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The spectrum scarcity and its allotment in the existing circum stances do not fulfill the aut horized clai m for new broadband technologies. It also confine and deny various companies to build up something inno vative such as producing new wireless services with more spectrum requirement. Such problems guide us entering into a new era of allowing unlicensed users to access licensed spectrum know as the Dynamic spectrum access initially introduced in by DARPA Defense Advanced Res earch Projects Ag ency .
A conclusion has been reached by cautious sp eculations that radio spectrum that we have in the present, will be inadequate soon and the MHz frequency for European G spect rum with billion dollar pri ce is an obvious confi rmation to the claim. FCC spectrum Policy Task Force with their inspection and measurement in different countries and cities have seen that the exploration they carried for the frequency band measurement substant iate the statement that more often than not, idle spectrum is re sulted when spectrum is allocated for various services . The idle spectrum shows the deficie ncy of spectrum but this is because a st atic policy is use for ma naging the spectrum, meaning that the real problem is not because there is not enough physical frequencies but is, because the spectrum usage in not proper.
Realizing the dilemma the researchers started working on innovative spectrum management schemes that may result in a proficient utilization of spectrum. This resulted in number of new techniques and so a famous scheme came into origin called Dynamic Spectrum Access which is far more competent and more diverse from the current static spectrum policy. It has three more models to go with it. The hierarchical model of the Dynamic spectrum access is given in fig:2  below:
Figure 2: Dynamic spectrum access, hierarchical model .
2.1 Hierarchical Model for Spectrum Access
The hierarchical makeup of this model helps to split the users into two categories explicitly Primary user which are licensed users and have high priority and Secondary user which are unlicensed, the licensed spectrum is open for access, for the secondary unlicensed users . This helps the secondary users to take maxim um utilization of the spectrum but only if there is no primary licensed user using the spectrum at that instance or that band has limited intervention, because the secondary users should not hamper with the primary users in that ar ea, allowing the primary system to operate normally. The model is further divided into two approaches .
2.1.1 Ultra Wideband Spectrum Underlay
Transmission pow er of the secondary user plays an important part in this approach. This scheme works by applying restriction on the power of the secondary user that allows it to transmit, so it means that for secondary users the noise level of primary user is the crite ria and they must do their operation below it . If the transmitted signal is spread over a wide frequency band then to keep away from interference, the secondary users communicates only through small rang es but an improvement is that they can also achieve hig h data rate by using Ultra Wide Band for the transmitted signal .
Since the secondary user in this approach are not confine to sense the spectrum for primary user signal, thus elim inating added burden on secondary user to initial detect the primary user signal using diverse techniques for detection .
Considering a supposition, that primary user will all the time be trans mitting on all probable fre quencies accessible, then to detect white spaces for usage and exploit the spectrum, it is very difficult to achieve .
2.1.2 Opportunistic Access or Spectrum Overlay
Mitola  introduced this approach using a term spectrum spooling and is considered to be one of the most well-known, of all the approaches. DARPA also studied it in prog ram for Next Gene ration and gave it the name Oppor tunistic Spectrum Access . In this approach the secondary unlicensed user can work with any normal power since not having any restraint on its trans mission power. The secondary user initially performs the discovery of the primary user signal on the licensed band on a particular region of interest but at the same time the secondary user must stay away from any interference to the primary user signal, if there is any signal det ected during the course of detection. If there is no signal detected then the secondary user can transmit till the primary user signal is detected, and at that instance the secondary user must depart the spectrum almost im mediately in order to stay away from intervention and for this, the secondary user must have competent signal detect ion techniques . Because of this approach it is compared to be well-suited and more com patible with the offered spectrum policies for wireless system and research is going on in this area to put into practice this approach and improve it more.
2.2 Exclusive Use Model for Dynamic Access
Even though similar in stature to that of existing spectrum policy, still there are various innovative features introduced in the model which makes it rather more flexible as compared to the aged static model. In this model licensing bands are introduced for explicit services and exclusive use  and this model also have further two approaches.
2.2.1 Spectrum Property Rights
The property rights for spect rum use was originally initiated in a sem inal paper publishes in by Ronald Coase  and Arthur De Vany , after some time then Law erence White also mentioned three parameters for the spectrum right policy including time, geographic area and spectrum band. In this scheme the licensees who are allotted the spectrum, are free to trade the spectrum, and they also can decide whichever technology they want to deal with, which allows the spectrum to be used to its best potential .
The model with this approach though must deal with some tribulations that may hinder its perfor mance, the first problem is that defining spectrum rights for the user to use are very difficult to put into pract ice because of channel inte rferences being adjoining and also because radio wave propagation are extremely erratic and unpredictable in both fr equency and space . The other problem that this model faces is that this model does not eliminates the whit e s paces in the spectrum by not entirely utilizing the spect rum and it is because in dig ital wireless communications data are send in small but su dden periods. From the management position the model faces an additional problem which is the regulation authorities, since they does not support the spectrum sharing, despite the fact that it is profit rewarding ,.
2.2.2 Spectrum Allotment
Being best fitting with the comm ercial applications such as UMTS and DVB-T dynamic exclusive use model with dy namic spect rum allotment approach also tries to enhance the spectrum competency in time and s pace-dep endent spectrum sharing, between radio se rvices that are synchro nized . The approach was brought by the European D RIVE project  because it assigns spect rum on tem poral and statistics of traffic dynamically. The major thought behind the app roach is that if a service requires extra bandwidth and have limited bandwidth, then it is allotted require bandwidth for certain time, along with a timer and after that time period as the timer expires, the extra bandwidth is then not permissible to the service but the one which was allotted to it in the start is still allowable .
This approach as compared to dynamic spectrum allocation has an added advantage that is, the fre quency allotment is this approach is incre dibly rapid and as compared to the stat ionary static approaches. This approach also provides f lexibility by minimizing the spectrum depletion to an immense amount however the approach also faces the same problem of white spaces which is desirable to be resolved .
2.3 Open Sharing Model
This is a diverse model that has been developed, also known as â€œSpectrum Commons Approachâ€Â bearing in mind that it helps the spectrum to be considered open for sharing and is used for s pectral reg ion manag ement, in addition to this, also in this approach the user owns no f raction of the spect rum by considering it as its property. Being a mo tivation of the ISM (Indust rial, Scie ntific and Medical) it is considered extremely prof icient, however still the approach does not addresses or deals with all the band admission issues and has some inadequacy to be solved .
2.4 Cognitive Radio
Software Defined Radio and Cognitive Radio exp ression were first put forward by Mitola in a seminar at The Royal Instit ute of Technology KTH, in and then published by Mitola and Ge rald Q in an article .
Sof tware Defined Radio being a multi band radio can hold mult iple protocols and air interfaces, it is reconfigured through software being a radio and have all the signal proper ties such as carrier fre quency, modulat ion type, sig nal bandwidth integrated in it. The net works entrance is distinct by software and modern SDR can also initiate features such as cryp tography, forward error cor rection coding, and source coding of voice, video and data additionally . A Cognitive Radio is im plemented on the basis of Sof tware Defined Radio and is co nsidered as an intelligent system, it can reconfigure itself according to the surroundings and also experts to new environment. The three important proper ties that make an SDR in CR are given as:
How can the spect rum be supe rvise and utilized to its complete potential
Wireless network interface, to construct use of network resources
An interface for human dealings so that they can get complete use of the resources.
The Dynamic Spectrum Access is an important part of the Cognitive Radio   and it uses Spect rum Overlay scheme.
Opport unistic spectrum access reduces the depletion of spectrum as compared to the contem porary rigid static spectrum supervision policy and the cognitive radio technology take advantage of the same opportunistic spectrum approach . Features and capabilities such as flexibility, agility, radio frequency, and networking are implemented by cognitive radio technology for enabling the use of spectrum .
Flexibility of this technology changes the configuration and waveform of a signal so that it can be used for two dive rse roles. Using the fle xibility feature, the radio reflect characteristics such as data range, latency, and packet error rate and extra feature such as agility attribute  which allows cognitive radio to modify the spectral band in which a device will mane uver to operate.
RF sensing is its skill to exam ine the system status linking both the radio and the surrounding and helps to evaluate its im pact and effect on the sur rounding  while networking is the capability of cognit ive radio to converse with various nodes and thus facilitate merge sensing .
2.4.2 Capability and Re-configurability of Cognitive Radio
It is that attribute of the cognitive radio which allows it to have authentic interactions with the surrou nding of the radio environment and assist it to access the vacant seg ment of the spect rum and it has the following three features .
Sensing and identifying White spaces: A cognitive radio can sense spectrum for white spaces which are fr equency bands either unemployed by licensed users or having par tial intervention .
Sharing the Band: A license involving two parties, permit the spectrum to be used and depending on the agreement the spectrum can be used in real time also as compared to makeshift basis .
Identifying the location: The ability to recognize other transmitter site for decent selection of parameters for instance pow er and freque ncy in that particular location .
The uniqueness of a cognitive radio to transmit on diverse frequencies and facilitating variety of acc ess techniques make it the first choice for the current circumstances .
2.4.3 Architecture of Primary Network
Known as the existing network the users of this network are called primary user and they have licensed to employ the frequency band for their operation having high priority and such a network have a Band manager to be in charge of the network operation ,.
2.4.4 Architecture of Secondary Network
Also called Cog nitive Radio Network, such a network have each and every one as unlicensed users identified as secondary user, furthermore the secondary user uses the primary licensed user frequency band for their operation but must also tries to steer clear of any intrusion with the users whose frequency band, it is trying to use.
In some cases the Secondary Network also have common secondary user substitute as a Secondary Base Station called Cognitive Radio Base Station (CRBS) or Band manager which helps the spectrum resources to be scattered among various Secondary Network users .
2.5 Cognitive Radio Network Operation
The Cognitive Radio Network can access equally the primary licensed network and unlicensed fraction of spectrum. Based on the operation that Cognitive Radio Network can carry out it can be categ orized in two subsequent types .
2.5.1 Operation on the Licensed Band
In this method the secondary user initially spot the primary user signal since they are the licensed user that can employ the licensed band plus have more priority than the secondary user, and if no primary user signal is detected the secondary user then uses the white spaces of the band for their operation . However during their processing if they notice the presence of the primary user signal the band must be emptied to stay away from any kind of clash. This type of operation requires a very cautious consideration on the secondary user side. In some cases if the interference is partial then base on the network operation for a while the secondary users are permitted to use the band .
2.5.2 Operation on Unlicensed Band
In unlicensed band operation secondary users require not to be concerned regarding the prim ary user signal and obst ruction with it, as the secondary user in the network has the same rig hts to employ the spect rum for their operation and there is no primary user. The secondary user sense to avoid interfering with other secondary users and then the spectrum is employed, also dive rse access methods are used by the cognitive radio network for communication among different users .
Cognitive radio can resolve numerous harms of spectrum depletion and a vital component of this technology is the spectrum sensing as the performance of Cognitive Radio Networks entirely depends on how perfectly has the band for spectrum access have been detected. The fig:3 below gives an idea of how the occupied band have some w hite spaces that need to be utilize by secondary users without having hindrance with primary user signal.