We propose a cross-layer optimization algorithm to minimize the average Age of Information (AoI) and at the same time maximizing the communication sum rate in a dual-functional reconfigurable intelligent surface (DF-RIS)-assisted Internet of Things (IoT) network, where the DF-RIS performs simultaneous information reflection and jamming to defend against an eavesdropper (Eve). Specifically, the transmission scheduling and resource allocation are adaptively co-designed to achieve secure low-latency communication in a more practical and detailed channel environment. For the case with perfect channel state information (CSI) of Eve and IoT devices, the transmission scheduling is transformed as a Markov decision process and optimized to find the optimal packet size by deep$\boldsymbol {Q}$-network. The resource allocation algorithm is performed to optimize subcarrier channels, transmit power, the scheduling matrix of the DF-RIS, and discrete phase shifts of the DF-RIS based on the alternating optimization method. For the imperfect CSI leads to an infinite number of inequality constraints, a robust resource allocation algorithm based on the transformed${\mathcal{S}}$-procedure is proposed, which can guarantee the security and low-latency of messages transmission. Simulation results show that the proposed algorithm can reduce the average AoI by at least 15.5% and increase communication sum rate by 9.2% than the existing baseline algorithms.

Unmanned aerial vehicle (UAV) communication system assisted by intelligent reflecting surface (IRS) has promising applications in future wireless communication. Considering the realistic high-efficient IRS-UAVs transmission system in the three-dimensional (3D) space, we propose a joint beamforming and UAVs’ trajectory optimization algorithm to maximize the secrecy energy efficiency (SEE) of the system, which is constrained by the mobility of UAVs and the number of access users. In the proposed algorithm, the beamforming vector, multi-IRS diagonal phase shift matrix, and UAVs’ trajectory are optimized alternately using block coordinate descent (BCD). We not only derive the optimal closed-form expression of the multi-IRS diagonal phase shift matrix, but also solve the 3D trajectory optimization of UAVs under the given beamforming matrix. Simulation results show the effectiveness of the proposed algorithm and the performance improvement for IRS-UAVs communication.

Due to the existence of imaginary interference in filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) systems, channel estimation technology is a difficult task. In this paper, an innovative preamble structure is suggested to increase the channel estimation accuracy. The suggested preamble structure employs five FBMC/OQAM symbols and every three adjacent subcarriers constitute a preamble sub-block (PSB). According to interference factors, the special pilots are designed for PSB, which can increase the central point’s pseudo-pilot power and therefore improve channel estimation performance. Experimental results testify that the suggested approach works better than its conventional counterparts.

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) has been studied as an alternative waveform for post-orthogonal frequency division multiplexing (OFDM) wireless communications. However, due to the intrinsic imaginary inter-carrier/inter-symbol interference of FBMC/OQAM signals, accurate channel estimation and equalization remain open issues for FBMC/OQAM systems in dispersive channels. In this article, two new signal models of the analysis filter bank (AFB) output are first derived in the absence of any channel assumption and in the presence of a relaxed channel assumption, respectively. Based on the new-built models, initial channel estimation with least squares (LS) (IniCE-LS) is presented and its output is used as the initial condition of recursive weighted LS (RWLS) estimation; moreover, non-iterative and iterative channel equalization combined with RWLS channel estimation (i.e., N-ICE-RWLS and ICE-RWLS) are proposed by using the real-time reconstructed FBMC/OQAM data symbols as pilots. The proposed N-ICE-RWLS and ICE-RWLS schemes not only allow to improve channel estimation and equalization performance, but also achieve high transmission efficiency. Considering the requirements of practical application, we further develop the low-complexity implementation methods of N-ICE-RWLS and ICE-RWLS. All these schemes are validated by analyzing the channel estimation accuracy and bit error rate (BER) over various frequency selective channels.

This paper considers a cooperative device-to-device (D2D) system with non-orthogonal multiple access (NOMA). We assume that the base station (BS) can simultaneously communicate with all users to satisfy the full information transmission requirement. In order to characterize the impact of the weak channel and different decoding schemes, two novel decoding strategies are introduced: single signal decoding scheme and maximum ratio combining (MRC) decoding scheme, respectively. With the single signal decoding scheme, the users decode the received signals immediately after the receptions from the BS. On the other hand, the MRC decoding scheme jointly decodes the received signals via MRC until the corresponding phase comes and the users jointly decode the received signals by employing MRC. Considering Rayleigh fading channels, the ergodic sum-rate (SR), outage probability and outage capacity of the proposed D2D-NOMA system are analyzed. Moreover, approximate expressions for the ergodic SR are also provided with a negligible performance loss. Numerical results demonstrate that the ergodic SR and outage probability of the proposed D2D-NOMA scheme overwhelm that of the conventional NOMA schemes. Furthermore, it is also revealed that the system performance including the ergodic SR and outage probability are limited by the weak channel for both the single signal decoding scheme and conventional NOMA schemes, but not for the MRC decoding scheme.

With the widespread use of Light Emitting Diode (LED) in vehicle lights and traffic lights, LED-based Visible Light Communication (VLC) has been regarded as an alternative candidate for data transmission for connected vehicles. In order to enhance the performance of non-uniform dynamic VLC, where the luminous intensity distribution is not uniform and the received optical power changes frequently with vehicle motion, we propose to integrate receiver spatial and angular diversity with VLC for connected vehicles. We take the position and the posture of the receiver into consideration since the performance of VLC varies significantly according to the spatial relationship between the receiver and the transmitter. In this paper, firstly, we present a Vehicle-to-Vehicle VLC (V2V-VLC) model and a Vehicle-to-Infrastructure VLC (V2I-VLC) model according to the spatial relationship model. Then, we present a layout scheme of receivers. In our scheme, we can not only achieve diversity gains but also obtain a forward total 180 degrees Field of View (FOV) and a backward total 180 degrees FOV respectively. Finally, we simulate and analyze the Bit Error Rate (BER) in both turning and lane change scenarios. Simulation results show that receiver spatial and angular diversity can significantly decrease the BER of VLC equipped vehicles.

Energy efficiency (EE) plays an indispensable role in the battery constrained systems. In this study, the EE optimisation problem for asymmetric analogue network coding (ANC) protocol is analysed in a two‐way relaying network, where the two sources have different transmission rate requirements. A generalised power consumption model is applied consisting of not only the transmit power and fixed circuit power, but also the most general rate‐dependent dynamic circuit power, with which a power allocation problem is formulated to achieve the maximal EE constrained by the rate requirements as well as the total transmit power budget. By exploiting the two‐layer optimisation method and non‐linear fractional programming theorem, the original EE maximisation problem is solved in an efficient way. Specially, when the rate‐dependent circuit power is modelled as the linear function of transmission rate, a closed‐form solution is obtained. Through simulations, the proposed power allocation algorithm is verified and the impacts of some system parameters such as relay location, transmit power and transmission rate asymmetry are also provided to gain useful insights, which facilitates the energy‐efficient design for the practical asymmetric ANC protocol.

Device-to-device (D2D) communication underlaying cellular networks improves spectral efficiency but causes interference to cellular users (CUs). Such interference can be utilized to help CUs prevent wiretapping. This paper aims to achieve the twofold goal of security provisioning for CUs and spectral efficiency enhancement for D2D links by optimizing the resource sharing of CUs and D2D links. We first provide the necessary and sufficient conditions for the accessibility of a CU channel by a D2D link. Then, we derive the jointly optimal resource sharing strategy, including the closed-form power control and the optimal channel pairing of CUs and D2D links. Numerical results show that the proposed strategy can improve both the CUs' security and D2D spectral efficiency.

This paper investigates joint relay and antenna selection schemes for two-way amplify-and-forward (AF) relaying system, where a multiantenna source exchanges information with a single-antenna source by exploiting a group of single-antenna relays. In view of performance and complexity tradeoffs, two optimal and one suboptimal joint selection schemes are proposed and compared. In contrast to the optimal selection schemes, the suboptimal counterpart is characterized by its low overhead and implementation complexity. The performance of these selection strategies has been investigated by deriving the outage probability approximations in closed form. Moreover, the achievable diversity orders and the corresponding coding gains are derived through high signal-to-noise ratio (SNR) approximations of the outage probability. Our theoretical and simulation results show that the performance gap between the optimal and suboptimal schemes disappears for specific scenarios, which provides useful design guidelines for practical two-way relaying system.

Two opportunistic transmission schemes, i.e., joint user-relay selection with beamforming (JURS-BF) and joint user-relay selection with antenna selection (JURS-AS), are proposed and analyzed for multi-user multi-relay bidirectional relaying networks (BRNs) composed of one N-antenna base station (BS)/ access point (AP), M single-antenna users, and K single-antenna relays. In particular, by considering the rate asymmetry between the BS/AP and the users, joint user-relay selection is tackled with beamforming and antenna selection to combine the benefits of multi-antenna diversity, multi-relay diversity, and multi-user diversity. To characterize the performance of the proposed JURS-BF and JURS-AS schemes, lower and upper bounds on the system outage probability are derived in closed form under Nakagami-m fading environment, followed by the diversity, coding gain analysis, and some discussions on the relay deployment. Our analytical results are verified through simulations, and it is proved that the proposed suboptimal schemes perform closely to the traditional outage-optimal counterparts while with much lower implementation complexity, which provides valuable insights into practical BRNs.

Summary With the rapid growth of cloud computing services, there is an increasing need to expand applications across data centers. In these paradigms, common information exchange between two data centers can be assisted via one or more substrate nodes, which correspond to bidirectional relaying communication. In this paper, we investigate the performance of time division broadcast (TDBC) protocol in bidirectional cloud networks in the presence of channel estimation errors (CEEs). The tight asymptotic expressions for individual and system outage probabilities are first presented in closed‐form. It is shown that CEE causes outage probability to remain fixed even if the channel is noiseless. Then, the symbol error rate performance for TDBC protocol with M‐ary Phase Shift Keying and M‐ary quadrature amplitude modulation signals is analyzed. Simulation results validate the accuracy of our analytical results. Furthermore, comparison of signal‐to‐noise ratio gap ratio shows that TDBC protocol is less sensitive to the effect of CEE than analog network coding protocol. Copyright © 2014 John Wiley & Sons, Ltd.

In downlink multiuser multiple‐input multiple‐output (MU‐MIMO) systems, block diagonalisation (BD) is a well‐known precoding technique that eliminates inter‐user interference. The number of simultaneously supportable users with BD is limited by the number of base station transmit antennas and the number of user receive antennas. Traditional MU‐MIMO scheduling algorithms focus on sum capacity. However, these user scheduling algorithms might not be optimal with respect to energy efficiency. Here, the authors consider the energy‐efficient MU‐MIMO scheduling that maximises the energy efficiency. The brute‐force search for the optimal user set, however, is computationally prohibitive. Therefore they propose a low‐complexity user scheduling algorithm for energy‐efficient MU‐MIMO systems. They first obtain an approximation expression for the energy efficiency by utilising the upper bound of the MU‐MIMO system capacity. Then they show that the maximum energy efficiency can be achieved if the scheduling user set is selected to obtain the largest matrix volume. The numerical results show that the proposed algorithm achieves a good tradeoff between energy efficiency and computational complexity. They also consider the problem of maintaining fairness among users, and propose a simplified proportional fair (PF) scheduling algorithm.

Energy efficiency (EE) is among the main considerations in the design of modern wireless networks. In this paper, we investigate the EE enhancement for asymmetric analog network coding (ANC) protocol of a two-way relay system based on statistical channel information. A power allocation problem is formulated as the system EE maximization problem with objective function quantified by Goodbit-per-Energy (GPE). Importantly, the EE optimization problem may not be convex and can be categorized into a nonlinear fractional programming problem. Therefore, to solve the problem, a nonlinear fractional programming based algorithm is proposed and closed-form solution is obtained, providing valuable insights into practical system designs. Simulation results highlight the effect of the proposed power allocation.

Finding the capacity region of a Gaussian X network is an open problem in information theory. In this study, the authors characterise the capacity regions of 2 × 2 user multiple‐input multiple‐output Gaussian X channels for very strong interference and generally strong interference. Moreover, the sum capacity is established for noisy interference. Sufficient conditions for Gaussian X networks to have very strong interference, generally strong interference and noisy interference are derived.

In this study, the authors present a cooperative spectrum sensing scheme based on opportunistic amplify‐and‐forward relaying (denoted by CSS‐OAFR), which provides higher detection performance by selecting the best cognitive relay and is interesting in distributed cognitive radio networks. Moreover, the authors focus on the detection performance using the energy detector over independent and non‐identically distributed Nakagami‐ m fading channels. In particular, a tight closed‐form lower bound of the average missed‐detection probability is derived for the convenience of performance evaluation in practice. Finally, simulations are provided to validate the CSS‐OAFR scheme and the derived closed‐form lower bound.

This study deals with the performance analysis and power allocation of a two-way amplify-and-forward relay system with channel estimation errors. Exact closed-form expressions for outage probability and average symbol error rate (SER) are first presented. To provide more insights, their closed-form asymptotic expressions are then obtained. It is shown that the presence of channel estimation error causes outage probability and average SER maintain a fixed level even when a noiseless channel is adopted. These results are applied further to study the optimal power allocation problem for each node. From the perspective of service quality, rather than minimise the outage probability as much as possible, the goal is to use the minimum energy consumed to satisfy the traffic requirements and thereby conserve the energy resource. Furthermore, based on the optimal solutions to power allocation, the optimal relay location is investigated, which indicates that the system has higher energy efficiency with the relay located around the middle point of the two end nodes for any asymmetric traffic requirement. The simulation results verify that the derived outage probability and average SER expressions are accurate and highlight the effect of power allocation under various traffic requirements and channel estimation errors.

An iterative interference alignment (IA) algorithm for the multi-user multi-input multi-output interference channel is proposed, which optimises on the Grassmann manifold to improve the performance of conventional interference subspace (ISS) alignment algorithm without the assumption of channel reciprocity. The proposed algorithm combines the extreme eigenvalues method and the modified steepest descent method on the Grassmann manifold to minimise the distances not only between the ISS and the subspace spanned by interference, but also between the desired signal subspace and the subspace spanned by useful signal. Utilising the subspace optimisation above, both the interference and useful signal are aligned to their respective subspaces. Numerical results show that the proposed algorithm can significantly improve the sum rate of interference channel. Moreover, by properly choosing the attenuation factor of step size, the proposed algorithm can achieve an effective tradeoff between sum rate performance and convergence speed, which gives the IA scheme more design flexibility.