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Nowadays due to the advancements in field of wireless communications, like the introduction of new applications and technologies the scarcity of the bandwidth has come out to the most prominent problem. However, the study conducted by the FCC (Federal Communications Commission) puts some light on this problem and shows that the large portion of the licensed spectrum remains free most of the time, approx. 80% of the spectrum is available and on an average only 20 % is occupied. So, the spectrum is almost 80% free and is under-utilized and is allocated to the licensed user that is the primary users only, to solve this problem FCC proposed that the secondary users to utilize the licensed spectrum when it is free without interfering or disturbing with the primary users and named it "Cognitive Radio".
Cognitive Radio is in itself a vast topic, it covers topics like the spectrum sensing, spectrum allocation, spectrum management etc. spectrum pooling is one of them, which in broad sense means the combining the whole licensed spectrum which are allocated to primary users or to the particular applications and technologies and bringing whole spectrum to one place from where all users can get access of it whenever they require it without interfering with the licensed users. The main area of research in Cognitive Radio is spectrum sensing. Till present day many of the spectrum sensing techniques has been proposed, but, the most studied spectrum sensing techniques are Energy Detection, Matched Filter Detection and Cyclostationary Feature detection. Every technique has its own drawbacks and advantages. The performance of Energy detection is susceptible to uncertainty in noise power, Matched filter detection needs a dedicated receiver for every primary user and in Cyclostationary feature detection the computational complexity is very high and requires a long observation time. This thesis compares the performance of these three major spectrum sensing techniques.
The simulation results in this paper show that Energy detection starts working at -7 dB s of SNR. Matched filter detection is better than energy detection as it starts working at low SNR of -30 dB s. Cyclostationary feature detection is better than both the previous detection techniques since it produces better results at lowest SNR, i.e. for values below -30 dB s. the results shows that the performance of energy detection gets better with increasing SNR as the "probability of primary detection" increases from zero at -14 dB s to 100% at +8 dB s and correspondingly the "probability of false detection" improves from 100% to zero. Similar type of performance is achieved using matched filter detection as "probability of primary detection" and the "probability of false detection" shows improvement in SNR as it varies from -30 dB s to +8 dB s. the cyclostationary feature detection outclasses the other two sensing techniques as 100% "probability of primary detection" and zero "probability of false detection" is achieved at -8 dB s, but the processing time of cyclostationary feature detection is greater than the energy detection and matched filter detection techniques.
Cognitive Radio is a real time intelligent wireless communication system which can smartly sense and adapt with the changing environment by altering its transmitting parameters, such as transmitting power, carrier frequency and modulation scheme. In simple words it is a communication system which gathers the information about the surrounding environment by sensing it and changes its transmitting parameters accordingly in accordance with the surrounding radio environment, so that the radio spectrum could be efficiently utilized. The main objective of the cognitive radio are highly reliable communications on whenever and wherever basis and to utilize the radio spectrum efficiently .
Cognitive Radio can be classified into following subsystems according to its operational area:
Multiband System: It supports more than one frequency band used by a wireless standards (e.g., GSM 900, GSM 1800, GSM 1900),
Multi-standard system: It supports more than one standard. Multi-standard systems can work within one standard family( e.g., UTRA-FDD, UTRA-TDD for UMTS) or across different networks ( e.g., DECT, GSM, UMTS, WLAN).
Multi-service system: It provides different services like telephony, data, video streaming etc.
Multi-channel system: It supports two or more independent transmission and reception channels simultaneously .
The main characteristic of cognitive radio is its reconfigurability, i.e. it can reconfigure dynamically according to the surrounding radio environment again and again. That is it can dynamically change its operational parameters to provide better Quality of Service. The reconfigurations are software-defined, i.e., they are accomplished by activating the appropriate software at the transceiver .
Reconfigurability can be done in following domains:
Frequency: By changing the operating frequency. This ability dynamically selects the appropriate operating frequency based on the sensing of signals from other transmitters or on some other method.
Adaptive Modulation/Coding: By selecting the appropriate modulation type for use with a particular transmission system to permit interoperability between systems.
Transmit Power Control: By dynamically switching between several transmission power levels in the data transmission process .
The word "cognitive" of cognitive radio is derived from the word "cognition", which refers to the process of knowing through perception, knowledge, reasoning and intuition with focus on information available from the environment .
Each of the transceiver in the cognitive radio system should be capable of adapting to its surrounding environment by itself, so that there is no nd of centralized management entity. This concept provides significant reduction in the complexity of the system . A good spectrum management scheme is required to efficiently manage and organize spectrum holes information among cognitive radios (spectrum resource management). Due to the stochastic nature of the cognitive radio networks the routing and topology information is more and more complex. By good mobility and connection management it can discover neighborhood, detect available internet access and support vertical handoffs, which will help cognitive radios to select rout and networks (mobility and connection management). Since cognitive radio networks are of heterogeneous nature, that is why the heterogeneities like wireless accss technology, system operators etc. introduces a lots of security issues, therefore security management is one of the main prerequisite of the cognitive radio networks .
Cognitive cycle includes detecting spectrum holes, selecting the best frequency bands, coordinating spectrum access with other users and vacating the spectrum when the primary user appears. Figure 1 shows the cognitive cycle.
Cognitive cycle has following functions:
Spectrum sensing and analysis.
Spectrum management and handoff.
Spectrum allocation and sharing.
Spectrum sensing and analysis: Cognitive radio can detect the spectrum holes or white spaces as shown in the Figure 2 below ( the frequency band that is empty and is not used by the primary users) and utilizes this free frequency band and when the primary user i.e. the licensed user wants to use the spectrum the cognitive radio senses it and frees it so that there is no interference generated by the secondary user transmission.
Figure 1: Cognitive Cycle 
Figure 2: Illustration of spectrum white space 
Spectrum management and handoff: After knowing the white spaces or spectrum holes by sensing the spectrum, spectrum management and handoff function of cognitive radio allows secondary users to choose the best frequency band from the available one and hop among the multiple free frequency bands accordingly with the time varying characteristics to achieve the Quality of Service (QoS). For example when a primary user wants to re-occupy his/her frequency band, the secondary user using that particular frequency band switches to the the other available frequency band, according to the channel capacity determined by the noise and interference levels, path loss, channel error rate, holding time etc.
Spectrum allocation and sharing: In dynamic spectrum access, the secondary user shares the spectrum resources with the primary users, other secondary users, or both. So, to achieve high spectrum efficiency, a good spectrum allocation and sharing mechanism is needed. When primary users and secondary users use the spectrum simultaneously the interference due to the secondary user should be limited by a certain threshold. When many secondary users are using the spectrum, their access should be coordinated to avoid collisions and interference with the primary users .
The necessity of Cognitive Radio
The revolutionary growth in the field of wireless communications i.e., due to the development of new wireless applications, standards and technology, the need of radio spectrum is increasing day by day, but since the radio spectrum for reliable communications cannot be increased, there is scarcity of radio spectrum. To account this problem a survey was done by FCC, which showed that the spectrum is under-utilized to a great extent most of the time. Since, FCC assigns spectrum to licensed holders, known as primary users, on a long term basis for large geographical regions . This assigned spectrum than cannot be used by other users except the primary users, and it has been observed that the primary users do not use this assigned spectrum the whole time, they use it only 10% to 20% of the time on an average. So, this means that on an average about 80% of the time the assigned radio spectrum is not used or under-utilized heavily. This shows that the spectrum scarcity is mainly due to inefficient fixed spectrum allocations rather than any shortage of spectrum. This observation prompted FCC to find a relatively new spectrum access technology . Cognitive Radio turned out to be the most promising solution to this problem that allows the secondary users to use the licensed spectrum when it is not in use by the primary users . Cognitive Radio technology can enhance spectrum efficiency drastically, it is also referred as the Dynamic Spectrum Access (DSA) network .
Cognitive Radio's key benefits
Cognitive Radio offers optimal diversity in frequency, power, modulation, coding, space, time, polarization and so on, which leads to following benefits:
Spectrum Efficiency: This will fulfill future demand for the spectrum, and is the basic behind the implementation of CR.
Higher bandwidth services: Demand of Multi Broadcast and Multicast Services (MBMS) is constantly on the rise which will be facilitated by the implementation of CR.
Graceful Degradation of Services: When conditions are not ideal, graceful degradation of service is provided, as opposed to the less desirable complete and sudden loss of service. This feature of CR is very important in providing services to the users especially when they are mobile and the base stations in contacts are constantly changing.
Improved Quality of Service (QoS) (latency, data rate, cost etc): Suitability, availability and reliability of wireless services will improve from the users perspective.
Commercial Exploitation: CR promotes spectrum liberalization (makes it much easier to trade spectrum between users). Indeed, a business case may exist for becoming a spectrum broker, whereby a third party manages the trade between supplier and demander and receives a commission.
Benefits to the Service Provider: More customers in the market and/or increased information transfer rates to existing customers. More players can come in the market.
Future-proofed product: A CR is able to change to services, protocols, modulation, spectrum etc. without the need for a user and/or manufacturer to upgrade to a new device.
Common hardware platform: Manufacturers will gain from economies of scale because they no longer need to build numerous hardware variants, instead using a single common platform run a wide range of software. This also assists in rapid service deployment.
Flexible regulation: By using a form of policy database, regulation could be changed relatively quickly as and when required, easing the burden on regulators.
Emergency service communications: Joint operations during major incident would benefit greatly as police, fire , ambulance and coastguard could be linked together in one radio user sensing the spectrum being used by the other parties and reconfiguring itself.
Benefits to licensee: CR can pave way of spectrum trading, where licensees would be allowed to lease a portion of their spectrum rights to the third parties on a temporal, spatial or other appropriate basis to recoup some of the expense of its 24 hr-a-day license and even make money.