The related research about physical layer security suitable for IoT is summarized in this section. Then, we discuss some existing work on PLS using untrusted relays and energy harvesting entities.
There are two main categories of PLS techniques: (1) intelligent designs to keep the information secure from the eavesdroppers where no secret key is needed; and (2) generation of secret keys over public channels by exploiting the wireless communication medium [26
]. In this paper, we are more interested in the first category, which does not require error-free two-way public channels. Moreover, keyless secrecy methods are more easily extended to large-scale sensor networks.
In the downlink communication network of the IoT, the controllers transmit signals, and they could be equipped with multiple antennas and adequate energy supply. The PLS schemes such as optimal precoding, artificial noise and secure space-time coding can be applicable in the IoT, and the pros and cons of these conventional PLS techniques have been summarized in [14
]. Secrecy rate and secrecy outage probability are main metrics to evaluate the secrecy performance. In the uplink communication network of the IoT, an IoT device, such as a sensor or a surveillance camera, transmits to the controller, and the IoT device is usually resource-constrained. The channel-aware encryption (CAE) scheme proposed in [27
] is an appealing solution in sensor networks where sensors have very low data rate. In the CAE scheme, a sensor may stay dormant, report a “flipped” decision, or report its unaltered local decision at each instant. How it acts depends on where its instantaneous channel fading gain to the legal controller falls among some known thresholds. How to optimize these comparison thresholds is not discussed by the authors in [27
]. In [28
], the optimal thresholds were derived to further improve the performance. When relays are used in IoT networks with passive eavesdroppers with locations, a randomize-and-forward relay scheme has been proposed in [29
]. The authors formulated a secrecy-rate maximization problem subject to a secrecy-outage-probability constraint, and designed the optimal power allocation and codeword rate [29
]. Considering the spectrum scarcity, a Cognitive Internet of Things (CIoT) has been proposed where the IoT device acts as a secondary user and accesses the primary spectrum by using the spectrum-leasing strategy [9
]. To achieve secure transmission, the authors utilized cooperative jamming performed by an energy harvesting helper. Based on the cooperative jamming scheme, an auction framework was proposed to build an incentive mechanism for the secondary users. The channel assignment problem in time-critical IoT-based cognitive radio networks under proactive jamming attacks was considered in [30
]. Subject to delay constraints, a probabilistic spectrum assignment algorithm that aimed at minimizing the packet invalidity ratio of each cognitive radio transmission has been proposed. Since energy harvesting is an appealing and promising technology [31
], more and more papers study PLS problems with EH nodes recently. In [10
], the PLS issue of cognitive sensor radio networks (CSRNs) with an external EH eavesdropper was investigated. Underlay spectrum sharing was used in CSRNs. The sensor node acts as the secondary user, and adjusts its transmit power to guarantee the primary user’s quality-of-service (QoS). Two scenarios with different interference power constraints were studied and the closed-form analytical expressions of secrecy outage probability for both cases were derived [10
]. Authors in [33
] considered an underlay cognitive radio system, where a source in a secondary system transmitted information to a full-duplex (FD) wireless EH destination node in the presence of an eavesdropper. The harvested energy at the destination was used to send jamming signals, so that the eavesdropper’s decoding capacity is degraded. Upper and lower bounds of probability of strictly positive secrecy capacity (SPSC) have been derived in [33
]. However, these existing secure schemes are based on the assumption that the nodes in the IoT or CIoT are trusted, and they are designed to prevent interception from the eavesdroppers outside.
When untrusted relays are considered, numerous PLS schemes have been proposed based on different relaying protocols. Authors in [34
] adopted a successive amplify-and-forward (AF) relaying scheme, where the multi-antenna source transmitted to two selected nodes alternately. The inter-relay interference, which is usually regarded as detrimental, was used to jam the untrusted nodes. The authors proposed several relay selection schemes with different complexities and derived the closed-form expressions of the lower bound of secrecy outage probability in [34
]. For multiple-antenna untrusted relay systems, a joint destination-aided cooperative jamming and precoding scheme was devised to maximize the secrecy rate by jointly designing the precoding matrices for the source, relay, and destination [35
]. Authors in [23
] proposed a modulo-and-forward (MF) protocol at the relay with nested lattice encoding at the source to improve the secrecy in a dual-hop untrusted relay network. A multi-hop line network was considered in [24
], where each node received signals transmitted by its neighbors, and the leftmost node sent messages to the rightmost node. When any or all of the relay nodes can be eavesdroppers, it has been shown in [24
] that it is possible to achieve end-to-end secure and reliable communication by utilizing nested lattice codes. Kalamkar, S.S. et. al. in [32
] investigated the problem of secure cooperative communication with the help of a wireless EH untrusted node. To realize the positive secrecy rate, destination-aided cooperative jamming was used. Analytical expressions were derived for the secrecy outage probability and the ergodic secrecy rate to evaluate the secrecy performance in [32
The goal of this paper is to solve the security problem in a CIoT network where a wireless EH untrusted node is used to relay the IoT device’s information. Underlay spectrum sharing is adopted by the CIoT network to relieve the stress of spectrum scarcity. As long as the interference threshold constraint is satisfied, the IoT device and the relay can access the primary spectrum, and the capability of spectrum sensing is not required. Although the IoT device and the relay may have strict energy constraints, the controller could have adequate energy supply. Therefore, destination-aided cooperative jamming is used to provide secure transmission. Together with the source signal, this jamming signal is also used for energy harvesting at the relay.