Optimal SVD-based Precoding for Secret Key Extraction from Correlated OFDM Sub-Channels Amir Aliabadian 1 , Mohammad Reza Zahabi 2,* , Majid Mobini 3 1- Electrical and Computer Engineering, Babol Noshirvani University of Technology, Iran, a.aliabadian@nit.ac.ir 2- Faculty of Electrical and Computer Engineering, Babol Noshirvani University of Technology, Iran, zahabi@nit.ac.ir (Corresponding Author) 3- Electrical and Computer Engineering, Babol Noshirvani University of Technology, Iran, mobini2002@gmail.com Abstract: Secret key extraction is a crucial issue in physical layer security and a promising technique for the next generation of 5G and beyond. Unlike previous works on this topic in which Orthogonal Frequency Division Multiplexing (OFDM) sub-channels are considered to be independent, the effect of correlation among sub-channels on the secret key rate is addressed in this paper. We assume a realistic model for dependency among the sub-channels. Moreover, a new approach with low computational complexity for key extraction and optimal Mutual Information (MI) is presented in our study. To do this, we utilize a Singular Value Decomposition-based (SVD-based) precoding to obtain an optimal SVD-based approach for key extraction. Simulation results show that the rate of the key exchange link may drop up to 72% when the signal to noise ratio (SNR) is considered to be 8dB. The low computational complexity of our proposed approach makes it a promising candidate for developing secure and high-speed networks. Keywords: Physical Layer Security, Secret Key, SVD-based Channel Decorrelation, OFDM. 1. Introduction Efficient physical layer secret key generation and authentication schemes based on the wireless channel are developing issues in physical layer security and especially in OFDM-based communication systems. Therefore, the channel parameters are considered as the most fundamental part of key construction techniques [1- 3]. Depending on the conditions of the environment, the Key Generation Rate (KGR) is defined as a parameter that specifies the secret key bits generated per second. Similarly, Key Disagreement Rate (KDR) is defined as the rate of the distinction of the bits of the keys established by Alice and Bob as the two ends of the communication link. In recent studies, OFDM structures are utilized for key generating with long sequences and increasing key generation rate such that a key is achieved in the coherence time for each sub-channel. Since KGR and KDR oppose each other, a reasonable tradeoff should set. This setting is considered between them by considering the demands of the system and the user interface [1-3]. In literature, the key generation method is separated into four stages including the channel probing, quantization, information reconciliation and privacy amplification [1-3]. The quantization scheme is utilized to optimize the operation of randomness, KGR, and KDR by adjusting the level of quantization and the threshold limit. To correct the errors and to set up a shared secret key, information reconciliation is unavoidable. This mismatch between legitimate users' keys can be repaired via channel coding [4-5]. Therefore, error correction codes are used to solve this problem as long as the difference between the keys is less than the number of correctable bits. A part of the information might be revealed at this stage, which will considerably affect the key generation performance. Another part of the information is also sent through the public channel during the information reconciliation stage, which can be heard by Eve as a wiretapper. This can potentially threaten the security of the key sequence. Privacy amplification is finally used for removing the revealed information from the agreed key sequence by legitimate users (Alice and Bob). It is worthwhile to point out that all mentioned stages can be independently modified and promoted. In the key generation, based on channel