2. 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.
3. 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.
4. 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
7. 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.
9. 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.
10. 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.
11. FDMA : Diagram
The broader frequency bandwidth is divided into narrow bands for
different users.
12. 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.
13. 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.
15. 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
16. 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
17. 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.
19. 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