Cognitive Radio Networks

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1. ?Introduction ?
Recent technological advances have resulted in the development of wireless ad hoc networks composed of devices that are self-organizing and can be deployed without infrastructure support. These devices generally have small form factors, and have embedded storage, processing and communication ability.
While ad hoc networks may support different wireless standards, the current state-of the- art has been mostly limited to their operations in the 900 MHz and the 2.4 GHz industrial, scientific and medical (ISM) bands. With the growing proliferation of wireless devices, these bands are increasingly getting congested.
At the same time, there are several frequency bands licensed to operators, such as in the 400?700 MHz range, that are used sporadically or under-utilized for transmission.

The licensing of the wireless spectrum is currently undertaken on a longterm basis over vast geographical regions. In order to address the critical problem of spectrum scarcity, the FCC has recently approved the use of unlicensed devices in licensed bands. Consequently, dynamic spectrum access (DSA) techniques are proposed to solve these current spectrum inefficiency problems. This new area of research foresees the development of cognitive radio (CR) networks to further improve spectrum efficiency. The basic idea of CR networks is that the unlicensed devices (also called cognitive radio users or secondary users) need to vacate the band once the licensed device (also known as a primary user) is detected. CR networks, however, impose unique challenges due to the high fluctuation in the available spectrum as well as diverse quality of- service (QoS) requirements. Specifically, in CR ad - hoc networks (CRAHNs), the distributed multi-hop architecture, the dynamic network topology, and the time and location varying spectrum availability are some of the key distinguishing factors. These challenges necessitate novel design techniques that simultaneously address a wide range of communication problems spanning several layers of the protocol stack.

Cognitive radio technology is the key technology that enables a CRAHN to use spectrum in a dynamic manner. The term, cognitive radio, can formally be defined as follows:
A ??Cognitive Radio? is a radio that can change its transmitter parameters based on interaction with the environment in which it operates. From this definition, two main characteristics of the cognitive radio can be defined as follows:

Cognitive? capability :

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Cognitive?Radio ?Features?

The idea of a cognitive radio extends the concepts of a hardware radio and a software defined radio (SDR) from a simple, single function device to a radio that senses and reacts to its operating environment. A Cognitive Radio incorporates multiple sources of information, determines its current operating settings, and collaborates with other cognitive radios in a wireless network. The promise of cognitive radios is improved use of spectrum resources, reduced engineering and planning time, and adaptation to current operating conditions.

Some features of cognitive radios include:

Sensing the current radio frequency spectrum environment:
Policy and configuration databases :
Mission-oriented configuration:
Adaptive algorithms :
Distributed collaboration :
Security :

Defining ? ?Cognitive ?Radio??
Tautologically, a cognitive radio could be defined as ?A radio that is cognitive,?

In the 1999 paper that first coined the term ?cognitive radio?, Joseph Mitola III defines a cognitive radio as [Mitola_99]: ?A radio that employs model based reasoning to achieve a specified level of competence in radio-related domains.?

However, in his recent popularly cited paper that surveyed the state of cognitive radio, Simon Haykin defines a cognitive radio as [Haykin_05]: ? An intelligent wireless communication system that is aware of its surrounding environment (i.e., outside world), and us es the methodology of understanding-by-building to learn from the environment and adapt its internal states to statistical variations in the incoming RF stimuli by making corresponding changes in certain operating parameters (e.g., transmit-power, carrier frequency, and modulation strategy) in real-time, with two primary objectives in mind:

? highly reliable communications whenever and wherever needed
? efficient utilization of the radio spectrum.
? It thinks, therefore it?s a cognitive radio

?An adaptive radio that is capable of the following:

a) Awareness of its environment and its own capabilities,
b) Goal driven autonomous operation,
c) Understanding or learning how its actions impact its goal,
d) Recalling and correlating past actions, environments, and performance.?

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