N-GEN Communications Web-based Seminars ©NGCS

Seminar - An Overview of Communications Technologies

Module 9 - Wireless Systems and Technologies                                                                                 Slide 17 of 80                                                                                                    ________________

    Cellular Communications Concepts

                                    Large Geographical Areas Divided into Cells

                                    Cell Clusters

                                    Cell Size

                                    Frequency Re-use

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Cellular mobile radio systems get their name from a technique used to divide a large geographic area, like a metropolitan area, into a series of small hexagonal shaped cells. A hexagonal shape chosen for a cell is very close to the circular coverage of a base station. The shapes fit together to cover the area completely. Individual cells have a typical radius of coverage from 2 to 8 miles. Low power transmitters and receivers with low antennas are placed in each cell and connected via wirline to the central switching computer known as the Mobile Switching Center. The Mobile Switching Center is connected to the PSTN.

A cell-cluster includes 7 cells.

The base stations communicate with the mobile and portable phones in their respective cells. The set of frequencies used in one cell can be used in a cell in other cluster within the same metropolitan area. A single conversation occupies a channel over a small geographic area, and the same channel can be re-used for another conversation in another cell within the same metropolitan area. FDMA is used to access the network within the cells.

For acceptable signal to noise ratio the nearest co-channel cell must be at least 6 cell radii away.

A Metropolitan Area without Cellular Radio System:

A Metropolitan Area with Cellular Radio System: