Cellular Systems Cellular Concepts The cellular concept was a ...

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EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications • Divide the total number of channels into two groups, one of which is used for fixed allocation to the cells, while the other is kept as a central poor to be shared by all users. • Mix the advantages the FCA and DCA, available schemes are scheduled and predictive. Channels need to be assigned to users to accommodate - new calls - handovers With the objective of increasing capacity and minimizing probability of a blocked call. System Expansion Techniques As demand for wireless services increases, the number of channels assigned to a cell eventually becomes insufficient to support the required number of users. More channels must therefore be made available per unit area. • This can be accomplished by dividing each initial cell area into a number of smaller cells, a technique known as cell-splitting. • It can also be accomplished by having more channels per cell, i.e. by having a smaller reuse factor. However, to have a smaller reuse factor, the cochannel interference must be reduced. This can be done by using antenna sectorization. Cell splitting • Cell splitting increases the number of BSs in order to increase capacity. There will be a corresponding reduction in antenna height and transmitter power. • Cell splitting accommodates a modular growth capability. This in turn leads to capacity increase Institute for Infocomm Research 73 National University of Singapore Institute for Infocomm Research 74 National University of Singapore
EE5401 Cellular Mobile Communications EE5401 Cellular Mobile Communications essentially via a system re-scaling of the cellular geometry without any changes in frequency planning. • Small cells lead to more cells/area which in turn leads to increased traffic capacity. • For new cells to be smaller in size, the transmit power must be reduced. If n=4, then with a reduction of cell radius by a factor of 2, the transmit power should be reduced by a factor of 2 4 (why?) • In theory, cell splitting could be repeated indefinitely. In practice it is limited 1. by the cost of base stations 2. handover (fast and low speed traffic) 3. not all cells are split at the same time : practical problems of BS sites, such as co-channel interference exist 4. Innovative channel assignment schemes must be developed to address this problem for practical systems. Sectorization • Keep the cell radius but decrease the D/R ratio. In order to do this, we must reduce the relative interference without increasing the transmit power. • Sectorization relies on antenna placement and directivity to reduce co-channel interference. Beams are kept within either a 60° or a 120° sector. Institute for Infocomm Research 75 National University of Singapore Institute for Infocomm Research 76 National University of Singapore
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