In an effort to improve the speed and quality of two-way radio communication, Birmingham University researchers have developed a self-adapting antenna for cognitive radio. This is a form of wireless communication in which a transceiver detects which channels are in use and which are not.
Dr Peter Gardner said the team is one of the first to try advances in the area by changing a radio’s antenna, rather than modifying the signal processing methods of cognitive radio.
‘The concepts behind cognitive radio are so far coming from the world of software and electronics, and some of the assumptions that have been made about the way that antennae operate are perhaps not entirely correct.
‘For example, you need a single, very broad band antenna that will cover a very wide frequency range. But if you have got a broad bandwidth, there are important trade-offs between bandwidth, size and efficiency,’ said Gardner.
Signals in old-fashioned radios are tuned, modulating the frequency or amplitude so that the receiver can extract the information, using fixed analogue circuits.
However, in cognitive radio, which Gardner described as a further extension of software-defined radio, a computer or signal-processing device is used to decode the radio signal. This means that if a transmitter sends a message using a different encoding technique, instead of changing the radio hardware to enable the receiver to pick up the message, suitable decoding software can simply be programmed into the radio.
‘A cognitive radio monitors the spectrum of all the signals within its range to see what other transmissions are going on and, based on that and on its own stored knowledge or its models of the propagation environment around it, it will decide what standard to use to transmit the data while causing the smallest possible interruption to everybody else.
‘They talk about spectrum etiquette — you know what other people are doing so you don’t try to interfere with it too much, but it’s done on an ad-hoc basis rather than by means of strict radio regulations. For example, some of the wireless LAN standards run around the 2.4GHz or 5GHz bands. If you go out of those bands you might end up interfering with someone else’s transmissions, such as the industrial, scientific and medical (ISM) bands, which are close to the wireless LAN,’ he said.
To help cognitive radios abide by this spectrum etiquette, the scientists are developing a reconfigurable antenna that is small enough to be fitted on to devices such as mobile phones and PDAs.
‘So you’ll have a small antenna that will have built into it switches, maybe in the form of microelectro and mechanical switches, or semiconductor-based switches that allow you to change the shape of the area being excited electrically by the antenna, in order to change its bandwidth coverage,’ said Gardner.
‘Coupled to the antenna you’ll have sensors that sense the spectral power across a wide bandwidth, and from that it will do an analysis of where in the spectrum most of the energy is.’
He said after the initial analysis, the radio then decides to use the bandwidth with least activity, such as the area between 2.15GHz and 2.2GHz, so it switches out all the bits of antenna except those required to set up a 2.15GHz antenna and blasts the message through on that bandwidth.
‘When that packet of data is dealt with, it then goes back to do another recce as the whole situation may have changed, so that for the next packet it may use a completely different bandwidth,’ he said.
In addition, Gardner said data could be spread over and transmitted over a very broad band if necessary, using either wide band code division multiple access or pulse transmission.
He also thinks it might be possible for the switches to change the polarisation properties of the antenna so that a different polarisation pattern could be used, or the radio energy could be radiated in a different direction as part of the etiquette.
‘Once many systems go over to this mode of operation, the whole radio spectrum will be better utilised because at any one time people can be using the whole spectrum, whereas previously just to regulate things, we were saying you can’t use that you can only use this. In a way it’s like deregulation, opening up the spectrum to a free market.’
Project partner Motorola will advise the researchers on the potential applications for the new technology.
Birmingham University researchers have developed a self-adapting antenna for cognitive radio