Transceiver Design of a Secure Multiuser FDSS-Based DFT-Spread OFDM System for RIS- and UAV-Assisted THz Communications

In this article, we design and implement a multiantenna configured secure multiuser discrete Fourier transform (DFT)-Spread orthogonal frequency division multiplexing (OFDM) system based on frequency-domain spectrum shaping (FDSS) for reconfigurable intelligent surfaces (RISs) and unmanned aerial vehicle (UAV)-assisted terahertz (THz) communications. Our proposed simulated system highlights more suitable performance matrices for a typical case of three users for color image transmission. We introduced a six-dimensional hyperchaotic system-based encryption algorithm to enhance the physical layer security (PLS) of a UAV-to-ground communication network. In addition, the block diagonalization (BD) precoding technique reduces multiuser interference (MUI). Furthermore, we included repeat and accumulate (RA) channel coding with Cholesky decomposition-based zero-forcing (CD-ZF) and minimum mean square error (MMSE) signal detection schemes to improve the bit error rate (BER). We adopted the FDSS scheme and considered null carriers to reduce the out-of-band (OOB) spectrum power. The simulation results demonstrate the effectiveness of the proposed system in terms of PLS enhancement for color image transmission, with a low image structural similarity index of 0.65%, 1.60%, and 0.70% for users 1, 2, and 3, respectively; an achievable OOB power emission of 337 dB; and estimated peak-to-average power ratios (PAPRs) ranging from 7.10 to 7.85 dB at a complementary cumulative distribution function (CCDF) of $1\times 10^{-4}$ for different ground-transmitting channels. At signal-to-noise ratios of 13.7, 9.4, and 7.5 dB, users 1, 2, and 3 achieve a BER of $1\times 10^{-3}$ under RA channel coding, MMSE, and binary phase shift keying (BPSK) digital modulation.