august 8, 2015 computer networks coe 549 directional antennas for ad- hoc networks tarek sheltami...
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April 19, 2023
Computer Networks COE 549Directional Antennas for Ad-
hoc NetworksTarek Sheltami
KFUPMCCSECOE
http://faculty.kfupm.edu.sa/coe/tarek/coe549.htm
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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
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Ad Hoc Networks
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A silenced node
A
B
C
D
Typically assume Omnidirectional antennas
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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 direction
Benefits:
better connectivity and lower end-to-end delay
2. Higher spatial reuse
Why?
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
B
E
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 802.1104/19/23
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
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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
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
AC
E
A
E
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 D
RTS/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
B
SD
X
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]04/19/23
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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 E
F
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 2500
Sending Rate (Kbps)
Avg
. E
nd
to
En
d D
elay
(s)
DMAC
MMAC
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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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References
1. 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