A cognitive radio is a kind of two-way radio that automatically changes its transmission or reception parameters, in a way where the entire wireless communication network -- of which it is a node -- communicates efficiently, while avoiding interference with licensed or licensed exempt 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 behaviour and network state. A cognitive radio, as defined by the researchers at Virginia Polytechnic Institute and State University, is "a software defined radio with a cognitive engine brain".[1]
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The idea of cognitive radio was first presented officially by Joseph Mitola III in a seminar at KTH, The Royal Institute of Technology in Stockholm, in 1998, published later in an article by Mitola and Gerald Q. Maguire, Jr in 1999.[2] It was a novel approach in wireless communications that Mitola later described as:
The point in which wireless personal digital assistants (PDAs) and the related networks are sufficiently computationally intelligent about radio resources and related computer-to-computer communications to detect user communications needs as a function of use context, and to provide radio resources and wireless services most appropriate to those needs.[3]
It was thought of as an ideal goal towards which a software-defined radio platform should evolve: a fully reconfigurable wireless black-box that automatically changes its communication variables in response to network and user demands.
Regulatory bodies in various countries (including the Federal Communications Commission in the United States, and Ofcom in the United Kingdom) found that most of the radio frequency spectrum was inefficiently utilized.[4] For example, cellular network bands are overloaded in most parts of the world, but many other frequency bands, such as military, amateur radio and paging frequencies are not. Independent studies performed in some countries confirmed that observation,[5][6][7] and concluded that spectrum utilization depends strongly on time and place. Moreover, fixed spectrum allocation prevents rarely used frequencies (those assigned to specific services) from being used by unlicensed users, even when their transmissions would not interfere at all with the assigned service. This was the reason for allowing unlicensed users to utilize licensed bands whenever it would not cause any interference (by avoiding them whenever legitimate user presence is sensed). This paradigm for wireless communication is known as cognitive radio.
The first phone call over a cognitive radio network was made on Monday 11 January 2010 in Centre for Wireless Communications at University of Oulu using CWC's cognitive radio network CRAMNET (Cognitive Radio Assisted Mobile Ad Hoc Network), that has been developed solely by CWC researchers.,[8][9]
Depending on the set of parameters taken into account in deciding on transmission and reception changes, and for historical reasons, we can distinguish certain types of cognitive radio. The main two are:
Also, depending on the parts of the spectrum available for cognitive radio, we can distinguish:
Although cognitive radio was initially thought of as a software-defined radio extension (Full Cognitive Radio), most of the research work is currently focusing on Spectrum Sensing Cognitive Radio, particularly in the TV bands. The essential problem of Spectrum Sensing Cognitive Radio is in designing high quality spectrum sensing devices and algorithms for exchanging spectrum sensing data between nodes. It has been shown[15] that a simple energy detector cannot guarantee the accurate detection of signal presence, calling for more sophisticated spectrum sensing techniques and requiring information about spectrum sensing to be exchanged between nodes regularly. Increasing the number of cooperating sensing nodes decreases the probability of false detection.[16]
Filling free radio frequency bands adaptively using OFDMA is a possible approach. Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe proposed a spectrum pooling system[7] in which free bands sensed by nodes were immediately filled by OFDMA subbands.
Applications of Spectrum Sensing Cognitive Radio include emergency networks and WLAN higher throughput and transmission distance extensions.
Evolution of Cognitive Radio toward Cognitive Networks is under process, in which Cognitive wireless mesh network (e.g. CogMesh) is considered as one of the enabling candidates aiming at realizing this paradigm change.
The main functions of Cognitive Radios are:[17][18]
The practical implementation of spectrum management functions is a very complex and multifaceted issue in itself, given that it has to address a mixture of technical and legal requirements. An example of the former is chosing appropriate sensing threshold to detect other users, while the latter is exemplified by the need to meet the rules and regulations set out for radio spectrum access in international (ITU Radio Regulations) and national (Telecommunications Law, etc.) legislation.[19][20]
Intelligent antenna (or smart antenna) is antenna technology using spatial beamforming and spatial coding to cancel interference; however, it requires intelligent multiple or cooperative antenna array. On the other hand, cognitive radio (CR) allows user terminals to sense whether a portion of the spectrum is being used or not, in order to share the spectrum among neighbor users. The following table compares the different points between two advanced approaches for the future wireless systems: Cognitive radio (CR) vs. Intelligent antenna (IA).
| Point | Cognitive radio (CR) | Intelligent antenna (IA) |
|---|---|---|
| Principal goal | Open Spectrum Sharing | Ambient Spatial Reuse |
| Interference processing | Avoidance by spectrum sensing | Cancellation by spatial pre/post-coding |
| Key cost | Spectrum sensing and multi-band RF | Multiple or cooperative antenna arrays |
| Challenging algorithm | Spectrum management tech | Intelligent spatial beamforming/coding tech |
| Applied techniques | Cognitive Software Radio | Generalized Dirty-Paper and Wyner-Ziv coding |
| Basement approach | Orthogonal modulation | Cellular based smaller cell |
| Competitive technology | Ultra wideband for the higher band utilization | Multi-sectoring (3, 6, 9, so on) for higher spatial reuse |
| Summary | Cognitive spectrum sharing technology | Intelligent spectrum reuse technology |
In response to the operator's commands, the cognitive engine is capable of configuring the radio system parameters. These parameters include "waveform, protocol, operating frequency, and networking".[21] It functions as an autonomous unit in the communication environment and frequently exchanges information about the environment with the networks it is able to access as well as with other CRs (Wipro Technologies, 2002).[21] A CR "monitors its own performance continuously, in addition to "reading the radio's outputs"; it then uses this information to "determine the RF environment, channel conditions, link performance, etc.", and adjusts the "radio's settings to deliver the required quality of service subject to an appropriate combination of user requirements, operational limitations, and regulatory constraints". These processes have been described as "reading the radio's meters and turning the radio's knobs".[1]
CR can sense its environment and without the intervention of the user can adapt to the users communication needs while conforming to FCC rules. Conceptually, the amount of spectrum is infinite, practically for propagation and other reasons it is finite because of the desirability of certain portions of the band. Even the spectrum which is assigned is far from being 100% utilized, hence efficient use of the spectrum is a growing concern. CR offers a solution to this problem. A CR can intelligently detect whether any portion of the spectrum is in use or not, and can temporarily latch into or out of it without interfering with the transmissions of other users thereby efficiently utilizing spectrum. According to Dr.Bruce Fette (2004), "Some of the radio's other cognitive abilities include determining its location, sensing spectrum use by neighboring devices, changing frequency, adjusting output power or even altering transmission parameters and characteristics. All of these capabilities, and others yet to be realized, will provide wireless spectrum users with the ability to adapt to real-time spectrum conditions, offering regulators, licenses and the general public flexible, efficient and comprehensive use of the spectrum".[22]
The phenomenal success of the unlicensed band in accommodating a range of wireless devices and services has led the FCC to consider opening further bands for unlicensed use. In contrast, the licensed bands are underutilized due to static frequency allocation. Realizing that CR technology has the potential to exploit the inefficiently utilized licensed bands without causing interference to incumbent users; the FCC released the Notice of Proposed Rule Making to allow unlicensed radios to operate in the TV broadcast bands. The IEEE 802.22 working group formed in November/2004 is equipped with the task of defining the air interface standard for Wireless Regional Area Networks based on CR sensing for the operation of unlicensed devices in the spectrum allocated to TV service.[23]