Using Directional Antennas in Ad Hoc Networks (UDAAN)

1. Using Directional Antennas in Ad Hoc Networks (UDAAN) Nitin H. Vaidya University of Illinois at Urbana-Champaign Joint work with Romit Roy Choudhury Xue Yang University of Illinois Ram Ramanathan BBN Technologies

2. Broad Theme • Impact of physical layer mechanisms on upper layers • Adaptive modulation • Power control • Directional antennas

3. Ad Hoc Networks • Formed by wireless hosts without requiring an infrastructure • May need to traverse multiple links to reach a destination A A B B

4. Mobile Ad Hoc Networks • Mobility causes route changes A A B B

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

6. Antennas • Wireless hosts typically use single-mode antennas • Typically, thesingle-mode = omni-directional • Much of the discussion here applies when the single-mode is not omni-directional

7. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F Pretending a circular range

8. IEEE 802.11 RTS = Request-to-Send RTS A B C D E F NAV = 10 NAV = remaining duration to keep quiet

9. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F

10. IEEE 802.11 CTS = Clear-to-Send CTS A B C D E F NAV = 8

11. IEEE 802.11 • DATA packet follows CTS. Successful data reception acknowledged using ACK. DATA A B C D E F

12. IEEE 802.11 ACK A B C D E F

13. Omni-Directional Antennas Red nodes Cannot Communicate presently X D C Y

14. Directional Antennas Not possible using Omni X D C Y

15. A Comparison

16. Question • How to exploit directional antennas in ad hoc networks ? • Medium access control • Routing

17. Antenna Model 2 Operation Modes: OmniandDirectional A node may operate in any one mode at any given time

18. Antenna Model In Omni Mode: • Nodes receive signals with gain Go • While idle a node stays in omni mode In Directional Mode: • Capable of beamforming in specified direction • Directional Gain Gd(Gd > Go) Symmetry: Transmit gain = Receive gain

19. Caveat Abstract antenna model • Results only as good as the abstraction On-going work: More accurate antenna models

20. Directional Communication • Received Power •  • (Transmit power) *(Tx Gain) * (Rx Gain) • Directional gain is higher

21. Potential Benefits ofDirectional Antennas • Increase “range”, keeping transmit power constant • Reduce transmit power, keeping range comparable with omni mode • Realizing only the second benefit easier

22. Neighbors • Notion of a “neighbor” needs to be reconsidered • Similarly, the notion of a “broadcast” must also be reconsidered

23. Directional Neighborhood Receive Beam Transmit Beam B A C • When C transmits directionally • Node A sufficiently close to receive in omni mode • Node C and A are Directional-Omni (DO) neighbors • Nodes C and B are not DO neighbors

24. Directional Neighborhood Transmit Beam Receive Beam A C B • When C transmits directionally • Node B receives packets from C only in directional mode • C and B are Directional-Directional (DD) neighbors

25. A Simple Directional MAC protocolObvious generalization of 802.11 • A node listens omni-directionally when idle • Sender transmits Directional-RTS (DRTS) towards receiver • RTS received in Omni mode (idle receiver in when idle) • Receiver sends Directional-CTS (DCTS) • DATA, ACK transmitted and received directionally

26. Directional NAV (DNAV) • Nodes overhearing RTS or CTS set up directional NAV(DNAV)for thatDirection of Arrival (DoA) B CTS D A C

27. Directional NAV (DNAV) • Nodes overhearing RTS or CTS set up directional NAV(DNAV)for thatDirection of Arrival (DoA) B D DNAV A C

28. Directional NAV (DNAV) • New transmission initiated only if direction of transmission does not overlap with DNAV,i.e., if (θ > 0) B D DNAV θ A C RTS

29. DMAC Example C E B D A B and C communicate D and E cannot: D blocked with DNAV from C D and A communicate

30. Data RTS Issues with DMAC • Two types of Hidden Terminal Problems • Due to asymmetry in gain B C A A is unaware of communication between B and C A’s RTS may interfere with C’s reception of DATA

31. Issues with DMAC • Two types of Hidden Terminal Problems • Due to unheard RTS/CTS D B C A • Node A beamformed in direction of D • Node Adoes nothear RTS/CTS from B & C

32. Issues with DMAC • Two types of Hidden Terminal Problems • Due to unheard RTS/CTS D B C A Node A may now interfere at node C by transmitting in C’s direction

33. Issues with DMAC • 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 Using omni antennas, X would be aware that A is busy, and defer its own transmission

34. Issues with DMAC • Uses DO links, but not DD links

35. DMAC Tradeoffs • Benefits • Better Network Connectivity • Spatial Reuse • Disadvantages • Hidden terminals • Deafness • No DD Links

36. Enhancing DMAC • Are improvements possible to make DMAC more effective ? • One possible improvement: Make Use of DD Links

37. Using DD Links Exploit larger range of Directional antennas Transmit Beam Receive Beam C A A and C are DD neighbors, but cannot communicate using DMAC

38. DO neighbors D E DD neighbors F C A B G Multi Hop RTS (MMAC) – 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

39. D E F A B C Impact of Topology Aggregate throughput 802.11 – 1.19 Mbps DMAC – 2.7 Mbps Nodes arranged in “linear” configuration reduce spatial reuse Aggregate throughput 802.11 – 1.19 Mbps DMAC – 1.42 Mbps A B C Power control may improve performance

40. Aligned Routes in Grid

41. Unaligned Routes in Grid

42. “Random” Topology

43. “Random” Topology: delay

44. MMAC - Concerns • Lower probability of RTS delivery • Multi-hop RTS may not reach DD neighbor due to • deafness or collision • Neighbor discovery overheads may offset the advantages of MMAC

45. Directional MAC: Summary • Directional MAC protocols show improvement in aggregate throughput and delay • But not always • Performance dependent on topology • “Random” topology aids directional communication

46. Routing

47. Routing Protocols • Many routing protocols for ad hoc networks rely on broadcast messages • For instance, flood of route requests (RREQ) • Using omni antennas for broadcast will not discover DD links • Need to implement broadcast using directional transmissions

48. Directional Routing Broadcast by sweeping Tradeoffs • Larger Tx Range Fewer Hop Routes • Few Hop Routes Low Data Latency • Small Beamwidth High Sweep Delay • More Sweeping High Overhead

49. F J J D K N Issues • Sub-optimal routes may be chosen if destination node misses shortest request, while beamformed • Broadcast storm: Using broadcasts, nodes receive multiple copies of same packet D misses request from K Optimize by having destination wait before replying RREP RREQ Use K antenna elements to forward broadcast packet

50. Performance • Preliminary results indicate that routing performance can be improved using directional antennas