Cognitive Radio – Future of Wireless Communication
Pragnesh V Patel and Snehal M Patel
Abstract —Increase in demand of something is good for mankind as it shows their economic development. But there is a natural resource which creates problems whenever there is increase in demand. That resource is radio spectrum and technology to deal with is this problem is called as cognitive radio. Cognitive radio is been developed under DARPA – XG. Cognitive radio is next generation technology in which wireless communicators can use spectrum that is allotted to someone for commercial use under certain restrictions. Section I introduces with the problems faced for spectrum and Cognitive radio and Cognitive Radio Networks are described in Section II and Section III respectively. Section IV deals with Spectrum Assignment and related problems whereas Spectrum Sensing techniques are addressed in Section V. Section VI discusses Applications of Cognitive Radio and w e finally conclude in Section VII
Index Terms —Cognitive Radio, Spectrum Holes, Spectrum Sensing Techniques, Spectrum Assignment, Primary Users, Secondary Users
Economy and communication infrastructure had never been closely dependent on each other as on today's date. Radio spectrum which is an important national and natural resource can be reused again and again under certain engineering and practical constraints. Radio spectrum is backbone for a large number of sectors such as telecommunications, military and defense services, political issues, emergency services, research and development and list lasts long.
Without any risk of midair collision pilots used to flew airplanes when they were newly introduced in market. As time passed more air planes both military and civil got added day by day and thus incremented the air traffic. Today airplanes without any collision fly at few meters distance from each other. This coordination is all due to perfect communication and planning of rules and regulations. Before the sky got densely packed with airplanes, there was a necessity to add new rules that governs pilot's flying range.  Today same problem is faced by radio spectrum. As number of users increase day by day, the spectrum gets more packed and thus there’s a need to invent new rules and technology to avoid this collision. To cop up with increased traffic, increased spectrum sharing is vital.
Management of radio spectrum is national issue and there are bodies that deal with it. For instance TRAI in India works in this field. On European level RSPG (Radio Spectrum Policy Group) and RSC (Radio Spectrum Committee) are responsible for usage and allocation of Radio Spectrums. National Regulatory Agency assign radio spectrum to private companies based on geographical areas. A normal user than pays to private companies to use this resource and service. The allocation of licenses is guided by the Radio Regulations published by International Telecommunication Union (ITU). High Quality of service and interference free environment is obtained by licensing method but unfortunately that leads to inefficient usage of radio resources.
National Allocation tables and Article 5 of Radio Regulations  show that Radio Spectrum's usage is already determined by the national authorities and respective boards. All public use spectrum is licensed to private companies. Current growth in demand for wireless communications predicts heavy demand of radio spectrum in future.  Theirs no way to increase the spectrum but the only way is to use the allotted radio spectrum efficiently. Studies by various survey companies show that most of the allocated radio spectrum is underutilized during most of the period of time. In  and  it is shown by FCC that there are large fluctuations in usage of the allotted radio resource in city San Diego, New Orleans and Atlanta. Shared Spectrum Company conducted survey on spectrum occupancy for bands between 30 MHz to 3 GHz at 6 locations in USA and found out that average occupancy was as low as 5.2%. [1-2][8-9]
Report by The Federal Communications Commission (FCC) classified the occupancy of frequency bands as follows:
1. Some bands are highly unoccupied
2. Some bands are moderately occupied
3. Some bands are heavily occupied
These inefficient occupancy of frequency served as nutrients for growth and development of cognitive radio.
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Figure 1 Spectrum Allocation in USA 
To get a solution for increasing demands for radio spectrum and reducing radio spectrum underutilization, cognitive radio approach has been proposed. In  a method for effective utilization of radio spectrum is proposed where a secondary user is allowed to get the access of radio spectrum in absence of primary user. This method is called as cognitive radio which is a concept that will take years to be realized. It faces a lot of research problems related to artificial intelligence, Digital Signal Processing and other fields.
II. cognitive radio
Studies from [17-18] shows that utilization of radio resources is quite low for large portions of spectrum which leads to waste of valuable radio resources. J Mitola in  proposed the concept of cognitive radio to exploit the unused parts of the radio spectrum. Cognitive radio is based on software defined radio that were proposed to make radios hardware independent. SDRs add the feature of programming which increases the flexibility of the radio to work on different spectrum bands. Thus SDR can access any part of radio spectrum depending upon the requirements and availability of spectrum bands. Current hardware radios don’t have this facility.
The licensing approach of radio spectrum has resulted in wastage of the valuable natural resource. The proposed concept of Cognitive radio exploits the unused frequency of the spectrum and thus make a fruit full use of this natural resource. A cognitive radio is an intelligent radio capable of learning the radio environment of the surrounding and modify itself to adjust with available radio spectrum. The Federal Communications Commission (FCC) gave following definition for cognitive radio.
"A cognitive radio is an intelligent system which adopts its operation according to the environment in which it operates via interaction. This interaction may involve active negotiation or communications with users in other spectrum and/or making decision based on passive sensing. Major requirement of cognitive radio is to be independent of software and being field reprogrammable. But today majority of them are software defined .
In cognitive radio networks, primary user and secondary user are most commonly used terms. Primary user is a user who has been assigned spectrum for long term use whereas secondary user is a user who has no license to access the spectrum but still accesses the radio spectrum using cognitive radio technology. A Radio device always looks for following thing during the scan process
1. Bands that have heavy Traffic i.e. almost occupied at all times
2. Bands that have moderate Traffic whose occupancy is high during certain time intervals
3. Bands that have very low traffic as a result has low occupancy and thus wastes the radio spectrum resource.
A Spectrum hole is a band of frequency allotted to primary user which is not utilized during certain period of time and place. It is also termed as white space.  The below figure gives a rough idea of spectrum hole.
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Figure 2 Spectrum Hole/White Space 
Various organizations have proposed the standards for Cognitive Radio. IEEE 802.22 is one of them which uses TV bands as it avoids interference problems . In 2005 IEEE I900 Standards Committee was jointly established by IEEE Communication Society and IEEE Electromagnetic Compatibility Society. The work of IEEE I900 is to standardize the key issues in the fields of spectrum management, Cognitive Radio and policy defined radio systems .
Temporal spectrum holes and Spatial Spectrum holes are two types of spectrum holes. The hole that is unoccupied during the time of sensing by secondary user is called as temporal spectrum hole. It works in time domain and demands less signal processing. On other side spatial spectrum hole is geographical dependent hole that is free when sensed by secondary user. This hole is in frequency band and hence intensive signal processing is required .
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 Image From - http://www.ijser.org/paper/Comparative-analysis-of-Signal-Processing-Techniques-for-Transmitter-Detection-for-spectrum-sensing-in-cognitive-radio.html