Frequency Division Multiplexing Access (FDMA)

Frequency Division Multiplexing
Access (FDMA)
SOUMEN SANTRA
MCA, M.Tech, SCJP, MCP

Multiple Access : Introduction
 It is require in wireless communication systems.
 It allows the subscriber to send data simultaneously from
one mobile station to the base station while receiving
information from the base station to the mobile station.
 It supports a communication system to connect multiple
stations using a common link.

Multiplexing : Features
 One path many user.
 It is used to increase the bandwidth to enhance the
capacity.
 The process of combining multiple signal into a single
transmission channel.
 The channel can accommodate more data.
 It is used for maximum utilization of channel capacity.

 Nodes or stations are connected and use a common link , called a
multipoint or broadcast link .
 We need a multiple access protocol or co-ordinate access to the link .
 Multiple access schemes are used to allow many mobile users to share
simultaneously a finite amount of radio spectrum.
 There are many formal protocols are getting used to access to a
shared link .
Multiplexing : Features

Multiple Access
Protocol
Random
Access
Protocol
Controlled
Access
Protocol
Channelization
Protocol
 ALOHA
 CSMA
 CSMA/CD
 CSMA/CA
 RESERVATION
 POLLING
 TOKEN
PASSING
 FDMA
 TDMA
 CDMA
 SDMA

Multiplexing : Channelization
 There is a Collision if more than one station tries to send
information at a time.
 Channelization protocol, a multiple access method overcome this
problem.
 Available bandwidth of a link is shared in time , frequency or
through code between different stations .
 The Channelization Protocols are FDMA ,TDMA,CDMA & SDMA.

Multiplexing : Channelization Diagram

Frequency division multiple access
FDMA
 FDMA is the basic technology for advanced mobile phone services .
Implemented at the media access control (MAC) layer.
Assigned a specific frequency band in the electromagnetic
spectrum,
During a call that user is the only one who has the right to access
the specific band.

FDMA : Features
 Continuous Transmission.
 Narrow bandwidth.
 It can be used both analog and digital signal.
 It is not vulnerable to the timing problem.
 It does not have near far problem, due to frequency filtering.
 Total system bandwidth is divided into narrow frequency slots with
guard band separation between the channels.
 No limitation of time for the users. But the cost of transceiver is high.
 A user is free to transmit or receive all the time on its allocated radio
channel.

FDMA : Diagram
The broader frequency bandwidth is divided into narrow bands for
different users.

FDMA : Guard Bands
 A narrow unused frequency band is left Between the different used
frequency channels.
This is known as the Guard band.
 This is used to avoid interference from the adjacent channels .
In the above shown FDMA system 10KHz guard bands are introduced between the
adjacent channels.

Guard Bands & Channel Assignment
 It is a small amount of bandwidth between different channels
which is not used.
 It is necessary to cater for instability of the sender.
 In Doppler effect the frequency shift due to movement.
 Channel assignment is carried out First Come First Basis.
 The number of channels and frequency spectrum depends upon
the modulation technique and the guard bands.

 It is implemented in narrowband system and less complex.
 The base station and the mobile station transmit and receive
simultaneously and continuously.
 Reduces the bit rate information and the use of efficient numerical
codes increases the capacity.
 Since the transmission is continuous , less number of bits are
required for synchronization and framing.
 No restriction regarding the type of baseband or type of modulation.
 Fairly efficient when the number of stations is small and the traffic is
uniformly constant.
 Capacity increase can be obtained by reducing the information bit
rate and using efficient digital code.
 No need for network timing & Channel bandwidth is relatively
narrow.
 It allots specific frequency band to every user.
FDMA : Advantages

 The maximum flow rate per channel is fixed and small.
 If channel is not in use it remains idle which results in wastage.
 Requires RF filtering to minimize adjacent channel interference.
 Does not differ significantly from analog system.
 Guard bands lead to a waste of capacity.
 Dedicated channels increases the call cost.
 Maximum bit rate per channel is fixed.
FDMA : Disadvantages

FDMA : Uses
 It is used in total access communication system.
 It is used in cellular communication and satellite communication.
 It is used for FM and AM radio broadcasting.

 In TDMA, the time domain is divided into different slots , whereas in
FDMA, allows more than one user to access the same frequency
band without interference.
 FDMA makes a good usage of transmitting and receiving power of
the signal since it has less inter modulation noise. But in TDMA, no
wastage of guard bands and also it has a flexible and faster bit rate.
Hence ,TDMA is more efficient and cost effective over FDMA.
FDMA vs. TDMA

Frequency Division Multiplexing Access (FDMA)

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