Distributed Relay Nodes Research Articles

Distributed Learning for MIMO Relay Networks

This paper studies a multi-antenna multi-user and multi-relay network, where the radio frequency (RF) power amplifiers (PA) of the nodes are subject to instantaneous power constraints. To optimize the nonlinear transceivers of the distributed nodes, we introduce a novel perspective of relating a relay network to an artificial neural network (ANN). With this perspective, we propose a distributed learning-based relay beamforming (DLRB) scheme. Based on a set of pilot sequences, the DLRB scheme can optimize the transceivers to minimize the mean squared error (MSE) of the data stream in a distributed manner. It can effectively coordinate the distributed relay nodes to form a virtual array to suppress interferences, even assuming neither the channel state information (CSI) nor information exchange between the relay nodes or between the users. We also present a frame design to support the DRLB so that it can adapt well with time-varying channels. Extensive simulations verify the effectiveness of the proposed scheme.

Relay Probing for Millimeter Wave Multi-Hop D2D Networks

The communications in the millimeter wave (mmWave) band, e.g., WiGig (60 GHz), is considered as one of the main components of 5G and beyond 5G (B5G) networks. However, it is characterized by short range transmissions along with high susceptibility to path blockage, e.g., human shadowing. Thus, mmWave multi-hop relaying using device to device (D2D) connections turns to be an efficient solution to extend its communication range and to route around blockages. In this context, relay probing is essential to discover/explore the candidate multi-hop routes from source-to-destination and then select the best route among them. However, a trade-off exists between relay probing and the required overhead coming from mmWave beamforming training. In this paper, taking advantage of the multi-band μW /mmWave relay nodes, an efficient multi-hop relay probing scheme is proposed for mmWave D2D routing. In this scheme, the collected μW received signal strengths (RSSs) among the distributed relay nodes are used to estimate the probability of their mmWave signal-to-noise power ratios (SNRs) while considering line-of-sight (LOS) and non-LOS (NLOS) path availabilities. Then, based on a proposed probabilistic metric, a hierarchical search algorithm is proposed to jointly discover the relay nodes along the candidate multi-hop routes and enumerate the number of routes expected to maximize the spectral efficiency of the whole path from source-to-destination. This is done in an offline phase, and only the relay nodes located within the pre-selected multi-hop routes are requested to do online relay probing. Numerical analysis confirms the superiority of the proposed mmWave multi-hop relay probing scheme over the candidate techniques.

Robust Iterative Transceiver Beamforming for Multipair Two-Way Distributed Relay Networks

In this paper, the transceiver beamforming problem is studied for multipair two-way distributed relay networks, particularly with multi-antenna user nodes and in the presence of channel state errors. With multi-antenna setting on the user nodes, some of the usual signal processing tasks are shifted from the relay nodes to the user nodes with the proposed transceiver beamforming designs. The transmit beamforming vectors, distributed relay coefficients, and the receive beamforming vectors are obtained by iteratively solving three sub-problems, each having a closed-form solution. The tasks of maximizing desired signal power, and reducing inter-pair interference and noise are thus allocated to different iteration steps. By this arrangement, the transmit and receive beamformers of each user are responsible for improving its own performance and the distributed relay nodes with simple amplify-and-forward protocol aim at creating a comfortable communication environment for all user pairs. With respect to the channel uncertainty, two relay strategies are proposed considering two different requirements from the communication network: sum relay output power and individual relay output power. Our simulation demonstrates that the performance improvement can be very significant through cooperation of the three components, especially when the number of relay nodes is large.

Joint Frequency Offset and Channel Estimation in Distributed MIMO-OFDM Systems

In recent times, cooperative communications or distributed multi-input multi-output (DMIMO) systems are being considered as key enabler for increasing spectral efficiency and coverage area in 4G and beyond wireless communication system. DMIMO system employs multiple relays each with single or multiple antennas in between the source and destination to form a virtual antenna array. However, a few of the fundamental difficulties in deployment of DMIMO networks with coherent communications are synchronization, tracking of independent oscillators, and channel estimation. The signal received at the destination is characterized by multiple carrier frequency offsets (MCFOs) and MIMO channel effects due to sparsely distributed relay nodes. In this paper, we address the issue of joint estimation of frequency offset and channel gains in a DMIMO orthogonal frequency division multiplexing (OFDM) systems deploying multiple antennas at the relays. We consider estimate-and-forward (EF) protocol for relaying where the coarse estimation process at the low cost relays generates inter carrier interference (ICI) in a multi-carrier system resulting in severe performance degradation and decrease in reliability of the network. The two iterative estimators, expectation conditional maximization (ECM) and space-alternating generalized expectation maximization (SAGE), are proposed to jointly estimate MCFOs and channel in the presence of ICI. The robustness of the estimators are evaluated through mathematical analysis and validated using simulations. The performance of the estimators is measured in terms of mean square error (MSE) and bit error rate (BER). Simulation results show that the proposed estimators provide a significant performance gain in DMIMO-OFDM systems with MIMO configuration at the relays compared to single-input single-output (SISO) system.

Zone-Based Clustering Approach for Separated Wireless Sensor Network Fields

Improved network lifetime is an important task to be achieved by these sensor networks. In this editorial note, a methodology for evaluation of clustering efficiency, routing efficiency, energy efficiency and lifetime of two dense sensor network fields by means of a distributed clustering approach, the hybrid energy efficient clustering algorithm (HEECA) has been proposed. It is a well-distributed and energy-efficient clustering algorithm which employs three innovative techniques: zone based transmission power (ZBTP) routing using distributed relay nodes (DRNs) and rapid cluster formation (RCF). The proposed scheme is compared with two well-evaluated existing distributed clustering algorithms O-LEACH and HEED.

Enhanced Zone-Based Clustering Method for Energy Efficient Wireless Sensor Network

Wireless sensor network contains a number of tiny sensors to get data from the surface where it deployed. Clustering is done to increase energy of the node to some certain period. Enhanced network lifetime is an important task to be achieved by these sensor networks. In this paper, a procedure for evaluation of clustering efficiency, routing efficiency, energy efficiency and lifetime of two impenetrable wireless sensor network fields, using a distributed clustering approach, the hybrid energy efficient clustering algorithm has been proposed, which targets mainly on effectively prolonging the lifetime of wireless sensor networks. It is a well-distributed and energy-efficient clustering algorithm which employs three novel techniques: zone based transmission power, routing by means of distributed relay nodes and rapid cluster formation. The clustering process is effectively controlled, thus the number of cluster heads selection and the number of packets delivered to the base station is also found to be effective.

Optimal Cooperative Relaying and Jamming for Secure Communication

This letter considers joint cooperative beamforming and jamming schemes in a cooperative relay network where distributed relay nodes perform cooperative beamforming. The problem of interest is to keep the eavesdropper's signal to interference and noise ratio (SINR) as low as possible while maintaining the legitimate receiver's SINR requirement. Applying semidefinite relaxation (SDR) technique, it is shown that a rank-one optimal solution always exists if perfect channel information is available. An optimal relay beamforming scheme has also been proposed for the scenario where only the second-order statistics of the eavesdropper's channel is available.

Evaluation of Clustering Parameters in WSN Fields Using Distributed Zone-Based Approach

Abstract Wireless sensor network (WSN) is a low-powered network formed by the sensor nodes that finds application in civilian, military, visual sense models, and many others. Improved network lifetime is an important task to be achieved by these sensor networks. In this paper, a methodology for evaluation of clustering efficiency, routing efficiency, energy efficiency, and the lifetime of two dense wireless sensor network fields—using a distributed clustering approach, the hybrid energy efficient clustering algorithm (HEECA)—was proposed, which mainly targeted effectively prolonging the lifetime of wireless sensor networks. It is a well-distributed and energy-efficient clustering algorithm that employs three novel techniques: zone based transmission power (ZBTP), routing using distributed relay nodes (DRNs), and rapid cluster formation (RCF). The proposed scheme was compared with two well-evaluated existing distributed clustering algorithms, O-LEACH and HEED. Simulation results clearly showed an excellent improvement in remaining energy, throughput, routing efficiency, and energy efficiency of the entire wireless sensor system. The clustering process was effectively controlled, thereby the number of cluster heads selection and the number of packets delivered to the base station was also found to be effective. Ultimately, the overall lifetime of the wireless sensor network was improved when compared to the two existing distributed clustering algorithms.

Investigating VANET dissemination protocols performance under high throughput conditions

The use of Vehicular Ad-Hoc Networks (VANETs) for the dissemination of data flows to mobile highway vehicles has gained recently attention. For this purpose, vehicle-to-vehicle multi-hop communications are employed. For the effective networking of packets among highway vehicles, it is necessary to implement efficient algorithms that vehicles use to elect themselves as relay nodes to forward packets that they receive from other vehicles. In this work, we investigate the behavior of distributed relay node election protocols. In particular, we consider a class of such protocols whose actions are determined at time instants induced by the expiration of properly calibrated timers. We then investigate the high throughput regime of dissemination protocols. We show that a key factor that limits the achievable throughput rate is the occurrence of spurious forwarding events. These events are shown to lead to message duplications and consequent throughput degradation. We characterize this phenomenon by considering a VANET system that is configured to disseminate a single message flow along a road, as well as a scenario that involves the dissemination of two data flows that are originated by two distinct source nodes. We propose the integration into the protocol of a probabilistic decimation logic, with minimal impact on the protocol complexity. We show that, combined with the use of timers, the probabilistic mechanism serves to alleviate the occurrence of spurious forwarding events and consequently enhances the system's throughput rate and packet delivery ratios. The results of our study provide key guidelines for the design and calibration of such high performance networking protocols for VANET systems.

Relay placement for restoring connectivity in partitioned wireless sensor networks under limited information

Several factors such as initial deployment, battery depletion or hardware failures can cause partition wireless sensor networks (WSNs). This results in most of the sensors losing connectivity with the sink node and thus creating disruption of the delivery of the data. To restore connectivity, one possible solution is populating relay nodes to connect the partitions. However, this solution requires information regarding the availability of the damaged area, number of partitions in the network and the location of the remaining nodes which may not be obtained for all applications. Thus, a distributed self-deployment strategy may better fit the application requirements. In this paper, we propose two distributed relay node positioning approaches to guarantee network recovery for partitioned WSNs by minimizing the movement cost of the relay nodes. The first approach is based on virtual force-based movements of relays while the second exploits Game Theory among the leaders of the partitions. Force-based approach stretches the network gradually with the deployment of additional relays. In the game-theoretic approach, the partition to be connected with is determined by the leader relay nodes based on the probability distribution function (pdf) of the partitions. Partitions with a higher pdf have priority over other partitions for recovery. Once the partition is connected with the relay nodes, it becomes the part of the connected network. Recovery proceeds with the partition having the next highest priority until network is completely recovered by reaching the system-wide unique Nash equilibrium. Both approaches are analyzed and evaluated extensively through simulation. Game-theoretic approach has been shown to outperform force-based approach as well as a centralized approach under most of the conditions.

Energy-efficient cooperative beamforming in clustered wireless networks

Recently, due to the growing concern over the ever-increasing energy consumption, much attention has been given to the energy-efficient design of wireless communication systems. Various strategies for improving the energy efficiency, which is defined as the transmitted bits per total energy consumed (in bits/Joule), have been proposed. In this paper, we analyze the energy efficiency of clustered cooperative beamforming where distributed relay nodes construct a virtual Multiple-Input Single-Output beamforming system with a maximum transmit power constraint, and compare its performance with that of direct communications. All the energy consumption overheads incurred are taken into account in the energy efficiency analysis assuming a constant circuit-power consumption model. We then study the number of relay nodes that maximizes the efficiency for a given outage constraint. In addition, we propose a mode switching algorithm which optimally chooses between using cooperative beamforming or direct communications for transmission. We show that cooperative beamforming significantly outperforms direct communications in energy efficiency, as well as in spectral efficiency, even though a much larger amount of circuit power is consumed.

Space–time coding scheme for time–frequency asynchronous two‐way relay networks

In this study, the authors develop a distributed space–time coding scheme for a two-way relay network that contains multiple distributed relay nodes. Both the time and frequency asynchronous nature of the distributed system are considered in the design. Distributed convolutional coding is employed to handle multiple timing errors in the networks. The authors prove that under perfect frequency synchronisation, the proposed scheme can achieve both spatial and multipath diversity by linear receivers, such as linear zero-forcing or minimum mean-square-error receiver, thus providing a low decoding complexity. The authors further find that frequency asynchronism has little effect on the designed scheme and show that the diversity can still be achieved almost surely (in the measured theoretic sense) under both time and frequency asynchronous scenarios. The authors also provide numerical results to corroborate the proposed studies.

Interference MIMO Relay Channel: Joint Power Control and Transceiver-Relay Beamforming

In this paper, we consider an interference multiple-input multiple-output (MIMO) relay system where multiple source nodes communicate with their desired destination nodes concurrently with the aid of distributed relay nodes all equipped with multiple antennas. We aim at minimizing the total source and relay transmit power such that a minimum signal-to-interference-plus-noise ratio (SINR) threshold is maintained at each receiver. An iterative joint power control and beamforming algorithm is developed to achieve this goal. The proposed algorithm exploits transmit-relay-receive beamforming technique to mitigate the interferences from the unintended sources in conjunction with transmit power control. In particular, we apply the semidefinite relaxation technique to transform the relay transmission power minimization problem into a semidefinite programming (SDP) problem which can be efficiently solved by interior point-based methods. Numerical simulations are performed to demonstrate the effectiveness of the proposed iterative algorithm.

Space–Frequency Convolutional Coding for Frequency-Asynchronous AF Relay Networks

In this paper, we design a space-frequency (SF) convolutional coding scheme for amplify-and-forward (AF) relay networks that contain multiple distributed relay nodes. The frequency-asynchronous nature of the distributed system is considered in our design. Orthogonal frequency-division multiplexing (OFDM) modulation is adopted, which is robust to certain timing errors. We exploit the signal space diversity technique and employ an extended cyclic prefix (CP) at the source node. The relay nodes need to perform only simple operations, e.g., convolution and amplification, and they need no information about the channels and the frequency offsets. Attributed to the extended CP, the multiple frequency offsets can directly be compensated at the destination. We further prove that both spatial and multipath diversity can be achieved by the proposed scheme. Numerical results are provided to corroborate the proposed studies.

Decentralized wireless relay network channel modeling: an analogous approach to mobile radio channel characterization

A novel channel model for a two-hop decentralized wireless relay network (DWRN) is presented, where the relays operate in a completely distributive fashion. The model is based on an analogous approach to the conventional description methods for wideband directional multipath channels. First, the concept of information azimuth-delay spectrum (IADS) is devised for a simplified DWRN with perfect receiver conditions. The IADS describes the distribution of the incoming information at the destination in the azimuth-delay domain. This definition, parallel to the widely-used power azimuth-delay spectrum (PADS) for physical multipath channels, represents a compact description of virtual DWRN channels. Subsequently, several key quantities derived from the IADS are introduced, which provide an intuitive way for analysis of a DWRN. The proposed approach is then applied to an elliptical random network (ERN) with uniformly distributed relay nodes. Numerical examples are used to demonstrate the usefulness of the suggested channel description method.