# Utilizing Multi-Dimensional MmWave MIMO Channel Features for Location Verification
## 1 Introduction  
Millimeter-wave (mmWave) multiple-input multiple-output (MIMO) systems serve as a critical technique for 5G (fifth-generation communication) and beyond wireless communication networks. The mmWave MIMO technique can lead to a significant increase in data rates, wide bandwidth, and higher spectrum efficiency. By integrating huge antenna arrays, 5G and beyond communication systems operating from 30 to 300 GHz can support huge connections among a bulk of smart devices, contributing to various applications such as the Industrial Internet of Things (IIoT) and the Internet of Vehicles (IoV). However, mmWave MIMO communication systems may become subject to malicious attacks because of the above important applications they can support. Moreover, the mmWave wireless communication medium is open in space and thus attackers can easily access the communication network and obtain the transferred information between legitimate users and base stations. One of the main threats in mmWave MIMO systems is identity-based spoofing attack, where an attacker eavesdrops the legitimate communication links and further impersonates a legitimate transmitter to inject harmful signals into the communication network. To tackle the spoofing attacks in mmWave systems, identity authentication presents a promising solution to secure mmWave MIMO communication systems. We note that the existing authentication schemes can be roughly divided into higher layer and physical layer authentication schemes. The higher layer authentication schemes are mainly dependent on secret keys and encryption/decryption algorithms to validate transmitter identities [1; 2; 3]. Meanwhile, the physical layer authentication schemes verify transmitter identities through physical layer features [4]. It is noted that the higher layer authentication may not completely adapt to the emerging mmWave MIMO systems which include distributed, dynamic and highly heterogeneous devices. Such mmWave MIMO systems may not support the cryptographic key distribution and management, high consumption of communication and computation resources in higher layer authenticationschemes. As a complement and an enhancement of the higher layer authentication, physical layer authentication possesses several characteristics such as high security, low complexity and high compatibility [5]. Thus, in this paper we focus on physical layer authentication in mmWave MIMO systems. Existing physical layer authentication schemes mainly include channel-based authentication and hardware-impairment-based authentication. The hardware-impairment-based method authenticates the legitimacy of a transmitter identity by extracting specific non-ideal hardware parameters from received signals. In contrast, channel-based authentication utilizes the fact that wireless channels decorrelate rapidly as the source (e.g., a transmitter or user using transmitter is transmitting signals to an intended destination/receiver) changes location in an environment with rich scatterers and reflectors. In fact, channel-based authentication verifies the location of the current transmitter rather than transmitter physics identity, and hence is referred to as location verification. There exists some research works on physical layer authentication for mmWave systems [6; 7; 8; 9; 10; 11; 12; 13; 14; 15]. In [6], the authors propose a physical layer authentication scheme by using sparse mmWave channel. A power allocation technique for the precoder and the authentication tag is first developed for fingerprint-based authentication in a multi-user mmWave UAV network [7]. Multipath time delay caused by planar specular reflectors is acquired to illustrate the authenticity of user position 5G mmWave communication with planar reflectors [8]. The intrinsic physical attributes for 5G communications are analyzed for physical layer authentication in detail and the future possible directions for the exploitation of physical attributes is demonstrated in [9].