Multiple access in wireless communication

Subject : Mobile Computing and Wireless Communication

CE-B

Maulik togadiya

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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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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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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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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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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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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Code division multiple access (CDMA)

Code division multiple access (CDMA) is a channel access method 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.

This allows several users to share a band of frequencies. To permit this without undue interference between the users, CDMA employs spread-spectrum technology.

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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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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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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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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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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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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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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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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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Carrier sense multiple access

Carrier sense multiple access (CSMA) is a probabilistic media access control (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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Type of CSMA: Persistence CSMA Non-persistence CSMA

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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.

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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