# Secure Image Signal Transmission Scheme Using Poly-Polarization Filtering and Orthogonal Matrix
###### Abstract  
In this paper, a novel secure image signal transmission scheme was proposed in wireless systems, in which the poly-polarization filtering and the orthogonal matrix (PPF-OM) were combined to protect the image signal and eliminate the polarization dependent loss (PDL) at the same time, which was caused by the non-ideal wireless channel. This scheme divided the image information sequence into two parts in order to modulate and reshape the results into symbol matrices with the same size. Then, two sets of polarization states (PSs) and orthogonal matrices (OMs) were designed to process the symbols in order to enhance information protection and eliminate the PDL. Legitimate users were able to apply the shared PSs and OMs, step by step, so the information could be recovered. However, for eavesdroppers, the received signals were random symbols that were difficult to demodulate. Then, the bit error rate and the secrecy rate were derived to evaluate the performance of the PPF-OM scheme. Finally, the simulations demonstrated the superior performance of the PPF-OM scheme for enhancing the information security and eliminating the PDL. wireless image signal transmission; secure transmission; polarization dependent loss; orthogonal matrix; PDL; polarization state +
Footnote †: journal: Applied Sciences  
while naturally distorted in other directions. In [8], QAM symbols were scrambled on a complex two-dimensional plane using the multi-fold encryption method. The wiretap channel model firstly proposed by [PERSON] on the basis of [PERSON]'s information theory was studied. Schemes to maximize the secrecy rate were proposed, such as artificial-noise-based techniques [9; 10], adaptive beam-forming [11; 12; 13], relay cooperative transmission [14], and so on. However, only a few of the existing physical layer security schemes have considered both degrading the BER and improving the secrecy rate. However, the development of wireless transmission technology has higher requirements for the transmission rate. To alleviate this problem, multi-input-multi-output (MIMO) appears to be very promising due to its more efficient use of communication channels [7; 13], which then generates multiple paths for information transmission based on multiple antennas. Furthermore, the transmission frequency continuously increases, which promotes the transfer of information at a higher bandwidth in order to improve the throughput, such as by using 6G, which operates in the terahertz (THz) band [15]. However, the multipath transmission of MIMO is weakened in high-frequency communication scenarios due to large signal attenuation, where the refraction and scattering components of the signal disappear. Fortunately, in this case, polarization signal processing technology, based on a dual-polarized antenna, could provide two independent channels and, thus, improve or even double the transmission efficiency in MIMO systems, as these mainly rely on cross-polarization discrimination (XPD) between two orthogonal polarizations [16]. Many advanced signal processing techniques have been proposed that adopt the use of dual-polarized antenna. The polarization state modulation (PM) technique in [4; 17] adapted a three-dimensional modulation technique and utilized the amplitude ratio and phase differential between the signals of the dual-polarized antennas as information-bearing parameters. The advantages of PM were summarized as three aspects:  
* PM could be combined with amplitude-phase modulation techniques to further improve the transmission efficiency [18]. * PM could be implemented at radio frequency; thus, the amplifier was operated in the linear region, which was energy efficient [19]. * PM could be used to improve the transmission security, such as through concealed transmission technology [20; 21] and combined constellation scrambling techniques [4].