Cellular Telephones

Cellular Telephones
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

The Cells

The Cells (cont.)

Properties:
- Appropriate size for low power transmissions (26 km² = 10 mi²)
- Non-adjacent cells can use same frequencies (i.e., each cell can use 1/7 of the available frequencies)
Handoff:
- The tower you are communicating with changes as you move from cell to cell
- Handoff occurs based on signal strength

Frequency Allocation and Channels

Frequency Allocation:
- A carrier typically has 832 frequencies
- 2 are used per call (full duplex)
- 42 are used for control
Channels:
- 832 - 42 = 790 frequencies are available for voice channels
- 790 / 7 = 112 (approximately) frequencies are available per cell
- 112 / 2 = 56 voice channels per carrier

Advanced Mobile Phone System (AMPS)

Frequency Division Multiple Access (FDMA)

Each call is on its own frequencies

Time Division Multiple Access (TDMA)

Assigns each call a certain portion of time on a designated frequency
Uses analog-to-digital conversion and compression
A channel (30 kHz wide and 6.7 milliseconds long) is split time-wise into three time slots
Operate in either the 800-MHz (IS-54) or 1900-MHz (IS- 136) frequency bands

Global System for Mobile Communications (GSM)

Code Division Multiple Access (CDMA)

Uses analog-to-digtal conversion
Gives a unique code to each call and spreads it over the available frequencies (spread spectrum)
Each phone has its own spreading code
Multiple calls (8-10) are overlaid on a channel
Operates in both the 800-MHz and 1900-MHz frequency bands

Personal Communications Services (PCS)