# Secure Image Signal Transmission Scheme Using Poly-Polarization Filtering and Orthogonal Matrix
###### Abstract
In addition, PS can be used as the parameter for polarization filtering (PF), based on which the information can be conveyed simultaneously by two- or three-way signals without mutual interference [22], thus improving the transmission efficiency. Furthermore, PF could also be developed for information security by combining it with artificial noise technology [23] or pseudo-random sequences [24]. However, despite all the aforementioned advantages, its performance is likely to deteriorate due to the use of non-ideal channels. This is because the channel is non-ideal, and the XPD of the dual-polarized antennas at the receiver side is limited, which causes mutual interference between the two polarized signals. This is called the polarization-dependent loss (PDL). Although there are methods to relieve the problem, such as the pre-compensation method and the zero-forcing method [4], the PDL can not be totally eliminated. In order to solve the problems mentioned above, a novel transmission scheme based on poly-polarization filtering and an orthogonal matrix (PPF-OM) was proposed to enhance the information security and eliminate the PDL. First, the information was divided into two parts and modulated into two symbols vectors with the same length \(u\). Then, they were reshaped into matrices of same size \(\frac{l}{2}\times\frac{2u}{l}\). For the first matrix, each row was multiplied with a PS vector, and for the second matrix, the corresponding row was multiplied with a corresponding orthogonal PS vector. After that, the horizon(H) and vertical(V) rows were extracted separately to construct two matrices (size \(\frac{l}{2}\times\frac{2u}{l}\)), which were then separately multiplied with a unique column vector from the unitary matrix (size \(M\times M\)), resulting in multiple matrices (size \(M\times\frac{2u}{l}\)). Finally, for each polarization, the matrices were added and reshaped into a vector (size \(1\times\frac{2 uM}{l}\)) for transmission. For legitimate users, the prior information regarding \(u\), \(l\), PSs, orthogonal matrix, and modulation mode were sharedwith the transmitter. The two polarized signals were isolated from the received signals; thus, they can be separately demodulated. However, for eavesdroppers, it was difficult to crack these parameters, which, therefore, indicated that the transmission security had been enhanced. Moreover, the PDL was also eliminated in the proposed method. Overall, the main advantages of the PPF-OM scheme could be summarized, as follows:  
1. Both the image information security and PDL elimination were considered in the PPF-OM scheme. The PDL was eliminated, and the information security was enhanced, which was evaluated by both the BER and secrecy-rate performances. 2. The information was conveyed by two signals, which were mixed twice by PSs and the OM, step by step. After two mixing procedures, signals became difficult to decipher. 3. The parameters of \(u\), \(l\), PSs, orthogonal matrix, and modulation mode should be determined in order, and if there is an error in a parameter, the latter parameters would produce greater errors. In addition, if eavesdroppers attempted to breach one parameter through the method of exhaustion, the recovered signals would be random plurals. Therefore, the PPF-OM scheme offered good protection for parameters, thus improving information security. 4. In the PPF-OM scheme, all power was used to transmit the signal, and no power was wasted. In this manner, there would still be self-interference in the eavesdropper's received signals, with an effect similar to the artificial noise that deteriorates the channel quality. Therefore, a positive secrecy capacity could be guaranteed to ensure information security. 5. In [25], the weight-weighted fractional Fourier transform (WFRFT) was used to process the transmit signal vectors to enhance information security. As described in the reference, the WFRFT operation consisted of a \(K\)-point discrete Fourier transform operation (O(\(K^{2}\))) and two reverse modulo-\(K\) operations (O(\(K\))).