In this paper, we investigate an energy-harvesting cooperative communication network, which comprises of a source, a destination, and multiple decode-and-forward (DF) relays in the presence of multiple passive eavesdropper (Es). Es attempt to intercept confidential information transmissions from the source to destination via DF relays. In this network, all the DF relays harvest energy from radio-frequency (RF) signals of a source through time-switching receivers. In order to improve the physical layer security of energy-harvesting cooperative communication networks, we propose a best relay selection (BRS) scheme, where the "best" relay is chosen to assist the source-destination transmission. For the purpose of comparison, we consider the classic direct transmission (DT) and equal relay selection (ERS) as benchmark schemes. We derive the exact closed-form expressions of outage probability (OP) and intercept probability (IP) for the ERS and BRS schemes over Rayleigh fading channels. Besides, the security-reliability tradeoff (SRT) is analyzed as a metric to evaluate the tradeoff performance of the proposed BRS scheme. Numerical results show that the SRT of the BRS scheme consistently outperforms that of the ERS scheme, which demonstrates the advantage of our proposed scheme against eavesdroppers. Besides, it is verified that total error rate (TER) defined as the sum of OP and IP can be minimized for both the ERS and BRS schemes through changing the time allocation factor between information transmission and energy harvesting phases. Moreover, there is a best energy conversation efficiency to obtain a maximal SRT value of the ERS and BRS schemes. In addition, as the number of DF relays increases, the SRT of BRS scheme improves notably, while that of ERS scheme remains unchanged. And as the number of Es increases, the SRT of both the ERS and BRS schemes become worse.

The authors investigate the security performance of the cell-free massive multiple-input multiple-output (MIMO) system over spatially correlated Rayleigh fading channels in the presence of an active pilot spoofing attack. There are a large number of distributed access points (APs) simultaneously serving a smaller number of users under the cell-free massive MIMO system, which is a promising technology in the future wireless communication. We focus on the downlink secure transmission employing the minimum mean square error (MMSE) estimator and performing the distributed conjugate beamforming technique. Generalized closed-form expressions of per-user achievable rate are derived. Also, we carry out the analytical results of the upper bound on the information rate leaked into the active eavesdropper. Numerical results reveal the ergodic secrecy rate (ESR) is influenced significantly by the active eavesdropper and the condition of channel fading correlation.

The faster‐than‐Nyquist (FTN) signalling is a bandwidth‐efficient technology which has drawn attention in the bandwidth‐starved world. However, inter‐symbol interference is introduced by transmitting signals at a higher signalling rate than allowed by the Nyquist criterion. Decision feedback equalisation (DFE) cancellation is an efficient signal detection scheme for FTN‐based communication system. However, since the DFE interference cancellation is executed by overall matrix computation, the required memory size is very large. In order to reduce the complexity of the interference cancellation process, a novel simple interference cancellation is proposed. Simulation results show that the scheme can perform better than DFE, and its complexity is very low because of the absence of QR decomposition.

This paper studies the secrecy performance of the cell-free massive multi-input multi-output (MIMO) system, where access points (APs) are equipped with a mixed analog-to-digital converter (ADC) and digital-to-analog converter (DAC) architecture. The distributed antenna structure is prone to multiple eavesdropping network threats. Specifically, legitimate transmissions can be affected by non-colluding and colluding eavesdroppers. Based on the above considerations, we determine the expressions of the sum achievable secrecy rate and secrecy energy efficiency (SEE) with applications of the additive quantization noise model and the conjugate performing precoding method. These provide the composite measures to analyze the effects of the mixed-ADC/DAC architecture on secrecy performance. We also propose a SEE-maximization power control (SEEMPC) scheme, which can be solved by the sequential convex approximation algorithm. In addition to verifying the effectiveness of the proposed SEEMPC algorithm, numerical simulations are carried out to evaluate the impact of some key design parameters, including the number of APs, quantization bit of coarse ADCs/DACs, ratio of ideal ADCs/DACs, and power control strategy.

The transmitter of multicarrier faster-than-Nyquist Signaling is a promising direction. In this paper, the bit and power allocation problem is discussed for the multicarrier FTN environment, with the aim to maximum mutual information while guarantee the practicality. An improved spectral shaping scheme with integer bit constraint is proposed for the transmitter. It is based on the modified iterative water-filling and greedy algorithm revised for our scenario. Correspondingly, there are two processes for the scheme called iterative power allocation and discrete bit replenishment. With the benefit of improved method, integer bits can be ensured for each subcarrier at the cost of a little mutual information loss. This is confirmed by simulation and numerical results.