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IEEE 802.11 DCF Based MAC Protocols for Multiple Beam Antennas and their Limitations
Vivek Jain, Anurag Gupta Dharma P.
AgrawalECECS DepartmentUniversity of Cincinnati
{jainvk, guptaag, dpa}@ececs.uc.edu
Dhananjay Lal
Research and Technology Center
Robert Bosch [email protected]
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Outline
Introduction Multiple Beam Antennas IEEE 802.11 DCF Proposed Variants Performance Evaluation Concurrent Packet Reception
Bounds Protocol Guidelines Conclusions
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Introduction
A B
C
DF
G
H
X
Nodes in Silent Zone
Omnidirectional Antenna – Low Throughput in Wireless Ad hoc networks due to poor spatial reuse
Omnidirectional Communication
A B
C
D
E
F
G
H
Directional Communication
Directional Antenna – Better Spatial reuse. But a node still unable to fully utilize “spatial bandwidth”.
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Introduction Multiple Beam Antenna – Exploits spatial bandwidth fully A node can initiate more than one simultaneous
transmissions (or receptions).
DATA
DATA
DATA
A
B
C
D
E
F
G
DATADATA
DATA
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Multiple Beam Antennas - Types
top view (horizontal)
Interferer 1
User 1
2
34
6
7
8
10
11
12
5User 3
9
User 2 Interferer 2
Interferer 3
1
Switched array
User 1
Interferer 1
top view (horizontal)
User 3
User 2
Interferer 3
Interferer 2
Adaptive array
Applications
Military networksCellular Communication NetworksWireless Local Area Networks
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Multiple Beam Antennas - Beam Forming
Therefore, a node can either transmit or receive simultaneously but not both.
… …
Direction of Arrival Estimation Beam Formation
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IEEE 802.11 DCF
TimeRTSDIFS
SIFS
DIFS RTS
Defer access
aSlotTime
RandomBackoff
Source
Destination ACK
Other
CTS
SIFS Data
SIFS
NAV (RTS)
NAV (CTS)
NAV (Data)
Physical Carrier Sensing
Virtual Carrier Sensing
De-facto medium access control for wireless LAN and ad hoc networks Originally designed for omnidirectional communication, its virtual
carrier sensing (VCS) mechanism is enhanced for directional communication to include directional of arrival also.
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IEEE 802.11 DCF for Multiple Beam Antennas
Random Backoff after DIFS wait
Beam-based Node-based
Transmission Control Packets (RTS/CTS)
Directional Omnidirectional
All nodes employ IEEE 802.11 DCF with directional virtual carrier mechanism (DVCS).
MMAC-NBMDMAC-NBMDMAC-BB MMAC-BB
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Performance Evaluation
1
23
4
8
7
Directional Coverage Area
Omnidirectional Coverage Area
5
6
The Antenna Model
Packet generation at each source node is modeled as Poisson process with specified mean arrival rate
Each packet has a fixed size of 2000 bytes and is transmitted at a rate of 2Mbps
Each node has maximum buffer of 30 packets Each packet has a lifetime of 30 packet durations Each simulation is run for 100 seconds.
Gains from spatial reuse only are
considered
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Performance Evaluation Omnidirectional protocols are able to achieve concurrent data
communications between node pairs A-B and C-D. Directional protocols on the other hand suffer from deafness
problem while omni-directional antenna from poor spatial reuse.
B
A
C
D
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Performance Evaluation None of the protocols are able to extract throughput of more
than 33% of the maximum possible value This implies only one route is active on an average and hence
concurrent packet reception is not occurring at node D.
A
B
C
D
E
G
F
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Performance Evaluation
Concurrent packet reception at node D
Concurrent packet transmission by node D
A
B
C
D
E
G
F
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Concurrent Packet Reception Bounds
N neighbors transmitting with probability p under saturation state
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Protocol Guidelines Transmit control packets in beams with
transmitting nodes – suppress deafness Common backoff timer for all beams –
support concurrent packet transmission Employ local synchronization – support
concurrent packet reception Employ hot-potato type of buffering, i.e.,
successive CPR and CPT cycles – minimize delay
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Conclusions Concurrent packet reception in multiple
beam antennas is highly improbable with IEEE 802.11 DCF based protocols
Asynchronous protocols are not suited for multiple beam antennas
A new MAC protocol based on the formulated guidelines is required.
Follow Up: IEEE Globecom 2005, St. Louis, Nov. 27-Dec.2.
V. Jain, A. Gupta, D. Lal, and D. P. Agrawal, "A Cross Layer MAC with Explicit Synchronization through Intelligent Feedback for Multiple Beam
Antennas,"
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Thank You!!!
Questions ???
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Performance Evaluation Deafness and route coupling do not affect omni-protocols,
but directional protocols experience performance degradation at higher loads.
A B
C D
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Performance Evaluation Omnidirectional protocols overwhelms node C leading to
data loss when the packet lifetime expires.
A
B
C
D
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Performance Evaluation Gains from omnidirectional communication
of control packets at medium loads Directional protocols perform better at
higher loads because of better spatial reuse.
D
H
I
J
C
G
B
FA
E
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Performance Evaluation Node-based backoff protocols for multiple beam
antennas achieve maximum throughput due to gains from concurrent packet transmissions
A B
C
D
E
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Performance EvaluationA B
C
D
E
Concurrent packet reception at any node
Concurrent packet transmission by any node
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Performance Evaluation
Energy expended in random and compete-5 topologies
Multiple beam omni-directional protocols expend more energy due to omni-directional transmission of control messages.
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Concurrent Packet Reception Bounds in Saturated State
The probability that a node receives data in b beams concurrently is given by ),(),()( MbQNbPbPCPR
bNb ppb
NNbP
)1(),(
P(b,N) is the probability that b out of N neighbors transmit in a slot concurrently and is given as
bM
bb
M
MbQ
!
),(
Q(b,M) is the probability of arranging b nodes in M beams such that CPR can happen, is given as
M
bCPRCPR bPP
2
)(
Thus the total probability of concurrent packet reception by a node is then given by