2. 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

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

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

5. Mobile Ad Hoc Networks Mobility causes route changes A B A B

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

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

8. CFABED RTS RTS = Request-to-Send IEEE 802.11 Pretending a circular range

9. CFABED RTS RTS = Request-to-Send IEEE 802.11 NAV = 10 NAV = remaining duration to keep quiet

10. CFABED CTS CTS = Clear-to-Send IEEE 802.11

11. CFABED CTS CTS = Clear-to-Send IEEE 802.11 NAV = 8

12. CFABED DATA DATA packet follows CTS. Successful data reception acknowledged using ACK. IEEE 802.11

13. CFABED ACK IEEE 802.11

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

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

16. A Comparison IssuesOmniDirectional Spatial Reuse LowHigh Connectivity LowHigh Interference OmniDirectional Cost & Complexity LowHigh

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

18. Antenna Model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time

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

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

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

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

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

24. B Directional Neighborhood A 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 C Transmit Beam Receive Beam

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

26. A Simple Directional MAC protocol Obvious 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

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

28. A B C D DNAV Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) Directional NAV (DNAV)

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

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

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

32. Issues with DMAC Node A beamformed in direction of D CB D A Two types of Hidden Terminal Problems –Due to unheard RTS/CTS Node A does not hear RTS/CTS from B & C

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

34. Issues with DMAC RTS X does not know node A is busy. X keeps transmitting RTSs to node A AB Using omni antennas, X would be aware that A is busy, and defer its own transmission X Z Y Deafness DATA

35. Issues with DMAC Uses DO links, but not DD links

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

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

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

39. Multi Hop RTS (MMAC) – Basic Idea A B C DE F G DO neighbors DD neighbors 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

40. Impact of Topology Nodes arranged in “linear” configuration reduce spatial reuse 802.11 – 1.19 Mbps DMAC – 2.7 Mbps 802.11 – 1.19 Mbps DMAC – 1.42 Mbps Aggregate throughput A FED BC A BC Power control may improve performance

41. Aligned Routes in Grid

42. Unaligned Routes in Grid

43. “Random” Topology

44. “Random” Topology: delay

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

46. 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

47. Routing

48. 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

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

50. 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 F J N J D K D misses request from K Optimize by having destination wait before replying RREP RREQ Use K antenna elements to forward broadcast packet

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

52. Conclusion Directional antennas can potentially benefit But also create difficulties in protocol design Other issues –Power control –Need better models for directional antennas –Capacity analysis –Multi-packet reception  Need to better understand physical layer

53. Thanks! www.crhc.uiuc.edu/~nhv for MAC: Mobicom 2002, Infocom 2000 Routing: Tech Report 2002

56. Performance Control overhead Throughput Vs Mobility Control overhead higher using DDSR Throughput of DDSR higher, even under mobility Latency in packet delivery lower using DDSR

57. Routing using Directional Antennas

58. Dynamic Source Routing [Johnson] Sender floods RREQ through the network Nodes forward RREQs after appending their names Destination node receives RREQ and unicasts a RREP back to sender node, using the route in which RREQ traveled

59. Route Discovery in DSR B A S E F H J D C G I K Z Y Represents a node that has received RREQ for D from S M N L

60. Route Discovery in DSR B A S E F H J D C G I K Represents transmission of RREQ Z Y Broadcast transmission M N L [S] [X,Y] Represents list of identifiers appended to RREQ

61. Route Discovery in DSR B A S E F H J D C G I K Z Y M N L [S,E] [S,C]

62. Route Discovery in DSR B A S E F H J D C G I K Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once Z Y M N L [S,C,G] [S,E,F]

63. Route Discovery in DSR B A S E F H J D C G I K Z Y M Nodes J and K both broadcast RREQ to node D N L [S,C,G,K] [S,E,F,J]

64. Route Reply in DSR B A S E F H J D C G I K Z Y M N L RREP [S,E,F,J,D] Represents RREP control message

65. DSR over Directional Antennas RREQ broadcast by sweeping –To use DD links

66. Route Discovery in DSR B A S E F H J D C G I K Z Y M Nodes J and K both broadcast RREQ to node D N L [S,C,G,K] [S,E,F,J]

67. Trade-off Larger Tx Range Fewer Hop Routes Few Hop Routes Low Data Latency Smaller Angle High Sweep Delay More Sweeping High Overhead

68. Route discovery latency … Single flow, grid topology (200 m distance) DSR DDSR4 DDSR6

69. Observations Advantage of higher transmit range significant only at higher distance of separation. Grid distance = 200 m --- thus no gain with higher tx range of DDSR4 (350 m) over 802.11 (250 m). –However, DDSR4 has sweeping delay. Thus route discovery delay higher

70. Throughput Sub-optimal routes chosen by DSR because destination node misses the shortest RREQ, while beamformed. DDSR18 DDSR9 DSR

71. Route Discovery in DSR F J D receives RREQ from J, and replies with RREP D misses RREQ from K N J RREP RREQ D K

72. Delayed RREP Optimization Due to sweeping – earliest RREQ need not have traversed shortest hop path. –RREQ packets sent to different neighbors at different points of time If destination replies to first arriving RREP, it might miss shorter-path RREQ Optimize by having DSR destination wait before replying with RREP

73. Routing Overhead Using omni broadcast, nodes receive multiple copies of same packet - Redundant !!! Broadcast Storm Problem Using directional Antennas – can do better ?

74. Use K antenna elements to forward broadcast packet. K = N/2 in simulations Routing Overhead Footprint of Tx  (No. Ctrl Tx)  (Footprint of Tx)  No. Data Packets Ctrl Overhead  =

75. Routing Overhead Control overhead reduces Beamwidth of antenna element (degrees)

76. Directional Antennas over mobile scenarios Frequent Link failures –Communicating nodes move out of transmission range Possibility of handoff –Communicating nodes move from one antenna to another while communicating

77. Directional Antennas over mobile scenarios Link lifetime increases using directional antennas. –Higher transmission range - link failures are less frequent Handoff handled at MAC layer –If no response to RTS, MAC layer uses N adjacent antenna elements to transmit same packet –Route error avoided if communication re-established.

78. Aggregate throughput over random mobile scenarios DSR DDSR9

79. Observations Randomness in topology aids DDSR. Voids in network topology bridged by higher transmission range (prevents partition) Higher transmission range increases link lifetime – reduces frequency of link failure under mobility Antenna handoff due to nodes crossing antenna elements – not too serious

80. Conclusion Directional antennas can improve performance But suitable protocol adaptations necessary Also need to use suitable antenna models … plenty of problems remain

81. Chicken and Egg Problem !! DMAC/MMAC part of UDAAN project –UDAAN performs 3 kinds of beam-forming for neighbor discovery –NBF, T-BF, TR-BF –Send neighborhood information to K hops –Using K hop-neighborhood information, probe using each type of beam-form –Multiple successful links may be established with the same neighbor

82. Nodes moving out of beam coverage in order of packet-transmission-time –Low probability Antenna handoff required –MAC layer can cache active antenna beam –On disconnection, scan over adjacent beams –Cache updates possible using promiscuous mode –Evaluated in [RoyChoudhury02_TechReport] Mobility

83. Side Lobes Side lobes may affect performance –Higher hidden terminal problems Node B may interfere at A when A is receiving from C BAC

84. Deafness in 802.11 Deafness 2 hops away in 802.11 C cannot reply to D’s RTS –D assumes congestion, increases backoff ABCD RTS

85. MMAC Hop Count Max MMAC hop count = 3 –Too many DO hops increases probability of failure of RTS delivery –Too many DO hops typically not necessary to establish DD link A B C DE F G DO neighbors DD neighbors

86. Broadcast Several definitions of “broadcast” –Broadcast region may be a sector, multiple sectors –Omni broadcast may be performed through sweeping antenna over all directions [RoyChoudhury02_TechReport] A Broadcast Region

87. DoA Detection Signals received at each element combined with different weights at the receiver

88. Why DO ? Antenna training required to beamform in appropriate direction –Training may take longer time than duration of pilot signal [Balanis00_TechReport] –We assume long training delay Also, quick DoA detection does not make MMAC unnecessary

89. Queuing in MMAC B C DE F G A

91. Impact of Topology Nodes arranged in linear configurations reduce spatial reuse for D-antennas 802.11 – 1.19 Mbps DMAC – 2.7 Mbps 802.11 – 1.19 Mbps DMAC – 1.42 Mbps Aggregate throughput A FED BC A BC

92. Protocol Stack Neighbor Discovery Routing Layer MAC Antenna Layer Transceiver Profile

93. Organization 802.11 Basics Related Work Antenna Model MAC Routing Conclusion