Conventional designs of OFDM-based cognitive radio (CR) systems mainly focus on the system throughput. Since nowdays the energy efficiency (EE) of wireless systems becomes more and more important, this paper intends to improve the system throughput for unit-energy consumption in OFDM-based CR systems. The energy-efficient power allocation problem of OFDM-based CR systems is studied under the total power constraint, the interference power constraint and the rate constraint. Since the energy-efficient power allocation problem is non-convex, in order to find the optimal power allocation at each subcarrier, we propose a novel method named water-filling factors aided search (WFAS) to solve the EE optimization problems with multiple constraints. The global ε-optimality and the convergence of the proposed method are presented. The numerical results confirm the theoretical conclusions. The comparisons of the energy-efficient power allocation and the spectrum-efficient power allocation in OFDM-based CR systems are given under different constraints via numerical simulations as well.

This study considers the immense problem of sum rate maximisation in K-user multiple-input multiple-output interference channels (IFC). In order to address this problem, the authors propose an iterative Taylor approximation algorithm to find the optimal transmit covariance matrices, where each user maximises the sum rate function through Taylor expansion. Owing to the distributed nature of the ITA algorithm, Taylor terms can be treated as prices, where each user collects current prices from the others to perform update. This process further leads us modelling it as a more general concave n-person game to analyse the existence and uniqueness of the Nash equilibrium (NE). Then an iterative covariance-price updating (ICP) algorithm is proposed for flexible updating and establishing a convergence process to obtain the optimal solution under NE uniqueness conditions. Simulation results show that both algorithms come with similar performance and both significantly outperform the conventional iterative waterfilling algorithm.

Radio frequency identification (RFID)-based sensor systems are emerging as a new generation of wireless sensor networks by inherently integrating identification, sensing, communications and computation capabilities. Security and privacy are critical issues in dealing with a large amount of sensed data. In the study, the authors propose a distributed key array authentication protocol (KAAP) that provides classified security protection. KAAP is synthetically analysed in three aspects: logic, security and performance. The logic analysis includes messages formalisation, initial assumptions and anticipant goals based on GNY Logic formal method to verify the design correctness of the protocol. The security analysis with respect to confidentiality, integrity, authentication, anonymity and availability is performed via the simulated attacks, which involves supposing the attacker's identity, simulating the attacker's authentication process and creating compromised conditions. Such analysis ensures that the protocol has an ability to resist both external attacks (spoofing, replay, tracking and Denial of Service) and internal forgery attacks. Additionally, the performance is evaluated and compared with other related protocols to show that KAAP can improve the reliability and efficiency of sensor systems with insignificantly increased complexity. The result indicates that the protocol is reliable and scalable in advanced RFID-based sensor systems.