Source Destination Research Articles

Small-m method for detecting all longest paths

Given a weighted directed graph without positive cycles, we construct a framework to detect all longest paths for pairs of nodes in a network. The interest is to identify all routes with the highest cumulative cost for each source–destination pair. The significance and need for this arises in several scheduling contexts, an example of which is called critical chain project management. All longest routes are enumerated and compared for each output to determine a bottleneck path referred to as critical chain. Besides finding longest paths, minimizing duration needs to be considered. This indicates that multiple types of optimization problems coexist in one methodology. We thus aim to contain the longest-paths problem through constraints, for which an optimal solution that minimizes duration can be detected by solving a single optimization problem. The framework is reduced to a constraint satisfaction problem in a mixed-integer linear-programming context, and the solution can be derived using a general purpose solver. Optimality for the longest-paths problem is proven using the small-m method. Since the developed framework does not require an objective function specification, the methodology can also be incorporated within other optimization based problem contexts.

Aided Opportunistic Jammer Selection for Secrecy Improvement in Underlay Cognitive Radio Networks

In this paper, we consider the physical-layer security for the primary system in an underlay cognitive radio model which comprises of a primary source–destination pair, a secondary receiver (SR), K secondary transmitters (STi, i = 1,…,K) and an eavesdropper (E). To protecting the transmission confidentiality of the primary transmission system against eavesdropping, we propose secondary transmitter (ST) aided opportunistic jamming transmission protocols. In our protocols, the selection of ST plays a crucial role. To this end, we propose the optimal secondary transmission selection (OSTS) and the Optimal Cooperative Jammer Selection (OCJS), respectively. For the non-security management, OSTS, OCJS schemes, we derive the closed-form expressions of the outage probability and the intercept probability for primary system, respectively. Furthermore, we also derive the closed-form expressions of the outage probability for the secondary user for the selective schemes. Numerical results show that the OCJS scheme has the best security performance and the conventional non-security management scheme has the worst security performance for primary system. In addition, the outage performances of OSTS and OCJS are better than the conventional non-security protocol in high secondary transmitting SNR region.

Outage Capacity of Rate-Adaptive Relaying for FSO Links With Nonzero Boresight Pointing Errors

An outage capacity analysis of a three-way free-space optical (FSO) setup based on the optical path selection, either source-destination (S-D) or source-relay-destination (S-R-D), with a greater value of channel gain along with the use of repetition coding (RC) is analyzed over atmospheric turbulence channels with nonzero boresight pointing errors. The developed expression, which is validated through Monte Carlo simulation, is used to carefully analyze the performance and design of rate-adaptive relaying FSO systems from an information theory point of view. It is highlighted that a greater outage diversity is achieved by the obtained results without investing in extra hardware at the expense of an information rate reduction.

Modified X–Y routing for mesh topology based NoC router on field programmable gate array

Network on chip (NoC) has been proposed as an enormously scalable solution to address communication problems in system on chip (SoC). The interconnections among multiple cores/multiple Intellectual Property modules on a chip have a major impact on communication and performance of the chip design in terms of area, throughput, latency and power. Hence, an efficient design of the NoC interconnect is of paramount importance. In this study, a novel idea of the NoC router using a single-side buffer in the input block and a programmable priority encoder in the scheduler is discussed and a 4 × 4 mesh topology based router design is implemented on a field programmable gate array device using these blocks. This gives an area optimisation of the router due to the use of a small buffer. Furthermore, a modified X–Y routing algorithm is discussed and implemented for the 8 × 8 mesh topology-based router. The result shows that the modified X–Y routing algorithm requires only two hops to reach any source destination pair resulting in latency optimisation of the NoC router. Thus, the router design with a single buffer and a modified X–Y routing algorithm provides the best solution for area and latency optimisation.

Low-Complexity Coordinated Relay Beamforming Design for Multi-Cluster Relay Interference Networks

Consider a multi-user multi-cluster relay interference network where each cluster contains a source–destination pair communicating through their own dedicated multi-antenna relays. We propose a coordinated relay beamforming design for interference management among relaying clusters, aiming to maximize the minimum signal-to-interference-and-noise ratio (SINR) at destinations, subject to per relay power budgets. We propose a structured relay beam matrix solution based on a weighted sum of two types of beam matrices: zero-forcing (ZF) beamforming and maximum ratio combining (MRC) beamforming, of which we obtain the optimal ZF and MRC beam matrices in closed form. Our proposed beamforming structure allows us to transform the original max–min SINR problem into a low-complexity weight optimization problem, which we solve via the semi-definite relaxation approach. Our solution is highly computationally efficient with the complexity not growing with the number of antennas at each relay. Comparing with the direct approach to obtain relay beam matrices for the original problem, our solution provides a very similar performance but with a significantly lower complexity and thus is scalable and suitable for large-antenna systems. The weights in our relay beamforming solution clearly reveal how the relay power splits and shifts between suppressing inter-cluster interference and maximizing signal beamforming gain as the channel strength or cluster distance changes. The simulation shows that our solution significantly outperforms the MRC-only or ZF-only solution under both the perfect channel-state information (CSI) and imperfect CSI over interference channels.

Relay Selection Schemes for FSO Communications over Turbulent Channels

Free Space Optics (FSO) Communication has attracted the attention of the researchers in the last decade due to its high data rate, security, and low cost. Relay-assisted techniques are used to divide the distance to shorter hops in order to mitigate the effects of turbulence, weather attenuation, pointing error, and geometric loss. Choosing an active relay per time slot has been proven to enhance the performance of the system and decrease the loading effect on the system when compared to all active relays. This paper investigates the best relay that can be selected according to the source to relay (S-R) channel coefficient, relay to destination (R-D) channel coefficient, and source to destination (S-D) channel coefficient. A comprehensive comparison is applied to the three following cases: (a) Broadcast phase from source to relay to select the best (Proactive-Relay); (b) Broadcast phase from relay to destination after broadcasting to all relays then select (Reactive-relays); and, (c) Direct link from source-to-best relay-to-destination to conclude which method is better for different scenarios, such as turbulence regime, number of relays, different pointing error effect, and severity of S-R as compared to R-D and vice versa. The selection methods regard to four aspects: (1) Number of relays (two or three relays); (2) Distance between Source-Relay and Relay-Destination (D = 400–600 m, 500–500 m, and 600–400 m); (3) The different turbulence of Log-normal channel and Gamma-Gamma channel (with a refractive index coefficient( C n 2 = 0.5 × 10−14, 2 × 10−14 and 5 × 10−14)); and finally, (4) Beam waist ω z (pointing error).

Wireless-Powered Full-Duplex Relay and Friendly Jamming for Secure Cooperative Communications

Wireless energy harvesting, physical-layer security, and full-duplex wireless are important, emerging fifth generation (5G) technologies. In this paper, we thus investigate a source–destination link with an energy-harvesting full-duplex relay and a jammer (to degrade the eavesdropper channel) in the presence of an eavesdropper. Thus, to exploit energy harvesting and to improve security, we propose a full-duplex jammer (FDJ) protocol and its half-duplex version (HDJ). Two cases for availability of the eavesdropper channel state information (ECSI) are considered: complete ECSI and incomplete ECSI. For both FDJ and HDJ protocols and for complete ECSI, we derive the instantaneous and average secrecy rates and compute optimal time split for energy harvesting. To gain more insights, we consider a practical interference-limited scenario and derive closed-form cumulative distribution function of the signal-to-interference plus noise ratio at the destination and eavesdropper nodes. Comparatively, we show that FDJ improves instantaneous secrecy rate over HDJ. However, the degree of improvement is highly dependent on time split for energy harvesting, amount of self-interference, the channel gains, and locations of the nodes. Our findings reveal that FDJ increases the average secrecy rate 150% over HDJ and 260% over HD relaying without jammer. For incomplete ECSI scenario, we derive asymptotic secrecy outage and show that FDJ performs better for small-to-medium values of source powers; otherwise, HDJ yields a higher gain.

Spectrum-Aware Routing in Full-Duplex Cognitive Radio Networks: An Optimization Framework

Routing and channel assignment schemes for cognitive radio networks (CRNs) are often designed assuming the half-duplex (HD) transmission capability per user. However, recent advances in full-duplex (FD) communications and self-interference suppression techniques challenge the traditional HD transmission capability, in which FD communication can significantly improve spectrum utilization. In this work, we investigate the routing and channel assignment problem in FD-based CRNs. Two types of FD communications are considered. The first type only allows for simultaneous transmission and reception over different channels, while the second type allows for simultaneous transmission and reception over the same channel. Specifically, for a given cognitive radio (CR) source–destination pair, we first formulate the channel assignment problem for each path between the communicating pair as an optimization problem with the main objective of minimizing the number of distinct assigned channels for that path such that the number of simultaneous active hops across the path is maximized. We show that the optimization problem is a binary linear programming problem, which is, in general, nondeterministic polynomial time-hard. Thus, we present a near-optimal solution based on a sequential fixing procedure, where the binary variables are iteratively determined by solving a sequence of relaxed programs. Accordingly, we develop a novel routing scheme that selects the best path along with the channel assignment such that the highest capacity is achieved. Simulation results are provided, which show that a careful routing and channel assignment scheme for FD CRNs can significantly improve the network performance.

Physical-Layer Security in Full-Duplex Multi-Hop Multi-User Wireless Network With Relay Selection

This paper investigates the relay selection (RS) problem for multi-hop full-duplex relay networks where multiple source-destination (SD) pairs compete for the same pool of relays, under the attack of multiple eavesdroppers. To enhance the physical-layer security, within a given coherence time, our objective is to jointly assign the available relays at each hop to different SD pairs to maximize the minimum secrecy rate among all pairs. Two RS schemes, optimal RS and suboptimal RS (SRS), are proposed for two-hop networks based on global channel state information (CSI) and only SD pairs CSI, respectively. Since all users can communicate within the same coherence time, our joint RS schemes are important for the user-fairness and ultra-reliable low-latency communications. To evaluate the performance, the exact secrecy outage probability of the SRS scheme is derived under two residual self-interference models. The asymptotic analysis shows that the SRS scheme achieves full diversity. A relay-based jamming scheme is also proposed by using unassigned relays for user communications. Finally, the two-hop RS schemes and the analysis are extended to the general multi-hop network with multiple eavesdroppers. The numerical results reveal interesting fundamental trends where the proposed schemes can significantly enhance the secrecy performance.

Performance Analysis of Secure Communications Over Dual Correlated Rician Fading Channels

Wireless secrecy under dual correlated Rician fading environments is theoretically studied for correlated source–destination and source–wiretapper channels. Without considering wiretapper active jamming capability, expressions for: 1) the ergodic secrecy capacity when the wiretapper is active and 2) secrecy outage probability when the wiretapper is passive are obtained. To further improve system security, new secrecy analyses employing an adaptive encoder with ON/OFF transmission are also given. The new findings exactly reduce to existing findings under dual correlated Rayleigh fading and advance theoretical developments for wireless secrecy under dual correlated Rician fading. Monte Carlo simulation results are given, which match theoretical predictions. Under wireless secrecy context, the disadvantage of line-of-sight (LoS) power and channel correlation is evidently shown, revealing the non-usefulness of LoS power and establishing a tradeoff between the transmission security and the transmission success. Benefits of a high-average transmit signal-to-noise ratio are discussed.

A PTAS for the metric case of the optimum weighted source–destination communication spanning tree problem

This work considers the optimum weighted source–destination communication spanning tree problem (WSDOCT), which is an NP-hard special case of the optimum communication spanning tree problem (OCT). Given an undirected graph G=(V,E) with non-negative lengths ω(e) associated to the edges and non-negative routing weights σ(u) and ρ(u) respectively to sending and receiving communication of nodes u∈V, the objective is to find a spanning tree T of G, that minimizes:∑u,v∈V(σ(u)ρ(v)+ρ(u)σ(v))d(T,u,v), where d(H,x,y) is the minimum distance between nodes x and y in a graph H⊆G. We present a polynomial time approximation scheme for the metric case of the WSDOCT. This result improves the until now best existing approximation algorithm for this problem. Also, we give a 2-approximation for the problem which improves the time complexity required by our PTAS to achieve this approximation ratio.

Scaling SDM Optical Networks Using Full-Spectrum Spatial Switching

In today's optical networks, the capacity supported by a single optical fiber is far higher than the demand between source–destination pairs. Thus, to take advantage of the installed capacity, lightpaths allocated between distinct source–destination pairs share the capacity provided by an optical fiber. This sharing is only possible if these lightpaths have different wavelengths. With the exponential evolution of traffic, the demand between source– destination pairs in a network can approach or exceed the capacity of a single fiber. In this scenario, spacedivision multiplexing (SDM) networks appear as a viable option, and new network-sharing technologies become relevant. In this paper, we study different switching approaches for SDM networks with uncoupled spatial superchannels—such as networks with multicore fibers or bundles of single-mode fibers—which result in different network-sharing strategies. We start by comparing three switching architectures considering the evolution of traffic over time: (1) full-spectrum spatial switching (full-spectrum SS); (2) wavelength switching (WS) of uncoupled spatial superchannels; and (3) independent switching (IS) of spatial and wavelength channels. IS provides high network flexibility at the cost of complex switching nodes. On the other hand, full-spectrum SS and WS simplify network nodes but reduce flexibility by switching superchannels. Simulation results indicate that WS offers poor utilization in the long term. Full-spectrum SS, in contrast, exhibits low utilization in the short term, but outperforms IS in the long term. We also propose alternative hybrid switching strategies that start with IS and change to full-spectrum SS after a certain number of spatial channels are activated. The simulations suggest that well-designed strategies for migration from IS to full-spectrum SS delay premature activation investments while saving on switching costs.

Variable-channel Threshold-based Hybrid AF-DF Cooperative Communications with Packet Splitting

Cooperative communications has been widely researched as an attractive way to improve wireless communications systems with a small number of antennas and low power consumption. However, forward processing in the relay terminals means consuming the power of relays in other terminals. Therefore, the waste of electricity from forwarding is to be reduced as much as possible. In this paper, we propose and analyze a new relay system for three-node, hybrid, cooperative communications systems, where the relay terminal may choose to transmit messages using amplifyand-forward (AF) with expansion of the channel matrix or using decode-and-forward (DF) with packet-splitting transmission. This is based on the qualities of the channels in the source–relay and source–destination links. Then, in the proposed system, the decision on the forwarding scheme is made in order to calculate a threshold based on the equation of the channel capacity. From the simulation results, the proposed system can improve the bit error rate and reduce the load from forward processing latency at the relay.

Diversity‐multiplexing tradeoff analysis in cooperative wireless networks with a multiple‐antenna relay

Multiple antennas can be used for increasing the amount of diversity or number of degrees of freedom in wireless communication systems. In this study, the authors consider a wireless half-duplex relay network consisting of one source–destination pair each with a single antenna and one relay with multiple antennas. They assume an approach where cooperation of the relay in the transmission depends on the decoding status at the relay. If the relay participates in transmission, the source and the relay use the distributed space-time code simultaneously. They derive the outage probability and the diversity-multiplexing tradeoff for the proposed protocol at high SNR. They have shown that the use of the centralised antennas with simultaneously decoding compared to the distributed antennas with independently decoding can improve the system performance.

Joint Transceiver Optimization for Wireless Information and Energy Transfer in Nonregenerative MIMO Relay Systems

In this paper, a two-hop nonregenerative multiple-input multiple-output relay system is investigated, where the relay node relies on harvesting the radio frequency energy transferred from the source node to forward information from source to destination. We consider the time switching (TS) protocol between wireless information and energy transfer. In particular, we propose a more general energy consumption constraint at the source node during the information and energy transfer, which includes the constant power constraints used in existing works as special cases. We study the joint optimization of the source precoding matrices, the relay amplifying matrix, and the TS factor to maximize the source–destination mutual information (MI). The optimal structure of the source and relay matrices is derived, which reduces the original transceiver optimization problem to a simpler power allocation problem. We propose a primal decomposition based algorithm and an upper bound based approach to efficiently solve the power allocation problem. The first algorithm achieves the global optimum, whereas the latter one has a lower computational complexity. Numerical simulations show that both proposed algorithms yield much higher system MI and better rate-energy tradeoff than existing approaches.

Analysis of capacity in vehicular device‐to‐device relay networks with multi‐user case

The transmission capacity of a vehicular network is still an interesting and challenging problem as capacity is impacted by transmission distance between both vehicles, density of vehicles, relay among vehicles, noise and interference. This study analyses the transmission capacity of vehicular device-to-device relay networks with multi-users (service vehicle (SV) and helper vehicle (HV)) in the overlay mode. SVs coexisting with HVs share the spectrum resources. Utilising stochastic geometry theory, SVs and HVs are modelled as the Poisson point process in vehicular networks, respectively. Subsequently, the HVs existence probability and the expectation distance with HV link are derived to calculate the successful transmission probabilities for SV to SV communication. With this characteristic HV relay transmission signals exceed transmission distance, the authors further calculate the transmission capacities with SV to SV communication assisted by HVs and reveal the influence of vehicles' density and transmission power. Besides, the relationship between transmission capacities, the variable direct link distance of SV to SV communication is also analysed. Simulation results indicate that transmission capacity can be improved by an assisted HV and influenced by a few system parameters, including the direct link distance between source SV and destination SV, SVs density and HVs density.

A Method of Optical Grooming Based on Dynamic Multicast Capable of Adaptive Splitting Under Differential Delay Constraint

Abstract Aiming at the problem of energy efficient transmission for multiple paths under differential delay constraint, we propose a method of optical grooming based on dynamic splitting under differential delay constraint algorithm (OG-DSDA) for optical networks. When a multicast demand cannot be groomed due to the bandwidth fragment, the method constructs two types of logic light-tree sets according to the differential delay constraint between source node and destinations. In order to reduce the network link costs caused by too many split parts, the method gives high priority to the first type logic light-trees with the maximum available bandwidth to meet the requirement. When the bandwidth of first type logic light-tree is insufficient for all sub-requests, the rest of sub-demands is adaptively groomed into a light-tree in second set of light-trees with the lower power consumption. Simulation results show that the proposed method can effectively decrease the network blocking probability and reduce network’s energy consumption.

Balancing flow table occupancy and link utilization in software-defined networks

Software-Defined Networking (SDN) employs a centralized control with a global network view and provides great opportunities to improve network performance. However, due to the limitation of flow-table space at the switches and unbalanced traffic allocation on links, an SDN may suffer from flow-table overflow and inefficient bandwidth allocation among flows, increasing the controller’s burden and degrading network performance. In this paper, we present a dynamic routing scheme named DIFF that differentiates flows based on their impact on network resource and adaptively selects routing paths for them to mitigate the problems of flow-table overflow and inefficient bandwidth allocation. DIFF pre-generates a set of paths for each pair of source–destination edge switches and intelligently selects the paths from the pre-generated path-sets for new flows with an objective to balance flow-table utilizations. It adaptively reroutes some elephant flows to achieve maximum throughput under the rule of max–min fair bandwidth allocation. Simulation results show that DIFF simultaneously balances the flow-table and link utilizations, reduces the controller’s workload and packet delay, while increasing network throughput, compared with baseline schemes.

Efficient cooperative ARQ protocols based on relay selection in underwater acoustic communication sensor networks

The characteristics of the underwater acoustic communication such as large propagation delay, high bit error rate and half-duplex, bring challenges to traditional automatic repeat request (ARQ) schemes. In this paper, we propose cooperative ARQ protocols based on relay selection in wireless sensor networks. These protocols use cooperative relays that provide an alternative path along a specific source-to-destination route. This alternative path has higher channel quality than that of the direct source–destination path. The main advantage of our methods is that we do not need to be aware of the relays locations. In fact, the relays will be organized by creating a time table at the destination. Furthermore, we evaluate the proposed schemes by comparing them with conventional stop and wait (S&W) ARQ and some other works in terms of throughput efficiency. Analytical and computer simulation results show that the proposed cooperative retransmission protocols can significantly improve the throughput even in a network with a few cooperative relay nodes.

Statistical Admission Control in Multi-Hop Cognitive Radio Networks

We address the problem of online admission control in multi-hop, multi-transceiver cognitive radio networks where the channel access is regulated by a bare-bones time-division multiple access protocol and the primary user activity is modeled as an ON/OFF process. We show that the problem of computing the available end-to-end bandwidth–necessary for admission control–is NP-Complete. Rather than working on an approximation algorithm and analyzing its worst-case performance, we relax the problem of online admission control by using a randomized scheduling algorithm and analyzing its average performance. Randomized scheduling is widely used because of its simplicity and efficiency. However, computing the resulting average throughput is challenging and remains an open problem. We solve this problem analytically and use the solution as vehicle for BRAND–a centralized heuristic for computing the average bandwidth available with randomized scheduling between a source destination pair in cognitive radio networks. Driven by practical considerations, we introduce a distributed version of BRAND and prove its correctness. An extensive numerical analysis demonstrates the accuracy of BRAND and its enabling value in performing admission control.