You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Advances in cognitive radio networks: A survey 
Chapter Title:  I. INTRODUCTION 
1. Abstract of Cited Paper (BIB001):  Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) Networks as well as Dynamic Spectrum Access (DSA) and cognitive radio networks. The term xG networks is used throughout the paper. The novel functionalities and current research challenges of the xG networks are explained in detail. More specifically, a brief overview of the cognitive radio technology is provided and the xG network architecture is introduced. Moreover, the xG network functions such as spectrum management, spectrum mobility and spectrum sharing are explained in detail. The influence of these functions on the performance of the upper layer protocols such as routing and transport are investigated and open research issues in these areas are also outlined. Finally, the cross-layer design challenges in xG networks are discussed. 
2. Abstract of Cited Paper (BIB002):  The interference temperature model was proposed by the FCC in 2003 as a way to dynamically manage and allocate spectrum resources. It would allow unlicensed radios to sense their current RF environment and transmit in licensed bands, provided their transmission does not raise the interference temperature for that frequency band over the interference temperature limit. It never received much interest because nobody was sure exactly how to use it or how if it would work. This research focuses on a mathematical analysis of the interference temperature model in an effort to examine the relationships between the capacity achieved by the unlicensed network and the interference caused to the licensed network. We develop a model for the RF environment and determine probability distributions governing interference temperature as a function of various elements in the model. We then determine bounds on the amount of interference caused by implementing such a system. We examine model environments for a wireless WAN and a wireless LAN, each coexisting with a licensed carrier. For each, we quantify both the impact on the licensed signal and also the capacity achieved by our underlay network. By substituting numeric values for RF environments in which the interference temperature model might be applied, we show that achievable capacity is very small, while the impact the licensee can be very large. Based on this, we propose alternate usages for interference temperature and ways to boost capacity. 
3. Abstract of Cited Paper (BIB003):  Merging the fundamental principles of resource allocation with the state-of-the-art in research and application examples, Han and Liu present a novel and comprehensive perspective for improving wireless systems performance. Cross-layer multiuser optimization in wireless networks is described systematically. Starting from the basic principles, such as power control and multiple access, coverage moves to the optimization techniques for resource allocation, including formulation and analysis, and game theory. Advanced topics such as dynamic resource allocation and resource allocation in antenna array processing, and in cooperative, sensor, personal area, and ultrawideband networks, are then discussed. Unique in its scope, timeliness, and innovative author insights, this invaluable work will help graduate students and researchers to understand the basics of wireless resource allocation whilst highlighting modern research topics, and will help industrial engineers to improve system optimization.

Literature Review:
T HE usage of radio spectrum resources and the regulation of radio emissions are coordinated by national regulatory bodies like the Federal Communications Commission (FCC). The FCC assigns spectrum to licensed holders, also known as primary users, on a long-term basis for large geographical regions. However, a large portion of the assigned spectrum remains under utilized as illustrated in Fig. 1 . The inefficient usage of the limited spectrum necessitates the development of dynamic spectrum access techniques, where users who have no spectrum licenses, also known as secondary users, are allowed to use the temporarily unused licensed spectrum. In recent years, the FCC has been considering more flexible and comprehensive uses of the available spectrum  , through the use of cognitive radio technology  . Cognitive radio is the key enabling technology that enables next generation communication networks, also known as dy- Fig. 1 . Spectrum usage BIB002 . namic spectrum access (DSA) networks, to utilize the spectrum more efficiently in an opportunistic fashion without interfering with the primary users. It is defined as a radio that can change its transmitter parameters according to the interactions with the environment in which it operates  . It differs from conventional radio devices in that a cognitive radio can equip users with cognitive capability and reconfigurability BIB001 , BIB002 . Cognitive capability refers to the ability to sense and gather information from the surrounding environment, such as information about transmission frequency, bandwidth, power, modulation, etc. With this capability, secondary users can identify the best available spectrum. Reconfigurability refers to the ability to rapidly adapt the operational parameters according to the sensed information in order to achieve the optimal performance. By exploiting the spectrum in an opportunistic fashion, cognitive radio enables secondary users to sense which portion of the spectrum are available, select the best available channel, coordinate spectrum access with other users, and vacate the channel when a primary user reclaims the spectrum usage right. Considering the more flexible and comprehensive use of the spectrum resources, especially when secondary users coexist with primary users, traditional spectrum allocation schemes BIB004 and spectrum access protocols may no longer be applicable. New spectrum management approaches need to be developed to solve new challenges in research related to cognitive radio, specifically, in spectrum sensing and dynamic spectrum sharing. As primary users have priority in using the spectrum, when secondary users coexist with primary users, they have to perform real-time wideband monitoring of the licensed spectrum to be used. When secondary users are allowed to transmit data simultaneously with a primary user, interference temperature limit should not be violated BIB003 . If secondary users are only allowed to transmit when the primary users are not using the spectrum, they need to be aware of the primary users' reappearance through various detection techniques, such as energy detection, feature detection, matched filtering and coherent detection. Due to noise uncertainty, shadowing, and multipath effect, detection performance of single user sensing is pretty limited. Cooperative sensing has been considered effective in improving detec-tion accuracy by taking advantage of the spatial and multi-user diversity. In cooperative spectrum sensing, how to select proper users for sensing, how to fuse individual user's decision and exchange information, and how to perform distributed spectrum sensing are issues worth studying. In order to fully utilize the spectrum resources, efficient dynamic spectrum allocation and sharing schemes are very important. Novel spectrum access control protocols and control channel management should be designed to accommodate the dynamic spectrum environment while avoid collision with a primary user. When a primary user re-appears in a licensed band, a good spectrum handoff mechanism is required to provide secondary users with smooth frequency transition with low latency. In multi-hop cognitive wireless networks, intermediate cognitive nodes should intelligently support relaying information and routing through using a set of dynamically changing channels. In order to manage the interference to the primary users and the mutual interference among themselves, secondary users' transmission power should be carefully controlled, and their competition for the spectrum resources should also be addressed. There have been many significant developments in the past few years on cognitive radios. This article surveys recent advances in research related to cognitive radios. In Section II, we overview the fundamentals of cognitive radio technology, architecture of a cognitive radio network and its applications. In Section III, we review existing works in spectrum sensing, including interference temperature, different types of detection techniques, and cooperative spectrum sensing. In Section IV, we discuss several important issues in dynamic spectrum allocation and sharing.