Search

Orthogonal frequency division multiplexing (OFDM) signals are widely employed in most wireless comm- unication systems, and the problem of spectrum sensing for OFDM signals has been extensively investigated. However, most of the available research reports on spectrum sensing for OFDM signals ignore the existence of pilots, which are usually inserted in the OFDM signals for synchronization, channel estimation and control information delivery. To sense weak OFDM signals with periodic pilots reliably in cognitive radio networks, we propose two likelihood ratio test (LRT) detectors for the scenarios with and without time synchronization information. In the ideal scenario that an unlicensed secondary user is perfectly synchronized with a licensed primary user, we propose a near-optimal LRT (NOLRT) detector, which constructs the test statistic by correlating the locally generated pilots with the received OFDM signal samples. Since the NOLRT detector relies on perfect time synchronization that is difficult to realize in practice, we further propose a more practical buffer-aided LRT (BALRT) detector that does not require time synchronization. To formulate the test statistic, the BALRT detector stores the averages of historical OFDM signal samples in a buffer and then correlates them with the currently received OFDM signal samples. Theoretical closed-form expressions for the probability of detection and probability of false alarm are derived for the proposed detectors. To alleviate the negative effects of noise uncertainty in low SNR scenarios, we also propose an effective method based on noise power estimation. Monte Carlo simulation results match with theoretical results very well and show that the NOLRT detector outperforms the counterparts significantly in additive white Gaussian noise (AWGN) channel with perfect time synchronization, while the BALRT detector outperforms the counterparts noticeably in multipath propagation environments without time synchronization. [ABSTRACT FROM AUTHOR]

In the hybrid communication scenario of the Heterogeneous Cloud Radio Access Network and Device-to-Device in 5G, spectrum efficiency promotion and the interference controlling caused by spectrum reuse are still challenges. In this article, a novel resource management method, consisting of power and channel allocation, is proposed to solve this problem. An optimization model to maximum the system throughput and spectrum efficiency of the system, which is constrained by Signal to Interference plus Noise Ratio requirements of all users in diverse layers, is established. To solve the non-convex mixed integer nonlinear optimization problem, the optimization model is decomposed into two sub-problems, which are all solvable quasi-convex power allocation and non-convex channel allocation. The first step is to solve a power allocation problem based on solid geometric programming with the vertex search method. Then, a channel allocation constructed by three-dimensional hypergraph matching is established, and the best result of this problem is obtained by a heuristic greed algorithm based on the bipartite conflict graph and µ -claw search. Finally, the simulation results show that the proposed scheme improves the throughput performance at least 6% over other algorithms. [ABSTRACT FROM AUTHOR]

In power-domain nonorthogonal multiple access, passive eavesdropping may appear among the users by successive interference cancellation. Furthermore, the external eavesdropper may do active eavesdropping (i.e., passive eavesdropping plus jamming) by a full-duplex technique. To combat this hybrid eavesdropping, we propose a novel physical layer security scheme. The main contributions in this scheme are as follows: 1) A time constellation is set up by the concept of timing channel, where each point contains several symbols and the intervals between them, exploits these intervals to carry privacy information, and can represent different values (i.e., multiple mapping rules are introduced). Based on this time constellation, different mapping rules are allocated to different users. 2) The beginning symbols of time constellation points are devised to take the information of mapping rule allocation. Moreover, an extra symbol transformation method for each beginning symbol is designed. According to these contributions, the advantages of our scheme are evident: 1) Each user only needs to judge the intervals and extract privacy information from them. It is predicted that the impact of jamming on this operation should be lower than that on directly using the modulated symbols to carry and transfer privacy information. In other words, the mistakes of privacy information acquisition with the time constellation should be less than that with the modulated symbols. 2) The mapping rule allocation and symbol transformation methods generate multiple possible values for one time constellation point. These possibilities bring the difficulty of determining the actual value represented by each point. Therefore, passive eavesdropping can be effectively suppressed. [ABSTRACT FROM AUTHOR]

We propose a novel physical layer scheme for improving multiple users' information security (e.g., vehicle devises and mobile terminals) in the next-generation communication systems. Owing to the limitation of service node's antennas, not all users can be concurrently served. Therefore, the node only provides services for some chosen users in a certain time. On the basis of this premise, in our scheme, the access status of each user is involved. With the information of access status, the node devises transmitters to increase the served users' transmission rates and decrease the possibility of each idle user's signal interception. Furthermore, the node executes active jamming according to the realistic situation of this multi-user network for confusing the idle users/potential eavesdroppers. In addition, with the gradually mature applications of full-duplex techniques for the terminals, each user can also implement active jamming in the full-duplex mode to interfere with the potential eavesdroppers. It is seen that our scheme is linear without iteration, and performance analysis and simulation results illustrate that it is feasible for multi-user security enhancement. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The coexistence of a macrocell and a number of femtocells often leads to a two-tier heterogeneous network, where the co-tier interference (CotIN) and cross-tier interference (CrotIN) both degrade users' quality of service. In order to suppress these interferences, we propose a precoding scheme in a heterogeneous network with cooperative femtocells, called CotIN elimination and CrotIN suppression with precoding criterion selection (ESPS) scheme. In this scheme, we first eliminate the CotINs of each user by applying the QR decomposition to channel matrix. Then the CrotINs of macrocell users and femtocell users are suppressed via the macrocell base station (MBS) and femtocell access points (FAPs) with precoding criterion selection, respectively. By using the ESPS scheme, spatial resources can be efficiently exploited. In addition, our ESPS scheme requires little information exchange between MBS and FAPs without iteration and thus significantly reduces the implementation complexity. Furthermore, the robustness is increased through introducing the information of channel uncertainty into the ESPS when channel estimation or quantization error exists. The performance analysis for the ESPS demonstrates that the ESPS is practical in the heterogeneous networks. Finally, simulation results show that the ESPS can decrease users' bit error rates and increase their transmission rates. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Doping is widely reported as an efficient strategy to enhance the performance of graphitic carbon nitride (g-CN). In the study of element-doped g-CN, the characterization of doped elements is an indispensable requirement, as well as a huge challenge. In this review, we summarize some useful characterization methods which can confirm the existence and chemical states of doped elements. The advantages and shortcomings of these characterization methods are discussed in detail. Various applications of element-doped g-CN and the function of doped elements are also introduced. Overall, this review article aims to provide helpful information for the research of element-doped g-CN., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)