General Cellular Concepts

In a cellular system the hand-sets carried by the users are called Mobile Stations (MS). The Mobile Stations communicate to the Base Stations (BS) through a pair of frequency channels, one for up-link and another for down-link. All the base stations of a Cellular systems are controlled by a central switching station called Mobile Switching Center (MSC) or Mobile Telephone Switching Office (MTSO). The MSC is responsible for all kinds of network management functions such as channel allocations, Handoffs, billing, power control etc. The MSC is also connected to the Public Subscriber Telephone Network (PSTN) or Public Land Mobile Network (PLMN) so as to allow the MS to talk to a Land Line telephone or vice versa.

Figure 1.1: Cellular Architecture (Source:www.iec.org)

Generally two range of frequencies are allocated to Mobile Cellular Systems one for up-link channels and other for down-link channels. This range of frequencies is divided into many channels so as to allow many MS to talk simultaneously. This technique of multiplexing several users over a given spectrum is called Frequency Division Multiple Access (FDMA). Other techniques like Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) are also employed by modern cellular systems. To support a large number of users in a given area a technique called ``Frequency Reuse'' is used by cellular systems. Under this technique the available channels are divided into smaller groups and a group of channels is allocated to a small region called ``cell'', typically of Hexagonal shape. In this way the whole range of frequency channels is used over a small group of cells and this group of cells is called ``cluster''. Typical cluster sizes are 4,7,9,12 and 19.

Figure 1.2: Frequency Reuse with Cluster size of 7 (Source:www.iec.org)

Another important feature of Mobile Cellular System is ``Handoff''. When a MS goes from one cell to anther then it needs to change to the BS of current cell cite and probably needs to be assigned different set of frequency channels. This phenomenon is called ``Handover'' or ``Handoff'' (HO). The HO decision may be taken entirely by controlling station or may be coordinated by MS.