Along with enhanced mobile broadband and ultrareliable low latency communication, massive connectivity has been one of the key requirements for enabling technologies of 5G. For IoT, low power consumption and wide area coverage for end devices (ED) are important figures of merit, for which LoRa, SigFox and Narrow Band-IoT are dominant technologies. In this letter, we analyze LoRa systems for increasing average system packet success probability (PSP) under unslotted ALOHA random access protocol. The lower bound for average system PSP is derived when many contending EDs are transmitting by stochastic geometry. And it is shown that the average system PSP can be maximized by properly allocating an SF to each traffic, which also maximizes connectivity of EDs. We formulate an optimization problem for maximizing the average system PSP to propose a sub-optimal SF allocation scheme to each traffic. Analysis on PSP is validated through simulations, and comparison with existing schemes reveals that our proposed scheme achieves the highest PSP and so the maximum connectivity.

We investigate the effect of outdated channel state information (CSI) on the secrecy performance of wireless-powered untrusted relay networks, in which the relay is a potential eavesdropper. To keep the information secret from this untrusted relay, the destination sends a jamming signal to the relay when the source transmits an information signal. At the same time, the relay harvests energy from the radio-frequency power, and forwards the received signals to the destination using this harvested energy. To determine the proportions of energy harvesting and information processing, the relay makes use of relaying based on power splitting or time switching policy. Although the destination tries to remove the jamming signal from the relaying signal, it cannot cancel out the jamming signal perfectly due to imperfect channel reciprocity caused by the outdated CSI; this residual jamming signal therefore has a negative impact on secrecy performance. In this scenario, we derive the closed-form expressions for the outage probability and average secrecy rate, and find the jamming power ratio, power splitting ratio, and time switching ratio to optimize these secrecy performance metrics. The numerical results demonstrate the accuracy of our analysis, and show that the proposed secure relaying protocols achieve a near-optimal secrecy performance, as well as outperforming the conventional scheme without the jamming power control.

To provide reliable connectivity in recent and future wireless communication systems, it is necessary to deploy several relay nodes. Further, a full-duplex (FD) technique has been in the spotlight since it can significantly improve spectral efficiency, and thus recent studies in relaying networks have considered FD relays. In relaying networks, confidentiality between the source and destination nodes from the relay node should be carefully kept since the relay node cannot be fully trusted, so called untrusted relay node. To this end, in this paper, we consider physical-layer security taking into account an untrusted FD relay node. We investigate a secure relaying protocol against the untrusted relay node where the destination generates artificial noise to prevent the untrusted relay from decoding the source information. We derive the analytical expression of the lower bound of the ergodic secrecy rate ( $\bar {R}$ ). We find two main factors affecting the secrecy performance: residual self-interference (RSI) at the FD-available nodes (i.e., relay and destination), and outdated channel state information (CSI) at the destination. Thereafter, we evaluate their effects on $\bar {R}$ and suggest the algorithm to find the sub-optimal artificial noise power level at the destination for maximizing $\bar {R}$ . Through simulations, we have verified our mathematical derivation and shown that our secure relaying protocol can achieve near-optimal secrecy performance. Numerical results imply that the artificial noise power level should be carefully considered when the channel is severely outdated and RSI is irresistible.

In this paper, we consider secure communication via a wireless-powered untrusted relay with outdated channel state information (CSI). The low security clearance of the relay means that both the source and destination wish to keep information confidential from it. To prevent the relay from decoding the source information, the destination emits a jamming signal to the relay during data transmission by the source. The wireless-powered relay uses both the data and the jamming signals as valuable sources of energy for the amplify-and-forward relaying process. In addition, it can adopt either a power splitting or a time switching policy to determine the proportions of energy harvesting and information processing. However, when the CSI of the relay-to-destination link is outdated, the destination cannot remove the jamming signal completely from the relaying signal due to imperfect channel reciprocity. To evaluate the effect of outdated CSI on the secrecy performance, we derive the analytical expressions of two performance metrics, outage probability ( $P_{out}$ ) for delay-limited transmission and ergodic secrecy rate (R) for delay-tolerant transmission. The results of the analysis also allow us to find the jamming power level and energy harvesting ratio to optimize Pout and R, respectively. Numerical simulations are used to verify the precision of our analysis, and confirm that the proposed secure relaying protocols accomplish near-optimal secrecy performances. Our results indicate that there is a need to reduce the jamming power level to ensure secure communication when the channel is severely outdated, while outdated CSI has a relatively low impact on the energy harvesting ratio.

In this paper, we examine Internet of Things (IoT) systems related to smart cities, smart factories, connected cars, etc. To support such systems in a wide area with low power consumption, energy harvesting technology utilizing wireless charging infrastructure is necessary for the longevity of networks. Considering that the position and amount of energy charged for each device could be unbalanced according to the distribution of nodes and energy sources, maximizing the minimum throughput among all nodes has become an NP-hard challenging issue. To overcome this challenge, we propose a machine learning based relaying topology algorithm with a novel backward-pass rate assessment method to present proper learning direction and an iterative balancing time slot allocation algorithm which can utilize a node with sufficient energy as the relay. To validate our proposed scheme, we conducted simulations on our established system model; thus, we confirm that the proposed scheme is stable and superior to conventional schemes.

The role of various internet-of-things (IoT) devices responsible for data collection and reporting becomes more important in the era of Industry 4.0. Due to the various advantages (e.g., wide coverage, robust security, etc.), the cellular networks have been continuously evolved to accommodate IoT scenario. In IoT scenario, connection establishment is essential and primary for enabling IoT devices to communicate with centralized unit (e.g., base station (BS)). This connection establishment procedure in cellular networks, random access procedure, is generally operated in a contention-based manner. So, it is vulnerable to simultaneous connection requests from multiple IoT devices to the BS, which becomes worse as the contention participants increase. In this article, we newly propose a resource-efficient parallelized random access (RePRA) procedure for resource-efficiently ensuring reliable connection establishment in cellular-based massive IoT networks. Key features of our proposed technique are twofold: (1) Each IoT device simultaneously performs multiple RA procedures in parallel to improve connection establishment success probability, and (2) the BS handles excessive use of radio resources based on newly proposed two types of redundancy elimination mechanisms. Through extensive simulations, we evaluate the performance of our proposed technique in terms of connection establishment success probability and resource efficiency under various combinations of control parameters. Consequently, we verify the feasibility of our proposed technique for reliably and radio-efficiently supporting a large number of IoT devices.