1 Directional Antennas in Ad 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 S D A B

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

15 Comparison OmniDirectional Spatial Reuse LowHigh (varies inversely with beamwidth) Connectivity LowHigh Interference OmniDirectional Cost & Complexity LowHigh

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 ABC 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/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 D C BA CTS (10) RTS (10) 10

21 IEEE  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 C Directional Neighborhood B A 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 B C A D E B and C communicate D & E cannot: D blocked with DNAV D and A communicate

32 Issues with DMAC  Hidden terminals due to asymmetry in gain  A does not get RTS/CTS from C/B C A B Data RTS 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 CB A beamformed in direction of D  A does not hear RTS/CTS from B/C A may now interfere at C D A

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

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 B A

37 DMAC Trade-off  Benefits  Better Network Connectivity  Spatial Reuse  Disadvantages –Increased hidden terminals –Deafness –Directional interference –Uses only DO links

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 Multi-Hop RTS: 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.

41 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 A B C DE F G H

42 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. A B C DE F G H

43 Performance  Simulation  Qualnet simulator  CBR traffic  Packet Size – 512 Bytes  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 = 1.19 Mbps DMAC = 2.7 Mbps IEEE = 1.19 Mbps DMAC = 1.42 Mbps

46 “Aligned” Flows MMAC DMAC

47 “Unaligned” Flows MMAC DMAC

48 “Unaligned” Flows & Topology MMAC 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

51 Routing using Directional Antennas

52 Motivation  Directional antennas affect network layer, in addition to MAC protocols

53 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

54 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

55 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

56 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]

57 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]

58 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]

59 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

60 DSR over Directional Antennas  RREQ broadcast by sweeping  To use DD links

61 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]

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

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

64 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

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

66 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

67 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

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

69 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  =

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

71 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

72 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.

73 Aggregate throughput over random mobile scenarios DSR DDSR9

74 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

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

76 Thanks!

77

78 Adaptive Modulation Joint work with Gavin Holland and Victor Bahl

79 Adaptive Modulation  Channel conditions are time-varying  Received signal-to-noise ratio changes with time AB

80 Adaptive Modulation  Multi-rate radios are capable of transmitting at several rates, using different modulation schemes  Choose modulation scheme as a function of channel conditions Distance Throughput Modulation schemes provide a trade-off between throughput and range

81 Adaptive Modulation  If physical layer chooses the modulation scheme transparent to MAC  MAC cannot know the time duration required for the transfer  Must involve MAC protocol in deciding the modulation scheme  Some implementations use a sender-based scheme for this purpose [Kamerman97]  Receiver-based schemes can perform better

82 Sender-Based “Autorate Fallback” [Kamerman97]  Probing mechanisms  Sender decreases bit rate after X consecutive transmission attempts fail  Sender increases bit rate after Y consecutive transmission attempt succeed

83 Autorate Fallback  Advantage  Can be implemented at the sender, without making any changes to the standard specification  Disadvantage  Probing mechanism does not accurately detect channel state  Channel state detected more accurately at the receiver  Performance can suffer Since the sender will periodically try to send at a rate higher than optimal Also, when channel conditions improve, the rate is not increased immediately

84 Receiver-Based Autorate MAC [Holland01mobicom]  Sender sends RTS containing its best rate estimate  Receiver chooses best rate for the conditions and sends it in the CTS  Sender transmits DATA packet at new rate  Information in data packet header implicitly updates nodes that heard old rate

85 Receiver-Based Autorate MAC Protocol D C BA CTS (1 Mbps) RTS (2 Mbps) Data (1 Mbps) NAV updated using rate specified in the data packet

86 Extra slides

87 Directional Antennas in Random Topologies Higher transmission range improves connectivity in addition to achieving fewer hop routes. E.g. Link a-b not possible using Omni transmission.

88 Effect of Beamwidth in Random Static Topologies