Index Modulation Concept Research Articles

Enhanced connectivity through double RIS uplink MIMO and advanced modulation

Reconfigurable Intelligent Surfaces (RISs) have garnered significant attention in the development of Internet of Things (IoT) networks and vehicular communication systems, offering improved spectral efficiency with reduced power consumption and complexity overhead. However, employing a single RIS (S-RIS) may limit certain application scenarios and prove insufficient for ensuring reliable communication between transmitters and receivers. Consequently, the use of multiple RISs is recommended to further enhance coverage, capacity, and spectral efficiency. In this paper, we introduce the concept of Index Modulation (IM) into the double RIS (D-RIS) uplink Multiple-Input Multiple-Output (MIMO) communications, utilizing the antenna indices at the base station (BS). IM is realized through a combination of Space-Shift Keying (SSK) and Spatial Modulation (SM) techniques. We present an analytical model for the Double RIS based on SSK/SM methods and obtain a closed-form expression for the theoretical bit error probability (BEP), employing a greedy detector. To validate our theoretical BEP derivations for d-RIS-SSK/SM, we compare them with simulated BEP results. Our simulation data reveals an excellent match between the derived BEP and the simulated BEP, especially at lower Signal-to-Noise Ratios (SNRs), specifically -34 dB for SSK and -24 dB for SM. Furthermore, we evaluate the performance of the proposed d-RIS-aided IM methods by contrasting them with conventional d-RIS and single RIS-aided IM systems. Our simulation results demonstrate that the d-RIS-assisted IM outperforms the conventional d-RIS and single RIS-aided IM, achieving lower BEP at lower SNRs. d-RIS aided IM saves 15 dB in SNR compared to d-RIS without IM. Moreover, it saves 9 dB and 7 dB in SNR compared to single RIS-SSK and single RIS-SM, respectively when N = 64 and saves 3.5 dB and 1 dB in SNR with N = 128.

Deployment of Energy-Efficient Aerial Communication Platforms With Low-Complexity Detection

Due to their flexibility, mobility and autonomy, unmanned aerial vehicles (UAVs) are considered as a potential candidate to operate as flying base stations to provide a specific geographical area with air-to-ground wireless communications services. Besides, the limitation of the on-board energy motivates the design of a more energy-efficient way to transmit information. Moreover, the deployment of multiple UAVs can affect the quality of experience (QoE) of users. In this paper, we propose a method that combines the concept of index modulation (IM) with the UAV communications systems, which we refer to as IM-UAV, to attain an improved energy-efficiency (EE). Furthermore, based on the proposed IM-UAV communication system, a gradient descent based UAV deployment scheme is designed to maximize the downlink average rate of the ground users (GUs) in the target area. On the other hand, the maximum likelihood (ML) detection for the IM-UAV requires a high computational complexity for detection at the GUs, while providing the best possible performance. Hence, we propose a low-complexity detection scheme that can separately detect the index symbols and data symbols to reduce the computational complexity at the receiver side. The simulation results demonstrate that the proposed deployment method is capable of attaining the appropriate positions to deploy the UAVs, while the EE is also improved by combining IM with UAV communication system. In addition, the proposed low-complexity detection scheme reduces the computation complexity by slightly sacrificing the bit error rate (BER) performance.

Channel Estimation and Multipath Diversity Reception for RIS-Empowered Broadband Wireless Systems Based on Cyclic-Prefixed Single-Carrier Transmission

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- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> 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.

Deep Learning-Based Soft Iterative-Detection of Channel-Coded Compressed Sensing-Aided Multi-Dimensional Index Modulation

The concept of Index Modulation (IM) has been actively researched as a benefit of its flexible trade-off between performance, achievable rate, energy efficiency, hardware cost and complexity. In order to fully exploit its the degrees of freedom, the concept of Multi-dimensional IM (MIM) has been developed in literature, where Compressed Sensing (CS) is often utilized to exploit the sparsity of the multi-dimensional transmitted signals. However, this flexibility and performance gains are attained at the cost of a substantially increased detection complexity. In this paper, we propose Deep Learning (DL) based detection for CS-aided MIM (CS-MIM), where both Hard-Decision (HD) as well as Soft-Decision (SD) detection combined with iterative decoding are conceived. More explicitly, firstly, we propose learning aided hard and soft detection for CS-MIM. Secondly, two novel neural network aided methods are proposed for Iterative Soft Detection (ISD), where iterations are carried out between the CS-MIM detector and a channel decoder. In contrast to the conventional detection of CS-MIM system, which critically relies on the knowledge of Channel State Information (CSI) at the receiver, the proposed learning-aided methods are capable of eliminating the overhead and complexity of Channel Estimation (CE), which results in an improved transmission rate. Explicitly, we develop an advanced DL architecture for blind-detection-aided MIM for the first time in the open literature, where the HD and SD CS algorithms are implemented by learning without the need for CE. Our simulation results demonstrate that the proposed learning techniques conceived for SD CS-MIM combined with iterative detection are capable of achieving near-capacity performance at a reduced complexity compared to the conventional model-based SD relying on CSI acquisition.

Index Modulation for STAR-RIS Assisted NOMA System

In this letter, we first incorporate the concept of index modulation (IM) into simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) aided non-orthogonal multiple access (NOMA) system to improve the spectral efficiency. Specifically, the proposed IM aided STAR-RIS-NOMA system enables extra information bits to be transmitted by allocating subsurfaces to different users in a pre-defined subsurface allocation pattern. Furthermore, an approximate closed form expression on bit error rate (BER) is derived. Simulation results demonstrate that the proposed IM aided STAR-RIS-NOMA system is able to acquire transmission rate improvement compared to the conventional STAR-RIS NOMA.

Index modulation aided generalized space-time block coding: A unified MIMO framework

In this paper, Index Modulation (IM) aided Generalized Space-Time Block Coding (GSTBC) is proposed, which intrinsically exploits the benefits of IM concept, diversity gain and spatial multiplexing gain. Specifically, the information bits are partitioned into U groups, with each being modulated by IM symbols (i.e. Spatial Modulation (SM), Quadrature SM (QSM),etc). Next, the structure of GSTBC is invoked for each K IM symbol, and a total of μ = U/K GSTBC codes are transmitted via T time slots. A Block Expectation Propagation (B-EP) detector is designed for the proposed IM-GSTBC structure. Moreover, the theoretical Average Bit Error Probability (ABEP) is derived for our IM-GSTBC system, which is confirmed by the simulation results and helpful for performance evaluation. Simulation results show that our proposed IM-GSTBC system is capable of striking an efficient trade-off between spatial multiplexing gain, spatial diversity gain as well as implementation cost imposed for both small-scale and large-scale MIMO antenna configurations.

Reconfigurable Intelligent Surface Assisted Space-Time Line Code for SIMO Transmission

In this contribution, we investigate reconfigurable intelligent surface assisted space-time line code (RIS-STLC) transmission for enhancing the signal quality when the direct link between the transmitter and receiver is blocked. In the context of intelligent transmission by RIS, the concept of index modulation (IM) is integrated into the pursuit of higher spectral efficiency, while a two-step detection is developed to improve the performance. Correspondingly, for blind transmission where the channel information at RIS is not available, the phase at RIS not associated with the channel is considered. Subsequently, the bit-error-rate (BER) performances of the proposed RIS-STLC system conditioned on both the intelligent and blind transmission are quantified. Theoretical analysis and simulation results demonstrate the effectiveness of the development of RIS-STLC.

Cyclic Delay Diversity With Index Modulation for Green Internet of Things

Cyclic delay diversity (CDD) is a simple and yet effective transmit diversity technique. Thanks to its strong power saving capability, CDD is very promising for green Internet of Things (IoT) networks. However, the reduced degrees of freedom in data transmission and the overhead of cyclic prefix lead to unsatisfactory spectral efficiency (SE), hindering the direct application of CDD to IoT networks where short-packet communication prevails. To tackle this problem, in this article, we propose to convey additional information via the varying cyclic delays for improving the SE of the CDD system based on the concept of index modulation (IM), which applies to both OFDM and cyclic-prefixed single-carrier (CPSC) signals. The methods to generate all possible cyclic delays and the optimal receivers are designed for both proposed systems. To aim at low computational complexity, we further propose a suboptimal receiver for the CDD-CPSC-IM system. Moreover, a closed-form upper bound on the bit error rate (BER) of the CDD-OFDM-IM system is derived, from which the coding gain is characterized. Simulation results in terms of BER corroborate the analysis and the superiority of the proposed systems over the pure CDD-OFDM and CDD-CPSC systems, uncovering the potential of CDD in the application to green IoT networks.

A Novel Downlink IM-NOMA Scheme

In the next generations of communication systems, resources' scarcity devolves to a more critical point as a consequence of the expected massive number of users to be served. Furthermore, the users' contradicting requirements on quality of service forces the network to behave in a dynamic way in terms of the multiple access orchestration and resource assignment between users. Hence, the linear relation between the number of served users and the required (orthogonal or semi-orthogonal) resources in the orthogonal multiple access (OMA) schemes is no longer sufficient. As promising candidates, Non-orthogonal multiple access (NOMA) schemes and index modulation (IM) techniques are emerging to satisfy the ever-increasing connectivity demands and spectral efficiency. In this article, a novel IM-based NOMA downlink scheme, termed as IM-NOMA, is proposed, where the base station (BS) selects a channel or more to serve each user based on the IM concept and the corresponding power level is allocated based on the NOMA concept. At the users' ends, maximum likelihood and successive interference cancellation (SIC) detection methods are considered and their error rate, outage probability and computational complexity are studied. Simulation results confirm the advantage of the proposed IM-NOMA scheme and that it can outperform the conventional NOMA system.

Multidimensional Index Modulation for 5G and Beyond Wireless Networks

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.

Secure Index-Modulation Based Hybrid Free Space Optical and Millimeter Wave Links

An enhanced security algorithm for wireless communication system deploying hybrid free space optical (FSO)–millimeter wave (mmWave) links is proposed in this clause. The activation of either links, FSO or mmWave, is determined through index modulation (IM) technique. In particular, the first bit to be transmitted determines the active link; while remaining bits modulate a conventional symbol to be transmitted over the active link. Yet, before transmitting the symbol, it is precoded by another signal that is extracted from the channel coefficient of the active link. The channel coefficients for both links are assumed to be available at the transmitter and the receiver through typical channel estimation algorithms. It is revealed that the security of the proposed system is significantly enhanced, where IM concept degrades the performance of an eavesdropper who is attacking any of the communication links. Also, the use of precoding worsens the functioning of an eavesdropper who is attacking both links. The impact of channel estimation errors along with the spectral efficiency, turbulence conditions, channel fading, and eavesdropper capability on the overall performance is investigated and thoroughly discussed.

NOMA-based downlink relaying with media-based modulation

In recent years, non-orthogonal multiple access (NOMA) has attracted considerable attention as a promising multi-access technique for next generation wireless communication systems. Media-based modulation (MBM), which is an emerging index modulation concept, is another technique that can be considered for next generation wireless communication systems due to its high spectral and energy efficiency. In this paper, we propose a NOMA-based decode-and-forward relaying system, where MBM is applied at the source to improve error performance at the relay. We consider a topology consisting of a single source with a single transmit antenna equipped with multiple radio frequency (RF) mirrors, one relay equipped with a single transmit and multiple receive antennas, and multiple users equipped with multiple receive antennas. Pairwise error probability expression for NOMA with multiple receive antennas is derived and the union bound on bit error rate (BER) for the overall system with imperfect successive interference cancellation (SIC) is obtained in closed-form. Analytical results are in agreement with the Monte Carlo simulation results. Moreover, two power allocation optimization problems are formulated in terms of user fairness and quality of service requirements of the users. BER performance of the proposed system is also investigated using the optimum coefficients according to the formulated problems. It is shown that the proposed system outperforms the conventional cooperative NOMA system in terms of error performance as the data rate increases.

Space-Frequency Index Modulation for Combating ICI in Intelligent Mobility Communications

Advanced wireless communications to support high vehicular speeds in the scenario of intelligent mobility have attracted considerable research attentions in both academia and industry. Multi-dimensional index modulation, which exploits transmission media resources for combining the concept of index modulation to achieve higher throughput with lower energy consumption, is regarded as a potential candidate for addressing the challenges in intelligent mobility communications. However, for the multi-dimensional index modulation system used in the mobile environment, the inter-carrier-interference (ICI) imposes a significant adverse impact on the system, especially in the context of multicarrier transmission. Therefore, by focusing on the two-dimensional index modulation system, the ICI impact on space-frequency index modulation is studied in this paper. Thanks to the grouping strategies and the Gaussian approximation to model the ICI, the approximate theoretical bounds for the error performance of the two dimensional systems are derived. Moreover, a novel robust space-frequency index modulation scheme is also developed. Finally, simulation results are presented to validate the system performances and the theoretical derivations.

Method of space-frequency block coding with dual-mode index modulation aided OFDM

The technology of combining Multiple Input-Multiple Output (MIMO) and Orthogonal Frequency Division Multiplexing (OFDM) for modern Wireless Communication Systems is considered. The areas of priority use of time and frequency variants of Alamouti’s diversity scheme for Wireless Communication Channels with limited frequency and energy resources are determined. The concept of Index Modulation (IM) is analyzed. The method of OFDM subcarriers index modulation is determined as the most promising means of increasing noise immunity and speed of information transmission for real frequency-selective channels of Wireless Communication Systems. The benefits of Dual-Mode OFDM-IM with optimized signal constellations based on BPSK, QPSK and 16-QAM are demonstrated. A novel Signal-Code design combining the Dual-Mode Index Modulation Aided OFDM and the Orthogonal Space-Frequency Block Coding with Alamouti’s core SFBC-DM-OFDM-IM is proposed. Compared to classical SFBC-OFDM the proposed method allows simultaneously to improve spectral and energy efficiency of information transmission systems in frequency and time-varying wireless communication channel. The SFBC-DM-OFDM-IM method is advisable for providing high-quality communications with highly dynamic ground objects, unmanned aerial vehicles and other aircrafts in the conditions of Signal-to-Noise Ratio critical reduction, which takes place under the influence of electronic suppression means. The obtained scientific results can be effectively used for designing new generation Wireless Communication Networks with limited frequency and energy resources.

Index Modulation for Frequency Diverse Array

The index modulation (IM) approach adds an extra signal modulation dimension by utilizing a subcarrier index domain, which is in addition to the traditional IQ domain. A frequency diverse array (FDA) technology is rapidly developing as a new technique for radar systems. In this letter, a novel transmitter architecture for the FDA, combining the IM concept with the FDA, namely IM-FDA, is presented. Compared with conventional frequency diverse arrays (CFDAs), the proposed architecture offers thumbtack-like beampatterns, which can decouple the range and angle dependence, an issue existing in the CFDA. An additional benefit is that, apart from sensing target location, information can be conveyed to the targets through the selected frequency offset indices in the proposed scheme. The transmitter architecture is elaborated along with its design details, and the efficacy of the proposed architecture is validated via numerical results for the transmit beampattern by comparison with the CFDA and random frequency diverse array.

Antenna Element Index Modulation for Frequency Diverse Array

In this paper, we propose a new architecture and provide performance analysis for frequency diverse array (FDA) Radar combined with the index modulation (IM) techniques. Here, the IM concept is applied upon frequency offsets in an FDA. Information is transmitted by a dynamically selected subset of antenna element indices applying predefined carrier frequency offsets, which generate different radiation patterns. It is shown that this enables the capability for simultaneous wireless communication and FDA Radar that has the range-angle coupling issue resolved.

Space-Time Media-Based Modulation

Media-based modulation (MBM), which utilizes radiation patterns of a reconfigurable antenna to convey information, appears as a promising index modulation (IM) scheme for beyond 5G networking. In this paper, we present a general framework for MBM from the perspective of space-time coding, and introduce a novel space-time coded IM concept, which is called space-time media-based modulation (ST-MBM). The proposed scheme is based on one of the prominent IM solutions, space shift keying, along with Hurwitz–Radon family of matrices in order to achieve transmit diversity gain with a single radio frequency (RF) chain by utilizing the unique RF mirror activation principle of MBM. We derive the theoretical pairwise error probability of the ST-MBM scheme for correlated and uncorrelated channel states and obtain the average bit error probability. Additionally, a lower bound is derived for the mutual information of the ST-MBM scheme to gain insights into the information theoretical bounds of the proposed scheme. Furthermore, extensive computer simulations are provided to show the superior error performance of the ST-MBM scheme over the state-of-the-art multiple-input multiple-output-based transmission systems.

Information-Guided Pilot Insertion for OFDM-Based Vehicular Communications Systems

Orthogonal frequency division multiplexing (OFDM) is widely considered as a promising technique for vehicular communications. In current OFDM-based vehicular communications systems, a portion of tones are dedicated for pilot symbols and transmitted along with the data-bearing tones to estimate residual frequency error and/or channel state information. However, these pilot symbols themselves do not carry any information, thus inevitably degrading the system spectral efficiency (SE). Motivated by the concept of index modulation, in this paper, we propose a novel pilot insertion technique to enhance the SE, in which the pilot positions are selected according to extra information bits. Specifically, three different types of pilot position selection schemes, which result in equal-spaced, unequal-spaced, and hybrid pilot placements, are studied, and the corresponding pilot position detection methods are designed, where the pilots can be used for either carrier phase tracking or channel estimation purpose. Computer simulation results corroborate the advantages of the proposed technique in terms of bit error rate.

Spectrally Efficient Frequency Division Multiplexing With Index-Modulated Non-Orthogonal Subcarriers

In this letter, we propose a novel index-modulated non-orthogonal spectrally efficient frequency-division multiplexing (SEFDM) scheme, in order to attain a higher bandwidth efficiency than the conventional orthogonal frequency-division multiplexing (OFDM) and SEFDM counterparts. More specifically, the recent index modulation concept is amalgamated with SEFDM, for the sake of reducing the effects of intercarrier interference while benefiting from SEFDM's increased spectral efficiency. We also formulate a low-complexity log-likelihood ratio-based detection algorithm, which allows the proposed SEFDM to operate in the configuration of an arbitrarily high number of subcarriers. Our simulation results demonstrate that the proposed SEFDM scheme outperforms the conventional SEFDM and OFDM, especially in a low-rate scenario.

Novel Index Modulation Techniques: A Survey

In fifth generation wireless networks, the escalating teletraffic and energy consumption has necessitated the development of green communication techniques in order to further enhance both the system’s spectral efficiency and energy efficiency. In the past few years, the novel index modulation (IM) has emerged as a promising technology that is widely employed in wireless communications. In this paper, we present a survey on IM in order to provide the readers with a better understanding of its principles, advantages, and potential applications. We start with a comprehensive literature review, where the concept of IM is introduced, and various existing IM schemes are classified according to their signal domains, including the frequency domain, spatial domain, time domain and channel domain. Then the principles of different IM-aided systems are detailed, where the transceiver design is illustrated, followed by descriptions of typical systems and corresponding performance evaluation. A range of challenges and open issues on IM are discussed before we conclude this survey.