Data Transmission Mobile Telephony

COMPUTER AIDED DESIGN

Data Transmission

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Spectral Bandwidths

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Cellular Access Technologies ► Cellular

access technologies are the methods used by mobile phone networks for transmitting information. ► There are three technologies used by cellphone networks for transmitting information: ƒ ƒ ƒ

FDMA TDMA CDMA 3

Frequency Division Multiple Access

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Each signal is modulated in a telephone office The signals are demodulated in a telephone office in the receiving area. FDMA is not an efficient method for digital transmission. 4

Time Division Multiple Access

TDM multiplexer takes one byte from each channel by rotation, compresses it and transmits as a stream in a pre-assigned sequence. ► Data is demultiplexed at the receiving end. ► Voice is compressed so that less transmission space is taken. ► TDMA systems operate in 800-1900 MHz frequency band. ►

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Code Division Multiple Access

Each call is spread out over the entire available bandwidth. Multiple calls are overlaid in the channel, with each being assigned a unique code and is transmitted in small pieces. ► At the receiving end, the same unique code is used to recover the signal. ► CDMA operates in 800-1900 MHz frequency band. ►

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GSM ► GSM,

Global System for Mobile Communication is an international standard which uses TDMA technology for access. ► GSM is in use in much of Asia, including India, Europe, Australia and Africa.

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Mobile Communication Cells

► The

geographical area is divided into a grid of small hexagonal cells. Each cell has a base station that consists of a tower and radio equipment. A mobile telephone switching office controls all the base stations in a region. 8

Mobile Communication Cells

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Each cell is a group of 50 to 70 radio channels, so that many simultaneous conversations can be made. Adjacent cells must use different groups of channels to avoid interference. In busy areas, cells can be of smaller size. 9

Mobile Communication Cells

As you move towards the edge of one cell, the cell’s base station notes the diminishing signal strength, while the base station in the cell you are moving towards senses the increasing signal strength. ► The base stations coordinate with each other and the mobile is given a signal to shift frequencies. ► This switches the phone to the new cell without dropping the call. 10 ►

Mobile Facts ► In

a walkie-talkie, same frequency is used by transceivers at both ends and so, only one party can talk at one time. ► But when two cell-phones are in conversation, they use difference frequencies. ► On a “Complexity per cubic inch” scale, cell phones are some of the most intricate devices people play with on a daily basis. 11

THANKS

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