COGNITIVE RADIOCOGNITIVE RADIO

INTRODUCTIONINTRODUCTION Cognitive radio is a paradigm for wireless communication in which either a network or a wireless node

changes its transmission or reception parameters to communicate efficiently avoiding interference with licensed or

unlicensed users. This alteration of parameters is based on the active monitoring of several factors in the

external and internal radio environment, such as radio frequency spectrum, user behavior and network

state.

ARCHITECTURE OF CRARCHITECTURE OF CR

A. Physical Layer A. Physical Layer Functions Functions

1) Spectrum Sensing: The main function of the physical layer is to sense the spectrum over all available degrees of freedom (time, frequency and space) in order to identify sub-channels currently available for transmission.

2) Channel Estimation 2) Channel Estimation

In order to set up the link, channel sounding is used to estimate the quality of sub-channels between SUs that want to communicate. The transmission parameters (transmit power, bit rate, coding, etc.) are determined based on the channel sounding results

3) Data Transmission 3) Data Transmission

CR’s optimally uses the available spectrum as determined by the spectrum sensing and channel estimation functions. Therefore it should have the ability to operate at variable symbol rates, modulation formats (e.g. low to high order QAM), different channel coding schemes, power levels and be able to use multiple antennas for interference nulling, capacity increase (MIMO) or range extension

B. Link Layer FunctionsB. Link Layer FunctionsGroup Management It is assumed that any secondary station will belong to

a SU Group. A newly arriving user can either join one of the existing groups or create a new one through the Universal Control Channel.

Link Management covers the setup of a link in order to enable the

communication between two SUs and afterwards the maintenance of this SU Link for the duration of the communication.

Medium Access Control As long as it can be assured that all Sub-Channels are used

exclusively, i.e. all Sub-Channels used by one SU Link cannot be used by any other SU Link this problem comes down to a simple token-passing algorithm ensuring that only one of the

two communication peers is using the link.

Types of Cognitive Types of Cognitive RadioRadio

Depending on the set of parameters taken into account in deciding on transmission and reception changes, we can distinguish certain types of cognitive radio.

The main two are: Full cognitive radio Spectrum sensing cognitive radio

(Contd.)(Contd.)depending on the parts of the

spectrum available for cognitive radio, we can distinguish:

Licensed Band Cognitive Radio (IEEE 802.22)

Unlicensed Band Cognitive Radio (IEEE 802.15)

FUNCTIONS OF CRFUNCTIONS OF CRThe main functions of Cognitive

Radios are:Spectrum Sensing : detecting the unused spectrum

and sharing it without harmful interference with other users.

(Contd.)(Contd.)Spectrum sensing techniques can be

classified into three categories: ◦Transmitter detection: cognitive radios must have the capability to determine if a signal from a primary transmitter is locally present in a certain spectrum, there are several approaches proposed: matched filter detection energy detection cyclostationary feature detection

Match filter detectionMatch filter detection

is obtained by correlating a known signal, or template, with an unknown signal to detect the presence of the template in the unknown signal.

(Contd.)(Contd.)

◦Cooperative detection: refers to spectrum sensing methods where information from multiple Cognitive radio users are incorporated for primary user detection.

(Contd.)(Contd.)Spectrum Management:

Capturing the best available spectrum to meet user communication requirements

management functions can be classified as: ◦spectrum analysis ◦spectrum decision

(Contd.)(Contd.) Spectrum Mobility: is defined as the

process when a cognitive radio user exchanges its frequency of operation. Cognitive radio networks target to use the spectrum in a dynamic manner by allowing the radio terminals to operate in the best available frequency band, maintaining seamless communication requirements during the transition to better spectrum .

(Contd.)(Contd.)Spectrum Pooling :is a resource

sharing strategy that organizes the available spectrum into a spectrum pool which is then optimized for a given application . Once a primary user appears, secondary users need to cease transmission if they will cause interference.

Cognitive radio networkCognitive radio networkThe cognitive radio network is an

intelligent multi user wireless communication system that embodies the following list of primary tasks:

•to perceive the radio environment (i.e., outside world) by empowering each user’s receiver to sense the environment on a continuous-time basis

•to learn from the environment and adapt the performance of each transceiver

(Contd.)(Contd.)•to facilitate communication

between multiple users through cooperation in a self-organized manner

•to control the communication processes among competing users through the proper allocation of available resources

Software defined radioSoftware defined radioSoftware-Defined Radio (SDR)

system is a radio communication system where components that have typically been implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors. etc.) are instead implemented using software on a personal computer or other embedded computing devices.

(Contd.)(Contd.)A basic SDR may consist of a computer

(PC) equipped with a sound card, or other analog-to-digital converter, preceded by some form of RF front end. Significant amounts of signal processing are handed over to the general purpose processor, rather than done using special-purpose hardware. Such a design produces a radio that can receive and transmit a different form of radio protocol (sometimes referred to as a waveform) just by running different software. It is the enabler of Cognitive radio.

Functions provided by Functions provided by SDRsSDRs1. The radio hardware. The radio

hardware includes radio frequency circuitry and signal processing devices.

2. Software modules. Software modules represent code that has been loaded into field programmable gate arrays (FPGAs), digital signal processors (DSPs), or embedded general purpose processors.

(Contd.)(Contd.)3. Middleware. The middleware

layer attempts to reduce the details of specific devices and software modules to common abstractions.

4. Device Manager: The device manager loads radio configurations into the hardware components and sets-up the logical radios.

(Contd.)(Contd.)

5. Logical Radio Layer: Depending on the radio configuration, the hardware and software can be programmed to act like multiple radio links .

6. Module Libraries: The module libraries are collections of radio functions.

(Contd.)(Contd.)7. Rules Engine and Policies:

Policies are used to limit the operation of the radio due to regulatory, geographical, or physical constraints.

8. Smart Controller: A “smart controller” manages all of the radio resources outlined above.

ISSUES IN CRISSUES IN CRSpectrum managementSpectrum utilization: presence of white spaces

Spectral co-existence Spectrum sharing

Spectrum managementSpectrum managementspectrum management involves

assigning particular frequencies to specified users. This can be done through administrative methods or by means of a market process, such as an auction. Additionally, some spectrum may be reserved for unlicensed use (this is sometimes referred to as the spectrum commons). All users satisfying certain restrictions, for example on power levels, might have access to unlicensed bands.

Spectral utilizationSpectral utilizationwhite spaces refer to

frequencies allocated to a broadcasting service but not used locally

unlicensed devices that can guarantee that they will not interfere with assigned broadcasts can use the empty white spaces in spectrum

Spectral coexistenceSpectral coexistenceTwo approaches have been given:Collaborative approach: radio of

different technologies exchange information regarding the frequency usage of the spectrum.

Non collaborative: radio devices sense the frequency spectrum occupancy and determine by itself the channel definition without communicating with other users.

Spectral sharingSpectral sharingStringent spectrum sensing

requirements◦Various types of primary users◦Low SNR environment ◦Fast wideband sensing◦Robust sensing (to noise uncertainty, interference …)

Can the acquired on/off status be used to protect primary users?◦Channel asymmetry ◦Shadowing, hidden terminal issue◦Transmission power asymmetry

CONCLUSIONCONCLUSIONThere are opportunities to explore

“spectrum white spaces”To promote white space reuse

◦technically, though challenging, effective solutions are needed to protect the primary users

◦economically, good business models are needed to make both primary users and secondary users happy!

(Contd.)(Contd.)Cognitive radio -> cognitive radio

networks◦ How to acquire cognition

PHY sensing Network layer traffic sensing

◦ Cognitive MAC: Control information sharing, cooperation, …

◦ Distributed processing◦ Other issues (security, trust, …)

Let us work together to make cognitive radio (networks) from imagination to reality!

(Contd.)(Contd.)Cognitive radio can be used in

the following fields:Signal processingCommunication theoryRadar systemsControl theory

bibliographybibliography

Wikipedia.orgIEEE xplorewww.technologyreview.comwww.cognitiveradio.wireless.vt.e

duIEEE papers

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