Polar codes combined with physical layer security on impulsive noise channels

Abstract

The need for secure communications is becoming more and more impor- tant in modern society as wired and wireless connectivity becomes more ubiquitous. Currently, security is achieved by using well established encryption techniques in the upper layers that rely on computational complexity to ensure security. However, processing power is continu- ally increasing and well-known encryption schemes are more likely to be cracked. An alternative approach to achieving secure communication is to exploit the properties of the communication channel. This is known as physical layer security and is mathematically proven to be secure. Phys- ical layer security is an active research area, with a significant amount of literature covering many different aspects. However, one issue that does not appear to have been investigated in the literature is the effect on physical layer security when the noise in the communication channel is impulsive. Impulsive noise adds large spikes to the transmitted signal for very short durations that can significantly degrade the signal. The main source of impulsive noise in wireless communications is electromag- netic interference generated by machinery. Therefore, this project will investigate the effect of impulsive noise on physical layer security. To ensure a high level of performance, advanced error-correcting codes are needed to correct the multiple errors due to this harsh channel. Turbo and Low-Density Parity-Check (LDPC) codes are capacity-approaching codes commonly used in current wireless communication standards, but their complexity and latency can be quite high and can be a limiting fac- tor when required very high data rates. An alternative error-correcting code is the polar code, which can actually achieve the Shannon capacity on any symmetric binary input discrete memoryless channel (B-DMC). Furthermore, the complexity of polar codes is low and this makes them an attractive error-correcting code for high data rate wireless commu- nications. In this project, polar codes are combined with physical layer security and the performance and security of the system is evaluated on impulsive noise channels for the first time. This project has three contributions: Polar codes designed for impulsive noise channels using density evo- lution are combined with physical layer security on a wire-tap chan- nel experiencing impulsive noise. The secrecy rate of polar codes is maximised. In the decoding of polar codes, the frozen bits play an important part. The posi- tions of the frozen bits has a significant impact on performance and therefore, the selection of optimal frozen bits is presented to opti- mise the performance while maintaining secure communications on impulsive noise wire-tap channels. Optimal puncturing patterns are investigated to obtain polar codes with arbitrary block lengths and can be applied to different modu- lation schemes, such as binary phase shift keying (BPSK) and M- ary Quadrature Amplitude Modulation (QAM), that can be rate compatible with practical communication systems. The punctured polar codes are combined with physical layer security, allowing the construction of a variety of different code rates while maintaining good performance and security on impulsive noise wire-tap chan- nels. The results from this work have demonstrated that polar codes are ro- bust to the effects of impulsive noise channel and can achieve secure communications. The work also addresses the issue of security on im- pulsive noise channels and has provided important insight into scenarios where the main channel between authorised users has varying levels of impulsiveness compared with the eavesdropper's channel. One of the most interesting results from this thesis is the observation that polar codes combined with physical layer security can achieve good perfor- mance and security even when the main channel is more impulsive than the eavesdropper's channel, which was unexpected. Therefore, this thesis concludes that the low-complexity polar codes are an excellent candidate for the error-correcting codes when combined with physical layer security in more harsh impulsive wireless communication channels.