In this paper, we focus on technologies that are suitable for low power wide area(LPWA) Internet of Things(IoT) and propose a performance improvement methods. Two important issues in LPWA IoT systems are massive connectivity and low power consumption.
The first study deals with improving the connectivity of LoRa (Long Range). Among the various LPWA IoT technologies, LoRa designed for the unlicensed band uses the CSS (Chirp Spread Spectrum) modulation in the PHY (Physical) layer. As the spreading factor of the CSS increases, the probability that a packet will be successfully arrived and processed in a gateway increases, realizing the long distance communication. However, a packet air time is also prolonged. Thus, it is not a good strategy where all sensors transmit with the highest spreading factor only for packet stability for the entire system’s profit. In this paper, we propose a sub-optimal spreading factor allocation method for each transmit node according to the distance from the receiving gateway for maximizing the connectivity of the whole system. Through the numerical analysis and simulation, we confirmed that the proposed method has advantages over the conventional system.
In the second study, for the shake of extending the communication distance and reducing the power consumption of nodes, we propose the introduction of untrusted relays with the help of energy harvesting. The relay can help reduce the transmit power consumption of the sensor nodes since nodes can transmit their packet to the nearest relay other than the gateway. Untrusted relays are those relays that want to help in transferring node’s packets but are not allowed to know the information. Under heterogeneous networks, devices that do not belong to the IoT network can be candidates for untrusted relays. In order to maintain information security from the relay, the gateway can emit a jamming signal, which is called
physical layer security communication. The relay can use the jamming signal and the received signal of the sensor nodes for energy harvesting, which contributes to reduce the power consumption of the relay. Reception of the jamming signal from the gateway and reception of packets from the sensor node are performed in the same phase, but signal transmission from the relay to the gateway is performed on the other phase. Since there is a time difference between the two phases, when the gateway transmits the jamming signal and when the jamming signal is removed from the received signal in a gateway, a residual jamming signal due to a time-varying channel exists. We propose an optimal security communication method in terms of the ergodic secrecy rate and the outage probability through the derivation of those expressions and verify it through numerical simulations.