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DOA-ALOHA: Slotted ALOHA for Ad Hoc

Networking Using Smart Antennas

Harkirat Singh & Suresh Singh

Portland State University, OR, USA

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• What is an ad hoc network• Smart Antenna Overview• Protocol description• Implementation of the protocol

within OPNET• Performance study of the protocol• Summary

Outline

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Ad Hoc Networks

A B

C D

Formed by co-operating wireless nodes No fixed network infrastructure

No centralized administration - Each node acts as a router

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MAC in Wireless networks

• Uses MAC protocol of IEEE 802.11 based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

• Basic channel access method can not combat hidden and exposed terminal problems

• RTS and CTS are used to reserve the channel for the entire duration of the transmission of data (including ACK)

• Physical and virtual carrier sensing is used for Collision Avoidance

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Antenna in Wireless networks

• Uses Omni-Directional Mode • Limited spatial reuse of the channel

A B

CD

A B

C D

If (C,D) are transmitting A & B cannot, with directionalantenna simultaneous sessions are possible

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Smart Antennas

Schematic of a smart antenna (adaptive linear array)

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Smart Antennas• Adaptive Antenna Arrays can direct the Radiation /

receiving pattern (main lobe) towards the desired node

• Signals received by multiple antennas are weighed and combined to maximize ‘SINR’ (Signal-to-Interference plus Noise ratio)

• Weight Vectors obtained will give information about the desired node position

• Weight Vectors can be computed to ‘Null’ undesired signals

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Smart Antennas

Received Power

(Transmit power) *(Tx Gain) * (Rx Gain)

Directional gain is higher, with Nulling Rx

Gain can be negligible

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Protocol Description

Direction-of-Arrival (DOA)-ALOHA is based on Slotted-ALOHA protocol

During DOA Minislot Tx and Rx discovers each other

Tx sends pure tone towards intended Rx

DOA Minislot ACK Minislot

DATA Transmission

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Protocol Description

The largest minislot is for the data transmission

Receiver rejects the packet if not an intended destination

Receiver sends ACK if data correctly received

Sender performs back-off if no ACK received (similar to Slotted-ALOHA)

Do not do Collision Avoidance (CA) but exploit Nulling!

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D

A

BC

E

F

Protocol Description

Node ‘A’ receives max power from node ‘B’, hence,

places main lobe towards B and Nulls towards D & F

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Implementation of the protocol within OPNET Adaptive Antenna Array is implemented in Matlab and antenna module calls the Matlab routines

A node has no packet scheduled for transmission issues a remote interrupt to antenna to compute weights for omni-direction mode

Transmitter MAC calls antenna module with desired direction which invokes Matlab routines to determine weights

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Implementation of the protocol within OPNET• During the duration of the DOA-Minislot, dra-power pipeline

stage computes the direction and the received power of all the signals

• Antenna module inserts (pw, dir) pair in a dynamic list

• Max power direction is the desired direction and all the other received signals are interfereres

• Antenna module invokes Matlab routine with input parameter (desired_DOA, interferers) and returns new weights

• We use Minimum Mean Square Error ( MMSE) algorithm for Nulling

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Implementation of the protocol within OPNET

a c and b d, ‘d’ mistakenlyForms a beam towards ‘a’

If a node beamforms incorrectly in a given timeslot, it remembers that direction in single-entry cache

During next slot node ignores maximum signal strength direction, if same, it selects second strongest signal Cache is not updated if a node correctly receives the packet and cache is reset if no signal from that direction

a

b

d

c

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Performance StudySimulation Parameters

Smart antenna implemented in Matlab and interfaced with Opnet

2 Mbps channel and free space propagation Grid Topology used

4 simultaneous flows of CBR traffic considered

512 Byte packet size used

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Some Aligned Routes in Grid

Sending rate (Tx) vs Aggregate Throughput

Ag

gre

gate

Th

rou

gh

pu

t (K

bp

s)

Sending Rate (Kbps)

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Unaligned Routes in Grid

Sending rate (Tx) vs Aggregate Throughput

Ag

gre

gate

Th

rou

gh

pu

t (K

bp

s)

Sending Rate (Kbps)

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“Random” Topology

Sending rate (Tx) vs Aggregate Throughput

Ag

gre

gate

Th

rou

gh

pu

t (K

bp

s)

Sending Rate (Kbps)

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• Power control• Impact on Routing• Extend study to multipath environments

Conclusion and Future Work

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Thank You

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• J. C. Liberti and T. S. Rappaport. Smart Antennas for Wireless Communications. Prentice Hall, 1999.

• Nitin H. Vaidya Romit Roy Choudhury, Xue Yang, and Ram Ramanathan. Using directional antennas for medium access control in ad hoc networks. In ACM/SIGMOBILE MobiCom 2002, 23 – 28 Sep 2002.

• www.eas.asu.edu/~trccomm/nsf/presentations/ Mar_21_Ravi_Govindarajula.pdf

• http://www.crhc.uiuc.edu/~croy/presentation.html

References