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Outline. Introduction Mobile Ad hoc Networks (MANETS) Antennas Multiple Access Protocols Mode Selection Criteria Motivations Assumptions Node Model Antenna Pattern Simulation Parameters Performance Evaluation Applicability of Mode Selection Criteria to Multiple Beam Antennas

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  1. Outline • Introduction • Mobile Ad hoc Networks (MANETS) • Antennas • Multiple Access Protocols • Mode Selection Criteria • Motivations • Assumptions • Node Model • Antenna Pattern • Simulation Parameters • Performance Evaluation • Applicability of Mode Selection Criteria to Multiple Beam Antennas • Conclusions • Future Work

  2. Mobile Ad Hoc Networks (MANETs) • Peer-to-peer connectivity • Lack of fixed infrastructure relays • Absence of centralized authority • Multi-hop forwarding to ensure network connectivity • Applications • Military.. Combat Systems, reconnaissance • Rescue, medical emergency, telemedicine

  3. Antenna Types • Omni-directional antenna • Transmits power equally in all directions • Directional antenna • Concentrates power in a directed zone • Smart Antenna • Has the in-built intelligence to change direction according to requirement (steer the beam) • Multiple-Beam Smart Antenna • Simultaneous transmission/reception in more than one directions • Multiple Input Multiple Output (MIMO) • Multiple streams of data in same channel.

  4. Smart Antenna System

  5. Antennas and MANETs • Omni-directional communication suffers from poor spatial reuse • Directional communication leads to better spatial reuse, reduces co-channel interference and provides range extension

  6. Multiple Access Protocols • MAC Proposals differ based on • How RTS/CTS transmitted (omni, directional) • Transmission range of directional antennas • Channel access schemes • Omni or directional NAVs • Antenna Model • Two Operation modes • Omni & Directional • Omni Mode: • Omni Gain = Go • Idle node stays in Omni mode • Directional Mode: • Capable of beamforming in specified direction • Directional Gain = Gd (Gd > Go) --> Range Extension • Directional Gain = Gd (Gd = Go) --> Spatial Reuse Range Extension Spatial Reuse

  7. Directional vs. Omni-directional • The Problem of utilizing directional antennas to improve the performance of ad hoc networks is non-trivial • Pros • Higher gain (Reduced interference) • Spatial Reuse • Cons • Potential possibility to interfere with communications taking place far away • Hidden Terminal • Deafness

  8. Motivations • Which mode? Omni-directional or directional • Analyze various topologies involving neighboring transmissions or receptions • Formulate mode selecting criteria for medium access control (MAC) for MANETs with heterogeneous technologies

  9. Assumptions • Two modes of operation: omni and directional • Directional transmission ofRTS/CTS/DATA/ACK in directional mode • Transmission range of directional antennas is same as that of omni-directional ==> Spatial Reuse • 4-Way CSMA for medium access control • The channel is symmetric

  10. Node Model • The node model of advance MANET available in OPNET is modified to facilitate directional mode of communication • In directional mode, the antenna (tx_rx_ant) points in the desired direction with the help of antenna pointing processor (tx_rx_point)

  11. Antenna Pattern Conical directional antenna pattern of main lobe having beam-width of 45 degrees and a gain of 0 dBi. The gain in remaining spherical side-lobe is confined to -20dBi

  12. Simulation Parameters Packets generated = 200 packets/sec/transmitter Maximum achievable throughput ~ 130 packets/sec/receiver (non-overlapping communication) ~ 65 packets/sec/receiver (two overlapping transmissions)

  13. Performance Evaluation – Deaf • Deaf communicating pair scenario • Receivers in same beam of the transmitter • Transmitters in same beam of the receiver • Both the transmitters are deaf to each other communication • Omni-directional mode performs better

  14. Performance Evaluation – Deaf Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode

  15. Performance Evaluation – Deaf Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver. However, average delay is nearly same in both cases

  16. Performance Evaluation – Common Receiver • Common receiver scenario • Two or more transmitters with common receiver • Usually both the transmitters are deaf to each other communication • Omni-directional mode performs better

  17. Performance Evaluation – Common Receiver Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode

  18. Performance Evaluation – Common Receiver Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver.

  19. Performance Evaluation – Linear_Pair_SameBeam • Another communicating pair in the same beam of the transmitter • Throughput of C-D pair suffers due to interference from A-B ongoing communication in directional mode • For optimal performance C switches to omni mode while other remains in directional mode

  20. Performance Evaluation – Linear_Pair_SameBeam Switching C to omni-directional mode while remaining nodes in directional mode gives optimal throughput

  21. Performance Evaluation – Linear_Pair_SameBeam Delay is less in directional mode as all newly generated packets are transmitted while packets in queue are dropped after maximum retransmission attempts

  22. Performance Evaluation – Linear_Pair_SameBeam Retransmission attempts by node C are much higher in directional mode owing to higher BER (i.e. collisions) at node D

  23. Performance Evaluation – Tx_0 • Another node transmitting in same direction • Again switching the mode of intermediate transmitting node to omni-directional mode while remaining with directional mode yields optimal performance

  24. Performance Evaluation – Tx_0 Average throughput in directional mode is about 15% lower than in omni-directional mode

  25. Performance Evaluation – Tx_0 BER is much higher in directional mode due to interference from transmitters as they are deaf to each other

  26. Performance Evaluation – Tx_90 and Rx_90 • Another non-interfering transmitter or receiver in the communicating beams • Omni-mode restricts simultaneous transmissions, hence directional mode is recommended Tx_90 Rx_90

  27. Performance Evaluation – Tx_90 and Rx_90 Directional communication achieves maximum possible throughput in all cases owing to better spatial reuse

  28. Performance Evaluation – Tx_90 and Rx_90 Delay is more in omni-directional communication due to increased media access delay at the transmitters

  29. Performance Evaluation – Tx_90 and Rx_90 Due to increased channel contention at the transmitters packet retransmission attempts are more in omni-directional mode

  30. Performance Evaluation – Linear, Parallel and X topologies • Only the intended receiver or transmitter in the communicating beams • Both the transmitters are deaf to each other communication • No other communicating node in those beams • Directional mode outperforms omni-directional mode of communication Linear X Parallel

  31. Performance Evaluation – Linear, Parallel and X topologies Traffic Received (packets/sec) vs. time

  32. Mode Selection Criteria • All nodes in omni-directional mode in the following cases: • Deaf communicating pair scenario • Receivers in same beam of the transmitter • Transmitters in same beam of the receiver • Both the transmitters are deaf to each other communication • Common receiver scenario • Two or more transmitters with common receiver • Intermediate transmitting node in omni-directional mode while other nodes in directional mode for the following cases: • Another communicating pair in the same beam of the transmitter • Another node transmitting in same direction • All nodes in directional mode, in the remaining cases including: • Another non-interfering transmitter or receiver in the communicating beams • Only the intended receiver or transmitter in the communicating beams

  33. Applicability of Mode Selection Criteria to Multiple Beam Antennas • Multiple beam antennas • Can either transmit or receive multiple packets simultaneously. This requires: • Packet receptions in different beams at the node to commence at the same time • Packet transmissions by a node in multiple beams to begin simultaneously • A node cannot both send and receive data at the same time • Can simulate omni-directional mode by transmitting in all possible beams simultaneously Can multiple beam antennas achieve optimal performance by transmitting control packets in beams having transmitters and receivers only ???

  34. Conclusions • Directional mode • Better spatial reuse • Enhances system capacity • Deafness and hidden terminal problems • However there are some cases where omni-directional mode performs better • Deaf communicating pair scenario • Interference from side-lobes cannot be ruled out • Common receiver scenario • Mode Selection Criteria forms the basis of developing MAC protocols for MANETs using heterogeneous antenna technologies • Dynamically switching a node from directional to omni-directional or vice versa depending on the neighboring nodes

  35. Future Work • Work needs to be extended for multi-hop topologies • Extensive study needs to be done with more communicating pairs within the vicinity so that performance varies with the node density • Game Theoretic approach for mode selection criteria in such scenarios • Performance of multiple beam antennas transmitting control packets in beams having transmitters and receivers only, need to be evaluated

  36. Physical Carrier Sense Physical Carrier Sensing Virtual Carrier Sensing IEEE 802.11 DCF – RTS/CTS access scheme IEEE 802.11

  37. Antenna System Phased Array Antenna

  38. Direction of Arrival Estimation

  39. Beam Formation • Beam Forming • Technique in which the gain pattern of an adaptive array is steered to a desired direction through either beam steering or null steering signal processing algorithms • Adaptive beam forming algorithms can provide substantial gains (of the order of 10log(M) dB, where M is number of array elements) as compared to omni directional antenna system Antenna Pattern of 7-element uniform equally spaced circular array.

  40. Switched Beam Consists of a set of predefined beams. Allows selection of signal from desired user. Beams have narrow main lobe and small side-lobes. Smart Antenna System • Signals received from side-lobes can be significantly attenuated. • Uses a linear RF network, called a Fixed Beam-forming Network (FBN) that combines M antenna elements to form up to M directional beams.

  41. General Smart Antenna Architecture Source: Chris Loadman, Zhizhang Chen and Dylan Jorgensen, “An Overview of Adaptive Antenna Technologies For Wireless Communications,” In Proc. o Communication Networks and Services Research Conference (CNSR), pp 15-19, 2003.

  42. Features and Benefits of Smart Antenna Systems Source: http://hosteddocs.ittoolbox.com/MI102204.pdf

  43. The global market for smart antennas growth Source: US analyst firm Visant Strategies

  44. A terminal with 16 antennas mounted on a laptop Source: Alexiou, A.and Haardt, M., “Smart antenna technologies for future wireless systems: trends and challenges,” IEEE Communications Magazine, Vol. 42, pp. 90-07, Sept. 2004

  45. MIMO PC Card Source: http://www.airgonetworks.com/pdf/Farpoint Group 2003-242.1 MIMO Comes of Age.pdf

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