April 22, 2023

Computer Networks COE 549Directional Antennas for Ad-

hoc NetworksTarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/coe549.htm

2

Outline Introduction

IEEE 802.11 (CSMA/CA) overview Motivations Problem statement

Beamforming: Definition, types and advantages. Basic DMAC Challenges in Ad-hoc Networks using directional

antennas. Multi-Hop MAC (MMAC) Beamforming with Power Control Performance Evaluation

Ad Hoc Networks

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A silenced node

A

B

C

D

Typically assume Omnidirectional antennas

Can Directional Antennas Improve Performance?

A

B

C

D

Not possible using Omni

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A Comparison

Issues Omni Directional

Spatial Reuse Low High

Connectivity Low High

Interference Omni Directional

Cost & Complexity

Low High

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Motivation

• Are directional antennas beneficial to medium access control in ad hoc networks ?

– To what extent ?– Under what conditions ?

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• Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS)

• RTS and CTS announce the duration of the imminent dialogue

• Nodes overhearing RTS/CTSdefer transmission for that duration– Network Allocation Vector (NAV) remembers duration

IEEE 802.11

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C FA B EDRTS

RTS = Request-to-Send

IEEE 802.11

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C FA B EDRTS

RTS = Request-to-Send

IEEE 802.11

NAV = 10

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C FA B EDCTS

CTS = Clear-to-Send

IEEE 802.11

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C FA B EDCTS

CTS = Clear-to-Send

IEEE 802.11

NAV = 8

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C FA B EDDATA

•DATA packet follows CTS. Successful data reception acknowledged using ACK.

IEEE 802.11

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C FA B EDACK

IEEE 802.11

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IEEE 802.11

• Channel contention resolved using backoff– Nodes choose random backoff interval from [0, CW]– Count down for this interval before transmission

Random backoff Data Transmit

Random backoff Wait

backoff

backoff Data Transmit

WaitA

B

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Antenna Model

2 Operation Modes: Omni and Directional

A node may operate in any one mode at any given time

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Antenna Model

In Omni Mode:• Let us assume that nodes receive signals with

Gain Go

In Directional Mode:• Directional Gain Gd (Gd > Go)

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Directional Communication

Received Power (Tx Gain) * (Rx Gain)• Tx Gain = Transmit gain in the direction of receiver• Rx Gain = Receive gain in the direction of the transmitter

AB C

Convention: A link shown by overlapping beams along the line joining the transmitter and receiver. Nodes C, A form a link. C, B do not.

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B

Directional Neighborhood

A

• When C transmits directionally•Node A sufficiently close to receive in omni mode

•Node C and A are Directional-Omni (DO) neighbors

•Nodes C and B are not DO neighbors

C

Transmit BeamReceive Beam

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Directional Neighborhood

AB C

•When C transmits directionally• Node B receives packets from C only in directional mode

•C and B are Directional-Directional (DD) neighbors

Transmit BeamReceive Beam

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• A technique in which the antenna pattern is switched (or steered) to a desired direction.

• Two types: switched & steered beam.

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Antenna Beamforming

- Steered beam: can direct the beam to the desired direction. (cost more but better performance)

- Switched beam: can select one from a set of predefined beams/antennas

S D S D

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1. Longer range Why?

higher antenna gain in the desired directionBenefits:

better connectivity and lower end-to-end delay

2. Higher spatial reuseWhy?

Reduced interference (narrower beamwidth)Benefits:

increased capacity and throughput

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Antenna Beamforming

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Identify the challenges encountered in MAC when beamforming antennas are used in Ad hoc networks and find the possible solutions of those problems in the literature.

Research Problem

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The two most impacted networking mechanisms as a result of using beamforming antennas are

1. Neighbor discovery identifies the one-hop neighbors

2. MAC provides distributed access to the channel

Challenges in Ad-hoc Networks

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DMAC is MAC with directional (beamforming) Antennas.

Two Operation Modes: OmniOmni and Directional

A node may operate in any mode at any given time

DMAC

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Basic DMAC

• Assumption: Location of neighbors is known.• Sender transmits Directional-RTS (DRTS) • A node listens omni-directionally when idle,

– RTS received in Omni mode.

• Receiver sends Directional-CTS (DCTS)• DATA, ACK transmitted and received directionally.• Operation is the same as 802.11 but with directional

antennas and , and with the use of DNAV (directional NAV)!!

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Basic DMAC

Why DNAV (directional Network allocation Vector)? Asnwer: to combat directional exposed terminal problem. increased spatial reuse and throughput

A C

BE

D

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Neighbor discovery

New notions of neighbors:

B

A

Nodes A and B are OOOO neighbors. Nodes C and A are not OOOO Nodes C and B are not DODO

C

but DODO neighbors.but DDDD neighbors.

Transmit antenna

Receive antenna

OO Omni Omni

DO Dir. Omni

DD Dir Dir

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Neighbor discovery • How to know the direction of the intended node?

– CTS, DATA, ACK are much easier than RTS– Two possible ways:

• From the AOA (Angle_of_Arrival ) of RTS and CTS.• Or from self location information included in RTS and CTS.

– Directing the beam towards the destination for DRTS is challenging. Possible solutions:

• Most MAC proposal assumes that this information is available by routing protocol. Each node know its location (by GPS or any location estimation method).

• By AoA cashing of overheard packets (ex. Takai et al.[2])• Circular DRTS • ORTS.

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DMAC by Takai et al. [2]• Goals: send RTS directionally without location knowledge.• Employs DNAV

– It is set according to AoA of the RTS/CTS dialog• Employs AoA cashing

– The direction of neighbors is cashed based on the estimation of AoA of the overheard packets.

• RTS is send directionally if the direction of the intended destination is available in the cash

• RTS is sent omnidirectionally if the direction of the destination is not available in the AoA cash or CTS is not received after directional RTS transmission.

• 3 to 4 times improvement in throughput compared to 802.11

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Neighbor discovery

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• Extended transmission range– Beamforming enables longer range– Advantages: reduced # of hops, e2e delays and better

connectivity (sparse networks)– Most of MAC proposals are not able to achieve the

maximum possible range• OO, OD link only,

– For Maximum range: • DD link

– MMAC by Choudhury et al. [3]

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Neighbor discovery

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MMAC by Choudhury et al. [3]- Knowledge of neighbors location is assumed- Goal: improve system performance (e2e delay and throughput)

by extending the range of transmission (DD link).- Similar to basic DMAC + DD link- DD link can be established by multi-hop RTS (MHRTS)

D

B

A

C

E

DO Link

DD LinkMHRTS

DATA

MHRTS

MHRTS

DRTS

DCTS

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Neighbor discovery

Multi Hop RTS – Basic Idea

A B

C

D EF

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

B

C

D E

F

G

H

A transmits RTS towards D

A

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MMAC protocol

B

C

D E

F

G

H

DNAV

A

H updates DNAV

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MMAC protocol

B

C

D E

F

G

H

A transmits M-RTS to DO neighbor B

A

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MMAC protocol

C

D E

F

G

H

A

B forwards M-RTS to C (also DO)

B

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MMAC protocol

B

C

D E

F

G

H

A beamforms toward D – waits for CTS

A

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MMAC protocol

B

D E

F

G

H

A

C forwards M-RTS to D

C

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MMAC protocol

B

E

F

G

H

C

A

D beamforms towards A – sends CTS

D

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MMAC protocol

B

E

F

G

H

C

D

A

A & D communicate over DD link

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MMAC protocol

B

E

FH

C

A

Nodes D and G similarly communicate

G

D

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Problems in DMAC

There are two main problems associated with DMAC:

1. New Hidden Terminals 2. Deafness

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Case 1. E is out of RTS/CTS range of

A/C communication

ACE

AE

D

The node is hidden to the ongoing communication of other node when it didn’t hear the RTS/CTS transmission while it can interfere

Case 2. Loss in channel state

D

C

Collision

Collision

The antenna of E is directed twards DRTS/CTS of A/C CANNOT be heard by E

Problems in DMAC 1. New Hidden Terminals

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• A node A is deaf with respect to nodes X, Z, if it cannot receive from nodes X, Z due to beam direction while it can receive if it was in omni mode.

• Effects:– Waste the capacity and energy (due unproductive control packets).– Introduce unfairness (increased backoff interval).

RTS

RTS A BX

Z

DATA

X and Z do not know node A is busy. They keep transmitting RTSs to node A

Problems in DMAC2. Deafness

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• Hidden terminals and deafness are the two critical problems in DMAC.

• Possible Solution:– Send RTS and/or CTS omnidirectionally while

DATA/ACK are sent directionally. Example:

DMAC by Ko et al. [5]

Problems in DMAC

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- Knowledge of neighbors location is assumed- Multiple directional antennas for each nodes (switched beam)- Goal: increase spatial reuse while reducing control packet collisions.- DATA/ACK is directional- CTS is omnidirectional = OCTS- Two schemes for RTS:- Scheme 1 : DRTS (Directional RTS) only- Scheme 2 : ORTS/DRTS

A

BS

DX

S can send to D but not to X Both schemes send DRTS

DS

Scheme 2 sends RTS in all directions (ORTS) if no antenna is blocked

A

B

Problems in DMACDMAC by Ko et al. [5]

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Performance • Offers about 50% better throughput compared to IEEE 802.11,

depends on Topology• Scheme 1 vs. Scheme 2:

– Scheme 2 tries to reduce collision of control packets at the source while scheme 1 tries maximize spatial reuse in the vicinity of the source.

– No significant performance difference

Problems in DMACDMAC by Ko et al. (Cont.)

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Problems with DMAC

Possible Solution to unfairness caused by Deafness:ToneDMAC by Choudury et al. [6]• Goal: to reduce the effect of unfairness caused by Deafness by

identify Deafness from congestion• RTS/CTS/DATA/ACK are sent directionally• After RTS/CTS/DATA/ACK exchange, A and B send their tones

omnidirectinally.• neighboring nodes that overhear the tones will know that node A or B

was engaged in communication.• Throughput is 2 times better than DMAC.

– Fairness is improved.

C will know that B was deaf. It will reset the backoff window to the minimum value.

A_TONE A B

CDATA

B_TONE

B_TONE

RTS

A_TONE

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DMAC Tradeoffs

• Benefits

– Better Network Connectivity

– Spatial Reuse

• Disadvantages

– Hidden terminals

– Deafness

– No DD Links

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Impact of Beamforming on Ad-hoc Networking:MAC , Neighbor discovery, Route discovery

Our Goal is to study the impact of Antenna beamforming on MAC.Examples: (Assume CSMA/CA )

Without beamforming With beamforming

A B C D A B C D

Exposed terminal problem No problem

A B A B

E

No problem Deafness Problem

C

D

E

C

D

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Beamforming with power control

• Power control by it self can achieve higher performance– Reduce interference– Lower energy consumption

• Power control + beamforming can substantially improve the performance

No power control or beamforming

Area = A

r/2

r

Power control only

Area = A/4

r/2

Beamforming only

Area = A/6

Power control or beamforming

Area = A/144 !!!

A rough comparison of relative interference reduction, assuming 10 degrees directional beamwidth, and r 4 propagation. [1]04/22/23

Performance

• Simulation– Qualnet simulator 2.6.1– Constant Bit Rate (CBR) traffic– Packet Size – 512 Bytes– 802.11 transmission range = 250meters– DD transmission range = 900m approx– Beamwidth = 60 degrees– Channel bandwidth 2 Mbps– Mobility - none

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MMAC Hop Count• Max MMAC hop count = 3

– Too many DO hops increases probability of failure of RTS delivery

– Too many DO hops typically not necessary to establish DD link

A

B

C D EF

G

DO neighbors

DD neighbors

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MMAC - Concerns

• Neighbor discovery overheads may offset the advantages of MMAC

• High traffic – lower probability of RTS delivery• Multi-hop RTS may not reach DD neighbor due to deafness or collision•No more than 3 DO links is used for each DD link

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Aligned Routes in Grid

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Agg

rega

te T

hrou

ghpu

t (K

bps)

802.11DMACMMAC

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Unaligned Routes in Grid

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Agg

rega

te T

hrou

ghpu

t (K

bps)

802.11DMACMMAC

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“Random” Topology

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Agg

rega

te T

hrou

ghpu

t

802.11DMACMMAC

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“Random” Topology: delay

0

0.5

1

1.5

2

0 500 1000 1500 2000 2500Sending Rate (Kbps)

Avg

. End

to E

nd D

elay

(s)

DMACMMAC

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• Nodes moving out of beam coverage in order of packet-transmission-time– Low probability

• Antenna handoff required– MAC layer can cache active antenna beam– On disconnection, scan over adjacent beams– Cache updates possible using promiscuous mode– Evaluated in [RoyChoudhury02_TechReport]

Mobility

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Broadcast• Several definitions of “broadcast”

– Broadcast region may be a sector, multiple sectors

– Omni broadcast may be performed through sweeping antenna over all directions [RoyChoudhury02_TechReport]

A

Broadcast Region

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References1. Basagni, M. Conti, S. Giordano, I. Stojmenovic, eds, Mobile Ad

Hoc Networking, IEEE Press/Wiley, August 2004. 2. M. Takai, et al., “Directional virtual carrier sensing for directional

antennas in mobile ad hoc networks”, ACM  MobiHoc 2002, pp 39-46, June 2002

3. R.R. Choudhury, X. Yang, N.H. Vaidya, and R. Ramanathan, “Using directional antennas for medium access control in ad hoc networks”, MOBICOM 2002, pp 59-70, September 2002

4. N.S. Fahmy, T.D. Todd and V. Kezys, “Ad hoc networks with smart antennas using IEEE 802.11-based protocols”, IEEE ICC 2002, pp 3144-3148, May 2002

5. Y-B Ko, V. Shankarkumar and N.H. Vaidya, “Medium access control protocols using directional antennas in ad hoc networks”, IEEE INFOCOM 2000, pp 13-21

6. Choudhury, R.R.and Vaidya, N.H., “Deafness: a MAC problem in ad hoc networks when using directional antennas” ICNP 2004, Proceedings of the 12th IEEE International Conference on Network Protocols, pp:283 - 292 , 2004