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.
|
