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The speed estimation has been widely used for tracking mobile device locations, providing essential information in location/mobility-aware communications, enhancing received signal quality/robustness, and reducing energy consumption and latency. Deep learning can be used to improve the performance constrained by signal/system model. This work focuses on the issues on machine learning (ML) based speed estimation using primary synchronous signal (PSS), which is embedded in the 5G standards, over general time-variant multipath channels. Aiming to reduce the complexity involved in the ML algorithms for the speed estimation in mobile networks, we propose a pipelined ML algorithm to decompose the original ML model into several smaller ones. The advantages of the proposed convolutional neural network (CNN) based approach have been verified by simulations.

Physical layer covert communication is a crucial secure communication technology that enables a transmitter to convey information covertly to a recipient without being detected by adversaries. Unlike typical cryptography and physical layer security systems that concentrate on protecting the sent signal content, covert communications seek to conceal the existence of legitimate transmission. Thus, with beyond fifth-generation (B5G) wireless communications, covert communications can operate in tandem or as a supplement to conventional security techniques. We provide an extensive overview of the basic theories and several strategies in physical layer covert communications in this article. In particular, we go into great detail about the basic theories of physical layer covert communications, such as channel models, codes, secret keys, and covertness metrics, as well as various covert schemes in progressively more complicated scenarios, such as covert communications in single-antenna and multiantenna three-node systems and covert communications in jammer- and relay-aided systems. In addition, we identify the challenges and future directions for research on covert communications in B5G wireless networks.

This article investigates several critical issues on the ergodic sum capacity and system outage performance of downlink non-orthogonal multiple access (NOMA) systems with different numbers of users per cluster sharing the same resource. The outcomes from this work suggest that NOMA systems with more users per cluster perform better under a low channel gain, while those with fewer users per cluster perform better under a high channel gain. It discloses a fact that a traditional NOMA system with a fixed user grouping scheme may not be a good choice to deal with complex channel conditions with diverse applications. To address the issue, this article introduces a concept of mixed user grouping NOMA, which outperforms OMA and traditional NOMA significantly. The work in this article lays a foundation for more further investigations on mixed user grouping NOMA systems.

In this paper, physical layer security (PLS) in an intelligent reflecting surface (IRS) assisted multiple-input multiple-output multiple-antenna eavesdropper (MIMOME) system is studied. In particular, we consider a practical scenario without instantaneous channel state information (CSI) of the eavesdropper and assume that the eavesdropping channel is a Rayleigh channel. To deal with the complexity of currently available IRS-assisted PLS schemes, we propose a low-complexity deep learning (DL) based approach to design transmitter beamforming and IRS jointly, where precoding vector and phase shift matrix are used to minimize the secrecy outage probability. Simulation results demonstrate that the proposed DL-based approach can achieve a similar performance of that with conventional alternating optimization (AO) algorithms with a significantly low computational complexity.

In this paper, we propose a multi-roadside unit (RSU) assisted mobile crowd computing framework for intelligently connected vehicle (ICV) networks, where vehicles within RSUs’ coverage act as workers to provide their computation and communication resources for computing resource limited vehicle user equipments (VUEs). Physical (PHY) layer security is used to secure computation task offloading and results feedback in time-varying vehicular channels. Artificial noise (AN) assisted adaptive wiretap coding is adopted to enhance the security of offloading links. With PHY security, the intended receiver can decode secret message while eavesdropper cannot. A modified exhaustive two-dimensional (2D) search algorithm is proposed to optimize transmission rate and secrecy rate in an effective secrecy throughput maximization problem, and a multi-agent twin delayed deep deterministic policy gradient algorithm (MATD3) is utilized to assign VUEs’ tasks without a central controller, where a reward function is defined according to the computing costs, including execution time, energy consumption, and price paid for computing. Finally, simulations verify the effectiveness of the proposed framework.

This work introduces a new method to estimate Doppler frequency which carries important characteristics feature of time-variant multipath fading channels. The proposed method employs the time diversity (TD) of general random processes with rich correlation characteristics introduced by various time intervals, where a maximum-likelihood (ML) algorithm is utilized. A closed-form weighted least-square equation is derived with the help of basis expansion approximation. In addition, the Cram $\acute{\text{e}}$ r-Rao lower bound (CRLB) of the estimation is analyzed. The results show that the proposed method performs well in terms of Doppler frequency estimation accuracy.

This article investigates physical layer security (PLS) in reconfigurable intelligent surface (RIS)-assisted multiple-input multiple-output multiple-antenna-eavesdropper (MIMOME) channels. Existing researches ignore the problem that secrecy rate can not be calculated if the eavesdropper’s instantaneous channel state information (CSI) is unknown. Furthermore, without the secrecy rate expression, beamforming and phase shifter optimization with the purpose of PLS enhancement is not available. To address these problems, we first give the expression of secrecy outage probability for any beamforming vector and phase shifter matrix as the RIS-assisted PLS metric, which is measured based on the eavesdropper’s statistical CSI. Then, with the aid of the expression, we formulate the minimization problem of secrecy outage probability that is solved via alternately optimizing beamforming vectors and phase shift matrices. In the case of single-antenna transmitter or single-antenna legitimate receiver, the proposed alternating optimization (AO) scheme can be simplified to reduce computational complexity. Finally, it is demonstrated that the secrecy outage probability is significantly reduced with the proposed methods compared to current RIS-assisted PLS systems.

Traditional intra-aircraft communication systems were designed based mainly on electrical wired links for information exchange, which significantly increases the weight and fuel consumption of aircrafts. It is extremely difficult to examine these systems or replace defective wiring. Wireless avionics intra-communication (WAIC) is an emerging technology to mitigate these problems. This article presents an architecture of a next-generation WAIC network that uses reconfigurable intelligent surface (RIS) and focuses mainly on the wideband (WB) and millimeter wave (mmWave) communications of WAIC systems. We discuss the propagation characteristics of WB mmWave WAIC channel models inside aircrafts. Then, we introduce channel codes for WAIC systems. In particular, we propose novel low-density parity-check (LDPC) codes called collective-encoded LDPC codes (CE-LDPC). In addition, several important physical-layer techniques - for example, transceiver design, multiple-access, beamforming, RIS, and so on - are discussed with their related issues.

This letter focuses on the problem of uplink channel estimation with the reduced fronthaul overhead in cell-free massive multiple-input multiple-output (mMIMO) systems. First, we propose a sub-sampling scheme to reduce the dimension of fronthaul. Then, we exploit the inherent channel sparsity and model the underdetermined channel estimation problem as an off-grid sparse signal recovery problem. Finally, an enhanced sparse Bayesian learning (ESBL) channel estimation algorithm is proposed to refine the sampled grid points and recover the sparse channel iteratively. Simulation results demonstrate that the proposed algorithm achieves a significant reduction on the fronthaul overhead and offers a better channel estimation performance.

Task-oriented caching in vehicular edge computing (VEC) has become a hot topic recently, which brings in many opportunities for capability limited vehicles and makes VEC task offloading and service provision more cost-effective. Most works on caching assisted task offloading in VEC focused mainly on the reuse of computation results and neglected the characteristics pertaining to VEC. In effect, task-oriented caching in VEC is time-sensitive, requiring multiple-choice and multi-granularity. We need to address the issues on different computing and storage capabilities between vehicles and edge server with their diverse applications. Therefore, caching enabled task offloading in VEC aims for exploitation and enhancement of the functions in edge computing. To investigate in depth the benefits of caching enabled task offloading in VEC, we first provide an architectural overview on caching enabled vehicular edge computing (CVEC). Then, we strive to address the most concerned issues, that is, where, what, how and when to cache the tasks in CVEC. Specifically, three caching strategies are introduced to address what to cache in CVEC. In addition, caching strategies are evaluated with respect to response latency and energy consumption at edge servers. Simulation results reveal that caching enabled task offloading in CVEC can remarkably improve the performance of VEC. We also take a look at the opportunities and discuss the challenges in CVEC to shed light on future research directions on CVEC.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

VLC can perform the functions of communications and lighting simultaneously, which is expected to be an important part of next-generation wireless communications. Compared to traditional RF wireless communications, VLC offers many salient features, such as its operation on unlicensed visible light spectra, its support of high communication rates, and its immunity against electromagnetic interferences. In this article, we focus on code division multiple access techniques used in VLC systems and discuss different application scenarios for VLC-CDMA technologies. In particular, two types of VLC-CDMA systems will be identified, i.e., unipolar codes and bipolar codes based VLC-CDMA systems, whose performances will be analyzed and compared explicitly.

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.

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.

Nowadays, computation is playing an increasingly more important role in the future generation of computer and communication networks, as exemplified by the recent progress in SDN for wired networks as well as C-RAN and mobile cloud computing (MCC) for wireless networks. This article proposes a unified concept (i.e., computation diversity) to describe the impact and diverse forms of the computation resources on both wired and wireless communications. By linking the computation resources to the communication networks based on quality of service requirements, we can show how computation resources influence the networks. Moreover, by analyzing the different functionalities of computation resources in SDN, C-RAN, and MCC, we can show the diverse and flexible forms that the computation resources present in different networks. The study of computation diversity can provide guidance in future networks design as far as how to allocate the resources jointly between computation (e.g., CPU capacity) and communication (e.g., bandwidth), thereby saving system energy and improving users' experiences.

Covert channels are communication channels to transmit information utilizing existing system resources without being detected by network security elements, such as firewalls. Thus, they can be utilized to leak confidential governmental, military, and corporate information. Malicious users, like terrorists, can use covert channels to exchange information without being detected by cyber-intelligence services. Therefore, covert channels can be a grave security concern, and it is important to detect, eliminate, and disrupt covert communications. Active network wardens can attempt to eliminate such channels by traffic modification, but such an implementation will also hamper innocuous traffic, which is not always acceptable. Owing to a large number of covert channel algorithms, it is not possible to deal with them on a case-by-case basis. Therefore, it necessitates a unified system model that can represent them. In this paper, we present an event-based model to represent timing covert channels. Based on our model, we calculate the capacity of various covert channels and evaluate their essential features, such as the impact of network jitter noise and packet losses. We also used simulations to obtain these parameters to verify its accuracy and applicability.

Bit-interleaved coded modulation (BICM) has been widely used in capacity-achieving communication systems. In this paper, we propose an enhanced BICM, namely, arithmetic-BICM (A-BICM), which is able to achieve high channel throughput and seamless rate adaptation in time-varying wireless channels. The A-BICM generates modulation symbols through arithmetic weighted sum operations, and it adapts to data rate via adjusting the number of transmitted symbols. Our contributions in the design of A-BICM are twofold. First, we use an extrinsic information transfer chart to analyze a demapping procedure, and this facilitates the selection of the weighting multiset, which is a key design parameter in A-BICM mapping. Second, we employ a systematic channel code in A-BICM and design an adaptation scheme to strike a balance between the performances in both high- and low-SNR regions. Simulation results show that the A-BICM outperforms the conventional BICM with length-864 low-density parity check coding in both additive white Gaussian noise and fading channels. The experiments in real WLAN environments reveal that A-BICM achieves 42.1% and 20.9% throughput gain over conventional BICM and hybrid automatic repeat request rate adaptation schemes. Furthermore, the channel coding rate and its constellation in A-BICM do not need to change within a fairly wide SNR region from 1 to 30 dB to reduce communication overhead.

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.

Due to the popularity of mobile user devices, D2D communications become a key technology to improve local services in the next generation cellular networks. However, the benefits count on efficient radio resource allocation between cellular links and D2D links, which poses critical challenges. By exploiting the network knowledge extracted from underlying mobile social networks (MSNs), this article proposes a socially aware D2D communication scheme to improve the spectrum and energy efficiency in cellular networks. First, by jointly considering the social and network knowledge, we propose a cluster formation scheme to categorize a group of UEs into multiple D2D clusters. Within each cluster, highly spectrum-efficient D2D multicasting (from the cluster head, i.e., a D2D transmitter UE to multiple cluster member UEs) is enabled. Next, a half-duplex scheme and a full-duplex scheme are proposed to coordinate the channel sharing between the cellular links and the D2D links. Simulation results show that the proposed schemes can form D2D clusters rationally among a group of UEs, and therefore largely increase the spectrum and energy efficiency of the network.

This paper proposes a new type of complementary code, i.e., 3-D complementary code (3DCC), which integrates space-time coding and code division multiple access under a unified framework for multiple-input-multiple-output (MIMO) system implementation. Owing to its ideal correlation properties, the 3DCC ensures orthogonal transmissions among users and antennas. Analytical studies given in this paper will show that the proposed 3DCC-MIMO system is able to support both transmit diversity and multiplex at the same time, with its unique agility to switch between them according to different channel environments and system configurations. Additionally, multiuser interference (MUI) is eliminated in a 3DCC-MIMO system for both synchronous and asynchronous transmissions over a flat-fading channel with a relatively low complexity. Simulation results are also provided to verify the correctness of analysis and demonstrate the superiority of the proposed MIMO system in terms of its MUI-free performance.

Motivated by growing demand for high-speed wireless communications, we proposed to implement code division multiple access systems based on orthogonal complementary codes (OCC-CDMA). While an OCC-CDMA system indeed offers ideal correlation properties, its capacity is strictly constrained by the number of element codes each user may use. In this paper, we suggest to use in-phase/quadrature (I/Q) column-wise complementary (IQCC) codes to implement a CDMA system to support 2M users, where M stands for the number of element codes in a flock of IQCC codes assigned to a user. The IQCC codes were designed to fit quadrature phase-shift keying modulation, where odd and even chips can be transmitted via I and Q carriers. We analyze correlation properties of the IQCC codes and introduce a CDMA system design based on the codes. The analytical results show that out-of-phase autocorrelation and cross-correlation functions are zero for any relative time shifts. Thus, a CDMA system implemented by the IQCC codes has an inherent capability to mitigate multiple access interference and multipath interference.

Presents the introductory editorial from the Editor-in-Chief.

Cyber-physical systems (CPSs) integrate the computation with physical processes. Embedded computers and networks monitor and control the physical processes, usually with feedback loops where physical processes affect computations and vice versa. CPS was identified as one of the eight research priority areas in the August 2007 report of the President's Council of Advisors on Science and Technology, as CPS will be the core component of many critical infrastructures and industrial control systems in the near future. However, a variety of random failures and cyber attacks exist in CPS, which greatly restrict their growth. Fortunately, an intrusion detection mechanism could take effect for protecting CPS. When a misbehavior is found by the intrusion detector, the appropriate action can be taken immediately so that any harm to the system will be minimized. As CPSs are yet to be defined universally, the application of the instruction detection mechanism remain open presently. As a result, the effort will be made to discuss how to appropriately apply the intrusion detection mechanism to CPS in this paper. By examining the unique properties of CPS, it intends to define the specific requirements first. Then, the design outline of the intrusion detection mechanism in CPS is introduced in terms of the layers of system and specific detection techniques. Finally, some significant research problems are identified for enlightening the subsequent studies.

In this paper, we propose a new system architecture using complete complementary (CC) codes for a multicarrier MIMO CDMA system to combat multipath fading in wireless channels. Unlike traditional MIMO systems, complementary codes were designed at chip-level to achieve time and frequency-spreading for each antenna. We analyze the system performance in multipath fading channels. A closed form expression of bit error rate (BER) performance is obtained. Extensive simulations were conducted to verify the analytical results. Our results indicate that a CC coded MIMO system can effectively decrease BER and take advantage of diversity gain.

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.

Wireless sensor networks (WSNs) consist of tiny sensor nodes that communicate with each other over wireless channels, often in a hostile environment where nodes can be captured and compromised. Consequently, an adversary may launch a clone attack by replicating the captured nodes to enlarge the compromised areas employing clones. Thus, it is critical to detect clone nodes promptly for minimizing their damage to WSNs. Recently, various clone detection schemes were proposed for WSNs, considering different types of network configurations, such as device types and deployment strategies. In order to choose an effective clone detection scheme for a given sensor network, the selection criteria play an important role. In this paper, we first investigate the selection criteria of clone detection schemes with regard to device types, detection methodologies, deployment strategies, and detection ranges. We then classify the existing schemes according to the proposed criteria. Simulation experiments are conducted to compare their performances. It is concluded that it is beneficial to utilize the grid deployment knowledge for static sensor networks; the scheme using the grid deployment knowledge can save energy by up to 94.44% in comparable performance (specifically in terms of clone detection ratio and the completion time), as compared to others. On the other hand, for mobile sensor networks, no existing approach works efficiently in reducing detection error rate.

In this letter, we propose an analytical approach to model uplink intercell interference (ICI) in hexagonal grid based orthogonal frequency division multiple access (OFMDA) cellular networks. The key idea is that the uplink ICI from individual cells is approximated with a lognormal distribution with statistical parameters being determined analytically. Accordingly, the aggregated uplink ICI is approximated with another lognormal distribution and its statistical parameters can be determined from those of individual cells using Fenton-Wilkson method. Analytic expressions of uplink ICI are derived with two traditional frequency reuse schemes, namely integer frequency reuse schemes with factor 1 (IFR-1) and factor 3 (IFR-3). Uplink fractional power control and lognormal shadowing are modeled. System performances in terms of signal to interference plus noise ratio (SINR) and spectrum efficiency are also derived. The proposed model has been validated by simulations.

This letter analyzes the performance of complementary coded code division multiple access (CC-CDMA) systems in frequency selective fading channels. Based on the evaluation of various combining algorithms for the correlation functions of M element sequences of a CC, a per-carrier parallel interference cancellation scheme based on minimum mean square error combining (MMSEC) is proposed for CC-CDMA to achieve a frequency diversity gain and multi-user interference resistance. Both analytical and simulation results verify the superiority of the proposed scheme. It is shown that combining algorithms and correlation properties of element sequences have a significant impact on the performance of CC-CDMA systems in frequency selective fading channels.

As the migration from code division multiple access (CDMA) to orthogonal frequency-division multiple access (OFDMA) involves a technological revolution in physical layer, the high speed packet access (HSPA) evolution (HSPA+) has been proposed for a smooth migration. HSPA+ is an effort to further improve radio performance of HSPA, and wideband CDMA (WCDMA) HSPA+ and time division-synchronous CDMA (TD-SCDMA) HSPA+ (TD-HSPA+) are two HSPA+ systems evolving from WCDMA and TD-SCDMA, respectively. Different from WCDMA HSPA+ system, TD-HSPA+ system utilizes eight antennas at Node B and two antennas at user equipment (UE) for 8 x 2 Multi-Input and Multi-Output (MIMO) transmission to achieve antenna array gain and diversity gain. Furthermore, to facilitate 8 x 2 MIMO configuration, the antenna elements are partitioned into two groups for dual-polarization transmissions. This paper focuses on 8 x 2 MIMO schemes in TD-HSPA+ systems, including the traditional single polarized MIMO (SP-MIMO), the separated and the joint singular value decomposition (SVD) based precoding dual-polarized MIMO (DP-MIMO) schemes. Performances of these TD-HSPA+ MIMO schemes are evaluated for both urban macro and micro scenarios. The simulation results show that the separated SVD based precoding DP-MIMO scheme performs better than the joint SVD based precoding DP-MIMO scheme, and the DP-MIMO scheme offers almost the same performance as the SP-MIMO due to the beamforming diversity gain and multiplex gain in TD-HSPA+ systems.

Orthogonal frequency division multiplexing (OFDM) technique has been widely used in high data rate wireless applications, but it suffers performance degradation at low signal-to-noise (SNR) regions due to the impairments caused by noise-induced channel estimation errors. In this article, we propose to use frequency-shift (FRESH) filtering technique to exploit temporal and spectral correlation of the OFDM signals for noise reduction purpose. A closed form time-varying minimum mean squared error (MMSE) expression has been derived to analyze the performance of FRESH filtering under additive white Gaussian noise (AWGN) and Rayleigh fading channels. The MMSE analysis carried out here can be a reference for the design of practical OFDM systems. In addition, theoretical SNR is analyzed in this article to show the performance gain achieved by FRESH filtering. The numerical results verify that the maximum SNR improvements offered by FRESH filtering are over 0.6 dB and 1.6 dB under AWGN and Rayleigh fading channels, respectively.