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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?
add spectrum
Add Spectrum
  • Multi-channel versions of IEEE 802.11
  • Practical limits on how much spectrum may be used
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
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
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
slide33

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 }

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

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

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 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

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
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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