# Multi-Antenna Jammer-Assisted Secure Short Packet Communications in IoT Networks
## 1 Introduction  
The Internet of Things (IoT), which is expected to connect various surrounding physical nodes such as different kinds of controllers and sensors, serves as a crucial architecture for e-health, home automation, smart cities, environmental monitoring, intelligent transportation, etc. [1; 2; 3]. Note that enormous amounts of confidential and sensitive information, e.g., financial files, trade secrets, and personal privacy, are exchanged via wireless channels in the IoT networks. For example, e-health contains personal confidential information such as physiological information and real-time location. If this private information is obtained by illegal eavesdroppers, it will result in very serious consequences. Therefore, privacy and security protection are a fundamental requirement in the design of IoT networks [4; 5]. Conventionally, security for IoT is addressed by cryptographic techniques. The encryption-based security techniques have inherent high complexity in secret key generation, distribution, and management, which makes it difficult to apply for IoT networks with many resource-constrained sensors and actuators [6; 7; 8]. Compared with cryptographic techniques, physical layer security utilizes the inherent randomness of the wireless medium, such as fading, noise, and interference, to guarantee secure information transmission. Since physical layer security does not need the complicated key exchange procedure and provides a low-complexity and effective security solution, it is more favorable for IoT networks [9; 10; 11; 12]. The physical layer security performance can befurther enhanced by a jamming strategy. By designing an artificial noise signal, the difference between the quality of the legitimate link and the eavesdropping link can be enlarged, which will lead to a high secrecy rate. In particular, a secure downlink IoT network was considered in [13], where the power allocation between the legitimate signal and the artificial noise signal and the number of transmit antennas were jointly optimized to maximize the network secrecy throughput. A cooperative jammer in [14] sends jamming signals to confuse the eavesdroppers for downlink transmission in IoT networks. The artificial noise sent by the IoT devices was proposed in [15] for secrecy performance enhancement and the sum secrecy rate was maximized, subject to the transmit power constraint. The authors in [16] derived the secrecy outage probability of multihop IoT networks with a cooperative jamming scheme. The above studies on IoT networks in the context of physical layer security assumed that the coding blocklength is large enough to achieve the secrecy capacity. However, the primary feature of IoT networks is the use of short packet transmissions to reduce communication latency. From [2; 5; 17; 18], we know that the packet length in IoT applications is only a few hundreds bits, e.g., 10-300 bytes of factory automation. In this case, perfect decoding cannot be achieved even when the data rate is below the Shannon capacity. Therefore, the physical layer security based on classic infinite blocklength design cannot be directly adopted to the IoT networks with short packet transmissions. The authors in [19] evaluated the maximal achievable secrecy rate for general wiretap channels under a given information leakage, error probability, and blocklength. Based on this achievable secrecy rate, [PERSON] [20] derived the secrecy throughput of an IoT network in the presence of a multi-antenna eavesdropper. Later, the work in [20] was extended to a wiretap channel with multiple eavesdroppers [21], multiuser multiple-input-multiple-output (MIMO) networks [22], and cognitive IoT networks with a multi-antenna relay [23]. Additionally, [PERSON] [24] considered the relationship between secrecy throughput and secrecy coding under finite blocklength, and provided the optimal code rates and blocklength in terms of maximizing the secrecy throughput.