Physical Layer Security in Multiuser Wireless Networks

Physical Layer Security in Multiuser Wireless Networks

Anish Prasad Shrestha (Inha University, South Korea) and Kyung Sup Kwak (Inha University, South Korea)
DOI: 10.4018/978-1-5225-0703-1.ch012
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Physical layer (PHY) security has recently gained considerable attention as an alternative option to cryptographic techniques since it can provide unconditional perfect secrecy. This chapter presents brief concept about PHY security and solutions along with quantitative measurement of secrecy performance for multiuser wireless networks. A typical transmitter in wireless communication may be required to send a confidential message to either a single or several users simultaneously. Aiming to provide comprehensive investigation of secrecy performance in such multiuser network, secure multicasting and opportunistic scheduling techniques are presented. Furthermore, to look into the effects of multiple antennas, system model under consideration employ maximal ratio combining at receiver.
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1. Introduction

The power of instant communication bestowed by proliferation of wireless communications has transformed the society and made our world a smaller place. The freedom of mobility that these wireless systems provide to the users has been the primary element to their popularity. However, it also vulnerable to leakage of confidential message resulted by open nature of wireless medium i.e. airwave which is exposed to all sorts of unwanted eavesdroppers. As such, confidentiality is one of the basic requirements to secure wireless communication. In common practice, a generalized layered protocol approach is considered by many wireless service providers to simplify networking designs, by dividing them into functional layers, and assigning protocols to perform each layers task. Traditionally, different security techniques are applied at different layers to secure information as shown in Figure 1. Confidentiality is mostly achieved at the upper layer using cryptographic encryption and decryption algorithm. These algorithms rely on the intractability of certain mathematical problems such as integer factorization and discrete logarithm problems which result into conditional security. A cryptographic algorithm is said to be computationally secure if the amount of work to break such an algorithm significantly exceeds the computational resources available to an adversary. With rapid progress in computational power, such techniques may be compromised in near future. In this regard, physical layer (PHY) security based on information-theoretic secrecy has emerged as a potential candidate.

Figure 1.

Security framework at various layers

Contrary to cryptographic algorithms, PHY security assumes no limit on the adversary's computing resources and provides unconditionally perfect secrecy. The concept of perfect secrecy was introduced by Shannon in (Shannon, 1949). The perfect secrecy implies that for all secret message X and eavesdroppers observation of secret message Y, the conditional probability Pr(X|Y) is same as the marginal probability Pr(X). The seminal work in (Wyner, 1975) commenced the idea of secrecy capacity under perfect secrecy constraints which laid the foundation for research works in PHY security. Channel coding and signal processing techniques are commonly used in PHY secrecy to safely transmit a message from the source to the destination while keeping confidentiality at eavesdropper. Although both techniques should be taken into account for optimal design, signal processing techniques often help to reduce the complexity of the channel coding operations especially in system models exploiting diversity where the spatial degrees of freedom can be taken into consideration. As such, this chapter focusses on signal processing techniques rather than channel coding.

This chapter aims to provide an overview of physical layer security paradigm and its implication in multiuser wireless networks where the transmitter could send an identical message to each user simultaneously or different messages to each user separately based on their channel conditions. Section 2 briefly presents the fundamental concept of PHY security and how it differs from traditional security approaches. Then, the basics metrics for measuring secrecy performances in PHY security are introduced to facilitate the discussion for next sections. Section 3 surveys the related works in the existing literature. Section 4 introduces the multiuser wireless networks with two transmission modes namely multicasting and opportunistic scheduling mode under PHY security constraint. This model is built upon the previous work (Shrestha & Kwak, 2014) and (Shrestha, Jung, & Kwak, 2013), however it bridges the gap between those works by joint comparison under one platform. As such, there is possibility of novel approach for switching between two modes for transmitter. Finally, the last section outlines the possible research directions for future and concludes the entire chapter.

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