Research On Physical Layer Secrecy Coding Based On Polar Code

The physical layer secrecy coding is an especial coding technique which matches with the transmission channels and guarantees both security and reliability simultaneously.The mother codes of physical layer secrecy coding are required to have good Shannon performance and be sensitive to the channel quality differences.As a result,a polar code is a promising mother code since it comes from the channel polarization and matches the channel quality naturally.This dissertation studies on the physical layer secrecy coding based on polar codes.By transferring the differences between the legitimate channel and the wiretap channel to the differences of the decoding performance,it prevents the confidential information from being decoded by the eavesdropper at the physical layer.However,the channel polarization is not perfect under the finite code length and the channel differences are not enough.So the secrecy rate of a practical secrecy polar code needs to be improved,and the following problems still exist.(1)The diversity of channel polarization is insufficient by the single-input parallel channels which limits the promotion of secrecy rate.(2)The artificial noise aided secrecy polar coding designed in signal domain has a limit on its validity and application scenarios.(3)The secrecy rate of traditional relay aided coding method declined rapidly when the relay does not cooperate during the whole course of communication.This dissertation studies the channel matching secrecy polar coding methods,and aims to improve the secrecy rate.The differences between legitimate channel and wiretap channel are expanded by multi-input parallel channels,scrambling and probabilistic relay,respectively.The main research contents are as follows:1.A secrecy polar coding method for multi-antenna systems is proposed to deal with the problem that the diversity of channel polarization is insufficient in the single-input parallel channels.The method uses multi-antenna to construct the multi-input parallel channels and increases the diversity of channel polarization in finite code length.The secrecy rate is increased by reducing the number of grey channels and increasing the number of polarized channels which the legitimate user and the wiretapper have big differences,respectively.To reduce the number of grey channels,an equal-capacity-difference mapping scheme is proposed and adopted among different transmission time slots,and the channel polarization is improved in finite code length.Meanwhile,a maximal-capacity-difference mapping scheme is proposed and used in the same transmission time slot to regulate the distribution of reliability of the polarized channels and increase the number of polarized channels which the legitimate user and the wiretapper have big differences.Finally the confidential bits number is increased and the secrecy rate is improved.Simulation results verify that the secrecy rate of proposed method can be increased from 0.029 and 0.004 to 0.042,compared to the random mapping-based secrecy polar coding method and Arikan's secrecy polar coding method at the average legitimate and wiretap channel capacity are 0.85 and 0.5,respectively,and the polarization order n(28)9,respectively.2.The traditional artificial noise aided secrecy polar coding method has low efficiency and limited application scenarios.To solve this problem,a scrambling based secrecy polar coding method is proposed.In this method,the confidential bits of last transmission code block are used as the scrambling bits and injected into the current codeword.Therefore the channel differences are expanded and the secrecy rate is improved without interrupting the legal communication.Since different bits have different effects on decoding,we propose the channel-mapping-based position selection algorithm and the bit error rate based position selection algorithm,respectively.The former has a lower complexity and the later has a better performance.According to the selected scrambling positions,parameters such as confidential bits,random bits and frozen bits positions are updated.Simulation results show that when the code length N(28)128 and the code block number T(28)10,the proposed bit-error-rate based scrambling secrecy polar coding method and channel-mapping based scrambling secrecy polar coding method have the secrecy rate of 0.41 and 0.4,respectively,which are much higher than random-selection-based scrambling secrecy polar coding method(0.33)and the traditional secrecy polar coding method(0.12).3.The secrecy rate of traditional relay aided secrecy polar coding method declined rapidly when the relay does not cooperate during the whole course of communication.To solve this problem,a relay aided secrecy polar coding method is proposed for the communication systems where the relay uses decode-and-forward(DF)in probability.In the single-relay system,the transmitter first encodes the secrecy bits in two layers: the first layer is designed over the virtual BEC that generated by the probabilistic DF relay,and the second layer is designed over the real transmission channels.After receiving the codeword,the relay decodes and extracts the frozen bits which the legitimate user cannot obtain directly in probability,and re-encodes them by classical secrecy polar coding.Finally,the receiver decodes the received codeword from the relay and the transmitter in turn.Afterwards,the above method is extended to the multi-relay system,and the optimization on transmission rate with the aim of maximizing the secrecy rate is established.The theoretical results verify that the bit error rate of the legitimate user tends to be zero with the code length and the timeslot number tends to infinite,and the proposed method can achieve weak security.Simulation results show that the secrecy rate of proposed method is higher than the traditional method in single-relay system,and the optimal transmission rate leads to a higher secrecy rate comparing with the random rate selection method in multi-relay system.