Question

Draw a diagram and briefly explain the multiple access techniques,

(1) Frequency Division Multiple Access

(2) Time Division Multiple Access

(3) Code Division Multiple Access

Subject: Mobile Communications Engineering

Answer 1

Frequency division multiple access -it allow multiple users to send data on a single communication channel in this method the shared frequency bandis divided into channels of equal bandwidth and these chachannels are separate by gaurd bands so that each conversation is carried on a different frequency. Guard bands are used so that no two bands overlap to avoid crosstalk and noise.

Since the signals are transmitted in allotted time so this decreases the probability of collision.

It is used in radio and TV transmission. Therefore to avoid interference between two successive channels Guard bands are used.

Time division multiple access -it is a channel access method which allows several users to share the same frequency channel by dividing the signal into different time slots in a share medium of network .It facilitates many users to share the same frequency without interference and it also increases the data carrying capacity It is used in digital mobile radio systems.The frequency of the system is divided into sub-bands, and TDMA is used for the multiple access in each sub-band.No guard band required in it.

code division multiple access -

CDMA is a channelization protocol for Multiple Access, where information can be sent simultaneously through several users over a single channel.

In this ,

A signal is generated which extends over a wide bandwidth the code which performs this action is known as spreading code. And a specific signal can be selected with a given code even in the presence of many other signals.

Unlike other schemes like FDMA or TDMA which divide the channel based on frequency or time slots, CDMA allows all stations to have access to the full bandwidth of the channel for the entire duration.