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In conventional hybrid beamforming approaches, the number of radio-frequency (RF) chains is the bottleneck on the achievable spatial multiplexing gain. Recent studies have overcome this limitation by increasing the update-rate of the RF beamformer. This paper presents a framework to design and evaluate such approaches, which we refer to as agile RF beamforming, from theoretical and practical points of view. In this context, we consider the impact of the number of RF-chains, phase shifters’ speed, and resolution to design agile RF beamformers. Our analysis and simulations indicate that even an RF-chain-free transmitter, which its beamformer has no RF-chains, can provide a promising performance compared with fully-digital systems and significantly outperform the conventional hybrid beamformers. Then, we show that the phase shifter’s limited switching speed can result in signal aliasing, in-band distortion, and out-of-band emissions. We introduce performance metrics and approaches to measure such effects and compare the performance of the proposed agile beamformers using the Gram-Schmidt orthogonalization process. Although this paper aims to present a generic framework for deploying agile RF beamformers, it also presents extensive performance evaluations in communication systems in terms of adjacent channel leakage ratio, sum-rate, power efficiency, error vector magnitude, and bit-error rates. [ABSTRACT FROM AUTHOR]

Magnetic random-access memory (MRAM) is a promising memory technology due to its high density, non-volatility, and high endurance. However, achieving high memory fidelity incurs high write-energy costs, which should be reduced for large-scale deployment of MRAMs. In this paper, we formulate a biconvex optimization problem to optimize write fidelity given energy and latency constraints. The basic idea is to allocate non-uniform write pulses depending on the importance of each bit position. The fidelity measure we consider is mean squared error (MSE), for which we optimize write pulses via alternating convex search (ACS). We derive analytic solutions and propose an alternating water-filling algorithm by casting the MRAM’s write operation as communication over parallel channels. Hence, the proposed alternating water-filling algorithm is computationally more efficient than the original ACS while their solutions are identical. Since the formulated biconvex problem is non-convex, both the original ACS and the proposed algorithm do not guarantee global optimality. However, the MSEs obtained by the proposed algorithm are comparable to the MSEs by complicated global nonlinear programming solvers. Furthermore, we prove that our algorithm can reduce the MSE exponentially with the number of bits per word. For an 8-bit accessed word, the proposed algorithm reduces the MSE by a factor of 21. We also evaluate MNIST dataset classification supposing that the model parameters of deep neural networks are stored in MRAMs. The numerical results show that the optimized write pulses can achieve 40% write-energy reduction for the same classification accuracy. [ABSTRACT FROM AUTHOR]

Channel estimation is one of the main challenges for implementing reconfigurable intelligent surface (RIS)-aided communication system with a large number of antennas at the base station (BS) because RIS is equipped with many passive reflective elements without active transmitting/receiving and signal processing abilities. In this paper, we focus on the channel estimation in a general RIS-aided multi-user mmWave communication system. Specifically, a novel two-phase channel estimation scheme consisting of on-line and off-line phases is proposed to estimate the BS-RIS channel, RIS-user channel, and BS-user channel, respectively. Inspired by the characteristics of few scatterers in mmWave communication systems, the antenna domain channels are converted into the virtual angular domain channels by using the discrete Fourier transform (DFT) matrix. Thus, the estimation problem can be formulated as a compressed sensing (CS) problem, and solved by using efficient vector approximate message passing (VAMP) algorithm with expectation-maximization (EM) to learn unknown parameters and obtain the estimates simultaneously. Simulation results show that, the above algorithm with two choices of prior distributions in the proposed mmWave RIS-aided multi-user system has fast convergence speed and desirable estimation performance. [ABSTRACT FROM AUTHOR]

This paper considers a cooperative multiple access (CMA) scheme where two full-duplex sources simultaneously send their own messages to a common destination. As part of their cooperation, each of the two sources also receives and relays part of the message from the other source. This paper develops algorithms for computing the optimal power allocation to achieve the maximal network throughput subject to residual self-interference (RSI) at the non-ideal full-duplex sources. As a generalization of a time-division duplex (TDD) scheme, we consider a three-phase CMA (3P-CMA) scheme which treats the TDD scheme as a special case. The throughput of the 3P-CMA scheme consistently exceeds that of the TDD scheme regardless of the RSI power gain. Also shown in this paper is a semi-closed form of the optimal power allocation when the three-phases are constrained to be identical, i.e., resulting in a one-phase CMA (1P-CMA) scheme. The optimal power allocation of the 1P-CMA scheme takes much less time to compute, and its network throughout is close to that of the 3P-CMA scheme when the RSI power gain is low. [ABSTRACT FROM AUTHOR]

This paper investigates the secrecy performance of a spectrum sharing network, where ${N}$ legitimate source-destination pairs orderly access the shared spectrum for communication, while an eavesdropper (E) attempts to tap the legitimate information transmission. To improve the physical layer security, we propose two jamming strategies that can intelligently switch between jamming and non-jamming, namely, suboptimal jammer selection (SJS) scheme and optimal jammer selection (OJS) scheme. Specifically, when a user pair is assigned to access the shared spectrum, another source is chosen as a friendly jammer in order to create intentional interference at E. For the purpose of comparison, we present the non-jammer selection (NJS) scheme as a benchmark. Analytical closed-form secrecy outage probability expressions of NJS, SJS and OJS schemes are derived over Nakagami- ${m}$ fading channels. We further present an asymptotic secrecy outage probability analysis to evaluate the secrecy diversity gain performance of NJS, SJS and OJS schemes. Numerical results show that the secrecy outage probability performance of OJS scheme is better than SJS and NJS schemes in the low average channel power gain ${\mathop \Omega \nolimits _{D} }$ region. Furthermore, the secrecy outage probabilities of NJS as well as SJS and OJS schemes converge to each other with the increase of ${\mathop \Omega \nolimits _{D} }$ , due to the fact that the OJS and SJS scheme will switch to NJS scheme when ${\mathop \Omega \nolimits _{D} }$ tends to infinity. [ABSTRACT FROM AUTHOR]

Data center networks typically adopt multi-rooted tree topologies to provide high bisection bandwidth. Various fine-grained load balancing schemes have been proposed to split flows across multiple paths. However, data center networks suffer from many uncertainties such as highly dynamic traffic. These uncertainties easily make network become asymmetric, resulting in significant packet reordering. Unfortunately, existing solutions passively deal with packet reordering based on a threshold and hardly adapt to asymmetric networks because of lacking the explicit reordering feedback. These solutions either fail to quickly respond to packet loss or cause unnecessary fast retransmission, which reduces link utilization and increases flow completion time. In this paper, we propose a fine-grained load balancing scheme RMC to eliminate the impact of packet reordering and handle uncertainties in asymmetric networks. To avoid unnecessary fast retransmission, the switch proactively identifies reordered packet according to local queue length and global path latency. Furthermore, we employ a coding technique with redundancy optimization to reduce long-tailed flow completion time under network asymmetry. Through a series of large-scale NS2 simulations and testbed experiments, we demonstrate that RMC effectively avoids unnecessary fast retransmission under different network scenarios and reduces flow completion time by up to 72% compared with state-of-the-art schemes. [ABSTRACT FROM AUTHOR]

Although the logically-centralized perspective is offered in Software-Defined Networking (SDN), the data plane is still distributed. Update commands sent by the centralized controller are executed asynchronously and independently in each switch. The timed SDN enables synchronous and coordinated update operations as each update command can be triggered by a pre-defined timestamp. Prior work on timed update mainly focuses on producing a congestion-free update sequence, whereas finding a congestion-free timed update sequence may prolong the update time. Even worse, such an update order may not exist. In this paper, we propose Chronus+, a novel timed update system that utilizes the switch buffer to shorten the update time while minimizing the switch buffer during updates. First, we formulate the Minimum Switch Buffer Problem (MSBP) as an integer linear programming and show its hardness. Then, we propose a set of efficient algorithms to solve the problem in polynomial time. Extensive evaluations in Mininet and large-scale simulations show that Chronus+ can reduce the update time by at least 17% and the switch buffer size by at least 27% compared with state-of-the-art approaches. [ABSTRACT FROM AUTHOR]

In this paper, the idea of offset spatial modulation (OSM) is integrated with orthogonal frequency division multiplexing (OFDM), toward an efficient design to bridge their advantages. Compared to its conventional counterpart as spatial modulation (SM)-OFDM, the proposed OSM-OFDM scheme aims at providing a simplified implementation structure with less number of radio frequency (RF) chains, by introducing an offset between the RF chain and the index of the activated transmit antenna on each subcarrier. Specifically, three types of offset antenna selection (OAS) methods are developed to meet different scene requirements for different number of available RF chains. Furthermore, through theoretical analysis, we quantify the bit error rate upper bounds of OSM-OFDM with different types of OAS methods. Finally, extensive computer simulations demonstrate that OSM-OFDM provides a flexible tradeoff among implementation cost, computation complexity and error performance. [ABSTRACT FROM AUTHOR]

Existing wireless mobile networks configure the uplink (UL) and downlink (DL) resources based on the orthogonal duplex principle, which has significantly evolved from the static frequency/time division duplex (FDD/TDD) to the semi-dynamic TDD in 4G and the fully-dynamic TDD in 5G. Fueled by the successful realizations of full duplex (FD) transmission, non-orthogonal duplex (NOD) emerges as an attractive candidate technique for future networks to improve the bidirectional throughput. In this paper, we investigate the performance limit of the NOD scheme, which imports the FD mode on the basis of fully-dynamic TDD. We first focus on a single-cell scenario by assuming no cooperation among cells, and strive to characterize the bidirectional capacity region by finding the capacity-optimal transmission mode scheduling policy. We obtain the optimal policies for the systems with and without modulation and coding scheme (MCS), respectively, where the policies are based on the instantaneous channel gains and the distribution of channels. We proceed to develop a scheduling policy that only relies on the instantaneous channel and queue information, and extend it to the multicell multiuser scenario with coordinated scheduling. Numerical and simulation results demonstrate the great potential of the NOD scheme in increasing bidirectional capacity and reducing the queuing delay. [ABSTRACT FROM AUTHOR]

This paper investigates the hybrid precoding design in millimeter-wave (mmWave) systems with a fully-adaptive-connected precoding structure, where a switch-controlled connection is deployed between every antenna and every radio frequency (RF) chain. To maximally enhance the energy efficiency (EE) of hybrid precoding under this structure, the joint optimization of switch-controlled connections and the hybrid precoders is formulated as a large-scale mixed-integer non-convex problem with high-dimensional power constraints. To efficiently solve such a challenging problem, we first decouple it into a continuous hybrid precoding (CHP) subproblem. Then, with the hybrid precoders obtained from the CHP subproblem, the original problem can be equivalently reformulated as a discrete connection-state (DCS) problem with only 0-1 integer variables. For the CHP subproblem, we propose an alternating hybrid precoding (AHP) algorithm. Then, with the hybrid precoders provided by the AHP algorithm, we develop a matching assisted fully-adaptive hybrid precoding (MA-FAHP) algorithm to solve the DCS problem. It is theoretically shown that the proposed MA-FAHP algorithm always converges to a stable solution with the polynomial complexity. Finally, simulation results demonstrate the superior performance of the proposed MA-FAHP algorithm in terms of EE and beampattern. [ABSTRACT FROM AUTHOR]

Major goal of ultra reliable and low latency communication (URLLC) is to reduce the latency down to a millisecond (ms) level while ensuring reliability of the transmission. Since the current uplink transmission scheme requires a complicated handshaking procedure to initiate the transmission, to meet this stringent latency requirement is a challenge in wireless system design. In particular, in the time division duplexing (TDD) systems, supporting the URLLC is difficult since the mobile device has to wait until the transmit direction is switched to the uplink. In this paper, we propose a new approach to support a low latency access in TDD systems, called channel aware sparse transmission (CAST). Key idea of the proposed scheme is to encode a grant signal in a form of sparse vector. This together with the fact that the sensing mechanism preserves the energy of the sparse vector allows us to use the compressed sensing (CS) technique in CAST decoding. From the performance analysis and numerical evaluations, we demonstrate that the proposed CAST scheme achieves a significant reduction in access latency over the 4G LTE-TDD and 5G NR-TDD systems. [ABSTRACT FROM AUTHOR]

In distributed Internet of Things (IoTs), channel hopping (CH) is an effective scheme for neighbor nodes to achieve blind rendezvous over common available channels and to establish communication links. When nodes are unaware of each other’s local clocks and the global channels and have no pre-assigned CH strategies or identifiers (IDs), it is particularly challenging to guarantee blind rendezvous within a finite period of time, which has not been solved yet by using only one radio. In this paper, we propose a novel quaternary-encoding-based CH (QECH) algorithm to tackle the above issue. The QECH algorithm encodes a randomly selected channel into a quaternary string according to the 6B/8B encoding. We also append a common prefix string as well as the randomly selected channel before the quaternary string to guarantee overlaps in the asynchronous scenario. For all kinds of quaternary digits, we construct four mutually co-prime numbers to enumerate all possible combinations of the common available channels. We theoretically analyze the deterministic rendezvous principle and the upper bounded rendezvous latency of the QECH algorithm. We also verify the effectiveness of the QECH algorithm through extensive simulations. Evaluation results show the superiority of the QECH algorithm in terms of rendezvous latency. [ABSTRACT FROM AUTHOR]

In this paper, we explore reduced-connectivity radio frequency (RF) switching networks for reducing the analog hardware complexity and switching power losses in antenna selection (AS) systems. In particular, we analyze different hardware architectures for implementing the RF switching matrices required in AS designs with a reduced number of RF chains. We explicitly show that fully-flexible switching matrices, which facilitate the selection of any possible subset of antennas and attain the maximum theoretical sum rates of AS, present numerous drawbacks such as the introduction of significant insertion losses, particularly pronounced in massive multiple-input multiple-output (MIMO) systems. Since these disadvantages make fully-flexible switching suboptimal in the energy efficiency sense, we further consider partially-connected switching networks as an alternative switching architecture with reduced hardware complexity, which we characterize in this work. In this context, we also analyze the impact of reduced switching connectivity on the analog hardware and digital signal processing of AS schemes that rely on received signal power information. Overall, the analytical and simulation results shown in this paper demonstrate that partially-connected switching maximizes the energy efficiency of massive MIMO systems for a reduced number of RF chains, while fully-flexible switching offers sub-optimal energy efficiency benefits due to its significant switching power losses. [ABSTRACT FROM PUBLISHER]

Based on the concept of reconfigurable antennas (RAs), space-shift keying (SSK)-RA has been recently proposed as a novel transmission scheme to improve the performance of SSK. In this context, it has been shown that RAs’ reconfigurable properties can be used as additional degrees of freedom to enhance the throughput, implementation complexity, and error performance of SSK. In this paper, we study the implementation of SSK-RA within underlay cognitive radio systems in an effort to improve the performance of the secondary user while verifying the constraints set by the primary user (PU). Taking advantage of the interplay between RAs and the propagation channels for both the secondary and interference links, we propose an RA-based scheme with beam-direction reconfiguration aiming at improving the secondary system’s performance while verifying an outage interference constraint to the PU. In this paper, we analyze the performance of the proposed scheme in Rician fading channels and provide simulation examples confirming these analytical results. The proposed schemes are shown to offer enhanced bit error rate performance and lower implementation complexity when compared with conventional antenna-based spectrum sharing systems. [ABSTRACT FROM PUBLISHER]

In channel-hopping (CH)-based distributed cognitive radio networks (CRNs), the time duration that secondary users (SUs) spend for establishing communication links is called access delay. To evaluate access delay, we propose an access delay model by jointly considering imperfect spectrum sensing and multi-channel multi-SU transmission, from the cross-layer perspective. The model considers two typical scenarios. The first scenario assumes that the SUs do not use contention scheme (CS) which indicates that the time slot is relatively shorter to just allow a transmission. The second scenario assumes that the SUs employ CS [i.e., modified Distributed Coordination Function (DCF)-based Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) in this paper], which indicates that the time slot is long enough to regulate multiple transmissions. We then propose a bio-inspired algorithm for the first scenario and a self-adaptive step-length algorithm for the second scenario to search for the optimal values of spectrum sensing parameters. The theoretical analysis and simulation results validate the proposed access delay model and show that the proposed algorithms can reduce the most redundant computation. They also show that the optimization of cross-layer parameters can significantly decrease SUs’ access delay. Moreover, we conduct a cost-benefit analysis to evaluate the performance of the two scenarios. [ABSTRACT FROM AUTHOR]

Small-cell architecture is widely adopted by cellular network operators to increase spectral spatial efficiency. However, this approach suffers from low spectrum temporal efficiency. When a cell becomes smaller and covers fewer users, its total traffic fluctuates significantly due to insufficient traffic aggregation and exhibits a large “peak-to-mean” ratio. As operators customarily provision spectrum for peak traffic, large traffic temporal fluctuation inevitably leads to low spectrum temporal efficiency. To address this issue, in this paper, we advocate device-to-device (D2D) load-balancing as a useful mechanism. The idea is to shift traffic from a congested cell to its adjacent under-utilized cells by leveraging inter-cell D2D communication, so that the traffic can be served without using extra spectrum, effectively improving the spectrum temporal efficiency. We provide theoretical modeling and analysis to characterize the benefit of D2D load balancing, in terms of total spectrum requirements and the corresponding cost, in terms of incurred D2D traffic overhead. We carry out empirical evaluations based on real-world 4G data traces and show that D2D load balancing can reduce the spectrum requirement by 25% as compared to the standard scenario without D2D load balancing, at the expense of negligible 0.7% D2D traffic overhead. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the secrecy outage probability and computation overhead of a multi-user multi-eavesdropper wireless system which is composed of one cluster head (CH) transmitting to multiple users, while multiple eavesdroppers attempt to tap the transmission from CH to multiple users in the presence of co-channel interference (CCI). To achieve high security and low computation overhead, we present two multi-user scheduling schemes, namely, the switch-and-examine combining with post-selection user scheduling (SECpsUS) scheme and the selection combining-based user scheduling (SCbUS) scheme. For the purpose of comparison, the conventional round-robin scheduling (RRS) is also considered as a benchmark scheme. Furthermore, we derive closed-form secrecy outage probability expressions and analyze computation overheads for the conventional RRS, SCbUS, and SECpsUS schemes. Numerical results and Monte Carlo simulations are provided to verify the correctness of our theoretical secrecy outage analysis and to demonstrate the effect of CCI on the system performance. It is shown that the SECpsUS and SCbUS schemes outperform the conventional RRS scheme in terms of their secrecy outage probabilities, where SCbUS achieves the best secrecy outage performance but at cost of the highest computation overhead, while the conventional RRS scheme has no computation overhead with the worst secrecy performance. Moreover, a flexible tradeoff between the secrecy outage performance and computation overhead can be achieved by the proposed SECpsUS scheme by adjusting a so-called switching threshold. In addition, as the number of users increases, the secrecy outage performance of the SECpsUS and SCbUS is improved significantly, whereas no secrecy benefit is achieved by the conventional RRS scheme. [ABSTRACT FROM AUTHOR]

While modern wireless devices are capable of using multiple WiFi interfaces, the Multipath TCP (MPTCP) protocol has been employed to make full use of the capacity of many radios by enabling multiple path communication simultaneously. To provide exceptional mobility support in wireless networks, a key question is to determine the best handoff strategy to switch between access points or among WiFi/3G interfaces during roaming. In this paper, we propose SERO, a novel model-driven SEamless ROaming framework to optimize layer-2 handoff and vertical handoff for multihomed devices using MPTCP. The proposed framework adopts a measurement-based method to derive the TCP throughput model for wireless communication during handoff. Based on the throughput model, we propose a hybrid handoff strategy that uses multiple WiFi interfaces for data transmission and employs 3G augmentation to bridge the network interruption caused by handoff and to guarantee the total throughput above a predefined threshold for roaming devices. We implement the SERO framework in a real-deployed wireless mesh network testbed, and evaluate its performance by extensive experiments, which shows that SERO achieves performance gain of 26%–180% compared with several existing handoff strategies. [ABSTRACT FROM AUTHOR]

The performance of millimeter wave (mmWave) multiple-input multiple-output (MIMO) systems is limited by the sparse nature of propagation channels and the restricted number of radio frequency chains at transceivers. The introduction of reconfigurable antennas offers an additional degree of freedom on designing mmWave MIMO systems. This paper provides a theoretical framework for studying the mmWave MIMO with reconfigurable antennas. Based on the virtual channel model, we present an architecture of reconfigurable mmWave MIMO with beamspace hybrid analog–digital beamformers and reconfigurable antennas at both the transmitter and the receiver. We show that employing reconfigurable antennas can provide throughput gain for the mmWave MIMO. We derive the expression for the average throughput gain of using reconfigurable antennas in the system, and further derive the expression for the outage throughput gain for the scenarios where the channels are (quasi) static. Moreover, we propose a low-complexity algorithm for reconfiguration state selection and beam selection. Our numerical results verify the derived expressions for the throughput gains and demonstrate the near-optimal throughput performance of the proposed low-complexity algorithm. [ABSTRACT FROM AUTHOR]

Enabled by new storage mediums, Computation-in-Memory is a novel architecture that has shown great potential in reducing the burden of massive data processing by bypassing the communication and memory access bottleneck. Suggested by Cassuto and Crammer, allowing for ultra-fast Hamming distance computations to be performed in resistive memory with low-level conductance measurements has the potential to drastically speed up many modern machine learning algorithms. Meanwhile, Hamming distance Computation-in-Memory remains a challenging task as a result of the non-negligible device variability in practical resistive memory. In this paper, build upon the work of Cassuto and Crammer, we study memristor variability due to two distinct sources: resistance variation, and the non-deterministic write process. First, we introduce a technique for estimating the Hamming distance under resistance variation alone. Then, we propose error-detection and error-correction schemes to deal with non-ideal write process. We then combine these results to concurrently address both sources of memristor variabilities. In order to preserve the low latency property of Computation-in-Memory, all of our approaches rely on only a single vector-level conductance measurement. We use so-called inversion coding as a key ingredient in our solutions and we prove the optimality of this code given the restrictions on bit-accessible information. Finally, we demonstrate the efficacy of our approaches on the k-nearest neighbors classifier. [ABSTRACT FROM AUTHOR]

This paper elaborates on the suboptimal greedy algorithm for target channel sequence selection as presented by Wang et al. in the paper, “Optimal Target Channel Sequence Design for Multiple Spectrum Handoffs in Cognitive Radio Networks,” IEEE Transactions on Communications, vol. 60, no. 9, pp. 2444–2455, September 2012. They claimed that the greedy algorithm requires comparing six target channel sequences. We prove that only five target channel sequences comparisons are sufficient. Hence, we present a modified algorithm and test it with a set of numerical results. It appears that the new algorithm reduces the processing time compared to the original one especially for a network with a large number of candidate channels. [ABSTRACT FROM PUBLISHER]

In this paper, the problem of joint mode switching and power adaptation is studied for simultaneous wireless information and power transfer (SWIPT) over a fading channel. The receiver dynamically switches between information decoding (ID) and energy harvesting (EH) modes while the transmitter dynamically adapts the transmit power. Considering the nonlinearity of practical EH circuits, a realistic nonlinear EH model is adopted rather than the idealistic linear EH model. To characterize the ultimate performance tradeoff between ID and EH, an optimization problem is formulated to maximize the average harvested energy under the constraints on the average achievable rate and the average transmit power, which is a nonconvex and combinatorial problem. To solve this problem, first, the optimal power adaptation scheme for the nonlinear EH receiver that operates only in the EH mode is proposed. Using this scheme, the jointly optimal solution for the mode switching and power adaptation is then derived. By comparing the obtained results to the existing results, various useful and interesting insights into the optimized SWIPT system with nonlinear EH are presented. An important insight into the impact of nonlinear EH is that, to exploit the high energy conversion efficiency of the nonlinear circuit, the EH mode has to be selected only in the moderate range of channel gains. Also, in the EH mode, the power has to be adapted to the short-term power threshold only for the moderate channel gains. [ABSTRACT FROM AUTHOR]

Caching and multicasting at base stations are two promising approaches to support massive content delivery over wireless networks. However, existing scheduling designs do not fully exploit the advantages of the two approaches. In this paper, we consider the optimal dynamic multicast scheduling to jointly minimize the average delay, power, and fetching costs for cache-enabled content-centric wireless networks. We formulate this stochastic optimization problem as an infinite horizon average cost Markov decision process (MDP).By using relative value iteration and special structures of the request queue dynamics, we analyze the properties of the value function and the state-action cost function of the MDP for both the uniform and nonuniform channel cases. Based on these properties, we show that the optimal policy, which is adaptive to the request queue state, has a switch structure in the uniform case and a partial switch structure in the nonuniform case. Moreover, in the uniform case with two contents, we show that the switch curve is monotonically non-decreasing. Motivated by the switch structures of the optimal policy, we propose a low-complexity suboptimal policy, which exhibits similar switch structures to the optimal policy, and design a low-complexity algorithm to compute this policy. [ABSTRACT FROM PUBLISHER]

In this paper, we propose to use variable- Nu generalized spatial modulation (VGSM), which is a specific type of generalized spatial modulation, in indoor line-of-sight (LOS) millimeter-wave (mmWave) communication. As compared with fixed- Nu generalized spatial modulation, VGSM needs less number of transmitter (TX) antennas to achieve the same data rate. Reducing the number of antennas is especially important for LOS mmWave MIMO communication, since it means reduced array lengths and routing loss in the RF front-end. We first derive and analyze the channel capacity for LOS VGSM, and then propose a novel switching structure for implementation of the VGSM TX at mmWave frequencies and analyze its performance in terms of power added efficiency and capacity. Two optimizations of the system performance of VGSM are performed: 1) power allocation optimization for the proposed TX, and 2) antenna separation optimization in order to optimize the capacity in LOS. Our results show that VGSM is a feasible and promising scheme for indoor LOS mmWave communication. For example, we show that an $8\times 8$ VGSM system with practical component values can achieve more than 20bpcu spectral efficiency within a distance of 1–5m and more than 17bpcu within 5–10m. [ABSTRACT FROM PUBLISHER]

This paper investigates the capacity and energy consumption metrics of small-cell networks that are enabled with sleep mode (SL) functionality. A novel method is introduced to systematically and accurately identify the potential SL cells that can maximize the spectrum reuse efficiency without the need for an exhaustive search. The performance of the proposed technique is assessed and compared with the always-on approach and an optimal benchmark. The results show that the proposed method significantly outperforms the always-on system and approaches the performance of the optimal benchmark with notably reduced computational burden. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a multiuser downlink wiretap network consisting of one base station (BS) equipped with A\mathrm{A} antennas, N\mathrm{B} single-antenna legitimate users, and N\mathrm{E} single-antenna eavesdroppers over Nakagami- $m$ fading channels. In particular, we introduce a joint secure transmission scheme that adopts transmit antenna selection at the BS and explores threshold-based selection diversity scheduling over legitimate users to achieve a good secrecy performance while maintaining low implementation complexity. More specifically, in an effort to quantify the secrecy performance of the considered system, two practical scenarios are investigated, i.e.: in Scenario I, the eavesdropper’s channel state information (CSI) is unavailable at the BS, and in Scenario II, the eavesdropper’s CSI is available at the BS. For Scenario I, novel exact closed-form expressions for the secrecy outage probability are derived, which are valid for general networks with an arbitrary number of legitimate users, antenna configurations, number of eavesdroppers, and the switched threshold. For Scenario II, we take into account the ergodic secrecy rate as the principle performance metric, and derive novel exact closed-form expressions for the ergodic secrecy rate. In addition, we also provide simple and asymptotic expressions for secrecy outage probability and ergodic secrecy rate under two distinct cases, i.e.: in Case I, the legitimate user is located close to the BS, and in Case II, both the legitimate user and eavesdropper are located close to the BS. Our important findings reveal that the secrecy diversity order is {A_{\mathrm{A}}}{m_{\mathrm{A}}} and the slope of secrecy rate is one under Case I, while the secrecy diversity order and the slope of secrecy rate collapse to zero under Case II, where the secrecy performance floor occurs. Finally, when the switched threshold is carefully selected, the considered scheduling scheme outperforms other well-known existing schemes in terms of the secrecy performance and complexity tradeoff. [ABSTRACT FROM AUTHOR]

Scheduling algorithms are crucial for most existing switches to improve the throughput. However, the delay of the switching fabric cannot be guaranteed with such scheduling algorithms. This paper aims to design a novel load-balanced wire-speed multicast switching fabric along with the attractive merits of network coding. We adopt a two-phase self-routing switching fabric constructed by Boolean-multicast concentrators (SRBMCs), where the first SRBMC distributes the incoming cells to its outputs uniformly and the second allows the distributed cells to be self-routed and multicast to their destinations concurrently. To further improve the switching performance, linear network coding is smoothly combined with SRBMC to reduce the packet loss rate. Theoretical analysis and numerical simulation demonstrate that the proposed switching fabric cannot only achieve wire-speed multicast switching but also be recursively constructed into an indefinite large-scale one with such merits as no internal buffers, low complexity, and guarantee in switching delay. Finally, we implement the proposed fabric on Field-Programmable Gate Array and verify its performance in multicast switching. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the secrecy capacity of a half-duplex energy harvesting (EH)-based multi-antenna amplify-and-forward relay network in the presence of a passive eavesdropper. During the first phase, while the source is in the transmission mode, the legitimate destination transmits an auxiliary artificial noise (AN) signal which has two distinct purposes: 1) to transfer power to the relay and 2) to improve system security. Since the AN is known at the legitimate destination, it is easily cancelled at the intended destination, which is not the case at the eavesdropper. In this respect, we derive new exact analytical expressions for the ergodic secrecy capacity for various well-known EH relaying protocols, namely, time switching relaying (TSR), power splitting relaying (PSR), and ideal relaying receiver (IRR). Monte Carlo simulations are also provided throughout our investigations to validate the analysis. The impacts of some important system parameters, such as EH time, power splitting ratio, relay location, AN power, EH efficiency, and the number of relay antennas, on the system performance are investigated. The results reveal that the PSR protocol generally outperforms the TSR approach in terms of the secrecy capacity. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the transmission strategy for cognitive radios (CRs), which opportunistically operate on various primary users’ channels with the aid of proactive spectrum handoff. In particular, a secondary user (SU) in the CR network proactively predicts the future spectrum status and decides whether to keep idle, or stay in the current channel, or switch to a new channel to resume its transmission. A problem of completing a target data packet size of \mathcal V bits within a predefined deadline D time slots is formulated as a discrete-time Markov decision process, in which the SU aims at minimizing its expected total cost, i.e., transmission cost, handoff cost, and overtime penalty. We solve the problem using dynamic programming, and propose a general optimal transmission with the proactive spectrum handoff (OTPH) algorithm whose complexity is 2\times \mathcal {V}\times D . Furthermore, we prove that for a convex penalty function, the optimal handoff-aided transmission exhibits a threshold structure. A monotone OTPH algorithm with a complexity of {\max }(\mathcal {V},D)$ is used in this case. Simulation results verify that our proposed scheme achieves both the minimal total cost and the highest data transmission efficiency as compared with the traditional always staying and always changing schemes. [ABSTRACT FROM AUTHOR]

In this paper, we propose adaptive techniques for multiuser multiple-input and multiple-output (MU-MIMO) cellular communication systems, to solve the problem of energy efficient communications with heterogeneous delay-aware traffic. In order to minimize the total transmission power of the MU-MIMO, we investigate the relationship between the transmission power and the M-ary quadrature amplitude modulation (MQAM) constellation size and get the energy efficient modulation for each transmission stream based on the minimum mean square error (MMSE) receiver. Since the total power consumption is different for MU-MIMO and multiuser single input and multiple output (MU-SIMO), by exploiting the intrinsic relationship among the total power consumption model, and heterogeneous delay-aware services, we propose an adaptive transmission strategy, which is a switching between MU-MIMO and MU-SIMO. Simulations show that in order to maximize the energy efficiency and consider different Quality of Service (QoS) of delay for the users simultaneously, the users should adaptively choose the constellation size for each stream as well as the transmission mode. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new wireless communication transmission architecture called switched phased-array (SPA), to enhance physical layer security. SPA works as a platform for three different transmission techniques: 1) conventional phased-array transmission; 2) antenna subset transmission (AST) technique; and 3) silent antenna hopping (SAH) transmission technique. SPA consists of a conventional phased-array blacktransmitter followed by antennas with an on–off switching circuit. The proposed solution maintains the objective of scrambling the constellation points in both amplitude and phase in undesired directions, while preserving a clear constellation in the target direction. The proposed solution—SPA—is different from previously used methods in the following ways: 1) SPA is not restricted to the use of phase modulation, and can accept any modulation type including QAM; 2) it does not need to modulate the signal in the radio frequency (RF) domain, where the conventional phased-array transmitter circuits remain unchanged; 3) in the far field, SPA scrambles the signal constellation by randomly switching-off some of the transmitting antennas (AST), or only one of them (SAH); 4) SPA can be easily integrated with the current infrastructure of phased-array transmitters; 5) SPA breaks up the correlation between the data rates and the switching speed; and 6) SPA performs a variety of DM transmission techniques. We present the potential transmission techniques including PA, AST, and SAH. We analyze the performance for all cases and derive an exact expression of the bit error probability, and also we analyze the secrecy capacity. The results show that SPA, and its variants AST and SAH, are simple and very efficient solutions to improve the physical layer security of mm-wave communication. [ABSTRACT FROM AUTHOR]

In this paper, we study a two-phase underlay cognitive relay network, where there exists an eavesdropper who can overhear the message. The secure data transmission from the secondary source to secondary destination is assisted by two decode-and-forward (DF) relays. Although the traditional opportunistic relaying technique can choose one relay to provide the best secure performance, it needs to continuously have the channel state information (CSI) of both relays, and may result in a high relay switching rate. To overcome these limitations, a secure switch-and-stay combining (SSSC) protocol is proposed where only one out of the two relays is activated to assist the secure data transmission, and the secure relay switching occurs when the relay cannot support the secure communication any longer. This security switching is assisted by either instantaneous or statistical eavesdropping CSI. For these two cases, we study the system secure performance of SSSC protocol, by deriving the analytical secrecy outage probability as well as an asymptotic expression for the high main-to-eavesdropper ratio (MER) region. We show that SSSC can substantially reduce the system complexity while achieving or approaching the full diversity order of opportunistic relaying in the presence of the instantaneous or statistical eavesdropping CSI. [ABSTRACT FROM AUTHOR]

In this paper, we optimize the use of existing DSL low-power sleeping modes (L2 and L3) in order to improve the energy efficiency of DSL access networks. Given that switching on a DSL line can cause instability by increasing the amount of time-varying crosstalk in the cable bundle, and that it takes energy and time to switch a DSL line on and off, we develop a method to optimally choose the appropriate sleeping state based on the line and traffic characteristics. We further develop and prove the structural properties of the optimal policy for switching to the appropriate sleeping state, allowing transitions between submodes with different power and transmit rate characteristics. We also present techniques that guarantee stable sleep mode operation. Using a realistic DSL simulator, we demonstrate the three-way tradeoff among energy consumption, delay performance, and stability. The increased flexibility of control introduced by our approach improves the energy-delay Pareto optimal tradeoff, and results in a more energy efficient and stable DSL operation compared to existing power saving policies. [ABSTRACT FROM PUBLISHER]

Quickest spectrum sensing seeks to optimize a balance between two opposing performance measures, one being the delay in identifying spectrum opportunities, and the other being the quality of the decision. The existing spectrum sensing approaches formed based on quickest detection theory rely on the assumption that the occupancy states of different spectrum bands over a wideband spectrum are statistically independent. This is an assumption that cannot be met in practice, especially in broadband communication schemes in which radio channels are dynamically grouped in bundles and allocated to different users based on the users' traffic needs. As a result of such channel grouping and allocation, the occupancy states of the channels, especially the adjacent ones, are correlated. This paper, in contrast to the existing literature on quickest spectrum sensing, considers a wideband spectrum in which the occupancy states of different channels follow a pre-specified dependency kernel. The objective is to design the quickest spectrum sensing approach for identifying spectrum holes, which aims to minimize the average delay in identifying spectrum opportunities while assuring, in parallel, certain guarantees on the quality of the decision. The closed-form characterization of the optimal sensing scheme is delineated and it is shown that this optimal scheme has low computational complexity. [ABSTRACT FROM PUBLISHER]

This paper considers bit-loading algorithms to maximize throughput under total power and spectral mask constraints in interference-free OFDM systems. The contribution is twofold. First, we propose a simple criterion to switch between two well-known algorithms from the literature: the conventional Greedy and Greedy-based bit-removing (with maximum allowable bit loading initialization) algorithms. Second, we present a new low-complexity loading algorithm that exploits the bit vector obtained by rounding the water-filling algorithm solution to the associated continuous-input rate maximization problem as an efficient initial bit vector of the Greedy algorithm. We theoretically prove that this bit vector has two interesting properties. The first one states that it is an efficient bit vector, i.e., there is no movement of a bit from one subcarrier to another that reduces the total used power. The second one states that the optimized throughput, starting from this initial bit vector, is achieved by adding or removing bits on each subcarrier at most once. Simulation results show the efficiency of the proposed algorithm, i.e., the achievable throughput is maximized with significant reduction of computation cost as compared to many algorithms in the literature. [ABSTRACT FROM PUBLISHER]

In this paper, we present a comprehensive investigation on the secrecy performance of opportunistic relay selection systems employing the decode-and-forward protocol over Rayleigh fading channels. Considering a practical setting where direct link between the source node (Alice) and the destination node (Bob) is available, we study the secrecy performance of three different diversity combining schemes, namely, maximum ratio combining (MRC), distributed selection combining (DSC), and distributed switch-and-stay combining (DSSC). Throughout the analysis, we consider two different scenarios based on the availability of the eavesdropper's channel state information (CSI), i.e., Scenario A, where the eavesdropper's CSI is not available at Alice and the relay, and Scenario B, where Alice and the relay have knowledge about the eavesdropper's CSI. For Scenario A, we derive exact closed-form expressions for secrecy outage probability and simple asymptotic approximations for the secrecy outage probability, which enable the characterization of the achievable secrecy diversity order and coding gains. For Scenario B, we derive closed-form expressions for the achievable secrecy rates. For both scenarios, we investigate the impact of feedback delay (outdated CSI) on the secrecy performance wherein exact and asymptotic secrecy outage probability and closed-form expressions of the secrecy achievable rates are obtained. Our analytical findings suggest that both the MRC and DSC schemes achieve the maximum diversity order of $K+1$ where $K$ is the number of relays. In addition, the feedback delay has a significant impact on the achievable secrecy performance by reducing the achievable diversity order to two. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a novel sequential channel estimation approach for multiband cognitive radio (CR) systems. We introduce a general model and test two scenarios of practical interest. The two scenarios are as follows: 1) CR users optimally estimate all the available bands; and 2) CR users find one good channel with a large gain. In particular, we use a sequential search in which the CR users estimate the available channels one by one. During the search, the CR users determine whether to terminate the current channel estimation process and switch to the next channel based on the training symbols received so far. Our objective is to design a switch function, an estimator, and a stopping rule that minimize a combination of estimation time and error. For the multiband estimation scenario, we show that the optimal rule is to find the optimal number of symbols required for each channel in a joint optimization problem. For the good channel search problem, we show that the optimal decision rules that minimize a properly chosen cost function have a simple structure. In particular, both the termination and switching rules are threshold based. Numerical results are provided to illustrate the effectiveness of the proposed algorithms. [ABSTRACT FROM PUBLISHER]