multiple access in wireless communication

multiple access

1. Multiple access in
wireless communication
Subject: Mobile Computing and Wireless Communication
CE-B
Maulik togadiya

2. What is Multiple access?
Multiple access methodallows several terminalsconnected
to the same multi-pointtransmission mediumto transmit
over it.Examples of shared physical media arewireless
networks,bus networks, ring networksandhalf
duplexpoint-to-point links.
Multiple access schemes to allow many users to share
simultaneously a finite amount of radio spectrum resources.
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4. 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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5. 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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6. 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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7. 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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8. 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. Code division multiple access
(CDMA)
Code division multiple access(CDMA) is achannel
access methodused by variousradio communication
technologies.
CDMA is an example ofmultiple access , where
several transmitters can send information
simultaneously over a single communication channel.
This allows several users to share a band of
frequencies. To permit this without undue interference
between the users, CDMA employsspread-spectrum
technology.
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10. In this system, a user has access to the whole bandwidth
for the entire duration. The basic principle is that
different CDMA codes are used to distinguish among the
different users.
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11. Space division multiple access
(CDMA)
If there are no separation in frequency or time or code
domain ,an alternative way of separating different users is
spatial separation
SDMA is a MIMO -based wireless communication network
architecture, primarily suitable for mobile ad-hoc networks,
which enables access to a communication channel by
identifying the user location and establishing a one-to-one
mapping between the network bandwidth division and the
identified spatial location.
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12. This figure shows a satellite system that uses spatial 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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13. Packet radio access
Packet radiois a form ofpacket switching technology used
to transmitdigital dataviaradio or wireless
communicationslinks. It uses the same concepts of data
transmission via Datagram that are fundamental to
communications via theInternet.
There are three medium access protocol appropriate for
packet radio networks:ALOHA,slotted ALOHA,
andCarrier Sense Multiple Access(CSMA).
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14. Pure aloha
Inpure 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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15. 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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16. 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 sloti.e.it misses the
time slot then the station has to wait until the beginning of the
next time slot.
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17. 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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18. Carrier sense multiple access
Carrier sense multiple access(CSMA) is a probabilistic
mediaaccesscontrol (MAC) protocol in which a node
verifies the absence of other traffic before transmitting on
transmission medium.
CSMA is set of rule that which the device attached to the
network first determine whether the channel or carrier is in
use or free and then act accordingly.
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19. Type of CSMA:
Persistence CSMA
Non-persistence CSMA
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20. Multiple access collision avoidance
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Multiple Access with Collision Avoidance (MACA) is a
protocol for slotted media access control used in wireless
LAN data transmission. MACA is used to avoid data
collisions caused by hidden station problems as well as
simplifying known exposed station problems.

21. 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.
WLAN data transmission collisions can still happen, and
MACA for Wireless (MACAW) is brought to extend the
functionality of MACA. It demands nodes to send
acknowledgments after every successful frame
transmission.
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