1

Directional Antennasin

Ad Hoc Networks

Nitin Vaidya

University of Illinois at Urbana-Champaign

Joint work with

Romit Roy Choudhury, UIUC

Xue Yang, UIUC

Ram Ramanathan, BBN

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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

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Mobile Ad Hoc Networks

May need to traverse multiple links to reach a destination

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Mobile Ad Hoc Networks (MANET)

Mobility causes route changes

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Why Ad Hoc Networks ?

Potential ease of deployment

Decreased dependence on infrastructure

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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

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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)

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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

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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

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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)

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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

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Directional Antennas

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Using Omni-directional Antennas

A Frozen node

S

D

A

B

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Directional Antennas

Not possible using Omni

S

D

A

B

C

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Comparison

Omni Directional

Spatial Reuse Low High

(varies inversely with beamwidth)

Connectivity Low High

Interference Omni Directional

Cost & Complexity Low High

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Questions

Are Directional antennas beneficial in ad hoc networks ?

To what extent ?

Under what conditions ?

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Research Direction

Identify issues affecting directional communication

Evaluate trade-offs across multiple layers

Design protocols that effectively use directional capabilities

Caveat: Work-in-Progress

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Preliminaries

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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

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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

BACTS (10)

RTS (10)

10

10

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IEEE 802.11

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)

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Antenna Model

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Antenna Model

2 Operation Modes: Omni & Directional

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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)

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C

Directional Neighborhood

B

A

A and B are Directional-Omni (DO) neighbors

B and C are Directional-Directional (DD) neighbors

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A Simple Directional MAC Protocol(DMAC)

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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

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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

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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.

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Directional NAV (DNAV)

E has DNAV set due to RTS from H. Can talk to B since E’s transmission beam does not overlap.

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Example

B

C

A

D

E

B and C communicate

D & E cannot: D blocked with DNAV

D and A communicate

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Issues with DMAC

Hidden terminals due to asymmetry in gain A does not get RTS/CTS from C/B

C

A B

DataRTS

A’s RTS may interfere with C’s reception of DATA

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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

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Issues with DMAC: Deafness

RTS

RTS

RTS

DATA

X does not know node A is busy. X keeps transmitting RTSs to node A

A B

With 802.11 (omni antennas), X would be aware that A is busy, and defer its own transmission

X

Z

Y

• Deafness

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Problems with DMAC

Shape of Silenced Regions

Region of interference for directional transmissionRegion of interference for

omnidirectional transmission

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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

CB

A

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DMAC Trade-off

Benefits

Better Network Connectivity

Spatial Reuse

Disadvantages

– Increased hidden terminals

– Deafness

– Directional interference

– Uses only DO links

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Solving DMAC Problems

Are improvements possible to make directional MAC protocols more effective ?

One possible improvement: Use DD links

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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

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Multi-Hop RTS: Basic Idea

A B

C

D E

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.

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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

D E

F

G

H

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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

D E

F

G

H

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Performance

Simulation Qualnet simulator 2.6.1 CBR traffic Packet Size – 512 Bytes 802.11 transmission range = 250 meters. Channel bandwidth 2 Mbps Mobility - none

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Impact of Topology

• Nodes arranged in linear configurations reduce spatial reuse for directional antennas

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Impact of Topology

IEEE 802.11 = 1.19 Mbps

DMAC = 2.7 Mbps

IEEE 802.11 = 1.19 Mbps

DMAC = 1.42 Mbps

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“Aligned” Flows

MMAC

DMAC

802.11

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“Unaligned” Flows

MMAC

DMAC

802.11

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“Unaligned” Flows & Topology

MMAC

DMAC

802.11

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Delay: “Unaligned” Flows & Topology

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Directional MAC: Summary

Directional MAC protocols can improve throughput and decrease delay But not always

Performance dependent on topology

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Routing using Directional Antennas

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Motivation

Directional antennas affect network layer, in addition to MAC protocols

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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

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

Represents a node that has received RREQ for D from S

M

N

L

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Represents transmission of RREQ

Z

YBroadcast transmission

M

N

L

[S]

[X,Y] Represents list of identifiers appended to RREQ

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

[S,E]

[S,C]

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

• 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]

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

• Nodes J and K both broadcast RREQ to node D

N

L

[S,C,G,K]

[S,E,F,J]

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Route Reply in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

N

L

RREP [S,E,F,J,D]

Represents RREP control message

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DSR over Directional Antennas

RREQ broadcast by sweeping To use DD links

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Route Discovery in DSR

B

A

S E

F

H

J

D

C

G

IK

Z

Y

M

• Nodes J and K both broadcast RREQ to node D

N

L

[S,C,G,K]

[S,E,F,J]

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Trade-off

Larger Tx Range Fewer Hop Routes

Few Hop Routes Low Data Latency

Smaller Angle High Sweep Delay

More Sweeping High Overhead

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Route discovery latency … Single flow, grid topology (200 m distance)

DSR

DDSR4DDSR6

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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 802.11 (250 m).

However, DDSR4 has sweeping delay. Thus route discovery delay higher

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Throughput

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

DDSR18

DDSR9

DSR

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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

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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

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Routing Overhead

Using omni broadcast, nodes receive multiple copies of same packet - Redundant !!!

• Broadcast Storm Problem

Using directional Antennas – can do better ?

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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 =

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Routing OverheadControl overhead reduces

Beamwidth of antenna element (degrees)

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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

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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.

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Aggregate throughput over random mobile scenarios

DSR

DDSR9

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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

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Conclusion

Directional antennas can improve performance

But suitable protocol adaptations necessary

Also need to use suitable antenna models

… plenty of problems remain

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Thanks!

www.crhc.uiuc.edu/~nhv

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Adaptive Modulation

Joint work with Gavin Holland and Victor Bahl

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Adaptive Modulation

Channel conditions are time-varying

Received signal-to-noise ratio changes with time

A B

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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 providea trade-off betweenthroughput and range

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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

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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

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Autorate Fallback

Advantage Can be implemented at the sender, without making any

changes to the 802.11 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

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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

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Receiver-Based Autorate MAC Protocol

D

C

BACTS (1 Mbps)

RTS (2 Mbps)

Data (1 Mbps)

NAV updated using rate

specified in the data packet

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Extra slides

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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.

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Effect of Beamwidth in Random Static Topologies