Hi everyone. My name is Seong Lyun Kim of Yonsei University. Like the other lecturers in this course, I'm with the W Department. The focus of my research and education has been in the area of wireless networks. And during one hour, I will try to give some brief overview of wireless resource and management. Well, let me start with the definition of wireless resource management. What is wireless resource management? Resource in communication, wireless communication means radio frequency spectrum which is very much limited. So we have to use this resource radio frequency spectrum very effectively. Optimization means management. So management means we have to maximize the number of users in the wireless system utilizing radio frequency spectrum resources very effectively. So talking about the wireless resource and management, I will try to answer six questions. The first question is, how does interference affect the capacity of wireless networks? Perhaps, you don't know the terminology interference, or the capacity or the wireless network. So the understanding the definition of this three key word terminology interference, the capacity, the wireless network is important to answer the first question. So let me start with an example of the cocktail party cases. Well, have you ever been in a cocktail party before? Maybe yes or maybe not. Well imagine you are in a cocktail party to celebrate something, your dissertation or the finalization of your report or whatever. And then in the beginning you can talk to your partners with a low volume and soft volumes. Because your partner listener can hear you very correctly. But as time goes, as the guest arrives in the cocktail room, room becomes very noisy. The so called interference is generating upon each communication pairs. So your partner listener actually can't hear your voice, or signal correctly due to increased interference that is generated by your other surrounding communication pairs. So in the cocktail party room, as time goes noise or interference goes to high and then no one can communicate. This is called cocktail party effect. And such cocktail party effect is very well used in communication to put an example or to illustrate the example of huge interference in a network. Well, the question is if interference is actually going up what will be happening? That was the first question. Of course you may understand that when interference goes up no one can communicate with each other. It's very difficult to talk to each other so there should be some coordinator such that some conversations may be possible. Well, to see more precisely we may visualize wireless network using a graph that's in your next slide. Well, network can be visualized using a graph sometimes to mathematically handle or to analyze the network more effectively. Of course, wireless network can be analyzed by using a graph. In the previous graph, we have two things. One is node, the other one is actually the edge. In a wireless network nodes means transmitter. In a cocktail party the speaker and another node is actually the receiver Rx in a cocktail party who is your partner or listeners. And the arrow, the direction arrow, means actually the wireless link. Which is actually the wireless channel which should be based on a radio frequency spectrum. So in the cocktail party room you have transmitting node and receiving node and link which is wireless link. So we can visualize cocktail party room utilizing such graph. For example, if we go to the next slide perhaps you'll see such random graph. In the random graph you see actually a connection of each node by one's link. By definition the random network means that your partner listener is actually the selected random name. That means Each communication parties are randomly selected. This is slightly different from cocktail party room. In the cocktail party room, speaker and listeners are actually very close and they are in a very closed proximity. However, in the random network, mathematical formula, mathematical form is actually transmitters and receivers are actually located in a random position, that is slightly different. But however, in many cases we can visualize cocktail party room utilizing this random network. Well, the question is if you increase the number of nodes or the number of speakers, in other words number of communication pairs. And this will generate lots of interferences because they are using the same resources, same radio frequencies, generating interferences and in certain level no one can communicate. So this actually by intuition we could understand. Well, question is, does node density increase? We know that capacity equals zero. Well, at this point let me define the meaning of capacity. In a wireless network capacity means the number of communication pairs that can successfully talk to each other. If their communication is failed due to large interference or some other reasons, then we count the capacity, I mean we do not count it as a capacity. So if capacity goes to zero means there is no one successful to communicate. So as interferences increases or as node density increases like in cocktail party room capacity, the number of successful communication pairs will be approaching to zero. So due to the interferences, we think that capacity is going to be equal to 0. Well, this is very intuitive and understandable. Our next question is, is there any accurate mathematical formula so that we can understand capacity and interfacing relationships more concisely or more accurately? That is related to answering the question more correctly. So if I go to the next slide, there is some mathematical formula which was actually originally published in a paper in the year 2000. Published in the Journal of IEEE Transactions on Information Theory by Gupta and Kumar. And if you look at this formula, it looks complicated. The formula itself is not that difficult to understand. The lambda is a function of n. Lambda means capacity. As I said, capacity is the number of successful communication pairs. So, Number of successful communication pairs is a function of n. What is n? n is node, number of nodes or node density. How many nodes in a given area? Or how many communication pairs? Is a function of n and the function is actually inversely proportional to n multiplied by log n. So that means if you understand this formula, as n goes to infinity, then lambda goes to zero. And we understand that lambda is approaching to zero as n goes to infinity, but however, we can more accurately understand the formula itself. Well, there is another terminology here, c and w. And c is actually the sum of constants. it's very much related to the system. So kind of the characteristics like antennas and some other physical layered techniques or something which are given parameters. Whereas w is a given frequency spectrum bandwidth. In the beginning, I said actually the radio resource or resource means frequency spectrum which is very much limited. So w is a given bandwidth or bandwidth or one wireless link. So as w becomes larger then capacity will be increasing. But in this formula the authors, Gupta and Kumar, assume that c and W are given and fixed. So this formula gives more accurate answer to the first question, as n goes to infinity, lambda goes to zero with such formula. And in their original paper, they defined the capacities slightly in a different way than the others. That is called the transport capacity. Transport capacity means, it is actually the multiplication. If you look at this line in the middle, these are bits multiplied by meters. So when Gupta and Kumar defines transport capacity, they are interested in number of bits that can be transmitted per second. However, they are also interested in the travel distance of bits. If one bit travels twice meters than the others, then transport capacity becomes twice of the others. So transport capacity, understanding transport capacity is quite important for answering the next questions in my other slides. But at the moment just understand that the capacity is a function of n which is inversely proportional to n. And as n goes to infinity then lambda goes to zero. Well this is actually the bad news for us because in a wireless network n is number of communication pairs. So people desire to communication more and number of communicating people will be increasing. However, if they are sharing the same radio channel, no one can communicate. So this is actually the dilemma for us. The question is how we handle such larger number of communication pairs while keeping this capacity rosier. Well, Radio Resource Management is, as I said in the beginning, is to maximize capacity of the wireless network while utilizing frequency spectrum effectively. How do we do that? Well, Radio Resource Management is, to say, to control interferences. We understand what is interference using this body effect. Well, by contouring interference amount effectively, then we can maximize the capacity or we can adjust it or we can increase the capacity wireless network. That will be the topic in my next questions and following questions as well. Well, of course you can think of a very simple resource management technique even though it is not desirable on a solution. For example, if you can't communicate with others like in a cocktail party room due to huge interferences. The best or very simple way of resource management is to make capacity zero. That means, the best way is to shut off. Of course this is very simple and actually of extreme cases radio resource management technique. Well, there might be some other cases, for example there will be some coordinators in the room. So the, for example, the imaginary situation, that the coordinator, actually making some traffic controls like this car should communicate first and the remaining will be staying there. As there will be some kind of round robin way of communication. For example, not in the cocktail party that mean the communication system, this is possible for example. So such a kind of radio resource management technique would be possible to handler interferences such that maximizing or in order to maximize the capacity of the wireless network. So to know a little bit about that kind of techniques let's take a look at the other questions following. Thank you.
