Physical Layer Security in Military Communications: A Three Levels Approach

Introduction

Military communications have significant differences when compared to commercial communications, especially at the tactical edge. They rely significantly on ad-hoc networks, multihop transmission, and face challenging conditions due to eavesdropping and jamming by the adversary (Vassiliou et al., 2013). New communications technologies, designed for tactical use, are affecting the battlefield with game-changing capabilities. Such technologies include Command, Control, Communications, Computers and Intelligence (C4I) technologies such as mobile and wireless networking, advanced antenna systems, jamming/anti-jamming capabilities, among others (Shea, 2016).

With the advent of machine-to-machine (M2M) communications, Internet of Things (IoT), and device-to-device (D2D) communications, their applications in military communications are gaining significant importance. Sensors collecting information from the battlefield, or vital signs of soldiers in the form of a body area network (BAN), follow an IoT approach when transmitting their data. D2D communications occur between communication devices in tactical networks whenever there is no communication infrastructure. In IoT/M2M and D2D networks, the overhead of traditional application-layer encryption techniques is becoming a limiting factor (Liu et al., 2015). Physical layer security (PLS) is a potential solution to address this problem. PLS implements techniques from the physical layer of the protocol stack without incurring additional encryption overhead (along with the associated energy consumption and computational power) at the application layer. It relies on signal processing, channel coding, and other physical layer techniques. For example, with steganography (a PLS technique), the signal is hidden within noise as far as an eavesdropper is concerned (Bash et al., 2015), while the destination recovers the signal without relying on computationally intensive decryption methods.

A common PLS approach to achieve the secure communications goal in small IoT networks (like BAN) is to rely on channel-aware encryption or bit flipping. For longer ranges, it is common to rely on cooperative relaying (Chen et al., 2014; Rodriguez et al., 2015), where certain relays are used to relay the signal from source to destination, while others act as jammers to prevent the eavesdropper from detecting the message. This often requires the use of antenna beamforming techniques, in order to avoid significant leakage of the signal in the direction of the eavesdropper (Chen et al., 2014; Chen et al., 2015). Additional overhead is incurred to determine the set of cooperative nodes acting as relays and the set of nodes acting as jammers, while selecting the suitable transmit power for each node.

When larger and more sophisticated devices are available, secure and reliable communications can be implemented with massive multiple-input multiple-output (MIMO) technologies (Yaacoub, 2016). In fact, when an infrastructure is present, simultaneous transmission (to the destination) and jamming (to the eavesdropper) can be performed without resorting to relays, by using concepts of massive MIMO systems at the source and/or destination. Massive MIMO deployments are becoming practically feasible due to millimeter wave communications that are being investigated for 5G deployments (Gao et al., 2015). This would allow the placement of a large number of antennas in a relatively small area. A particular antenna disposition to form cylindrical antenna arrays for joint transmission and jamming in military communications was proposed in (Yaacoub, 2016). It enjoys circular symmetry while providing high antenna gain in desired directions, with limited leakage in other directions.

Thus, different security solutions can be used at the Section (5-10 soldiers), Platoon (3-5 Sections), Company (3-5 Platoons), or Battalion (3-5 Companies) levels. This paper focuses on reducing energy consumption and enhancing the efficiency of military ad-hoc network formation and communication when sensors and low power devices are involved. In addition, secure and reliable communications with the advances in massive MIMO technologies are investigated when larger and more sophisticated devices are available. Physical layer security techniques are studied, in addition to, and sometimes in combination with, lightweight encryption methods.