By Dr Kho Yau Hee
(Published in’Campus & Beyond’, a weekly column written by Swinburne academics in the Borneo Post newspaper)
Modern wireless communications system, such as the mobile phone network, has grown to become an integral part of our everyday lives, enabling us to stay connected 24/7 anytime, anywhere. Today, it has become so indispensable that it is hard to imagine a life without it. But have you ever wondered how a wireless communications system works?
The operation of a wireless communications system is based on the principle of electromagnetic waves radiation. This radiation carries with it energy from a source point, dispersing it in all directions, very much like the ripple effects when a pebble hits the surface of a pond, and it can propagate through different kinds of channel, for example water, air and space (i.e. vacuum). It is through this radiation that many different kinds of wireless communications systems are made possible, such as underwater sonar communications (water), mobile communications (air) and satellite communications (space).
For a transmission to take place, for example sending a text message via a mobile phone, the message is converted into an electrical signal and sent (i.e. radiated) via an antenna that acts as a point source. In general the signal is radiated in all directions, but this can be made uni-directional by focusing the radiation in a particular direction. How far the radiation can propagate depends on the energy content of the signal; the more powerful the signal is the farther it can be transmitted, and vice versa. To receive the signal at the other end, a receiver antenna is required. This receiver antenna operates in the reverse sequence of the transmitter antenna, i.e. it converts the radiation back into electrical signals which the mobile phone then converts back to the text message.
The transmission (and receipt) of the electromagnetic radiation is done at a particular frequency. Many such frequencies combine to form what is known as the electromagnetic spectrum, and the frequency range that is suitable for mobile communications is known as the radio spectrum. This is very similar to the colour spectrum of a rainbow where the colours correspond to a particular frequency, and these frequencies combine to form the rainbow.
Owned and regulated by the relevant government authority, specific frequencies are assigned to an organisation for various purposes. For example, a segment of the spectrum may be licensed for daily commercial radio and television broadcast, mobile phone transmission, etc while some may be allocated for free community usage such as community radio and emergency rescue purpose.
As a key player in establishing communication links, the radio spectrum is obviously an important resource. Governments around the world control the spectrum management and generate significant revenues from commercially licensing it. For example, in the auction of radio spectrum for third generation (3G) communications systems at the turn of the new millennium, telecommunication companies paid billions of dollars to own a slice of the spectrum.
There is, however, a limitation in the capacity of this communication link in terms of the number of users or the size of data that can be transmitted. A simple way to relate this would be to think of a highway where the number of lanes (users) or the amount of cars (data) that it can support is fixed. In communication systems, however, the number of users and data can be expanded by adding more frequencies. However, it is expensive to obtain such frequencies and the frequency spectrum is also increasingly congested. Therefore, new technology is needed to solve this bottleneck.
Today, wireless devices such as modems with two or more antennas are commonplace. A device configured in this manner makes use of a technology generally known as the multiple-input multiple-output (MIMO) technique. Simply, this technology utilises multiple antennas at the transmitter and receiver end of a communication system. With multiple antennas at both ends, it is possible to create multiple links. Research has confirmed that the MIMO technique can increase the capacity of a communications system at no additional cost to the frequency spectrum or the need to increase the transmission power.
Currently, numerous researches are being carried out to increase the spectral efficiency through the MIMO technique. Due to the multiple signals received by the many receiver antennas, more complicated algorithms are needed to recover the transmitted signals. An algorithm is a sequence of mathematical instructions to solve a problem. Being more complicated, the execution of the algorithms consumes more processing time and computing power, so algorithms with reduced complexity but that could still achieve comparable performance will be needed.
At Swinburne Sarawak research into reduced complexity algorithms for a MIMO system is being investigated. This includes algorithms for a reduced complexity channel estimator that is used to estimate the effects of the transmission channel so that at the receiver end those effects can be eliminated, and the original transmitted signal recovered. Apart from this, different receiver designs for a MIMO system, integrated with the above algorithms, are also being investigated so that an overall improvement in the processing time and computing power can be achieved.
While our telecommunication companies are starting to offer 3G services, behind the scenes many advanced research are already underway to develop 4G. We can expect a faster, more efficient and seamless system in the near future but no matter how advanced our telecommunication systems may have grown, it still depends on the basic principle – the invisible yet magical electromagnetic radiation that is all around us.
Dr Kho Yau Hee is a lecturer with the School of Engineering, Computing and Science at Swinburne University of Technology Sarawak Campus. He can be contacted at firstname.lastname@example.org.