This paper provides an overview of the use of small cells (e.g., femtocells) in the Internet of Things (IoT) environments. As a result of rapid increase in the number of mobile connected devices such as smart-phones and tablets, the demand for data traffic is exponentially increasing. In order to satisfy mobile users’ requests and meet the requirements of high data traffic, mobile operators have to increase the network capacities dramatically. One of the promising solutions for the network operators to improve coverage and capacity, and provide high data rate services in a less costly manner is the deployment of femtocells related technologies. Femtocells will help mobile operators to provide a basis for the next generation of services which are a combination of voice, video, and data services to mobile users. Moreover, proper traffic modelling and deployment strategies of femtocells will improve the overall performance of the femtocell network. Therefore, in this survey paper, we overview modelling traffic and deployment strategies in femtocells and provide a review for the use of femtocells and their applications in the IoT environments. In addition, we present open research issues associated with IoT-femtocell based applications.

Small Cells in the Forthcoming 5G/IoT: Traffic Modelling and Deployment Overview

Consider a multi-user orthogonal frequency division multiple access (OFDMA) system where multiple users share multiple discrete subcarriers, but at most one user is allowed to transmit power on each subcarrier. To adapt fast traffic and channel fluctuations and improve the spectrum efficiency, the system should have the ability to dynamically allocate subcarriers and power resources to users. Assuming perfect channel knowledge, two formulations for the joint subcarrier and power allocation problem are considered in this paper: the first is to minimize the total transmission power subject to the quality of service constraints and the OFDMA constraint, and the second is to maximize some system utility function subject to the total transmission power constraint per user and the OFDMA constraint. In spite of the existence of various heuristics approaches, little is known about the computational complexity status of the above problem. This paper aims at filling this theoretical gap, i.e., characterizing the complexity of the joint subcarrier and power allocation problem for the multi-user OFDMA system. It is shown in this paper that both formulations of the joint subcarrier and power allocation problem are strongly NP-hard. Several subclasses of the problem which can be solved efficiently in polynomial time are also identified. These complexity results suggest that there are not polynomial time algorithms that are able to solve the general joint subcarrier and power allocation problem to global optimality (unless P=NP), and determining an approximately optimal subcarrier and power allocation strategy is more realistic in practice.

On the Complexity of Joint Subcarrier and Power Allocation for Multi-User OFDMA Systems

A timely addition to the understanding of IMT-Advanced, this book places particular emphasis on the new areas which IMT-Advanced technologies rely on compared with their predecessors. These latest areas include Radio Resource Management, Carrier Aggregation, improved MIMO support and Relaying.Each technique is thoroughly described and illustrated before being surveyed in context of the LTE-Advanced standards. The book also presents state-of-the-art information on the different aspects of the work of standardization bodies (such as 3GPP and IEEE), making global links between them.Explores the latest research innovations to assess the future of the LTE standardCovers the latest research techniques for beyond IMT-Advanced such as Coordinated multi-point systems (CoMP), Network Coding, Device-to-Device and Spectrum SharingContains key information for researchers from academia and industry, engineers, regulators and decision makers working on LTE-Advanced and beyond

Mobile and Wireless Communications for IMT-Advanced and Beyond

Consider a multi-user Orthogonal Frequency Division Multiple Access (OFDMA) system where multiple users share multiple discrete subcarriers, but at most one user is allowed to transmit power on each subcarrier. To adapt fast traffic and channel fluctuations and improve the spectrum efficiency, the system should have the ability to dynamically allocate subcarriers and power resources to users. Assuming perfect channel knowledge, two formulations for the joint subcarrier and power allocation problem are considered in this paper: the first is to minimize the total transmission power subject to quality of service constraints and the OFDMA constraint, and the second is to maximize some system utility function (including the sum-rate utility, the proportional fairness utility, the harmonic mean utility, and the min-rate utility) subject to the total transmission power constraint per user and the OFDMA constraint. In spite of the existence of various heuristics approaches, little is known about the computational complexity status of the above problem. This paper aims to fill this theoretical gap, i.e., characterizing the complexity of the joint subcarrier and power allocation problem for the multi-user OFDMA system. It is shown in this paper that both formulations of the joint subcarrier and power allocation problem are strongly NP-hard. The proof is based on a polynomial time transformation from the so-called 3-dimensional matching problem. Several subclasses of the problem which can be solved to global optimality or $\epsilon$-global optimality in polynomial time are also identified. These complexity results suggest that there are not polynomial time algorithms which are able to solve the general joint subcarrier and power allocation problem to global optimality (unless P$=$NP), and determining an approximately optimal subcarrier and power allocation strategy is more realistic in practice.

This article highlights particular challenges inherent in the design and operation of future mobile communications systems This paper also discusses the various degrees of freedom involved, and points out which design paradigms appear most promising and which major fields of future research remain.

Future Mobile Communication Networks: Challenges in the Design and Operation

This paper presents a framework based on self-organization to jointly self-optimize the spectrum assignment and the transmission power in the context of downlink OFDMA femtocell deployments. It is a distributed framework where each femtocell acts as an autonomous entity. Femtocells perform spectrum and transmission power assignment based on users’ reported measurements, which are employed to sense intercell interference, including that from other femtocells or from macrocells in two-layer deployments. Results have been obtained for a realistic indoor femtocell deployment with and without macrocell interference. The paper shows that, thanks to self-organization, sensible performance improvements can be achieved in terms of spectral efficiency and power consumption reductions.

Self-Optimization of Spectrum Assignment and Transmission Power in OFDMA Femtocells

We present an optical dilatometer for high-accuracy and high-resolution absolute measurement of the linear coefficient of thermal expansion (CTEl). Based on a highly symmetric differential heterodyne interferometer, dimensional changes of a tubular shaped specimen under controlled thermal conditions can be characterized. Our measurement facility is located in vacuum where the test specimen can be temperature controlled in a temperature range between 20 °C and 60 °C. A thermally stable support and two identical isostatic mirror clamps were specifically designed to fix a reference and a measurement mirror inside the tube enabling a measurement, where no load in the axial direction was applied to the device under test (DUT). We measured the linear CTE of two carbon-fibre reinforced plastic (CFRP) tubes with different predicted linear CTEs at room temperature: −0.647 × 10−6 K−1 and 0 ± 2.5 × 10−9 K−1, respectively. Currently, we are investigating the manufacture limitations of the CFRP and the limitations of our apparatus in terms of measurement accuracy. In the next step, we will characterize a specifically manufactured zero-class Zerodur™ tube with a CTEl value <10 × 10−9 K−1. Due to its high thermal stability and non-directional structural isotropy this material has been chosen for macroscopic calibration of the metrology system. The results of these measurements will thus provide the resolution limitations of our facility and can be taken as an absolute accuracy reference.

Interferometry based high-precision dilatometry for dimensional characterization of highly stable materials

The gravitational wave detector LISA utilizes as current baseline a high sensitivity Optical Readout (ORO) for measuring the relative position and tilt of a free flying proof mass with respect to the satellite housing. The required sensitivities in the frequency band from 30 μHz to 1Hz are ∼ pm/ √ Hz for the translation√ and ~ nrad/√ Hz for the tilt measurement. EADS Astrium, in collaboration with the Humboldt University Berlin and the University of Applied Sciences Konstanz, has realized a prototype ORO over the past years. The interferometer is based on a highly symmetric design where both, measurement and reference beam have a similar optical pathlength, and the same frequency and polarization. The technique of differential wavefront sensing (DWS) for tilt measurement is implemented. With our setup noise levels below 5pm/ √Hz for translation and below 10nrad/ √Hz for tilt measurements – both for frequencies above 10mHz – were demonstrated. We give an overview over the experimental setup, its current performance and the planned improvements. We also discuss the application to first verification of critical LISA aspects. As example we present measurements of the coefficient of thermal expansion (CTE) of various carbon fiber reinforced plastic (CFRP) including a "near-zero-CTE" tube.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10566/1056612/A-high-sensitivity-heterodyne-interferometer-as-a-possible-optical-readout/10.1117/12.2308257.pdf

A high sensitivity heterodyne interferometer as a possible optical readout for the LISA gravitational reference sensor and its application to technology verification


 In a global context aiming to unlock a low carbon future by industry decarbonization, developing the infrastructure for capturing and storing CO2 emissions is a key target of countries, energy companies and regulatory bodies. Injection for geological storage in suitable reservoirs is an advantageous option which presents challenges related to the completion accessories and string exposed to the injected fluid and the thermodynamical loads during injection and the well life.
 The purpose of this work is to simulate by numerical analysis and full-scale test, the behavior of a gas-tight Metal-to-Metal OCTG premium dope-free connection when subjected to low temperatures and loads generated by the effect of a sudden CO2 high pressure drop during injection in depleted reservoirs. Extreme temperature drop down caused by the Joule-Thompson (J-T) effect between injection conditions (P-T) inside the tubular and those in the annulus, may expose tubing connections to a thermal shock reaching a temperature near the theoretical figure of -78.5°C. This temperature drop assumed as worst-case scenario is also explored. The analysis is performed considering estimated loads for a CO2 injection case study.
 The numerical analysis and full-scale test performed confirm the structural and sealability performance of the connection is not affected by the exposure to such low temperatures. Additionally, transient thermal loads, with a drop of approximately 100°C, appears to be not critical for the metal-to-metal dope-free seal integrity and also not affecting the structural integrity of the connection. The challenges setting up of a prototype testing frame, simulating the cooling by thermal shock, lead to a methodology for assessing CCS projects premium connection able to define a robust testing protocol for cryogenic temperatures. The numerical and full-scale results collected on the tested connection size, together with the ones previously tested, allow extrapolation to near sizes of the same premium thread family.
 The results achieved by testing a premium connection which has been subjected to a thermal shock approaching -78.5°C represent a forefront in the industry, demonstrating the reliability of the product not only in operative conditions during CO2 injection, but also after an extreme event, assessing performance for the CCUS storage projects.

Jorge Cordero

Papers

Self-Optimization of Spectrum Assignment and Transmission Power in OFDMA Femtocells

Interferometry based high-precision dilatometry for dimensional characterization of highly stable materials

A high sensitivity heterodyne interferometer as a possible optical readout for the LISA gravitational reference sensor and its application to technology verification

Qualification Tests of OCTG Premium Connection under Cryogenic conditions for CCS projects