Cell Fundamentals
In practice, cells are of arbitrary shape(close to a circle) because it has the same power on all sides and has same sensitivity on all sides, but putting up two-three circles together may result in interleaving gaps or may intersect each other so order to solve this problem we can use equilateral triangle, square or a regular hexagon in which hexagonal cell is close to a circle used for a system design. Co-channel reuse ratio is given by:
DL/RL = Square root of (3N) Where, DL = Distance between co-channel cells RL = Cell Radius N = Cluster Size
The number of cells in cluster N determines the amount of co-channel interference and also the number of frequency channels available per cell.
Cellular Networks
A Cellular Network is formed of some cells. The cell covers a geographical region and has a base station analogous to 802.11 AP which helps mobile users attach to the network and there is an air interface of physical and data link layer protocol between mobile and base station. All these base stations are connected to the Mobile Switching Center which connects cells to a wide-area net, manages call setup, and handles mobility.
There is a certain radio spectrum that is allocated to the base station and to a particular region and that now needs to be shared. There are two techniques for sharing mobile-to-base station radio spectrum:
- Combined FDMA/TDMA: It divides the spectrum into frequency channels and divides each channel into time slots.
- Code Division Multiple Access (CDMA): It allows the reuse of the same spectrum over all cells. Net capacity improvement. Two frequency bands are used one of which is for the forwarding channel (cell-site to subscriber) and one for the reverse channel (sub to cell-site).