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.

In this paper, a novel design of amplitude phase shift keying-aided space-time block coded differential spatial modu-lation (APSK-STBC-DSM) is proposed. This design conceived not only maintains the diversity benefits of the space-time block coded differential spatial modulation (STBC-DSM) system, but also conveys extra information bits with the aid of amplitude phase shift keying (APSK), so as to enhance the transmission rate. Moreover, an improved low-complexity detector is also proposed, in order to achieve near-optimal detection performance. Ultimately, simulation results exhibit that the proposed APSK-STBC-DSM system is capable of providing significant bit error rate (BER) performance gains over conventional STBC-DSM and other differential spatial modulation (DSM) counterparts.

In this contribution, the system of quasi-orthogonal space-time block coded spatial modulation (QOSTBC-SM) with minimum decoding complexity (MDC) is proposed toward linear detection complexity by utilizing the corresponding single-symbol decoding algorithm at the receiver. In addition, we compare the performance of the MDC-QOSTBC-SM system with different decoding algorithms in conjunction with comparing this scheme with space-time block coded spatial modulation (STBCSM). Furthermore, we extend it to uplink multi-user multiple-input multiple-output (MIMO) communications and compare the performance with STBC-SM in the context of quasi-static flat Rayleigh fading channels. The simulation results indicate that the proposed scheme achieves a significant reduction in decoding complexity at the cost of a slight performance loss when employing the single-symbol decoding algorithm.

To enable secure and effective downlink multiple-input multiple-output (MIMO) transmission, it has recently been proposed to integrate space-time block code (STBC) with spatial modulation (SM) and artificial noise (AN). However, this technique suffers from high detection complexity at the receiver side. In this contribution, therefore, we present an AN-assisted transmission scheme for the quasi-orthogonal space–time block coded receive spatial modulation (QOSTBC-RSM) system while developing two low-complexity detection algorithms. Our simulation results demonstrate that the proposed scheme and the corresponding detection methods are capable of guaranteeing safe transmission and achieving near-optimal bit error rate (BER) performance while significantly reducing computational overhead.

The introduction of space-time block code (STBC) can efficiently increase the diversity of original spatial modulation (SM). However, the spectral efficiency of STBC will be reduced gradually with the increase of the transmit antennas. In this contribution, we conceive a novel quasi-orthogonal space-time block coded spatial modulation (QOSTBC-SM) design for multiple-input and multiple-out (MIMO) transmission by reaping their respective benefits, while improving the spectral efficiency compared to conventional space-time block coded spatial modulation (STBC-SM). More specifically, in the proposed QOSTBC-SM structure, the information bits are conveyed via the active antenna index, as well as the QOSTBC blocks at the transmitter, while at the receiver, a low-complexity detection scheme is proposed. Furthermore, a closed-form union bound of the bit error rate (BER) is also quantified by theoretical derivation. Finally, our simulation results demonstrate that QOSTBC-SM is capable of outperforming the conventional counterpart as STBC-SM with higher spectral efficiency.

Orthogonal Frequency Division Multiplexing (OFDM) with Index Modulation (OFDM-IM), which conveyed information bits via the activated indices and constellation symbols is a promising technique in the next wireless communications. In the OFDM-IM scheme, only part of subcarriers are activated to transmit information, the inactive subcarriers transmit zero symbols, so that the conventional differential coding is not suitable for the adjacent subcarriers. In order to address this issue, in this paper, a novel Rectangular Differential OFDM-IM (RD-OFDM-IM) scheme is proposed to exploit the benefits of OFDM-IM dispensing with Channel State Information (CSI). In the proposed RD-OFDM-IM scheme, N subcarriers are partitioned into G subblocks and index modulation is employed in each subblock first. Then rectangular differential coding is invoked during two adjacent subblocks, so that non-coherent detection can be employed for the proposed RD-OFDM-IM scheme. Simulation results are shown that the proposed RD-OFDM-IM scheme is capable of providing considerable performance gain over conventional Differential OFDM (D-OFDM) scheme with lower Peak Average Power Ratio (PAPR).

In this paper, a novel scheme dubbed space-time block coded rectangular differential spatial modulation (STBC-RDSM) is proposed for multiple-input and multiple-out (MIMO) systems, which combines space-time block coding (STBC) and rectangular differential spatial modulation (RDSM) to reap their respective benefits while avoiding the drawbacks of conventional differential spatial modulation (DSM) systems. More specifically, in the proposed STBC-RDSM scheme, information bits are conveyed via the rectangular differentially encoded antenna index matrices, as well as the STBC blocks. Furthermore, a low-complexity detection scheme is proposed. Our simulation results demonstrate that STBC-RDSM outperforms its conventional DSM counterparts in various spectral efficiencies. Finally, a closed-form union bound on the bit error rate (BER) is derived and validated by our simulation results.

In this paper, a novel space-time block coded rectangular differential spatial modulation (STBC-RDSM) is proposed for multiple-input and multiple-out (MIMO) system, which combines the space-time block coding (STBC) and rectangular differential spatial modulation (RDSM) to take the advantages of RDSM and STBC systems, while avoiding the drawbacks of conventional differential spatial modulation (DSM) systems. More specifically, in the proposed STBC-RDSM scheme, the information bits are conveyed via the rectangular differential encoded antenna indices matrices, as well as the STBC blocks. Our simulation results demonstrate that STBC-RDSM outperforms the existing DSM systems for various spectral efficiencies.