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In this paper, we consider deploying an intelligent reflecting surface (IRS) to enhance the downlink (DL) energy transfer and uplink (UL) information transmission efficiency for secure multiple-input multiple-output (MIMO) wireless powered communication networks (WPCNs). We aim to maximize the secrecy throughput of all users by jointly optimizing the DL/UL time allocation, the energy transmit covariance matrix of hybrid access point (AP), the information transmit beamforming matrix of users and the phase shifts of IRS in DL/UL, subject to constraints of energy/information transmit power at the hybrid AP/users and that of unit-modulus IRS phase shifts for DL/UL. To tackle the non-convex problem, we first transform the original problem into an equivalent form based on the mean-square error (MSE) method given time allocation, and then apply the alternating algorithm to update the optimization variables iteratively. Specifically, the energy covariance matrix and the information beamforming matrix are obtained based on the dual subgradient method. For the IRS phase shifts, we investigate two IRS beamforming reflection setups, namely different DL/UL IRS beamforming and identical DL/UL IRS beamforming. For the former case, the second-order cone programming technique and the Majorization-Minimization algorithm/element by element iterative algorithm are applied to obtain the DL and UL IRS phase shifts, respectively. For the latter case, the IRS phase shifts are obtained by the successive convex approximation technique. To further reduce the computational complexity of the single-user system, we derive the closed-form solutions of IRS phase shifts in each iteration for the two different reflection setups. Simulation results show that all the proposed algorithms can greatly improve the secrecy throughput compared to the conventional system without IRS. [ABSTRACT FROM AUTHOR]

In this paper, the performance analysis of non-orthogonal multiple access (NOMA) in a cloud radio access networks (C-RAN) is carried out. The problem of jointly optimizing user association, muting and power-bandwidth allocation is formulated for NOMA-enabled C-RANs. To solve the mixed integer programming problem, the joint problem is decomposed into two subproblems as 1) user association and muting 2) power-bandwidth allocation optimization. To deal with the first subproblem, we propose a centralized and heuristic algorithm to obtain a feasible solution to the remote radio head (RRH) muting problem for given bandwidth and transmit power. The second subproblem is then reformulated for tractibility purpose and a low-complexity algorithm is proposed to bandwidth and power allocation subject to users data rate constraints. Moreover, for given user association and muting states, the near optimal power allocation is derived in a closed-form expression. Simulation results show that the proposed NOMA-enabled C-RAN outperforms orthogonal multiple access (OMA)-based C-RANs in terms of total achievable rate, interference mitigation and can achieve significant fairness improvement. [ABSTRACT FROM AUTHOR]

This paper studies a secure multiuser multiple-input single-output (MISO) communication system aided by an intelligent reflecting surface (IRS), where multiple colluding eavesdroppers (EVEs) coexist. We aim to maximize the sum secrecy rate (SSR) via jointly optimizing the beamforming vectors, the artificial noise (AN) and the phase shifts at the IRS subject to the maximum transmit power constraint and unit modulus constraints. To address the non-convex optimization problem, we first propose an alternating optimization (AO) algorithm based on semidefinite relaxation (SDR) and obtain a high-quality sub-optimal solution. In order to reduce the high computational complexity, a low-complexity alternating optimization (LC-AO) algorithm is developed, in which the beamforming vectors, AN and the IRS phase shifts are optimized alternately by the generalized power iteration (GPI) and the Riemannian manifold conjugate gradient (RMCG) algorithm, respectively. Simulation results show the advantages of deploying the IRS in improving the system secrecy performance. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the anti-jamming problem of a directional modulation (DM) system with the aid of intelligent reflecting surface (IRS). As an efficient tool to combat malicious jamming, receive beamforming (RBF) is usually designed by using the statistical properties of the jamming received by Bob. Thus, it is very necessary to estimate the receive jamming covariance matrix (JCM) at Bob. To achieve a precise JCM estimation, three JCM estimation methods, including eigenvalue decomposition (EVD), parametric estimation method by gradient descend (PEM-GD) and parametric estimation method by alternating optimization (PEM-AO), are proposed. Here, the proposed EVD is derived according to the rank-2 constraint of JCM. The PEM-GD method fully explores the structure features of JCM and the PEM-AO is proposed to decrease the computational complexity of the former via dimensionality reduction. The simulation results show that the proposed three methods perform better than directly using sample covariance matrix. Additionally, the proposed PEM-GD and PEM-AO outperform EVD method and the clutter and disturbance covariance estimator RCML. [ABSTRACT FROM AUTHOR]

This paper proposes a new simultaneous terahertz (THz) information and power transfer (STIPT) system, which utilizes a reconfigurable intelligent surface (RIS) for both the data and power transmission. We aim to maximize the information users’ (IUs’) sum data rate while guaranteeing the power harvesting requirements of energy users (EUs) and RIS. To solve the formulated non-convex problem, the block coordinate descent (BCD) based algorithm is adopted to alternately optimize the transmit precoding of IUs, RIS’s reflecting coefficients, and the position of RIS. Additionally, the penalty constrained convex approximation (PCCA) algorithm is proposed to optimize the deployment of the RIS, where the introduced penalties ensure that the solution is always feasible. The simulation results show that the proposed solution outperforms the benchmark schemes, and the proposed BCD algorithm can greatly improve the performance of the STIPT system. [ABSTRACT FROM AUTHOR]

With the very stringent demand for real-time transmission of wireless communication services, the requirement of sub-millisecond ultra-low end-to-end delay has been initially proposed in the sixth generation (6G) communication networks. Finite blocklength transmission is one of the potential technologies to meet such a low end-to-end delay demand for the next generation networks. However, as the finite blocklength decreases, the transmission delay decreases while the queuing delay increases, which results in the tradeoff between the transmission delay and the queuing delay. To achieve the optimal balance, in this paper we propose an adaptive blocklength transmission framework to minimize the important part of the end-to-end delay of wireless networks, where we focus on the transmission delay and queuing delay. A dynamic buffering model for variable transmission time interval (V-TTI) is introduced for the time-varying arrival of packets adaptation. Then, we propose the Flexible proximal Alternating direction method of multipliers based Blocklength Optimization (FaBo) scheme to minimize the important part of the end-to-end delay for the single user case. We also propose the Multiple deep Q-learning network based Resource Allocation (MuRa) scheme, which can efficiently balance the transmission delay and queuing delay, to minimize the important part of the end-to-end delay for the multi-user case. Numerical results show that the proposed adaptive blocklength framework can reduce the important part of the end-to-end delay compared with that of long-term evolution and the fifth generation (5G) new radio. We also show that our proposed schemes can quickly converge to the minimum end-to-end delay. [ABSTRACT FROM AUTHOR]

In this article, we present a new scenario of direction of arrival estimation using massive multiple-input multiple-output receive array with low-resolution analog-to-digital convertors (ADCs), which can strike a good balance between performance and circuit cost. Based on the linear additive quantization noise model, the inpact of low-resolution ADCs on methods, such as Root-MUSIC, is analyzed. Also, the closed-form expression of Cramer–Rao lower bound (CRLB) is derived to evaluate the performance loss caused by the low-resolution ADCs. The simulation results show that the Root-MUSIC methods can achieve the corresponding CRLB. Furthermore, adopting ADCs with 2–3 b is an efficient choice for most applications if 1 dB performance loss is acceptable. [ABSTRACT FROM AUTHOR]

In this paper, an intelligent reflecting surface (IRS) is deployed to assist the terahertz (THz) communications. The sum-rate of user equipments (UEs) is maximized while guaranteeing the rate requirement of each UE. A block coordinate searching (BCS) algorithm is proposed to jointly optimize the IRS’s coordinates, phase shifts, THz sub-bands allocation and power control. Specifically, the relaxation with penalties based (RPB) algorithm is developed to obtain feasible coordinates of the IRS and guarantee the monotonicity of objective value. In addition, the IRS phase shifts are formulated as closed-form expressions with introduced pricing factors. Simulation results show that the proposed scheme can significantly enhance system performance. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) are envisioned to be a disruptive wireless communication technique that is capable of reconfiguring the wireless propagation environment. In this paper, we study a free-space RIS-assisted multiple-input single-output (MISO) communication system in far-field operation. To maximize the received power from the physical and electromagnetic nature point of view, a comprehensive optimization, including beamforming of the transmitter, phase shifts of the RIS, orientation and position of the RIS is formulated and addressed. After exploiting the property of line-of-sight (LoS) links, we derive closed-form solutions of beamforming and phase shifts. For the non-trivial RIS position optimization problem in arbitrary three-dimensional space, a dimensional-reducing theory is proved. The simulation results show that the proposed closed-form beamforming and phase shifts approach the upper bound of the received power. The robustness of our proposed solutions in terms of the perturbation is also verified. Moreover, the RIS significantly enhances the performance of the mmWave/THz communication system. [ABSTRACT FROM AUTHOR]

The application of non-orthogonal multiple access (NOMA) to multicarrier systems can improve the spectrum efficiency and enable massive connectivity in future mobile systems. Resource allocation in multicarrier NOMA systems is a non-deterministic polynomial time-hard problem requiring exhaustive search, which has prohibitive computational complexity. Instead, efficient algorithms that provide a good trade-off between system performance and implementation practicality are needed. In this paper, exact values of the optimal channel gain ratios between a pair of NOMA users are presented for the first time for quadrature amplitude modulation (QAM) schemes. Further, numerical limits are derived for the values of channel gain ratios that fulfill the system constraints. These findings are used to propose a user pairing algorithm with quasi-linear complexity. Further, a novel scheme for data rate and continuous power allocation is proposed. Through numerical simulations, it is proved that the proposed scheme yields an achievable sum-rate close to the performance of exhaustive search, and it outperforms other suboptimal resource allocation schemes. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) or intelligent reflecting surfaces (IRSs) are regarded as one of the most promising and revolutionizing techniques for enhancing the spectrum and/ or energy efficiency of wireless systems. These devices are capable of reconfiguring the wireless propagation environment by carefully tuning the phase shifts of a large number of low-cost passive reflecting elements. In this article, we aim to answer four fundmental questions: 1) Why do we need RISs? 2) What is an RIS? 3) What are RIS's applications? 4) What are the relevant challenges and future research directions? In response, eight promising research directions are pointed out. [ABSTRACT FROM AUTHOR]

In this letter, an intelligent reflecting surface (IRS)-aided secure wireless information and power transfer system is studied. To maximize the harvested power of energy harvesting receiver (EHR), we optimize the secure transmit beamforming at the access point (AP) and phase shifts at the IRS subject to the secrecy rate (SR) and the reflecting phase shifts at the IRS constraints. Due to the non-convexity of optimization problem and coupled optimization variables, we convert the optimization problem into a semidefinite relaxation (SDR) problem and a sub-optimal solution is obtained. To reduce the high-complexity of the proposed SDR method, a low-complexity alternating optimization (LC-AO) algorithm is proposed. Simulation results show that the harvested power of the proposed SDR and LC-AO methods approximately double that of the existing method without IRS with the same SR. In particular, the proposed LC-AO achieves almost the same performance as the proposed SDR but with a much lower complexity. [ABSTRACT FROM AUTHOR]

High-speed railway (HSR) has demonstrated tremendous growth worldwide, and is currently attaining a maximum velocity of 575 km/h. Such a high speed makes mobile wireless communications extremely challenging for HSR since the handover (HO) rate increases with speed, resulting in a high call dropping probability. By employing a dedicated distributed antenna system (DAS) along with the two-hop network architecture, in this paper, we propose a frequency switch (FSW) scheme to mitigate the persistent HO issue in conventional HSR wireless communication systems. The proposed scheme significantly alleviates interruption time associated with the traditional HO process, providing an effective scheme when the train moves across small cells covered by remote antenna units (RAUs) belonging to the same central unit (CU). In order to further reduce the HO command failure probability, an enhanced fast predictive HO mechanism is proposed by starting the HO process in advance, when the train moves from one RAU to the next one controlled by the adjacent CU. Analytical results show that compared to the traditional HO scheme, the proposed schemes can improve the system performance substantially by delivering ultra-reliable low-latency communications with a reduced HO/FSW rate. [ABSTRACT FROM AUTHOR]

This letter investigates a multiuser mobile-edge computing (MEC) system with data compression technique to reduce the redundancy of sensed data, save the energy consumption and reduce the latency for wireless transmission. In this regard, the problem of minimizing the total energy consumption by jointly optimizing the transmission bandwidth allocation and data compression ratio is firstly considered under the constraints of latency and limited computation resource. Moreover, Lagragian and iterative-based algorithms are proposed to solve the non-convex optimization problem. Finally, simulation results are shown to verify the efficiency of the proposed algorithms and demonstrate that the application of data compression technique into MEC system can effectively reduce the energy consumption and latency. [ABSTRACT FROM AUTHOR]

Based on directional modulation (DM), robust beamforming matrix design for sum secrecy rate maximization is investigated in this article in multiuser systems. First, in case of random estimation error of direction angles toward eavesdroppers, a Von Mises distribution-sum secrecy rate maximization (VMD-SSRM) method is proposed to maximize the sum secrecy rate. Semidefinite relaxation and first-order approximation based on Taylor expansion are employed to solve the optimization problem in VMD-SSRM. Then in order to optimize the sum secrecy rate in the case of the worst estimation errors of direction angles, a maximum angle estimation error-SSRM (MAEE-SSRM) method is proposed. The optimization problem is constructed based on the upper and lower bounds of the estimated eavesdropping channel related coefficient and then solved by the change of the variable. Simulation results show that our two proposed methods have better sum secrecy rates than the state-of-the-art methods. Furthermore, when the estimation error of direction angle toward each eavesdropper is random but within a certain range, the secrecy performance of our VMD-SSRM method is better than that of our MAEE-SSRM method. On contrary, when the direction angle estimation error reaches its maximum, our MAEE-SSRM method performs better. [ABSTRACT FROM AUTHOR]

An intelligent reflecting surface (IRS) is invoked for enhancing the energy harvesting performance of a simultaneous wireless information and power transfer (SWIPT) aided system. Specifically, an IRS-assisted SWIPT system is considered, where a multi-antenna aided base station (BS) communicates with several multi-antenna assisted information receivers (IRs), while guaranteeing the energy harvesting requirement of the energy receivers (ERs). To maximize the weighted sum rate (WSR) of IRs, the transmit precoding (TPC) matrices of the BS and passive phase shift matrix of the IRS should be jointly optimized. To tackle this challenging optimization problem, we first adopt the classic block coordinate descent (BCD) algorithm for decoupling the original optimization problem into several subproblems and alternately optimize the TPC matrices and the phase shift matrix. For each subproblem, we provide a low-complexity iterative algorithm, which is guaranteed to converge to the Karush-Kuhn-Tucker (KKT) point of each subproblem. The BCD algorithm is rigorously proved to converge to the KKT point of the original problem. We also conceive a feasibility checking method to study its feasibility. Our extensive simulation results confirm that employing IRSs in SWIPT beneficially enhances the system performance and the proposed BCD algorithm converges rapidly, which is appealing for practical applications. [ABSTRACT FROM AUTHOR]

In this work, a detection strategy based on multiple antennas with beam sweeping is developed to detect the potential transmission of unmanned aerial vehicle (UAV) in wireless networks. Specifically, suspicious angle range where the UAV may present is divided into different sectors to potentially increase detection accuracy by using beamforming gain. We then develop the optimal detector and derive its detection error probability in a closed-form expression. We also utilize the Pinsker's inequality and Kullback-Leibler divergence to yield low-complex approximation for the detection error probability, based on which we obtain some significant insights on the detection performance. Our numerical results show that there exists an optimal number of sectors that can minimize the detection error probability in some scenarios (e.g., when the number of measurements is limited). Intuitively, this can be explained by the fact that there exists an optimal accuracy of the telescope used to find an object in the sky within limited time period. [ABSTRACT FROM AUTHOR]

We consider an underlay wiretap cognitive two-way relay network (CTWRN), where two secondary sources exchange their messages via multiple secondary decode-and-forward digital network coding relays in the presence of an eavesdropper by using a three-phase time division broadcast protocol and sharing the licensed spectrum of primary users. To mitigate eavesdropping attacks, an artificial noise (AN)-aided opportunistic relay selection scheme, called generalized max-min (GMM) relay selection is proposed to enhance physical layer security for the wiretap CTWRNs. The performance of the GMM scheme is analyzed, and evaluated by the exact closed-form outage probability and intercept probability. Additionally, we also provide asymptotic approximations for the outage probability and intercept probability at high signal-to-noise ratio. For comparison, we analyze the performance of the conventional max-min (MM) relay selection scheme as well. It is shown that the GMM scheme outperforms the MM scheme in terms of the security-reliability tradeoff (SRT), where the security and reliability are quantified by the intercept probability and outage probability, respectively. Moreover, the SRTs of the MM and GMM schemes can be substantially improved by increasing the number of secondary relays, while the improvement of the GMM scheme is more evident than that of the MM scheme. [ABSTRACT FROM AUTHOR]

In directional modulation networks, two practical random-subcarrier-selection methods are proposed to transmit confidential message, and by constructing random subcarrier set and performing a randomization procedure, only a single receive power peak is formed in the desired user position per orthogonal frequency division multiplexing (OFDM) symbol. This scheme completely addresses the crucial problem facing secure precise wireless transmission (SPWT); how to achieve the SPWT per OFDM symbol while the traditional SPWT holds only in the statistically average sense. Several necessary conditions for SPWT per OFDM are derived and proposed: random, distributed, and distinct subcarrier spacing pattern mapped onto transmit antenna array. Two random subcarrier index sets, quadratic subcarrier set (QSS), and prime subcarrier set (PSS) are constructed. Subsequently, a random metric is defined, and a randomization procedure (RP) is proposed. Its detailed process include the following steps: integer mod, ordering, blocking, and block interleaving (BI) where BI is repeated until the random metric is greater than the predefined value. This yields a single high energy main peak at the desired position while at other positions, i.e., outside the main peak, only weak received energy peak seriously contaminated by the artificial noise are harvested. Simulation results confirm that our proposed two methods, QSS plus RP and PSS plus RP, perform significantly better than conventional methods such as linear subcarrier set plus RP. They actually form a single high energy main peak at desired position, and generate only extremely small leakage outside the main peak. [ABSTRACT FROM AUTHOR]

This paper examines the possibility, performance limits, and associated costs for a self-sustained relay to transmit its own covert information to a destination on top of forwarding the source’s information. Since the source provides energy to the relay for forwarding its information, the source does not allow the relay’s covert transmission and to detect it. Considering the time switching (TS) and power splitting (PS) schemes for energy harvesting, where all the harvested energy is used for transmission at the self-sustained relay, we derive the minimum detection error probability $\xi ^{\ast }$ at the source based on which we determine the maximum effective covert rate $\Psi ^{\ast }$ subject to a given covertness constraint on $\xi ^{\ast }$. Our analysis shows that $\xi ^{\ast }$ is the same for the TS and PS schemes, which leads to the fact that the cost of achieving $\Psi ^{\ast }$ in both the two schemes in terms of the required increase in the energy conversion efficiency at the relay is the same, although the values of $\Psi ^{\ast }$ in these two schemes can be different in specific scenarios. For example, the TS scheme outperforms the PS scheme in terms of achieving a higher $\Psi ^{\ast }$ when the transmit power at the source is relatively low. If the covertness constraint is tighter than a specific value, it is the covertness constraint that limits $\Psi ^{\ast }$ , and otherwise, it is upper bound on the energy conversion efficiency that limits $\Psi ^{\ast }$. [ABSTRACT FROM AUTHOR]

The mobility management will be more complex and will have a great impact on the quality of service in the future cellular networks, as these networks will have to handle a huge number of user equipments (UEs) and their frequent hand offs due to very dense short-footage small cells. This paper presents a framework to model and derive the coverage of small cells, the cell sojourn time, and the hand off rate in multi-tier small cell networks. The distribution of the small cells around a reference UE’s path is studied by taking into consideration the overlaps among the small cells. Two types of hand off rates are introduced to estimate the load managed by different cells, where inter-frequency hand off rate and intra-frequency hand off rate represent the fraction of hand offs managed by the first tier and the other tiers, respectively. Our analysis shows that ignoring the overlaps among the small cells affects the accuracy of the results significantly. The simulation results validate the accuracy of the analytical results and also show the impact of different parameters, such as the small cell density, the number of tiers and the size of the small cells on the small cell sojourn time, the macro cell sojourn time, and the hand off rate. [ABSTRACT FROM AUTHOR]

Covert wireless communication aims to hide the very existence of wireless transmissions in order to guarantee a strong security in wireless networks. In this paper, we examine the possibility and achievable performance of covert communication in amplify-and-forward one-way relay networks. Specifically, the relay is greedy and opportunistically transmits its own information to the destination covertly on top of forwarding the source’s message, while the source tries to detect this covert transmission to discover the illegitimate usage of the resource (e.g., power and spectrum) allocated only for the purpose of forwarding the source’s information. We propose two strategies for the relay to transmit its covert information, namely rate-control and power-control transmission schemes, for which the source’s detection limits are analyzed in terms of detection error probability and the achievable effective covert rates from the relay to destination are derived. Our examination determines the conditions under which the rate-control transmission scheme outperforms the power-control transmission scheme, and vice versa, which enables the relay to achieve the maximum effective covert rate. Our analysis indicates that the relay has to forward the source’s message to shield its covert transmission and the effective covert rate increases with its forwarding ability (e.g., its maximum transmits power). [ABSTRACT FROM AUTHOR]

In this paper, a tradeoff content caching strategy is proposed to jointly minimize the cell average outage probability and fronthaul usage in a cloud radio access network. At first, an accurate closed-form expression of the outage probability conditioned on the user's location is presented, and the cell average outage probability is obtained through the composite Simpson's integration. The caching strategy for jointly optimizing the cell average outage probability and fronthaul usage is then formulated as a weighted sum minimization problem, which is a nonlinear 0–1 integer problem. Two heuristic algorithms are proposed to solve the problem. First, a genetic algorithm (GA)-based approach is proposed. Numerical results show that the performance of the proposed GA-based approach with significantly reduced computational complexity is close to the optimal performance achieved by an exhaustive-search-based caching strategy, and the GA-based approach can improve the performance by up to 47.5% on average than the typical probabilistic caching strategy. Second, in order to further reduce the computational complexity, a mode selection approach is proposed. Numerical results show that this approach can achieve near-optimal performance over a wide range of weighting factors through a single computation. [ABSTRACT FROM AUTHOR]

Ultra-dense networks (UDNs) constitute one of the most promising techniques for supporting the 5G mobile system. By deploying more small cells in a fixed area, the average distance between users and access points can be significantly reduced; hence, dense spatial frequency reuse can be exploited. However, severe interference is the major obstacle in UDNs. Most contributions deal with the interference by relying on cooperative game theory. This article advocates the application of dense user-centric C-RAN philosophy to UDNs, thanks to the recent development of cloud computing techniques. Under dense C-RAN, centralized signal processing can be invoked for supporting CoMP transmission. We summarize the main challenges in dense user-centric C-RANs. One of the most challenging issues is the requirement of the global CSI for the sake of cooperative transmission. We investigate this requirement by only relying on partial CSI, namely, on inter-cluster large-scale CSI. Furthermore, the estimation of the intra-cluster CSI is considered, including the pilot allocation and robust transmission. Finally, we highlight several promising research directions to make the dense user-centric C-RAN become a reality, with special emphasis on the application of big data techniques. [ABSTRACT FROM AUTHOR]

In this paper, a practical wireless transmission scheme is proposed to transmit confidential messages to the desired user securely and precisely by the joint use of multiple techniques, including artificial noise (AN) projection, phase alignment/beamforming, and random subcarrier selection (RSCS) based on orthogonal frequency division multiplexing (OFDM), and directional modulation (DM), namely RSCS-OFDM-DM. This RSCS-OFDM-DM scheme provides an extremely low-complexity structure for the desired receiver and makes the secure and precise wireless transmission realizable in practice. For illegal eavesdroppers, the receive power of confidential messages is so weak that their receivers cannot intercept these confidential messages successfully once it is corrupted by AN. In such a scheme, the design of phase alignment/beamforming vector and AN projection matrix depends intimately on the desired direction angle and distance. It is particularly noted that the use of RSCS leads to a significant outcome that the receive power of confidential messages mainly concentrates on the small neighboring region around the desired receiver and only small fraction of its power leaks out to the remaining large broad regions. This concept is called secure precise transmission. The probability density function of real-time receive signal-to-interference-and-noise ratio (SINR) is derived. Also, the average SINR and its tight upper bound are attained. The approximate closed-form expression for average secrecy rate is derived by analyzing the first-null positions of the SINR and clarifying the wiretap region. Simulation and analysis show that the proposed scheme actually can achieve a secure and precise wireless transmission of confidential messages in line-of-propagation channel, and the derived theoretical formula of average secrecy rate is verified to coincide with the exact results well for medium and large scale transmit antenna array or in the low and medium SNR regions. [ABSTRACT FROM AUTHOR]

Providing high data rate wireless transmissions has been difficult in indoor environments, particularly in multi-floor buildings. One way to achieve high data rate wireless transmissions is to reduce the radio transmission distance between the transmitter and the receiver by using distributed antenna systems (DASs) and employing frequency reuse. However, due to the reuse of the limited available spectrum, co-channel interference can severely degrade system capacity. In this paper, the uplink spectral efficiency of an in-building DAS with frequency reuse is studied, where remote antenna units (RAUs) deployed on each floor throughout the building are connected to a central unit (CU) where received signals are processed. The impact of co-channel interference on system performance is investigated by using a propagation channel model derived from multi-floor, in-building measurement results. The proposed scheme exploits the penetration loss of the signal through the floors, resulting in frequency reuse in spatially separated floors, which increases system spectral efficiency and also reduces co-channel interference. A comparative analysis with conventional co-located antenna deployment at the floor center is provided. Location based RAU selection and deployment options are investigated. System performance is evaluated in terms of location-specific spectral efficiency for a range of potential mobile terminal (MT) locations and various in-building propagation characteristics. [ABSTRACT FROM PUBLISHER]

In the conventional receiver for pulse-amplitude-modulated discrete multitone (PAM-DMT), the real parts of the subcarriers are distorted by the clipping noise and ignored in detection. In this letter, the signal analysis shows that the real parts of the subcarriers also contain the information of the transmitted signal. An iterative receiver is then proposed to improve the transmission performance of PAM-DMT by exploiting both the real and imaginary parts of the subcarriers. Simulation results show that the proposed iterative receiver can significantly improve the transmission performance of PAM-DMT systems. [ABSTRACT FROM PUBLISHER]

Radio resource allocation is very important in relay orthogonal frequency-division multiple-access (OFDMA) networks. Since perfect channel state information (CSI) is hardly obtained, the robust design that takes into account the CSI imperfection plays a crucial role in the development of efficient resource-allocation schemes. As to the chunk-based resource-allocation scheme, this paper focuses on the imperfect CSI-based resource-allocation problem to jointly optimize the chunk assignment, transmission link selection (direct or relay link), and power allocation in multirelay OFDMA systems. The optimization formulation minimizes the total transmit power with transmission requirement constraints and is considered a mixed-integer optimization problem, and a dual-maximization-based joint resource-allocation algorithm is proposed. The proposed dual-maximization-based algorithm is shown to be optimal. We also develop a low-complexity but near-optimal resource-allocation algorithm. The numerical results validate the performance gains offered by the proposed joint resource-allocation algorithms. The results also show that the impact of imperfect CSI is nonnegligible, which highlights the importance of robust designs in practical wireless networks. [ABSTRACT FROM PUBLISHER]

The distributed antenna system (DAS) is a promising system to provide high data rate transmissions in wireless communications. So far, most researches in the performance analysis for the DAS have been focused on single-user cases. This paper aims to investigate and present an optimal resource allocation scheme in downlink multiuser DASs. Different from the resource allocation in a conventional cellular system, the resource allocation in the multiuser DAS must consider multiple channel conditions of the transmission links between multiple remote antenna unites (RAUs) and any user, which increases the complexity of the resource allocation. In order to provide insights in the multiuser DAS, the effect of the number of RAUs used to communicate with each user is investigated extensively. In addition to power allocation, subcarrier allocation and bit allocation are studied in the orthogonal frequency division multiple access (OFDMA) DAS. To reduce the complexity of the resource allocation in the downlink multiuser DAS, the chunk allocation technique is adopted by grouping a set of contiguous subcarriers into one chunk and allocating resources chunk by chunk to users and an equivalent channel fading factor per subcarrier for a user is proposed to represent the combined channel gain from multiple RAUs to the user. The numerical results show that by using the proposed resource allocation method, each user only needs to be connected to a few RAUs, depending on the location of the user. [ABSTRACT FROM AUTHOR]

With the deployment of high speed train (HST) systems increasing worldwide and their popularity with travelers growing, providing broadband wireless communications (BWC) in HSTs is becoming crucial. In this paper, a tutorial is presented on recent research into BWC provision for HSTs. The basic HST BWC network architecture is described. Two potential cellular architectures, microcells and distributed antenna systems (DASs) based cells, are introduced. In particular, the DAS is discussed in conjunction with radio over fiber (RoF) technology for BWC for HSTs. The technical challenges in providing DAS-based BWC for HSTs, such as handoff and RoF are discussed and outlined. [ABSTRACT FROM PUBLISHER]