Exploiting antenna capabilities in wireless networks
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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

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

  • Wireless capacity limited

  • In dense environments, performance suffers

  • How to improve performance?


Improving per flow capacity

Improving Per-Flow Capacity


Add spectrum

Add Spectrum

  • Multi-channel versions of IEEE 802.11

  • Practical limits on how much spectrum may be used


Power control to improve spatial reuse

A

B

C

D

A

B

C

D

Power Controlto Improve Spatial Reuse


Improving communication locality

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


Exploit infrastructure

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


Improving per flow capacity1

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


Exploiting antennas

Exploiting Antennas


Antennas many possibilities

Antennas: Many Possibilities

  • Directional antennas

  • Diversity antennas

  • Reconfigurable antennas


Exploiting antennas1

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


Our research

Our Research

  • Past and present: Directional antennas

  • Present and future: Diversity and reconfigurable antennas


This talk protocols for ad hoc networks using directional antennas

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


Outline

Outline

  • Preliminaries

  • A simple MAC protocol and the “deafness” problem

  • MAC-layer anycasting


Ad hoc networks

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


Antenna model

Antenna Model

  • 2 Operation Modes: Omni & Directional

  • Directional mode typically has sidelobes

  • Not all antennas represented by this model


Antenna model1

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


Benefits of directional antennas greater received power

A

B

D

C

Benefits of Directional AntennasGreater Received Power

  • Longer links may be formed

  • May lower Tx power, reducing interference to others


Benefits of directional antennas

Benefits of Directional Antennas

  • Low gain in unwanted directions

  • Reduces interference to others

  • Example ….


Using omni directional antennas

Using Omni-directional Antennas

  • When C receives from D, B cannot transmit

D

A

B

C


Using directional antennas

D

A

B

C

Using Directional Antennas

  • C may receive from D, and simultaneously B may transmit to A


A detour

A detour …


Hidden terminal problem

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


Rts cts handshake in 802 11

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


Outline1

Outline

  • Preliminaries

  • A simple MAC protocol and the “deafness” problem

  • MAC-layer anycasting


Directional mac dmac

Directional MAC(DMAC)

  • Idle node listens in omni-directional mode

  • Sender sends a directional RTS towards intended receiver

  • Receiver responds with directional CTS


Directional mac 802 11 variant

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


Directional mac

A

RTS

B

C

D

Directional MAC

  • C remembers not to transmit in A’s direction

  • C may transmit towards


Issues with dmac

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


Issues with dmac deafness

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


Issues with dmac deafness1

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


Illustration

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


Exploiting antenna capabilities in wireless networks

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


Impact of deafness

Impact of Deafness

  • Unnecessary transmissions of RTS

  • Increased packet drops

  • Increased delay and variance

  • Unfairness among flows


Another problem

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


Solutions to deafness

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


Solutions to deafness1

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


Solution tone dmac

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


Tone dmac

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


Tone dmac1

Tone DMAC

  • Node i transmit tone fifor durationti

  • fiand ti functions of the node identifier i

    fi = i mod F

    ti = i mod T


Tone dmac2

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 }


Tone dmac example

Tone DMAC Example


Backoff two flows to common receiver

Backoff: Two flows to common receiver

Backoff Counter for DMAC flows

  • Another possible improvement:

Backoff Values

Backoff Counter for ToneDMAC flows

time


Packet drops three flows common receiver

Packet Drops: Three flows, common receiver

DMAC

ToneDMAC

time


Udp throughput multiple multihop flows

UDP Throughput: Multiple multihop flows

  • ToneDMAC outperforms DMAC, ZeroToneDMAC

    ZeroToneDMAC = DMAC with only omnidirectional Backoff


Delay performance 2 flows common rx

Delay Performance: 2 flows, common Rx

  • Large fluctuation in DMAC packet delay  Higher variance


Tcp throughput multiple multihop flows

TCP Throughput: Multiple multihop flows

  • RTT estimation of TCP better with ToneDMAC due to low delay variance


Dmac summary

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


Mac layer anycasting

MAC-Layer Anycasting


Observation

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


Improvement

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 802.11)

  • 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


Our proposal

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


Mac layer anycasting1

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 unicaststhe packet


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

MAC-Layer Anycast Framework

Network Layer

Anycast

Module

MAC Layer

Physical Layer


Directional mac1

Directional MAC

X

DRTS

S

D

Y


Directional mac2

Directional MAC

Remember to not transmit towards D

X

DCTS

S

D

Y


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}

MAC Constraints

X

Y

S

B

D

A

Directional Beam

Patterns


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

DNAV Constraints

S

X

F

A

E


Dnav constraints1

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

Not Allowed

DNAV Constraints

S

X

F

A

E


Dnav constraints2

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

DNAV Constraints

Allowed

S

X

F

A

E


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}

MAC Constraints – Omni Antennas


Power constraints

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}

Power Constraints

N

R

T

P


Power constraints1

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}

Power Constraints

N

R

T

P


Power constraints2

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}

Power Constraints

N

R

T

P


Design issues and tradeoffs

Design Issues and Tradeoffs


Digression

“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


Digression1

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}


Out of order delivery

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


Source routing

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


Preliminary evaluation anycasting

Preliminary Evaluation(Anycasting)


Grid topology 5 flows 3 hops

Grid topology, 5 flows, 3 hops


Large grid topology 10 flows 5 hops

Large Grid topology, 10 flows, 5 hops


Anycast summary

Anycast: Summary

  • MAC-Layer anycasting can improve performance

  • Several tradeoffs arise

    On-going work


Conclusion

Conclusion

  • Directional antennas can benefit performance

  • But need suitable protocols

  • On-going work:

    • Cheaper antennas that can improve performance

    • Testbed deployment


Thanks

Thanks!

www.crhc.uiuc.edu/wireless

Acknowledgements

Romit Roy Choudhury, UIUC

Ram Ramanathan, BBN

Xue Yang, UIUC


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