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The broadcast nature of wireless communications makes transmission susceptible to eavesdropping. Recently, physical-layer security has been extensively studied to secure the wireless transmissions, but the security is severely compromised when the eavesdropper has the superiority in the channel quality. Toward this issue, we in this paper propose a joint ARQ and network coding method exploiting the characteristics of both the physical layer and the link layer. Specifically, we relate the message to be transmitted in the current slot with the successfully decoded messages in previous slots, where the ARQ is employed at the legitimate side to guarantee the successful transmission and decoding. Once one or more received messages fail to be decoded at the eavesdropper, it further prevents the eavesdropper from decoding the transmissions in later slots and thus the message transmitted currently and later is secured at the legitimate side. Moreover, for the more disadvantageous case that the eavesdropper is geographically closer to the transmitter, we further propose a destination-aided network coding scheme, which secures private information no matter where the eavesdropper is. The closed-form expressions of intercept probability and loss probability of the private information are analytically presented. The simulation results are provided to confirm our theoretical findings. [ABSTRACT FROM AUTHOR]

Future vehicle-to-infrastructure communications in a multi-vehicle environment highly depend on the acquisition of wireless orthogonal frequency division multiplexed (OFDM) channels through the conventional pilot-tone based channel training protocol. However, this protocol can be easily compromised by a pilot-aware attacker spoofing/jamming/nulling those pilot tones, which have been designated for multiple vehicles. To solve this issue, we propose merging the multi-vehicle signal characteristic into the well-known cover-free coding technique for securely and retrievably coding pilot tones of multiple vehicles. In particular, multiple vehicles encode and convey their own pilot tones into diversified subcarrier activation patterns (SAPs). Then, road side unit observes the superposition of those encoded SAPs and re-designs the decoding procedure using signal independence characteristic such that those SAPs, though camouflaged by malicious signals in wireless environment, could be separated, identified, and decoded into original pilots. The protocol reliability is formulated using pilot identification error probability (IEP), and the protocol $\xi$ -accessibility is defined by the ratio of the number of accessible vehicles to the consumed OFDM symbols under the constraint of IEP less than $\xi$. We prove that a win–win relationship between reliability and accessibility always exists and also demonstrate how more number of antennas of the attacker could improve the reliability better. Finally, numerical results verify how the proposed method is enabled to support ultra-reliable ($1-\xi \geq 99.999\%$) and ultra-low-latency (up to 1.5 ms) multi-vehicle access for channel training under an Long Term Evolution (LTE) 20 MHz bandwidth. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) is becoming important in 5G, therefore, it is widely researched. Note that the users served in NOMA are often paired to prevent excessive interference. However, if the channel condition of direct downlink from the service node to one user is serious, the node may require the other user with better channel condition to relay this user’s signals, especially when the pure relay is difficult to deploy. This relay transmission is helpful for NOMA communication, but two problems should be considered: (i) how to persuade the relay user (i.e., the user with better channel condition) to expend extra resources for relay; (ii) how to suppress eavesdropping in the relay transmission, especially for the signals of indirect communication user (i.e., the user with worse channel condition). To solve these problems, we propose a novel signal-level scheme. In this scheme, on one hand, the node increases the spectral efficiency of relay user, and does the wireless power transfer to make the relay user supplement his/her energy by energy harvesting. On the other hand, a signal transformation method is designed to deal with each signal of indirect communication user. This transformation hides privacy information of indirect communication user, but does not disrupt the relay transmission. Utilizing the proposed scheme, the above problems are effectively solved by the compensation for the relay user and the signal protection for the indirect communication user. [ABSTRACT FROM AUTHOR]

Among the plenty of studies on physical layer security, multi-antenna signal processing and nodes cooperation are two widely adopted techniques. However, in practical systems there are many single-antenna devices that do not support cooperative transmission either. To guarantee the information security in this scenario, we propose a novel transmission strategy that exploits the temporal characteristics of a multipath fading channel. Specifically, we apply this strategy to a time-reversal-based communication system. We consider a joint design of signal waveform and artificial noise. Based on this approach, specific transmission schemes are designed for the cases with and without eavesdropper's channel state information (CSI), respectively. The numerical results show the superiority of our proposed schemes in terms of security enhancement even when the eavesdropper's CSI is unavailable. [ABSTRACT FROM AUTHOR]

In cognitive Ad Hoc networks (CAHN), because the contentions and mutual interferences among secondary nodes are inevitable as well as secondary nodes usually have limited power budget, spectrum efficiency and energy efficiency are critically important to the CAHN, especially for the medium access control (MAC) protocol design. Aiming at improving both spectrum and energy efficiencies, we in this paper propose a diversity technology called Channel-Aggregation Diversity (CAD), through which each node can utilize multiple channels simultaneously and efficiently allocate the upper-bounded power resource with only one data radio. Based on the proposed CAD technology, we further develop a CAD-based MAC (CAD-MAC) protocol, which enables the secondary nodes to sufficiently use available channel resources under the upper-bounded power and transmit multiple data packets in one transmission process subject to the transmission-time fairness constraint. In order to improve the performance of CAHNs, we propose two joint power-channel allocation schemes. In the first scheme, we aim at maximizing the data transmission rate. By converting the joint power-channel allocation to the Multiple-Choice Knapsack Problem, we derive the optimal allocation policy through dynamic programming. In the second scheme, our objective is to optimize the energy efficiency and we obtain the corresponding allocation policy through fractional programming. Simulation results show that our proposed CAD-MAC protocol can efficiently increase the spectrum and energy efficiencies as well as the throughput of the CAHN compared with existing protocols. Moreover, the energy efficiency of the CAHN can be further improved by adopting the energy efficiency optimization based resource allocation scheme. [ABSTRACT FROM AUTHOR]