1. Utilizing 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. UDAAN DARPA FCS communications project
Focus on exploiting directional antennas for ad hoc networking

4. UDAAN Protocol Stack Neighbor Routing Layer Discovery BBN UIUC MAC
Transceiver Profile
MAC
UIUC
Antenna
Black box

5. 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

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

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

8. 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

9. IEEE 802.11 Pretending a circular range RTS = Request-to-Send RTS A BF
Pretending a circular range

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

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

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

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

14. IEEE
ACK A B C D E F

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

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

17. A Comparison Issues Omni Directional Spatial Reuse Connectivity
Low
High
Connectivity
Interference
Cost & Complexity

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

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

20. 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

21. Antenna Model
More recent work models sidelobes approximately

22. Caveat Abstract antenna model Results only as good as the abstraction
Need more accurate antenna models

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

24. 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

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

26. 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

27. Directional Neighborhood
Receive Beam
Transmit 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

28. 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

29. Directional MAC Pretending a circular range RTS = Request-to-Send XB C D E F
Pretending a circular range

30. Directional MAC
CTS = Clear-to-Send X
CTS A B C D E F

31. Directional MAC
DATA packet follows CTS. Successful data reception acknowledged using ACK. X
DATA A B C D E F

32. Directional MAC X
ACK A B C D E F

33. Directional NAV (DNAV)
Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) D
CTS C X Y

34. Directional NAV (DNAV)
Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) D C
DNAV X Y

35. 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

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

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

38. 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 A does not hear RTS/CTS from B & C

39. 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

40. X does not know node A is busy. X keeps transmitting RTSs to node A
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

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

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

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

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

45. Multi Hop RTS (MMAC) – Basic IdeaF 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

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

47. Aligned Routes in Grid

48. Unaligned Routes in Grid

49. “Random” Topology

50. “Random” Topology: delay

51. 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

52. 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

53. Routing

54. 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

55. 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

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

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

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

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

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

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

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

63. Directional Routing Tradeoffs Larger Tx Range Fewer Hop Routes
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

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

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

66. 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

67. Related papers at www.crhc.uiuc.edu/~nhv
Thanks!
Related papers at

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

71. Routing using Directional Antennas

72. 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

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

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

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

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

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

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

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

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

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

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

83. 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 (250 m).
However, DDSR4 has sweeping delay. Thus route discovery delay higher

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

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

86. 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

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

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

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

90. 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

91. 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.

92. Aggregate throughput over random mobile scenarios
DDSR9
DSR

93. 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

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

95. 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

96. Mobility
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]

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

98. Deafness in 802.11 Deafness 2 hops away in 802.11
C cannot reply to D’s RTS
D assumes congestion, increases backoff A B C D
RTS

99. 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 C
DO neighbors D E
DD neighbors F B A G

100. 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]
Broadcast Region A

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

102. 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

103. Queuing in MMAC D E F C A B G

105. Impact of Topology
Aggregate throughput A F E D B C
– 1.19 Mbps
DMAC – 2.7 Mbps
Nodes arranged in linear configurations reduce spatial reuse for D-antennas
Aggregate throughput
– 1.19 Mbps
DMAC – 1.42 Mbps A B C

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