A Review of Cross-layer Design in Dynamic Spectrum Access for Cognitive Radio Networks <s> Introduction <s> Since early 1980 an exponential blowup of cellular mobile systems has been observed, which has produced, all over the world, the definition of a plethora of analog and digital standards. In 2000 the industrial competition between Asia, Europe, and America promises a very difficult path toward the definition of a unique standard for future mobile systems, although market analyses underline the trading benefits of a common worldwide standard. It is therefore in this field that the software radio concept is emerging as a potential pragmatic solution: a software implementation of the user terminal able to dynamically adapt to the radio environment in which it is, time by time, located. In fact, the term software radio stands for radio functionalities defined by software, meaning the possibility to define by software the typical functionality of a radio interface, usually implemented in TX and RX equipment by dedicated hardware. The presence of the software defining the radio interface necessarily implies the use of DSPs to replace dedicated hardware, to execute, in real time, the necessary software. In this article objectives, advantages, problem areas, and technological challenges of software radio are addressed. In particular, SW radio transceiver architecture, possible SW implementation, and its download are analyzed. <s> BIB001 </s> A Review of Cross-layer Design in Dynamic Spectrum Access for Cognitive Radio Networks <s> Introduction <s> As the cellular and PCS world collides with wireless LANs and Internet-based packet data, new networking approaches will support the integration of voice and data on the composite infrastructure of cellular base stations and Ethernet-based wireless access points. This article highlights some of the past accomplishments and promising research avenues for an important topic in the creation of future wireless networks. We address the issue of cross-layer networking, where the physical and MAC layer knowledge of the wireless medium is shared with higher layers, in order to provide efficient methods of allocating network resources and applications over the Internet. In essence, future networks will need to provide "impedance matching" of the instantaneous radio channel conditions and capacity needs with the traffic and congestion conditions found over the packet-based world of the Internet. Furthermore, such matching will need to be coordinated with a wide range of particular applications and user expectations, making the topic of cross-layer networking increasingly important for the evolving wireless buildout. <s> BIB002 </s> A Review of Cross-layer Design in Dynamic Spectrum Access for Cognitive Radio Networks <s> Introduction <s> 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. <s> BIB003 </s> A Review of Cross-layer Design in Dynamic Spectrum Access for Cognitive Radio Networks <s> Introduction <s> With the rapid deployment of new wireless devices and applications, the last decade has witnessed a growing demand for wireless radio spectrum. However, the fixed spectrum assignment policy becomes a bottleneck for more efficient spectrum utilization, under which a great portion of the licensed spectrum is severely under-utilized. The inefficient usage of the limited spectrum resources urges the spectrum regulatory bodies to review their policy and start to seek for innovative communication technology that can exploit the wireless spectrum in a more intelligent and flexible way. The concept of cognitive radio is proposed to address the issue of spectrum efficiency and has been receiving an increasing attention in recent years, since it equips wireless users the capability to optimally adapt their operating parameters according to the interactions with the surrounding radio environment. There have been many significant developments in the past few years on cognitive radios. This paper surveys recent advances in research related to cognitive radios. The fundamentals of cognitive radio technology, architecture of a cognitive radio network and its applications are first introduced. The existing works in spectrum sensing are reviewed, and important issues in dynamic spectrum allocation and sharing are investigated in detail. <s> BIB004
Cognitive radio is an intelligent radio which will change its transceiver parameters dynamically depending upon the environment conditions (I. F. BIB003 . In cognitive radio networks, each secondary user can dynamically access the spectrum without interfering with primary users in the network. A user node can join in a cognitive radio network, if it has wirelesscommunication, networking and cognitive radio capability. There are two types of users using the network; Primary Users (PUs) and Secondary Users (SUs). Primary Users are also called licensed users who will utilize the already allocated spectrum. Secondary Users are also called Cognitive Users or unlicensed users because, for communication, no fixed spectrum is allocated for SUs (E. BIB001 . SUs can utilize the spectrum of PUs without giving interference to the PUs. The Quality of Service (QoS) parameters of network are bandwidth, end-to-end delay, jitter, throughput, etc. All the networks offer two types of quality of services: guaranteed QoS and best-effort QoS (Sanjay BIB002 . For PUs, guaranteed Quality of Service is provided. For SUs, best-effort QoS is provided, as they use the unused spectrum of PUs when there is a need for communication. When a particular licensed spectrum is free, SUs will be using that licensed spectrum. When PUs come to access that spectrum, SUs will move to the other vacant spectrum without giving interference to the PUs. The vacant spectrum is called white space or spectrum hole and is Infrastructured cognitive radio network is also called centralized cognitive radio network (Beibei BIB004 . In infrastructured cognitive radio networks, there will be separate base station for SUs and PUs. The base station used by SUs is called Secondary base station. Infrastructureless cognitive radio network is also called Cognitive radio adhoc network. In infrastructureless cognitive radio network, there is no base station. The SUs in the network will communicate among themselves typically like adhoc network, using the vacant licensed spectrum band. The Semi-static network infrastructure cognitive radio network is also called Cognitive wireless mesh network (Matteo Cesana et al., 2011). Wireless mesh network is self-organized and self-configured. It consists of mesh nodes which can get internet access through mesh routers. Mesh nodes are wirelessly connected in adhoc manner. Cognitive node uses two different approaches to access the spectrum: spectrum overlay and spectrum underlay (I. F.  . In overlay approach, secondary users occupy the unoccupied spectrum holes when the primary users are not occupying it. When the PU starts transmitting in that spectrum, SU move to another unoccupied spectrum hole and do not give interference to the primary users. On the other hand, in underlay cognitive system, secondary users use the spectrum when the primary users are transmitting. SUs operate at low power and below the interference threshold of the primary users. The information available at network layer is hand-off initiation/completion events and network interface currently in use. The transport layer information is round-trip time, maximum transmission unit, receiver window, congestion window, number of packets lost and actual throughput. The application layer communicates to the lower layers for its quality-ofservice needs such as delay tolerance, acceptable delay variation, required throughput and acceptable packet loss rate.