The use of physical layer methods for improving the security of wireless links has recently become the focus of a considerable research effort. Such methods can be used in combination with cryptography to enhance the exchange of confidential messages over a wireless medium in the presence of unauthorized eavesdroppers, or they can be used to enable secrecy in the absence of shared secret keys through the use of coding strategies, jamming or beamforming. Indeed, one of the driving forces behind the recent emergence of physical layer techniques for security is the push toward adding extra degrees of freedom in the form of multiple antennas at the transmitter and receiver of the link. Multiple-input multiple-output (MIMO) wireless systems have been extensively studied during the past two decades, and their potential gains in throughput, diversity, and range have been well quantified. MIMO approaches are now an integral part of the WiFi and 4G standards in use today. It is not surprising that MIMO architectures are useful in improving wireless security as well, since they can provide focused transmit selectivity of both information and noise toward desired and undesired receivers. In this chapter, we discuss a number of different ways that physical layer security can be achieved in wireless networks with MIMO links. We will focus primarily on signal processing related issues (e.g., beamforming, power control, resource allocation) that enable reliable reception at intended recipients and minimize data leakage to eavesdroppers, and we will consider a variety of different MIMO settings including point-to-point, broadcast, interference, and multi-hop networks. We cannot offer an exhaustive survey of such methods in just a single chapter; instead, we present a few representative approaches that illustrate

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Physical Layer Security in Wireless

the gains that multiple antennas provide. Additional and more complete treatments of the physical layer security problem can be found in [1-4] as well as other chapters in this book.