DOAALOHA: Slotted ALOHA for Ad Hoc Networking Using Smart Antennas

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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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Outline

• What is an ad hoc network
• Smart Antenna Overview
• Protocol description
• Implementation of the protocol within OPNET
• Performance study of the protocol
• Summary

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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
A B
C D
C
D
If (C,D) are transmitting A B cannot, with
directional antenna 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

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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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Protocol Description
B
D
C
A
E
F
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
c

• a ? c and b ? d, d mistakenly
• Forms a beam towards a
• If a node beamforms incorrectly
• in a given timeslot, it remembers
• that direction in single-entry cache

a
d
b

• 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
  

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Performance Study
Simulation 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
Aggregate Throughput (Kbps)
Sending Rate (Kbps)
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Unaligned Routes in Grid
Sending rate (Tx) vs Aggregate Throughput
Aggregate Throughput (Kbps)
Sending Rate (Kbps)
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Random Topology
Sending rate (Tx) vs Aggregate Throughput
Aggregate Throughput (Kbps)
Sending Rate (Kbps)
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Conclusion and Future Work

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

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Thank You
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References

• 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