Towards D2D Physical-layer Covert Communication Methods In Hybrid Eavesdropping Cellular Networks

Recent years have witnessed the emergence of advanced wireless communication technologies such as 5G and beyond 5G mobile networks.Meanwhile,the proliferation of wireless communication devices has made the best of these booming technologies for ubiquitous data sensing and flexible information sharing across end-users.People are also more likely to share personal information,such as location information and electronic health information,through wireless terminals.Such user-centric and data-intensive services have drawn the attention of adversaries to launch severe attacks for their own benefits.Device-to-Device(D2D)communications have been one of the most important communication methods in the fifth-generation network,with advantages of high spectral efficiency,low latency,and low power consumption.The continuous improvement of D2 D communication technologies and application standards has brought more end-users configured with this communication mode,resulting in an expose of a large amount of privacy and sensitive information to the wireless environment through D2 D links.Considering the broadcast nature of wireless networks and resource-limited terminals(e.g.,power-limited nodes configured with single antenna),D2 D communication links are more vulnerable to potential eavesdropping.Particularly,eavesdropping and complex interference coexist in cellular networks.How to achieve effective security and privacy protection for D2 D links has become a crucial problem.In order to address these threats and challenges,traditional cryptography and informationtheoretic physical-layer security have received much research efforts.However,both of these approaches are devoted to protecting the contents of information,while the existence of signals can still be observed by adversaries.There are application scenarios,where not only the contents but also the existence of privacy-sensitive data should preferably be hidden.Physical-layer covert communications are trying to protect the existence of covert signals by using the characteristics of wireless channels,e.g.,multipath,noise,and interference.In hybrid eavesdropping cellular networks,the eavesdropping environment is gradually harsh with respect to legitimate users(i.e.,as the number of eavesdroppers increases,less amount of adversaries’ information will be obtained by legitimate users),and meanwhile,more complex interactions between multiple links are emerging.Existing efforts can hardly realize D2 D covert communications in the proposed networks.Furthermore,without deliberately introducing jamming nodes into the networks,we have realized D2 D covert communications by using the existing resource of the hybrid eavesdropping cellular networks,as explained in details in the follows.(1)D2D covert communications based on cooperative interference in cellular networks with a single eavesdropper: we proposed a covert communication scheme that allows D2 D devices to transmit covert signals with a low probability of detection with respect to an eavesdropper.The cellular and D2 D non-covert signals are designed as cooperation jammers to meet the covertness requirement without introducing external nodes.Power domain non-orthogonal multiple access and successive interference cancellation are used to improve covert throughput.Considering that the D2 D transmitter cannot obtain the adversary’s detection threshold,we derive a minimum error detection probability to measure the covertness.Numerical results are presented to reveal the impacts of different system parameters,including transmit power,communication rate,and noise variances,etc.,on the covert communication performance of the proposed scheme.It is shown that the covert throughput can be improved with the help of cellular and D2 D non-covert signals by adjusting the transmission rate and maximum transmit power.(2)D2D covert communications assisted by a full-duplex base station in cellular networks with multiple eavesdroppers: we proposed two D2 D covert communication schemes with the assistance of a full-duplex base station(BS).To confuse multiple eavesdroppers and reduce the complexity of cooperation between nodes comparing with the first scheme,the antennas at BS are designed to transmit independent artificial noises.According to the number of antennas at the BS,two covert communication schemes are designed.The performance of the proposed schemes is evaluated in terms of covert throughput,which is defined as the maximum achievable D2 D communication rate under the given covertness requirement.Considering the fact that only channel distribution information of the adversary links is known to the BS,we derive the average minimum error probability(AMEP)to ensure the covertness requirement.The closedform expressions of the minimum AMEP and D2 D available communication rate are derived to analyze covert throughput.It is concluded that the derivation of minimum AMEP can be simplified if the appropriate number of(i.e.,one,two,or a large number of)transmit antennas is allocated for artificial noises.The analytical results are compared with Monte-Carlo simulation results to verify the effectiveness of the proposed schemes.We also investigate the impact of system parameters,e.g.,signal power,communication rate,and residual self-interference factor,on AMEP and covert throughput.It is shown that the covert throughput can be improved by properly setting system parameters,e.g.,the number of antennas configured at BS and the maximum power allocated for artificial noise transmission,etc.(3)D2D covert communications assisted by energy nodes in cellular networks with randomly distributed eavesdroppers: we proposed a D2 D covert communication scheme for cellular networks with power beacons(i.e.,energy nodes).We divide the communications slot into two sub-slots,i.e.,wireless energy harvesting sub-slot and covert communication sub-slot.To confuse all the randomly distributed eavesdroppers,the power beacons are designed to send jamming signals in the covert communication sub-slot without the assistance of the BS.In the wireless energy harvesting sub-slot,the D2 D transmitter collects all the energy from power beacons and the cellular link to support covert communications in the next sub-slot.The proposed scheme is measured by covert throughput,which is defined as the available D2 D communication rate with several constraints,e.g.,the energy outage probability,the covert probability,and the connect outage probability of cellular link.The closed-form expressions of the proposed constraints are derived.Moreover,a feasible solution is provided to solve the covert throughput optimization problem.The analytical results are compared with Monte-Carlo simulation results to verify the effectiveness of the proposed schemes.We also investigate the impact of system parameters,e.g.,D2 D transmit power,the density of power beacons,and the number of eavesdroppers,on covertness and covert throughput.It is shown that the covert throughput can be improved by properly setting system parameters,e.g.,the density of power beacons,and the transmit power of D2D.