Need for Cellular Hierarchy
Extending the coverage to the areas that are difficult to cover by a large cell. Increasing the capacity of the network for those areas that have a higher density of users. An increasing number of wireless devices and the communication between them.
Cellular Hierarchy
- Femtocells: The smallest unit of the hierarchy, these cells need to cover only a few meters where all devices are in the physical range of the uses.
- Picocells: The size of these networks is in the range of a few tens of meters, e.g., WLANs.
- Microcells: Cover a range of hundreds of meters e.g. in urban areas to support PCS which is another kind of mobile technology.
- Macrocells: Cover areas in the order of several kilometers, e.g., cover metropolitan areas.
- Mega cells: Cover nationwide areas with ranges of hundreds of kilometers, e.g., used with satellites.
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).