TYPES OF MULTIPLE ACCESS IN WIRELESS COMMUNICATION

1. Multiple access in wireless
communication
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2. What is Multiple access?
 Multiple access method allows several terminals connected
to the same multi-point transmission medium to transmit
over it. Examples of shared physical media are wireless
networks, bus networks, ring networks and half
duplex point-to-point links.
 Multiple access schemes to allow many users to share
simultaneously a finite amount of radio spectrum resources.
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3. Multiple Access Techniques
Frequency division multiple access
(FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)
Space division multiple access (SDMA)
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4. Frequency division multiple
access (FDMA)
 The frequency band is divided into channels of equal
bandwidth so that each conversation is carried on a
different frequency.
 A specific frequency band is given to one person, and it
will received by identifying each of the frequency on the
receiving end. It is often used in the first generation of
analog mobile phone.
 Each station assigned a fixed frequency.
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5.  In FDMA system signaling dimensions are divided along
the frequency axis into non overlapping channels and
each end users assigned a different frequency channel as
shown in fig.
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6. Time division multiple access
(TDMA)
 Time frame is divided into slots (channels). Each user is
allocated a particular time slot or channel.
 Time Division Multiple Access (TDMA) is a digital
cellular telephone communication technology. It facilitates
many users to share the same frequency without
interference. Its technology divides a signal into different
timeslots, and increases the data carrying capacity.
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7.  In TDMA system signaling dimensions are divided along
the Time axis into non overlapping channels and each
end users assigned a different timeslot channel as shown
in fig.
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9. Space division multiple
access (SDMA)
 It control the energy radiated in space for every users.
 It offers service to different users by using different spot bean
antennas . These spot beans are satellite signals sent by a high
gain antenna which concentrate on power and covers service
to small geographical area.
 Highly directional antennas are use for frequency reuse at a
sufficiently large distance.
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10.  This figure shows a satellite system that uses space division multiple
access (SDMA) technology. In this example, a single satellite
contains several directional antennas. Some of these antennas use
the same frequency. This allows a single satellite to simultaneously
communicate to two different satellite receivers that operate on the
same frequency.
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11. Code division multiple
access (CDMA)
 In Code division multiple access (CDMA)
chipping codes/orthogonal codes /pseudorandom
codes are used shared between sender and the
receiver.
 When a user sent a message they encrypt it by
XOR-ing message with codes(unreadable form).
 On receiver side receiver can decrypt it using that
particular code only .hence it maintains
confidentiality.
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12.  It allows multiple access through a common channel
i.e. multiple users can share a common channel as
different codes are assign to them. .
 It is also not affected by noise and fading of
signal(reduce in strength) as it uses spread spectrum
technique.
 PROBLEM: Far-Near Problem.
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14. Packet radio access
 Packet radio is a form of packet switching technology
used to transmit digital data via radio or wireless
communications links. It uses the same concepts of
data transmission via Datagram that are fundamental to
communications via the Internet.
 There are three medium access protocol appropriate for
packet radio networks: ALOHA, slotted ALOHA,
and Carrier Sense Multiple Access (CSMA).
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15. Pure aloha
 In pure ALOHA, the stations transmit frames whenever they
have data to send.
 When two or more stations transmit simultaneously, there is
collision and the frames are destroyed.
 In pure ALOHA, whenever any station transmits a frame, it
expects the acknowledgement from the receiver.
 If acknowledgement is not received within specified time, the
station assumes that the frame (or acknowledgement) has
been destroyed.
 If the frame is destroyed because of collision the station waits
for a random amount of time and sends it again.
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16.  Therefore pure ALOHA dictates that when time-out period
passes, each station must wait for a random amount of time
before resending its frame. This randomness will help avoid more
collisions.
 Figure shows an example of frame collisions in pure ALOHA.
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17. Slotted aloha
 Slotted ALOHA was invented to improve the efficiency
of pure ALOHA as chances of collision in pure ALOHA
are very high.
 In slotted ALOHA, the time of the shared channel is
divided into discrete intervals called slots.
 The stations can send a frame only at the beginning of the
slot and only one frame is sent in each slot.
 In slotted ALOHA, if any station is not able to place the
frame onto the channel at the beginning of the slot i.e. it
misses the time slot then the station has to wait until the
beginning of the next time slot.
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18.  In slotted ALOHA, there is still a possibility of collision if
two stations try to send at the beginning of the same time
slot as shown in fig.
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19. PROBLEM
 Pure and slotted Aloha protocols do not
listen to the channels before transmitting
the data and hence they are unaware of
the information of other users in the
channel.
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20. CARRIER SENCE MULTIPLE
ACCESS CONTROL
 Carrier sense multiple access (CSMA) is based on a
protocol that each terminal in the network is able to
monitor the status of the channel before transmitting the
data.
 If the channel is ideal then the user is allowed to transmit
the data based on certain algorithm that is common to all
the transmitters on the network
 Here detection delay (time required to sense whether
the channel is ideal)and propagation delay (time
required to sent the data)are two important parameters.
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21.  Type of CSMA:
 1-Persistence CSMA ( continuous sensing of channel)
 Persistence CSMA( sense and if not ideal wait for
random
period of time and again start sensing)
 Non-persistence CSMA(combination of both)
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22. Multiple access collision
avoidance
 Multiple Access with Collision Avoidance (MACA) is a
protocol avoids the collision when simultaneous transmission
of data by more than one user(station) occurs that cannot sense
each other(HIDDEN STATION PROBLEM) .
 Also avoids Exposed node problem
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23.  Station A can communicate with Station B. Station C
can also communicate with Station B. However, Stations
A and C cannot communicate with each other since they
cannot sense each other on the network, because they are
out of range of each other.
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24.  The exposed node problem occurs when a node is
prevented from sending packets to other nodes because
of a neighbouring transmitter.
 EXAMPLE:
Consider an example of 4 nodes labeled R1, S1, S2, and R2,
where the two receivers (R1, R2) are out of range of each other,
yet the two transmitters (S1, S2) in the middle are in range of
each other. Here, if a transmission between S1 and R1 is taking
place, node S2 is prevented from transmitting to R2 as it will
interfere with the transmission by its neighbour S1. However
note that R2 could still receive the transmission of S2 without
interference because it is out of range of S1.
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25.  In MACA, a wireless network node announces that it is
going to send the data frame, informing the other nodes to
remain silent. When a node intends to transmit the data
frame, it communicates using a signal known as Request-
To-Send (RTS) that includes the length of the data frame
to transmit. If the recipient permits the transmission, it
responds back to the sender with a signal known as Clear-
To-Send (CTS), which includes the length of the data
frame that it is about to receive.
 In the meantime, the nodes that listen to the RTS signal
must remain silent until the data is fully transmitted in
order to avoid conflict with CTS.
 Nodes send acknowledgments after every successful
frame transmission.
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REQUEST TO
SENT
CLEAR TO
SENT
OTHERS
NODE
REMAINS
SILENT

27. REFERENCES
 1. https://en.wikipedia.org/wiki/Exposed_node_proble
 2.
http://www.mosharaf.com/blog/2009/09/30/macaw-a-media-access-protocol
 3. www.powershow.com
 4. slideplayer.com/slide/5101644/
 Wireless Communications & Networks
Author : Pearson
Publisher : William Stallings
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