Best method gives reception in tunnels
You are sitting on the train and have just started to watch a film on your mobile phone when the picture moves ever more slowly and then freezes – the train is in a tunnel. Twenty minutes later and you have only seen five minutes of the film. The people talking on their phones have been forced to call back a dozen times, so they finally give up and send a text message. Recognise the scenario? Irritating isn't it? Majid Nasiri Khormuji is currently writing his doctoral thesis on a subject that is attracting many researchers right now: What is the best, most problem-free method of transmiting a signal to a receiver irrespective of material obstacles?
"If it is not possible to find the best way to communicate via a third node, then neither will we be able to solve communication problems between many more nodes," explains Majid, who is a doctoral student approaching the end of his thesis on Communication Theory.
And this is important as there are now almost six billion mobile phones in the world today. In ten years' time there will be 50 billion apparatus connected to Internet, electronic components and mobile phones who must all share frequency space. Ericsson has made these calculations and has begun to test relays in LTE Advance, a future wireless network that will allow speeds of up to 950 Mbps.
"Industry has begun to realise that this has created energy savings opportunities on the net. We transmit enormous amounts of images, music, film etc. and volumes are increasing. These relays are necessary to maintain good quality," states Majid.
The problem is that researchers have not yet managed to figure out the best way of transmitting information via relays – base stations, sensor nodes, other mobile users etc. They do not know which is most energy-efficient, or which method gives the best quality.
The weaker the signal the more users on the same frequency
Majid illustrates the principle of relays by drawing a triangle on the board. A text message is sent from the transmitter in the left-hand corner via the top of the triangle – the relay – which then transmits it forward to the receiver in the area's right hand corner. By using the relay it is possible to reduce the strength of the signal so that many more people will be able to use the same frequency space.
"My signal cannot be heard as well when it is weaker and consequently it also disturbs the other signals less. In addition, a large signal may take up the entire frequency, while a weaker one allows others to use the same frequency," explains Majid.
Energy utilisation is still the greatest advantage of transmitting information via relays.
Location affects energy efficiency
"If you have an unlimited amount of energy you can transmit a very large signal. But if a lot of people are talking at the same time it is not possible to transmit large signals or if an area does not have so many resources like in developing countries. Transmitting a weak signal sending information via a relay means that a mobile unit uses less battery energy so it is possible to speak for longer or send more information," says Majid.
In the case of road or railway tunnels, relays will have to be located on the edges of the tunnels so that no material obstacles block transmission of the signal forward. Will this mean, in the end, a considerable reduction of energy use when so many battery-driven relays will be set up in so many locations?
"You save energy by spreading the signals out over more nodes and optimising the location of the relays. Location is enormously influential as to how much energy they use. At the same time research is underway into the streamlining of the battery-powered relays so that they use a minimum of energy," Majid explains.
However Majid is merely examining the theory of how these relays are to be constructed, his algorithms will be like the brains of these relays. The people who develop and produce relays of various types are then able to take the best method and copy it into their sensors and electrical apparatus. However there is still a long way to pret-a-porter algorithms!
Currently there are many different ways of transmitting signals containing information. However the identification of the best way is keeping many researchers like Majid busy.
With all this uncertainty - what is your driving force?
"It is only when we have found the best way that we can begin to calculate how good or bad the methods used today are and consequently be able to justify why we need to improve or retain a system."
How close to the best way are you?
"I have managed to identify the quickest way and how to calculate it in order to transmit information to a relay in certain cases. But we still do not know how much there is to win by doing it in another way. Neither do we know what is the most cost-efficient method. That also depends on the material available in ten years' time, which is something we know nothing about right now."
For more information please contact Majid Nasiri Khormuji, +46(0)8790 8445 or at khormuji@ee.kth.se.
