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MAC for Directional Antenna

Redes Ad Hoc Sem FioProf. Marco Aurélio Spohn

DSC UFCG2010.1

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MAC for Directional Antenna

Benefits of Directional Antenna

More spatial reuse With omni-directional antenna, packets intended to one

neighbor reaches all neighbors as well

Node A

Node B

Node C

Node A

Node B

Node C

Omni-directional transmission Directional transmission

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MAC for Directional Antenna

Benefits of Directional Antenna

Increase “range”, keeping transmit power constant

Reduce transmit power, keeping range comparable with omni mode

Reduces interference, potentially increasing spatial reuse

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More Spatial Reuse

A B

C D

Omni-directional antenna

While A is transmitting to B, C cannottransmit to D

A B

C D

Directional antenna

Both A and C can transmit simultaneously

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Antenna Model

2 Operation Modes: Omni and Directional

A node may operate in any one mode at any given time

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Antenna Model

In Omni Mode:

Nodes receive signals with gain Go

While idle a node stays in omni mode

In Directional Mode:

Capable of beamforming in specified direction

Directional Gain Gd (Gd > Go)

Symmetry: Transmit gain = Receive gain

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Multi-Beam Adaptive Array (MBAA) directional Antennas

An MBAA antenna can successfully receive and transmit one or more overlapping packets at the same time by pointing its beams toward individual packet directions, while annulling all other unwanted directions.

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Multi-Beam Adaptive Array (MBAA) directional Antennas

Node d transmitting to nodes b and c at the same time.

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Two “directional” models

Steerable: can point to any direction (there is a minimum beam-width)

Switched-beam: many antenna components cover all directions (i.e., 4 antenna elements covering each 90 degrees)

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Directional Packet Transmission

A B

B’s omni receive range

D-O transmission

A B

B’s directional receive beam

D-D transmission

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MAC Designs for Directional Antenna

Most proposals use RTS/CTS dialog

They differ in how RTS/CTS are transmitted

Omni-directional transmit: ORTS, OCTS

Directional transmit: DRTS, DCTS

Current proposals: ORTS/OCTS [Nasipuri’00]

DRTS/OCTS [Ko’00]

DRTS/DCTS [Choudhury’02]

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ORTS/OCTS

Sender sends omni-directional RTS Receiver sends omni-directional CTS

Receiver also records direction of sender by determining the antenna on which the RTS signal was received with highest power level

Similarly, the sender, on receiving CTS, records the direction of the receiver

All nodes overhearing RTS/CTS defer transmissions

Sender then sends DATA directionally to the receiver

Receiver sends directional ACK

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ORTS/OCTS – cont.

Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA

All neighbours of sender/receiver defer transmission on receiving omni-directional RTS/CTS

spatial reuse benefit not realized

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D-MAC

Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally

Directional RTS (DRTS) andOmni-directional CTS (OCTS)

RTS and CTS block antenna elements, making it possible to transmit/receive even though a node hear a RTS/CTS

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DATA

DRTS(B)

OCTS(B,C) OCTS(B,C)

ACK

A B C ED

DRTS(D)

DATA

ACK

OCTS(D,E)

DMAC: DRTS/OCTS

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Directional NAV

Physical carrier sensing still omni-directional

Virtual carrier sensing directional – directional NAV

When RTS/CTS received from a particular direction, record the direction of arrival and duration of proposed transfer

Channel assumed to be busy in the direction from which RTS/CTS received

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Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)

X

D

Y

CCTS

Directional NAV (DNAV)

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Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA)

X Y

Directional NAV (DNAV)

D

C DNAV

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Directional NAV (DNAV)

A

B

Cθ

DNAVD

New transmission initiated only if direction of transmission does not overlap with DNAV, i.e., if (θ > 0)

RTS

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Deafness: A MAC problem with directional antennas

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ToneDMAC

Use out-of-band tone acting as a feedback mechanism to all neighbors

After the completion of every dialog, a communicating node transmits an out-of-band tone, OMNIDIRECTIONALLY!

This tone helps other nodes to distinguish deafness from congestion

If the tone is received from the intended receiver, a node deduces deafness and sets backoff timer appropriately

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Acknowledgments

Presentation adapted from the following sources: Prasant Mohapatra, UC Davis, ://www.cs.ucdavis.edu/~prasant/ECS257/NOTES/Adhoc-Sensor.ppt Katia Obraczka and J.J. Garcia-Luna-Aceves, UCSC http://www.cse.ucsc.edu/classes/cmpe257/Spring05