Transcript Document

Utilizing Directional Antennas
in Ad Hoc Networks
(UDAAN)
Nitin H. Vaidya
University of Illinois at Urbana-Champaign
Joint work with
Romit Roy Choudhury
Xue Yang
University of Illinois
Ram Ramanathan
BBN Technologies
Broad Theme
• Impact of physical layer mechanisms on
upper layers
– Adaptive modulation
– Power control
– Directional antennas
UDAAN
• DARPA FCS communications project
• Focus on exploiting directional antennas for
ad hoc networking
UDAAN Protocol Stack
Neighbor
Discovery
Routing Layer
BBN
Transceiver Profile
MAC
Antenna
UIUC
Black box
Ad Hoc Networks
• Formed by wireless hosts without requiring
an infrastructure
• May need to traverse multiple links to reach a
destination
A
A
B
B
Mobile Ad Hoc Networks
• Mobility causes route changes
A
A
B
B
Why Ad Hoc Networks ?
• Ease of deployment
• Decreased dependence on infrastructure
Antennas
• Wireless hosts typically use
single-mode antennas
• Typically, the
single-mode = omni-directional
• Much of the discussion here applies when the
single-mode is not omni-directional
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
Pretending a circular range
IEEE 802.11
RTS = Request-to-Send
RTS
A
B
C
D
E
F
NAV = 10
NAV = remaining duration to keep quiet
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
Omni-Directional Antennas
Red nodes
Cannot
Communicate
presently
X
D
C
Y
Directional Antennas
Not possible
using Omni
X
D
C
Y
A Comparison
Issues
Omni
Directional
Spatial Reuse
Low
High
Connectivity
Low
High
Interference
Omni
Directional
Cost & Complexity
Low
High
Question
• How to exploit directional antennas in
ad hoc networks ?
– Medium access control
– Routing
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)
Symmetry: Transmit gain = Receive gain
Antenna Model
• More recent work models sidelobes
approximately
Caveat
Abstract antenna model
 Results only as good as the abstraction
Need more accurate antenna models
Directional Communication
Received Power

(Transmit power) *(Tx Gain) * (Rx Gain)
Directional gain is higher
Potential Benefits of
Directional Antennas
• Increase “range”, keeping transmit power
constant
• Reduce transmit power, keeping range
comparable with omni mode
• Realizing only the second benefit easier
Neighbors
• Notion of a “neighbor” needs to be
reconsidered
– Similarly, the notion of a “broadcast” must also be
reconsidered
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
A Simple Directional MAC protocol
Obvious generalization of 802.11
• A node listens omni-directionally when idle
• Sender transmits Directional-RTS (DRTS) towards
receiver
• RTS received in Omni mode (idle receiver in when idle)
• Receiver sends Directional-CTS (DCTS)
• DATA, ACK transmitted and received directionally
Directional MAC
RTS = Request-to-Send
X
RTS
A
B
C
D
E
F
Pretending a circular range
Directional MAC
CTS = Clear-to-Send
X
CTS
A
B
C
D
E
F
Directional MAC
•DATA packet follows CTS. Successful data reception
acknowledged using ACK.
X
DATA
A
B
C
D
E
F
Directional MAC
X
ACK
A
B
C
D
E
F
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up directional NAV
(DNAV) for that Direction of Arrival (DoA)
D
CTS
C
X
Y
Directional NAV (DNAV)
• Nodes overhearing RTS or CTS set up directional NAV
(DNAV) for that Direction of Arrival (DoA)
D
C
X
DNAV
Y
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 omni antennas, X would be aware that A is
busy, and defer its own transmission
Issues with DMAC
• Uses DO links, but not DD links
DMAC Tradeoffs
• Benefits
• Disadvantages
– Better Network
Connectivity
– Hidden terminals
– Spatial Reuse
– Deafness
– 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
using DMAC
Multi Hop RTS (MMAC) – 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
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” configuration
reduce spatial reuse
Aggregate throughput
802.11 – 1.19 Mbps
DMAC – 1.42 Mbps
Power control may
improve performance
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
• Lower probability of RTS delivery
• Multi-hop RTS may not reach DD neighbor due to
deafness or collision
• Neighbor discovery overheads may offset the
advantages of MMAC
Directional MAC: Summary
• Directional MAC protocols show improvement
in aggregate throughput and delay
– But not always
• Performance dependent on topology
– “Random” topology aids directional communication
Routing
Routing Protocols
• Many routing protocols for ad hoc networks
rely on broadcast messages
– For instance, flood of route requests (RREQ)
• Using omni antennas for broadcast will not
discover DD links
• Need to implement broadcast using
directional transmissions
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
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
Route Discovery in DSR
Y
Broadcast transmission
[S]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
[X,Y]
Represents list of identifiers appended to RREQ
Route Discovery in DSR
Y
Z
S
E
[S,E]
F
B
C
A
M
J
[S,C]
H
G
K
I
L
D
N
Route Discovery in DSR
Y
Z
S
E
F
B
[S,E,F]
C
M
J
A
L
G
H
I
[S,C,G] K
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
• Nodes J and K both broadcast RREQ to node D
N
Route Reply in DSR
Y
Z
S
E
RREP [S,E,F,J,D]
F
B
C
M
J
A
L
G
H
K
I
Represents RREP control message
D
N
DSR over Directional Antennas
• RREQ broadcast by sweeping
– To use DD links
Directional Routing
Broadcast by sweeping
Tradeoffs
Larger Tx Range
Few Hop Routes
Fewer Hop Routes
Low Data Latency
Small Beamwidth
More Sweeping
High Sweep Delay
High Overhead
Issues
• Sub-optimal routes may be chosen if destination node
misses shortest request, while beamformed
F
D misses request from K
J
RREP
J
D
K RREQ
N
Optimize by having
destination wait before
replying
• Broadcast storm: Using broadcasts, nodes receive
multiple copies of same packet
Use K antenna elements to
forward broadcast packet
Performance
• Preliminary results indicate that routing
performance can be improved using
directional antennas
Conclusion
• Directional antennas can potentially benefit
• But also create difficulties in protocol design
• Other issues
–
–
–
–
Power control
Need better models for directional antennas
Capacity analysis
Multi-packet reception
 Need to better understand physical layer
Thanks!
Related papers at
www.crhc.uiuc.edu/~nhv
Performance
Throughput Vs Mobility
Aggregate Throughput (Kbps)
Control overhead
DSR
DDSR4
DDSR6
DDSR9
DDSR18
2000
1500
1000
500
0
0
5
10
15
20
25
mobility (m/s)
• Control overhead higher using DDSR
• Throughput of DDSR higher, even under mobility
• Latency in packet delivery lower using DDSR
30
Routing using Directional Antennas
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
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents a node that has received RREQ for D from S
Route Discovery in DSR
Y
Broadcast transmission
[S]
S
Z
E
F
B
C
M
J
A
L
G
H
K
I
D
N
Represents transmission of RREQ
[X,Y]
Represents list of identifiers appended to RREQ
Route Discovery in DSR
Y
Z
S
E
[S,E]
F
B
C
A
M
J
[S,C]
H
G
K
I
L
D
N
Route Discovery in DSR
Y
Z
S
E
F
B
[S,E,F]
C
M
J
A
L
G
H
I
[S,C,G] K
D
N
• Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
• Nodes J and K both broadcast RREQ to node D
N
Route Reply in DSR
Y
Z
S
E
RREP [S,E,F,J,D]
F
B
C
M
J
A
L
G
H
K
I
Represents RREP control message
D
N
DSR over Directional Antennas
• RREQ broadcast by sweeping
– To use DD links
Route Discovery in DSR
Y
Z
S
E
[S,E,F,J]
F
B
C
M
J
A
L
G
H
D
K
I
[S,C,G,K]
• Nodes J and K both broadcast RREQ to node D
N
Trade-off
Larger Tx Range
Few Hop Routes
Fewer Hop Routes
Low Data Latency
Smaller Angle
More Sweeping
High Sweep Delay
High Overhead
Route discovery latency … Single
flow, grid topology (200 m distance)
DDSR4
DDSR6
DSR
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
Throughput
DDSR18
DDSR9
DSR
Sub-optimal routes chosen by DSR because destination
node misses the shortest RREQ, while beamformed.
Route Discovery in DSR
F
J
RREP
J
D
K
RREQ
D receives RREQ from J, and replies with RREP
D misses RREQ from K
N
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
Routing Overhead
• Using omni broadcast, nodes receive multiple
copies of same packet - Redundant !!!
• Broadcast Storm Problem
• Using directional Antennas – can do better ?
Routing Overhead
Use K antenna elements to forward broadcast packet.
K = N/2 in simulations
Footprint
of Tx
Ctrl Overhead 
=
 (No. Ctrl Tx)  (Footprint of Tx)
 No. Data Packets
Routing Overhead
Control overhead reduces
Beamwidth of antenna element (degrees)
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
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.
Aggregate throughput over random
mobile scenarios
DDSR9
DSR
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
Conclusion
• Directional antennas can improve performance
• But suitable protocol adaptations necessary
• Also need to use suitable antenna models
•
… plenty of problems remain
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
Organization
•
•
•
•
•
•
802.11 Basics
Related Work
Antenna Model
MAC
Routing
Conclusion