Directional Antennas in Ad Hoc Networks

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