Preface

Technology is rapidly transforming our daily live, local community, as well as entire globe and everyone should be able to participate in this transformation. Currently, we rely on the wireless connected devices and systems to not only enable on-demand, pervasive communications for a large proportion of the population, but also other critical application areas such as scientific and medical research, industrial control and automation, and public safety. Thus, the communication technologies are crafting new corporate social responsibility initiatives which address global problems, support innovative ideas, and enable opportunities for all in recent increasingly digital world. As the wireless communication technologies and its applications continue to flourish, the demand for precious spectrum resources, which is an essential foundation block to support the wireless communications in the globally connected world, will continue to grow. Recently, due to exponential increase in the number of wireless connected devices with other bandwidth hungry services have exploded huge communication traffic and we expect that the demand for spectrum will continue to increase as new wireless technologies and applications requirements continue to emerge in the foreseeable future which results in the spectrum scarcity. This voracious enthusiasm for additional spectrum resources cannot be met by simply allocating new spectrum. Therefore, the usable capacity of spectrum must be expanded with innovative technologies, regulatory reforms, and removal of market barriers. The cognitive radio is one of the innovative technologies which has the potential to effectively address the spectrum scarcity problem and radically change the way we utilize spectrum. Due to its potential impact, various stakeholders - including regulatory policymakers, wireless device manufacturers, telecommunication service provider, and academic researchers - have shown strong interest in it, especially with respect to research and development.

The cognitive radio technology has emerged as a prone candidate for exploiting the increasingly flexible licensing (dynamic spectrum access) of the spectrum for the wireless communication system. The regulatory bodies have come to realize that most of the time, a large portion of certain licensed frequency band remain empty/unused. Therefore, to redress this, a new regulation would allow for devices which are able to sense and adapt to their spectral environment, such as cognitive radio to become secondary user and such users are wireless devices that opportunistically employ the spectrum already licensed to the primary users. The primary users generally associate with the primary spectral licensed holder and thus have higher priority right to the spectrum. The intuitive objective behind secondary spectrum licensing is to improve the spectral efficiency of the network, whereas depending on the type of licensing without affecting higher priority users.

In the cognitive radio network, the spectrum prediction, spectrum monitoring, and medium access control protocols play an important role to exploit the spectrum opportunities, manage the interference to the primary users, and coordinate. The dynamic leasing, in which some wireless devices opportunistically employ the spectrum rather than choose for a long-term sub-lease. In order to exploit the spectrum, we require a device which is able to sense the communication opportunity and then take actions based on the sensed environment. The cognitive radio offers a novel way of solving spectrum underutilization problems. The emergence of Federal Communication Commission's secondary market initiative has been brought both the obvious desire for spectral efficiency as well as empirical measurements showing that most of the time certain licensed frequency remain unused. The goal of secondary market initiative is to remove unnecessary regulatory barriers to new secondary market-oriented policies. The key points of the book that benefits the readers are as follows:

• Provides decent background about the fundamentals of the spectrum sharing techniques.
• Explores the advanced frame structures with spectrum accessing techniques.
• Role of spectrum prediction and spectrum monitoring techniques for interference-free spectrum sharing as well as for effective spectrum mobility is analyzed.
• Due to the demand of distributed architectures in various applications, the energy and spectral efficient frameworks are presented by using the self-scheduled medium access control (SMC-MAC) protocol in the CRNs.
• The frameworks of CR-NOMA for further improvement in the spectral efficiency as compared to CR are also the unique contributions of this book.
• The interference management schemes for the spectrum sharing are illustrated.
• The exploitation of CR for the internet-of-vehicles, that is CR-inspired internet-of-vehicles, adds as a novel contribution in the book.

Thus, it is perceived that this book enables readers about the recent advances in the field of spectrum sharing, strategies of mathematical modeling, theories of spectrum sharing in addition to the primary activities of cognitive radio network. This book puts together a rich set of research articles featuring recent advances in theory, design, and analysis of cognitive radio networks with its connected environments. The book consists of 13 chapters, which cover a wide range of topics related to the cognitive radio technology, in particular, the topics covered in this book include fundamental challenges and issues in designing cognitive radio systems, information-theoretic analysis of cognitive radio systems, spectrum sharing, spectrum sensing and coexistence issues, adaptive physical layer protocols and link adaptation techniques for cognitive radio, different techniques for spectrum access by distributed cognitive radio, and cognitive medium access control protocols. The book is organized as follows. Chapter 1 starts with the connected environments, evolution of the wireless communication as well as technical perspectives by using the Third Generation Partnership Project (3GPP) with state-of-the-art spectrum sharing/access techniques and the fundamental issues related to cognitive radio networks with its several connected parameters and the major research challenges mostly from a signal processing and communication-theoretic perspective are well presented. The potential advantages, limiting factors, and characteristic features of the existing cognitive radio spectrum sharing domains are thoroughly discussed. The comparison of various spectrum accessing techniques such as interweave spectrum access, underlay spectrum access, overlay spectrum access, and hybrid spectrum access is presented. As the complexities of wireless technologies increase, novel multidisciplinary approaches for the spectrum sharing/management are required with inputs from the technology, economics, and regulations. To identify the available spectrum resource, decision on the optimal sensing and transmission time with proper coordination among the users for spectrum access are the important characteristics of spectrum sharing methods.

Chapter 2 describes a novel hybrid-cum-improved spectrum access technique to improve the throughput and data loss of the cognitive radio networks. The hybrid-cum-improved spectrum access technique consists of two advanced frame structures which explore the hybrid spectrum accessing strategy to utilize the channel in the presence of primary user. The closed-form expressions of the throughput and data loss for the proposed cognitive radio networks are derived numerically and the simulation results are the witness of superior performance with reference to the throughput and data loss.

Chapter 3 explores the cognitive radio networks in the high traffic environments where the spectrum prediction plays an important role to select a particular channel for spectrum sensing from the pool of channels on the basis of idle prediction probability. The considered frame structure has spectrum prediction phase before the spectrum sensing and data transmission phase. We have exploited the concept of hybrid spectrum access strategy to improve the throughput of the considered frame structure. The probability of primary users' emergence in the data transmission period is very significant which needs to be detected to avoid data loss and interference with primary user; however, there is no mechanism to perform this function. The spectrum monitoring technique has been used simultaneously with the data transmission period which is an advanced technique that exploits the received signal characteristics to detect the emergence of primary user. The use of spectrum monitoring improves the performance of high-traffic cognitive radio networks in terms of data loss, power loss, and interference-at-primary user.

Chapter 4 discusses the potential issues concerning the random selection of spectrum sensing channel after the spectrum prediction phase in the cognitive radio networks. A novel approach of improved channel selection is proposed which relies on the probabilities of channels by which predicted idle and the closed-form expressions for the throughput of cognitive user are derived. To achieve the prediction probabilities, the pre-channel-state-information is prerequisite, which may be unavailable for particular scenarios; therefore, a modified selection method is introduced to avoid the sense and stuck problem. For additional improvement in...