Angela Doufexi

Bio: Angela Doufexi is an academic researcher from University of Bristol. The author has contributed to research in topics: MIMO & Orthogonal frequency-division multiplexing. The author has an hindex of 26, co-authored 276 publications receiving 3668 citations. Previous affiliations of Angela Doufexi include Kyocera & University College London.

A comparison of the HIPERLAN/2 and IEEE 802.11a wireless LAN standards

Abstract: At present, WLANs supporting broadband multimedia communication are being developed and standardized around the world. Standards include HIPERLAN/2, defined by ETSI BRAN, 802.11a, defined by the IEEE, and HiSWANa defined by MMAC. These systems provide channel adaptive data rates up to 54 Mb/s (in a 20 MHz channel spacing) in the 5 GHz radio band. An overview of the HIPERLAN/2 and 802.11a standards is presented together with software simulated physical layer performance results for each of the defined transmission modes. Furthermore, the differences between these two standards are highlighted (packet size, upper protocol layers etc.), and the effects of these differences on throughput are analyzed and discussed.

Expanding window fountain codes for unequal error protection

Abstract: A novel approach to provide unequal error protection (UEP) using rateless codes over erasure channels, named Expanding Window Fountain (EWF) codes, is developed and discussed. EWF codes use a windowing technique rather than a weighted (non-uniform) selection of input symbols to achieve UEP property. The windowing approach introduces additional parameters in the UEP rateless code design, making it more general and flexible than the weighted approach. Furthermore, the windowing approach provides better performance of UEP scheme, which is confirmed both theoretically and experimentally.

Hotspot wireless LANs to enhance the performance of 3G and beyond cellular networks

Abstract: At present, WLANs supporting broadband multimedia communications are being developed and deployed around the world Standards include HIPERLAN/2 defined by ETSI BRAN and the 80211 family defined by the IEEE These systems provide channel adaptive data rates up to 54 Mb/s (in a 20 MHz channel spacing) over short ranges up to 200 m The HIPERLAN/2 standard also specifies a flexible radio access network that can be used with a variety of core networks, including UMTS It is likely that WLANs will become an important complementary technology to 3G cellular systems and will typically be used to provide hotspot coverage In this article the complementary use of WLANs in conjunction with UMTS is presented In order to quantify the capacity enhancement and benefits of cellular/hotspot interworking we have combined novel ray tracing, software-simulated physical layer performance results, and optimal base station deployment analysis The study focuses on an example deployment using key lamppost mounted WLAN access points to increase the performance (in terms of capacity) of a cellular network

Energy-Efficient Spectrum Sensing and Access for Cognitive Radio Networks

Abstract: We consider a cognitive radio system with one secondary user (SU) accessing multiple channels via periodic sensing and spectrum handoff We propose an optimal spectrum sensing and access mechanism such that the average energy cost of the SU, which includes the energy consumed by spectrum sensing, channel switching, and data transmission, is minimized, whereas multiple constraints on the reliability of sensing, the throughput, and the delay of the secondary transmission are satisfied Optimality is achieved by jointly considering two fundamental tradeoffs involved in energy minimization, ie, the sensing/transmission tradeoff and the wait/switch tradeoff An efficient convex optimization procedure is developed to solve for the optimal values of the sensing slot duration and the channel switching probability The advantages of the proposed spectrum sensing and access mechanism are shown through simulations

Application of cooperative sensing in radar-communications coexistence

Abstract: The feasibility of coexistence between radar and a communication system is investigated under log-normal shadowing conditions. A key element that enables coexistence is a sensing receiver to detect the presence of a radar system and prevent transmissions when interference may occur. Initial studies where each communication terminal does its own sensing and decision-making shows that while bandsharing is possible, under stringent interference requirements, communication efficiency can be limited. Analytical and simulation results illustrate that cooperative sensing leads to an improved detection range, higher detection probability and lower complexity of sensing nodes than that of single terminal sensing. It is concluded that cooperative spectrum sensing is able to enhance detection capabilities to maximise radar spectrum-sharing usage efficiency.

IEEE Standard for Local and Metropolitan Area Networks Part 16: Air Interface for Fixed Broadband Wireless Access Systems Draft Amendment: Management Information Base Extensions

Abstract: This document provides updates to IEEE Std 802.16's MIB for the MAC, PHY and asso- ciated management procedures in order to accommodate recent extensions to the standard.

Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency

Abstract: Massive multiple-input multiple-output MIMO is one of themost promising technologies for the next generation of wirelesscommunication networks because it has the potential to providegame-changing improvements in spectral efficiency SE and energyefficiency EE. This monograph summarizes many years ofresearch insights in a clear and self-contained way and providesthe reader with the necessary knowledge and mathematical toolsto carry out independent research in this area. Starting froma rigorous definition of Massive MIMO, the monograph coversthe important aspects of channel estimation, SE, EE, hardwareefficiency HE, and various practical deployment considerations.From the beginning, a very general, yet tractable, canonical systemmodel with spatial channel correlation is introduced. This modelis used to realistically assess the SE and EE, and is later extendedto also include the impact of hardware impairments. Owing tothis rigorous modeling approach, a lot of classic "wisdom" aboutMassive MIMO, based on too simplistic system models, is shownto be questionable.

A Survey on Smart Grid CommunicationInfrastructures: Motivations, Requirements andChallenges

Abstract: A communication infrastructure is an essential part to the success of the emerging smart grid. A scalable and pervasive communication infrastructure is crucial in both construction and operation of a smart grid. In this paper, we present the background and motivation of communication infrastructures in smart grid systems. We also summarize major requirements that smart grid communications must meet. From the experience of several industrial trials on smart grid with communication infrastructures, we expect that the traditional carbon fuel based power plants can cooperate with emerging distributed renewable energy such as wind, solar, etc, to reduce the carbon fuel consumption and consequent green house gas such as carbon dioxide emission. The consumers can minimize their expense on energy by adjusting their intelligent home appliance operations to avoid the peak hours and utilize the renewable energy instead. We further explore the challenges for a communication infrastructure as the part of a complex smart grid system. Since a smart grid system might have over millions of consumers and devices, the demand of its reliability and security is extremely critical. Through a communication infrastructure, a smart grid can improve power reliability and quality to eliminate electricity blackout. Security is a challenging issue since the on-going smart grid systems facing increasing vulnerabilities as more and more automation, remote monitoring/controlling and supervision entities are interconnected.

Cross-Layer combining of adaptive Modulation and coding with truncated ARQ over wireless links

Abstract: We developed a cross-layer design which combines adaptive modulation and coding at the physical layer with a truncated automatic repeat request protocol at the data link layer, in order to maximize spectral efficiency under prescribed delay and error performance constraints. We derive the achieved spectral efficiency in closed-form for transmissions over Nakagami-m block fading channels. Numerical results reveal that retransmissions at the data link layer relieve stringent error control requirements at the physical layer, and thereby enable considerable spectral efficiency gain. This gain is comparable with that offered by diversity, provided that the maximum number of transmissions per packet equals the diversity order. Diminishing returns on spectral efficiency, that result when increasing the maximum number of retransmissions, suggest that a small number of retransmissions offers a desirable delay-throughput tradeoff, in practice.