With the development of CR technologies, secondary users who are not authorized with spectrum usage rights can utilize the temporally unused licensed frequency bands owned by the primary users. Therefore, in a CR network architecture, the components include both a secondary network and a primary network, as shown in Figure 4.

A Secondary Network refers to a network composed of a set of secondary users with/without a secondary Base Station. Secondary users can only access the licensed spectrum when it is not occupied by a primary user. The opportunistic spectrum access of secondary users is usually coordinated by a secondary Base Station, which is a fixed infrastructure component serving as a hub of the secondary network. Both secondary users and secondary Base Stations are equipped with CR functions. If several secondary networks share one common spectrum band, their spectrum usage may be coordinated by a central network entity, called spectrum broker. The spectrum broker collects operation information from each secondary network, and allocates the network resources to achieve efficient and fair spectrum sharing.

A Primary Network is composed of a set of primary users and one or more primary Base Stations. Primary users are authorized to use certain licensed spectrum bands under the coordination of primary Base Stations. Their transmission should not be interfered by secondary networks. Primary users and primary Base Stations are in general not equipped with CR functions. Therefore if a secondary network share a licensed spectrum band with a primary network, besides detecting the spectrum white space and utilizing the best spectrum band, the secondary network is required to immediately detect the presence of a primary user and direct the secondary transmission to another available band so as to avoid interfering with primary transmission.

Cognitive Transceiver Architecture: The basic structure of a cognitive transceiver is illustrated in Figure 5. It consists of three parts: Radio Frequency (RF), Analog to Digital Converter (ADC), and baseband processing. Depending on the particular type of cognitive radio, one or more of those components is made adaptive. In a spectrum-sensing cognitive radio, only the RF front end is adaptive and different from a conventional RX. For a fully cognitive radio, the baseband processing also has to be adaptive. This can be most easily achieved by implementing the baseband processing as software on a Digital Signal Processor (DSP).

Consequently, fully cognitive radio has also been called software radio by many of its proponents.

Figure 6 shows a typical RF RX front end structure. This front end has to be adaptive to provide CR functionality. It consists of The receiver (RX) filter, The Low Noise Amplifier (LNA), The Local Oscillator, The RX down converter, The RX low pass filter, The Automatic Gain Control (AGC) amplifier and RX Analog to Digital Converter (ADC). The function of various components are described as follows,

The RX Filter: It performs a rough selection of the received band. The bandwidth of the filter is tunable corresponds to the bandwidth of the received signal. The out of band signals are rejected to avoid saturation of Low Noise Amplifier.

The Low-Noise Amplifier (LNA): It amplifies the signal received by RX filter, so that the noise added by later components of the RX chain has less effect on the Signal-to-Noise Ratio (SNR). Further amplification occurs in the subsequent steps of down conversion.

The Local Oscillator (LO): It provides sinusoidal signals corresponding to signals received by RX filter to perform down conversion so as to get desired signal by CR. The frequency of the LO can be fine-tuned by using VCO to make sure that the LO produces the signal, which after down conversion gives the correct detected base band signal.

The RX Down Converter: It converts the received signal (in one or several steps) into baseband. In baseband, the signal is available as a complex analog signal.

The RX Low-Pass(Base band) Filter: It provides a selection of desired frequency bands for one specific user (in contrast to the RX bandpass filter that selects the frequency range in which the service operates). It eliminates adjacent channel interference as well as noise. This filter should affect the desired signal as little as possible.

The Automatic Gain Control (AGC): It amplifies the signal such that its level is well adjusted to the quantization at the subsequent ADC.

The RX ADC: It converts the analog signal into values that are discrete in time and amplitude. The required resolution of the ADC is determined essentially by the dynamics of the subsequent signal processing. The sampling rate is of limited importance as long as the conditions of the sampling theorem are fulfilled. Oversampling increases the requirements for the ADC, but simplifies subsequent signal processing.

The main requirements of this structure are,

Wide Band Operation: The components of RF RX front end i.e. antennas, Low Noise Amplifier (LNA), local oscillator, and automatic gain control, all have to be wideband enough so that they can operate at all possible frequencies at which the cognitive radio operates.

Tunable : The RF front end must be able to select the channel which the cognitive radio wants to use. This can be achieved by an adjustable local oscillator (e.g., a Voltage Controlled Oscillator, VCO )

Interference Rejection: Since the channel selection occurs only after the down converter (mixer), a major challenge in cognitive radio lies in the possibility of RX saturation through strong out-of-band signals. RF components like the LNA can be saturated by a signal that is not in the band that the RX wants to demodulate at a specific time, but still within the overall receive band of the cognitive radios. Such strong interference can be reduced by tunable RF notch filters that can be placed before the LNA. However, the hardware cost of such filters is high, and the tunability is rather limited.

Since CRs are able to sense, detect, monitor the surrounding RF environment and reconfigure their own operating characteristics according to situations, cognitive communications increase spectrum efficiency and support higher bandwidth service. Moreover, the burdens of centralized spectrum management can be reduced by efficient spectrum sharing. However, one can think why primary users allow secondary users to use their spectrum, for which they are already paying. The reasons for this could be,

• Profit: Using CR technology, Spectrum owners might be able to charge secondary users in real time for a particular frequency band used for a short time to make profit. For this auction algorithms have been proposed in the literature. This is useful for some applications, but might not always be practical as the costs for monitoring and billing could become higher than the revenue from the auctioning of the spectrum.

• Regulatory Requirements: The frequency regulator can mandate that a certain spectrum range can be used by cognitive devices as long as they do not interfere with primary users. Such an approach can be used for that part of the spectrum that primary users never pay for example, TV. In the U.S.A., as well as in many other countries, TV stations did not buy the spectrum they use, but rather got it for free because they are deemed to perform a public service. This makes it easy for the frequency regulator to demand that TV stations coexist with other services “in public interest.”

• Emergency Services: Another form of cognitive radio occurs in times of emergency, when services that normally count as “primary users” have to give up spectrum for emergency services like natural disaster or terrorist attacks.

Based on these facts, CRs can be employed in many applications such as military communications, public safety and security, support for existing wireless technologies and many more.

Applications of Cognitive Radio:

-As, the capacity of military communications is limited by radio spectrum scarcity. It is due to static frequency assignments in many unused applications, where a large amount of spectrum is idle. Using CR concept bandwidth crunch problem can be solved by allocating bandwidth as per need and application. Therefore, CR can provide military an adaptive, seamless, and secure communications.

-A CR network can also be used to provide public safety and security. A natural disaster or terrorist attack can destroy existing communication infrastructure. To provide search and rescue operations, an emergency network is required. CR networks can play an important role to provide communication as they have capability to detect unused spectrum and reconfigure itself to provide service without delay. Thus it provides public safety and security with dynamic spectrum selection.

-Next, CR networks can also provide commercial markets for existing wireless technologies. Since CR can intelligently determine which communication channels are available and which are in use, can automatically switches to an unused or unoccupied channel. Thus provides additional bandwidth for rapidly growing data applications. Also this adaptive channel allocation avoids the need of expensive redeployment.

Over the last couple of years, the CR techniques have been the subject of research and standardization. IEEE802.22 working group has developed an air interface (PHY & MAC) based on unlicensed operation in TV broadcast bands without causing harmful interference to primary users (TV receivers) of this band. In addition to the TV service, other services such as wireless microphones are also allowed to operate on vacant TV channels on the basis of non interfering basis. The key parameters of 802.22 are summarized in Table 10.1.