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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. Ad Hoc Networks. A. B. C. D. Formed by co-operating wireless nodes No fixed network infrastructure

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

  2. 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

  3. 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

  4. A B A B C D C D If (C,D) are transmitting A & B cannot, with directional antenna simultaneous sessions are possible Antenna in Wireless networks • Uses Omni-Directional Mode • Limited spatial reuse of the channel

  5. Smart Antennas Schematic of a smart antenna (adaptive linear array)

  6. 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

  7. Smart Antennas Received Power  (Transmit power) *(Tx Gain) * (Rx Gain) Directional gain is higher, with Nulling Rx Gain can be negligible

  8. Example (Beamforming) Antenna pattern with 8 antenna elements Desired=45 deg, Nulls=20,70 deg Desired=45 deg, Nulls=10, 20, 30, 70, 80, 90 deg

  9. DOA Minislot ACK Minislot DATA Transmission D B C A E F Protocol Description • Direction-of-Arrival (DOA)-ALOHA is based on Slotted-ALOHA protocol Node ‘A’ receives max power from node ‘B’ hence places main lobe towards B and Nulls towards D & F

  10. 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!

  11. 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

  12. 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

  13. MUSIC Spatial Spectrum - Receiver Power (in dB) Direction of Arrival (DOA) deg

  14. 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

  15. Performance Study Simulation Parameters

  16. Some Aligned Routes in Grid Sending rate (Tx) vs Aggregate Throughput Aggregate Throughput (Kbps) Sending Rate (Kbps)

  17. Unaligned Routes in Grid Sending rate (Tx) vs Aggregate Throughput Aggregate Throughput (Kbps) Sending Rate (Kbps)

  18. “Random” Topology Sending rate (Tx) vs Aggregate Throughput Aggregate Throughput (Kbps) Sending Rate (Kbps)

  19. Summary • Power control • Impact on Routing • Extend study to multipath environments

  20. Thank You

  21. 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

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