 Hello everyone, welcome to a video lecture on Improving Coverage and Capacity of Cellular System. I am K. R. Biradar, Assistant Professor, Department of Electronics and Telecommunication Engineering Walsh and the Institute of Technology, Sallapur. Let us start with the learning outcomes first. At the end of this video, you will be able to explain different techniques to improve the capacity of cellular system. Introduction As demand for wireless service increases, the number of channels assigned to a cell is not enough to support the required number of users. Solution is to increase channels per unit coverage area. That means system designers have to provide more channels per unit coverage area. The various system expansion techniques are, first one, cell splitting. The cell splitting increases the number of base stations deployed and allows an orderly growth of the cellular system. Second one, sectoring. In sectoring uses directional antennas to further control interference and frequency reuse. Third one, micro cell zone concept, which is relatively new technique, distributes the coverage of a cell and extends the cell boundary to hard to reach places. Cell splitting. Cell splitting is the process of subdividing a congested cell into smaller cells. Each with its own base station, a corresponding reduction in antenna height and a corresponding reduction in transmitter power. Basing the cells reduces the cell size and thus more number of cells. So, these smaller cells within the large cell have a very interesting characteristic. All of them have their own base station, but these base stations are not so high. The antennas are shorter and they also transmit less power. More number of cells leads to more number of clusters, leads to more number of channels. These results into higher capacity. Cell splitting allows a system to grow by replacing large cells by small cells. Cell splitting. So, let us look at an example. Earlier we had these big cells because the demand was not so high. So, large cell in low density areas however, some of the reasons being supported by the cells have experienced an increase in the number of users. So, as a first step we split the cells. Please note we have done the splitting partially. Certain cells are being split into 2 or 3 cells, but these cells can only handle high density. But in the event that more number of users have to be supported per cell may be in the central cell we try to subdivide. The cells further by putting smaller and smaller base stations therefore, these smaller cells would imply greater capacity. Cells are split to add channels with no new spectrum usage. Depending on the tropic pattern the smaller cells may be activated or deactivated in order to efficiently use cell resources. Sectoring the technique for decreasing co-channel interference and thus increasing system capacity by using directional antenna is called sectoring. This is the co-channel interference and keep the cell radius are unchanged by replacing the single omnidirectional antenna by several directional antennas. Radiating within a specified sector cell is normally partitioned into sector. These sectors could be 120 degree sectors. In that case we will only have 3 sectors per cell or you can divide into 490 degree sectors or you can divide it into 660 degree sectors cell sectoring. Let us look at some typical geometries of cell sectoring. So, one is our standard omnidirectional. We have not done by any sectoring. We have one base station is free to radiate at all portions of the cell, but what you can do additionally is to take that hexagon and first subdivide into 3 sectors on the same tower which earlier supported my base station. Now we have 3 antennas and not 1 antenna 1 could be radiating in sector A, antenna 2 could be radiating in sector B and antenna 3 could be radiating in sector C. There would be slight overlap between the sectors. The sector boundaries will not have a very clear distinction between A, B and C. There have to be some overlap otherwise I cannot carry on successful hand ups, but in this case we have used a sector of angle 120 degrees. The other possibility of using the same 120 degrees sector is to subdivide my cell in other passion. So, different geometries would require you to see whether you are most distant base station and an average is what distance from serving base station antenna. Yet another method to sectorize the cell is to use 90 degree sector antennas. Here I have A, B, C and D at 90 degrees. Another method is to have a 60 degree sector. So, I divide the original cell into 6 sectors of 60 degrees each. Normally anything below this is not preferred because too many sectors would require too many hand ups and too many antennas. What are the problems with cell sectoring? Past the video think and try to answer from the cellular structure basics. I think you might have written the answer. Thank you. Let us discuss now. The first problem is the increase in number of antennas at each base station. They are expensive. They have to be maintained. They consume power. There is a degree in the truncating efficiency due to sectoring. Clearly when you subdivide a cell into sectors and we are not asking for extra bandwidth, we are cutting the full into smaller subsections and each sector is using a part of the pie. So, individual truncating efficiency goes down. Then we have the additional problem of increased number of hand ups. Each sector is being treated separately and then you move from one sector to another. You have to be handed out. But the good news is many modern base station support sectoring and the related hand up without the help of mobile switching center. Therefore, we are sectoring provides an alternative to the cell splitting, micro cell zone concept. Large control base station is replaced by several low power transmitters on the edge of the cell. The mobile retains the same channel and the base station simply switches the channel to a different zone site and the mobile moves from one zone to another zone. Since a given channel is active only in a particular zone in which mobile is travelling, base station radiation is localized and interference is reduced. The channels are distributed in time and space by all three zones are reused by co-channel cells. Advantage is that while the cell maintains a particular coverage radius, co-channel interference is reduced due to zone transmissions on the edge of the cell. The figure shows each of the three zone sites are connected to a single base station and share the same radio equipment. The zones are connected by coaxial cable, fiber optic cable or micro link to the base station. Multiple zones and a single make up a call as a mobile travels within the cell it is served by the zone with the strongest signal. This approach is superior to sectoring since antennas are placed at outer edge of the cell and any base station channel may be assigned by the base station. As a mobile travels from one zone to another within the cell it retains the same channel. Thus unlike in sectoring a hand up is not required at the MSC and the MSC or when the mobile travels between zones within the cell. The base station simply switches the channel to a different zone site. In this way a given channel is active only in the particular zone in which the mobile is travelling and hence the base station radiation is localized and interference is reduced. The channels are distributed in time and space by all three zones and are also reused in co-channel cells in the normal fashion. This technique is particularly useful along highways or in along urban traffic corridors. These are the references I referred to prepare the above presentation. Thank you.