Constant envelope orthogonal frequency-division multiplexing (CE-OFDM) has attracted increasing attention as a power-efficient modulation scheme for Internet of Things (IoT) networks, particularly in energy-constrained scenarios such as space–air–ground integrated networks (SAGINs). Despite its inherently low peak-to-average power ratio (PAPR), CE-OFDM suffers from significant bit error rate (BER) degradation due to nonlinear phase wrapping caused by phase modulation. To address this limitation, we propose a selective mapping (SLM)-aided CE-OFDM scheme, in which the transmit sequence with the minimum number of phase jumps is selected from a pre-defined set of candidates. This strategy effectively mitigates the impact of phase wrapping and significantly improves BER performance. Furthermore, to eliminate the spectral overhead caused by conventional side information transmission, we propose an embedded side information (ESI) mechanism that seamlessly incorporates index data into the transmitted signal without requiring additional bandwidth. Simulation results verify that the proposed SLM-aided CE-OFDM scheme, equipped with ESI, achieves substantial BER improvements over conventional CE-OFDM systems, while maintaining the low-PAPR property essential for energy-constrained IoT devices.

To support passive Internet-of-Things (IoT) for future 6G networks, backscatter communication (BC) has emerged as a promising solution due to its ultra-low-power consumption nature. In this paper, we propose a novel multistatic passive IoT architecture which allows the reader to recover the information from multiple backscatter devices (BDs), excited by multiple remote continuous-waveform (CW) transmitters. At each BD, multiple backscatter antennas, in conjunction with cyclic delay transmission (CDT) and interleaved frequency division multiple access (IFDMA) framework, are deployed to achieve antenna gain, diversity advantage, as well as transmission orthogonality among the BDs. Furthermore, transmit beamforming at the CW transmitters and cyclic delays across the BDs are optimized to enhance the performance and ensure fairness among the BDs. Extensive simulation results have validated the effectiveness of the proposed framework under various scenarios, demonstrating substantial improvements in both reliability and fairness compared with conventional schemes.

The space–air–ground integrated network (SAGIN) has emerged as a promising architecture for future wireless communication systems. However, its open and heterogeneous nature introduces significant physical-layer security risks. To overcome the security concerns in SAGIN, we propose a constant-envelope orthogonal frequency division multiplexing with index modulation (CE-OFDM-IM) based transmission framework, where the constant-envelope property ensures compatibility with hardware-constrained SAGIN environments, while the index modulation mechanism inherently supports covert signaling via implicit subcarrier activation patterns. To enable reliable detection at legitimate receivers, we design both optimal and low-complexity receiver architectures, and further introduce a clipping-based technique to suppress phase wrapping and enhance demodulation robustness. Additionally, the average bit error probability (ABEP) is analytically characterized through performance analysis. Finally, simulation results demonstrate that the proposed CE-OFDM-IM system achieves robust transmission with zero peak-to-average power ratio (PAPR), offering a practical and energy-efficient solution for secure communications in SAGIN environments.

Recently, the pinching-antenna system (PASS) has emerged as a promising architecture owing to its ability to reconfigure large-scale path loss and signal phase by activating radiation points along a dielectric waveguide. However, existing studies mainly focus on line-shaped PASS architectures, whose limited spatial flexibility constrains their applicability in multiuser and indoor scenarios. In this paper, we propose a novel two-dimensional (2D) pinching-antenna system (2D-PASS) that extends the conventional line-shaped structure into a continuous dielectric waveguide plane, thereby forming a reconfigurable radiating plane capable of dynamic beam adaptation across a 2D spatial domain. An optimization framework is developed to maximize the minimum received signal-to-noise ratio (SNR) among user equipments (UEs) by adaptively adjusting the spatial configuration of pinching antennas (PAs), serving as an analog beamforming mechanism for dynamic spatial control. For the continuous-position scenario, a particle swarm optimization (PSO)-based algorithm is proposed to efficiently explore the nonconvex search space, while a discrete variant is introduced to accommodate practical hardware constraints with limited PA placement resolution. Simulation results demonstrate that the proposed 2D-PASS substantially improves the minimum SNR compared with conventional line-shaped PASS and fixed-position antenna (FPA) benchmarks, while maintaining robustness under varying user distributions and distances.

Backscatter communication has emerged as a promising technology for future Internet of Things (IoT) due to its ultra-low power consumption. However, its performance is limited by the double fading effect in the backscatter channel. In this paper, we incorporate multiple-input multiple-output (MIMO) technique into backscatter communications, where the sinusoidal radio frequency (RF) source, backscatter device (BD), and receiver are all equipped with multiple antennas. Furthermore, cyclic delay diversity (CDD) is employed across antennas at the BD to achieve array gain as well as transmit diversity gain. To cope with the frequency-selectivity introduced by CDD, two frequency-domain signal detection schemes are utilized based on the maximum ratio combining (MRC) and minimum mean square error (MMSE) criteria, and the closed-form expressions for the bit error rates (BERs) of backscatter communications are derived. The transmit beamformer at the RF source is optimized to minimize the BER of the backscatter communication. With properly designed beamforming, the overall backscatter link can be effectively strengthened, thereby enabling more reliable backscatter communications. Simulation results show that deploying multiple antennas at both the RF source and the receiver significantly improves the BER performance, while the CDD-based transmission scheme can help to achieve additional array gain as well as transmit diversity gain for backscatter communications.

Symbiotic radio (SR) has emerged as a promising technology for enabling efficient spectrum and power sharing between active and backscattering transmissions. In this paper, we investigate the reconfigurable intelligent surface (RIS)-assisted SR system, where the primary transmission uses orthogonal frequency division multiplexing (OFDM) and the RIS transmits the secondary signal by backscattering the primary signal. The primary OFDM block and the secondary symbol have identical symbol periods but may not be perfectly synchronized, which can introduce inter-carrier interference (ICI) in the received OFDM blocks, thereby hindering joint signal detection. To address this issue, we propose a novel pilot structure and receiver design for SR. Specifically, the RIS sent a training sequence at the beginning of the secondary transmission, enabling the receiver to detect the presence of ICI and estimate essential parameters. If ICI is detected, two effective methods for synchronization offset estimation are proposed. Then, joint signal detection is improved by properly decoupling primary and secondary signals, mitigating the impact of synchronization offsets. On the other hand, if ICI is absent, the secondary signal arrival is identified using the training sequence, and joint signal detection is directly performed without suffering ICI. Simulation results validate the accuracy of the proposed estimation methods and show that the proposed detection methods ensure the reliable detection of both primary and secondary signals, even in the presence of ICI.

In the field of constant envelope orthogonal frequency division multiplexing (CE-OFDM), the use of phase modulation heralds a paradigm with an extremely reduced peak-to-average power ratio (PAPR) of zero. However, the non-linear characteristics inherent to phase modulation pose a formidable challenge to the integration of modern channel coding strategies aimed at improved bit error rate (BER) results. To address this issue, we present an efficient low-complexity log-likelihood ratio (LLR) computation strategy based on the traditional phase receiver, anchored in an effective approximation to the phase noise variance. Furthermore, this foundation paves the way for an innovative receiver architecture, namely the near-maximum likelihood (NML) receiver, designed to improve decoding performance without incurring significant computational overhead. Comparative analyses of encoded CE-OFDM across different receiver models not only confirm the accuracy and superiority of the proposed LLR estimation, but also underscore the dual advantages of our proposed NML receiver in optimising BER and detection complexity, positioning CE-OFDM as an optimal candidate for scenarios requiring high-efficiency power amplifiers (PAs).

Backscatter communication is a promising technology to support massive connectivity for passive Internet-of-Things (IoT) in a cost-effective and power-efficient manner. However, the double fading effect and the passive backscatter operation in backscatter devices (BDs) pose significant challenges to reliable transmissions and efficient multiple access. To overcome these challenges, in this paper, we propose a multiple access scheme for the multi-BD bistatic backscatter communication (BBC) system using multiple antennas. Specifically, the single-carrier frequency domain equalization (SC-FDE) transmission scheme with cyclic delay diversity (CDD) is adopted by each BD to combat the double fading. Furthermore, interleaved frequency division multiple access (IFDMA) is used to facilitate multiple access of BDs. By equipping BDs with multiple backscatter antennas and leveraging the CDD technique, our proposed scheme can not only increase the received signal-to-noise (SNR) performance but also introduce spatial diversity received by the reader. Frequency domain orthogonality among BDs is achieved by assigning unique frequency sets to each, which enables more efficient energy use and reduces synchronization requirements, thereby offering substantial improvements over traditional time division multiple access (TDMA). Besides, the proposed scheme thoughtfully considers BD hardware constraints, by optimizing the trade-off between the cost and connectivity. Simulation results have verified the effectiveness and robustness of the proposed multi-BD BBC system with various configurations.

To support the massive Internet-of- Things (IoT) network, symbiotic radio (SR) has emerged as a promising solution that enables passive IoT connections by exploiting active primary transmissions. Realizing the enhanced spectrum- and energy-efficiency promised by SR requires symbol synchronization between the primary and IoT signals, which, however, remains challenging for cost-limited IoT devices. In this paper, we investigate reconfigurable intelligent surface (RIS)-assisted SR (RSR) with imperfect symbol synchronization. Specifically, the primary transmission employs orthogonal frequency division multiplexing (OFDM), while the RIS enhances the primary transmission and concurrently transmits its secondary signal by passively backscattering the incident primary signal. Due to the unknown synchronization offset (SO) between primary and secondary signals, the reflected channel via the RIS exhibits variations within each OFDM block, consequently leading to inter-carrier interference (ICI) in the received signal. To mitigate this unfavorable effect, we propose a novel receiver design by utilizing virtual subcarriers within each OFDM block. By employing energy detection at the virtual subcarriers, the receiver can detect the arrival of the secondary signal based on the ICI. Furthermore, by compensating the loss of orthogonality in the received OFDM block, the receiver can blindly estimate the SO, thereby facilitating joint detection of primary and secondary signals. Simulation results validate that our proposed receiver significantly improves the bit error rate (BER) performance for RSR with imperfect symbol synchronization.

In previous studies on reconfigurable intelligent surface (RIS)-aided spectrum sharing cognitive radio (CR), the potential of RIS in supporting secondary transmission may not be fully unleashed, due to the insufficient attention to its capacity to mitigate interference from the primary transmitter at the secondary receiver. To bridge this gap, this paper investigates a general active RIS (RIS)-aided CR system, in which the secondary user (SU) aims to minimize the transmit power while satisfying its own SINR constraint and the interference temperature constraint at the primary receivers. The SU needs to jointly optimize the transmit beamforming at the SU transmitter and the reflection coefficients at the active RIS. An improved alternating optimization (AO) algorithm is first proposed, which exploits the active RIS not only to enhance the transmission channel for the secondary transmission, as commonly addressed in most CR literatures, but also to mitigate interference from the primary transmitter. Additionally, a novel low-complexity channel customization (CC) algorithm is then proposed, which can efficiently customize the SU transmission channel toward a desired direction without the need for AO iterations. Simulation results show that the SU transmit power can be effectively reduced by exploiting the active RIS with the PU interference mitigation. Moreover, the proposed CC algorithm achieves the transmit power reduction with moderate performance loss as compared with the AO algorithm, but offers an efficient means to facilitate beamforming management in CR.

This paper characterizes the achievable rate region of the active reconfigurable intelligent surface (RIS)-aided multiple-input single-output (MISO) interference channel, where an active RIS is used to help multiple multi-antenna transmitters send information to their intended receivers in the presence of strong interference among them. All the transmitters are subject to the transmit power constraints, while the active RIS must satisfy the power budget constraint and the maximum amplitude constraint for each reflecting element (RE). Under these constraints, the rate-profile method is employed to approach the Pareto boundary of the rate region, which needs to solve a series of feasibility problems for a given rate profile. These problems can be solved by an alternating optimization algorithm. In each iteration, the sum of the rate tuples is sequentially optimized by the transmit beamforming vectors at the transmitters and the reflection coefficients matrix at the active RIS. Specifically, the transmit beamforming vectors are obtained by solving a sequence of second-order cone programming (SOCP) problems, and the reflection coefficients matrix is obtained by solving a sequence of semidefinite programming (SDP) problems along with Gaussian randomization. Simulation results show that, even with strong interference, the active RIS can offer a significant improvement in the rate region compared to the passive RIS under the same power budget.

Symbiotic radio (SR) is a promising solution to achieve high spectrum- and energy-efficiency due to its spectrum sharing and low-power consumption properties, in which the secondary system achieves data transmissions by backscattering the signal originating from the primary system. In this paper, we are interested in the pilot design and signal detection when the primary transmission adopts orthogonal frequency division multiplexing (OFDM). In particular, to preserve the channel orthogonality among the OFDM sub-carriers, each secondary symbol is designed to span an entire OFDM symbol. The comb-type pilot structure is employed by the primary transmission, while the preamble pilot structure is used by the secondary transmission. With the designed pilot structures, the primary signal can be detected via the conventional methods by treating the secondary signal as a part of the composite channel, i.e., the effective channel of the primary transmission. Furthermore, the secondary signal can be extracted from the estimated composite channel with the help of the detected primary signal. The bit error rate (BER) performance with both perfect and estimated CSI, the diversity orders of the primary and secondary transmissions, and the sensitivity to symbol synchronization error are analyzed. Simulation results show that the performance of the primary transmission is enhanced thanks to the backscatter link established by the secondary transmission. More importantly, even without the direct link, the primary and secondary transmissions can be supported via only the backscatter link.

Interference poses a significant challenge in wireless communications due to the broadcast and superposition nature of wireless media. Reconfigurable intelligent surfaces (RIS), a promising 6G technology, can manage interference by customizing radio propagation with controllable reflecting elements (REs). Active RIS, which amplifies signal amplitudes and adjusts phases with enhanced REs, shows potential in addressing strong interference. This paper investigates the active RIS-aided multiple-input single-output (MISO) interference channel, characterizing the achievable rate region under transmit-power constraints, maximum amplitude constraint on each RE, and a power budget constraint at the active RIS. Utilizing multiple antennas at transmitters and multiple enhanced REs at the active RIS, we explore the Pareto Boundary of the achievable rate region with the rate profile method. A general alternating optimization framework iteratively optimizes the transmit beamforming vector and the reflection coefficient matrix through convex optimization techniques. We also consider a stringent case where active RIS ensures interference-free transmissions, deriving a reflection coefficient matrix structure via subspace decomposition. Additionally, a suboptimal algorithm combining this structure with maximum-ratio-transmission (MRT) and zero-forcing (ZF) beamforming characterizes the achievable rate region. Simulation results demonstrate that active RIS significantly improves the rate region and ensures interference-free transmissions compared to passive RIS and relay under the same power budget.

This paper investigates the reconfigurable intelligent surface (RIS) assisted symbiotic radio (RSR) system, where the primary transmission adopts orthogonal frequency division multiplexing (OFDM), and the RIS enables the secondary transmission by backscattering the primary signal. In particular, we propose a novel transmission protocol for the RSR system. With the help of the proposed protocol, the primary signal can be successfully detected without the knowledge of the secondary transmission at the RIS. Moreover, based on the detected primary signals, efficient channel estimation is achieved with a scalable training overhead. Furthermore, with the estimated channel state information (CSI), the passive beamforming vector of the RIS is optimized to maximize the weighted sum-rate of the primary and secondary transmissions. To solve this problem, we develop an effective algorithm based on the direct fractional programming (FP) approach, enabling the RIS to enhance the primary transmission and simultaneously transmit its own secondary signal. Simulation results validate the effectiveness of our proposed scheme and demonstrate that our proposed scheme outperforms the conventional ones with the same training length.

Symbiotic radio (SR) is a promising solution to achieve high spectrum- and energy-efficiency due to its spectrum sharing and low-power consumption properties, in which the secondary system achieves its data transmission by backscattering the signal originating from the primary system. In this paper, we are interested in the pilot design and signal detection when the primary transmission adopts orthogonal frequency division multiplexing (OFDM) scheme. In particular, in order to preserve the channel orthogonality among the OFDM sub-carriers, each secondary symbol is designed to span one OFDM symbol. The comb-type pilot is employed by the primary transmission, while the preamble pilot is used by the secondary transmission. With the designed pilot structures, the primary signal can be detected via the conventional methods by treating the secondary signal as a part of the composite channel. Furthermore, the secondary signal can be extracted from the estimated composite channel with the help of the detected primary signal. The bit error rate (BER) performance of the primary and secondary transmissions with both perfect and estimated CSI is analyzed. Simulation results show that the performance of the primary transmission is enhanced thanks to the backscatter link established by the secondary transmission. More importantly, even without the direct link, the primary and secondary transmissions can be supported via only the backscatter link.

Orthogonal time frequency space (OTFS) is a two-dimensional modulation scheme designed in the delay-Doppler domain, for combating the high mobility environment toward future wireless communications. In this paper, we propose a novel generalized spatial modulation combined OTFS (GSMOTFS) system, aiming at achieving better system performance compared to the conventional multiple-input multiple-output OTFS (MIMO-OTFS) and GSM orthogonal frequency division multiplexing (GSM-OFDM) systems. Furthermore, a decision feedback detector based on the minimum mean square error (MMSE) criterion is also developed toward efficient signal detection for the proposed GSM-OTFS system.

In this paper, a pair of detectors based on the local search (LS) and${M}$algorithms are proposed for the multiple-input multiple-output subcarrier-index modulation orthogonal frequency division multiplexing (MIMO-SIM-OFDM) systems. Specifically, the${M}$algorithm is employed in the LS algorithm to avoid the local optimal solution in the first proposed detector, which is named as enhanced LS (E-LS) detector. Furthermore, a threshold is employed to the E-LS detector to reduce the complexity in the second proposed detector, which is named as low-complexity E-LS (LC-E-LC) detector. Simulation results show that the proposed detectors can obtain a flexible tradeoff between performance and complexity, compared to the traditional maximum likelihood (ML) and log-likelihood ratio (LLR) detectors.