Cooperative Access in Cognitive Radio Networks: Stable Throughput and Delay Tradeoffs

In this paper, we study and analyze fundamental throughput-delay tradeoffs in cooperative multiple access for cognitive radio systems. We focus on the class of randomized cooperative policies, whereby the secondary user (SU) serves either the queue of its own data or the queue of the primary user (PU) relayed data with certain service probabilities. The proposed policy opens room for trading the PU delay for enhanced SU delay. Towards this objective, stability conditions for the queues involved in the system are derived. Furthermore, a moment generating function approach is employed to derive closed-form expressions for the average delay encountered by the packets of both users. Results reveal that cooperation expands the stable throughput region of the system and significantly reduces the delay at both users. Moreover, we quantify the gain obtained in terms of the SU delay under the proposed policy, over conventional relaying that gives strict priority to the relay queue.Comment: accepted for publication in IEEE 12th Intl. Symposium on Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks (WiOpt), 201

Cooperation and Underlay Mode Selection in Cognitive Radio Network

In this research, we proposes a new method for cooperation and underlay mode selection in cognitive radio networks. We characterize the maximum achievable throughput of our proposed method of hybrid spectrum sharing. Hybrid spectrum sharing is assumed where the Secondary User (SU) can access the Primary User (PU) channel in two modes, underlay mode or cooperative mode with admission control. In addition to access the channel in the overlay mode, secondary user is allowed to occupy the channel currently occupied by the primary user but with small transmission power. Adding the underlay access modes attains more opportunities to the secondary user to transmit data. It is proposed that the secondary user can only exploits the underlay access when the channel of the primary user direct link is good or predicted to be in non-outage state. Therefore, the secondary user could switch between underlay spectrum sharing and cooperation with the primary user. Hybrid access is regulated through monitoring the state of the primary link. By observing the simulation results, the proposed model attains noticeable improvement in the system performance in terms of maximum secondary user throughput than the conventional cooperation and non-cooperation schemes

Prerequisites for Mesenchymal Stem Cell Transplantation in Spinal Cord Injury

We have aimed at distinguishing obligatory prerequisites for mesenchymal stem cell transplantation in spinal cord injury from those prerequisites which are unnecessary or are prerequisites that have to be further investigated. Obligatory prerequisites include the following. First, the site of injury is extensively gliotic, constituting an unsuitable medium for stem cell transplantation. It has to be dissolved by neurolyzing agents, chondroitinase ABC as an example. Second, stem cells need a suitable biomaterial scaffold for their proper integration. Third, the biomaterial scaffold necessitates a tissue filler harboring stem cells, other cells and neurotrophic factors in a combinatorial approach. Fourth, the efficiency of mesenchymal stem cells themselves has to be increased (by reducing oxidative stress-induced apoptosis, by hypoxic preconditioning, by modulating the extracellular matrix and by other measures). Prerequisites that have to be further investigated include the ideal source, mode, quantity, time point and number of injections of mesenchymal stem cells; which growth factors and cells to be used in the combinatorial approach; transforming mesenchymal stem cells into motor neuron-like cells or Schwann cells; increasing the homing effect of stem cells and how to establish a continuous drug and cell delivery system

Bridging Defects in Chronic Spinal Cord Injury Using Peripheral Nerve Grafts: From Basic Science to Clinical Experience

Nerve grafting of the injured spinal cord should pursue a sixfold attack: lysing the fibrosis/gliosis to an extent that allows settling of the basal lamina preventing meanwhile collapse of the neural tissue matrix; supplying the tissue matrix with a suitable scaffold, on which the basal lamina can settle; basal lamina synthesis; seeding the basal lamina with cell adhesion molecules; providing the axonal growth cone with neurite outgrowth promoting factors that allow its distal progression; supplying the axonal growth cone with neurotrophic factors that power its continued growth. In addition to this, the intrinsic properties of the neurons should be stimulated, possibly through modulating the function of astrocytes by heparin, aspirin and other factors. Nerve side grafting of the cord increases the incidence of nerve regeneration by applying additional grafts extending from the side of the donor end of the cord to the side of the recipient end. Also, it allows the surgeon to enhance regeneration through a partially regenerated cord. During surgery, after establishment of CSF circulation, a long-lasting indwelling catheter has to be inserted for postoperative drug and cell delivery. This allows for continual lysis of the gliosis by chondroitinase ABC, sialidase and other factors

Speculations on the Use of Marine Polysaccharides as Scaffolds for Artificial Nerve ‘Side-’Grafts

Marine polysaccharides (e.g., chitosan, alginate and agarose) meet the requirements of artificial nerve grafting. These include the following: (1) prerequisites of biopolymers used as scaffolds; (2) conditions required by nerve autografts; (3) macroengineering requirements (form, design); (4) microengineering requirements (microgrooves, inclusion filaments); (5) mechanical conditions required by nerve autografts; (6) molecular aspects of peripheral nerve regeneration; space and adherence for: (i) chondroitin sulfate proteoglycans, (ii) neurite outgrowth promoting factors, (iii) neurotrophic factors, (iv) cells; (7) artificial side grafts should be compatible with autologous nerve grafts; (8) spatial distribution of neurotrophic factor gradients; (9) modulation of fibrosis; (10) renewal of luminal fillers. The mechanical stability of chitosan should be increased by adding other polymeric chains and cross-linking. As a result of deacetylation of chitin, chitosan scaffolds have got numerous positively charged free amino groups that provide adherence for growth factors and cells. To modulate fibrosis, heparin cross-linked chitosan microspheres have proved effective for delivering/transplanting cells, heparin and growth factors. To renew luminal fillers, chitosan microspheres may be injected through an indwelling catheter incorporated into the nerve conduit. Agarose and alginate have gained more acceptance as hydrogel lumen fillers. Interacting with positively charged chitosan, negatively charged alginate may form versatile chitosan-calcium-alginate microspheres

Modeling the Environmental Impact of Sustainability Policies in the Construction Industry Using Agent Based Simulation and Life Cycle Analysis

The construction industry, with its long supply chain and long lifetime of projects, is blamed to be one of the main contributors to environmental concerns including accelerated resource consumption and harmful emissions. Industry stakeholders, including developers, designers, contractors and suppliers, are, therefore, continuing to explore different options to reduce this impact. Various approaches have been adopted in different countries with building rating systems like the Leadership in Energy & Environmental Design (LEED) certification program being the most common way reflecting stakeholders’ efforts to go green. Governments and concerned authorities at national and state levels are expected to foster the trend of sustainable construction by motivating these stakeholders and pursuing policies that would help the green momentum. However, decision makers at such governmental and state levels face a challenge of prioritizing the policies and regulations that should be imposed. The objective of this paper is to present the development of a framework of an Agent Based Model (ABM) that simulates the effect of different possible policies in the construction market using Life Cycle Analysis (LCA), which is to be used by decision makers to assess and prioritize different policies or combination of policies. The framework was developed using Anylogic software and a sample construction market from the state of Qatar was used as an example for implementing the proposed framework. Results of running the model on this sample market illustrate the effectiveness of using this ABM as a support tool for decision makers in the area of sustainable construction