Cognitive radio basics

- an overview or tutorial about the basics of Cognitive Radio and how with the Software Defined Radio, Cognitive Radio is a technology for future radio communications systems.

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) 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 is able to select the frequency band, the type of modulation, and power levels most suited to the requirements and prevailing conditions.

In view of this cognitive radio is effectively an environmentally friendly radio communicatons technology, adapting to the prevailing conditions. In fact the term "Cognitive Radio" was coined by Joseph Mitola while he was writing his doctoral thesis on the topic. He is now programme manager on the Cognitive Radio development for the US Military.

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.

Acceptance

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.