In this paper, we analyze the secrecy and throughput of multiple-input single-output (MISO) energy harvesting (EH) Internet of Things (IoT) systems, in which a multi-antenna base station (BS) transmits signals to IoT devices (IoTDs) with the help of relays. Specifically, the communication process is separated into two phases. In the first phase, the BS applies transmit antenna selection (TAS) to broadcast the signal to the relays and IoTDs by using non-orthogonal multiple access (NOMA). Here, the relays use power-splitting-based relaying (PSR) for EH and information processing. In the second phase, the selected relay employs the amplify-and-forward (AF) technique to forward the received signal to the IoTDs using NOMA. The information transmitted from the BS to the IoTD risks leakage by the relay, which is able to act as an eavesdropper (EAV) (i.e., an untrusted relay). To analyze the secrecy performance, we investigate three schemes: random-BS-best-relay (RBBR), best-BS-random-relay (BBRR), and best-BS-best-relay (BBBR). The physical layer secrecy (PLS) performance is characterized by deriving closed-form expressions of secrecy outage probability (SOP) for the IoTDs. A BS transmit power optimization algorithm is also proposed to achieve the best secrecy performance. Based on this, we then evaluate the system performance of the considered system, i.e., the outage probability and throughput. In addition, the impacts of the EH time, the power-splitting ratio, the numbers of BS antennas, and the numbers of untrusted relays on the SOP and throughput are investigated. The Monte Carlo approach is applied to verify our analytical results. Finally, the numerical examples indicate that the system performance of BBBR is greater than that of RBBR and BBRR.

Introduction

The Internet of Things (IoT) has attracted the attention of many researchers worldwide [1]–[۳]; the main drive behind the future IoT relates to smart sensor technologies, including in farm monitoring, vehicular tracking, healthcare, and industrial environments [4]–[۶]. Although the term IoT has been around for almost a decade, the corresponding technologies and protocols, such as massive connectivity, energy constraints, scalability and reliability limitations, and security, are still open research issues [7]–[۹]. An important problem caused by the usage of massive IoT devices (IoTDs) is spectrum scarcity [5]. The nonorthogonal multiple access (NOMA) technique has been used as a promising solution to overcome this drawback. This is because NOMA can increase the connectivity and improve spectrum utilization in IoT systems [10]. For example, E. Hossain et al. investigated the system on a large scale using NOMA and concluded that NOMA not only improves spectral efficiency but also increases power efficiency [10]. I. Khan et al. proved that NOMA is a promising approach for future mobile Internet and IoT applications, which will require handling enormous increases in data traffic, massive connectivity, and low latency [9].