Defining cognitive radio can be a very difficult and a controversial task, through the term Cognitive Radio was firstly described by Joseph Mitola. From his description we can define the Cognitive Radio as a radio capable of analyzing the environment such as channels and users, learning and predicting the most suitable and efficient way of using the available spectrum and adapting all of its operation parameter, for example carrier frequency, transmission power, and modulation strategy in real time [1-3]. The main reason for introducing the cognitive radio is the apparent scarcity of the radio spectrum due to the static allocation of radio spectrum by FCC.
Cognitive radio is an enhancement of the traditional software radio concept because the radio
is aware of its environment and its capabilities,
able to alter the behavior of physical layer independently,
capable of following complex adaptation strategies by learning from previous experiences and dealing with situations that are not planned at the preliminary design,
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intelligently optimize their own performance in response to user requests, and
in conformity with FCC rules by sensing, adaptation and learning.
Cognitive radio can be modeled for wireless communication in which either a wireless node or particular network changes its transmission or reception parameters, to communicate efficiently and yet preventing interference with licensed or unlicensed users. The changing of parameters is based on active monitoring of several factors obtained from external and internal radio environment, for example user behavior, radio spectrum and network state .
Therefore, cognitive radio , ,  is a new promising technology of wireless communication that allows an unlicensed user (SU) can intelligently detect which of the communication channels are busy and which are idle, and immediately change to other vacant channels while avoiding occupied ones. Thus, it not only optimizes the use of available radio spectrum but also minimizing interference to other users.
2.1.1 Types of Cognitive Radio
The set parameters that can be taken into account in deciding on transmission and reception changes, and for historical reasons, cognitive radio can be distinguished into certain types. The main two types are:
Spectrum Sensing Cognitive Radio: The frequency spectrum parameter is only considered.
Full Cognitive Radio: Every possible parameter that is observable by a wireless network can be taken into account.
Also depending on parts of the spectrum available of cognitive radio, we can distinguish:
Licensed Band Cognitive Radio: The cognitive radio is capable of utilizing the bands that assigned to the licensed users apart from the unlicensed frequency bands, such as U-NII band or the ISM band.
Unlicensed Band Cognitive Radio: The cognitive radio can only operate in the unlicensed part of radio spectrum. One such of the system is described in IEEE 802.15 Task Group 2 specifications , which focus on the coexistence between the IEEE 802.11 and Bluetooth.
2.1.2 Functions of Cognitive Radio
Recently most of the research works are based on the spectrum sensing cognitive radio. However, we are focusing on the spectrum mobility function in this thesis. The main goal of cognitive radio is to enhance the spectrum management system. The execution of cognitive radio system is based on following four main functions:
Spectrum sensing: Priority of the licensed user (PU) comes first in the cognitive radio concept. Thus, the detection of the spectrum holes is one of the most important requirements for cognitive radio. SU will detect the unused spectrum and share it without causing interference to PU. The detection of the PU signal is the most efficient way to detect the spectrum holes. The spectrum sensing techniques include transmitter detection, interference based detection, and cooperative detection.
Spectrum management: Cognitive radios should find out the best spectrum available spectrum bands to meet the QoS requirement. Therefore, spectrum management function is required for cognitive radio network. The function includes spectrum analysis and followed by spectrum decision to select the band according to user requirements.
Spectrum mobility: The cognitive radio networks are designed to utilize the spectrum in a dynamic manner by allowing the users to operate in a best available frequency band. The spectrum mobility occurs when the SU needs to switch to another frequency upon detection of PU signal. It helps to maintain seamless communication requirements during the spectrum transition process.
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Spectrum sharing: Once a cognitive radio knows its transmitting frequency, it informs its receiver about the band chosen. Besides that, a fair spectrum scheduling method is to be provided.
Other functions of cognitive radio that include the ability of a transceiver to:
encrypt or decrypt signals,
identify and authorize its user,
determine its geographic location,
sense neighboring wireless devices in operation, and
adjust output power and its modulation characteristics.
Figure 2.1 shows the architecture of a cognitive radio that described a cognitive function to analyze existing spectrum users, and measure properties of its own communication channels and a set of rules expressed through the policy engine which define what the radio is allowed to do, and what it is not allowed to do .
Figure 2.1: Architecture of a Cognitive Radio 
The cognitive radios can be explained as machines that can sense their environment (radio spectrum) and respond intelligently to it. Just like animals and people, they allow to
seek their own kind (other radios which they want to communicate),
find a place to live or stay (usable spectrum),
avoid or outwit enemies (interfering radios),
make a living (deliver the services that their user wants),
conform to the etiquette of their society (the regulation agency, e.g. FCC), and
deal with completely new situations and learn from experience.
2.1.4 Cognitive Radio Advantages
Cognitive radio is a powerful tool for solving two major problems:
With spectrum access: Finding an open frequency and using it.
With Interoperability: Talking to legacy radios using a variety of incompatible waveforms.
Other benefits include:
improved link reliability
stay away from bad channels
improved spectrum utilization and efficiency of the radio spectrum
increase data rate on goods channels
stay away from over occupied radio spectrum
significant interest from FCC, DoD
possible use in TV band reframing, high speed internet in the rural areas and high data rate application networks, for example video conferencing.
Cognitive Tasks and Cognitive Cycle
Analysis & Modeling
(Spectrum Sensing, Estimation, Predictionââ‚¬Â¦)The tasks of the cognitive radio are described by the cognitive cycle as shown in figure 2.2 below.
(Transmit power control, dynamic spectrum managementââ‚¬Â¦)
Figure 2.2 Basic Cognitive Cycle
The fundamental tasks of cognitive radio can generally be categorized in the following three types: Analysis, Predictive Modeling and Decision Making. Analysis and predictive modeling are carried out in the receiver, whereas decision making is carried in the transmitter. It should be noted that the transmitter and receiver should work in harmony all the times. In order for that to happen, we must need a feedback channel connected from the receiver to the transmitter. The receiver is allowed to pass information about the performance of forward link back to the transmitter through the feedback channel. Therefore, the cognitive radio is an example of a feedback communication system.
Cognitive Tasks 1: Analysis
Cognitive radio should have the intelligence to observe the surrounding environment and analyze its characteristics. Obtaining those characteristics is the first step for cognition. Some examples for these kinds of task and their explanation will be introduced.
Spectrum sensing is the main task in the analysis part. It can be defined as studying the spectrum and find the unused channels. Some terminologies need to be introduced to further discuss the spectrum sensing.
Interference Temperature: It is a measure of the sensed power in a certain frequency band. Therefore, by obtaining this measure, two important limits: maximum level where any signal exceeds it will be, and minimum level where any signal below it can be neglected and thus that certain band can be considered as empty or unoccupied, and can be used by other users.
Spectrum Holes: It is a frequency band which is free enough to be used and finding the spectrum holes is the main goal of the spectrum sensing.
By the above definitions, the spectrum sensing operation can be performed in two stages: estimation of the interference temperature and detection of spectrum holes. Both stages are performed periodically. The interference temperature is suggested to be estimated for the whole targeted frequency ranges. Then depending on the current interference and the interference temperature on the previous iterations, all channels can be classified into three types of spectrum holes:
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White spectrum holes, are not fully used
Gray spectrum holes, are partially used
Black spectrum holes, are fully used
After the sensing operation finishes, the users will be allowed to use the white holes freely and partially use the gray holes in such a way that it will not disturb the PU. However, they are not allowed to use the black holes, because the black holes are assumed to be fully used and any additional use will cause interference to the ongoing communication of PU.
Basically, there are two sensing modes, reactive sensing and proactive sensing that depending on the way to initiate the sensing. Each one of those two modes has its advantages and disadvantages, thus both of them might be used depending on the application and the environment conditions.
Reactive sensing: It is initiated only when the user has data to send, therefore it is also called on-demand sensing. If all channels were occupied, the user will wait for a predefined time and restart sensing again until the user sends all data that he was trying to send.
Advantage: The sensing overhead is reduced.
Disadvantage: The data is delayed until the sensing is performed with a good accuracy.
Proactive sensing: It is done periodically even when the user is not intending to send any data. The time between the sensing iteration is called the sensing period. These sensing periods may differ between the channels since each channel has its own unique behavior. The sensing period should be optimized separately for each channel to compensate for the unique traffic pattern on that channel.
Advantage: The delay is reduced since the users will know the holes even before they need them.
Disadvantage: Large amount of time and effort is wasted on sensing even when it is not needed, thus increasing the sensing overhead.
Channel estimation was also considered to be a part of the cognitive radio. This operation aims in analyzing the channel behavior and its effects on the transmitted signal and estimating the impulse response of the channel. By knowing the channel impulse response, its effects can be neutralized on the receiver by using an equalizer or on the transmitter by transmitting a signal that can absorb those effects.
Cognitive Tasks 2: Predictive Modeling
Due to the dynamic behavior of the communication channels and environment, analyzing only the current channel and using the results directly to select a free channel or to equalize the channel might leads to an inefficient use of the resources.
Therefore, an improvement to develop the way to use the knowledge obtained from the normal analysis result is the predictive modeling. It aims on finding models that predicts the behavior of the channels on the future and even the traffic patterns. Those models currently used will raise the efficiency of using the analysis results, and will improve and ease the decision making procedures.
The predictive modeling uses the current observations along with the previous observations and based on some statistical measures, it tries to find the model that will most likely suits the channel or the traffic in the near future.
Basically, prediction implies the possibility for some errors. However, it will significantly improve the performance of the system to an extent where those errors can be neglected.
Cognitive Tasks 3: Decision Making
Decision making is the core of the cognitive radio, because the task of cognitive radio is to decide the best configuration for both transmitter and receiver intelligently. Many tasks require intelligent and fast decisions to be made by the radios in cognitive radio system. The operation of finding the best decisions can be considered a sort of optimization with variable complexity depending on the task nature. Some examples of decision making include dynamic spectrum management and transmit power control.
2.3 WiMAX standard