1 Exploiting Antenna Capabilities in Wireless Networks Nitin Vaidya Electrical and Computer Engineering, and Coordinated Science Lab (CSL) University of Illinois at Urbana-Champaign

2 Wireless Capacity  Wireless capacity limited  In dense environments, performance suffers  How to improve performance?

3 Improving Per-Flow Capacity

4 Add Spectrum  Multi-channel versions of IEEE  Practical limits on how much spectrum may be used

5 Power Control to Improve Spatial Reuse ABCD ABCD

6 Improving Communication Locality  Local communication (among nearby nodes) uses less “space”  Allows spatial reuse among different flows  Improves per-flow capacity  Not always feasible: Application-dependent

7 Exploit Infrastructure  Infrastructure provides a “tunnel” through which packets can be forwarded  Can effectively improve locality of communication  Infrastructure access can become a bottleneck E A BS1BS2 X Z infrastructure Ad hoc connectivity

8 Improving Per-Flow Capacity  Previous techniques are all useful, but have limitations  Dense networks likely to require further improvements in capacity  Exploit other forms of diversity  Mobility  Antennas

9 Exploiting Antennas

10 Antennas: Many Possibilities  Directional antennas  Diversity antennas  Reconfigurable antennas  …

11 Exploiting Antennas  Need protocol adaptations to exploit available antenna capabilities  Not sufficient to modify physical layer alone  Higher layer adaptation often necessary: medium access control (MAC) and routing

12 This Talk Protocols for Ad Hoc Networks using Directional Antennas Issues of interest  Medium access control  Neighbor discovery  Routing  Longer links, shorter routes  Longer times to failure  Broadcast-based discovery harder This talk  Deafness problem  MAC-Layer Anycasting

13 Outline  Preliminaries  A simple MAC protocol and the “deafness” problem  MAC-layer anycasting

14 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  Hidden terminals

15 Antenna Model  2 Operation Modes: Omni & Directional  Directional mode typically has sidelobes  Not all antennas represented by this model

16 Antenna Model  Omni Mode:  Omni Gain = Go  Directional Mode:  Capable of beamforming in specified direction  Directional Gain = Gd (Gd > Go) Received power  Transmit power * G tx * G rx

17 Benefits of Directional Antennas Greater Received Power  Longer links may be formed B A C D  May lower Tx power, reducing interference to others

18 Benefits of Directional Antennas  Low gain in unwanted directions  Reduces interference to others  Example ….

19 Using Omni-directional Antennas  When C receives from D, B cannot transmit C B A D

20 Using Directional Antennas  C may receive from D, and simultaneously B may transmit to A C B A D

21 A detour …

22 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

23 RTS/CTS Handshake in  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

24 Outline  Preliminaries  A simple MAC protocol and the “deafness” problem  MAC-layer anycasting

25 Directional MAC (DMAC)  Idle node listens in omni-directional mode  Sender sends a directional RTS towards intended receiver  Receiver responds with directional CTS

26 Directional MAC ( Variant)  DATA and ACK transmitted and received directionally  Nodes overhearing RTS or CTS remember not to transmit in corresponding directions  Overhearing nodes may transmit in other directions

27 Directional MAC  C remembers not to transmit in A’s direction  C may transmit towards D D A C B RTS

28 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

29 Issues with DMAC: Deafness  Deafness: C does not know why no response from A  Cannot differentiate between collision, and busy node A  Conservative response is to “backoff” and try later D AB C RTS

30 Illustration  B initiates communication to A  While A is busy, C transmits RTS to A  No response from A  C waits a while, tries again  No response, C waits longer …  When A becomes free, C in wait mode  A become busy again, …. Repeat AB C RTS

31 RTS Backoff Data RTS CTS ACK Data CTS RTS  B initiates communication to A  While A is busy, C transmits RTS to A  No response from A  C waits a while, tries again  No response, C waits longer …  When A becomes free, C in wait mode  A become busy again, …. Repeat Illustration Packet drop AB C

32 Impact of Deafness  Unnecessary transmissions of RTS  Increased packet drops  Increased delay and variance  Unfairness among flows

33 Solutions to Deafness  Deafness since C does not know A is busy  Make C aware that A is busy  Require A to transmit a busy signal while receiving  Alternative: A transmits a “free” signal after it become idle RTS Backoff Data RTS CTS ACK Data CTS RTS Packet drop AB C

34 Solution: Tone DMAC  Nodes unable to communicate with A adapt backoff based on the “tone” from A  Think of it as “free-tone” as opposed to a “busy- tone”  A node need only use tone or data channel at any time, not both RTS Backoff Data RTS CTS ACK AB C Tone RTS CTS Data Backoff

35 Tone DMAC  Why a narrow-band tone?  Save bandwidth  Trade-off  Narrow-band signal prone to fading: Use long enough tone duration  Aliasing, since C cannot tell who transmitted a tone –Use multiple tones –One tone per node too expensive –Share tones

36 Tone DMAC  Node i transmit tone f i for duration t i  f i and t i functions of the node identifier i f i = i mod F t i = i mod T

37 Tone DMAC  When a node, such as C in our example, hears a tone f for duration t, node C determines whether the tone could have been sent by its intended traget (node A in our example)  If C determines that A is the tone sender, C reduces its waiting time before next RTS  Aliasing can occur since multiple nodes can hash to the same tuple { f, t }

38 Tone DMAC Example

39 Backoff: Two flows to common receiver  Another possible improvement: Backoff Counter for DMAC flows Backoff Counter for ToneDMAC flows time Backoff Values

40 Packet Drops: Three flows, common receiver DMAC ToneDMAC time

41 UDP Throughput: Multiple multihop flows  ToneDMAC outperforms DMAC, ZeroToneDMAC ZeroToneDMAC = DMAC with only omnidirectional Backoff

42 Delay Performance: 2 flows, common Rx  Large fluctuation in DMAC packet delay  Higher variance

43 TCP Throughput: Multiple multihop flows  RTT estimation of TCP better with ToneDMAC due to low delay variance

44 DMAC Summary  Deafness aggrevated by directional communication  “Free” tones, or other alternative mechanisms, appear useful to reduce degradation caused by deafness  Practicality issue:  Tone assignment  Fading Topic of ongoing research

45 MAC-Layer Anycasting

46 Observation  Network layer typically selects one “optimal” route  MAC layer required to forward packet to next hop neighbor on this route  “Optimal” route selection based on a long-term view of the network  Independent of instantaneous channel conditions at each hop

47 Improvement ?  MAC layer aware of local link conditions  Congestion, channel fluctuations at smaller time scale  Power constraints for transmission  Virtual carrier sensing information (NAV in )  Exploit MAC layer awareness  Especially when using directional antennas  Forward packets based on combination of  Long-term directives of routing layer, and  Short-term knowledge at MAC layer

48 Our Proposal  Make forwarding decisions at the MAC layer  Utilize information already available to the MAC layer (as opposed to explicitly gathering feedback)  With DMAC, a node already knows that it cannot transmit in certain directions  Our approach can be combined with mechanisms that gather information explicitly

49 MAC-Layer Anycasting  Source often has multiple “good” routes to sink  Typically, one random downstream neighbor chosen  Supply multiple downstream neighbors to MAC layer  MAC layer chooses any one of the neighbors based on available information, and unicasts the packet

50 MAC-Layer Anycast Framework  Anycast module receives group of downstream neighbors  Anycast group = {A, B, X}  Anycast module forms anycast sequence (based on chosen policy)  Seq. = {X, X, B, A, X, B, A}  MAC layer attempts to transmit to “available” neighbors Network Layer MAC Layer Physical Layer Anycast Module

51 Directional MAC X DS DRTS Y

52 Directional MAC X DS DCTS Remember to not transmit towards D Y

53 MAC Constraints  Route from S to D: {S,A,B,D}  Assume A communicating with B  S cannot send packet to A  Multiple retransmissions can be avoided by forwarding packet to X instead Specify anycast group specified as {A, X} A S Y D B X Directional Beam Patterns

54 DNAV Constraints  Communication between E and F requires S to set DNAV in direction of E  Communication between S and A not possible until E completes transmission  Communication between S and X may be possible Anycasting with group {A,X} can improve performance F E X A S

55 Not Allowed DNAV Constraints F E X S A  Communication between E and F requires S to set DNAV in direction of E  Communication between S and A not possible until E completes transmission  Communication between S and X may be possible Anycasting with group {A,X} can improve performance

56 DNAV Constraints F E XS A Allowed  Communication between E and F requires S to set DNAV in direction of E  Communication between S and A not possible until E completes transmission  Communication between S and X may be possible Anycasting with group {A,X} can improve performance

57 MAC Constraints – Omni Antennas  Route from S to D: {S,A,B,D}  While F communicating to E, A is silenced by CTS from E  S transmits RTS to A, receives no reply, retransmits  Multiple retransmission can be avoided by forwarding packet to X Anycast group specified to S can be {A, X}

58 Power Constraints RT P N  With PCMA, node R announces additional interference that it can tolerate  To initiate communication to N, T must choose power level according to this tolerance  Power level to transmit to N is too high. However, transmission to P is feasible MAC-Layer anycasting can forward packets with PCMA. Anycast group {P, N}

59 Power Constraints RT P N  With PCMA, node R announces additional interference that it can tolerate  To initiate communication to N, T must choose power level according to this tolerance  Power level to transmit to N is too high. However, transmission to P is feasible MAC-Layer anycasting can forward packets with PCMA. Anycast group {P, N}

60 Power Constraints RT P N  With PCMA, node R announces additional interference that it can tolerate  To initiate communication to N, T must choose power level according to this tolerance  Power level to transmit to N is too high. However, transmission to P is feasible MAC-Layer anycasting can forward packets with PCMA. Anycast group {P, N}

61 Design Issues and Tradeoffs

62 “Digression”  Anycasting can bypass unavailable links  Each intermediate node locally performs anycasting  Local (greedy) decisions can cause  Route to digress significantly from global optimal  Need to restrict digression below tolerance

63 Digression  Say, Anycast group = Neighbors on the minimum and (minimum+1)-hop routes  {S,X,J,P,K,Z,D} digresses 3 hops more that {S,A,B,D}

64 Out-of-Order Delivery  MAC-Layer anycasting performed on per-packet basis  Delay on the different routes can be different  Out of order packet delivery possible  TCP-like transport protocols may encounter problems

65 Source Routing  Source routing – source specifies all possible routes  To perform anycasting with source routing  Source includes enough information for intermediate nodes to form anycast group  Possible implementation – include a directed acyclic graph (DAG)  Including DAG in packet – larger control overhead

66 Preliminary Evaluation (Anycasting)

67 Grid topology, 5 flows, 3 hops

68 Large Grid topology, 10 flows, 5 hops

69 Anycast: Summary  MAC-Layer anycasting can improve performance  Several tradeoffs arise On-going work

70 Conclusion  Directional antennas can benefit performance  But need suitable protocols  On-going work:  Cheaper antennas that can improve performance  Testbed deployment

71 Thanks! Acknowledgements Romit Roy Choudhury, UIUC Ram Ramanathan, BBN Xue Yang, UIUC

72 Another Problem Performing directional carrier sensing when in wait mode leads to another instance of deafness While C waits to transmit to A, it beamforms and performs carrier sensing  C cannot hear RTS from D AB C RTS D

73 Solutions to Deafness  Nodes required to switch to omni mode during back-off  C can hear D while waiting for A  Trade-off: C may receive transmission from E to F, and not be able to receive from D, or transmit to A AB C RTS D E