Physical-layer security based on pseudo-noise (PN) superposition is a promising approach for mitigating eavesdropping in future wireless systems. However, under Shannon's capacity formulation with Gaussian signaling, achieving secrecy typically requires allocating substantial transmit power to PN, resulting in a significant reduction in achievable information rate and limiting practical applicability. This limitation is alleviated when finite-alphabet modulation schemes, such as M-ary Quadrature Amplitude Modulation ($M$-QAM), are employed, as expected in practical 6G transceivers. In this work, we analyze the information rate performance of PN-assisted systems under $M$-QAM signaling using mutual information and derive the corresponding achievable secrecy rate. The impact of PN power allocation on both the legitimate user and the eavesdropper is investigated across different modulation orders and channel conditions. Monte Carlo simulations are conducted to evaluate system behavior under varying user and eavesdropper channel conditions and to examine how PN power allocation influences secrecy performance. The results show that, at sufficiently high signal-to-noise ratio (SNR), the information rate becomes largely insensitive to PN power allocation, enabling near-perfect secrecy with $M$-QAM modulation-highlighting a key departure from Shannon-capacity-based secrecy analyses and underscoring the practicality of finite-alphabet security mechanisms for 6G wireless systems.
Fernando Moya Caceres, C. Divarathne, Yapeng Xie et al.· 0 citations
The integration of non-orthogonal multiple access (NOMA) with sub-Terahertz and Terahertz (sub-THz/THz) wireless communications can be a key enabler for future 6G networks, offering enhanced spectral efficiency and massive connectivity in highly directional and bandwidth-rich frequency bands. In this work, we investigate the performance of a power-domain downlink terrestrial-to-UAV NOMA system under realistic propagation conditions and hardware impairments. We develop a comprehensive analytical framework to evaluate the outage probability (OP) and average bit error probability (ABEP) of the considered system operating over multi-cluster fluctuating two-ray (MFTR) fading channels, which provide a flexible and physically motivated model for small-scale fading in sub-THz and THz propagation environments. The proposed model jointly accounts for frequency-dependent path loss with molecular absorption, pointing error–induced beam misalignment, and imperfect successive interference cancellation (SIC), while supporting practical square $M$ -QAM modulation. An exact analytically tractable expression is derived for OP and a high-signal-to-noise ratio (SNR) approximation is obtained for ABEP, along with asymptotic results in the high-SNR regime that provide insight into the system’s coding gain and diversity behavior. The analytical results reveal two distinct operating regimes. Under strong-interference conditions, persistent multiuser interference leads to irreducible ABEP floors, whereas in mild-interference scenarios, the ABEP decays with the average SNR according to a power-law behavior, with performance limited by the dominant physical impairment, either multipath fading or beam misalignment. Monte Carlo simulations validate the analysis and highlight key design trade-offs involving power allocation, SIC imperfection, modulation order, beamwidth, carrier frequency, and channel richness.
H. Mora, Fernando Moya Caceres, N. V. Garzón et al.· IEEE Open Journal of the Com...· 0 citations
