Multiple access techniques in wireless communication

What is Multiple Access?


The transmission bandwidth in different frequency band varies in range. It is more than the bandwidth needed for a message to be transmitted. Hence, in order to utilized the frequency spectrum efficiency "Multiple Access" method is introduced.

In telecommunication and computer networks, a channel access method or multiple access methods allow more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. 

Multiple access technique permits multiple access to a channel. A channel represents a system resource assigned to a given mobile user that enables the user to establish communication with over users in the network. 

The ability of the system to carry many signals at the same time is called Multiple Access. Through Multiple Access, it is possible to share the communication capacity of the cellular system among a large number of base stations.

Depending on the channel type, specific multiple access techniques can be used for communication. The channel access types and the associated multiple access techniques are as follows.  

FDMA : 


FDMA is a wireless system that allows multiple users to send data through a single communication channel, such as a coaxial cable or microwave beam, by dividing the bandwidth of the channel into separate non-overlapping frequency subchannels. Each subchannel allocating to a separate user. 

The channel or frequency band is unique for each subscriber. The channel allocation can be done on a basis to the users to request service.

In FDMA systems the users are assigned a pair of frequency, one for the forward channel and other for the reverse channel.

Example - FM radio where multiple users can transmit simultaneously; however, on different frequency channels. 


TDMA : 


TDMA is a wireless system for shared-medium networks that allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot allows multiple stations to share the same transmission medium while using only a part of its channel capacity. 

TDMA system divides the radio spectrum into a time slot only one user is allowed to transmit or receive. 

Various users can transmit the same frequency band at different times. 

CDMA : 


CDMA is a wireless system used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communication channel that allows several users to share a band of frequencies.

In CDMA the narrowband message signal is multiplied by a large bandwidth called spreading signal. This spreading signal is actually a pseudo-noise code sequence and it has a higher chip rate than the data rate of the message signal.

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