Directional Antennas in Ad Hoc Networks

1
Directional AntennasinAd Hoc Networks

• Nitin Vaidya
• University of Illinois at Urbana-Champaign
• Joint work with
• Romit Roy Choudhury, UIUC
• Xue Yang, UIUC
• Ram Ramanathan, BBN

2
Mobile Ad Hoc Networks

• Formed by wireless hosts which may be mobile
• Without necessarily using a pre-existing
  infrastructure
• Routes between nodes may potentially contain
  multiple hops

3
Mobile Ad Hoc Networks

• May need to traverse multiple links to reach a
  destination

4
Mobile Ad Hoc Networks (MANET)

• Mobility causes route changes

5
Why Ad Hoc Networks ?

• Potential ease of deployment
• Decreased dependence on infrastructure

6
Many Applications

• Personal area networking
• cell phone, laptop, ear phone, wrist watch
• Military environments
• soldiers, tanks, planes
• Civilian environments
• taxi cab network
• meeting rooms
• sports stadiums
• boats, small aircraft
• Emergency operations
• search-and-rescue
• policing and fire fighting

7
Many Variations

• Fully Symmetric Environment
• all nodes have identical capabilities and
  responsibilities
• Asymmetric Capabilities
• transmission ranges and radios may differ
• battery life at different nodes may differ
• processing capacity may be different at different
  nodes
• Asymmetric Responsibilities
• only some nodes may route packets
• some nodes may act as leaders of nearby nodes
  (e.g., cluster head)

8
Many Variations

• Traffic characteristics may differ in different
  ad hoc networks
• bit rate
• timeliness constraints
• reliability requirements
• unicast / multicast / geocast
• host-based addressing / content-based addressing
  / capability-based addressing
• May co-exist (and co-operate) with an
  infrastructure-based network

9
Many Variations

• Mobility patterns may be different
• people sitting at an airport lounge
• New York taxi cabs
• kids playing
• military movements
• personal area network
• Mobility characteristics
• speed
• predictability
• direction of movement
• pattern of movement
• uniformity (or lack thereof) of mobility
  characteristics among different nodes

10
Challenges

• Limited wireless transmission range
• Broadcast nature of the wireless medium
• Hidden terminal problem
• Packet losses due to transmission errors
• Mobility-induced route changes
• Mobility-induced packet losses
• Battery constraints
• Potentially frequent network partitions
• Ease of snooping on wireless transmissions
  (security hazard)

11
Question

• Can ad hoc networks benefit from the progress
  made at physical layer ?
• Efficient coding schemes
• Power control
• Adaptive modulation
• Directional antennas
• Need improvements to upper layer protocols

12
Directional Antennas
13
Using Omni-directional Antennas
A Frozen node
B
D
S
A
14
Directional Antennas
Not possible using Omni
B
D
S
C
A
15
Comparison
16
Questions

• Are Directional antennas beneficial in ad hoc
  networks ?
• To what extent ?
• Under what conditions ?

17
Research Direction

• Identify issues affecting directional
  communication
• Evaluate trade-offs across multiple layers
• Design protocols that effectively use directional
  capabilities

Caveat Work-in-Progress
18
Preliminaries
19
Hidden Terminal Problem

• Node B can communicate with A and C both
• A and C cannot hear each other
• When A transmits to B, C cannot detect the
  transmission using the carrier sense mechanism
• If C transmits, collision may occur at node B

20
RTS/CTS Handshake

• Sender sends Ready-to-Send (RTS)
• Receiver responds with Clear-to-Send (CTS)
• RTS and CTS announce the duration of the transfer
• Nodes overhearing RTS/CTS keep quiet for that
  duration

C
10
B
A
D
10
21
IEEE 802.11

• Physical carrier sense
• Virtual carrier sense using Network Allocation
  Vector (NAV)
• NAV is updated based on overheard
  RTS/CTS/DATA/ACK packets, each of which specified
  duration of a pending transmission
• Nodes stay silent when carrier sensed busy
  (physical/virtual)

22
Antenna Model
23
Antenna Model

• 2 Operation Modes Omni Directional

24
Antenna Model

• Omni Mode
• Omni Gain Go
• Idle node stays in Omni mode.
• Directional Mode
• Capable of beamforming in specified direction
• Directional Gain Gd (Gd gt Go)

25
Directional Neighborhood
C
B
A
A and B are Directional-Omni (DO) neighbors B
and C are Directional-Directional (DD) neighbors
26
A Simple Directional MAC Protocol(DMAC)
27
DMAC Protocol

• A node listens omni-directionally when idle
• Only DO links can be used
• Sender node sends a directional-RTS using
  specified transceiver profile
• Receiver of RTS sends directional-CTS

28
DMAC Protocol

• DATA and ACK transmitted and received
  directionally
• Nodes overhearing RTS or CTS sets up NAV for that
  DOA (direction of arrival)
• Nodes defer transmitting only in directions for
  which NAV is set

29
Directional NAV (DNAV)

• Node E remembers directions in which it has
  received RTS/CTS, and blocks these directions.
• Transmission initiated only if direction of
  transmission does not overlap with blocked
  directions.

30
Directional NAV (DNAV)

• E has DNAV set due to RTS from H. Can talk to B
  since Es transmission beam does not overlap.

31
Example
C
E

B
D
A
B and C communicate D E cannot D blocked with
DNAV D and A communicate
32
Issues with DMAC

• Hidden terminals due to asymmetry in gain
• A does not get RTS/CTS from C/B

C
B
A
Data
As RTS may interfere with Cs reception of DATA
33
Problems with DMAC

• Hidden terminals due to directionality
• Due to unheard RTS/CTS

D
C
B
A
A beamformed in direction of D ? A does not
hear RTS/CTS from B/C A may now interfere at C
34
Issues with DMAC Deafness

• Deafness

Z
RTS
A
B
DATA
RTS
Y
RTS
X does not know node A is busy. X keeps
transmitting RTSs to node A
X
With 802.11 (omni antennas), X would be aware
that A is busy, and defer its own transmission
35
Problems with DMAC

• Shape of Silenced Regions

Region of interference for directional
transmission
Region of interference for omnidirectional
transmission
36
Problems with DMAC

• Since nodes are in omni mode when idle, RTS
  received with omni gain
• DMAC can use DO links, but not DD links

C
B
A
37
DMAC Trade-off

• Disadvantages
• Increased hidden terminals
• Deafness
• Directional interference
• Uses only DO links

• Benefits
• Better Network Connectivity
• Spatial Reuse

38
Solving DMAC Problems

• Are improvements possible to make directional MAC
  protocols more effective ?
• One possible improvement Use DD links

39
Using DD Links

• Possible to exploit larger range of directional
  antennas.

C
A
A C are DD neighbors, but cannot communicate
with DMAC If A C could be made to point
towards each other, single hop communication may
be possible
40
Multi-Hop RTS Basic Idea
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.
41
MMAC protocol

• A transmits RTS in the direction of its DD
  neighbor, node D
• Blocks H from communicating in the direction H-D
• A then transmits multi-hop RTS using source route
• A beamforms towards D and now waits for CTS

42
MMAC protocol

• D receives MRTS from C and transmits CTS in the
  direction of A (its DD neighbor).
• A initiates DATA communication with D
• H, on hearing RTS from A, sets up DNAVs towards
  both H-A and H-D. Nodes B and C do not set DNAVs.
• D replies with ACK when data transmission
  finishes.

43
Performance

• Simulation
• Qualnet simulator 2.6.1
• CBR traffic
• Packet Size 512 Bytes
• 802.11 transmission range 250 meters.
• Channel bandwidth 2 Mbps
• Mobility - none

44
Impact of Topology

• Nodes arranged in linear configurations reduce
  spatial reuse for directional antennas

45
Impact of Topology
IEEE 802.11 1.19 Mbps DMAC 2.7 Mbps
IEEE 802.11 1.19 Mbps DMAC 1.42 Mbps
46
Aligned Flows
MMAC
802.11
DMAC
47
Unaligned Flows
MMAC
802.11
DMAC
48
Unaligned Flows Topology
MMAC
802.11
DMAC
49
Delay Unaligned Flows Topology
50
Directional MAC Summary

• Directional MAC protocols can improve throughput
  and decrease delay
• But not always
• Performance dependent on topology

51
Routing using Directional Antennas
52
Motivation

• Directional antennas affect network layer, in
  addition to MAC protocols

53
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

54
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D
from S
55
Route Discovery in DSR
Y
Broadcast transmission
Z
S
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
X,Y Represents list of identifiers appended
to RREQ
56
Route Discovery in DSR
Y
Z
S
S,E
E
F
B
C
M
L
J
A
G
S,C
H
D
K
I
N
57
Route Discovery in DSR
Y
Z
S
E
F
S,E,F
B
C
M
L
J
A
G
H
D
K
S,C,G
I
N

• Node C receives RREQ from G and H, but does not
  forward
• it again, because node C has already forwarded
  RREQ once

58
Route Discovery in DSR
Y
Z
S
E
F
S,E,F,J
B
C
M
L
J
A
G
H
D
K
I
N
S,C,G,K

• Nodes J and K both broadcast RREQ to node D

59
Route Reply in DSR
Y
Z
S
RREP S,E,F,J,D
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents RREP control message
60
DSR over Directional Antennas

• RREQ broadcast by sweeping
• To use DD links

61
Route Discovery in DSR
Y
Z
S
E
F
S,E,F,J
B
C
M
L
J
A
G
H
D
K
I
N
S,C,G,K

• Nodes J and K both broadcast RREQ to node D

62
Trade-off
Larger Tx Range Fewer Hop
Routes Few Hop Routes Low Data
Latency Smaller Angle High Sweep
Delay More Sweeping High Overhead
63
Route discovery latency Single flow, grid
topology (200 m distance)
DDSR4
DDSR6
DSR
64
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

65
Throughput
DDSR18
DDSR9
DSR
Sub-optimal routes chosen by DSR because
destination node misses the shortest RREQ, while
beamformed.
66
Route Discovery in DSR
F
J
RREP
J
D
K
N
RREQ
D receives RREQ from J, and replies with RREP D
misses RREQ from K
67
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

68
Routing Overhead

• Using omni broadcast, nodes receive multiple
  copies of same packet - Redundant !!!
• Broadcast Storm Problem
• Using directional Antennas can do better ?

69
Routing Overhead
Use K antenna elements to forward broadcast
packet. K N/2 in simulations
Footprint of Tx
70
Routing Overhead

• Control overhead reduces

Beamwidth of antenna element (degrees)
71
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

72
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.

73
Aggregate throughput over random mobile scenarios
DDSR9
DSR
74
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

75
Conclusion

• Directional antennas can improve performance
• But suitable protocol adaptations necessary
• Also need to use suitable antenna models
• plenty of problems remain

76
Thanks!

• www.crhc.uiuc.edu/nhv

77
(No Transcript)
78
Adaptive Modulation

• Joint work with Gavin Holland and Victor Bahl

79
Adaptive Modulation

• Channel conditions are time-varying
• Received signal-to-noise ratio changes with time

A
B
80
Adaptive Modulation

• Multi-rate radios are capable of transmitting at
  several rates, using different modulation schemes
• Choose modulation scheme as a function of channel
  conditions

Modulation schemes provide a trade-off
between throughput and range
Throughput
Distance
81
Adaptive Modulation

• If physical layer chooses the modulation scheme
  transparent to MAC
• MAC cannot know the time duration required for
  the transfer
• Must involve MAC protocol in deciding the
  modulation scheme
• Some implementations use a sender-based scheme
  for this purpose Kamerman97
• Receiver-based schemes can perform better

82
Sender-Based Autorate Fallback Kamerman97

• Probing mechanisms
• Sender decreases bit rate after X consecutive
  transmission attempts fail
• Sender increases bit rate after Y consecutive
  transmission attempt succeed

83
Autorate Fallback

• Advantage
• Can be implemented at the sender, without making
  any changes to the 802.11 standard specification
• Disadvantage
• Probing mechanism does not accurately detect
  channel state
• Channel state detected more accurately at the
  receiver
• Performance can suffer
• Since the sender will periodically try to send at
  a rate higher than optimal
• Also, when channel conditions improve, the rate
  is not increased immediately

84
Receiver-Based Autorate MAC Holland01mobicom

• Sender sends RTS containing its best rate
  estimate
• Receiver chooses best rate for the conditions and
  sends it in the CTS
• Sender transmits DATA packet at new rate
• Information in data packet header implicitly
  updates nodes that heard old rate

85
Receiver-Based Autorate MAC Protocol
C
RTS (2 Mbps)
B
A
D
86
Extra slides
87
Directional Antennas in Random Topologies
Higher transmission range improves connectivity
in addition to achieving fewer hop routes. E.g.
Link a-b not possible using Omni transmission.
88
Effect of Beamwidth in Random Static Topologies