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Directional Antennas in Ad Hoc Networks

Explore the benefits of adding directional antennas in ad hoc networks, overcoming challenges, comparison with omnidirectional antennas, and the potential impact on network performance and protocols.

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Directional Antennas in Ad Hoc Networks

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  1. Directional AntennasinAd Hoc Networks Nitin Vaidya University of Illinois at Urbana-Champaign Joint work with Romit Roy Choudhury, UIUC Xue Yang, UIUC Ram Ramanathan, BBN

  2. Mobile Ad Hoc Networks • Formed by wireless hosts which may be mobile • Without necessarily using a pre-existing infrastructure • Routes between nodes may potentially contain multiple hops

  3. Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination

  4. Mobile Ad Hoc Networks (MANET) • Mobility causes route changes

  5. Why Ad Hoc Networks ? • Potential ease of deployment • Decreased dependence on infrastructure

  6. Many Applications • Personal area networking • cell phone, laptop, ear phone, wrist watch • Military environments • soldiers, tanks, planes • Civilian environments • taxi cab network • meeting rooms • sports stadiums • boats, small aircraft • Emergency operations • search-and-rescue • policing and fire fighting

  7. Many Variations • Fully Symmetric Environment • all nodes have identical capabilities and responsibilities • Asymmetric Capabilities • transmission ranges and radios may differ • battery life at different nodes may differ • processing capacity may be different at different nodes • Asymmetric Responsibilities • only some nodes may route packets • some nodes may act as leaders of nearby nodes (e.g., cluster head)

  8. Many Variations • Traffic characteristics may differ in different ad hoc networks • bit rate • timeliness constraints • reliability requirements • unicast / multicast / geocast • host-based addressing / content-based addressing / capability-based addressing • May co-exist (and co-operate) with an infrastructure-based network

  9. Many Variations • Mobility patterns may be different • people sitting at an airport lounge • New York taxi cabs • kids playing • military movements • personal area network • Mobility characteristics • speed • predictability • direction of movement • pattern of movement • uniformity (or lack thereof) of mobility characteristics among different nodes

  10. Challenges • Limited wireless transmission range • Broadcast nature of the wireless medium • Hidden terminal problem • Packet losses due to transmission errors • Mobility-induced route changes • Mobility-induced packet losses • Battery constraints • Potentially frequent network partitions • Ease of snooping on wireless transmissions (security hazard)

  11. Question • Can ad hoc networks benefit from the progress made at physical layer ? • Efficient coding schemes • Power control • Adaptive modulation • Directional antennas • … • Need improvements to upper layer protocols

  12. Directional Antennas

  13. Using Omni-directional Antennas A Frozen node B D S A

  14. Directional Antennas Not possible using Omni B D S C A

  15. Comparison

  16. Questions • Are Directional antennas beneficial in ad hoc networks ? • To what extent ? • Under what conditions ?

  17. Research Direction • Identify issues affecting directional communication • Evaluate trade-offs across multiple layers • Design protocols that effectively use directional capabilities Caveat: Work-in-Progress

  18. Preliminaries

  19. A B C Hidden Terminal Problem • Node B can communicate with A and C both • A and C cannot hear each other • When A transmits to B, C cannot detect the transmission using the carrier sense mechanism • If C transmits, collision may occur at node B

  20. RTS (10) CTS (10) RTS/CTS Handshake • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • RTS and CTS announce the duration of the transfer • Nodes overhearing RTS/CTS keep quiet for that duration C 10 A B D 10

  21. IEEE 802.11 • Physical carrier sense • Virtual carrier sense using Network Allocation Vector (NAV) • NAV is updated based on overheard RTS/CTS/DATA/ACK packets, each of which specified duration of a pending transmission • Nodes stay silent when carrier sensed busy (physical/virtual)

  22. Antenna Model

  23. Antenna Model • 2 Operation Modes: Omni & Directional

  24. Antenna Model • Omni Mode: • Omni Gain = Go • Idle node stays in Omni mode. • Directional Mode: • Capable of beamforming in specified direction • Directional Gain = Gd (Gd > Go)

  25. Directional Neighborhood C A B A and B are Directional-Omni (DO) neighbors B and C are Directional-Directional (DD) neighbors

  26. A Simple Directional MAC Protocol(DMAC)

  27. DMAC Protocol • A node listens omni-directionally when idle • Only DO links can be used • Sender node sends a directional-RTS using specified transceiver profile • Receiver of RTS sends directional-CTS

  28. DMAC Protocol • DATA and ACK transmitted and received directionally • Nodes overhearing RTS or CTS sets up NAV for that DOA (direction of arrival) • Nodes defer transmitting only in directions for which NAV is set

  29. Directional NAV (DNAV) • Node E remembers directions in which it has received RTS/CTS, and blocks these directions. • Transmission initiated only if direction of transmission does not overlap with blocked directions.

  30. Directional NAV (DNAV) • E has DNAV set due to RTS from H. Can talk to B since E’s transmission beam does not overlap.

  31. Example C E B D A B and C communicate D & E cannot: D blocked with DNAV D and A communicate

  32. RTS Issues with DMAC • Hidden terminals due to asymmetry in gain • A does not get RTS/CTS from C/B B C A Data A’s RTS may interfere with C’s reception of DATA

  33. Problems with DMAC • Hidden terminals due to directionality • Due to unheard RTS/CTS D B C A A beamformed in direction of D  A does not hear RTS/CTS from B/C A may now interfere at C

  34. Issues with DMAC: Deafness • Deafness Z RTS A B DATA RTS Y RTS X does not know node A is busy. X keeps transmitting RTSs to node A X With 802.11 (omni antennas), X would be aware that A is busy, and defer its own transmission

  35. Problems with DMAC • Shape of Silenced Regions Region of interference for directional transmission Region of interference for omnidirectional transmission

  36. Problems with DMAC • Since nodes are in omni mode when idle, RTS received with omni gain • DMAC can use DO links, but not DD links C A B

  37. DMAC Trade-off • Disadvantages • Increased hidden terminals • Deafness • Directional interference • Uses only DO links • Benefits • Better Network Connectivity • Spatial Reuse

  38. Solving DMAC Problems • Are improvements possible to make directional MAC protocols more effective ? • One possible improvement: Use DD links

  39. Using DD Links • Possible to exploit larger range of directional antennas. C A A & C are DD neighbors, but cannot communicate with DMAC If A & C could be made to point towards each other, single hop communication may be possible

  40. DO neighbors D E DD neighbors F C A B G Multi-Hop RTS: Basic Idea A source-routes RTS to D through adjacent DO neighbors (i.e., A-B-C-D) When D receives RTS, it beamforms towards A, forming a DD link.

  41. D E H F C A B G MMAC protocol • A transmits RTS in the direction of its DD neighbor, node D • Blocks H from communicating in the direction H-D • A then transmits multi-hop RTS using source route • A beamforms towards D and now waits for CTS

  42. D E H F C A B G MMAC protocol • D receives MRTS from C and transmits CTS in the direction of A (its DD neighbor). • A initiates DATA communication with D • H, on hearing RTS from A, sets up DNAVs towards both H-A and H-D. Nodes B and C do not set DNAVs. • D replies with ACK when data transmission finishes.

  43. Performance • Simulation • Qualnet simulator 2.6.1 • CBR traffic • Packet Size – 512 Bytes • 802.11 transmission range = 250 meters. • Channel bandwidth 2 Mbps • Mobility - none

  44. Impact of Topology • Nodes arranged in linear configurations reduce spatial reuse for directional antennas

  45. Impact of Topology IEEE 802.11 = 1.19 Mbps DMAC = 2.7 Mbps IEEE 802.11 = 1.19 Mbps DMAC = 1.42 Mbps

  46. “Aligned” Flows MMAC 802.11 DMAC

  47. “Unaligned” Flows MMAC 802.11 DMAC

  48. “Unaligned” Flows & Topology MMAC 802.11 DMAC

  49. Delay: “Unaligned” Flows & Topology

  50. Directional MAC: Summary • Directional MAC protocols can improve throughput and decrease delay • But not always • Performance dependent on topology

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