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Exploiting Antenna Capabilities in Wireless Networks. Nitin Vaidya Electrical and Computer Engineering, and Coordinated Science Lab (CSL) University of Illinois at Urbana-Champaign www.crhc.uiuc.edu/wireless/. Wireless Capacity. Wireless capacity limited

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Exploiting Antenna Capabilities in Wireless Networks


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    1. Exploiting Antenna Capabilities in Wireless Networks Nitin Vaidya Electrical and Computer Engineering, and Coordinated Science Lab (CSL) University of Illinois at Urbana-Champaign www.crhc.uiuc.edu/wireless/

    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 802.11 • Practical limits on how much spectrum may be used

    5. A B C D A B C D Power Controlto Improve Spatial Reuse

    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. infrastructure BS1 BS2 E A Z Ad hoc connectivity X Exploit Infrastructure • Infrastructure provides a “tunnel” through which packets can be forwarded • Can effectively improve locality of communication • Infrastructure access can be become a bottleneck

    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. Our Research • Past and present: Directional antennas • Present and future: Diversity and reconfigurable antennas

    13. This TalkProtocols for Ad Hoc Networks usingDirectional 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

    14. Outline • Preliminaries • A simple MAC protocol and the “deafness” problem • MAC-layer anycasting

    15. 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

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

    17. Antenna Model • Omni Mode: • Omni Gain = Go • Directional Mode: • Capable of beamforming in specified direction • Directional Gain = Gd (Gd > Go) Received poweratransmit power*Gtx*Grx

    18. A B D C Benefits of Directional AntennasGreater Received Power • Longer links may be formed • May lower Tx power, reducing interference to others

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

    20. Using Omni-directional Antennas • When C receives from D, B cannot transmit D A B C

    21. D A B C Using Directional Antennas • C may receive from D, and simultaneously B may transmit to A

    22. A detour …

    23. A B C 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

    24. RTS (10) CTS (10) RTS/CTS Handshake in 802.11 • 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 C 10 A B D 10

    25. Outline • Preliminaries • A simple MAC protocol and the “deafness” problem • MAC-layer anycasting

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

    27. Directional MAC(802.11 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

    28. A RTS B C D Directional MAC • C remembers not to transmit in A’s direction • C may transmit towards

    29. RTS Issues with DMAC • Hidden terminals due to asymmetry in gain • A does not get RTS/CTS from C/B B C A Data A’s RTS may interfere with C’s reception of DATA

    30. Z RTS A B DATA RTS Y RTS X Issues with DMAC: Deafness • Deafness: X does not know why no response from A • Cannot differentiate between collision, and busy node A

    31. A B RTS C Issues with DMAC: Deafness • Deafness: X 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

    32. A B RTS C Illustration • A initiates communication to B • 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

    33. RTS RTS CTS Data Backoff A B RTS ACK C RTS CTS Data Packet drop 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

    34. Impact of Deafness • Unnecessary transmissions of RTS • Increased packet drops • Increased delay and variance • Unfairness among flows

    35. A B RTS C 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 D RTS

    36. A B RTS C 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 E D RTS

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

    38. A B C RTS RTS CTS Data Backoff RTS ACK Backoff Tone RTS RTS CTS Data 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

    39. 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

    40. Tone DMAC • Node i transmit tone fifor durationti • fiand ti functions of the node identifier i fi = i mod F ti = i mod T

    41. 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 } 

    42. Tone DMAC Example

    43. Backoff: Two flows to common receiver Backoff Counter for DMAC flows • Another possible improvement: Backoff Values Backoff Counter for ToneDMAC flows time

    44. Packet Drops: Three flows, common receiver DMAC ToneDMAC time

    45. UDP Throughput: Multiple multihop flows • ToneDMAC outperforms DMAC, ZeroToneDMAC ZeroToneDMAC = DMAC with only omnidirectional Backoff

    46. Delay Performance: 2 flows, common Rx • Large fluctuation in DMAC packet delay  Higher variance

    47. TCP Throughput: Multiple multihop flows • RTT estimation of TCP better with ToneDMAC due to low delay variance

    48. 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

    49. MAC-Layer Anycasting

    50. 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