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With 5G mobile communication systems having been commercially rolled out, research discussions on next generation mobile systems, that is, 6G, have started. On the other hand, vehicular technologies are also evolving rapidly, from connected vehicles called V2X (vehicle to everything) to autonomous vehicles to the combination of the two, that is, the networks of connected autonomous vehicles (CAV). How fast the evolution of these two areas will go hand-in-hand is of great importance, which is the focus of this article. After a brief overview of the technological evolution of V2X to CAV and 6G key technologies, this article explores two complementary research directions, namely, 6G for CAVs versus CAVs for 6G. The former investigates how various 6G key enablers, such as THz, cell free communication and artificial intelligence (AI), can be utilized to provide CAV mission-critical services. The latter discusses how CAVs can facilitate effective deployment and operation of 6G systems. This article attempts to investigate the interactions between the two technologies to spark more research efforts in these areas. [ABSTRACT FROM AUTHOR]

Machine-type data traffic volume in 5G networks will grow rapidly with widespread applications of MTC. The MTC devices, including home appliances, sensors, wearable devices, vehicles, and even low-altitude UAVs, will be made smarter with embedded advanced communication modules. However, a massive number of MTCDs with diverse QoS requirements cannot be served well by traditional wireless systems, which were primarily designed for human-type devices. New wireless technologies are needed to support distinct MTC network applications. In this article, five emerging MTC technologies, including Bluetooth 5, WiFi HaLow, LTE-M, NB-IoT, and WURs, are discussed, followed by a survey of 3GPP activities on vehicle-based and UAV-based MTC technologies, such as LTE-V2X and C-UAV for aerial vehicles. In the end, we also address the issues on the new trends for MTC networks with identified research challenges. [ABSTRACT FROM AUTHOR]

This paper investigates an integrated wireless communication system including non-orthogonal multiple access, full-duplex relaying, and energy harvesting techniques (named as EH-FD-NOMA). In this scheme, an energy-limited full-duplex relay harvests energy from a source at the first stage. Then, the relay detects the superimposed signal from the source and transmits the decoded signal to destination. Closed-form outage probabilities and ergodic rates at the relay and destination are derived. Numerical results verify the analytical results and show the superior performance of the EH-FD-NOMA if compared to its counterparts. [ABSTRACT FROM AUTHOR]

Device-to-device (D2D) communication is a promising technique to enhance the spectrum efficiency of a cellular network. However, one of the major challenges in D2D communications is to select appropriate communication modes for all possible D2D pairs in a D2D network. In this paper, we study the mode selection problems in a multi-mode and multi-pair D2D network, where the eNB can assign one of the three D2D communication modes (including local route mode, direct D2D mode, and relay D2D mode) to each D2D pair. To achieve the maximal cell-wise capacity, an optimal mode selection algorithm is proposed in this paper with the aim to share cellular users’ uplink channels. The optimal mode selection problem can be modeled by an integer programming problem. Particularly, in a fully loaded cell, we discover that the optimal mode selection problem can be solved using a linear programming algorithm with a significantly reduced complexity. [ABSTRACT FROM AUTHOR]

The fifth generation communication systems aim to achieve higher capacity and more user access. Full-duplex (FD) communication can achieve twice the spectral efficiency of the conventional half-duplex systems under ideal conditions, and FD relay is a common application of FD technology. In this paper, we propose a modified signal-to-leakage-and-noise ratio (SLNR) precoding scheme for the FD multiuser MIMO relay system, in which the self-interference (SI) of the relay is considered. In contrast to block diagonalization (BD) precoding and zero-forcing (ZF) precoding, SLNR precoding does not have an antenna dimension limitation, which enables it to support more antennas and users. In addition, SLNR precoding balances the interference and noise of a system, whereas BD precoding and ZF precoding only consider the interference. The numerical results show that the proposed SLNR precoding scheme can achieve better performance with low SI. With a reasonable sum capacity loss, the SLNR precoding scheme provides more user access despite a limited number of transmit antennas. [ABSTRACT FROM AUTHOR]

Device-to-Device (D2D) communications have been proposed to improve spectrum and energy efficiency of cellular networks. One of the most critical issues in D2D networks is the selection of the communication modes for multiple D2D pairs in a cell. In this article, relay-assisted mode, in addition to basic cellular and direct D2D modes, is taken into account to improve the link performance of D2D pairs. We investigate the optimal mode selection schemes to maximize cell-wise capacity based on the reuse of cellular uplink channels in different multi-pair D2D communication scenarios. We discover that the optimal mode selection problem can be transformed into a linear programming problem with polynomial computational complexity. The exact analytical solutions for optimal mode selection are derived and can be useful to cellular operators to maximize the cellwise capacity. [ABSTRACT FROM AUTHOR]

D2D communication is an effective approach for traffic offloading in future cellular systems. However, only using direct D2D communication is not enough to fully exploit the advantages of D2D communications due to long distances or poor link quality between source and destination D2D UEs. This work aims to enhance the traffic offloading capacity of cellular-assisted D2D communication, taking into account D2D relays. In particular, we propose a unified model for D2D communication, in which two other D2D communication modes, that is, relay-assisted D2D and local route based D2D, are also supported by UEs in addition to direct D2D mode. We design radio protocol architectures for the three D2D modes and propose an eNB scheduling algorithm, which enables the eNB to perform joint scheduling on mode selection, radio resource allocation, and power coordination for all D2D pairs in each subframe. The effectiveness of our designs is validated by simulations. [ABSTRACT FROM PUBLISHER]

With an increasing demand to improve spectrum efficiency and energy efficiency in cellular networks, device-to-device (D2D) communications have drawn a lot of attention from both academia and industry. Cooperative D2D networking, which allows for cooperation among mobile users (MUs) via D2D links while communicating with other MUs or base stations (BSs), is one of the most promising techniques in D2D communications. In cooperative D2D communications, two major issues need to be investigated: network performance evaluation and peer selection. In order to enhance evaluation accuracy while maintaining data privacy, it is essential to employ social interactions among MUs in cooperative D2D communications. In this article, we investigate social aware cooperative D2D networks. We first introduce social relationship formation, and then social learning based cooperative network evaluation and social relationship based peer selection are investigated, respectively. Numerical results show the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

Wireless information and power transfer (WIPT) enables more sustainable and resilient communications due to the fact that it avoids frequent battery charging and replacement. However, it also suffers from possible information interception due to the open nature of wireless channels. Compared to traditional secure communications, secrecy wireless information and power transfer (SWIPT) has several distinct characteristic features. Wireless power transfer may increase vulnerability to eavesdropping, since a power receiver as a potential eavesdropper usually has a shorter access distance than an information receiver. On the other hand, wireless power transfer can be exploited to enhance wireless security. This article reviews the security issues in various SWIPT scenarios, with an emphasis on revealing the challenges and opportunities for implementing SWIPT. Furthermore, we provide a survey on a variety of physical layer security techniques to improve secrecy performance. In particular, we propose to use massive MIMO techniques to enhance power transfer efficiency and secure information transmission simultaneously. Finally, we discuss several potential research directions to further enhance the security in SWIPT systems. [ABSTRACT FROM PUBLISHER]

Physical-layer network coding (PNC) is an attractive technique because it can significantly enhance the throughput of a wireless network with a relatively simple implementation, as compared with conventional network coding (NC) schemes. In this paper, we first investigate PNC with frequency shift keying (FSK) through the derivation of its bit-error rate (BER) and capacity, as well as the impact of imperfect synchronization to the overall performance. Based on the discussions on PNC with 2FSK, we then further expand the analysis to PNC with $M$-ary FSK (MFSK). Theoretical results indicate that the BER performance of PNC with 2FSK is a bit worse than that of PNC with 2PSK. As the constellation dimension $M$ increases, the BER performance of PNC with $M$-ary FSK improves at the expense of bandwidth efficiency degradation. [ABSTRACT FROM PUBLISHER]

With the growing popularity of mobile Internet, providing secure wireless services has become a critical issue. Physical layer security (PHY-security) has been recognized as an effective means to enhance wireless security by exploiting wireless medium characteristics, for example, fading, noise, and interference. A particularly interesting PHY-security technology is cooperative relay due to the fact that it helps to provide distributed diversity and shorten access distance. This article offers a tutorial on various multi-antenna relaying technologies to improve security at physical layer. The state-of-the-art research results on multi-antenna relay aided PHY-security as well as some secrecy performance optimization schemes are presented. In particular, we focus on large-scale MIMO relaying technology, which is effective in tackling various challenging issues for implementing wireless PHY-security, such as short-distance interception without eavesdropper CSI and with imperfect legitimate CSI. Moreover, the future directions are identified for further enhancement of secrecy performance. [ABSTRACT FROM AUTHOR]

The performance analysis of a distributed access wireless network, in which nodes are randomly distributed [namely, a random wireless network (RWN)] over a large area, is challenging. An effective mathematical modeling of these networks can help network designers to predict whether a given set of quality-of-service (QoS) requirements can be satisfied prior to the real network deployment. Previous models for the performance analysis of multihop RWNs fail to give accurate results due to the assumption of a single-hop, limited, and a priori known number of stations; negligence of the hidden terminals; and the use of a Boolean logic collision model. In this paper, we develop an alternative probabilistic modeling approach to analyze the performance of these networks in medium-access-control (MAC) and network layers. Using the proposed modeling approach and obtained metrics, we can identify a set of optimal parameters to maximize the performance of the networks. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a denoise-and-forward network coding (DNF-NC) transmission scheme for its applications in two-way relay multiple-input and multiple-output (MIMO) systems. We first consider a scenario with a single pair of source nodes, and minimum mean square error (MMSE) receiver is applied at each node. The global optimal precoding design based on the mean square error (MSE) criterion can be achieved by solving two independent convex optimization problems. To achieve a better tradeoff between performance and complexity, an alternative power optimization approach is proposed using a channel diagonalization technique. Then, we proceed to considering a more challenging bidirectional communication scenario with multiple pairs of source nodes. With intrapair coordination at the sources, we modify DNF-NC by employing a signal alignment technique to combat interpair interference. The numerical results demonstrate that the proposed DNF-NC schemes can significantly improve bit error rate (BER) performance in both two scenarios, and such performance gains can be achieved with relatively low computational complexity. [ABSTRACT FROM PUBLISHER]

Device-to-device (D2D) communications help improve the performance of wireless multicast services in cellular networks via cooperative retransmissions among multicast recipients within a cluster. Resource utilization efficiency should be taken into account in the design of D2D communication systems. To maximize resource efficiency of D2D retransmissions, there is a tradeoff between multichannel diversity and multicast gain. In this paper, by analyzing the relationship between the number of relays and minimal time-frequency resource cost on retransmissions, we derive a closed-form probability density function (pdf) for an optimal number of D2D relays. Motivated by the analysis, we then propose an intracluster D2D retransmission scheme with optimized resource utilization, which can adaptively select the number of cooperative relays performing multicast retransmissions and give an iterative subcluster partition algorithm to enhance retransmission throughput. Exploiting both multichannel diversity and multicast gain, the proposed scheme achieves a significant gain in terms of resource utilization if compared with its counterparts with a fixed number of relays. [ABSTRACT FROM PUBLISHER]

Earlier works studied the tradeoff between sensing performance and throughput of secondary users (SUs) for cooperative spectrum sensing. However, The tradeoff between sensing performance and energy consumption has not been well investigated in cognitive radio (CR) networks with relays. In this paper, the issues on cooperative spectrum sensing in a CR network with amplify-and-forward relay are studied. In particular, an optimization problem is formulated to analyze the tradeoff between sensing performance and energy consumption. The objective is to minimize energy consumption for spectrum sensing under given detection probability and false alarm probability constraints. We do so via finding an optimal pair of the parameters, i.e., the number of samples (proportional to sensing time) and the amplification gain. First, the case of fixed amplification gain is considered, in which the minimum number of samples is identified. Then, the case of varying amplification gain is further discussed, where an appropriate amplification gain is found such that the energy consumption is minimized while the sensing performance is kept above a threshold and the required throughput of the SU can be satisfied. The analytical and simulation results show that there exists an optimal pair of the number of samples and the amplification gain to strike the best tradeoff between sensing performance and energy consumption. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a secrecy relaying communication scenario where all nodes are equipped with multiple antennas. An eavesdropper has the access to the global channel state information (CSI), and all the other nodes only know the CSI not associated with the eavesdropper. A new secrecy transmission protocol is proposed, where the concept of interference alignment is combined with cooperative jamming to ensure that artificial noise from transmitters can be aligned at the destination, but not at the eavesdropper due to the randomness of wireless channels. Analytical results, such as ergodic secrecy rate and outage probability, are developed, from which more insightful understanding of the proposed protocol, such as multiplexing and diversity gains, can be obtained. A few special cases, where outage probability cannot be decreased to zero regardless of SNR, are also discussed. Simulation results are provided to demonstrate the performance of the proposed secrecy transmission protocol. [ABSTRACT FROM AUTHOR]

This paper proposes a cooperative network-coding (CNC) scheme for a dual-hop decode-and-forward (DF) multiple-input–multiple-output (MIMO) system, which consists of multiple source stations (SSs), one relay station (RS), and one base station (BS). Both the RS and the BS use multiple antennas, whereas the SS has only one antenna. Through a MIMO configuration between the RS and the BS, a cooperative network coded DF (CNC-DF) relay protocol is proposed to transmit the composite data flows from multiple SSs via the RS to the BS simultaneously to reduce delay and to provide network-coded cooperation gain. To demonstrate its effectiveness, CNC-DF is compared with time-division multiple-access (TDMA) DF and orthogonal space–time coded (OSTC) DF protocols. The ergodic capacity and outage probability for these three protocols are explicitly derived, showing that the proposed CNC-DF scheme can significantly improve the performance, which is closely related to the minimum of the numbers of SSs and the numbers of antennas used by the RS and the BS. [ABSTRACT FROM PUBLISHER]

This article presents a hierarchical cooperative relay-based heterogeneous network (HCRHeNet) to support both unicast and multicast services, where hierarchical cooperative relay nodes are deployed to provide a cost-effective coverage extension based on the convergence of heterogeneous radio networks. The underlined HCR-HeNet divides its coverage into three layers: hierarchical cooperative basic layer, homogeneous cooperative enhanced layer, and heterogeneous cooperative extended layer. In the hierarchical cooperative basic layer, highspeed data transmission is enabled using highorder modulation and coding schemes for unicast services, and hierarchical modulation schemes for multicast services. In the homogeneous cooperative enhanced layer, where users may be located near a cell boundary and thus need the help of relay nodes, cooperative homogeneous diversity gain can be achieved. In the heterogeneous cooperative extended layer, heterogeneous cooperative diversity gain guarantees the convergence and interworking of multiradio access networks. The key techniques in the physical and MAC layers are identified. Issues in the application of cognitive radio and self-organized networking techniques in HCR-HeNet are also discussed. Finally, a physical layer testbed for the proposed HCR-HeNet with multicast services is introduced. [ABSTRACT FROM PUBLISHER]