Communication Pairs Research Articles

A Learning-Based Thermal-Sensitive Power Optimization Approach for Optical NoCs

Optical networks-on-chip (NoCs) based on silicon photonics have been proposed as emerging on-chip communication architectures for chip multiprocessors with large core counts. However, due to the thermal sensitivity of optical devices used in optical NoCs, on-chip temperature variations cause significant thermal-induced optical power loss, which would counteract the power advantages of optical NoCs. To tackle this problem, in this work, we propose a learning-based thermal-sensitive power optimization approach for mesh- or torus-based optical NoCs in the presence of temperature variations. The key techniques proposed include an initial device-setting and thermal-tuning mechanism that is a device-level optimization technique, and a learning-based thermal-sensitive adaptive routing algorithm that is a network-level optimization technique. Simulation results of an 8x8 mesh-based optical NoC show that the proposed initial device-setting and thermal-tuning mechanism confines the worst-case thermal-induced optical energy consumption to be on the order of tens of pJ/bit, by avoiding significant thermal-induced optical power loss caused by temperature-dependent wavelength shifts. Besides, it shows that the learning-based thermal-sensitive adaptive routing algorithm is able to find an optimal path with the minimum estimated thermal-induced optical power consumption for each communication pair. The proposed routing has a greater space for optimization, especially for applications with more long-distance traffic.

Enhancing Fault Tolerance and Resource Utilization in Unidirectional Quorum-Based Cycle Routing

Cycle-based optical network routing, whether using synchronous optical networking rings or p-cycles, provides sufficient reliability in the network. Light trails forming a cycle allow broadcasts within a cycle to be used for efficient multicasts. Optimal communication quorum sets forming optical cycles based on light trails have been shown to flexibly and efficiently route both point-to-point and multipoint-to-multipoint traffic requests. Commonly, cycle routing techniques use pairs of cycles to achieve both routing and fault tolerance, which use substantial resources and create the potential for underutilization. Instead, we intentionally utilize $R$ redundancy within the quorum cycles for fault tolerance such that every point-to-point communication pairs occur in at least $R$ cycles. We develop a generalized $R$ redundancy cycle technique that provides optical networks high fault-tolerant communications capability. When applied using only the single unidirectional cycles rather than the standard paired cycles, the generalized $R$ redundancy technique has been shown to almost halve the necessary light-trail resources in the network. However, due to unidirectional nature, a small percentage of node pairs for one-to-one communication may not have exactly two paths. For this reason, we further develop a greedy cycle direction heuristic and show a reduction of missing pairs. More importantly, we show that the resource requirement is reduced while maintaining the fault tolerance and dependability expected from cycle-based routing. The result is a set of cycles with 96.6%–99.37% fault coverage, while using 42.9%–47.18% fewer resources.

Optimal Node Placement and Resource Allocation for UAV Relaying Network

Utilizing unmanned aerial vehicle (UAV) as the relay is an effective technical solution for the wireless communication between ground terminals faraway or obstructed. In this letter, the problems of UAV node placement and communication resource allocation are investigated jointly for a UAV relaying system for the first time. Multiple communication pairs on the ground, with one rotary-wing UAV serving as relay, are considered. Transmission power, bandwidth, transmission rate, and UAV’s position are optimized jointly to maximize the system throughput. An optimization problem is formulated, which is non-convex. The global optimal solution is achieved by transforming the formulated problem to be a monotonic optimization problem.

Characterizing Mutual Information of Multistream MIMO-SVD Systems in Heterogeneous Random Networks

The main focus of this paper is on multistream multiantenna (multiple-input multiple-output) communications in large-scale heterogeneous random networks. Previous work in this area mainly studies the outage probability from the perspective of a given stream, which is inaccurate as it generally overlooks possible interference-driven cross stream signal-to-interference correlation in each communication link. To tackle this issue, in this paper, we focus on a link-level performance evaluation by characterizing the complementary cumulative distribution function (CCDF) of random mutual information (RMI) under heterogeneous interference. We focus on the scenarios that no coordination among transmitters is persuaded, and the receivers do not attempt to cancel out part of interuser interference. Assume that channel state information is known at both the transmitter and receiver of each communication pair, Rayleigh fading, that the singular value decomposition (SVD) is applied to construct pre-coding/post-coding filters for single-user detection, and that transmitted signals of antennas are i.i.d. Gaussian. We first provide accurate expression for the achievable data rate (ADR) of each link, and show that, regardless of the interference power, when transceivers are equipped with equal number of antennas it grows sublinearly with the number of antennas. We then approximate RMI via Gaussian distribution that its mean is the ADR and its variance is derived based on network's parameters. It is seen that the Gaussian approximation is credible even when two streams of data are transmitted. This paper provides various insights regarding the impact of system parameters, including density of transmitters, path-loss exponent, and the number of antennas, on the ADR and CCDF of RMI. We finally spot tremendous benefits of the SVD scheme over other prominent multiplexing systems, including maximum ratio combining, zero-forcing, and diagonal BLAST superstructure with minimum mean square error receivers.

Energy Efficiency and Spectrum Efficiency Tradeoff in the D2D-Enabled HetNet

We investigate the tradeoff between energy efficiency (EE) and spectrum efficiency (SE) in the heterogeneous network composed of a macro base station (BS), several pico BSs, and device-to-device (D2D) communication pairs during the uplink transmission. A utility function of the tradeoff between EE and SE is defined first. Then, we formulate the tradeoff utility maximization problem as a joint channel allocation and power control problem for cellular and D2D users. The original problem is transformed into a more tractable subtractive form, and we further decompose the problem into several subproblems that can be solved separately. Numerical results confirm the effectiveness of the proposed scheme and offer valuable insights. Compared with EE, SE is much less sensitive to the variation of the tradeoff factor, which indicates that our proposed scheme can achieve high EE while guaranteeing a fairly large value of SE with a properly chosen EE–SE tradeoff factor.

Exploiting Trust Degree for Multiple-Antenna User Cooperation

For a user cooperation system with multiple antennas, we consider a trust degree based cooperation techniques to explore the influence of the trustworthiness between users on the communication systems. For the system with two communication pairs, when one communication pair achieves its quality of service (QoS) requirement, they can help the transmission of the other communication pair according to the trust degree, which quantifies the trustworthiness between users in the cooperation. For given trust degree, we investigate the user cooperation strategies, which include the power allocation and precoder design for various antenna configurations. For SISO and MISO cases, we provide the optimal power allocation and beamformer design that maximize the expected achievable rates while guaranteeing the QoS requirement. For a SIMO case, we resort to semidefinite relaxation (SDR) technique and block coordinate update (BCU) method to solve the corresponding problem, and guarantee the rank-one solutions at each step. For a MIMO case, as MIMO is the generalization of MISO and SIMO, the similarities among their problem structures inspire us to combine the methods from MISO and SIMO together to efficiently tackle MIMO case. Simulation results show that the trust degree information has a great effect on the performance of the user cooperation in terms of the expected achievable rate, and the proposed user cooperation strategies achieve high achievable rates for given trust degree.

Distributed MAC protocol for multichannel cognitive radio ad hoc networks based on power control

Spectrum for wireless communications is traditionally assigned by inefficient spectrum access schemes. Portions of the spectrum are exclusively used by specific wireless systems. In recent studies, measurements indicated that allocated spectrum bands are not efficiently used in a certain geographic area, and the utilization of licensed spectrum is highly dependent on location and time. In cognitive radio ad hoc networks (CRAHNs), Secondary Users (SUs) can opportunistically utilize spectrum that is available from Primary Users (PUs). SUs must hop from one spectrum band to another to obtain the spectrum opportunities. In this paper, we propose a distributed medium access control (DMAC) protocol based on power control for CRAHNs, to mitigate the hidden and exposed terminal problems of multichannel PUs and SUs by building an appropriate number of monitor nodes in suitable positions. We develop the Markov chain model to characterize the performance of our proposed DMAC protocol for the saturation network. In this study, transmitter power control is based on the estimated distance between SU communication pairs; this increases channel spatial reuse and throughput, and reduces PU outage probability. In addition, DMAC uses the neighbors of PUs to inform SU communication pairs in the one-hop range; this also reduces PU outage probability. We also compare our proposed scheme to existing MAC protocols for CRAHNs. We show that DMAC will improve channel spatial reuse and normalized throughput, and reduce PU outage probability.

Monogamy relation in multipartite continuous-variable quantum teleportation

Quantum teleportation (QT) is a fundamentally remarkable communication protocol that also finds many important applications for quantum informatics. Given a quantum entangled resource, it is crucial to know to what extent one can accomplish the QT. This is usually assessed in terms of output fidelity, which can also be regarded as an operational measure of entanglement. In the case of multipartite communication when each communicator possesses a part of $N$-partite entangled state, not all pairs of communicators can achieve a high fidelity due to monogamy property of quantum entanglement. We here investigate how such a monogamy relation arises in multipartite continuous-variable (CV) teleportation particularly using a Gaussian entangled state. We show a strict monogamy relation, i.e. a sender cannot achieve a fidelity higher than optimal cloning limit with more than one receiver. While this seems rather natural owing to the no-cloning theorem, a strict monogamy relation still holds even if the sender is allowed to individually manipulate the reduced state in collaboration with each receiver to improve fidelity. The local operations are further extended to non-Gaussian operations such as photon subtraction and addition, and we demonstrate that the Gaussian cloning bound cannot be beaten by more than one pair of communicators. Furthermore we investigate a quantitative form of monogamy relation in terms of teleportation capability, for which we show that a faithful monogamy inequality does not exist.

A location-aware power saving mechanism based on quorum systems for multi-hop mobile ad hoc networks

IEEE 802.11 specifies a Power Saving Mode (PSM) in order to save the consumption of energy or power for mobile stations in wireless ad hoc networks. Following the standard of PSM, a number of studies [1–5] further discussed the wake-up/sleep scheduling of beacon intervals based on Quorum systems so that any communication pair of stations has common awake intervals for data exchange. However, most of them did not take into consideration the fact that non-neighbor stations did not require common wake up intervals since they were unable to communicate with each other. Hence, the energy conservation and the transmission delay could not be significantly improved. This paper utilizes the location information and proposes a location-aware power saving mechanism. In the proposed scheme, the network area is partitioned into regular hexagon cells and every station determines the basic quorum intervals based on its cell location information such that only neighboring cells have common beacon intervals for data exchange. In addition, a collision avoidance strategy is proposed to improve the network throughput. Experiment results reveal that the proposed scheme outperforms existing approaches in terms of energy conservation and transmission delay.

Stochastic geometry based asymptotic analysis for two‐tier HetNets with massive MIMO relay employing MRC/MRT and ZF processing

AbstractThis paper focuses on the multi‐pair amplify‐and‐forward massive multiple‐input multiple‐output (MIMO) relay enabled two‐tier heterogeneous cellular networks in terms of aggregate spectral efficiency (SE), where the macro cells are overlaid with dense small cells. Specially, the maximum ratio combining/maximal ratio transmission and zero‐forcing processing schemes are employed at massive MIMO relay, respectively. The macro base stations located in the center of the cells are equipped with massive MIMO antenna array, while the randomly distributed macro users (MUs), small cell base stations, and small cell users are equipped with single antenna. The positions of MUs, small cell users and small cell base stations are modelled as independent Poisson point processes. For such two‐tier heterogeneous cellular networks, to obtain the aggregate SE in target cell, we first formulate the end‐to‐end signal‐to‐interference noise ratios for a communication pair as well as the expression of power amplification factor at target macro base stations so that the total spectrum efficiency per cell is formulated. Then, by employing stochastic geometry, the closed‐form expressions of the power amplification factor and the expectation of the reciprocal of the equivalent end‐to‐end signal‐to‐interference noise ratios are achieved. With these results, we derive the lower bounds of SEs for maximum ratio combining/maximal ratio transmission and zero‐forcing schemes. The derivations display a clear perspective about the impact of network parameters on the system‐level performance, which includes macro cell radius, small cell radius, macro cell guard radius, the mean number of MUs per cell and so on. To achieve optimal (or suboptimal) system performance, we find that the radius of small cell should be less than the guard radius. For the guard radius, there exists an upper bound, only under which the increase of the guard radius would contribute to the improvement of SE. At the same time, our results show that with given network parameters, especially the guard radius and the small cell radius, there exists a unique greatest lower bound in terms of macro cell radius. When the macro cell radius is less than the greatest lower bound, the aggregate SE is increasing with the macro cell radius. On the contrary, the achievable aggregate SE saturates to the corresponding constant, approximately. In addition, the greatest lower bound of the macro cell radius is the function of the guard radius and the small cell radius. Copyright © 2016 John Wiley & Sons, Ltd.

A Low-Complexity Power Allocation Strategy to Minimize Sum-Source-Power for Multi-User Single-AF-Relay Networks

Because of its outstanding performance in extending coverage and reducing transmit power, wireless relay cooperation is deemed as one of the promising techniques to realize green-broadband communication in the future. In such relay cooperative systems, optimal power allocation not only can prolong the lifetime of users, but also is an effective way to reduce radiation in the environment. In this paper, for amplify-and-forward-relay cooperative networks, where multiple communication pairs share a common relay, we propose a novel power allocation scheme, which has considerably lower complexity compared with the existing solution schemes. The objective of this paper is to minimize sum-source-power consumption under the conditions that the transmit power of all nodes in the network is constrained and their predetermined target signal-to-noise ratios are satisfied. By providing sufficient analytical evidence, we study the optimality, convergence, and computational complexity of our proposed scheme. Through numerical simulation, we further justify the effectiveness and efficacy of our scheme compared with the existing works.

Local Congestion Avoidance in Network-on-Chip

Network-on-Chip (NoC) has been made the communication infrastructure for many-core architecture. NoC are subject to congestion, which is claimed to be avoided by many researchers. However, there is no completely understanding of congestion in literature, which hinders its solution. Toward this direction, we firstly carry out study on congestion in this paper. We find that congestion usually occurs at a portion of nodes in a local network region. Moreover, local congestion will significantly decrease system performance and mostly impact some particular communication pairs. Then we attempt to solve local congestion by addressing different local region size, based on Divide-Conquer approach and routing pressure. It avoids congestion in every local region by keeping routing pressure of every local region minimum. Using different local region size will create different routings. Our study shows that the local region size is closely related with the routing performance. When local region size is $5\times 5$ the optimal routing performance of large size network could be achieved.

SplayNet: Towards Locally Self-Adjusting Networks

This paper initiates the study of locally self-adjusting networks: networks whose topology adapts dynamically and in a decentralized manner, to the communication pattern $\sigma$ . Our vision can be seen as a distributed generalization of the self-adjusting datastructures introduced by Sleator and Tarjan, 1985: In contrast to their splay trees which dynamically optimize the lookup costs from a single node (namely the tree root), we seek to minimize the routing cost between arbitrary communication pairs in the network. As a first step, we study distributed binary search trees (BSTs), which are attractive for their support of greedy routing. We introduce a simple model which captures the fundamental tradeoff between the benefits and costs of self-adjusting networks. We present the SplayNet algorithm and formally analyze its performance, and prove its optimality in specific case studies. We also introduce lower bound techniques based on interval cuts and edge expansion, to study the limitations of any demand-optimized network. Finally, we extend our study to multi-tree networks, and highlight an intriguing difference between classic and distributed splay trees.

Soft tissue coverage on the segmentation accuracy of the 3D surface-rendered model from cone-beam CT

ObjectivesThe aim of this study is to investigate the effect of soft tissue presence on the segmentation accuracy of the 3D hard tissue models from cone-beam computed tomography (CBCT).Materials and methodsSeven pairs of CBCT Digital Imaging and Communication in Medicine (DICOM) datasets, containing data of human cadaver heads and their respective dry skulls, were used. The effect of the soft tissue presence on the accuracy of the segmented models was evaluated by performing linear and angular measurements and by superimposition and color mapping of the surface discrepancies after splitting the mandible and maxillo-facial complex in the midsagittal plane.ResultsThe linear and angular measurements showed significant differences for the more posterior transversal measurements on the mandible (p < 0.01). By splitting and superimposing the maxillo-facial complex, the mean root-mean-square error (RMSE) as a measurement of inaccuracy decreased insignificantly from 0.936 to 0.922 mm (p > 0.05). The RMSE value for the mandible, however, significantly decreased from 1.240 to 0.981 mm after splitting (p < 0.01).ConclusionsThe soft tissue presence seems to affect the accuracy of the 3D hard tissue model obtained from a cone-beam CT, below a generally accepted level of clinical significance of 1 mm. However, this level of accuracy may not meet the requirement for applications where high precision is paramount.Clinical relevanceAccuracy of CBCT-based 3D surface-rendered models, especially of the hard tissues, are crucial in several dental and medical applications, such as implant planning and virtual surgical planning on patients undergoing orthognathic and navigational surgeries. When used in applications where high precision is paramount, the effect of soft tissue presence should be taken into consideration during the segmentation process.

Transmission capacity analysis for cellular based cognitive radio VANETs

This paper presents a framework of cellular based cognitive-radio vehicular ad hoc networks (CCR-VANETs) that consists of a cellular network (primary network) and VANET (secondary network). These coexisting networks share the uplink spectrum of the cellular network. Although some scaling law-based results on VANETs have been presented for the description of performance changing patterns, they are unsuitable for CCR-VANET and cannot be directly used for estimating the capacity of the communication pairs in the entire network. In this study, we apply the interference-based transmission capacity analysis for CCR-VANET scenario. The classic car-following model is used to describe the moving pattern of all buses and vehicles, and a two-phase CSMA based on the opportunistic spectrum access (OSA) protocol of the secondary network is adopted. Then the calculation of the transmission opportunity is obtained. After that, the outage probability is analyzed, in which just the first layer of interferers in both primary and secondary networks are considered, as a result of the bound effect of interference power in the wireless network. With the obtained results on outage probability under the worst interfered scenario, we characterize a lower bound on transmission capacity of the secondary network. Finally, the simulation results are conducted to validate our analytical results, and also to give an analysis of the average transmission capacity trade-off between the primary and secondary networks.

The Degrees of Freedom of Full-Duplex Bidirectional Interference Networks With and Without a MIMO Relay

In a full-duplex bidirectional interference network with $2K$ transceivers, there are $K$ communication pairs: each user transmits a message to and receives a message from one intended user and interferes with and experiences interference from all other users. All nodes may interact, or adapt inputs to past received signals, and may thus co-operate with each other. We derive a new outer bound, and use interference alignment to demonstrate that the optimal degrees of freedom (DoF, also known as the multiplexing gain) is $K$ : full-duplex operation doubles the DoF, but interaction and co-operation does not further increase the DoF. We next characterize the DoF of a full-duplex bidirectional interference network with a MIMO full-duplex relay. If the relay is noncausal/instantaneous (at time $k$ forwards a function of its received signals up to time $k$ ) and has $2K$ antennas, we demonstrate a one-shot scheme where the relay mitigates all interference to achieve the interference-free $2K$ DoF. In contrast, if the relay is causal (at time $k$ forwards a function of its received signals up to time $k-1$ ), we show that a full-duplex MIMO relay cannot increase the DoF of the full-duplex bidirectional interference network beyond $K$ , as if no relay or interaction is present.

On Interference Alignment With Imperfect CSI: Characterizations of Outage Probability, Ergodic Rate and SER

In this paper, we give a unified performance analysis of interference alignment (IA) over MIMO interference channels. Rather than the asymptotic characterization, i.e. degree of freedom (DOF) at high signal-to-noise ratio (SNR), we focus on the other practical performance metrics, namely outage probability, ergodic rate and symbol error rate (SER). In particular, we consider imperfect IA due to the fact that the transmitters usually have only imperfect channel state information (CSI) in practical scenario. By characterizing the impact of imperfect CSI, we derive the exact closed-form expressions of outage probability, ergodic rate and SER in terms of CSI accuracy, transmit SNR, channel condition, number of antennas, and the number of data streams of each communication pair. Furthermore, we obtain some important guidelines for performance optimization of IA under imperfect CSI by minimizing the performance loss over IA with perfect CSI. Finally, our theoretical claims are validated by simulation results.

Analysis of Link Lifetime With Auto-Correlated Shadowing in High-Speed Railway Networks

In high-speed railway (HSR) networks, wireless link connection is fragile because of fast train speed and severe fading environment. Link lifetime is an important metric for many safety and non-safety applications that require a radio connection to be maintained for an entire communication session. An analytical modeling structure is proposed for estimating the probability density function (PDF) of link lifetime under auto-correlated shadow fading environment in HSR networks. A discrete-time Markov chain (DTMC) based model is proposed to model both pathloss and auto-correlated shadowing components into a transition process. Based on the proposed DTMC model, the PDF of the link lifetime between a communication pair can then be more accurately derived. Performance of the model is verified by a measurement campaign conducted along a part of the Zhengzhou-Xi'an HSR networks in China.

Distributed formation of degree constrained minimum routing cost tree in wireless ad-hoc networks

During several decades, there have been many researches on approximation algorithms for constructing minimum routing cost tree (MRCT) that minimizes the sum of routing cost of all pairs in a tree topology. Existing algorithms have been mainly studied in the field of graph theory, thus it is difficult to apply them to multi-hop wireless ad-hoc networks due to the theoretical and centralized methodology. In addition, wireless ad-hoc network protocols restrict the maximum degree, which is the maximum number of children a parent may have, in order to prevent excessive concentration of traffic. However, this limitation has not been considered by any existing algorithms. In this paper, we define the degree constrained MRCT (DC-MRCT) problem and extract the characteristics of DC-MRCT by analyzing all possible tree topologies for the given number of nodes. Based on these characteristics that DC-MRCT has the minimum sum of tree level and the maximum square sum of subtree sizes, we propose a distributed DC-MRCT Formation (DC-MRCTF) algorithm that can be applicable to any type of wireless ad-hoc network protocols working on tree topology. Performance evaluation shows that DC-MRCTF gives noticeable benefit for up to 80% of individual communication pair compared with the representative tree formation algorithm in ZigBee as well as significantly reduces the sum of routing cost of all pairs regardless of network density.

An Efficient Pilot Scheme in Large-Scale Two-Way Relay Systems

This letter considers an efficient pilot scheme for two-way relay system with massive antennas in the relay base station (BS). For such a system, we prove that user terminals (UTs) in the same communication pair can reuse the pilot sequence without performance loss. Hence, only $K$ pilot symbols are required to support $2K$ UTs while $2K$ pilot symbols are needed in conventional cases. We analyze the asymptotic performance when the BS's antenna number grows infinite. In such a case, the inter-pair interference and noise vanish, and the self-interference determined by the second-order statistics of channel vectors can be feasibly tackled. In this way, the desired signal can be obtained by subtracting the self-interference from the received signal, and the capacity increases with the BS's antenna number.