Cognitive Radio Tutorial
- an overview or tutorial about the basics of Cognitive Radio detailing a definition, history, Cognitive radio technology, architecture and Cognitive Radio networks.
Cognitive radio (CR) is one of the new long term developments taking place and radio receiver and radio communications technology. After the Software Defined Radio (SDR) which is slowly becoming more of a reality, cognitive radio (CR) and cognitive radio technology will be the next major step forward enabling more effective radio communications systems to be developed.
The idea for cognitive radio has come out of the need to utilise the radio spectrum more efficiently, and to be able to maintain the most efficient form of communication for the prevailing conditions. By using the levels of processing that are available today, it is possible to develop a radio that is able to look at the spectrum, detect which frequencies are clear, and then implement the best form of communication for the required conditions. In this way cognitive radio technology is able to select the frequency band, the type of modulation, and power levels most suited to the requirements, prevailing conditions and the geographic regulatory requirements.
Cognitive radio definition
There are likely to be a variety of different views of what exactly what a cognitive radio may be. Accordingly a definition of a cognitive radio may be of use in a number of instances.
A cognitive radio may be defined as a radio that is aware of its environment, and the internal state and with a knowledge of these elements and any stored pre-defined objectives can make and implement decisions about its behaviour.
In general the cognitive radio may be expected to look at parameters such as channel occupancy, free channels, the type of data to be transmitted and the modulation types that may be used. It must also look at the regulatory requirements. In some instances a knowledge of geography and this may alter what it may be allowed to do.
In some instances it may be necessary to use a software defined radio, so that it can reconfigure itself to meet the achieve the optimal transmission technology for a given set of parameters. Accordingly Cognitive radio technology and software defined radio are often tightly linked.
Cognitive radio history
There have been many factors that have lead to the development of cognitive radio technology. One of the major drivers has been the steady increase in the requirement for the radio spectrum along with a drive for improved communications and speeds. In turn this has lead to initiatives to make more effective use of the spectrum, often with an associated cost dependent upon the amount of spectrum used. In addition to this there have been many instances where greater communications flexibility has been required. Along the way, there have been several significant milestones along the road to develop cognitive radio technology.
One example that exemplified the need for flexible communications occurred in the Netherlands in 2000 when a fireworks factory exploded killing 23 people, destroying much of the town and injuring more than a thousand people. While dealing with this catastrophe, the emergency services (fire, medical, police, etc) experienced real communications difficulties because they all had different communications systems and were unable to communicate with the other services.
Another major emergency was the 9-11 terrorist attacks in the USA. Again communications difficulties were experienced.
While often a variety of radios would be needed for intercommunications, this would not be viable for small groups of people, and reconfigurable radios would have enabled far more effective communications to be achieved.
With spectrum becoming a more scarce resource many radio regulatory bodies started to look at how it might be more effectively used.
In the UK a report by Professor Cave was published in 2002 detailing the possibility of selling spectrum dependent upon the bandwidth required. This cognitive radio technology would lend itself to this approach of spectrum management as it would be able to utilise areas that were temporarily free and thereby maximise the use of particular areas.
Similarly others had been working on the possibility of self configuring radios. In fact the term "Cognitive Radio" was coined by Joseph Mitola while he was writing his doctoral thesis on the topic in 2002.
Intelligence and flexibility
Work is under way to determine the best methods of developing a radio communications system that would be able to fulfil the requirements for a CR system. Although the level of processing required may not be fully understood yet, it is clear that a significant level of processing will be needed. The radio will need to determine the occupancy of the available spectrum, and then decide the best power level, mode of transmission and other necessary characteristics. Additionally the radio will need to be able to judge the level of interference it may cause to other users. This is an equally important requirement for the radio communications system if it is to operate effectively and be allowed access to bands that might otherwise be barred.
Cognitive radio architecture
In addition to the level of processing required for cognitive radio, the RF sections will need to be particularly flexible. Not only may they need to swap frequency bands, possibly moving between portions of the radio communications spectrum that are widely different in frequency, but they may also need to change between transmission modes that could occupy different bandwidths.
To achieve the required level of performance will need a very flexible front end. Traditional front end technology cannot handle these requirements because they are generally band limited, both for the form of modulation used and the frequency band in which they operate. Even so called wide band receivers have limitations and generally operate by switching front ends as required. Accordingly, the required level of performance can only be achieved by converting to and from the signal as close to the antenna as possible. In this way no analogue signal processing will be needed, all the processing being handled by the digital signal processing.
The conversion to and from the digital format is handled by digital to analogue converters (DACs) and analogue to digital converters (ADCs). To achieve the performance required for a cognitive radio, not only must the DACs and ADCs have an enormous dynamic range, and be able to operate over a very wide range, extending up to many GHz, but in the case of the transmitter they must be able to handle significant levels of power.
Currently these requirements are beyond the limits of the technology available. Thus the full vision for cognitive radio cannot yet be met.
Nevertheless in the future the required DAC and ADC technology will undoubtedly become available, thereby making cognitive radio a reality.
Cognitive radio examples
While a considerable degree of research has been undertaken and a number of limited trials of cognitive radio technology have taken place, one of the first examples of a widely manufactured and distributed cognitive radio system is to be found in the cellular telecommunications arena. Here the idea of femtocell base stations has been developed to allow users to have achieve far better 3G coverage within their homes. Using the ADSL backhaul to link into the network, the new femtocells are effectively a form of cellular access point.
To operate correctly these femtocells must not cause interference to the main network, nor to any adjacent femtocells. To achieve this, cognitive radio technology has been used. By using cognitive radio, the femtocells are able to monitor their environment, select which geographic area they are in to ensure they comply with regulatory standards, and then choose a suitable channel frequency.
With wire-less and radio communications becoming far more widely used, and the current levels of growth looking to increase, ideas such as cognitive radio will become more important. Some areas of the spectrum are very heavily used while others are relatively free.
Additionally the ability to change modes, frequencies and power levels will not only make communication possible for the cognitive radio system itself, but should also reduce the overall levels of interference to other users.
This is because the most spectrum or interference efficient modes can be chosen by the cognitive radio system.
In view of the possibility of CR radio communications systems utilising the spectrum more efficiently some regulatory bodies such as the FCC in the USA and Ofcom in the UK are looking favourably at the idea of cognitive radio. When the idea becomes a reality it would enable greater efficient use of the radio spectrum, which is not an infinite resource as it once was considered. Accordingly the way may be opened from this viewpoint to assist the development of cognitive radio communications technology.
By Ian Poole
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