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Recently wireless networks have been growing very rapidly with the insertion of Wi-fi devices into our life and the "smart" telecommunication devices as the mobile phones("smartphones"). In order to meet this huge growth in wireless technologies and services provided, researchers as well as industry have been working toward new techniques and standardizations to make technology more useful for the nowadays many needs of humans for communication.
The most critical consequences of this growth in wireless networks are the ones related to spectrum usage and management as the electromagnetic radio spectrum is the most precious "natural"(air) resource when there is discussion about wireless networks and telecommunications in general. The existing policies for spectrum management were based on static spectrum allocation for a specific technology and services controlled by regulation agencies like the Federal Communications Commission (FCC) and the European Telecommunications Standards Institute (ETSI). With the appearance of wireless personal communications (WPC) technologies it became unreasonable to use these policies and rely on static spectrum allocation due to economical-competition and technical considerations. In order to solve this, Industrial, Scientific and Medical (ISM)  bands have been provided as a good solution to handle these types of wireless networks. Nevertheless, after a while ISM bands get congested and over-utilized which affects the quality of communication or the Quality of Service (QoS) as we call it today, on those bands and to overcome this a new way promising technology was introduced, the software defined radio (SDR) followed by cognitive radio (CR) networks based on dynamic spectrum access and network's intelligence.
2.1.2 Cognitive Radio
The term cognitive radio was first introduced by Joseph Mitola . "The Cognitive radio is a radio that adapts to the conditions of the environment by analyzing, observing and learning. The cognitive network makes use of these adaptations for future decisions" . Cognitive radio is basically used for maximum utilization of the radio bandwidth and as a way to conserve bandwidth and make the most out of it in the best possible way. Basis of the performance optimization is the cognitive process which is shared by the cognitive radio and the cognitive networks and was described by Mitola and Haykin. The main part of this process is to learn from the past decisions and make use of it for future decisions. The radio Knowledge Representation Language ("RKRL") is a language which the cognitive radio uses for knowledge. The cognitive radios need variable parameters for the description of the optimization space. These parameters come from the Software Defined Radio (SDR) .
2.1.3 Requirements and Reasons for improvement in technology
The main purpose of any technology is to utilize the needs in the best possible way with a minimum cost and minimum impact on the ongoing industry. The cognitive network is be able to provide high performance in a better time period than the non-cognitive radio networks, with better Quality of Service (QoS), higher throughput and smaller costs for the end user. The cost of the cognitive networks calculated with respect to the communication should justify and satisfy the performance. For the implementation of the actual functionality of the network, cognitive radio requires a Software Adaptable Network (SAN) and in the same way for the modification of radio operation e.g. waveform, bandwidth, time, spatiality etc., the cognitive radio depends on a Software Define Radio (SDR) .
2.1.4 Cognitive Radio Operation
Fig 2.1 Basic Cognition cycle
Fig. 2.1 shows the basic cognitive radio tasks, spectrum sensing, spectrum analysis and spectrum decision in the radio environment.
2.2 Software Defined Radio (SDR)
Wireless communications basically depend on the signals, physical hardware and their attributes. In the past communication technology had straightforward signaling, analog hardware and very limited functionality with high costs. The Software Defined Radio (SDR) was introduced for handling more than one communication technology (e.g. GSM,CDMA,UMTS and the upcoming 4G networks) . so that the terminals can change their operation with respect to the software and the needs of the user who controls them. Nowadays many signaling methods have been proposed and used in various communication technologies all over the world. Software defined radio enhances the wireless devices with cognition abilities like awareness, learning, sensing and reasoning. Moreover, it has the ability to resolve the emerging interoperability issues by providing a global seamless connection. Before the invention of cognitive radio, SDR was focused on multi-mode and multi-standard devices. SDR plays a vital role, to realize the features of cognitive radio and for the future with the creation of a truly Full cognitive Radio (intelligent CR) .
2.2.2 SDR and its relationship with Cognitive Radio
Adaptability is the main property of the cognitive radio where frequency, power, modulation and bandwidth can be changed according to the current radio environment. To avoid analog circuits and components, SDR provides variable radio functionality. The cognitive radio is basically a SDR which already knows the condition, state, position and automatically adjusts its functions according to the desired objectives and goals of the user. The relation between the SDR and the cognitive radio can be demonstrated in Fig 2.2. It is clear from the below diagram that the cognitive radio encompasses the SDR. The SDR is developed in software based on Digital Signal Processing with the modifiable Radio Frequency components. So, the SDR is a generic radio platform which has the capability to operate in different bandwidths over a large number of frequencies as well as using different modulation schemes and waveform formats. As a result of this, the SDR can support multiple standards such as GSM, WCDMA, WIMAX, UMTS, Wi-Fi etc., and multiple access schemes such as TDMA, OFDM and SDMA etc.
Fig 2.2: The relationship between SDR and Cognitive Radio.
2.3 Cognitive Radio Architecture
Almost all the wireless spectrum is licensed for different tasks and few bands are still unlicensed. There is a discussion about all the possible scenarios for a better description of the communication protocols and standars. The components of the cognitive radio architecture are shown in Fig 2.3. In Fig 2.3 it is clear that there are two main groups, the primary networks and the cognitive radio networks (the next generation networks) or neXt Gen Networks as they are called.
2.3.2 Primary networks
Primary networks have special rights to specific bands and usually access to more premium services and better QoS if there is no congestion in the band. The primary network includes the primary user and the primary base-station.
â€¢ Primary user: Primary users also called licensed users, operate in specific spectrum bands. This operation is entirely controlled by the only primary base-station. These primary users do not require any further enhancements for the coexistence of the primary base-stations and the primary users.
â€¢ Primary base-station: The primary base station has a standar infrastructure. Primary networks don't have the ability of cognitive radio, sharing the spectrum with cognitive users but it can be demanded to have both legacy and cognitive radio protocols for primary network access of cognitive radio users.
2.3.3 Cognitive radio networks
Cognitive radio networks do not have the permission to operate in the required band. The CR networks can be deployed both with infrastructure and without infrastructure networks as illustrated in fig 2.3. The fundamentals of the network are as following:
â€¢ Cognitive Radio user: The CR user (the unlicensed user) has no spectrum license, so extra functionalities and privileges are needed for sharing the spectrum band.
â€¢ Cognitive Radio base-station: The CR base-station (the unlicensed base station)
has a standar infrastructure component with CR capabilities. Cognitive Radio can access the different networks by providing the single hop network connection to CR user . Single hop connection is used to reduce the propagation delay, it has now become essential to have single hop network connection which connects the user terminals. The CR network architecture in Fig. 2.3 shows different types of networks primary network access, with infrastructure based CR network, without infrastructure based CR network (adhoc network). The CR networks operate both in licensed and unlicensed bands (mixed spectrum environment). There are three access types are:
â€¢ CR network access: The CR users can access the CR base-station not only the
licensed bands but also the unlicensed spectrum bands.
â€¢ CR ad hoc access: The CR users communicate with different CR users through the ad hoc connection on licensed and unlicensed bands.
â€¢ Primary network access: The licensed bands are means for the CR users through
which they access the primary base-station.
Fig 2.3 Cognitive radio Architecture
2.4 Spectrum Sensing:
Due to an increasing demand of high data rates, static frequency cannot fulfill the demand of these high data rates. As a result of this, new methods for exploiting the spectrum are being introduced. Cognitive radio, exploiting the unused spectrum is a new way to access the spectrum in a more efficient way than before. Spectrum sensing is measuring the interferences over the spectrum to find the unused channels . In this way efficient use of spectrum is utilized. Spectrum sensing is involved in determining the type of the signal like the carrier frequency, the modulation scheme, the waveform and others .
The mostly used spectrum sensing techniques are given as ,
â€¢ Matched Filtering
â€¢ Waveform-Based Sensing
â€¢ Cyclostationary Based Sensing
â€¢ Energy Detector Based Sensing
â€¢ Radio Identification
â€¢ Other Sensing Methods
2.4.3 Current Challenges in Spectrum sensing and CR
There are some challenges which needs to be solved for efficient spectrum sensing which are gives as,
â€¢ Hardware Requirements (fast electronics and costly hardware (antennas and filters))
â€¢ The Hidden Primary User Problem
â€¢ Spread Spectrum Primary Users
â€¢ Sensing Time
2.5 Spectrum Management
The goal is to find the best available spectrum to fulfill the needs of the communication. The licensed, unlicensed and unused spectrum bands are spread over a large number of frequencies in the cognitive radio networks. These unused spectrum bands show different properties and attributes according to the time varying radio environment and the probabilistic events. The Cognitive radio has to decide the best available spectrum band, such that it fulfills the QoS requirements and the most bandwidth efficient way .
2.5.2 Spectrum analysis and Spectrum holes sensing
Spectrum analysis discovers the different functionalities of the spectrum bands, to make productive use of the spectrum band according to the requirements and needs of its users. Each spectrum hole (Band of frequencies assigned to the primary user, but at a specific time and geographic location, these bands is not fully utilized by that user or group of users .) should be defined according to the time varying environment and the information of the band like frequency and bandwidth. To represent the quality of the spectrum band, parameters are defined such as interference, holding time, path loss, link layer delay, wireless link errors etc.
â€¢ Interference: The interference characteristics of the channel can be determined from the spectrum band in used. The permissible power of a CR user can be calculated, from the amount of interference which is use for the calculation of the channel capacity.
â€¢ Holding time: Holding time is an expected time, from which the CR user occupy the licensed band before its interruption. For better quality holding time should be as long as possible.
â€¢ Path loss: If the operating frequency increases, the path loss will also be increased. If the cognitive users have the constant transmission power then at higher frequencies their transmission range decreases. In order to compensate the increased path loss if we increase the transmission power this yields in higher interference to the other users.
â€¢ Wireless link errors: This error rate of the channel changes according to the change in modulation scheme and interference level of the spectrum band.
â€¢ Link layer delay: Different link layer protocols are required to address path loss,
interference and wireless link errors.
2.5.3 Spectrum decision or Spectrum Allocation
When an analysis of all the spectrum bands is completed, a spectrum band should be selected for the transmission according to the QoS requirements. The decision rules are focused on the cost of communication and fairness of use .
2.5.4 Spectrum management challenges
Challenges for the spectrum management are listed below; a lot of research is undergoing for the issues below.
â€¢ How to integrate all the parameters of the spectrum, for the spectrum decision-allocation.
â€¢ Multiple spectrum bands used for simultaneous transmission.
â€¢ Spectrum decision and reconfiguration is needed in a cooperative framework.
â€¢ Spectrum decision-allocation over heterogeneous spectrum bands and heterogeneous networks.
For the decision of the best spectrum band over the heterogeneous environment, the CR network supports the spectrum decision operation both for licensed and unlicensed bands under different characteristics.
2.6 Spectrum mobility
In cognitive radio networks, spectrum mobility occurs when the frequency of operation changes. For better transition of the spectrum, spectrum mobility maintains all the requirements of the communication. Spectrum mobility has a vital role while designing cognitive protocols . Two main factors affect the spectrum mobility. The first one is the delay incurred during the spectrum handoff. This delay affects the communication at different layers. When the primary user appears, the cognitive radio performs a new type of handoff which we call a spectrum handoff.
2.6.2 Spectrum Handoff
The cognitive radio has the ability to adapt to the frequency operation. Due to this the network protocol changes its mode of operation from one mode to another. The main goal of mobility management is that these transitions are completed without any disturbance and in a time efficient manner. The mobility management should have awareness of the duration of the spectrum handoff, from the sensing algorithm. When the mobility management learns the latency, its job is to confirm that the communication of the Cognitive Radio user should undergo a minimum performance degradation , . In the same way, multi-layer documents are needed for accomplishing the functionalities of spectrum mobility. The interaction between spectrum management and spectrum mobility can be illustrated as in Fig 2.4. The spectrum sensing and spectrum sharing are keen to enhance the spectrum efficiency. The spectrum management functionalities cooperate with communication layers. The spectrum management needs QoS information, sensing, scheduling, transport and routing for the decision of the appropriate spectrum band. From Fig. 2.4, link layer information and sensing delays information are required for the estimation of spectrum handoff time latency. The transport layer and application layer should know the latency, for the route recovery by using the spectrum handoff. Due to this, spectrum handoff is very important in the communication layers.
Fig 2.4 Handoff decision and network communication
2.6.3 Spectrum mobility challenges
The below are some open research challenges for efficient spectrum mobility in cognitive networks .
â€¢ Spectrum mobility in time domain. The available channels change with respect to time, so to maintain QoS in this environment is a challenge. The physical radio goes through the spectrum to fulfill the QoS requirements.
â€¢ Spectrum mobility in space. As user changes its position from one place to another, the available bands also changes. To assign a spectrum is a major issue in the CR networks.
â€¢ If CR user moves to another place, the available spectrum bands also changes and due to this spectrum handoff takes place. So the required spectrum handoff scheme should be integrate inter cell handoff. The spectrum handoff in different networks is referred as vertical handoff which takes place in the CR networks.
â€¢ At a particular instance, many frequency bands are available for a CR user. For the selection of the best available spectrum, algorithms are required.
â€¢ When the operational frequency becomes busy in the communication by a CR user. Then the node applications have to move to other available frequency bands.
â€¢ Designing a new mobility management, to reduce the loss and the delay in a handoff.
2.7 Spectrum Sharing
Spectrum sharing is the major challenge which open spectrum usage faces. Spectrum sharing is related to medium access control (MAC) problems in the current system; however, there are different challenges for the spectrum sharing in cognitive radio. Spectrum sharing consists of five steps which are,
â€¢ Spectrum sensing: The CR can allot a specific part of the spectrum if it is not used by the licensed user. When a CR wants to transmit data, it will first sense its surrounding spectrum usage.
â€¢ Spectrum allocation: When spectrum is available, a channel is allocated. This allocation depends on the availability of the channel and also internal/external policies.
â€¢ Spectrum access: When the nodes are trying to access the available spectrum, spectrum access helps to prevent colliding and overlapping of the spectrum.
â€¢ Transmitter-receiver handshake: The transmitter-receiver handshake is essential for effective communication in cognitive radio, after the determination of the spectrum.
â€¢ Spectrum mobility: The spectrum mobility is important in the communication between the nodes. If a particular part of the spectrum is required by the licensed user, communication should be continued by utilizing another free part of the spectrum.
2.7.2 Classification of spectrum sharing
Spectrum sharing can be classified into three main parts, i.e. architecture, spectrum allocation behavior and spectrum access techniques which is illustrated in Fig 2.5 .
Fig. 2.5 Classification of spectrum sharing in Cognitive radio
â€¢ Centralized spectrum sharing: In centralized spectrum sharing, spectrum
allocation and access procedures are controlled by a centralized entity . Each entity in the CR network forwards the measurements of spectrum allocation to the central entity.
â€¢ Distributed spectrum sharing: when the construction of an infrastructure is not suitable, then distributed solutions are proposed.
â€¢ Cooperative spectrum sharing: The interference measurements are distributed among other nodes, the centralized solution is also referred as cooperative.
â€¢ Non-cooperative spectrum sharing: Non-cooperative solutions only think about
the nodes in hand that's why also called selfish solutions. The Non-cooperative solutions are reduced spectrum utilization and minimal communication requirements.
â€¢ Overlay spectrum sharing: This overlay spectrum sharing is also known as the
spectrum access technique. The node accesses the network by using that portion which is not under usage of the licensed user (LU).
â€¢ Underlay spectrum sharing: The underlay spectrum sharing technique take
advantage of the spread spectrum techniques which are specifically developed for cellular networks . The underlay spectrum sharing requires such spread spectrum technique from which it can utilize high bandwidth.
2.7.3 Spectrum sharing challenges
There is a huge amount of ongoing research issues in spectrum sharing, which should be properly investigated for the efficient use of the spectrum. A few challenging issues in CR along with their possible solutions are :
â€¢ Common control channel (CCC): In spectrum sharing solutions, when the primary user has selected a channel, this should be vacated without any interference. As a result, implementation is not feasible in fixed CCC CR networks. When we are not using CCC, the handshaking between the transmitter and the receiver becomes a challenge.
â€¢ Dynamic radio range: In CR networks, huge amounts of spectrum are used. Node neighbours change with respect to the variation of the operating frequency. The changing in the neighbour node affects the interference profile and the routing decisions. For minimum interference, control channels will be selected from the lower portion (high transmission range and selection of data channels in the high part of the spectrum.) and data channels will be selected from the higher portion.
â€¢ Spectrum unit: The channels can be defined with respect to the frequency dimension, as frequency bands . Spectrum sharing is a challenge in advanced algorithms with respect to the definition of the channel behaving as a spectrum unit. The properties of the channel are not constant due to the influence of the operating frequency. The cognitive radio spectrum can be designed based on the generic spectrum unit. In a cognitive radio network it is difficult to find a common spectrum for efficient utilization.
2.8 OFDM Based Cognitive Radio
New challenges and aspects arise in the applications of orthogonal frequency multiplexing (OFDM) in CR. For the configuration of radio and physical parameters, the cognitive engine is responsible for smart decisions. When all the information is abstracted, the decision unit can make conclusions for the best system. A decision includes suitable channel coding, operation, frequency and bandwidth. At this point, OFDM has the edge over the same transmission technologies and their adaptive features. By changing the configuration of OFDM, the cognitive system will be able to communicate with different radio access technologies. The radio circuit splits into a digital and an analog part, digital parts are IF, ADC and DAC and the analog part are STAR. Both digital and analogy parts are keen to enhance the flexibility. The abilities of spectrum shaping together with adaptiveness, make OFDM the best for CR systems. In the table 2.1 this can be evaluated that the requirements of CR and how these requirements can be fulfilled by OFDM.
Table 2.1 OFDM Cognitive Radio 
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