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Wireless energy harvesting (WEH) enabled Device-to-Device (D2D) communication emerges as an effective technique to improve spectral and energy efficiencies. However, D2D users are usually power-constrained devices that may be monitored or attacked by adversaries easily. To confuse randomly distributed adversaries in WEH-enabled D2D underlaying cellular networks, power beacons (PBs) can be used to send jamming signals in idle time slots. A time slot is divided into two sub-slots, i.e., WEH sub-slot and covert transmission sub-slot. The energy collected by a D2D transmitter in the first sub-slot is used to support possible covert transmission in the second sub-slot. The performance of the proposed scheme is measured by covert throughput, which is defined as D2D communication rate with several constraints, e.g., wireless energy harvesting, covertness, and cellular link communication requirements. The closed-form expressions of energy outage probability, covert probability, and desired link connection outage probabilities are derived. The minimum covert probability is used to measure covertness. Moreover, an alternating optimization algorithm is adopted to maximize covert throughput. The analytical results are compared with Monte-Carlo simulation results to verify the analytical approach. In addition, the impacts of different parameters (e.g., density of PBs and covert signal transmit power) on covert performance are evaluated. [ABSTRACT FROM AUTHOR]

Due to its flexible deployment and high mobility of unmanned aerial vehicles (UAVs), the UAV-assisted cognitive radio (CR) network has attracted a lot of attention as one of the most promising techniques to address spectrum congestion issues in futuristic networks. However, its performance can be severely affected by the blocked line-of-sight (LoS) channel in its air-to-ground (A2G) links. In this paper, we propose a UAV CR system enabled by a reconfigurable intelligent surface (RIS), which helps to reconstruct reliable links in UAV-assisted cognitive radio (CR) networks. Our goal is to maximize the achievable rate of a secondary receiver (SR) through the proper selection of the UAV trajectory, transmit power, and RIS phase shifts based on a given interference temperature threshold and other practical constraints. In addition, we solve the corresponding non-convex optimization problem using block coordinate descent (BCD) and successive convex approximation (SCA) algorithms. The simulation results will demonstrate the effectiveness of the proposed algorithms. [ABSTRACT FROM AUTHOR]

Although 5G networks have enabled mobile users to get a better experience, task scheduling remains challenging for massive Internet of Things (IoT) devices in remote areas. This paper investigates the task scheduling problem for delay-aware IoT applications in civil aircraft-augmented space-air-ground integrated networks (CAA-SAGIN), where the normalized sky access platforms (SAPs) can collect and forward the terrestrial tasks. Specifically, we first propose an access control scheme for a non-preemptive priority queuing system and a transmission control scheme with cross-layer optimization. Secondly, considering the uncertain distribution of the transmission numbers and generated data, we formulate a robust two-stage stochastic optimization problem of delay minimization. With the proposed robust task scheduling with risk aversion (RTS-RA) algorithm, the original problem can be decomposed into two subproblems, which can be further transformed into tractable semi-definite program (SDP) problems respectively. Simulation results show that the cross-layer optimization scheme can achieve a good tradeoff between delay and throughput. Also, the RTS-RA algorithm outperforms the exiting offloading schemes in terms of end-to-end delay, transmitted data, and energy consumption with lower computational complexity. [ABSTRACT FROM AUTHOR]

In an orthogonal frequency division multiplexing (OFDM) system, cyclic prefix (CP) has to be added to eliminate multipath-induced inter-symbol interference (ISI) with a severely sacrificed bandwidth efficiency. Peak-to-average ratio (PAPR) may impair its performance further if a large number of carriers are used. On the other hand, traditional multi-carrier modulation suffers a prohibitive complexity, which can not be implemented with colorred current available silicon technologies. This work proposes a cost-effective way to implement multi-carrier modulation, namely multi-layer quadrature (MLQ) modulation, which requires only two oscillators. As an application example, we apply MLQ modulation to a complementary coded code division multiple access (MLQ-CC-CDMA) system, whose multi-carrier transmission is based on MLQ modulation to mitigate ISI and multi-user interference (MUI) with ideal auto- and cross-correlations of complementary codes. Compared to a frequency domain (FD) CC-CDMA system, MLQ-CC-CDMA has a lower RF complexity. Compared to OFDM direct sequence (DS) CDMA and single carrier frequency division multiple access (SC-FDMA) CDMA schemes, MLQ-CC-CDMA achieves a higher spectrum efficiency (SE) and a lower bit error rate with Rake and minimum mean square error (MMSE) detection due to its powerful multipath and multi-carrier diversity gains. [ABSTRACT FROM AUTHOR]

With the deployment of the fifth generation (5G) in many countries, people start to think about what the next-generation of wireless communications will be. The current communication technologies are already approaching the Shannon physical capacity limit with advanced encoding (decoding) and modulation techniques. On the other hand, artificial intelligence (AI) plays an increasingly important role in the evolution from traditional communication technologies to the future. Semantic communication is one of the emerging communication paradigms, which works based on its innovative “semantic-meaning passing” concept. The core of semantic communication is to extract the “meanings” of sent information at a transmitter, and with the help of a matched knowledge base (KB) between a transmitter and a receiver, the semantic information can be “interpreted” successfully at a receiver. Therefore, semantic communication essentially is a communication scheme based largely on AI. In this article, an overview of the latest deep learning (DL) and end-to-end (E2E) communication based semantic communications will be given and open issues that need to be tackled will be discussed explicitly. [ABSTRACT FROM AUTHOR]

Industrial Wireless Networks (IWNs) play a critical role in industry 4.0 applications. However, industrial control processes require deterministic data delivery with an extremely high reliability, which is challenging in harsh IWN environments. Existing scheduling schemes rely mainly on accurate global synchronization to ensure deterministic transmission, which incurs additional hardware cost and a large communication overhead. To eliminate the synchronization requirements, we propose an asynchronous scheduling model that supports arbitrary time offset between any node pairs. As finding a solution with a given reliability guarantee in this model is a NP-hard problem, we design a probabilistic algorithm based on the Monte Carlo tree search (MCTS) method to generate and verify feasible schedules. The simulation results show that E-MCTS is significantly better in transmission reliability if compared to the classic Monte Carlo tree search algorithm. [ABSTRACT FROM AUTHOR]

Spectrum efficiency is a critical performance metric in wireless communications due to an increasing demand for radio spectrum to support high data rate applications. Direct sequence spread spectrum technology enables a code division multiple access (CDMA) system a strong anti-interference capability but with a relatively low spectrum efficiency. To address this issue, complementary coded scrambling multiple access (CCSMA) (X. Liu et al., in 2018) was proposed to use three-dimensional (space-time-frequency) resources for performance enhancement. However, in a CCSMA system, if the number of multipath returns is larger than the number of transmit (Tx) antennas, a code reuse problem occurs to degrade bit error rate performance significantly. This work designs a new scrambling code, namely generalized complementary coded scrambling multiple access (GCCSMA) codes, to overcome the code reuse problem in CCSMA. GCCSMA uses complementary codes, scrambling modulation, and multiple-input-multiple-output (MIMO) techniques jointly, which works well even in multipath channels with more paths than the number of Tx antennas. The analysis and simulations are conducted to validate the effectiveness of the proposed GCCSMA system. [ABSTRACT FROM AUTHOR]

Sparse code multiple access (SCMA), which helps improve spectrum efficiency (SE) and enhance connectivity, has been proposed as a non-orthogonal multiple access (NOMA) scheme for 5G systems. In SCMA, codebook design determines system overload ratio and detection performance at a receiver. In this paper, an SCMA codebook design approach is proposed based on uniquely decomposable constellation group (UDCG). We show that there are $N+1$ ($N\ge 1$) constellations in the proposed UDCG, each of which has $M (M\ge 2)$ constellation points. These constellations are allocated to users sharing the same resource. Combining the constellations allocated on multiple resources of each user, we can obtain UDCG-based codebook sets. Bit error ratio (BER) performance will be discussed in terms of coding gain maximization with superimposed constellations and UDCG-based codebooks. Simulation results demonstrate that the superimposed constellation of each resource has large minimum Euclidean distance (MED) and meets uniquely decodable constraint. Thus, BER performance of the proposed codebook design approach outperforms that of the existing codebook design schemes in both uncoded and coded SCMA systems, especially for large-size codebooks. [ABSTRACT FROM AUTHOR]

Growing computation requirements in emerging intelligently connected vehicle (ICV) networks has posed a big challenge for vehicles to process computation-intensive tasks on-board, and thus computation partition and offloading plays a critical role. Owing to broadcast nature of wireless channels, confidential messages without proper security protection in ICVs may be easily exposed to malicious users. In this paper, a mobile crowdsourcing (MCS) based mobile crowd computing framework for ICV networks is proposed, where multiple vehicles act as workers to provide computing services for end user (EU). In particular, artificial noise (AN) assisted physical (PHY) layer security approaches are used to enhance the security in offloading links. Ergodic secrecy rates in different offloading phases in time-varying channels are derived. In addition, an incentive-driven mechanism is introduced to encourage workers to share their idle computing resources, and an optimization problem is formulated to minimize the overall price paid for computing tasks, subject to energy consumption and delay constraints. Finally, Monte Carlo simulations verify the analysis on the ergodic secrecy rates. [ABSTRACT FROM AUTHOR]

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]

Ultra-dense network (UDN) is one of the most important techniques to support massive number of different types of user equipments (UEs). It is essential to understand the impact of densification on various system performance metrics in future cellular networks. Two contradictory conclusions were made from the existing research works. Some works indicated that the network densification has reached its upper limit and deploying more UEs may only reduce UDN performance; whereas the others claimed that more UEs can still be supported by deploying more small cell base stations (SBSs). The existence of above different conclusions is due mainly to the difficulties to find an effective means to take into account different path loss models in UDNs. In this paper, we aim to propose a unified analytical approach, which is effective to simplify UDN performance analysis based on different path loss models with a low computation complexity. Using the proposed approach with idle mode of SBSs enabled, we can identify the impact of the densities of SBSs and UEs on various performance metrics, i.e., coverage probability and average area spectral efficiency (ASE). The conclusions will be supported by the analytical and simulation results verifying the effectiveness of the proposed unified analytical approach. [ABSTRACT FROM AUTHOR]

In wireless network virtualization processes, load imbalance in substrate nodes and substrate links may significantly reduce the number of virtual network requests served by a substrate network. In addition, load imbalance also impairs resource utilization and increases embedding cost of the virtual networks. To tackle these issues, this work proposes a re-embedding scheme for virtual node (RSVN). Based on the load condition of a substrate network, RSVN first determines whether to start re-embedding procedure or not, and then selects proper re-embedded virtual nodes according to their re-embedding factors. Next, RSVN re-embeds the selected virtual nodes to balance the loads of a wireless substrate network. The extensive simulation results showed that RSVN improves the acceptance ratio of virtual network requests, increases the resource utilization of substrate networks, and reduces the average embedding cost of the entire virtual networks. [ABSTRACT FROM AUTHOR]

Vehicle-to-vehicle (V2V) underlay cellular communication plays an important role in the fifth-generation (5G) and beyond communications and its security issues attract a lot of attention. Physical layer security, which explores channel characteristics to protect wireless communications, is very attractive due to its low latency. This paper focuses on secrecy capacity maximization in V2V underlay cellular communications, where the secrecy capacity of vehicular user equipments (VUEs) is optimized first, followed by sum proportional fairness function optimization for both VUEs and cellular user equipments (CUEs). The optimization problems can be solved in two sub-problems, e.g., power and subcarrier allocation problems. The power allocation can be done with the first strategy using a one-dimensional searching algorithm, whereas a genetic algorithm (GA) is used to solve the second optimization problem. In addition, as VUEs share uplink resources with CUEs, the co-channel interference exists and the subcarrier allocation sub-problem is a 3D search problem, which is solved by a two-step iterative Hungarian algorithm (IHA). Finally, simulations are performed to validate the performance of the proposed schemes. [ABSTRACT FROM AUTHOR]

Ocean monitoring network (OMN) is a remote oceanic data collection system, which integrates communication and networking technologies. As underwater acoustic communication has been widely used for data transmission between battery- powered underwater sensor nodes (USNs) and sink nodes (SNs), an oceanic data collection system requires energy efficient deployment to prolong the lifetime of the whole network. This work aims to propose an unmanned aerial vehicle (UAV) assisted OMN architecture, in which sensing data are transmitted first from USNs to sea surface SNs in each data collection cycle using underwater acoustic communication, and then a UAV hovering above the SNs collects and relays all the data to a ground base station via wireless communication links. To extend the network lifetime, we model SNs and UAV deployment and resource allocation as a mixed integer non-convex optimization problem. To solve the problem efficiently in a heuristic way, we design a SNs deployment scheme with the help of time division USN-to-SN access and NOMA based SN-to-UAV access schemes. Computer simulations validate the superiority of the proposed deployment and access schemes on OMN lifetime performance. in particular, increasing the number of time slots in the USN-to-SN access process can improve the performance significantly. [ABSTRACT FROM AUTHOR]

In a frequency division complementary coded CDMA (FD-CC-CDMA) system, multiple tuners have to be used for multi-carrier modulation, which has a very high complexity and power consumption. In this paper, we propose a CP-free OFDM scheme to implement multi-carrier modulation in an FD-CC-CDMA system, which is named as CP-free OFDM-CC-CDMA system. The transmitter in the CP-free OFDM-CC-CDMA system offers a unique flexibility to adjust the amount of data processed at the same time to ensure that the OFDM block size exceeds the delay spread of a multipath channel, which attributes to effective mitigation of multipath induced inter-symbol interference (ISI). At the receiver of a CP-free OFDM-CC-CDMA system, minimum mean square error (MMSE) equalization and MMSE combining are used to mitigate multi-user interference (MUI) and inter-carrier interference (ICI). The obtained results show that a CP-free OFDM-CC-CDMA system offers a much higher transmission efficiency than FD-CC-CDMA and OFDM-DS-CDMA systems. In addition, the proposed CP-free OFDM-CC-CDMA system outperforms OFDM-DS-CDMA system in terms of bit error rate due to its inherent powerful diversity gain. [ABSTRACT FROM AUTHOR]

This article aims to provide a comprehensive overview on PHY layer security assisted ICV networks. Beginning with a discussion from a general perspective of the ICV network architecture, this work summarizes the major security threats faced by ICV networks. To achieve confidential data transmission with a low latency and a high data rate, several physical layer security schemes are surveyed in this article, which include RRM, cooperative jamming, multi-antenna technologies, and PHY-layer key generation. In addition, the performances of the aforementioned schemes are compared in terms of different aspects, followed by a discussion on some major challenges that need to be tackled in the implementation of ICV networks. [ABSTRACT FROM AUTHOR]

A device-to-device (D2D) communication link is vulnerable because it is relatively easy to be compromised by adversaries due to the fact that D2D terminals are power limited nodes. This article proposes a covert communication scheme that allows D2D communication links to transmit covert signals (e.g., privacy sensitive data) to ensure a low probability of detection. We allocate transmit powers of cellular and D2D noncovert signals following a fading D2D channel to add the uncertainty in adversary's background noises. With the help of base station, cooperative power domain nonorthogonal multiple access and successive interference cancellation are used to decode covert signal. Depending on whether a D2D transmitter has the knowledge of adversary's detection threshold, we derive minimum error probability to measure the covertness performance. Furthermore, we evaluate covert throughput, which is defined as a maximum average transmission rate of covert signal, subject to a given covertness performance. [ABSTRACT FROM AUTHOR]

In device-to-device (D2D) underlaying vehicle-to-vehicle (V2V) communications, radio resource blocks (RBs) are allocated to primary cellular users, which yields sub-optimal rates and a long access latency for secondary D2D-enabled vehicles. This work investigates a joint radio resource and power management (RRPM) problem for secure cellular underlaying V2V communications, where cellular users and vehicles have the same priority, giving vehicles more opportunities to access the RBs. Specifically, we aim to maximize secrecy rates of V2V channels under the condition that eavesdropper has an adaptive receiving detection vector to maximize the received signal-to-interference-plus-noise ratio (SINR) from vehicles. For a single pair of a vehicle and a cellular user, the closed-form optimal power allocation expressions are derived for the interference-limited scenario and the noise-limited scenario, respectively. Moreover, for multiple pairs of vehicles and cellular users, a 3-partite hypergraph based 3-dimensional matching approach is proposed to solve a mixed-integer and non-convex problem, which achieves a near-optimal result with an $O(n^4)$ time complexity. Simulations in different scenarios show that the secrecy rate of the proposed scheme can be improved by 50% if compared to existing schemes. [ABSTRACT FROM AUTHOR]

The existing investigations on artificial noise (AN) security systems assumed that only null spaces are used to send AN signals, and all eigen-subchannels should be used to transmit messages. Our previous work proposed an AN scheme that allocates some of eigen-subchannels to transmit AN signals for improving secrecy rates. Nevertheless, our previous work considered only uncorrelated MIMO Rayleigh fading channels. In fact, the correlations among antennas exist in realistic scattering channel environments. In this paper, we extend our previous AN scheme to spatially correlated Rayleigh fading channels at both legitimate receiver and eavesdropper sides and derive an exact theoretical expression for the ergodic secrecy rate of the AN scheme, along with an approximate analysis. Both numerical and simulation results show that the proposed AN scheme offers a higher ergodic secrecy rate than the existing schemes, revealing a fact that the correlation among eavesdropper’s antennas can potentially improve the secrecy rate of an MIMO system. [ABSTRACT FROM AUTHOR]

To support high speed transmission and traffic offloading from cellular networks, LTE-unlicensed (LTE-U) standard was proposed by 3GPP to enable cellular data transmission in unlicensed band. The primary challenge of LTE-U is to successfully coexist with the incumbent systems (e.g., WiFi networks) in the unlicensed bands, while still maintaining the quality-of-service (QoS) for LTE-A users. This paper proposes a new framework for implementing LTE-U standard based on deployed LTE-A infrastructure and cognitive radio (CR) technology to sense radio environment and construct a spectrum map, based on which the LTE-U system can perform joint power and channel allocation to maximize the overall system throughput. We introduce a spectrum map construction scheme and evaluate the Cramer-Rao bound of the scheme. It is shown that the original NP-hard joint power and channel allocation problem can be transformed into a linear programming problem for its solution. Simulation results verify that the proposed LTE-U scheme helps improve system throughput via access collision avoidance. [ABSTRACT FROM AUTHOR]

Due to its high speed and low latency, D2D communication plays an important role in providing proximity data services in 5G systems, which may contain privacy sensitive data. Covert communication is useful to protect the privacy of user data against adversaries. In a D2D underlaying cellular network, an antenna array can be used at a base station (BS) to transmit artificial noise to confuse adversaries that try to detect D2D covert signals. Based on the numbers of antennas at BS, two covert schemes are presented in this work and their performances are evaluated in terms of D2D covert throughput, i.e., maximum achievable D2D data rate with the given covertness requirements. As only channel distribution information (CDI) of adversary links is known to the BS, the average minimum error probability (AMEP) is used as a metric to measure the covertness performance. In the proposed schemes, closed-form expressions of the minimum AMEP and achievable D2D data rate are derived. It is shown that the derivation of minimum AMEP can be simplified if either one or massive antennas are allocated for sending artificial noise. The analytical results are compared to Monte-Carlo simulation results to verify the feasibility of the proposed schemes. It is also revealed that covert throughput can be further improved by setting system parameters properly, e.g., the number of antennas at BS and artificial noise transmit power. [ABSTRACT FROM AUTHOR]

A traditional direct sequence CDMA (DS-CDMA) system works based on spreading codes, which should have good auto-correlation and cross-correlation properties to eliminate multiple access interference (MAI) and multipath interference (MI). However, to find a large number of good spreading codes is extremely difficult, and thus the capacity of a DS-CDMA system is limited strictly by the number of available spreading codes. This work proposes a downlink DS-CDMA design to enable multiple access using only one spreading code, namely identical code cyclic shift (ICCS) code, where in-phase and quadrature channels are used to transmit data and pilot, respectively. In particular, the ICCS code is used at a transmitter to perform DS modulation for user data, which is then sent to the in-phase channel. At the same time, the pilots are sent over the quadrature channel in a time-division multiplex (TDM) mode to construct a correlation matrix of ICCS code. At a receiver, MAI is removed by so-designed pilots plus a matched filter-bank. Simulation results show that the proposed ICCS multiple access (ICCSMA) can achieve a better performance than a traditional DS-CDMA system in a high signal to noise ratio region. ICCSMA serves as a bridge to turn CDMA to NOMA to enable massive connectivity in 5 G and beyond communications. [ABSTRACT FROM AUTHOR]

Ultra-dense network (UDN) is one of the enabling technologies in the fifth generation (5G) wireless communications, and the application of joint transmission (JT) is extremely important to deal with severe inter-cell interferences in UDNs. However, most of the current works done in performance analysis on JT schemes in the literature were based largely on simulation results due to the difficulties in quantitatively identifying the number of desirable and interfering transmitters in a UDN setup. In this work, we are motivated to propose an analytical approach to investigate the performance of JT schemes with a unified approach based on stochastic geometry, which is in particular useful for studying different JT schemes as well as conventional transmission schemes without JT. Using the proposed approach, we can unveil the statistical characteristics (i.e., expectation, moment generation function, and variance) of desirable signal and interference powers of a given user equipment (UE), and thus the system performances, such as average signal-to-interference-plus-noise ratio (SINR) and area spectral efficiency, can be evaluated analytically. The simulation results are illustrated to verify the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

VLC performs the dual-functions of communication and lighting and will play an important role in next-generation wireless communications. Proper multiple access methods should be identified for VLC in multi-user scenarios. This article proposes a VLC-CDMA system using OCCs, which are unipolar codes with a multiple sub-codes structure. OCCs offer good autocorrelation and cross-correlation functions. The issues on encoding criteria and application of OCCs in VLC-CDMA systems are discussed. The transmitters use LEDs with different wavelengths to distinguish different sub-codes. Compensator circuits are used to achieve equal gain combining at receivers to exploit the good correlation properties of OCCs. The receivers use the filters with different center wavelengths to distinguish different sub-codes. The performance of VLC-CDMA systems with OCC is compared to OOCs and multi-carrier CDMA systems, to verify that the proposed system is robust for its applications in multi-user scenarios. [ABSTRACT FROM AUTHOR]

Broadband satellite services, such as satellite- based Internet of Things and cloud services, have attracted much attention recently. To achieve the goal of anytime and anywhere high-speed satellite communications, there is a big demand for satellite spectrum. The push for improving spectrum efficiency in satellite systems has been an important issue, mainly from the perspectives of spectrum reuse and dynamic spectrum access. In this article, we investigate the feasibility of improving satellite spectrum efficiency from another viewpoint, that is, introducing a game-theoretical approach, including spectrum pricing and auction, for satellite spectrum dynamic allocation. Although spectrum pricing and auction have been suggested for terrestrial networks, this work aims to establish a market-inspired model to address distinct characteristic features of satellite communications. In particular, we propose a generic satellite spectrum pricing scheme, taking into account three factors: spectrum reuse in multi-beam satellite systems (MSS), where intercell interference should be mitigated; the heterogeneous nature of satellite transmission links in different spectrum bands, in which a differential pricing scheme is desirable; and predictable and periodic satellite switching from one non-GEO satellite link to another, where traffic-oriented pricing should be used. Finally, the challenges and future works in satellite mobile communications are identified. [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]

In this paper, multiple-access techniques are viewed as generalized coding schemes to support both multiuser separation and coding gain. In this sense, the traditional coding schemes, e.g., low-density parity-check (LDPC) code, can be viewed as a special case. Note that still a big difference exists between multiple-access techniques and LDPC codes. In particular, both binary and nonbinary LDPC codes are generated based on finite field theory, whereas multiple-access works based on not necessarily a finite field. This paper presents a mathematical framework, called “multiuser coding,” to unify multiple-access and traditional coding theories. To verify the proposed framework, we propose a uniquely decodable mapping based codebook for futuristic multiple-access communications. A three-dimensional Tanner graph based message passing algorithm is introduced for multiuser detection. Simulation results show that the proposed scheme can support a large number of users with a promising performance. [ABSTRACT FROM AUTHOR]

Visible light communication (VLC) can offload the traffic from existing radio frequency networks and provide illumination at the same time. In order to mitigate intercell interference and to support multiple users, this paper proposes a user-centric quality-of-experience (QoE) optimization scheduling scheme with multicolor light emitting diodes (LED) to improve illumination quality. We design a greedy algorithm to allocate lightwave bandwidths and search for the optimal access point versus user equipment (AP–UE) links dynamically according to the traffic load and locations of UEs. The simulation results show that the proposed scheme offers a guaranteed UEs’ QoE and helps schedule more UEs while ensuring illumination quality at the same time. [ABSTRACT FROM AUTHOR]

This article discusses various issues on intelligent radio (IR) from a perspective of intelligent UE and intelligent BS. Unlike a traditional UE, an intelligent UE can automatically access channel resources, cooperate with other UEs, and manage interferences. To implement such an intelligent UE, innovative techniques should be used, such as smart resource sensing and grant-free transmission. The core of IR lies in the development of thinking hardware, which can collect/process vital information of the environment and manage its resources optimally to support ultra-broadband communications. In particular, an integrated radar-communication unit is one of the most critical environment sensors in an intelligent UE, which helps it to acquire accurate location and mobility information of its neighbors for cooperation and interference avoidance. The traditional physical layer designs (such as beamforming and MIMO) should be revolutionized for the development of IR. ML and other AI algorithms play an important role in both BS and UEs. The communication protocols should be re-designed as well. The IR can offer a very high spectrum efficiency and power efficiency and is one of the most important research topics for future-generation wireless communications. [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]

Traditional orthogonal frequency-division multiplexing (OFDM) systems mitigate multipath interferences with the help of a cyclic prefix (CP) appended between two adjacent OFDM blocks. The length of the CP must be made longer than channel delay spread to avoid intersymbol interference (ISI). The use of CPs degrades spectrum efficiency significantly, which is a deal price paid in all existing OFDM systems. Motivated to deal with this issue, this paper proposes a CP-free OFDM scheme with successive multipath interference cancellation (SMIC), which does not require CPs and removes ISI before fast Fourier transform (FFT) at a receiver using stored feedback equalization (SFE). The SFE operation leaves a CP gap between adjacent OFDM blocks, and the CP is regenerated with estimated signals. In this way, we convert linearly shifted OFDM blocks induced by multipath propagation back to cyclically shifted OFDM blocks for successful FFT operation. SMIC-OFDM achieves a much higher spectrum efficiency than traditional OFDM owing to the save of CPs in transmission signals. The performance of SMIC-OFDM in terms of the bit error rate, capacity, and computational complexity is compared to that of traditional OFDM to verify the effectiveness of the proposed SMIC-OFDM scheme. [ABSTRACT FROM AUTHOR]

Small cell base stations (SBSs) in 5G and beyond networks will play multiple roles in addition to radio access, and will be deployed in a very high density. They can facilitate mobile edge computing (MEC), such that SBSs can store, process, and control network data signals, becoming important elements in ultra-dense networks (UDNs). However, intercell interference may jeopardize the gains from network densification, and thus coordinated multi-point (CoMP) transmission is important in UDNs. MEC and CoMP are different technologies, but they can work jointly to benefit UDNs. In this article, we aim to bridge the gap between MEC and CoMP based on three MEC functions. We will show that CoMP transmission reduces UDN complexity and transmission delay with collaboration among MEC servers in UNDs. In return, MEC-enabled UDNs can alleviate the backhaul pressure on CoMP transmission and make it possible to implement scalable CoMP. [ABSTRACT FROM AUTHOR]

In wireless powered communication networks (WPCNs), base station (BS) and power beacons (PBs) can offer supplement power for uplink transmission of user equipments (UEs). However, the aggregate power consumption of massively deployed PBs may exceed that of a BS. We propose a non-uniform deployment scheme for PBs in WPCNs, where a cell is divided into inner and outer areas, such that BS and PBs can cooperate to power UEs. To be more specific, a BS located in the center of a cell provides downlink power supply for the inner area UEs and uplink information decoding for all the UEs in the cell; while the PBs power UEs in the outer area. With multiple antennas, maximum ratio transmission and maximum ratio combining are adopted for downlink energy beamforming and uplink information reception. Considering a finite area of the network, we derive the distribution of the distance from a non-center-located UE to its nearest PB in the outer area. An optimization problem is formulated to minimize total average power consumption while satisfying BS average transmission power constraint and coverage probability threshold. Moreover, coverage probability is derived for performance evaluation. The numerical results show that the power consumption of the proposed scheme is reduced significantly compared to PB-only WPCNs. [ABSTRACT FROM AUTHOR]

Millimeter-wave (mmWave) communication is one of the most promising technologies in the fifth generation (5G) mobile networks due to its capability to access a large amount of available spectrum resources. Despite theoretical potential of a high data rate, there are still several key technical challenges to use mmWave in mobile networks, such as severe path loss, high penetration loss, and narrow beamwidth. Hence, accurate and reliable knowledge on mmWave channel propagation characteristics is essential for developing 5G wireless communication systems. In this article, the fundamental characteristics of mmWave are presented first. Then two main channel modeling methods are discussed. In order to investigate the channel characteristics at mmWave bands, measurements using three different large-scale array topologies are carried out, and typical channel parameters are extracted and analyzed. [ABSTRACT FROM AUTHOR]

This paper presents a probabilistic decision-making framework for rear-end collision avoidance systems, focusing on modeling the impact of major collision-causing factors on the occurrence of accidents. Decisions on when and how to assist drivers are made using a Bayesian network approach according to collision risk evaluation results, given a prior probabilistic knowledge. The structure of the Bayesian network model is learnt using a K2 algorithm with a practical dataset. To provide adequate response time for drivers, we also predict collision probability in the next monitoring interval using a Kalman filter model. The prediction accuracy is evaluated with different use cases and compared to real scenarios with the obtained dataset. To make our framework more relevant to practical applications, we also discuss the corresponding safety control strategies by classifying collision risk into high and low levels. In addition, the proposed model is evaluated through experiments including simulations and road tests. In the simulations, the algorithms are tested in different scenarios with various configurations of weather conditions, driver response capability, and vehicular dynamics. In order to demonstrate collision avoidance performance, the proposed model is also compared to existing schemes. In the road tests, the algorithm is embedded into an unmanned vehicle with predefined parameters to evaluate the impact of computational complexity on rear-end collision avoidance. Numerical results show that the proposed model provides an accurate estimation for car-following collision risk with a relatively low complexity, taking into account the impacts of vehicle dynamics, driver reaction capacity, and external environment on rear-end collisions. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) is a promising candidate technology for 5G cellular systems. In this paper, design and optimization of scheduling and NOMA algorithms is investigated. The impact of power allocation for NOMA systems with round-robin scheduling is analyzed. A statistic model is developed for network performance analysis of joint scheduling of spectrum resource and power for NOMA algorithms. Proportional fairness scheduling for NOMA is proposed with a two-step approach, with objective of achieving high throughput and user fairness with low computational complexity. In the first step, an optimal power allocation strategy is developed with an objective of maximizing weighted sum rate. In the second step, three fast and scalable scheduling and user pairing algorithms with QoS guarantee are proposed, in which only a few user pairs are checked for NOMA multiplex. The algorithms are extended to the cases with imperfect channel state estimation and more than two users being multiplexed over one resource block. Numerical results show that the proposed algorithms are faster and more scalable than the existing algorithms, and maintain a higher throughput gain than orthogonal multiple access. [ABSTRACT FROM AUTHOR]

Cognitive radio technology, which helps to alleviate spectrum scarcity problem, plays an important role in future wireless communication systems. Spectrum sensing is the key technique of cognitive radio. The upcoming fifth generation (5G) communications need to deal with fast time-varying channels (i.e., channel state is non-static within a sensing window), while existing sensing methods are based mainly on conventional quasi-static channels. Consequently, traditional sensing methods may not work well in 5G communications. This paper aims to propose a sliced sensing technique, which can support 5G cognitive radio applications working in fast time-varying channels. We analyze the impact of non-static channels on the performance of traditional sensing methods and disclose a negative factor, i.e., out-of-phase feature distortion effect. Accordingly, we propose a sliced spectrum sensing scheme and probe its feasibility via several relevant trials. Based on the aforementioned works, a complete version of the sliced sensing technique is designed, which can adapt to channel variation and choose the optimal slicing strategy. Simulation results manifest that the sliced sensing technique outperforms traditional sensing methods in both Rayleigh fading and 3GPP spatial channel models. [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]

The articles in this special section provide a comprehensive overview on the recent development of the intelligent radio. The advances in wireless communications have continuously been pushing the limit of radio technologies. Nowadays, radio networks can provide extremely high data rate, ultra-low latency, and high reliability to serve communication needs of sectors that could not be imagined before. However, radio technologies have become highly complex and call for new solutions. The recent advances in artificial intelligence (AI), including machine learning (ML), data mining, and big data analysis, bring significant promise for addressing hard problems in radio networks. It has been the increasing trend to move the intelligence beyond the spectrum access, which is primarily targeted by cognitive radio, to address various challenges in radio networks, including, but not limited to, channel modeling, modulation, beamforming, radio resource allocation, and network management. Radio technologies are on the way evolving to the intelligent radio, in which AI/ML frameworks and algorithms are applied to learn from environments and explore hidden characteristics of networks for new capacity, performance, and services. We believe the intelligent radio will be the prominent feature of next generation wireless networks. It calls for interdisciplinary research to integrate the advances in AI/ML, communications, computing, and cloud technologies. Both theoretical and applied breakthroughs are expected in this new area. [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]

Virtual multiple-input multiple-output (V-MIMO) is an effective way to achieve a diversity gain in uplink channels. A well-designed user pairing scheme in V-MIMO systems can exploit multiuser diversity to yield a significant performance gain. This paper investigates joint user pairing and power allocation problems in V-MIMO systems to maximize energy efficiency (EE) while guaranteeing spectrum efficiency (SE) under a total power consumption constraint. We formulate such a joint problem as a complex nonlinear optimization problem, which can be solved in two steps in $M\times K$ V-MIMO systems, i.e., power allocation with known user group and joint user pairing and power allocation. A multi-level water-filling method is used, and an iterative algorithm based on the outputs from the first step is employed in the second step. Finally, we propose a joint user pairing and power allocation scheme to find a user group that can achieve the maximum EE with power allocation. Simulation results and performance comparisons illustrate improved performance of the proposed scheme. Compared to the existing user pairing approaches, the proposed scheme achieves a significantly high EE gain while guaranteeing SE of a V-MIMO system. [ABSTRACT FROM AUTHOR]

Cyclic prefix (CP) is used in OFDM/OFDMA systems for data transmission over multipath channels at a dear price of spectral efficiency degradation. This work aims to design a CP-free OFDM/OFDMA system for futuristic high-speed communications, where data block length can be shorter than channel delay spread. In particular, we utilize a time domain precoding scheme working jointly with a multiblock decision feedback equalization algorithm to mitigate intersymbol interference and intercarrier interference caused by multipath propagation. The satisfactory performance of the scheme is validated by simulation results, showing that the proposed CP-free OFDM/OFDMA scheme with known channel state information at transmitter and receiver offers a significantly higher spectral efficiency than traditional CP-based OFDM/OFDMA schemes. [ABSTRACT FROM AUTHOR]

In a traditional complementary coded CDMA (CC-CDMA) system, signals are spread using the direct sequence (DS) technique to suppress multipath interference (MI) and multiple access interference (MAI). However, DS spreading suffers from very poor bandwidth efficiency. This paper proposes a scrambling-based CC-CDMA system, namely complementary coded scrambling multiple access (CCSMA). A CCSMA system offers a unified platform to integrate CC-CDMA, multiple-input multiple-output (MIMO), and scrambling technique seamlessly. Utilizing complementary codes in scrambling modulation, a CCSMA system is able to suppress MI and MAI effectively. With the help of space domain scrambling, the CCSMA system offers much higher bandwidth efficiency than a CC DS-CDMA system. Bit error rate and system capacity are evaluated explicitly in this paper, verifying that CCSMA is a viable multiple access technique for providing high capacity in downlink MIMO channels. [ABSTRACT FROM PUBLISHER]

This paper studies the issues on wireless networks virtualization in terms of two important performance metrics, i.e., energy efficiency (EE) and delay. Different from existing works on physical layer, we aim to achieve a good tradeoff between EE and delay in wireless networks virtualization using cross-layer stochastic optimization approach. In particular, we formulate a cross-layer problem using fractional programming and Lyapunov optimization method. The EE and delay tradeoff solution is given explicitly by deriving their analytical bounds that are verified by simulation results. [ABSTRACT FROM PUBLISHER]

Statistical signal transmission (SST) is an emerging orthogonal frequency-division multiplexing (OFDM) technology that can carry additional information over conventional OFDM signal streams without extra bandwidth cost. This additional information can be exploited to carry control information, location/speed data, or cooperative schedule signals, etc., to improve the performance of wireless communications. However, existing detection methods used in this technique work-based mainly on time-invariant channels (i.e., channel state is quasi-static within a detection period), and cannot work well in diverse channel variations caused by different user velocities. In this paper, we propose a mobility adaptive detection method for SST. First, we analyze the influence of nonstatic channels on SST detection and identify a negative factor, called cyclic autocorrelation function whitening effect. Correspondingly, we probe the feasibility of antiwhitening concept and carry out relevant experiments for verification purpose. Based on this, a complete version of mobility adaptive detection method is proposed. Numerical results manifest that the proposed method outperforms the existing ones in high velocity scenarios. In addition, the properties of the proposed method are investigated, based on which we show a specific application for SST to reveal the potential for SST to be used in 5G wireless communications. [ABSTRACT FROM AUTHOR]

Heterogeneous network (HetNet) plays an important role in the upcoming 5G communications. A HetNet in futuristic communications is envisaged to provide full access data services, such that users will always have the best service with their available radio resources. To realize a full access HetNet, statistical signal transmission (SST) can be an important enabling technique. SST exploits statistical features of ordinary signals as a data transmission scheme to carry information data. Due to its inherent unique properties, SST signal is more robust than ordinary signals in conventional quasi-static channels. However, detection of SST signals in fast time-varying channels will face challenges. Motivated by this fact, this paper aims to analyze the impact of fast time-varying channels to SST, and reveals negative effects of burst of deep fading channel-state (BDFC) and feature dissipation with out-of-phase (FDOP). Accordingly, we design an automatic re-transmission scheme and a fragmental feature capturing method to mitigate the aforementioned issues, respectively. Based on these studies, we propose a channel condition aware detection scheme for SST, aiming to enhance SST detection performance in fast time-varying channels. Numerical results show that with the proposed scheme, a maximum gain of more than 10 dB can be obtained under either BDFC or FDOP condition. Furthermore, it is also demonstrated that the proposed scheme is robust with imperfect channel coherence time information, which is attractive for practical applications. [ABSTRACT FROM PUBLISHER]

In wireless networks, retransmissions are used by senders to recover lost packets. The conventional retransmission schemes usually retransmit only one lost packet each time, resulting in a low retransmission efficiency. However, using broadcasting nature of a wireless channel with network coding (NC), we can recover more than one lost packet per transmission. Different from single-sender multiple-receiver wireless networks as considered in the existing works, this paper explores the ways to use NC for retransmissions in multiple-sender multiple-receiver scenarios. In addition, this paper also aims to investigate the advantages and shortcomings of using NC in retransmissions. In order to maximize the gain and minimize the impact in NC retransmissions, a probabilistic network coding retransmission (PNCR) scheme is proposed, where a buyer/seller game-theoretic approach is adopted to optimize the performance. The experimental results confirm the effectiveness of the PNCR scheme. [ABSTRACT FROM PUBLISHER]

Radio frequency energy transport is a viable technology to power small-sized sensors in wireless industrial sensor networks (WISNs). 60 GHz wireless communication emerges as one of the leading technologies for next generation wireless networks, and it can also be useful to build up WISNs. In this article, we investigate the efficiency of RF energy transport in WISNs with 60 GHz communications. We present a spherical antenna design with supported analytics on directional antennas in 60 GHz wireless communications. We introduce and solve the spherical distribution problem of directional antennas to maximize the energy transport efficiency in WISNs. The extensive simulations given in this article verify that the proposed antenna configuration performs much better than the existing solutions in WISN applications. [ABSTRACT FROM PUBLISHER]