We propose a two-way generalized code index modulation scheme with hexagonal QAM (TW-GCIM-HQAM) over Nakagami-m fading. The design combines GCIM and physical layer network coding with energy-efficient HQAM constellations. Using orthogonal Walsh-Hadamard spreading codes, two users exchange data via a half-duplex relay in two time slots (instead of three in conventional two-way DF). Information is carried by both the HQAM symbols and the indices of the spreading codes on the I/Q branches. We compare the scheme with two-way decode and forward at matched rates and evaluate the average bit error rate via Monte Carlo simulations on Nakagami-m channels for several HQAM constellations. Results show competitive or improved error performance, together with better energy use and time slot efficiency.

The growing demands in wireless communication technologies necessitate the development of more advanced and efficient systems. Therefore, this paper introduces a high-performance and data rate index modulation technique called the double media-based modulation (DMBM) system. The proposed system enhances the conventional media-based modulation (MBM) system by doubling the mirror activation patterns (MAPs) and the transmitted symbols within the same transmission duration. Consequently, the DMBM system achieves double the spectral efficiency of MBM while improving error performance through an increased number of bits encoded in the indices. The proposed DMBM scheme undergoes performance evaluation using$M$-ary quadrature amplitude modulation ($M$-QAM) over Rayleigh, Rician, and Nakagami-$m$fading channels. Its error performance is compared with alternative techniques such as spatial modulation (SM), quadrature SM (QSM), MBM, and double SM (DSM) over the Rayleigh channel. Also, to further improve reliability, especially in high-mobility scenarios envisioned for sixth-generation (6G) networks, orthogonal time frequency space (OTFS) modulation is integrated into the proposed DMBM system. The proposed OTFS-based DMBM (OTFS-DMBM) system is compared with the conventional OTFS system and offers better error performance at the same spectral efficiency. Furthermore, comprehensive analyses of throughput, complexity, energy efficiency, spectral efficiency, and capacity are conducted for the DMBM system alongside the benchmark system. The impact of imperfect channel state information (CSI) for the proposed DMBM system is also analyzed, and performance comparisons are presented for both perfect and imperfect CSI conditions. The findings demonstrate that the DMBM system outperforms its counterparts, highlighting its potential as a superior solution for modern wireless communication networks' demands.

In this paper, we present a novel communication system model that integrates reconfigurable intelligent surfaces (RIS), spatial shift keying (SSK), and code index modulation (CIM) based on Hadamard coding called RIS based transmit SSK-CIM (RIS-CIM-TSSK). By leveraging RIS, the system adapts rapidly to dynamic environments, enhancing error rates and overall reliability. SSK facilitates the transmission of additional passive information while eliminating the need for multiple radio frequency (RF) chains, thereby reducing complexity. CIM enhances passive information transmission through frequency domain spreading, which may increase signal obfuscation. This proposed scheme not only improves energy efficiency but also offers a robust solution for reliable communication in modern wireless networks, paving the way for smarter and more adaptable implementations. We consider a suboptimal, low-complexity detector for the proposed scheme and also address the blind case for phase adjustment of the RIS. Finally, we present the simulation results for the proposed system model across various configurations, including different numbers of receive and transmit antennas, varying reflecting elements of the RIS, and different code lengths.

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.

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.

Reconfigurable intelligent surface (RIS) structures reflect incident signals by adjusting phase adaptively according to the channel condition during transmission to increase signal quality at the receiver. Spatial modulation (SM) technique is a potential candidate for future energy-efficient wireless communications due to its ability to provide better throughput, low-cost implementation, and good error performance. Also, Alamouti's space-time block coding (ASBC) is an important space and time coding technique for the diversity gain and simplified maximum likelihood (ML) detection. In this paper, we propose the RIS-assisted received spatial modulation (RSM) scheme with ASBC, namely RIS-RSM-ASBC, for next-generation wireless networks. The RIS in the proposed system model is separated into two parts, each used as an access point (AP) to transmit its Alamouti coded information while reflecting passive signals to the selected receive antenna. We design the optimal ML detector for the proposed RIS-RSM-ASBC scheme. Furthermore, we present the greedy detector (GD) to reduce the complexity of the ML detector. Extensive computer simulations are performed to corroborate theoretical derivations. The results show that the RIS-RSM-ASBC system is highly reliable and provides data rate enhancement compared to traditional RIS-assisted transmit SM (RIS-TSM), RIS-assisted transmit quadrature SM (RIS-TQSM), RIS-assisted received SM (RIS-RSM), RIS-assisted transmit space shift keying with ASBC (RIS-TSSK-ASBC), and RIS-assisted TSSK with Vertical-Bell Laboratories Layered Space-Time (RIS-TSSK-VBLAST) schemes.

Reconfigurable intelligent surfaces (RISs) are software-controlled passive devices that can be used as relay$(R)$systems to reflect incoming signals from a source$(S)$to a destination$(D)$in a cooperative manner with optimum signal strength to improve the performance of wireless communication networks. The configurability and flexibility of an RIS deployed in an Internet of Things (IoT)-based network can enable network designers to devise stand-alone or cooperative configurations that have considerable advantages over conventional networks. In this article, two new deep neural network (DNN)-assisted cooperative RIS (CRIS) models, namely, DNN$_{R} -$CRIS and DNN$_{R, D} -$CRIS, are proposed for cooperative communications. In DNN$_{R} -$CRIS model, the potential of RIS deployment as an IoT-based relay element in a next-generation cooperative network is investigated using deep-learning (DL) techniques for RIS phase optimization. In addition, to reduce the maximum-likelihood (ML) complexity at$D$, a new DNN-based symbol detection method is presented with the DNN$_{R, D} -$CRIS model combined with DNN-assisted phase optimization. For a different number of relays and receiver configurations, the bit error rate (BER) performance results of the proposed DNN$_{R} -$CRIS and DNN$_{R, D} -$CRIS models and traditional CRIS scheme (without a DNN) are presented for a multirelay cooperative communication scenario with path loss effects. It is revealed that the proposed DNN-based models show promising results in terms of BER, even in high-noise environments with low system complexity.

This study proposes an iterative, joint channel estimation, equalization, and data detection method in the presence of high mobility for a multicarrier downlink system that communicates over rapidly time‐varying channels. The proposed method uses a basis expansion method (BEM) which has low computational complexity and helps to reduce the number of coefficients needed to represent a time‐varying channel and therefore is extremely easy to implement practically. Unlike the current literature, which is almost entirely focused on the uplink communication systems due to their computational costs, this method prioritizes the goal of being feasible in a downlink system with a reasonable performance. The proposed suboptimal algorithm is based on the space‐alternating generalized expectation‐maximization (SAGE) algorithm and the time‐varying channel is represented by orthogonal basis functions obtained by means of discrete Walsh‐Hadamard transform (DWHT). The resulting receiver iterates between maximum a posteriori (MAP) based channel estimation in the subspace spanned by the orthogonal basis functions and successive interference cancellation. Numerical examples show that the proposed algorithm has a satisfactory symbol error rate with low computational complexity and also has a reasonable peak‐to‐average power ratio (PAPR) reduction effect.

In this paper, a cooperative relaying scheme where optimum threshold value based relay selection algorithm is considered for decode and forward (DF) based cooperative relaying networks is proposed over the Nakagami-m fading channels. Relay selection scheme with optimum threshold value is an effective method to decrease the error propagations that occur at the relays due to detection errors. In order to decrease performance degradation due to error propagations in DF based cooperative relaying networks and increase the error performance of the system, virtual noise (VN), MRC and cooperative-MRC (C-MRC) based detection techniques with the optimal threshold value based relay selection are considered. Functions of optimum threshold value for the considered techniques are derived theoretically.

In this paper, two different antenna selection models based on signal-to-noise ratio (SNR) have been proposed in order to reduce the end-to-end bit error rate (BER) of decode and forward (DF) relaying cooperative system where source and destination terminals contain single antenna while relay terminal has multiple antennas. Specifically, by using instantaneous and average SNR of source-to-destination, source-to-relay and relay-to-destination links, optimal decision rules are derived theoretically for antenna selection at the relay terminal. Later, BER expressions of considered system have been derived for two antenna selection models over Rayleigh fading channels. Theoretical and simulation results show the validity of proposed mathematical analysis and point out the confirmation of theoretical results.

This work proposes a novel iterative blind receiver to jointly estimate channel parameters and transmitted data sequence on the multipath fading channel in Direct Sequence Code Division Multiple Access (DS CDMA) systems. Since time delay of the transmission channel represents nonlinear characteristic, joint estimation of these parameters and data sequence is difficult for multipath fading channels. To derive of the proposed iterative method we used completely Bayesian approach and then, Sequential Monte Carlo (SMC) technique is adapted to proposed method.