Multiuser FDSS-Based DCT-Spread Massive MIMO OFDM System for Secure RIS-Assisted UAV-Enabled Networks

Enhancing spectral efficiency (SE), reducing out-of-band (OOB) power emission, minimizing multiuser interference (MUI), and ensuring secure data communication are key challenges in the development of future-generation unmanned aerial vehicle (UAV)-enabled wireless communication systems. To address these challenges and emphasize effective solutions, we propose a novel and secure reconfigurable intelligent surface (RIS)-assisted UAV-enabled multiuser frequency-domain spectrum shaping (FDSS)-based discrete cosine transform (DCT)-Spread massive multiple-input multiple-output (mMIMO) orthogonal frequency division multiplexing (OFDM) system. The system incorporates physical layer security (PLS) based on two-dimensional cosine-sine interleaved chaotic map (2D-CSIM), block diagonalization (BD) precoding, FDSS with null carriers, turbo and repeat-and-accumulate (RA) channel coding, Cholesky decomposition-based zero-forcing (CD-ZF), and lattice reduction-based minimum mean square error (LR-MMSE) signal detection schemes. The 2D-CSIM enhances confidentiality in audio signal transmission, while the BD precoding reduces MUI. In addition, the FDSS with null carriers reduces OOB emissions and enhances the system’s SE. The turbo and RA channel coding with CD-ZF and LR-MMSE signal detection schemes improve the bit error rate (BER) performance. Simulation results show the effectiveness of the proposed system. It achieves significant PLS for audio transmission, with low correlation coefficients of <2% for four users between the original and encrypted signals. We also quantify the effectiveness of the 2D-CSIM-based encryption technique using the National Institute of Standards and Technology (NIST) Statistical Test Suite. The system achieves an OOB reduction of 334 dB, SE of 33.30 bps/Hz, and peak-to-average power ratio (PAPR) of 8.89 dB at a complementary cumulative distribution function (CCDF) of $1 \times 10^{-3}$ , which are comparable to state-of-the-art systems. At signal-to-noise ratios (SNRs) of 27–28.5 dB, the four users achieved a BER of $1\times 10^{-4}$ with RA coding, LR-MMSE detection, and lower-order quadrature amplitude modulation (QAM) constellation.