Medium Access Control for Wireless Networks using Directional Antennas

1
Medium Access Control for Wireless Networks using
Directional Antennas
ECE 256
2
Applications
Several Challenges, Protocols
3
Omnidirectional Antennas
4
IEEE 802.11 with Omni Antenna
M
Y
S
RTS
D
CTS
X
K
5
IEEE 802.11 with Omni Antenna
silenced
M
Y
silenced
S
Data
D
ACK
silenced
X
K
silenced
6
IEEE 802.11 with Omni Antenna
silenced
M
Interference management A crucial challenge
for dense multihop networks
S
Data
D
ACK
silenced
X
K
silenced
7
Managing Interference

• Several approaches
• Dividing network into different channels
• Power control
• Rate Control

Recent Approach Exploiting antenna capabilities
to improve the performance of wireless multihop
networks
8
From Omni Antennas
silenced
M
S
D
silenced
X
K
silenced
9
To Beamforming Antennas
M
S
D
X
K
10
To Beamforming Antennas
M
S
D
X
K
11
Outline / Contribution

• Antenna Systems ? A closer look
• New challenges with beamforming antennas
• Design of MAC and Routing protocols
• MMAC, ToneDMAC, CaDMAC
• DDSR, CaRP
• Cross-Layer protocols Anycasting
• Improved understanding of theoretical capacity
• Experiment with prototype testbed

12
Antenna Systems

• Signal Processing and Antenna Design research
• Several existing antenna systems
• Switched Beam Antennas
• Steerable Antennas
• Reconfigurable Antennas, etc.
• Many becoming commercially available

For example
13
Electronically Steerable Antenna ATR Japan

• Higher frequency, Smaller size, Lower cost
• Capable of Omnidirectional mode and Directional
  mode

14
Beam Steering

• Steering
• Mechanical steering (rotating dish antennas,
  cellular, etc.)
• Electronic steering (wireless cards, vehicle
  mounted, etc.)

15
For Mesh Networks

• On poletop or vehicles
• Antennas bigger
• No power constraint

16
Antenna Abstraction

• 3 Possible antenna modes
• Omnidirectional mode
• Single Beam mode
• Multi-Beam mode
• Higher Layer protocols select
• Antenna Mode
• Direction of Beam

17
Antenna Beam

• Energy radiated toward desired direction

Main Lobe (High gain)
A
Sidelobes (low gain)
Pictorial Model
18
Directional Communication

• Directional Gain (Gd ) Omni Gain (Go)
• Friss Equation

A
B
C
19
Directional Reception

• Directional reception Spatial filtering
• Interference along straight line joining
  interferer and receiver

C
C
Signal
Signal
A
B
A
B
Interference
D
Interference
D
No Collision at A
Collision at A
20

• Will attaching such antennas at the radio layer
• yield most of the benefits ?
• Or
• Is there need for higher layer protocol support ?

21

• Lets study a simple baseline MAC protocol
• (a directional version of 802.11)
• Call this protocol DMAC and investigate
• its behavior through simulation

22
Lets design a Directional MAC

• Lets assume 2 antenna modes
• Omni (beamwidth 360) and directional
• Switching between modes require negligible
  latency
• Several design choices appear
• Mode of channel access (omni or directional)
• Backing off mechanism
• Mode of RTS/CTS transmissions
• Mode of Data/ACK transmissions
• Mode of Idle state
• Omni or directional NAVs
• Several others
• Ko00,Ramanathan01,Nasipuri00, Balanis00,
  Takai02,Bandyopadhay01, Bao02, Sanchez01,
  Ephremides98, Elbatt02, Sivakumar03, Gossain03,
  Tang05, Vasudevan05, Korakis03, etc.

23
Design Choices Carrier Sensing

• How should a node carrier sense ?
• Omnidirectionally or directionally ?

24
Design Choices Carrier Sensing

• How should a node carrier sense ?
• Omnidirectionally or directionally ?
• Omni carrier sensing inhibits spatial reuse ?
  unsuitable

Channel busy
Channel idle
a
b
a
b
25
Design Choices Backing Off

• While backing off, should a node be
• Omnidirectional or directional ?

26
Design Choices Backing Off

• While backing off, should a node be
• Omnidirectional or directional ?
• Omni back off prevents paralle communication ?
  unsuitable

Freeze Backoff
Continue backoff
a
b
a
b
27
Design Choices Virtual Carrier Sensing

• Inhibit transmissions only in unsafe directions
• Directional antennas extend NAV to Directional
  NAV

DNAV allows transmission
a
b
DNAV inhibits transmission
28
Design Choices Exchanging RTS/CTS

• Omnidirectional RTS/CTS
• Directional RTS/CTS
• Multiple directional RTS/CTS (sweep)

29
Design Choices Exchanging RTS/CTS

• Omnidirectional RTS/CTS
• Limits spatial reuse cannot always transmit
  omni on account of directional NAV
• Limits the distance between communicable
  neighbors
• Directional RTS/CTS
• Improves reuse but requires direction of intended
  receiver
• Suffers from a problem called deafness
  (explained later)
• Multiple directional RTS/CTS (sweep)
  KorakisMobihoc
• No deafness, but large overhead

30
Design Choices Exchanging Data/ACK

• Omnidirectional Data/ACK
• Lower spatial reuse
• Lower link quality
• Lower communication range
• Directional Data/ACK
• The intuitive choice ? higher spatial reuse,
  better link quality, longer range/lower power

31
Design Choices Idle Mode

• While idle a node should be
• Omnidirectional or directional ?

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Design Choices Idle Mode

• While idle a node should be
• Omnidirectional or directional ?
• In absence of traffic information, idle node has
  to be omni

Desired Signal for a
Unwanted Interference for a
c
c
d
a
a
33

• Lets combine the design choices -- DMAC

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DMAC Example

• Remain omni while idle
• Nodes cannot predict who will trasmit to it

Y
S
D
X
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DMAC Example

• Assume S knows direction of D

Y
S
D
X
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DMAC Example
Y
S
D
X
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Intuitively
Performance benefits appear obvious
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However
Throughput (Kbps)
Sending Rate (Kbps)
39

• Clearly, attaching sophisticated antenna hardware
  
• is not sufficient
• Simulation traces revealed
• various new challenges
• Motivates higher layer protocol design

40
New Challenges

• Self Interference
• with Directional MAC

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Unutilized Range

• Longer range causes interference downstream
• Offsets benefits
• Network layer needs to utilize the long range
• Or, MAC protocol needs to reduce transmit power

Data
A
D
B
C
route
42
Enhancing MAC

• MMAC
• Transmit multi-hop RTS to far-away receiver
• Synchronize with receiver using CTS (rendezvous)
• Communicate data over long links

43
New Challenges II

• New Hidden Terminal Problems
• with Directional MAC

44
New Hidden Terminal Problem

• Due to gain asymmetry
• Node A may not receive CTS from C
• i.e., A might be out of DO-range from C

CTS
RTS
Data
B
C
A
45
New Hidden Terminal Problem

• Due to gain asymmetry
• Node A later intends to transmit to node B
• A cannot carrier-sense Bs transmission to C

RTS
CTS
Data
Carrier Sense
B
C
A
46
New Hidden Terminal Problem

• Due to gain asymmetry
• Node A may initiate RTS meant for B
• A can interfere at C causing collision

Collision
Data
RTS
B
C
A
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New Challenges II

• New Hidden Terminal Problems
• Due to missed out RTS/CTS

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New Hidden Terminal Problem II

• While node pairs communicate
• X misses Ds CTS to S ? No DNAV toward D

Y
S
Data
Data
D
X
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New Hidden Terminal Problem II

• While node pairs communicate
• X misses Ds CTS to S ? No DNAV toward D
• X may later initiate RTS toward D, causing
  collision

Collision
Y
S
Data
D
RTS
X
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New Challenges III

• Deafness
• with Directional MAC

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Deafness

• Node N initiates communication to S
• S does not respond as S is beamformed toward D
• N cannot classify cause of failure
• Can be collision or deafness

M
Data
S
D
RTS
N
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Channel Underutilized

• Collision N must attempt less often
• Deafness N should attempt more often
• Misclassification incurs penalty (similar to TCP)

M
Data
S
D
RTS
N
Deafness not a problem with omnidirectional
antennas
53
Deafness and Deadlock

• Directional sensing and backoff ...
• Causes S to always stay beamformed to D
• X keeps retransmitting to S without success
• Similarly Z to X ? a deadlock

Z
DATA
RTS
S
D
RTS
X
54
Impact on Backoff
Backoff Counter for DMAC flows
Backoff Values
Backoff Counter for ToneDMAC flows

• Another possible improvement

time
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New Challenges IV

• MAC-Layer Capture
• The bottleneck to spatial reuse

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Capture

• Typically, idle nodes remain in omni mode
• When signal arrives, nodes get engaged in
  receiving the pkt
• Received packet passed to MAC
• If packet not meant for that node, it is dropped

Wastage because the receiver could accomplish
useful communication instead of receiving the
unproductive packet
57
Capture Example
Both B and D are omni when signal arrives from A
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Outline / Contribution

• Antenna Systems ? A closer look
• New challenges with beamforming antennas
• Design of Capture-aware MAC and Routing protocols
• Experiment with prototype testbed

59
Impact of Capture

• Beamforming for transmission and reception only
• is not sufficient
• Antenna control necessary during idle state also

60
MAC Layer Solution
Idle Beam

• Capture-Aware MAC (CaDMAC)
• D monitors all incident traffic
• Identifies unproductive traffic
• Beams that receive only
• unproductive packets are
• turned off
• However, turning beams off
• can prevent useful communication in future

C
D
A
B
61
CaDMAC Time Cycles

• CaDMAC turns off beams periodically
• Time divided into cycles
• Each cycle consists of
• Monitoring window 2. Filtering window

cycle
1
2
1
1
2
2
time
All beams remain ON, monitors unproductive beams
Node turns OFF unproductive beams while it is
idle. Can avoid capture
62
CaDMAC Communication
C

• Transmission / Reception uses
• only necessary single beam
• When node becomes idle, it
• switches back to appropriate
• beam pattern
• Depending upon current time window

D
A
B
63
Spatial Reuse in CaDMAC

• During Monitoring window, idle nodes are omni

C
E
D
A
B
F
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Spatial Reuse in CaDMAC

• At the end of Monitoring window CaDMAC identifies
  unproductive links

C
E
D
A
B
F
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Spatial Reuse in CaDMAC

• During Filtering window ? use spatial filtering

Parallel Communications CaDMAC 3
DMAC others 2 Omni 802.11 1
C
E
D
A
B
F
66
Network Transport Capacity

• Transport capacity defined as
• bit-meters per second
• (like man-miles per day for airline companies)
• Capacity analysis

67
Directional Capacity

• Existing results show
• Capacity improvement lower bounded by
• Results do not consider side lobes of radiation
  patterns
• Consider main lobe and side lobe gains (gm and
  gs)
• Capacity upper bounded by
• i.e., improvement of

CaDMAC still below achievable capacity
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Discussion

• CaDMAC cannot eliminate capture completely
• Happens because CaDMAC cannot choose routes
• Avoiding capture-prone links ? A routing problem

A
B
X
Y
69

• Routing using Beamforming Antennas
• Incorporating capture-awareness

70
Motivating Capture-Aware Routing

• Find a route from S to D, given A?B exists
• Options are SXYD, SXZG

Z
Z
D
D
A
A
B
B
X
X
Y
Y
S
S
No Capture
Capture
71
Protocol Design

• Source routing protocol (like DSR)
• Intermediate node X updates route cost from S - X
• Destination chooses route with least cost
  (Uroute)
• Routing protocol shown to be loop-free

C1
USX
X
C2
C5
S
D
C3
USD USX C2 C5 PD 1
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Unified Routing Metric

• Uroute Weighted Combination of
• 1. Capture cost (K)
• 2. Participation cost (P)
• 3. Hop count (H)
• Weights chosen based on sensitivity analysis

73
CaRP Vs DSR
2
1
3
4
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CaRP Vs DSR
75
CaRP Vs DSR
76
CaRP Vs DSR
77
CaRP Vs DSR
78
CaRP Vs DSR
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CaRP Vs DSR
80
CaRP Vs DSR
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CaRP Vs DSR
DSR
CaRP
CaRP prefers a traffic-free direction Squeezes
in more traffic in given area
82
Performance of CaDMAC
CaDMAC
DMAC
Aggregate Throughput (Mbps)
CMAC
802.11
CBR Traffic (Mbps)
83
Throughput with CaRP
CaRP CaDMAC
Random Topologies
Aggregate Throughput (Mbps)
DSR CaDMAC
DSR 802.11
Topology Number
84
Conclusion / Criticism

• State of the art used omnidirectional antennas
• Antenna community advancements was critical
• However, smart antennas cannot be used
• Unless, protocols become antenna-aware
• MMAC paper identifies several awareness issues
• Hidden terminal, deafness, capture, interference,
  etc.
• Proposes one solution
• Many missing pieces - neighbor discovery,
  multipath, mobility
• Capture
• Intelligence even during idle state
• Solution assumes stable traffic, high resolution
  antennas

85

• Questions

86
Announcements

• Example reviews
• Posted on course websites
• Students not signed up for ppt
• One marathon class with all presentations
• I will stay through, you are welcome to attend
• Project Groups
• Please start thinking about it
• Feb 21 is deadline for emailing project topic
  rough plan
• If you dont have partners
• Please stay back after class next Tuesday

87

• BackUp Slides

88
Testbed Prototype

• Network of 6 laptops using ESPAR antennas
• ESPAR attached to external antenna port
• Beams controlled from higher layer via USB
• Validated basic operations and tradeoffs
• Neighbor discovery
• Observed multipath
• 60 degrees beamwidth useful
• Basic link state routing
• Improves route stability
• Higher throughput, less delay

89
Neighbor Discovery

• Non LOS and multipath important factors
• However, wide beamwidth (60 degrees) ? reasonable
  envelope

Anechoic Chamber
Office Corridor
90
Route Reliability

• Routes discovered using sweeping DO links
• Data Communication using DD links
• Improved SINR improves robustness against fading

91
Summary

• Future Dense wireless networks
• Better interference management necessary
• Typical approach Omni antennas
• Inefficient energy management
• PHY layer research needs be exploited

92
Impact of Hidden Terminals, Deafness, Capture,
unutilized range
93
Conclusion

• Directional antennas intuitively beneficial
• However, closer examination shows several
  tradeoffs
• We designed a simple DMAC protocol
• Considered several design choices, including
  carrier-sensing, backoff, RTS/CTS mode, idle
  mode, etc.
• Observed several problems with DMAC
• Such problems do not appear with omnidirectional
  antennas
• Glanced at some approaches to optimize DMAC
• Optimizations offer encouraging benefits in
  performance
• But several problems still remain to be resolved

94
Many open problems good project topics

• Neighbor discovery with directional antennas
• Especially under mobile scenarios
• Directional antennas and routing
• Vectorial, Zig-Zag routing ? Choose routes using
  direction info.
• Beamwidth power control
• Control network topology based on user need
• Capacity of directional communication
• How much theoretical improvements possible over
  omni ?
• TDMA based protocols
• Probably worth considering with directional
  antennas
• and many more

95
Thoughts !!

• Directional/MIMO antennas heavily considered for
  next generation networks
• 802.11s (for mesh) advocating such technologies
• Lot of research papers in the near past ? many
  open problems
• However, can mobility be supported ??
• Antennas impact higher layers
• Impact on performance of omni routing protocol
  studied
• Routing protocols designed for directional
  antennas
• Is cross layer necessary ?
• Testbed prototypes being built
• Both for adhoc and mesh networks

96
Why adopt new antenna technology ?

• Previous protocols assumed omnidirectional
  antennas
• Omni antennas radiate energy in unwanted
  directions
• Wasteful / unnecessary
• Recent advances in signal processing and antenna
  design principles
• Interfere only toward desired direction
• Feasible at smaller size and lower cost
• We ask
• Can we utilize directional antennas in multihop
  networking
• What are the benefits ? What protocols should be
  designed ?

97

• But first,
• Some basic antenna concepts

98
Shifting from WLAN to Multihop
99
Intuitive Benefit (1) Spatial Reuse
Omni Communication
Directional Communication
e
f
f
c
a
b
c
a
b
d
d
100
Intuitive Benefit (2) Range Extension
Omni Communication
Directional Communication
c
a
b
c
a
b
d
d
Many more benefits ...
101
No Free Lunch

• Several issues arise with directional beams
• Determining direction to transmit
• New hidden terminal problems
• Broadcast with directional beams
• Deafness
• Capture
• And many more

Insufficient to only add directional antennas,
without appropriate support from protocols
102
Why Carrier Sense ?

• Is carrier sensing necessary at all ?
• Transmission from M to N does not interfere B
• If B transmitting data to C, then collision
  likely anyway
• Directional carrier sensing seems unnecessary
• In reality, node B has sidelobes
• Carrier sensing necessary for situation when M
  close to B

Carrier Sense
Data
M
B
C
N
103
Combining Design Choices DMAC

• Idle nodes remain omni
• When packet arrives from network layer
• Consult directional NAV
• Carrier sense directionally toward receiver
• Wait for backoff in directional mode
• Transmit directional RTS
• RTS received omnidirectionally
• Receiver determines Direction of Arrival (DoA) of
  RTS
• Following CTS, Data, ACK exchange directional
• Switch back to omnidirectional mode

104
Routing with Higher Range

• Directional routes offer
• Better connectivity, fewer-hop routes
• However, broadcast difficult
• Sweeping necessary to emulate broadcast
• Evaluate tradeoffs ? Designed directional DSR

105
Todays Discussions

• Introducing the role of antennas
• Recall lecture 2 ? Motivate need for antenna
  technology
• Basic directional antenna concepts
• Applying directional antennas to networking
• Design choices and tradeoffs
• Designing the first simple protocol ? DMAC
• Several problems / challenges with DMAC
• Investigate optimizations ? MMAC, CaMAC
• What lies ahead ?
• Research issues in MAC, routing, and higher
  layers

106
Measuring Route Cost

• Sum capture costs of all beams on the route
• Capture cost of a Beam j
• how much unproductive traffic incident on Beam j
• Routes hop count
• Cost of participation
• How many intermediate nodes participate in cross
  traffic

X
S
D