spatial cross-correlation functions (CCFs), temporal auto-correlation functions (ACFs), and frequency correlation functions (FCFs), are derived and investigated. Simulation results are provided to show the performance of the proposed sub-array partition based hybrid far- and near-field modeling solution for RIS-assisted V2V channels.

In this paper, a cyclic-prefixed single-carrier (CPSC) transmission scheme with phase shift keying (PSK) signaling is presented for broadband wireless communications systems empowered by a reconfigurable intelligent surface (RIS). In the proposed CPSC-RIS, the RIS is configured according to the transmitted PSK symbols such that different cyclically delayed versions of the incident signal are created by the RIS to achieve multipath diversity. A practical and efficient channel estimator is developed for CPSC-RIS and the mean square error of the channel estimation is expressed in closed-form. We analyze the bit error rate (BER) performance of CPSC-RIS over frequency-selective Nakagami-$m$fading channels. An upper bound on the BER is derived by assuming maximum-likelihood detection. Furthermore, by applying the concept of index modulation (IM), we propose an extension of CPSC-RIS, termed CPSC-RIS-IM, which enhances the spectral efficiency. In addition to conventional constellation information of PSK symbols, CPSC-RIS-IM uses the full permutations of cyclic delays caused by the RIS to carry information. A sub-optimal receiver is designed for CPSC-RIS-IM to aim at low computational complexity. Our simulation results in terms of BER corroborate the performance analysis and the superiority of CPSC-RIS(-IM) over the conventional CPSC without an RIS and orthogonal frequency division multiplexing with an RIS.

Reconfigurable intelligent surface (RIS)-empowered communication has emerged as a novel concept for customizing future wireless environments in a cost- and energy-efficient way. However, due to double path loss, existing fully passive RIS systems that purely reflect the incident signals into preferred directions attain an unsatisfactory performance improvement over the traditional wireless networks in certain conditions. To overcome this bottleneck, we propose a novel transmission scheme, named hybrid reflection modulation (HRM), exploiting both active and passive reflecting elements at the RIS and their combinations, which enables to convey information without using any radio frequency (RF) chains. In the HRM scheme, the active reflecting elements using additional power amplifiers are able to amplify and reflect the incoming signal, while the remaining passive elements can simply reflect the signals with appropriate phase shifts. Based on this novel transmission model, we obtain an upper bound for the average bit error probability (ABEP), and derive achievable rate of the system using an information theoretic approach. Moreover, comprehensive computer simulations are performed to prove the superiority of the proposed HRM scheme over existing fully passive, fully active and reflection modulation (RM) systems.

This paper proposes a reconfigurable intelligent surface (RIS)-aided and angular-based hybrid beamforming (AB-HBF) technique for the millimeter wave (mmWave) massive multiple-input multiple-output (MIMO) systems. The proposed RIS-AB-HBF architecture consists of three stages: (i) RF beamformer, (ii) baseband (BB) precoder/combiner, and (iii) RIS phase shift design. First, in order to reduce the number of RF chains and the channel estimation overhead, RF beamformers are designed based on the 3D geometry-based mmWave channel model using slow time-varying angular parameters of the channel. Second, a BB precoder/combiner is designed by exploiting the reduced-size effective channel seen from the BB stages. Then, the phase shifts of the RIS are adjusted to maximize the achievable rate of the system via the nature-inspired particle swarm optimization (PSO) algorithm. Illustrative simulation results demonstrate that the use of RISs in the AB-HBF systems has the potential to provide more promising advantages in terms of reliability and flexibility in system design.

Design of simple transceiver architectures is inevitable in order to provide low computational complexity, low power consumption and affordable cost in beyond 5G (B5G) wireless systems, but it results in hardware impairments that significantly degrade the performance reliability of transmission. In this paper, among these hardware impairments, we discuss in-phase and quadrature (IQ) imbalance in orthogonal time frequency space (OTFS), which is a recent waveform considered as a potential candidate for B5G systems to relax the vulnerability against time-variant wireless channels. To mitigate the effect of IQ imbalance for OTFS, we propose an energy and spectral efficient IQ imbalance compensation scheme with the aid of index modulation (IM), which provides an attractive flexibility in the system design. In contrast to conventional solutions used in classical wireless technologies, such as iterative and pilot-based techniques, the proposed scheme avoids additional energy consumption and significant spectral efficient loss during the estimation and compensation of the IQ imbalance effect. The obtained bit error rate (BER) results validate the accuracy of the proposed compensator for different OTFS system configurations considering perfect/imperfect channel state information in practical scenarios.

Abstract Reconfigurable intelligent surface (RIS)‐empowered communications is on the rise and is a promising technology envisioned to aid in 6G and beyond wireless communication networks. RISs can manipulate impinging waves through their electromagnetic elements enabling some sort of control over the wireless channel. The potential of RIS technology is explored to perform a sort of virtual equalization over‐the‐air for frequency‐selective channels, whereas equalization is generally conducted at either the transmitter or receiver in conventional communication systems. Specifically, using an RIS, the frequency‐selective channel from the transmitter to the RIS is transformed to a frequency‐flat channel through elimination of inter‐symbol interference (ISI) components at the receiver. ISI is eliminated by adjusting the phases of impinging signals particularly to maximize the incoming signal of the strongest tap. First, a general end‐to‐end system model is provided and a continuous to discrete‐time signal model is presented. Subsequently, a probabilistic analysis for elimination of ISI terms is conducted and reinforced with computer simulations. Furthermore, a theoretical error probability analysis is performed along with computer simulations. It is analysed and demonstrated that conventional RIS phase alignment methods can successfully eliminate ISI, and the RIS‐aided communication channel can be converted from frequency‐selective to frequency‐flat.

In this study, we present a unifying framework for future reconfigurable intelligent surface (RIS)-assisted space shift keying (SSK) systems; we additionally propose two novel transmission schemes. The key strategies surrounding this concept, namely power-sensing RIS-SSK and partitioned RIS-SSK, grant knowledge of the activated transmitter (Tx) antenna index at the RIS, which, when using SSK, allows us to adjust the reflection phases. The performance of the proposed scheme is investigated in terms of the theoretical average bit error rate (ABER), and the effect of non-ideal transceivers. The performance is then compared to that of a reference RIS-SSK scheme, and a complexity analysis is provided. The obtained results, verified by extensive computer simulations, demonstrate that the partitioned and power-sensing RIS-SSK schemes achieve a higher ABER than the reference one. Moreover, hardware impairments clearly have a critically degrading impact on the system performance for all schemes, and should be carefully taken into account in future communication systems.

Reconfigurable intelligent surface (RIS)-assisted communications recently appeared as a game-changing technology for next-generation wireless communications due to its unprecedented ability to reform the propagation environment. One of the main aspects of using RISs is the exploitation of the so-called passive beamforming (PB), which is carried out by adjusting the reflection coefficients (mainly the phase shifts) of the individual RIS elements. However, practically, this individual phase shift adjustment is associated with many issues in hardware implementation, limiting the RIS achievable gain. In this paper, we propose a low-cost, phase shift-free and novel PB scheme by only optimizing the on/off states of the RIS elements while fixing their phase shifts. The proposed PB scheme is shown to achieve the same scaling law (quadratic growth with the RIS size) for the signal-to-noise ratio as in the classical phase shift-based PB scheme, yet, with far less sensitivity to spatial correlation and phase errors. We provide a unified mathematical analysis that characterizes the performance of the proposed PB scheme and obtain the outage probability for the considered RIS-assisted system. Based on the provided computer simulations, the proposed PB scheme is shown to have a clear superiority over the classical one under different performance metrics.

Media-based modulation (MBM), exploiting rich scattering properties of transmission environments via different radiation patterns of a single reconfigurable antenna (RA), has brought new insights into future communication systems. In this study, considering this innovative transmission principle, we introduce the realistic, two-dimensional (2D), and open-source SimMBM channel simulator to support various applications of MBM systems at sub-6 GHz frequency band in different environments.

Index modulation (IM) has been recognized as a compelling candidate for the next generation wireless networks. This paper considers index modulation-orthogonal frequency division multiplexing (IM-OFDM) with single subcarrier activation, where a single subcarrier is activated for transmitting an amplitude-phase modulated (APM) symbol in each IM group. Furthermore, a reduced-complexity approximate message passing aided symbol detector (AMPD) is proposed for reducing the detection complexity at the receiver side. The system performance of IM-OFDM with single subcarrier activation is investigated in terms of spectral efficiency (SE), peak-to-average power ratio (PAPR), bit error ratio (BER), as well as detection complexity. Our studies show that IM-OFDM with single subcarrier activation is capable of attaining low PAPR. Moreover, IMOFDM with single subcarrier activation outperforms the classic OFDM using APM for a SE of 1 bit/s/Hz, when high power amplifier (HPA) is employed. Furthermore, our investigation shows that low detection complexity can be achieved by the proposed AMPD for IM-OFDM with single subcarrier activation at the cost of a certain BER performance loss.

Abstract In 5G and beyond communication systems, distinct numerologies can coexist to serve diverse requirements for users and applications. However, the inter‐numerology interference (INI) is a main challenge that significantly impacts the system performance. Therefore, the performance under INI has become an essential evaluation metric for the suitability of the different transmission schemes in the future communication systems. This paper analyzes the impact of INI on the performance of orthogonal frequency division multiplexing with index modulation (OFDM‐IM) systems. Specifically, an analytical expression of the INI level in OFDM‐IM systems is presented as a function of the subcarrier activation ratio (SAR) and subcarrier activation probability (SAP). Furthermore, aiming at reducing the INI level, an adaptive subcarrier mapping scheme (SMS) is proposed based on the conventional combinatorial mapping scheme. Moreover, analysis and evaluation of SAR and SAP are performed regarding the requirements of 5G and beyond services. It is proved that the INI level in OFDM‐IM systems is highly dependent not only on the number of active subcarriers but also on their position in an OFDM block.

Index modulation (IM) provides a novel way for the transmission of additional data bits via the indices of the available transmit entities compared with classical communication schemes. This study examines the flexible utilization of existing IM techniques in a comprehensive manner to satisfy the challenging and diverse requirements of 5G and beyond services. After spatial modulation (SM), which transmits information bits through antenna indices, application of IM to orthogonal frequency-division multiplexing (OFDM) subcarriers has opened the door for the extension of IM into different dimensions, such as radio frequency (RF) mirrors, time slots, codes, and dispersion matrices. Recent studies have introduced the concept of multidimensional IM by various combinations of 1-D IM techniques to provide higher spectral efficiency (SE) and better bit error rate (BER) performance at the expense of higher transmitter (Tx) and receiver (Rx) complexity. Despite the ongoing research on the design of new IM techniques and their implementation challenges, proper use of the available IM techniques to address different requirements of 5G and beyond networks is an open research area in the literature. For this reason, we first provide the dimensional-based categorization of available IM domains and review the existing IM types regarding this categorization. Then, we develop a framework that investigates the efficient utilization of these techniques and establishes a link between the IM schemes and 5G services, namely, enhanced mobile broadband (eMBB), massive machine-type communications (mMTCs), and ultrareliable low-latency communication (URLLC). In addition, this work defines key performance indicators (KPIs) to quantify the advantages and disadvantages of IM techniques in time, frequency, space, and code dimensions. Finally, future recommendations are given regarding the design of flexible IM-based communication systems for 5G and beyond wireless networks.

Reconfigurable intelligent surface (RIS)-assisted communication appears as one of the potential enablers for sixth generation (6G) wireless networks by providing a new degree of freedom in the system design to telecom operators. Particularly, RIS-empowered millimeter wave (mmWave) communication systems can be a remedy to provide broadband and ubiquitous connectivity. This paper aims to fill an important gap in the open literature by providing a physical, accurate, open-source, and widely applicable RIS channel model for mmWave frequencies. Our model is not only applicable in various indoor and outdoor environments but also includes the physical characteristics of wireless propagation in the presence of RISs by considering 5G radio channel conditions. Various deployment scenarios are presented for RISs and useful insights are provided for system designers from the perspective of potential RIS use-cases and their efficient positioning. The scenarios in which the use of an RIS makes a big difference or might not have a big impact on the communication system performance, are revealed. The open-source and comprehensiveSimRIS Channel Simulatoris also introduced in this paper.

Reconfigurable intelligent surface (RIS)-empowered communication is one of the promising 6G technologies that allows the conversion of the wireless channel into an intelligent transmit entity by manipulating the impinging waves using man-made surfaces. In this paper, the potential benefits of using RISs are investigated for indoor/outdoor setups and various frequency bands (from sub 6 GHz to millimeter-waves). First, a general system model with a single RIS is considered and the effect of the total number of reflecting elements on the probabilistic distribution of the received signal-to-noise ratio and error performance is investigated under Rician fading. Also for this case, the path loss exponent is analyzed by considering empirical path loss models. Furthermore, transmission models with multiple RISs are developed and analyzed for indoor and outdoor non line-of-sight (NLOS) scenarios. The conventional RIS selection strategies are also integrated for systems equipped with multiple RISs for the first time. Through extensive simulations, it is demonstrated that the RIS-assisted systems provide promising solutions for indoor/outdoor scenarios at various operating frequencies and exhibit significant results in error performance and achievable data rates even in the presence of system imperfections such as limited range phase adjustment and imperfect channel phase estimation at RISs.

Edge caching can effectively reduce backhaul burden at core network and increase quality-of-service at wireless edge nodes. However, the beneficial role of edge caching cannot be fully realized when the offloading link is in deep fade. Fortunately, the impairments induced by wireless propagation environments could be renovated by a reconfigurable intelligent surface (RIS). In this paper, a new RIS-aided edge caching system is proposed, where a network cost minimization problem is formulated to optimize content placement at cache units, active beamforming at base station and passive phase shifting at RIS. After decoupling the content placement subproblem with the hybrid beamforming design, we propose an alternating optimization algorithm to tackle the active beamforming and passive phase shifting. For active beamforming, we transfer the problem into a semidefinite programming (SDP) and prove that the optimal solution of SDP is always rank-one. For passive phase shifting, we introduce the block coordinate descent method to alternately optimize the auxiliary variables and the RIS phase shifts. Further, a conjugate gradient algorithm based on manifold optimization is proposed to deal with the non-convex unit-modulus constraints. Numerical results show that our RIS-aided edge caching design can effectively decrease the network cost by improving the quality of offloading links.

This study presents a multiuser (MU) multiple‐input multiple‐output (MIMO) downlink transmission scheme based on the quadrature spatial modulation (QSM) concept, which uses the indices of the non‐zero entries in its transmission vector to modulate and transmit an independent sequence of bits for each user in the system. The MU interference is removed by using a matrix precoding technique based on channel state information (CSI) known as block diagonalisation (BD). The performance of the proposed scheme is compared with the conventional MU–MIMO–BD system for uncorrelated Rayleigh fading channels and correlated fading channels with imperfect CSI in the reception. Additionally, a low‐complexity near maximum likelihood (ML) detection algorithm for the MU–MIMO–QSM signal's detection is proposed. For the considered cases, the proposed MU–MIMO–QSM scheme exhibits gains up to 1 dB in bit error rate performance and a reduction in detection complexity up to 93% as compared to the conventional MU–MIMO–BD scheme for the optimal ML detection. The proposed algorithm performs very near to the optimal ML detector whilst achieving a complexity reduction of up to 58%.

Transmission through reconfigurable intelligent surfaces (RISs), which control the reflection/scattering characteristics of incident waves in a deliberate manner to enhance the signal quality at the receiver, appears as a promising candidate for future wireless communication systems. In this paper, we bring the concept of RIS-assisted communications to the realm of index modulation (IM) by proposing RIS-space shift keying (RIS-SSK) and RIS-spatial modulation (RIS-SM) schemes. These two schemes are realized through not only intelligent reflection of the incoming signals to improve the reception but also utilization of the IM principle for the indices of multiple receive antennas in a clever way to improve the spectral efficiency. Maximum energy-based suboptimal (greedy) and exhaustive search-based optimal (maximum likelihood) detectors of the proposed RIS-SSK/SM schemes are formulated and a unified framework is presented for the derivation of their theoretical average bit error probability. Extensive computer simulation results are provided to assess the potential of RIS-assisted IM schemes as well as to verify our theoretical derivations. Our findings also reveal that RIS-based IM, which enables high data rates with remarkably low error rates, can become a potential candidate for future wireless communication systems in the context of beyond multiple-input multiple-output (MIMO) solutions.

Spatial modulation (SM) has been shown to be a promising low-complexity alternative to the state-of-art multiple-input multiple-output (MIMO) schemes due to its novel transmission approach. This paper investigates the performance of SM-MIMO systems in the presence of two practical undesirable effects, namely in-phase (I) and quadrature-phase (Q) imbalance (IQI) and imperfect channel state information (ICSI). An optimum maximum likelihood detection (MLD) method is proposed to tackle the effects of self-interference and signal distortion caused by IQI impairment by adapting the traditional MLD technique in accordance with the asymmetric characteristics of the IQI. More particularly, upper-bounds of the closed-form average pairwise error probability (APEP) and the average bit error rate (ABER) are derived for generalized Beckmann fading channels. As erroneously interpreted channel coefficients at the receiver (Rx) cause the error rate to increase and the detection to fall short, Cramer-Rao bound, which is a lower bound on the variance of the channel estimator, is utilized to assess the estimation accuracy. The system performance is evaluated by analytical derivations that are corroborated with computer simulations. The obtained results show that ICSI and IQI should be seriously considered while designing the future SM-based wireless communication systems.

Orthogonal frequency division multiplexing (OFDM) with index modulation (OFDM-IM) appears as a promising multi-carrier waveform candidate for beyond 5G due to its attractive advantages such as operational flexibility and ease of implementation. However, OFDM-IM may not be a proper choice for 5G services such as enhanced mobile broadband (eMBB) since achieving high data rates is challenging because of its null subcarriers. One solution to enhance the spectral efficiency of OFDM-IM is the employment of multiple distinguishable constellations (modes) by also exploiting its null subcarriers for data transmission. This article proposes a novel IM technique called super-mode OFDM-IM (SuM-OFDM-IM), where mode activation patterns (MAPs) and subcarrier activation patterns (SAPs) are jointly selected and conventional data symbols are repetition coded over multiple subcarriers to achieve a diversity gain. For the proposed scheme, a low-complexity detector is designed, theoretical analyses are performed and a bit error rate (BER) upper bound is derived. The performance of the proposed system is also investigated through real-time experiments using a software-defined radio (SDR) based prototype. We show that SuM-OFDM-IM exhibits promising results in terms of spectral efficiency and error performance; thus, appears as a potential candidate for 5G and beyond communication systems.

In this paper, a novel opportunistic spectrum sharing scheme, based on orthogonal frequency division multiplexing with index modulation (OFDM-IM), is proposed for cognitive radio (CR) networks. In the considered OFDM-IM based CR (OFDM-IM-CR) model, the primary transmitter (PT) communicates with the primary receiver with the aid of an amplify-and-forward (AF) relay by transmitting OFDM-IM signals. Meanwhile, the secondary transmitter (ST) passively senses the spectrum and transmits its own information over those inactive subcarriers of the primary network to the secondary receiver if the signal-to-noise ratio of the PT$\to $ST link is above a predefined threshold; otherwise, the ST stays in silent mode. Two different types of maximum-likelihood (ML) detectors are designed for the primary network, based on the knowledge of either the estimated channel state information or the statistical channel information of the secondary network. A complexity-reducing method, which is applicable to both types and achieves near optimal performance, is further proposed. To evaluate the performance, a tight upper bound on the bit error rate (BER) is derived, assuming the first type of ML detection. Simulation results corroborate the analysis and show that OFDM-IM-CR has the potential of outperforming OFDM-CR and OFDM-IM-AF in terms of BER with higher spectral efficiency.

Multiple-input multiple-output (MIMO) transmission approaches have been recently considered in the context of molecular communications due to desirable improvements they provide in terms of communication efficiency. Among these methods, molecular index modulation (molecular-IM) schemes yield a significant improvement in throughput and show promising results for future molecular MIMO research. However, existing molecular-IM methods rely on perfect spatial alignment between corresponding antennas, which may not be the case in a possible practical scenario. Motivated by this practical constraint, this study proposes a novel receiver design for molecular-IM. The proposed decoder is an augmented version of the maximum count decoder (MCD) and operates by merging MCD with a simple linear combining technique. Our numerical results show that the proposed approach yields a desirable robustness against antenna misalignments while still maintaining a simplistic receiver structure.

In this paper, a novel circular space-time block coding (CSTBC)-based index modulation (IM) technique, which achieves various orders of transmit diversity gains, is introduced. In the proposed system, a promising IM concept, media-based modulation (MBM), which carries additional information bits through the available radio frequency (RF) mirrors of transmit antennas, is considered. Unlike traditional STBC-based IM concepts, the proposed technique provides various orders of transmit diversity gains and attains higher spectral efficiency values with a single RF chain. The bit error rate (BER) performance of the proposed system is theoretically analyzed and an upper bound expression for the average bit error probability (ABEP) is derived. Moreover, via comprehensive computer simulations, the improved BER performance of the proposed CSTBC-IM is demonstrated over the-stateof-the-art IM schemes.

In this paper, a novel low-complexity detection algorithm for spatial modulation (SM), referred to as the minimum-distance of maximum-length (m-M) algorithm, is proposed and analyzed. The proposed m-M algorithm is a smart searching method that is applied for the SM tree-search decoders. The behavior of the m-M algorithm is studied for three different scenarios: 1) perfect channel state information at the receiver side (CSIR); 2) imperfect CSIR of a fixed channel estimation error variance; and 3) imperfect CSIR of a variable channel estimation error variance. Moreover, the complexity of the m-M algorithm is considered as a random variable, which is carefully analyzed for all scenarios, using probabilistic tools. Based on a combination of the sphere decoder (SD) and ordering concepts, the m-M algorithm guarantees to find the maximum-likelihood (ML) solution with a significant reduction in the decoding complexity compared with SM-ML and existing SM-SD algorithms; it can reduce the complexity up to 94% and 85% in the perfect CSIR and the worst scenario of imperfect CSIR, respectively, compared with the SM-ML decoder. The Monte Carlo simulation results are provided to support our findings as well as the derived analytical complexity reduction expressions.

Future wireless communication systems face several transceiver hardware imperfections that may significantly degrade their performance. In-phase (I) and quadrature-phase (Q) imbalance (IQI), which causes self-interference effects on the desired signal, is an important and practical example to these impairments. In this paper, a channel state information-assisted dual-hop amplify-and-forward (AF) relaying system in the presence of IQI is analyzed. The error performance of the relevant AF cooperative protocol is firstly studied by considering the traditional maximum likelihood detection (MLD) algorithm as a benchmark. Then, two compensation methods, weighting and zero-forcing, are proposed to mitigate the IQI effects. Finally, an optimal MLD solution is introduced by adapting the traditional MLD technique in compliance with the asymmetric characteristics of the IQI. The system performance is evaluated in terms of average symbol error probability (ASEP) through the computer simulations. The ASEP is calculated analytically for the optimal MLD method as well under the assumption of point-to-point communication, which has been envisioned as an allied technology of the fifth generation (5G) wireless systems, between the source and the relay nodes. A power allocation algorithm is provided for this specific case. The extensive computer simulations and analytical results prove that the proposed optimal MLD method provides the best results.

Molecular communication via diffusion (MCvD) is a molecular communication method that utilizes the free diffusion of carrier molecules to transfer information at the nanoscale. Due to the random propagation of carrier molecules, intersymbol interference (ISI) is a major issue in an MCvD system. Alongside ISI, interlink interference (ILI) is also an issue that increases the total interference for the MCvD-based multiple-input-multiple-output (MIMO) approaches. Inspired by the antenna index modulation (IM) concept in traditional communication systems, this paper introduces novel IM-based transmission schemes for MCvD systems. In this paper, molecular space shift keying (MSSK) is proposed as a novel modulation for molecular MIMO systems, and it is found that this method combats ISI and ILI considerably better than the existing MIMO approaches. For nanomachines that have access to two different molecules, the direct extension of MSSK, quadrature MSSK (QMSSK) is also proposed. QMSSK is found to combat ISI considerably well while not performing well against ILI-caused errors. In order to combat ILI more effectively, another dual-molecule-based novel modulation scheme called the molecular spatial modulation (MSM) is proposed. Combined with the Gray mapping imposed on the antenna indices, MSM is observed to yield reliable error rates for molecular MIMO systems.

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.

Index modulation (IM) is an attractive concept for next generation communication systems. However, as an emerging technique, there are still some challenges need to be tackled in this frontier. In this paper, we consider two aspects of IM, which are diversity and detection complexity. Specifically, we propose a linear precoding assisted index modulation (LPIM) scheme for orthogonal frequency division multiplexing (OFDM) systems. We commence by analyzing the diversity and coding gains of the proposed scheme. Moreover, a detailed codebook design criterion is proposed. Then, our LPIM codebook is designed based on the maximum diversity and coding gain criteria. In contrast to the signaling model of the existing full diversity precoder designed for OFDM, we introduce a modeling method to link the zero-valued IM symbols to the origin of a Lattice for implementing our full diversity precoder designed for OFDM-IM. Both analytical and computer simulation results are provided for characterizing the attainable performance of OFDM-LPIM, demonstrating that it is capable of achieving full diversity as well as an attractive coding gain. However, the maximum-likelihood (ML) detection complexity of OFDM-LPIM is excessive, hence a low-complexity generalized iterative residual check detector (GIRCD) is proposed, which is inspired by the existing sparse recovery algorithms. Finally, computer simulation results are provided for demonstrating that GIRCD can provide a beneficial trade-off between bit error ratio performance and complexity.

Space shift keying (SSK) has many advantages through its unique transmission manner as compared to other multiple-input multiple-output (MIMO) techniques. Nevertheless, the practicality of SSK in the presence of real-time imperfections such as channel estimation errors and hardware impairments (HWIs) is still an open research problem. On the other hand, the effects of HWIs are assumed as zero-mean circularly-symmetric complex Gaussian random variable (RV) in the literature. However, this model does not reflect the asymmetric characteristics of different HWIs. Therefore, the aim of this paper is to shed light on the joint effect of improper Gaussian noise (IGN) and imperfect channel state information (ICSI) on the performance of SSK receiver. Particularly, an optimal maximum likelihood (ML) detector is designed, and pairwise error probability (PEP) expression is derived. Additionally, an exact closed-form Cramer-Rao bound expression is calculated for evaluating the channel estimation accuracy under the effect of IGN. The results obtained by using computer simulations prove that the proposed optimal detector is superior to the traditional ML detector in the presence of IGN and ICSI.

Due to its several implementation advantages, orthogonal frequency division multiplexing (OFDM) has become the most frequently used waveform in modern wireless communication technologies. However, OFDM has some vulnerabilities that may limit its advantages. In order to alleviate OFDM's vulnerabilities, several waveforms have been proposed within the context of 5G research activities. Universal-filtered OFDM (UF-OFDM) is one of the most effective waveforms due to its OFDM-like structure, easier implementation procedure and robustness against synchronization errors. Most of the recent studies on novel waveforms generally target filter optimization or performance improving techniques. In this paper, as a novel approach, UF-OFDM with filter shift keying (OFDM-FSK) is proposed. Here, we consider the filter realizations to carry information through index modulation mechanism. Accordingly, utilized filter of each UF-OFDM subband is selected from a filter set based on the incoming data bits, and an increase in data rate is obtained. At the receiver side, accumulated maximum-likelihood decision metric in each subband is computed. The filter realization, as well as data symbols, that provide the minimum decision metric in each subband are detected. Hence the throughput of the conventional waveform is increased in an efficient and practical manner. The performance of OFDM-FSK system is evaluated through extensive computer simulations. To assess its real-time performance, real-time experiments are also carried out by using software defined radio (SDR) nodes. In the created testbed, a wireless synchronization structure is provided and the considered waveforms are tested in a setup with imperfect synchronization. This experimentation provides realistic insights about the benefits of OFDM-FSK. As it will be demonstrated via both computer simulation and test results, OFDM-FSK provides significant throughput and error performance benefits over UF-OFDM.

In this letter, generalized frequency division multiplexing (GFDM) is combined with index modulation (IM) exploiting the block-based structure of GFDM in an innovative way. The proposed GFDM-IM scheme uses flexible IM numerology to reduce out-of-band (OOB) emission and provides a novel multilayer system model to further benefit from the enhanced distance spectrum of IM. The proposed scheme adapts maximum likelihood successive interference cancellation based near-optimum detector to improve the error performance and reduce the computational complexity. It has been shown that the proposed GFDM-IM scheme achieves better error performance than classical and IM-based orthogonal frequency division multiplexing and GFDM schemes at the same spectral efficiency, and provides an interesting tradeoff among OOB emission, spectral efficiency, and latency.

As an emerging multi-carrier transmission scheme, multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM) conveys information through multiple distinguishable constellations (or modes, alternatively) and their full permutations, increasing the spectral efficiency of OFDM-IM and classical OFDM. However, both MM-OFDM-IM and its extension, that is, MM-OFDM with in-phase/quadrature index modulation (MM-OFDM-IM-IQ), cannot provide any transmit diversity gain, which may be critical for ultra-reliable communications. In this paper, aiming at enhancing the diversity gain of MM-OFDM-IM(-IQ) schemes, coordinate interleaved (CI-)MM-OFDM-IM and linear constellation precoded (LCP-) MM-OFDM-IM-IQ are proposed, both of which achieve a diversity order of two without loss of spectral efficiency. The optimal rotation angle for CI-MM-OFDM-IM and the optimal precoding matrix for LCP-MM-OFDM-IM-IQ, in the sense of maximizing their coding gains, are derived in closed form. Computer simulations corroborate the advantages of the proposed schemes in terms of diversity and bit error rate performance toward next-generation wireless networks.

In this paper, the end-to-end bit error performance of a dual-hop (DH) energy harvesting (EH) amplify-and-forward/decode-and-forward (AF/DF) system is investigated. In this system, the source communicates with the destination over an intermediate relay when the direct link between the source and the destination is in deep fade. The channels are assumed to be exposed to Nakagami-m fading and all nodes are equipped with one antenna. The relay uses power splitting (PS) and time switching (TS) relaying protocols to harvest energy and uses it to forward the decoded signal or to transmit the amplified version of the received signal to the destination. The bit error probabilities of the considered DH-AF and DH-DF systems are analytically derived for both EH protocols and their performances are comparatively evaluated. Moreover, a system optimization is performed to maximize the bit error rate (BER) performance. For the TS mode, a list of mixture of modulations is provided for different spectral efficiency values and different time allocation parameters. Our comprehensive results on the BER performance of AF- and DF-aided DH networks with EH in the relay, provide basic guidelines for the design of the future DH-EH systems.

Orbital angular momentum (OAM)-based mode division multiplexing (MDM) is an emerging physical layer solution for short-distance line-of-sight wireless communications by providing high spectral efficiency with a considerably low detection complexity. Similarly, index modulation (IM) schemes have been extensively studied over the past few years due to their attractive features, such as improved error performance, low complexity, and high energy efficiency. In this paper, we propose the scheme of orbital angular momentum with index modulation (OAM-IM) by using the activated OAM modes themselves to carry additional information through the principle of IM. A general guideline is presented for the construction of optimal OAM-IM schemes along with the formulation of a low-complexity maximum likelihood detector. It is shown via computer simulations as well as theoretical bit error probability calculations that the proposed OAM-IM schemes can achieve considerably better error performance than the reference OAM-MDM schemes, while having the same level of detection complexity.

In this paper, we propose an index modulated orthogonal frequency division multiplexing spread spectrum (IM-OFDM-SS) scheme, which combines the techniques of SS and IM under the framework of OFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional$M$-ary modulated symbols. A low-complexity maximal ratio combining (MRC) detector is designed, in which the receiver first detects the spreading codes and then de-spreads and demodulates the symbols. The bit error rate (BER) performance of IM-OFDM-SS systems in the presence of channel estimation errors is analyzed. An upper bound and approximate average bit error probability associated with maximum-likelihood and MRC detection, respectively, are derived. Subsequently, we extend the idea of IM-OFDM-SS to multi-code and multi-user scenarios, proposing generalized (G-)IM-OFDM-SS and IM-based multi-carrier code division multiple access (IM-MC-CDMA), respectively. Simulation results verify the analyses and show that the proposed IM/ GIM-OFDM-SS outperforms the existing OFDM-IM and OFDM-SS schemes significantly. In addition, the IM-MC-CDMA exhibits a lower BER than MC-CDMA at the same SEs for either multi-user or single-user detection.

Multiple-mode orthogonal frequency division multiplexing with index modulation (MM-OFDM-IM), which transmits an OFDM signal with information bits embedded onto multiple distinguishable signal constellations of the same cardinality and their permutations, is a recently proposed IM technique in the frequency domain. It is capable of achieving higher spectral efficiency and better error performance than classical OFDM and existing frequency-domain IM schemes. In this paper, we propose the scheme of generalized (G-) MM-OFDM-IM, which allows a different subcarrier to utilize a signal constellation of a different size while conveying the same number of IM bits. Considering phase shift keying constellations, we present design guidelines for GMM-OFDM-IM to achieve an optimal error performance in the asymptotically high signal-to-noise ratio region. A computationally efficient and near-optimal detector based on the idea of sequential decoding is also tailored to GMM-OFDM-IM, which avoids the detection of an illegitimate constellation permutation. Monte Carlo simulations are conducted to validate the inherent properties and advantages of GMM-OFDM-IM.

Multiple-input multiple-output orthogonal frequency division multiplexing with index modulation (MIMO-OFDM-IM), which provides a flexible trade-off between spectral efficiency and error performance, is recently proposed as a promising transmission technique for energy-efficient 5G wireless communication systems. However, due to the dependence of subcarrier symbols within each subblock and the strong interchannel interference, it is challenging to detect the transmitted data effectively while imposing low computational burden to the receiver. In this paper, we propose a low-complexity detector based on the sequential Monte Carlo (SMC) theory for the detection of MIMO-OFDM-IM signals. The proposed detector, which draws samples based on the importance weights at the subblock level, achieves near-optimal error performance with considerably reduced computational complexity. Simulation and numerical results in terms of bit error rate (BER) and number of complex multiplications (NCM) corroborate the superiority of the proposed detector.

In this study, the authors propose novel cooperative spatial modulation (SM) systems with two main relaying techniques [amplify‐and‐forward (AF) and decode‐and‐forward (DF)], where all nodes have multiple transmit and/or receive antennas. Most of the studies in the literature of cooperative SM systems, which combine the advantages of cooperative communications and SM systems, consider only the space shift keying (SK) scheme with single receive/transmit antenna at relay and destination. Since the error performance of SM highly depends on the number of receive antennas and more flexible cooperative communications systems can be obtained by using SM with multiple antennas, it is essential to investigate multi‐antenna cooperative SM systems. They derive analytical expressions of the average bit error probability for both the newly proposed cooperative SM‐DF and cooperative SM‐AF systems and validate with the computer simulation results. Furthermore, they present the bit error rate comparison of considered systems with classical M ‐ary phase SK/quadrature amplitude modulation cooperative systems. Computer simulation results indicate that multiple antennas cooperative SM systems provide better error performance than classical cooperative systems for both relaying techniques.

In this study, a new cooperative spectrum sharing protocol based on overlay system paradigm is proposed for different relaying strategies. In the considered model, a single‐antenna primary transmitter (PT) communicates with a primary receiver (PR) by the assistance of a secondary transmitter (ST) which seeks an opportunity to realise spectrum sharing by adopting spatial modulation (SM). The proposed protocol consists of two time slots. In the first time slot, primary M ‐ary modulated signal is transmitted from PT to ST and PR. ST processes the received signal according to the relaying strategy it adopts and transmits the processed signal by SM to PR and secondary receiver (SR) in the second time slot. Thanks to SM at ST, primary information is carried by conventional M ‐ary modulation whereas secondary information is conveyed by antenna indices. Therefore, the need for superposition of different users' information and power allocation at ST are avoided. Upper bounds on bit error probability according to three different relaying strategies: fixed and incremental amplify‐and‐forward, and selective decode‐and‐forward, are derived for both users. The theoretical results are supported via computer simulations and error performance comparisons with two existing reference systems are performed to show the superiority of the proposed protocol.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) performs transmission by considering two modes over OFDM subcarriers, which are the null and the conventional$M$-ary signal constellation. The spectral efficiency (SE) of the system, however, is limited, since the null mode itself does not carry any information and the number of subcarrier activation patterns increases combinatorially. In this paper, a novel IM scheme, called multiple-mode OFDM-IM (MM-OFDM-IM), is proposed for OFDM systems to improve the SE by conveying information through multiple distinguishable modes and their full permutations. A practical and efficient mode selection strategy, which is constrained on the phase shift keying/quadrature amplitude modulation constellations, is designed. Two efficient detectors that provide different tradeoffs between the error performance and detection complexity are also proposed. The principle of MM-OFDM-IM is further extended to the in-phase and quadrature components of OFDM signals, and the method of generating multiple modes from the$M$-ary pulse amplitude modulation constellation for this modified scheme is also introduced. Bit error rate (BER) analyses are provided for the proposed schemes. Monte Carlo simulations on BER corroborate the analyses and show that the proposed schemes appear as promising multi-carrier transmission alternatives by outperforming the existing OFDM-IM counterparts.

Visible light communications (VLC) is a promising and uncharted new technology for the next generation of wireless communication systems. This paper proposes a novel generalized light emitting diode (LED) index modulation method for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM)-based VLC systems. The proposed scheme avoids the typical spectrum efficiency losses incurred by time- and frequency-domain shaping in OFDM signals. This is achieved by exploiting spatial multiplexing along with LED index modulation. Accordingly, real and imaginary components of the complex time-domain OFDM signals are separated first, then resulting bipolar signals are transmitted over a VLC channel by encoding sign information in LED indexes. As a benchmark, we demonstrate the performance analysis of our proposed system for both analytical and physical channel models. Furthermore, two novel receiver designs are proposed. Each one is suitable for frequency-flat or selective channel scenarios. It has been shown via extensive computer simulations that the proposed scheme achieves considerably better bit error ratio versus signal-to-noise-ratio performance than the existing VLC-MIMO-OFDM systems that use the same number of transmit and receive units [LEDs and photo diodes (PDs)]. Compared with the single-input single-output (SISO) DC biased optical (DCO)-OFDM system, both spectral efficiency and DC bias can be doubled and removed respectively simply by exploiting a MIMO configuration.

Index modulation concept has attracted considerable research interest in the past few years. As a realization of index modulation in the frequency domain, orthogonal frequency division multiplexing with index modulation (OFDM-IM) has recently been proposed, which conveys information bits through both the subcarrier activation patterns and the amplitude phase modulation constellation points. This paper proposes two enhanced OFDM-IM schemes aimed at achieving higher spectral efficiency and diversity gain, respectively. The first one, termed OFDM with hybrid in-phase/quadrature index modulation (OFDM-HIQ-IM), explores the I- and Q- dimensions jointly for index modulation, allowing transmission of more index modulation bits in each subcarrier group. The second one, termed linear constellation precoded OFDM-IQ-IM (LP-OFDM-IQ-IM), spreads information symbols across two adjacent active subcarriers through linear constellation precoding to harvest additional diversity gain. By maximizing the minimum squared Euclidean distance, two different realizations of LP-OFDM-IQ-IM are derived, which leads to a rotated and a diamond-shaped constellation, respectively. The proposed OFDM-HIQ-IM and LP-OFDM-IQ-IM, as revealed by both theoretical analyses and computer simulations, enable low-complexity detection and exhibit superior error rate performance over the existing OFDM-IM schemes.

In this paper, we propose a generalized light emitting diode (LED) index modulation scheme for multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) visible light communications (VLC) systems. The proposed scheme generalizes the LED index modulation concept by using the spatial multiplexing principle to transmit complex OFDM signals through VLC channels by separating these signals into their real-imaginary and positive-negative parts. The maximum a posteriori (MAP) estimator of the proposed scheme, which relies on quadratic programing (QP) problem, is presented for flat VLC channels. It is shown via computer simulations that the proposed scheme achieves considerably better error performance than the existing VLC-MIMO-OFDM systems due to its power efficiency and improved transceiver structure.

In this paper, a new cooperative spectrum sharing protocol which avoids mutual interference between primary and secondary users by employing spatial modulation (SM) at the secondary transmitter (ST), is proposed for overlay cognitive radio networks. In the proposed protocol, ST cooperates with selective decode-and-forward (DF) relaying strategy to prevent error propagation. Primary transmitter (PT) exploits M-ary phase shift keying (M-PSK) modulation whereas ST utilizes SM (Nt transmit antennas and M- PSK). Since ST transmits PT's information by M-PSK symbols and its own information by antenna indexes, mutual interference at primary receiver (PR) and secondary receiver (SR) is removed. Upper bounds on the bit error probability (BEP) of the primary and secondary users are analytically derived and supported via computer simulation results which show that the proposed protocol significantly improves the bit error performance of both primary and secondary users compared to the non-cooperation case and the cooperative DF spectrum sharing protocol using superposition coding.

Visible light communication (VLC) involves the dual use of illumination infrastructure for high speed wireless access. Designing such optical based communication systems, realistic indoor optical channel modeling becomes an important issue to be handled. In this paper, first we obtain new realistic indoor VL channel characterizations and models, in a multiple-input multiple-output (MIMO) transmission scenario, using non-sequential ray tracing approach for the channel impulse responses (CIRs). Practical issues such as number of light emitting diode (LED) chips per luminary, spacing between LED chips, objects inside the room and cabling topology are also investigated. On the other hand, since indoor optical channels exhibit frequency selectivity, multi-carrier communication systems, particularly orthogonal frequency division multiplexing (OFDM) is used to handle the resulting inter-symbol interference in VLC systems. Hence, we propose a new MIMO-OFDM based VLC system, called MIMO enhanced unipolar OFDM (MIMO-eU-OFDM) by combining MIMO transmission techniques with the recently proposed eU-OFDM scheme. The bit error rate (BER) performance of the proposed system is investigated in the presence of the 2 × 2 and 4 × 4 realistic MIMO VLC channels and its BER performance is compared with the reference optical MIMO-OFDM systems.

A combination of space‐time block coding and spatial modulation (STBC‐SM) has been recently proposed for multiple input‐multiple output systems to obtain both spatial diversity gain and more capacity simultaneously while assuming the perfect channel state information (P‐CSI) was available at the receiver. However, in practical scenarios the CSI is unknown to the receiver and should be estimated in order to detect the transmitted data in a reliable way. Therefore, channel estimation (CE) is a major challenge in designing the STBC‐SM systems. In this study, the problem of CE is investigated and a new pilot‐aided channel estimation (PA‐CE) technique, coupled with an interpolation, is proposed for the STBC‐SM systems operating in the presence of rapidly time‐varying mobile channels. Several interpolation schemes such as the linear, nearest neighbour, piecewise cubic Hermite and the low‐pass interpolations are applied and their performances are compared to determine the best suitable interpolation technique to be employed in STBC‐SM systems. Bit error rate (BER) performance of the proposed CE technique is then investigated in time‐varying channels with different modulation. Moreover, the pairwise error probability of the STBC‐SM scheme is derived and its average bit error probability is evaluated analytically in the presence of CE errors.

Orthogonal frequency division multiplexing with index modulation (OFDM-IM) is a novel multicarrier transmission technique which has been proposed as an alternative to classical OFDM. The main idea of OFDM-IM is the use of the indices of the active subcarriers in an OFDM system as an additional source of information. In this work, we propose multiple-input multiple-output OFDM-IM (MIMO-OFDM-IM) scheme by combining OFDM-IM and MIMO transmission techniques. The low complexity transceiver structure of the MIMO-OFDM-IM scheme is developed and it is shown via computer simulations that the proposed MIMO-OFDM-IM scheme achieves significantly better error performance than classical MIMO-OFDM for several different system configurations.

A new multiple-input multiple-output (MIMO) transmission scheme named\textit{ trellis coded space-shift keying} (TC-SSK) is proposed in this paper. This scheme is obtained by the combination of trellis coding and SSK for a given spectral efficiency, number of transmit antennas and trellis states. A soft decision Viterbi decoder is employed at the receiver to decode the transmitted SSK symbols. Using systematic methods, new TC-SSK schemes are obtained for 1 and 2 bits/s/Hz spectral efficiencies. An upper bound for the pairwise error probability (PEP) of the TC-SSK scheme is calculated for quasi-static Rayleigh fading channels. Computer simulation results show that the proposed TC-SSK schemes achieve significantly better error performance than the uncoded SSK, and the theoretical error probability curves are in close match with the computer simulation results. Furthermore, TC-SSK provides an attractive trade- off between complexity and performance compared to the spatial modulation with trellis coding.

Successive relaying has recently emerged as an effective technique for cooperative networks and provides significant improvements in bandwidth efficiency over traditional relaying techniques; however, to achieve full-diversity, the available successive relaying protocols generally assume noise-free source-relay and interference-free inter-relay channels. In this paper, a novel successive relaying protocol is proposed for N-relay wireless networks by removing these optimistic assumptions. The proposed protocol benefits from distributed space-time block codes (STBCs) with coordinate interleaving and relay selection. It achieves a diversity order of two and high transmission rate under realistic network conditions with single-symbol maximum likelihood (ML) detection. A general N-relay signaling protocol is presented, and specific design examples are given for N=2, 3 and 4-relay cooperative networks. The average symbol error probability (ASEP) is analytically derived and shown to match with computer simulation results. It is also shown via computer simulations that the proposed scheme achieves significantly better error performance and is more robust to channel estimation errors than its counterparts given in the literature under realistic network conditions.

Emerging two-path successive relaying protocols generally rely on error-free source-relay channels and/or interference-free inter-relay channels to achieve high-rate and full-diversity. In this paper, by removing these optimistic assumptions, a novel two-path successive relaying scheme that benefits from relay selection and distributed space-time block coding (STBC), and transfers the data from the source to the destination via relays in a reliable fashion is proposed. The proposed scheme can achieve full diversity without the requirement of perfect decoding at relays since not only the destination but also the relays benefit from distributed STBC and relay selection. As the target STBC, coordinate interleaved orthogonal design (CIOD) for two transmit antennas is considered. The average symbol error probability of the proposed scheme is derived and its error performance is compared with reference systems.

In the above named paper [ibid., vol. 61, no. 5, pp. 1776–1785, May 2013], the authors correct an error from equation (12) and (13), on the variance of z given g and s that is expressed correctly in (14). This correction affects (15), (16), (17) and finally the optimal combining coefficient αMDC,igiven in (18) which should be modified as noted by the authors. Note that, the approximated maximum deflection coefficient given in Lemma 2 should be similarly corrected and (32) should be modified as further noted.

In this paper, a novel orthogonal frequency division multiplexing (OFDM) scheme, which is called OFDM with index modulation (OFDM-IM), is proposed for frequency-selective fading channels. In this scheme, inspiring from the recently introduced spatial modulation concept for multiple-input multiple-output (MIMO) channels, the information is conveyed not only by M-ary signal constellations as in classical OFDM, but also by the indices of the subcarriers, which are activated according to the incoming bit stream. Different transceiver structures are proposed and a theoretical error performance analysis is provided for the new scheme. It is shown via computer simulations that the proposed scheme achieves significantly better error performance than classical OFDM due to the information bits carried in the spatial domain by the indices of OFDM subcarriers.

Spatial modulation (SM), which employs the indices of multiple transmit antennas to transmit information in addition to the conventional M-ary signal constellations, is a novel transmission technique that has been proposed for multiple-input multiple-output systems. In this study, a new class of space-time trellis codes, called ‘super-orthogonal trellis-coded SM’ (SOTC-SM), is proposed. These codes combine set partitioning and a super set of space-time block coded SM (STBC-SM) codewords to achieve maximal diversity and coding gains by exploiting both SM and space-time block codes. Unlike super-orthogonal space-time trellis codes (SOSTTCs), which parametrise the orthogonal STBCs, these new codes expand the antenna constellation using the principle of SM. Systematic construction methods are presented for the SOTC-SM scheme and design examples are given for 2, 4 and 8 trellis states, at 2, 3 and 4 bits/s/Hz spectral efficiencies. The approximate bit-error probability performance of SOTC-SM is derived and shown to match computer simulation results. A simplified maximum likelihood detection method for the proposed scheme is given. It is shown through computer simulations that the proposed SOTC-SM schemes achieve significantly better error performance than SOSTTCs with comparable complexity.

In this paper, we propose a novel MIMO transmission scheme, called trellis coded spatial modulation (TC-SM) in which the trellis (convolutional) encoder and the spatial modulation (SM) mapper are jointly designed similar to the conventional trellis coded modulation (TCM). A soft decision Viterbi decoder, which is fed with the soft information supplied by the optimal SM decoder, is used at the receiver. We derive the pairwise error probability (PEP) upper bound for the TC-SM scheme in uncorrelated quasi-static Rayleigh fading channels. From the PEP upper bound, code design criteria are given and then used to obtain new 4-, 8- and 16-state TC-SM schemes using QPSK (quadrature phase-shift keying) and 8-PSK modulations for 2 bits/s/Hz and 3 bits/s/Hz spectral efficiencies, respectively. We show by computer simulations that the proposed TC-SM schemes achieve significantly better error performances than their counterparts at the same spectral efficiency with reduced decoding complexity.

A novel multiple-input multiple-output (MIMO) transmission scheme, called space-time block coded spatial modulation (STBC-SM), is proposed. It combines spatial modulation (SM) and space-time block coding (STBC) to take advantage of the benefits of both while avoiding their drawbacks. In the STBC-SM scheme, the transmitted information symbols are expanded not only to the space and time domains but also to the spatial (antenna) domain which corresponds to the on/off status of the transmit antennas available at the space domain, and therefore both core STBC and antenna indices carry information. A general technique is presented for the design of the STBC-SM scheme for any number of transmit antennas. Besides the high spectral efficiency advantage provided by the antenna domain, the proposed scheme is also optimized by deriving its diversity and coding gains to exploit the diversity advantage of STBC. A low-complexity maximum likelihood (ML) decoder is given for the new scheme which profits from the orthogonality of the core STBC. The performance advantages of the STBC-SM over simple SM and over V-BLAST are shown by simulation results for various spectral efficiencies and are supported by the derivation of a closed form expression for the union bound on the bit error probability.

Spatial modulation (SM), in which multiple antennas are used to convey information besides the conventional M-ary signal constellations, is a new multiple-input multiple-output (MIMO) transmission technique, which has recently been proposed as an alternative to V-BLAST (vertical Bell Labs layered space-time). In this paper, a novel MIMO transmission scheme, called spatial modulation with trellis coding (SM-TC), is proposed. Similar to the conventional trellis coded modulation (TCM), in this scheme, a trellis encoder and an SM mapper are jointly designed to take advantage of the benefits of both. A soft decision Viterbi decoder, which is fed with the soft information supplied by the optimal SM decoder, is used at the receiver. A pairwise error probability (PEP) upper bound is derived for the SM-TC scheme in uncorrelated quasi-static Rayleigh fading channels. From the PEP upper bound, code design criteria are given and then used to obtain new 4-, 8- and 16-state SM-TC schemes using quadrature phase-shift keying (QPSK) and 8-ary phase-shift keying (8-PSK) modulations for 2,3 and 4 bits/s/Hz spectral efficiencies. It is shown via computer simulations and also supported by a theoretical error performance analysis that the proposed SM-TC schemes achieve significantly better error performance than the classical space-time trellis codes and coded V-BLAST systems at the same spectral efficiency, yet with reduced decoding complexity.

Space-time block coded spatial modulation (STBC-SM), which employs space-time block coding (STBC) for spatial modulation (SM), is proposed as a new multiple-input multiple-output (MIMO) transmission scheme. In the STBC-SM scheme, the transmitted information symbols are expanded not only to the space and time domains but also to the spatial (antenna) domain, therefore both core STBC and antenna indices carry information. A general framework is presented for the design of the STBC-SM scheme for any number of transmit antennas. The proposed scheme is optimized by deriving its diversity and coding gains to exploit the diversity advantage of STBC. A low-complexity maximum likelihood (ML) decoder is given for the new scheme. It is shown by computer simulations that STBC-SM provides approximately 3–5 dB (depending on the spectral efficiency) better error performance than SM and V-BLAST systems.

The authors deal with the design of high-rate, full-diversity, low-maximum likelihood (ML) decoding complexity space-time block codes (STBCs) with code rates of 2 and 1.5 complex symbols per channel use for multiple-input multiple output (MIMO) systems employing three and four transmit antennas. The authors fill the empty slots of the existing STBCs from coordinate interleaved orthogonal designs (CIODs) in their transmission matrices by additional symbols and use the conditional ML decoding technique, which significantly reduces the ML decoding complexity of non-orthogonal STBCs while ensuring full-diversity and high coding gain. First, two new schemes with code rates of 2 and 1.5 are proposed for MIMO systems with four transmit antennas. The authors show that our low-complexity rate-2 STBC outperforms the corresponding best STBC recently proposed by Biglieri et al. (2008) for quadrature phase shift keying (QPSK), due to its superior coding gain while our rate-1.5 STBC outperforms the full-diversity quasi-orthogonal STBC (QOSTBC). Then, two STBCs with code rates of 2 and 1.5 are proposed for three transmit antennas, which are shown to outperform the corresponding full-diversity QOSTBC. The authors prove by an information-theoretic analysis that the capacities of new rate-2 STBCs for three and four transmit antennas are much closer to the actual MIMO channel capacity than the capacities of classical OSTBCs and CIODs.

In this paper, we present a new full-rate, full-diversity coordinate interleaved design (CID) for multiple-input multiple-output (MIMO) systems with two transmit antennas. When compared with the conventional space-time block codes (STBCs) and coordinate interleaved orthogonal designs (CIODs), the proposed scheme is above the predefined full-rate limits since we ignore the pure orthogonality principle while transmitting extra symbols from available slots. The higher complexity of this structure due to non-orthogonality is reduced by using a simple conditional maximum-likelihood (ML) decoder. The new CID achieves full-diversity with a high minimum determinant, and outperforms its counterpart given by Sezginer and Sari with certain constellations, which make it to be considered for the future high data rate wireless applications.