In a cellular system, the entire region is divided into regular hexagonal patterns called as cells. The area covered by the cell is called Cell Size and there is no standard fixed value defined for this. It varies from service provider to service provider. The cell size ranges from 10 Km to 50 Km based on the population in that area. Figure 1 shows the basic structure and elements of a cellular system.

2.1.1 Elements Of a Cellular system

A cellular system consists of three important elements. They are: Mobile Stations (MS), Base Stations (BS) and Mobile Switching Centre (MSC).

Mobile Station commonly called as cell phone is a transmitting/receiving portable station in the cellular Radio service used for communication. Mobile stations may be hand held ( portable units) or installed in vehicles (mobiles).

Base Station/Cell sites refers to a fixed station used for radio communication with mobile stations. Base stations are located at the center or edge of the cell. It consists of transmitter and receiver antennas mounted on a tower and some circuitry to process the mobile signals.

Initially the cell sites were designed to cover a radius of 10 miles ( 16 Kms) or more. The transmitted signal power and height of the antenna determines the final cell size.

Mobile Switching Centre/ Mobile Telephone Switching Office (MSC/MTSO): In a cellular radio system, MSC is very important element which connects the cellular Base Stations and the mobiles to the Public Switched Telephone Network (PSTN). MSC coordinates the routing of calls in a large service area and does very important functions such as finding frequency channels, provides billing information etc.

2.1.2 Control and Voice Channels

The BS, MSC and PSTN communicate with each other using wired connectivity. The wireless connection is only between the Base Station and Mobile Station. A full Duplex communication exists between these two entities. Both are connected to each other by a pair of voice channels or control channels.

Control Channels refer to the radio channels used for transmission or reception of call set up, call requests, call initiation and other beacon or control purposes.

Voice Channels are radio channels used for transmission or reception of actual information from subscriber (user) .

Bandwidth of these channels vary from technology to technology. For example, 1G technology AMPS uses 30KHz as each channel’s bandwidth. 2G technology GSM uses 200 KHz as each channel’s bandwidth.

The channels used for transmission from Base station to the mobile are called Forward Channels or Downlink Channels. The channels used for transmission from Mobile station to the Base Station are called Reverse Channels or Uplink Channels.

The forward channel frequency is always higher than the Reverse channel frequency by 45 MHz. This is due to the reason that higher frequencies require higher processing power. The mobile station side which is portable has to work on limited power levels. Hence the lower spectrum out of the two bands is assigned for the mobile station transmission. Also, Duplexers available in those days could separate out the transmitted and received frequencies with minimum interference only if both were at a minimum frequency separation of 45 MHz. Channel connectivity between MS and BS is illustrated in Figure 2.

There are four channels using which a mobile can communicate with the Base station. They are namely, FCC, RCC, FVC ,RVC. The Reverse Control Channel carries signaling information like call requests or call termination from Mobile Station to Base station.

The Forward Control channel carries signaling information like Power control, information of channels etc. from Base station to Mobile station.

The Forward and Reverse Voice Channels carry the desired voice signals between MS and BS.

2.1.3 Cell Geometry

A decision had to be made in the initial phases of Cellular deployment regarding shape of the cell. Ideally, the coverage area of the Base Station antenna , known as the footprint of the Base Stations antenna should be taken as the cell size and shape. But, the real footprint of antenna is amorphous in nature as shown in Figure 3.

Hence, circular shaped cells can be considered. However it is observed that overlap occurs when circular cells are deployed along a hexagonal pattern. If we try to avoid the overlapping, then there is a non covered area or void formed in the middle of any three cells. Both the cases are shown in Figure 4.

Hence , a polygonal shape has to be selected in which there are no overlaps or voids. There are three regular polygonal shapes, Equilateral triangle, square and hexagonal for which it is possible to completely cover an area without overlap as shown in Figure 5.

Figure 6 shows the coverage area pattern for each of these shapes and how a polygon can be inscribed in a circle of radius R, the cell (coverage) radius. It is observed that the maximum area $(3/2 \sqrt {3\ R^2})$ coverage is obtained in hexagonal pattern.

Apart from maximum coverage area and less overlapping, other advantages of hexagonal cellular system also offers minimum interference between cells.

For these reasons, the Hexagonal geometry is preferred for designing cellular systems. It is the most economically efficient cell structure as it requires the least number of cells to cover a given area. Now onwards, the hexagonal cell structure is followed in all discussions and it provides a guide line for planning and design of systems. Readers should note that it is never precisely true that always hexagonal geometry is formed. In actual practice the cell formations are illustrated as shown in Figure 7.