Study On Physical Layer Secrecy Coding Based On Polar Codes

Physical layer security can approach the information-theoretic security using the feature of physical channel itself such as noise and fading. It has drawn much attention in recent years. Polar codes are a class of error correction codes which are constructed based on the channel characteristics and can achieve the Shannon channel capacity limit in theory, so they are suitable for the physical layer security. Physical layer secrecy coding methods based on Polar codes for the wiretap channel and relay-eavesdropper channel are studied in this thesis, which have important theoretical significance and reference value.The main works are as follows:Firstly, a puncturing mechanism is introduced to enhance the communication security by hiding part of Polar code-bits transmitted in the channel aiming at the wiretap channel secrecy coding problem. At first, a novel puncturing Polar codes method is proposed. In the proposed method, Bhattacharyya parameter is used for calculating the reliability of each code-bit, which contributes to the decision of the punctured position, thus a reliability puncturing Polar codes method is presented. Then the reliability puncturing Polar codes is applied to maximize the differences in recovery degrees of the received signals between the legitimate user and the eavesdropper. Simulation results show that the proposed method has about 1.7dB and 2.0dB decrease in the security gap parameter in comparison with those of random puncturing method and stopping-tree puncturing method respectively. The reliability puncturing Polar codes has the minimum security gap, so it can guarantee the security and reliability of message transmission even in a small channel degradation condition.Secondly, the relay nodes can be divided into two cases which are respectively called the trusted node and the untrusted node. Two relay-eavesdropper channel secrecy coding methods based on Polar codes under the two cases are proposed in the thesis. At first, a trusted-relay wiretap channel secrecy coding improved method based on Polar codes is proposed. In the improved method, the DF relay protocol is adopted and the wiretap channel is not limited to degraded condition during the second transmission time slot, therefore, there are no extra random sequences in the secrecy coding process. Secondly, an untrusted-relay wiretap channel secrecy coding method based on Polar codes is given. In the method, the AF relay protocol is adopted. The generation matrix of Polar code acts as private key and the encryption coding is performed by public key which is published from private key using McEliece cryptosystem. Simulation results show that in the proposed trusted-relay wiretap channel secrecy coding method, the secure transmission rate increases from 0.16 bits/Sys to 0.42 bits/Sys when the SNR of the relay channel is 3dB; and the secure transmission rate increases from 0.36 bits/Sys to 0.62 bits/Sys when the SNR of the relay channel is 8dB. The proposed method can increase the coding rate in the source node. And in the proposed untrusted-relay wiretap channel secrecy coding method, when the SNR of relay channel is 4dB, the legitimate receiver’s BER is about 3.6×10-5 small enough for the reliable communication, the relay and the eavesdropper’s BER always stay around 0.5. The relay node can’t get secret information, thus the security of the system is improved.