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5G Mobile and Wireless Communications Technology 1st Edition Afif Osseiran
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Preface
We are experiencing the dawn of what is called the digital age. The Internet and digital technologies are transforming our world. Wireless communication systems constitute a basic component of the current and the future information society. The fifth generation of communication systems, or 5G, will be, in the years to come, the most critical building block of what is referred to as a digital society. It is expected that 5G will be a truly converged network environment, because not only mobile communications systems but also other wireless systems and wired networks will coexist and use the same infrastructure.
The vision ahead is that 5G will provide virtually ubiquitous, ultra-fastbroadband, connectivity not only to millions of individual users but also to billions of connected objects. Therefore, 5G systems should have the capabilities to (a) serve the datahungry devices, for example, smartphones and tablets, providing rates of Gbps; (b) enable machine-to-machine (M2M) communications, for example, vehicle-tovehicle (V2V) networks; (c) allow for interconnectivity of massive devices, for example, sensors and e-health equipment; and (d) support a wide range of applications and sectors. All these expectations set a wide variety of technical requirements for the design of 5G systems, including higher peak and user data rates, extremely low latency and response times, enhanced indoor and outdoor coverage, significantly increased number of devices, reaching the astronomical number of 100 billion, seamless mobility, security and privacy, increased battery lifetime, and improved quality of service (QoS) while maintaining low operational costs.
All the predictions available in the literature lead to the conclusion that beyond 2020, wireless communication systems will be able to support more than 1000 times the traffic volume served today by the current telecommunication systems. This extremely high traffic load is one of the major issues faced by the 5G designers, manufacturers, and researchers, alike. It appears that this challenge will be addressed by a combination of parallel techniques that will use more spectrum more flexibly, realize higher spectral efficiency, and densify cells. Therefore, novel techniques and paradigms should be developed to support such challenges.
In this context, this book addresses diverse key-point issues of the nextgeneration wireless communication systems and attempts to identify promising solutions. The core of the book deals with the techniques and methods belonging
to what is generally referred to as radio access network. The increased needs and the users’ expectations from the next-generation systems have been based for long on enabling technologies provided by the physical layer. These technologies are mainly developed to combat signal degradations imposed by the wireless channel and to support increased user data rates and improved QoS.
Chapter 1 presents an overview of the wave propagation and modelling of radio channels with characteristics that vary in time and space. Then, the influence of multipath propagation on the signal-distorting characteristics of wireless channels and the resulting effects on digital communications are outlined. Both narrowband and wideband channel modeling is discussed with suggestions for assessing whether a wide-sense stationary (WSS) model is appropriate and for modeling channel processes with nonstationary characteristics and correlated scattering. Spatial channel characterization and MIMO channel models are reviewed and the measurement equipment and techniques are presented. Moreover, in Chapter 2 , various channel models are compared, modeling techniques involved (stochastic, ray-tracing, etc.) are analyzed, and the applications of the involved theory in optimizing the cell planning procedures are demonstrated.
The millimeter-wave (mm-Wave) band has been foreseen for the development of 5G networks. A large amount of spectral space, from 28 to 95 GHz, is available to establish such systems providing increased data rates. Hence, the design issues of such networks necessitate the thorough knowledge of mm-Wave propagation characteristics. In Chapter 3, the state-of-the art channel models, derived from the latest measurement campaigns in various propagation scenarios (indoor, outdoor, etc.) are presented. In addition, models that describe the spatial–temporal variations of the mm-Wave channel are explained. Recent advances of MIMO and massive MIMO technology exploitation in mm-Wave propagation are also examined.
Chapter 4 presents the fundamental concepts and basic theoretical tools for the qualitative analysis of fading channels. This section helps the reader to understand the nature and types of fading, the basic characteristics of a wireless channel, and the impact of fading on signal transmission. Fading mitigation techniques are presented, and a complete classification of the existing transmit/receive diversity techniques that exploit the spatial, frequency, time, and the polarization domains, is also provided.
OFDM modulation and OFDM-based transmission schemes (e.g., OFDMA) have dominated the current modern wireless standards. However, despite OFDM’s implementation simplicity, some key weaknesses such as the need for extended guard bands leading to reduced spectral efficiency, the increased peak-to-average power ratio, and its inherent sensitivity to frequency offset impairments have motivated in the research of alternative multicarrier or single-carrier modulation techniques based on filter banks. Therefore, Chapter 5 reviews the most important modulation and multicarrier schemes, including OFDM that have become strong candidates for the future generations of wireless standards.
The 5G radio access will be built on both new radio access technologies (RATs) and the evolved existing wireless technologies (LTE, HSPA, GSM, and WiFi). Chapter 6 provides an overview of radio network planning techniques from 2G to 4G. Then it describes the energy-efficient green radio network planning concept for heterogeneous 4G wireless networks and presents a cellular layout adaptation method.
The incorporation of MIMO transmission in the modern wireless standards has provided a significant improvement in the achieved spectral efficiency and system capacity. As technology moves toward 5G, the concept of MIMO has been evolved with the definition of complex centralized schemes, such as massive MIMO, or decentralized schemes, that is, network MIMO and coordinated multipoint transmission. Moreover, alternative advanced MIMO schemes reducing the number of required RF chains, enabling full duplex communications, and allowing the use of compact antennas have been considered. Chapter 7 reviews the most important trends and challenges in MIMO transmission with emphasis on their application to 5G systems.
Alternative enabling technologies are also presented. As an example, Chapter 8 introduces the load-controlled parasitic antenna arrays (LC-PAA) that resemble the operation of conventional antenna arrays with many elements, thus boosting the performance of multiantenna wireless communication systems, whereas at the same time providing cost and energy consumption savings and size reduction. Therefore, a novel method is presented that enables one to perform arbitrary channel-dependent precoding with LC-PAAs. The possible application of this technique in point-to-point MIMO, multiuser MIMO (MU-MIMO), coordinated MIMO, and massive MIMO setups is investigated. The design and implementation of various types of LC-PAAs for these frequency bands are also shown.
Spatial modulation (SM) has been recently proposed as a promising transmission concept that reduces the complexity and the cost of multiple-antenna schemes. At the same time, it guarantees high data rates, improved system performance, and energy efficiency. Working principles of SM are presented in Chapter 9 and the advantages and disadvantages of SM-MIMO as compared to the state-of-theart MIMO communications are also discussed. Various transmission techniques for SM-MIMO, namely space shift keying (SSK), generalized space shift keying (GSSK) and generalized spatial modulation (GSM) are also further discussed. An analytical framework for the performance evaluation of SM-MIMO over fading channels in terms of the average bit error probability (ABEP) is also presented. Some MIMO transmission schemes closely related to the SM-MIMO paradigm, that is, single RF MIMO schemes, the incremental MIMO, and the antenna subset modulation (ASM) schemes, are briefly discussed. Moreover, several applications of SM-MIMO for future 5G wireless communications are presented, including MIMO implementations that exploit the massive MIMO paradigm as well as the combination of both orthogonal frequency division multiplexing (OFDM) and single carrier with SM-MIMO.
Device-to-device (D2D) communication is recognized as one of the technology components of the evolving 5G architecture. The reuse of cellular resources by D2D links that are located randomly inside a macro-cell imposes a cochannel interference to the base station (BS), cellular users and to other D2D receivers. Several aspects of D2D communications are investigated in Chapter 10, such as interference statistics of interferers scattered according to a homogeneous Poisson point process, D2D neighbor discovery based on signal-to-interference-and-noise ratio association metric, D2D link performance in the presence of interference and power control imposed by the BS, and the impact of mobility on the D2D link performance. In addition, a V2V use case is considered and the performance of a multiuser cooperative V2V communication system is studied.
Next, Chapter 11 addresses the virtualization of wireless access in order to provide the required capacity to a set of virtual base stations (VBSs) with diverse requirements, instantiated in a given geographical area. A network architecture is presented, based on a generic network virtualization environment, in which both physical and virtual perspectives are considered and the main stakeholders are taken into account. A new tier of radio resource management (RRM) is proposed for inter-VNets (virtual networks) RRM aiming at transposing the cooperative set of functionalities to the virtualization environment.
Cooperative techniques have significantly contributed toward improving the capacity of the 4G networks and are considered as a basic element of the imminent 5G networks. This topic is addressed in Chapter 12 of this book. Among cooperative techniques, cooperative relaying (CR) has received significant attention from researchers due to the gains that it offers to the network. The techniques that have received a large amount of contributions are the opportunistic relay selection (ORS) and successive relaying (SuR) and several policies, such as successive opportunistic relaying (SOR) with full duplex (FD) operation and SOR for networks with buffer-aided (BA) relays that offer additional degrees of freedom in inter-relay interference (IRI) mitigation. In addition, an overview of interference mitigation techniques designed for relay networks is provided.
Cognitive radio networks (CRNs) have been proposed as a promising solution to cope with the spectral scarcity, and this is the main topic of Chapter 13. In addition, as the technology of CRN can help to unlock the full potential of 5G wireless systems, the 5G key enablers are described and finally the role of CRN in 5G networks is also highlighted. Furthermore, several formulations of different CRN’s resource allocation problems are presented based on various mathematical approaches such as optimization techniques, game theory, matching theory, multicriteria decision-making theory, and machine learning. The basic concepts of each approach are described and an extensive list of scientific works is also presented. Finally, various future research avenues for the application of cognitive systems to 5G and the investigation of novel flexible algorithms in radio resource management are also discussed.
The last chapter (Chapter 14) focuses on transformational wireless technologies for health care, discussing their potential and the challenges that rose. As it is well known, the significant advances in wireless communications, sensing technologies, and sensor data analytics are opening new opportunities in medicine, and are promising to address the unsustainability of current health care provision models. Notably, health care challenges, including rising health care costs, aging populations, and emerging disease threats rank among the most serious concerns in the world. Wireless technology can empower both patients and medical providers by providing round-the-clock health status information. Examples include wireless on-body (wearable, epidermal) and in-body (implantable, ingestible) medical devices that may be used as sensors, actuators, and/or drug delivery devices. Remote diagnosis, vital parameter control, elderly monitoring, and chronic disease management are just some of the examples of applications of wireless technologies. Exploitation of wireless technologies and sensor data analytics in health care can lead to healthier citizens, reduced hospital stays, and lower costs.
This book is dedicated to Philip Constantinou, an inspiring professor, a valuable colleague, and a loyal friend. He was an exceptional professor who was always there for students, who could understand your thoughts and concerns, who was the inspiration, and who gave confidence to the team. A testimony to this was the overwhelming response we received from many colleagues who wanted to contribute to this book because they wanted to join us in celebrating his research and educational achievements in the general field of wireless communications for the past 25 years.
Philip was a kind human being, a generous and a welcoming person open to the community, showing great concern and affection for people with special needs. Before joining the National Technical University of Athens, Athens, Greece, he and his family have lived for many years in Canada, where he was granted the Master of Applied Science by University of Ottawa and started his great telecom career in Telesat in Ottawa, Canada. Then he worked for several years for the Government of Canada in the Department of Communications, while at the same time pursuing his PhD degree at the Carleton University, Ottawa, Canada. Being an international man, he believed in openness, valued collaboration, and always tried to impart the importance of communicating engineering novelty to his students. He was always supportive to the junior communications engineers’ ideas and encouraged them to act on their own initiative following their dream. Philip was a workaholic lab enthusiast who loved to deliver working systems. Although he was a zealous and motivated leader, he had a methodic approach regularly reminding to his colleagues two of his favorite phrases: “One step at a time” and “I am always a practical man.” His students and his colleagues, alike, owe him a great debt of gratitude for making us feel special, strong, and capable of doing things.
Athanasios (Thanasis) G. Kanatas Konstantina (Nantia) S. Nikita Panagiotis (Takis) Mathiopoulos
About the Editors
Athanasios (Thanasis) G. Kanatas is a professor at the Department of Digital Systems and dean of the School of Information and Communication Technologies at the University of Piraeus, Greece. He received his diploma in electrical engineering from the National Technical University of Athens (NTUA), Greece, in 1991, MSc degree in satellite communication engineering from the University of Surrey, Surrey, UK in 1992, and earned his PhD degree in mobile satellite communications from NTUA, Greece in February 1997. From 1993 to 1994 he was with National Documentation Center of the National Research Institute. In 1995, he joined SPACETEC Ltd. as technical project manager for VISA/EMEA VSAT Project in Greece. In 1996, he joined the Mobile Radiocommunications Laboratory as a research associate. From 1999 to 2002, he was with the Institute of Communication and Computer Systems responsible for the technical management of various research projects. In 2000, he became a member of the board of directors of OTESAT S.A. In 2002, he joined the University of Piraeus as an assistant professor. He has published more than 150 papers in international journals and international conference proceedings. He is the author of 6 books in the field of wireless and satellite communications. He has been the technical manager of several European and National R&D projects. His current research interests include the development of new digital techniques for wireless and satellite communications systems, channel characterization, simulation, and modeling for mobile, mobile satellite, and future wireless communication systems, antenna selection and RF preprocessing techniques, new transmission schemes for MIMO systems, V2V communications, and energy efficient techniques for wireless sensor networks. He has been a senior member of IEEE since 2002. In 1999, he was elected chairman of the Communications Society of the Greek Section of IEEE. He has been a member of the TPC of more than 40 international IEEE conferences. He was a corecipient of two best paper awards for papers published in the International Conference on Advances in Satellite and Space Communications (SPACOMM), Athens, Greece, (2010) and Global Wireless Summit, Wireless VITAE, Atlantic City, United States, (2013).
Konstantina (Nantia) S. Nikita received her diploma in electrical engineering and earned her PhD degree from the National Technical University of Athens (NTUA), as well as MD degree from the Medical School, University of Athens. From 1990 to 1996, she worked as a researcher at the Institute of Communication and Computer Systems. In 1996, she joined the School of Electrical and Computer Engineering, NTUA, as an assistant professor, and since 2005, she serves as a professor at the same school. Moreover, she is an adjunct professor of Biomedical Engineering and Medicine, Keck School of Medicine and the Viterbi School of Engineering, University of Southern California. She has authored or coauthored 165 papers in refereed international journals, 41 chapters in books, and more than 300 papers in international conference proceedings. She is the editor of seven books in English and author of two books in Greek. She holds three patents. She has been the technical manager of several European and National R&D projects. She has been honorary chair/chair of the program/organizing committee of several international conferences, and has served as a keynote/invited speaker at international conferences, symposia, and workshops organized by NATO, WHO, ICNIRP, IEEE, URSI, and so on. She has been the advisor of 27 completed PhD theses, several of which have received various awards. Her current research interests include biomedical telemetry, biological effects and medical applications of radiofrequency electromagnetic fields, biomedical signal and image processing and analysis, simulation of physiological systems, and biomedical informatics. She is an associate editor of the IEEE Transactions on Biomedical Engineering, the IEEE Journal of Biomedical and Health Informatics, the IEEE Transactions on Antennas and Propagation, the Wiley Bioelectromagnetics, and the Journal of Medical and Biological Engineering and Computing. She has received various honors/awards, with the Bodossakis Foundation Academic Prize (2003) for exceptional achievements in “Theory and Applications of Information Technology in Medicine” being one of them.
She has been a member of the board of directors of the Atomic Energy Commission and of the Hellenic National Academic Recognition and Information Center, as well as a member of the Hellenic National Council of Research and Technology. She has also served as the deputy head of the School of Electrical and Computer Engineering of the NTUA. She is a member of the Hellenic National Ethics Committee, a founding fellow of the European Association of Medical and Biological Engineering and Science (EAMBES), a fellow of the American Institute of Medical and Biological Engineering (AIMBE), and a member of the Technical Chamber of Greece and of the Athens Medical Association. She is also a member of the BHI Technical Committee, the founding chair and ambassador of the IEEE–EMBS, Greece chapter and has served as the vice-chair of the IEEE Greece Section.
Panagiotis (Takis) Mathiopoulos is a professor of telecommunications at the Department of Informatics and Telecommunications, University of Athens, Greece. Prior to that, he was with the Institute for Space Applications and Remote Sensing (ISARS) of the National Observatory of Athens, first as its director (2001–2005)
and then as director of research (2006–2014). From 1989 to 2003, he was a faculty member in the Department of Electrical and Computer Engineering, the University of British Columbia (UBC), where he was a professor from 2000 to 2003. From 2008 to 2013, he was appointed as guest professor at the Southwest Jiaotong University, China. He has been also appointed by the Government of People’s Republic of China as a senior foreign expert at the School of Information Engineering, Yangzhou University (2014–2016) and by Keio University as a visiting (Global) professor in the Department of Information and Computer Science (2015–2016 and 2017–2018) under the Top Global University Project of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Government of Japan.
For the past 25 years, he has been conducting research mainly on the physical layer of digital communication systems for terrestrial and satellite applications, including digital communications over fading and interference environments. He coauthored a paper in GLOBECOM’89 establishing for the first time in the open technical literature the link between maximum likelihood sequence estimation (MLSE) and multiple (or multisymbol) differential detection for the AWGN and fading channels. He is also interested in channel characterization and measurements, modulation and coding techniques, synchronization, SIMO/MIMO, UWB, OFDM, software/cognitive radios, green communications, and 5G. In addition, since 2010, he has been actively involved in research activities in the fields of remote sensing, LiDAR systems, and photogrammetry. In these areas, he has coauthored more than 110 journal papers, mainly published in various IEEE and IET journals, 4 book chapters, and more than 120 conference papers.
Prof. Mathiopoulos has been or currently serves on the editorial board of several archival journals, including the IET Communications, and the IEEE Transactions On Communications (1993–2005). He has regularly acted as a consultant for various governmental and private organizations. Since 1993, he has served on a regular basis as a scientific advisor and a technical expert for the European Commission (EC). In addition, from 2001 to 2014, he has served as a Greek representative to high level committees in the European Commission (EC) and the European Space Agency (ESA). He has been a member of the TPC of more than 70 international IEEE conferences, as well as TPC vice-chair for the 2006-S IEEE VTC and 2008-F IEEE VTC as well as cochair of FITCE2011. He has delivered numerous invited presentations, including plenary and keynote lectures, and has taught many short courses all over the world. As a faculty member at the ECE of UBC, he was elected as ASI fellow and a Killam research fellow. He was a corecipient of two best paper awards for papers published in the 2nd International Symposium on Communication, Control, and Signal Processing (2008) and 3rd International Conference on Advances in Satellite and Space Communications (2011).
Contributors
Georgia E. Athanasiadou Department of Informatics and Telecommunications University of Peloponnese Tripoli, Greece
Petros S. Bithas Department of Digital Systems University of Piraeus Piraeus, Greece
Robert Bultitude Department of Systems and Computer Engineering Carleton University Ottawa, Ontario, Canada
Luisa Caeiro
ESTSetúbal/INESC-ID University of Lisbon Lisbon, Portugal
Filipe Cardoso
ESTSetúbal/INESC-ID University of Lisbon Lisbon, Portugal
Theofilos Chrysikos Wireless Telecommunications Laboratory Department of Electrical and Computer Engineering University of Patras Patras, Greece
Luis M. Correia IST/INESC-ID University of Lisbon Lisbon, Portugal
George P. Efthymoglou Department of Digital Systems University of Piraeus Piraeus, Greece
Athanasios G. Kanatas Department of Digital Systems University of Piraeus Piraeus, Greece
Vasileios M. Kapinas Department of Electrical and Computer Engineering
Aristotle University of Thessaloniki Thessaloniki, Greece
George K. Karagiannidis
Department of Electrical and Computer Engineering
Aristotle University of Thessaloniki
Thessaloniki, Greece
Konstantinos Karathanasis
School of Electrical and Computer Engineering
National Technical University of Athens Athens, Greece
Stavros Kotsopoulos
Wireless Telecommunications Laboratory
Department of Electrical and Computer Engineering University of Patras Patras, Greece
Konstantinos Maliatsos
Department of Digital Systems University of Piraeus
Piraeus, Greece
Panagiotis Mathiopoulos
Department of Informatics and Telecommunications University of Athens Athens, Greece
Nektarios Moraitis
School of Electrical and Computer Engineering
National Technical University of Athens
Athens, Greece
Konstantina S. Nikita
School of Electrical and Computer Engineering
National Technical University of Athens Athens, Greece
Nikolaos Nomikos
Department of Information and Communication Systems Engineering
University of Aegean Mytilene, Greece
Dimitrios Ntaikos
Broadband Wireless & Sensor Networks (B-WiSE) Research Laboratory
Athens Information Technologies Marousi, Greece
Konstantinos Ntougias
Broadband Wireless & Sensor Networks (B-WiSE) Research Laboratory
Athens Information Technologies
Marousi, Greece
Georgia D. Ntouni
Department of Electrical and Computer Engineering
Aristotle University of Thessaloniki
Thessaloniki, Greece
Athanasios D. Panagopoulos
School of Electrical and Computer Engineering
National Technical University of Athens
Athens, Greece
Constantinos B. Papadias
Broadband Wireless & Sensor Networks (B-WiSE) Research Laboratory
Athens Information Technologies Marousi, Greece
Konstantinos Peppas
Department of Informatics and Telecommunications University of Peloponnese
Tripoli, Greece
Marios I. Poulakis School of Electrical and Computer Engineering
National Technical University of Athens Athens, Greece
Anargyros J. Roumeliotis School of Electrical and Computer Engineering
National Technical University of Athens Athens, Greece
George V. Tsoulos Department of Informatics and Telecommunications University of Peloponnese Tripoli, Greece
Ioannis Valavanis Department of Informatics and Telecommunications University of Peloponnese Tripoli, Greece
Stavroula Vassaki School of Electrical and Computer Engineering
National Technical University of Athens Athens, Greece
Demosthenes Vouyioukas Department of Information and Communication Systems Engineering University of Aegean Mytilene, Greece
Dimitra Zarbouti Department of Informatics and Telecommunications University of Peloponnese Tripoli, Greece
Chapter 1 Propagation Measurement-Based Wireless Channel Characterization and Modeling
1.3.5 Characterization of Received Signal Envelope Variations ................23
1.3.6 Characterization of Direction Dispersion on Multipath Radio Channels .............................................................................. 25
1.4 Data Qualification Processing prior to the Estimation of Parameters from Measured Data ..................................................................................30
1.4.1 Rejection of Noise from Measured Impulse Response Estimates .... 32
1.4.2 Assessment of Conformance to a Wide-Sense-Stationary Model ... 34
1.4.3 Modeling Estimates for Probability Distribution Functions 40
1.4.4 Estimation of Frequency Correlation Functions and the Confirmation of Uncorrelated Scattering ....................................... 42
1.4.5 Considerations Related to Assumptions of Gaussianity and Ergodicity .......................................................................................43
44
1.1 Introduction
Any communications link can be modeled as shown in Figure 1.1, the model having three major components: (1) the source, usually referred to as the transmitter; (2) the transmission channel; and (3) the sink, usually referred to as the receiver, or the destination of the transmitted signals.
After final design and implementation of the transmitter, receiver, and antennas (if the communication signals are to be carried via radiowave propagation), any distortion of the transmitted waveform, st(), on its path from the transmitter to the receiver comes from either noise or the physical characteristics of the radio channel. These characteristics can primarily be categorized as transmission loss, the dispersion (or spreading) of the transmitted signal in time as a result of propagation delays, its dispersion in frequency as a result of Doppler effects, or its dispersion in three-dimensional space. On a static or nonvarying radio channel, which exists any time neither the communications link antennas nor anything in the environment of operation is moving and the physical characteristics of the environment are not changing, such distortion can be corrected using fixed technology, including hardware or software. However, when the transmission characteristics of a radio channel are randomly varying because changes in the environment are random, the correction of distortion becomes challenging. Models, and knowledge of the parameters
thereof, for the random variations are thus needed to assess whether the correction of distortion is needed to achieve the desired quality of service on the communication link, and if so, to determine design criteria for distortion correcting hardware and software, and for use in simulation models to test its effectiveness. This chapter focuses on the modeling of randomly varying radio channels, the estimation of model parameters from radio propagation measurements, and the characterization of measured channels based upon model parameters, which is the first step in assessing the requirement for distortion correcting technology.
Following this introduction, this chapter has three main sections. Section 1.2 begins with a discussion of the most fundamental impairment to wireless transmission: transmission loss in an environment where there are no reflectors, scatterers, or obstructions. Then, the subject of multipath propagation when there are objects in the environment of operation that can reflect, scatter, and obstruct radiowaves is introduced and the review of a method for modeling transmission loss in a multipath environment based on radio propagation measurements is discussed. Next, in Section 1.3, the influence of multipath propagation on the signal-distorting characteristics of wireless channels and resulting effects on digital communications are outlined. This is accompanied by a review and discussion of a classical model, applicable to the analysis of single-input-single-output systems, for modeling randomly varying channels as a Gaussian wide-sense-stationary process. It is followed by the general assessment of channel quality, with particular emphasis on methods by which measured channels can be classified in accordance with the subject model and by which model parameters can be estimated from measured data for use in channel modeling and simulation. Section 1.3.6 summarizes an extension from the literature of the classical model to make it applicable for multiple-input multiple-output systems, which are envisaged as a key to the success of 5G systems in meeting anticipated transmission capacity requirements.
Section 1.4 focuses on the data qualification preprocessing of measured data that is necessary for the reduction of errors in analyzing such data for the estimation of model parameters. First, a very general overview of radio channel sounding is given. A method from the literature for the reduction of noise from wideband channel measurements is then reviewed. A discussion follows on the assessment of measured data to determine if modeling of random variations in the time domain using a wide-sense-stationary model is appropriate. Section 1.4.3 is a review of modeling cumulative probability distributions based on estimates thereof from measured data. Then, the estimation of spaced-frequency correlation functions for randomly time varying channels is outlined, with emphasis on how results can be used to verify conditions required for modeling variations in the frequency domain as a wide-sense-stationary process. Section 1.4.5 is a very short note on considerations regarding the assessment of whether a Gaussian model is appropriate and whether assumptions of ergodicity are reasonable.
1.2 Transmission Loss
What is referred to as free-space transmission occurs anytime radiowaves travel between a radiating source and a receiving antenna without interacting with anything, such as physical obstructions, particles, or electric charges (e.g., in an ionospheric plasma). If one considers the transmission of radio waves from a point source, or isotropic antenna, which is a fictitious construct that cannot be realized in the real world, radiation occurs equally in all direction and the radiated waves spread out in spheres of ever-increasing radius, r, with the isotropic antenna at their center. Since the area of the surface of a sphere is given by
the power loss (most frequently referred to as propagation loss), at any frequency, as a result of the propagation of radiowaves in free space is inversely proportional to r 2 . A practical transmit (Tx) antenna focuses energy in specific directions such that the ratio of power density it radiates in any direction, θ, to that which would be radiated by an isotropic antenna, is given by its directive gain (often referred to as gain), Gt () θ . Similarly, the ratio of power received by a receive (Rx) antenna from any direction, θ, to the power that would be received by an isotropic antenna, is given by its gain, Gr () θ . Associated with a receive antenna is an effective area, Ae , which can be thought of as a capture area, over which power incident upon it can be received. This effective area [1] is related to the gain of the antenna in accordance with the equation
Using Equation 1.1 and knowledge of the focusing ability of a Tx antenna, the power density, S , incident on any surface as a result of the radiation of power, Pt , by the Tx antenna can be computed as
where E is the magnitude of the electric field and η is the intrinsic impedance of free space [2]. Then, using Friis equation [2], the power received through the Rx antenna is given by
and the free-space transmission loss is given by
Figure 1.2 The difference of transmission loss between equal gain antennas at the specified frequencies and at 2 GHz.
Note that because the effective area of the Rx antenna is dependent upon wavelength, λ, free-space transmission loss has a direct-quadratic dependence upon λ , and hence, an inverse-quadratic dependence upon the frequency of operation, f. The fact that the frequency dependence of transmission loss is an antenna property rather than a property of the propagation environment is often overlooked. If Equation 1.5 were used to calculate the ratio of transmission loss at two different frequencies f 1 and f 2 over a free space radio link having equal gain antennas at the two frequencies, it is clear that this ratio would be given by ∆ = L (/ ) λλ21 2 .
Figure 1.2 is a plot of this ratio for the case of f 1 2 = GHz , which is a ratio of interest to many in the consideration of the design of 5G systems, which are proposed for operation above 6 GHz.
The quadratic relationship, resulting in a decreasing slope of the ratio of transmission loss with respect to that at 2 GHz as frequency increases, is worth noting.
1.2.1 Multipath Propagation
In many instances free-space transmission is impossible. This is a result of the fact that as the radiated waves spread out in spheres of ever-increasing radius, they impinge upon obstacles, or encounter particles or, as in the ionosphere, regions where there are concentrated charge densities. In the troposphere, the interaction of radiowaves with obstacles causes reflection, diffraction, and scattering. These phenomena, in turn, generate secondary waves that can interfere with waves that arrive at a Rx antenna directly from the Tx antenna. Such interference is referred to
as multipath interference, and it can either enhance or diminish the performance of a radio communication link, depending upon the overall link design.
In the region of an Rx antenna, the electromagnetic fields from the multipath waves add vectorally such that the magnitude and phase of the total electric field, E tot , can be written as
where, using Equation 1.3 and assuming single interactions, (/ ) EP Gr it t =η πρ 4 2 ii , and ρi is a complex coefficient resulting from the interaction of the “ i th” multipath wave with some obstacle or surface before it is reflected, diffracted, or scattered toward the Rx antenna. If radio path “ i ” is the direct path between the Tx and Rx antennas, ρi = 1. The vector sum of the multipath waves can either enhance or diminish received power, depending on the phases of the interfering waves at any specific receive location. This, in turn, can modify the dependence of transmission loss on distance, r, of what must now be referred to as average transmission loss, from the inverse quadratic dependence appropriate for free space, and increases the exponent to which r is raised, or decreases it, depending upon the circumstance. In a reverberation chamber, for example, where all waves that impinge upon the chamber walls are reflected with a reflection coefficient near unity, this exponent is reduced, resulting in lower average transmission loss than that which would occur in free space. This is because energy is prevented from leaving the chamber by its walls. On a radio path between two antennas where there is one reflection point and the angle between the direction of travel of the impinging wave and the reflecting surface is small, the reflection coefficient is near unity, and if the electric field is perpendicular to the plane containing the direction (or Poynting) vectors of both the incident and reflected waves, there is a phase shift of π radians on reflection. In this case, received power can diminish in proportion to r 4 in accordance with the well-known two-ray model [2] at Tx–Rx ranges beyond a breakpoint, after which the direct and reflected waves begin to have an inverse phase relationship and their vector addition is therefore destructive. This distance is a function of the offset of the antennas from the reflecting surface (i.e., antenna heights if the surface is the earth) and the wavelength at the frequency of operation.
Multipath propagation and its effects can be exemplified by considering the simple case, at a frequency of 2.35 GHz, of equal-height antennas in a small empty room of dimensions 5 m long × 3.6 m wide × 4 m high, where the direct wave from the Tx antenna, as well as reflections from the four walls, the floor, and the ceiling are received. A simple simulation using Equation 1.6 and assuming vertical quarter-wavelength monopoles with radiation patterns as in [1] at the Tx and Rx yields a received power pattern along the centerline of the room as shown in Figure 1.3. For this simulation, the transmit power, Pt , was set equal to 7 dBm, transmission line losses were equal to 2.2 dB, the walls were modeled as being
Plasterboard and concrete surfaces
Frii's eqn
Perfectly reflecting surfaces
Figure 1.3 Results from a simulation of the received sum of multipath waves as a function of distance along the centerline of a small, empty room.
constructed of plasterboard on a wooden support structure, and the floor and ceiling were modeled as being made of dry concrete. Equations for the reflection coefficients and the material constants were taken from [2]. Results for the case in which the walls and floor are perfectly conducting are also shown in Figure 1.3
The undulations of the multipath sums as the constituent waves add with different phase relationships as a function of the distance between the Tx and the Rx can clearly be seen. It can also be seen that the average power of the multipath sum for the case of perfectly reflective surfaces decreases less rapidly than the curve given by Friis equation, leading to a model transmission loss is proportional to r raised to an exponent that is less than 2.
1.2.2 Modeling of Transmission Loss in a Multipath Environment
The modeling of transmission loss is something that has received much attention by many. This is an important part of communication system design, since the capacity of any radio link is directly related to the ratio of received signal power to noise power, or the SNR, at the receiver.
The modeling of Pr as a function of Tx–Rx separation, say, from simulations such as above, or from propagation measurements, is best done by expressing Pr
in terms of the logarithm of distance. When plotted, this relationship is a straight line and is easy to model. However, transmission loss, the ratio of Pr -to-Pt is modeled more often than Pr . In a multipath environment, by convention, before such modeling, averaging is conducted to eliminate transmission loss variations that result from the vector addition of multipath waves. This averaging can be conducted over small ranges of time, space, or frequency, since the physics associated with the seemingly random variations that result from the vector addition are the same in all three domains. If the averaging is done in time, motion of either the antennas, or objects in the operating environment is needed to stir the component multipath waves (referred to as multipath components, or MPCs) to achieve a variation in time that covers as much as possible as the full dynamic range commensurate with the multipath sum where the measurements are being made. If the averaging is over space, one of the link antennas must be moved in order to achieve the variations of interest. A wideband channel sounding signal [2] facilitates ease in averaging over frequency. The range over which averaging must be conducted is best determined by the observation of the absence of variations in averaged results.
Modeling of the log–log relationship between average transmission loss and Tx–Rx antenna separation (sometimes herein referred to as range) can be effected using a standard least-mean-squared (LMS) error approach. This can be done by considering a system of multiple equations, each for the average transmission loss at a different position along the trajectory of the Rx antenna. The modeling process begins by representing the logarithm of the Tx–Rx separation (d ) as
where d ref is an arbitrary short distance from the Tx antenna, in its far field. The mean transmission loss can then be modeled as
where L ref represents the path loss at d ref . If measured or simulated path loss samples are ordered as a function of d , one can write the system of equations as
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For a moment there was absolute silence, broken only by the slap of Jake Blue’s palm against the butt of his gun.
But, swiftly as he drew, Hashknife shaded him by a second and fired —— from his hip. Blue spun off the porch, splintering one of the porch-posts with his misdirected bullet.
Spot Easton had thrown himself sidewise and fired across his horse’s neck, but his horse threw its head wildly, and the bullet buzzed through the doorway—doing no damage. A second later one of the cowboys crashed his horse into Easton’s mount, knocking Easton from his saddle.
Doc Clevis, insane from the disclosures, and knowing what it would mean, drew a heavy pistol from under his coat and spurred straight at the porch, only to meet Skelton’s riot-gun at close range. He was literally blown out of his saddle.
From the ground, among the milling horses, Spot Easton shot wildly at Sleepy, who was churning up the dirt around Easton’s head with bullets. Hagen fired once, and his bullet ripped along Hashknife’s forearm just as Hashknife shot. The jar of the bullet threw Hashknife’s gun far enough aside to miss Hagen but caught his horse, which whirled wildly, unseating its rider. Hagen’s foot hung in the stirrup.
Bucking and kicking, the bronco whirled into the tangle of tombstones where Hagen fell free. Easton’s gun was empty and he tried to fight his way out of the milling horses, but Sleepy dove after him and, locked together, they rolled into the open.
Dell Blackwood forced his horse to the porch and held up his hands.
“I’m out of it,” he yelled. “I’m admittin’ that I stole some 33 calves for Easton, but I never shot nobody.”
He tossed his reins to the ground and slid out of his saddle.
Came the rattle of a wagon, and Mrs. Frosty Snow and Jane drove into the yard. Two cowboys helped Sleepy rope Spot Easton, and then all eyes turned to the two women in the wagon.
“Lonesome Lee, here’s yore daughter!” called Mrs. Snow.
Lonesome went slowly out to the wagon to meet Jane and held up his arms to her. The crowd watched them silently.
“I been waitin’ for you, Jane,” said Lonesome slowly as she climbed down over the wheel.
He held her in his arms for a moment and turned to the crowd.
“Hashknife, I want you to meet my daughter; you and Sleepy Stevens.”
“Why, I know them!” exclaimed Jane. “They ——” “Know us?” grinned Sleepy. “My ——, I sung all night to her once.”
A grizzled cowman leaned over the shoulder of his horse and said to Hashknife—
“You can prove all the things you said?”
“Yeah,” nodded Hashknife. “I sure can.”
“What about him?”
The man pointed at Blackwood, who stood beside the porch, guarded by another cowboy.
“Him?” Hashknife squinted at Blackwood seriously. “Pardner, I—I dunno. He kept out of this. He admits that he mis-branded calves for Easton, and we could likely send him—” Hashknife shook his head slowly. “Lookin’ at it from a cold-blooded angle, suppose we give him his horse and tell him to git to —— out of here.”
“But he’s a rustler!” exclaimed another cowman.
“That’s a fact,” nodded Hashknife. “That sure is a fact. He admits it, don’t he?”
Hashknife looked around at his listeners.
“How many of us would admit the truth?”
Somewhere in the crowd a man laughed and smiles began to appear. Hashknife had won his point. He turned to Blackwood, who could scarcely believe his ears.
“Blackwood, you’re free to drift. I ain’t preachin’ to you, but kinda remember what might ’a’ happened.”
“You mean—” Blackwood licked his dry lips. “You mean, I’m free to —go?”
“You’ve got good ears, old-timer.”
Blackwood swung into his saddle, looked at Hashknife for several moments as though wanting to say something, but was unable to begin. Then as he turned slowly and rode out of the yard, unbelieving that any man or men could be so generous.
The cattlemen roped Easton to a horse, picked up Doc Clevis and Jake Blue, and strung out in a long cavalcade toward town.
“What happened here?” asked Jane wonderingly.
“Well, ma’am,” said Hashknife slowly, looking back at the tumbled tombstones, “you see, a front-yard ain’t no place for a cemetery, so we held a meetin’ today to start a new one some’ers else.”
“And that ain’t such a big lie, at that,” said Mrs. Frosty Snow slowly. Then to Lonesome and Jane, she said:
“Pile in here and go back to the ranch for supper with me. I hope that danged Swede cook don’t take the things to heart that I told him today, ’cause I need him for one more meal. You fellers better come along, too, ’cause I want you to tell me all about it.”
“Please do,” Jane pleaded. “Perhaps Mr. Stevens will sing for us.”
Mrs. Frosty Snow turned the team around and headed for the gate, while Hashknife, Sleepy and Skelton stood together and watched them disappear around the bend. Hashknife went over to the porch and kicked loose the pegs and broken wires.
“Do we go over to Snow’s to supper?” asked Sleepy.
“Uh-huh,” grunted Hashknife. “I’d like to get used to Jane Lee, ’cause she’s sure as —— got a wide streak of humor in her system. You goin’, Skelton?”
“After what you’ve done for me? My ——, I’d even do a little singin’ m’self, Hashknife.”
“Thassall right,” said Hashknife hastily as he wound a handkerchief around his scored forearm, where Hagen’s bullet had left its mark.
“Your appreciation is accepted—but don’t sing. There’s such a thing as carryin’ humor to excess, Skelton.”
Skelton grinned widely and put his hand on Hashknife’s shoulder.
“Cowboy, you shore made history in Lodge-Pole County today, and jist t’ show you how much I appreciates it, I’m splittin’ the 33 into three parts right now. From this here date, me and you and Sleepy
own this place. No arguments a-tall—no sir She ain’t worth a —— of a lot for cows, but if there’s a gold mine—anyway, we’re pardners; the three of us.”
Hashknife looked closely at the old man and at Sleepy, who was busily rolling a cigaret. It was very quiet now A string of dustylooking cattle were coming down past the corner of the ranch-house fence, heading for the creek.
A magpie flew past the house, swerved sharply at sight of the three men, and perched on a corner of the corral, scolding earnestly Fleecy clouds flecked the blue sky beyond the timbered ridges; from the hillside came the whistling bark of a ground-squirrel; down in the willows a cow bawled softly for her calf.
Hashknife turned slowly and took a deep breath, as he said— “Whatcha say t’ havin’ a song by the Tombstone trio?”
***
Transcriber’s Note: This story appeared in the December 30, 1922 issue of Adventure Magazine.
END OF THE PROJECT GUTENBERG EBOOK THE RANCH OF THE TOMBSTONES
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