28 August 2011


Cells

            A cell is the basic geographic unit of a cellular system. The term cellular comes from the honeycomb shape of the areas into which a coverage region is divided. Cells are base stations transmitting over small geographic areas that are represented as hexagons. Each cell size varies depending on the landscape. Because of constraints imposed by natural terrain and man-made structures, the true shape of cells is not a perfect hexagon.

Clusters

            A cluster is a group of cells. No channels are reused within a cluster. Figure  illustrates a seven-cell cluster.



Cell Splitting

          Cell splitting is the process of splitting a mobile cell into several smaller cells. The increased number of cells would increase the number of clusters over the coverage region.  The purpose of cell splitting is to increase the channel capacity and improve the availability and reliability of a cellular telephone network. 

            Splitting cell areas creates new cells, providing an increase in the degree of frequency reuse, thus increasing the channel capacity of a cellular network. Cell splitting provides for orderly growth in a cellular system. The major drawback of cell splitting is that it results in more base station transfers (handoffs) per call and a higher processing load per subscriber.

            Cell splitting is the resizing or redistribution of cell areas. In essence, cell splitting is the process of subdividing highly congested cells into smaller cells each with their own base station and set of channel frequencies. Cell splitting occurs when traffic levels in a cell reach the point where channel availability is jeopardized. If a new call is initiated in an area where all the channels are in use, a condition called blocking occurs. A high occurrence of blocking indicates that a system is overloaded. 

            Providing wide-area coverage with small cells is indeed a costly operation. Therefore, cells are initially set up to cover relatively large areas, and then the cells are divided into smaller areas when the need arises. The area of a circle is proportional to its radius squared. Therefore, if the radius of a cell is divided in half, four times as many smaller cells could be created to provide service to the same coverage area. If each new cell has the same number of channels as the original cell, the capacity is also increased by a factor of 4. Cell splitting allows a system’s capacity to increase by replacing large cells with several smaller cells while not disturbing the channel allocation scheme required to prevent interference between cells. 

 

Figure illustrates the concept of cell splitting. Macrocells are divided into minicells, which are then further divided into microcells as traffic density increases. Each time a cell is split, its transmit power is reduced. As figure shows, cell splitting increases the channel capacity of a cellular telephone system by rescaling the system and increasing the number of channels per unit area (channel density). Hence, cell splitting decreases the cell radius while maintaining the same cochannel reuse ratio (D/R).