Using Directional Antennas for
Medium Access Control
in
Ad Hoc Networks
Romit Roy Choudhury, UIUC
Xue Yang, UIUC
Ram Ramanathan, BBN
Nitin H. Vaidya, UIUC
Ad Hoc Networks
Typically assume Omnidirectional antennas
A silenced
node
C
B
A
D
Can Directional Antennas
Improve Performance?
Not possible
using Omni
C
B
A
D
A Comparison
Issues
Omni
Directional
Spatial Reuse
Low
High
Connectivity
Low
High
Interference
Omni
Directional
Cost &
Complexity
Low
High
Motivation
• Are directional antennas beneficial
to medium access control in ad
hoc networks ?
– To what extent ?
– Under what conditions ?
Protocol Stack
Neighbor
Discovery
Routing Layer
Transceiver Profile
Hello, Data/Control Pkts
DMAC / MMAC
Antenna Layer
Organization
•
•
•
•
•
•
•
•
802.11 Basics
Related Work
Antenna Model
Simple Directional MAC protocol (DMAC)
Problems with DMAC – Insights
Multi-Hop MAC (MMAC)
Performance (comparison with 802.11)
Conclusion
IEEE 802.11
• 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/CTS
defer transmission for that duration
– Network Allocation Vector (NAV) remembers duration
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
NAV = 10
C
D
E
F
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
F
IEEE 802.11
CTS = Clear-to-Send
CTS
A
B
C
D
E
NAV = 8
F
IEEE 802.11
•DATA packet follows CTS. Successful data reception
acknowledged using ACK.
DATA
A
B
C
D
E
F
IEEE 802.11
ACK
A
B
C
D
E
F
IEEE 802.11
• Channel contention resolved using backoff
– Nodes choose random backoff interval from [0, CW]
– Count down for this interval before transmission
A
Random
backoff
Data Transmit
backoff
Wait
B
Random
backoff
Wait
backoff
Data Transmit
Antenna Model
2 Operation Modes: Omni and Directional
A node may operate in any one mode at any given
time
Antenna Model
In Omni Mode:
• Nodes receive signals with Gain Go
• While idle a node stays in Omni mode
In Directional Mode:
• Capable of beamforming in specified direction
• Directional Gain Gd (Gd > Go)
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
B
A
C
Convention: A link shown by overlapping beams
along the line joining the transmitter and receiver
in question. Nodes C, A form a link. C, B do not.
Directional Neighborhood
Receive Beam
B
Transmit Beam
A
C
• 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
Directional Neighborhood
Transmit Beam
Receive Beam
B
A
C
•When C transmits directionally
• Node B receives packets from C only in directional mode
•C and B are Directional-Directional (DD) neighbors
Related Work
• Many proposals/analyses of directional antennas
[Ko00,Ramanathan01,Nasipuri00, Balanis00,
Takai02,Bandyopadhay01, Bao02, Sanchez01,
Ephremides98]
• MAC Proposals differ based on
–
–
–
–
How RTS/CTS transmitted (omni, directional)
Transmission range of directional antennas
Channel access schemes
Omni or directional NAVs
Simple DMAC protocol
Similar protocols proposed in
[Takai02, Nasipuri00, Ramanathan01]
• A node listens omni-directionally when idle
• Sender transmits Directional-RTS (DRTS) using
specified transceiver profile
• RTS received in Omni mode (only DO links used)
• Receiver sends Directional-CTS (DCTS)
• DATA,ACK transmitted and received directionally
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up
directional NAV (DNAV) for that Direction of
Arrival (DoA)
B
CTS
D
A
C
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up
directional NAV (DNAV) for that Direction of
Arrival (DoA)
B
D
A
DNAV
C
Directional NAV (DNAV)
• New transmission initiated only if direction of
transmission does not overlap with DNAV, i.e.
if (θ > 0)
B
D
A
DNAV
θ
RTS
C
DMAC Example
C
E
D
B
B and C communicate
D and E cannot: D blocked with DNAV from C
D and A communicate
A
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to asymmetry in gain
Data
RTS
A
B
C
A is unaware of communication between B and C
A’s RTS may interfere with C’s reception of DATA
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to unheard RTS/CTS
D
B
A
C
• Node A beamformed in direction of D
• Node A Does not hear RTS/CTS from B & C
Issues with DMAC
• Two types of Hidden Terminal Problems
– Due to unheard RTS/CTS
D
B
A
C
Node A may now interfere at node C by transmitting
in C’s direction
Issues with DMAC
• Deafness
Z
RTS
A
B
DATA
RTS
Y
RTS
X
X does not know node A is busy.
X keeps transmitting RTSs to node A
Using 802.11 (omni antennas) X would be aware
that A is busy, and defer its own transmission
DMAC Tradeoffs
• Benefits
– Better Network
Connectivity
• Disadvantages
– Hidden terminals
– Deafness
– Spatial Reuse
– No DD Links
Enhancing DMAC
• Are improvements possible to make
DMAC more effective ?
• One possible improvement:
Make Use of DD Links
Using DD Links
Exploit larger range of Directional antennas
Receive Beam
A
Transmit Beam
C
A and C are DD neighbors, but cannot communicate
in DMAC
Multi Hop RTS – Basic Idea
D
C
A
B
DO neighbors
E
DD neighbors
F
G
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
MMAC protocol
A transmits RTS towards D
D
H
E
F
C
A
B
G
MMAC protocol
H updates DNAV
DNAV
D
H
E
F
C
A
B
G
MMAC protocol
A transmits M-RTS to DO neighbor B
D
H
E
F
C
A
B
G
MMAC protocol
B forwards M-RTS to C (also DO)
D
H
E
F
C
A
B
G
MMAC protocol
A beamforms toward D – waits for CTS
D
H
E
F
C
A
B
G
MMAC protocol
C forwards M-RTS to D
D
H
E
F
C
A
B
G
MMAC protocol
D beamforms towards A – sends CTS
D
H
E
F
C
A
B
G
MMAC protocol
A & D communicate over DD link
D
H
E
F
C
A
B
G
MMAC protocol
Nodes D and G similarly communicate
D
H
E
F
C
A
B
G
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, Main-lobe Gain = 10 dBi
Side lobes ignored
Channel bandwidth 2 Mbps
Mobility - none
Impact of Topology
D
A
A
E
B
B
F
C
C
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 2.7 Mbps
Nodes arranged in
linear configurations
reduce spatial reuse
for D-antennas
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 1.42 Mbps
Aggregate Throughput (Kbps)
Aligned Routes in Grid
1200
802.11
DMAC
MMAC
1000
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
Aggregate Throughput (Kbps)
Unaligned Routes in Grid
1200
1000
802.11
DMAC
MMAC
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
“Random” Topology
Aggregate Throughput
1200
1000
802.11
DMAC
MMAC
800
600
400
200
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
Avg. End to End Delay (s)
“Random” Topology: delay
2
1.5
1
DMAC
MMAC
0.5
0
0
500
1000
1500
Sending Rate (Kbps)
2000
2500
MMAC - Concerns
• High traffic – lower probability of RTS delivery
• Multi-hop RTS may not reach DD neighbor due to
deafness or collision
• We use no more than 3 DO links for each DD link
• Neighbor discovery overheads may offset the
advantages of MMAC
Future Work
• Impact of directional antennas on the
performance of routing protocols
[RoyChoudhury02]
• DMAC/MMAC does not implement power
control – plan to incorporate in future
• Protocols to alleviate deafness through MAC
layer per-antenna packet scheduling
Conclusion
• Directional MAC protocols show improvement
in aggregate throughput and delay
– But not always
• Performance dependent on topology
– Random topology aids directional communication
• MMAC outperforms DMAC & 802.11
– 802.11 better in some scenarios
Thank you
www.crhc.uiuc.edu/~croy
Chicken and Egg Problem !!
• DMAC/MMAC part of UDAAN project
– UDAAN performs 3 kinds of beam-forming for
neighbor discovery
– NBF, T-BF, TR-BF
– Send neighborhood information to K hops
– Using K hop-neighborhood information, probe using
each type of beam-form
– Multiple successful links may be established with the
same neighbor
Mobility
• 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]
Side Lobes
• Side lobes may affect performance
– Higher hidden terminal problems
B
A
C
Node B may interfere at A when A is receiving from C
Deafness in 802.11
• Deafness 2 hops away in 802.11
RTS
A
B
C
D
• C cannot reply to D’s RTS
– D assumes congestion, increases backoff
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
C
B
A
D
DO neighbors
E
DD neighbors
F
G
Broadcast
• Several definitions of “broadcast”
– Broadcast region may be a sector, multiple
sectors
Broadcast Region
A
– Omni broadcast may be performed through
sweeping antenna over all directions
[RoyChoudhury02_TechReport]
DoA Detection
• Signals received at each element
combined with different weights at the
receiver
Why DO ?
• Antenna training required to beamform in
appropriate direction
– Training may take longer time than duration
of pilot signal [Balanis00_TechReport]
– We assume long training delay
• Also, quick DoA detection does not make
MMAC unnecessary
Queuing in MMAC
D
E
F
C
A
B
G
Impact of Topology
D
A
A
E
B
B
F
C
C
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 2.7 Mbps
Nodes arranged in
linear configurations
reduce spatial reuse
for D-antennas
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 1.42 Mbps