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3527 results in Wireless Communications

Page 28 of 177

  • 11 - Resource Allocation in Relay-Based Networks

  • from Part IV - Link Layer Resource Allocation in Wireless Networks
  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp 308-351

  • Summary

    Introduction

    Cooperative diversity via relaying communications is an important technology to enhance the robustness and network capacity of wireless systems. Cooperative transmission between a source node and a destination node is performed with the assistance of a number of relay nodes where the source and relay nodes collaboratively transmit information to the destination node [1–5]. It is intuitive that to make cooperative transmission efficient or even possible, the source node has to carefully choose one or several good relays. Then the resource first forwards its data to those relays. After that the source and selected relays can coordinate their transmissions in such a way that maximum multiplexing or diversity gains can be achieved at the destination node.

    Although cooperative diversity is simple in concept, there are many technical issues to be resolved for practical implementation. First, protocol design for cooperative diversity is one important research issue [1, 2, 4–6]. Second, most practical cooperative diversity protocols have multiple phases. For example, the source node broadcasts its message to assisting relays in the first phase, and then the relays collaboratively transmit the received information to the destination in the second phase. Therefore, cooperative transmission may not be always beneficial or even necessary because direct transmission from the source to the destination node may already be successful.

    Adaptive cooperative protocols, where nodes cooperate only when necessary or they cooperate using incremental transmissions, usually result in significantly better performance than straightforward protocols [6, 7]. In addition, emerging technology such as network coding can be employed to design even smarter cooperative protocols [8]. Finally, other important issues such as relay selection and synchronization among relays’ transmissions need to be considered for practical implementation [9–15]. Research work in cooperative communication has covered different aspects of design and performance analysis of cooperative protocols as well as resource allocation.

    In general, resource allocation has significant impacts on system performance [16–20]. In practice, source nodes and assisting relays usually have limited radio resources such as bandwidth and power, and these resources must be shared by several wireless nodes.

  • 6 - Power Control in Cellular Wireless Networks

  • from Part III - Physical Layer Resource Allocation in Wireless Networks
  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp 207-230

  • Summary

    For a wireless network, the transmit power is one of the main radio resources. Two major objectives of power control in a wireless network are to extend UEs’ battery life and to maintain an acceptable QoS (e.g., in terms of the SINR or throughput) for all UEs by minimizing interferences to UEs. Data services require a higher SINR (as a measure of QoS) as compared to the voice service, because the latter is more tolerant to bit errors. In contrast to the voice service for which the QoS is measured by a step function of the SINR [1], the commonly used QoS measure for the data service is, in general, an increasing function of the SINR.

    A distributed scheme for power control is preferred to a centralized one, because in the former, the transmit power level of a user is decided by that user by utilizing the locally available information and uses minimal feedback from the BS. In this way, the need for frequent power setting commands by the BS are avoided, and the processing capabilities at the BS needed to obtain the instantaneous uplink power levels of all UEs are substantially reduced. In contrast, a centralized approach needs to have information about path-gains and throughput requirements for all UEs at the BS.

    In this chapter, we first discuss why power control is needed and state the objectives of power control, followed by a discussion on conventional open/closed loop and centralized power control algorithms. Then various existing distributed power control algorithms are presented and evaluated according to different criteria.

    Objectives of Power Control

    In Chapter 5, we studied the relation between transmit power and SINR vectors. We know that the achieved SINRs by UEs at uplink or downlink determine their experienced QoS. Now the question is how the predetermined target QoS for users can be achieved. The transmit power cannot be set at random or at a fixed level by the UEs. For example, if the UEs served by the same cell transmit at the same fixed power level, the SINR for the UEs with good path-gains (e.g., those near the BS) are high, whereas for the far UEs it will be low. This is called the near-far problem, which can be addressed if the transmit power is dynamically controlled.

  • 2 - Wireless Networks and Resource Allocation

  • from Part I - Basics of Wireless Networks
  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp 49-114

  • Summary

    Protocol Layers for Data Communication

    Data communication between two processes (or applications) can be implemented by performing several tasks (i.e., modules) in a hierarchical manner. These modules, when arranged in a vertical stack, form a layered protocol stack. Each layer performs a subset of functions (related to transmission and/or reception) required for communication between the two processes. Such a layer depends on more primitive functions performed by its lower layer, and it also provides services to the upper layers. During the communication process between two processes, the peer layers in the corresponding devices communicate by using a defined set of rules or conventions. This set of rules or conventions is referred to as a protocol at the corresponding layer.

    The open system interconnection (OSI) model proposed by the International Organization for Standardization (ISO) [2] defines a generic protocol stack for a data communication network. The OSI model consists of the following seven layers depicted in Figure 2.1: physical, data link, network, transport, session, presentation, and application layers. The lower layers are closer to hardware-based physical data transmission procedures, while the higher layers mostly perform software-based operations. Each layer communicates with its own counterpart; for example, the network layer in the transmitter communicates with the network layer in the receiver. This independently layered structure enables very versatile and reliable networks such as the Internet. In this section, we briefly discuss each of the layers in the OSI protocol stack.

    Physical Layer

    The physical layer is concerned with the transmission of individual bits over the transmission medium. That is, it is responsible for the modulation and demodulation of the signals at the transmitter and receiver, respectively. The modulation and demodulation techniques depend on the physical medium (e.g., whether it is a “wireline” or a “wireless” medium). For wireless communications, modulation techniques such as PSK, PAM, and QAM and their variants can be used (which were briefly discussed in Chapter 1). For communications over fiber optic cables, typical transmitters are laser diodes that can be modulated to switch between ON and OFF states, which correspond to transmission of 1 and 0, respectively.

  • 1 - Introduction

  • from Part I - Basics of Wireless Networks
  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp 3-48

  • Summary

    Basics of a Wireless Communication System

    Wireless connectivity is one of the most prominent features of modern communications. A basic wireless communication system consists of three main components: transmitter, wireless medium, and receiver. The transmitter transforms the information into physical signals and transmits it over a transmission medium. The wireless medium, which is also called the communication channel, carries the signal, e.g., the free space carries the electromagnetic waves. The receiver receives physical signals from the medium and converts them into information. Usually, the medium corrupts and distorts the transmitted signal. The aim of any communication system is to obtain the transmitted information at the receiver with the lowest possible error rate. Figure 1.1 depicts the block diagram of a digital communication system. For successful multiuser communication, specific network structures and protocols need to be employed.

    Electromagnetic Spectrum and Frequency Range

    To transmit information over free space, electromagnetic waves are used. The properties of electromagnetic waves vary with frequency, and each frequency band is suitable for certain applications. The natural electromagnetic spectrum consists of a wide range of frequencies, starting from a few Hz up to 1022 Hz. Figure 1.2 lists the frequency bands in the electromagnetic spectrum along with the corresponding applications.

    Wireless communication is usually possible in a specific frequency range called the radio frequency (RF) band. The RF band consists of frequencies as low as 30 MHz up to 30 GHz. One advantage of RF band is, for most of the part of this band, relatively small antennas can be used to receive and transmit electromagnetic signals. When impinging on the earth's atmosphere, the RF signals penetrate the ionosphere and the earth's curvature does not hinder the transmission. The low-frequency RF signals, i.e., shortwave signals, are reflected by the ionosphere and thus can be used to broadcast signals.

    While military use of the RF spectrum may vary in different countries, the International Telecommunications Union (ITU) allocates and standardizes the commercial spectrum worldwide. The commercial spectrum bands can be divided into licensed and unlicensed bands.

  • 12 - Channel Allocation for Infrastructure-Based 802.11 WLANs

  • from Part IV - Link Layer Resource Allocation in Wireless Networks
  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp 352-376

  • Summary

    Introduction

    Due primarily to its unlicensed frequency band of operation and low-cost equipment, the IEEE 802.11-based wireless access technology, also known as WiFi, has been widely deployed in local area networks (LANs). A typical deployment of this technology is shown in Figure 12.1. Based on how they are managed, wireless LANs (WLANs) can be categorized into one of the following: (1) Centrally managed or (2) Uncoordinated [2]. Centrally managed deployments are usually seen in places such as university campuses, offices, or airports where all access points (APs) and associated clients are managed by a central entity. On the other hand, uncoordinated WLANs operate in the absence of a central control and are typical in places such as residential neighborhoods or private hotspots managed by different service providers (e.g., restaurants, coffee shops, etc.).

    Successful deployment in either case requires efficient mechanisms for addressing performance issues such as excessive interference, which usually translates into low throughputs. In the literature, several techniques have been proposed to address such performance issues. In particular, association control (or load balancing), in which a central entity associates (respectively, disassociates) clients with (respectively, from) APs in order to balance traffic in a network, is usually proposed for the centrally managed deployments [3]. Proposed for the uncoordinated deployments, on the other hand, are such techniques as power control [4] and careful carrier-sensing [5], in which transmission power is dynamically tuned and unnecessary carrier sensing is avoided, respectively. One other technique that is extensively considered and applicable to both centrally managed and uncoordinated environments is channel assignment, in which a frequency channel is assigned to each AP for use for a certain duration of time. In this chapter, we present a survey of such channel assignment techniques. We identify and discuss several major approaches applicable to the different deployment scenarios. Subsequently, a qualitative comparison is made among these approaches. Some comments on current practice in channel assignment are also presented. Finally, several important future research directions are outlined.

  • Preface

  • Ekram Hossain, University of Manitoba, Canada, Mehdi Rasti, Long Bao Le, Université du Québec, Montréal
  • Book:
    Radio Resource Management in Wireless Networks
    Published online:
    11 May 2017
    Print publication:
    27 April 2017, pp xiii-xvi

  • Summary

    Wireless communications and networking technology are advancing at a very rapid pace. Newer technologies and standards are evolving to serve the ever-increasing number of users demanding different types of mobile applications and services. Research and development activities on wireless technology constitute one of the most important segments of research and development in the telecommunications area today. Radio resources such as the radio spectrum and transmission power are the fundamental ingredients for any wireless system. Radio resource management is a fundamental problem in wireless networks. Efficient allocation and management of radio resources to serve the mobile users with different requirements is essential for practical deployment and operation of wireless communications systems and networks.

    Radio resource allocation is a very broad topic that cannot be fully covered in a single book. This book with the title Radio Resource Management in Wireless Networks: An Engineering Approach particularly focuses on resource management issues related to power control, interference management, joint power control and cell association, channel assignment, and multiple access control in traditional as well as emerging wireless networks such as multi-tier cellular, relay-based cellular, and cognitive radio networks (CRNs). This book intends to provide background knowledge on resource allocation in wireless networks from a system-centric or engineering point of view, review the existing related literature, discuss the research challenges, present different techniques to model and analyze the resource allocation problems, and develop both centralized or distributed resource allocation algorithms. This book includes the classical as well as recently developed models and analyses for allocation of different resources (e.g., spectrum and transmission power) in cellular wireless networks. It will be useful for graduate students (M.Sc. and junior Ph.D. students) to understand different resource allocation problems and algorithms in different types of wireless networks (i.e., both infrastructure-based and infrastructure-less wireless networks). It can be used as a textbook for a graduate course on “Wireless Networks” as well as a reference book for researchers and engineers working in the area of wireless communications and networks.

  • 10 - Advanced MIMO Detection Techniques

  • Rakhesh Singh Kshetrimayum, Indian Institute of Technology, Guwahati
  • Book:
    Fundamentals of MIMO Wireless Communications
    Published online:
    23 July 2017
    Print publication:
    17 April 2017, pp 213-244

  • Summary

    Introduction to spatially multiplexed MIMO systems

    We have observed in chapter 2 that the capacity of wireless communication links is increased by using multiple antennas at the transmitter and the receiver. To achieve these capacities, a transmission scheme, called Diagonal-Bell Laboratories Layered Space-time (D-BLAST) has been proposed by G. J. Foschini (1996). In an i.i.d. Rayleigh scattering environment, this processing structure leads to theoretical rates which grow linearly with the number of antennas (for NR = NT ) with these rates approaching 90% of Shannon's capacity. But this has large computational complexity required for implementation of this scheme.

    A simplified version, called Vertical BLAST (V-BLAST) has been proposed by G. J. Foschini et al., (1999). They have demonstrated spectral efficiencies of 20–40 bps/Hz at average signal-to-noise ratio (SNR) ranging from 24 to 34 dB could be achieved in indoor environments. The essential difference between D-BLAST and V-BLAST lies in the vector encoding process. The D-BLAST code blocks are organized along diagonals in space-time. In V-BLAST, however, the vector encoding process is simply a demultiplexing (DMUX) operation followed by independent bit-to-symbol mapping of each sub-stream. No inter sub-stream coding, or coding of any kind, is required.

    Note that BLAST detection scheme can be done in one of the following ways:

  • 1. interference nulling to reduce the effect of the other (interfering) signals on the desired one is employed in successive interference cancellation (SIC) which will be discussed in section 10.4;

  • 2. ordering to select the sub-stream with the largest signal-to-noise ratio (SNR) or other criteria along with SIC, which will be discussed later in ordered successive interference cancellation (OSIC) (section 10.5).

    • Vertical/horizontal layered space-time transmission

    Vertical Bell Laboratories Layered Space-time Transmission (V-BLAST) suggests simultaneous transmission of independent uncoded data sub-streams. The input data is separated into several substreams with demultiplexing (DMUX) operation which are associated with each transmit antenna.

    This process is followed by independent bit-to-symbol mapping (M-ary) of each sub-stream and may be interleaved (optional) as depicted in Fig. 10.1 (a). We consider a V-BLAST system with NT transmitting antennas. At the transmitter the data is passed through a serial-to-parallel converter (S/P converter) and transformed into NT sub-streams, where each sub-stream is sent through a different transmit antenna.

Page 28 of 177