1

Medium Access Control for Wireless Networks using Directional Antennas

ECE 256

2

ApplicationsSeveral Challenges, Protocols

Internet

3

Internet

Omnidirectional Antennas

4

CTS = Clear To Send

RTS = Request To Send

IEEE 802.11 with Omni Antenna

D

Y

S

M

K

RTS

CTS

X

5

IEEE 802.11 with Omni Antenna

D

Y

S

X

M

Ksilenced

silenced

silenced

silencedData

ACK

6

IEEE 802.11 with Omni Antenna

DS

X

M

Ksilenced

silenced

silenced

Y

silencedData

ACK

D

silenced

E

silencedA

silencedC

silenced

F

silenced

B

silenced

G

silenced

`` Interference management ``A crucial challenge for dense multihop networks

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 …

DS

X

M

Ksilenced

silenced

silenced

Y

silenced

D

silenced

E

silencedA

silencedC

silenced

F

silenced

B

silenced

G

silenced

9

To Beamforming Antennas

DS

X

M

K

Y

D

E

A

C

F

B

G

10

To Beamforming Antennas

DS

X

M

K

Y

D

E

A

C

F

B

G

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

A

Pictorial Model

A

Main Lobe (High gain)

Sidelobes (low gain)

18

Directional Communication

Directional Gain (Gd ) ≥ Omni Gain (Go) Friss’ Equation

A B CDO OO

DD

19

Directional Reception

Directional reception = Spatial filtering Interference along straight line joining interferer and

receiver

A BC

D

Signal

Interference

No Collision at A

A B

C

D

Signal

Interference

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

Let’s study a simple baseline MAC protocol(a directional version of 802.11)

Call this protocol DMAC and investigate its behavior through simulation

22

Let’s design a “Directional MAC”

Let’s 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

a b

c

d

a b

c

d

Channel busy

Channel idle

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

a b

c

d

a b

c

d

Freeze Backoff

Continuebackoff

27

Design Choices – Virtual Carrier Sensing Inhibit transmissions only in unsafe directions

Directional antennas extend NAV to Directional NAV

a b

c

d

DNAV inhibits transmission

DNAV allows 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 ?

32

Design Choices – “Idle” Mode

While “idle” a node should be Omnidirectional or directional ?

In absence of traffic information, idle node has to be omni

a

c

a

c

d

Desired Signal for “a”

UnwantedInterference for “a”

33

Let’s combine the design choices -- DMAC

34

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

DMAC Example

D

Y

S

X

35

RTS

DMAC Example

D

Y

S

X

Assume S knows direction of D

36

RTS

RTSCTS

DMAC Example

D

Y

S

X

DATA/ACK

X silenced … but only toward direction of D

37

Intuitively

Performance benefits appear obvious

38

However …

0

100

200

300

400

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Aggregate Throughput (Kbps)

802.11

DMAC

Sending Rate (Kbps)

Th

rou

gh

pu

t (K

bp

s)

39

Clearly, attaching sophisticated antenna hardware is not sufficient

Simulation traces revealed various new challenges

Motivates higher layer protocol design

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Self Interferencewith Directional MAC

New Challenges

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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

A B CData

D

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 Hidden Terminal Problemswith Directional MAC

New Challenges II …

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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

B CAData

RTSCTS

45

New Hidden Terminal Problem

Due to gain asymmetry

Node A later intends to transmit to node B A cannot carrier-sense B’s transmission to C

B CAData

Carrier Sense

RTSCTS

46

New Hidden Terminal Problem

Due to gain asymmetry

Node A may initiate RTS meant for B A can interfere at C causing collision

B CADataRTS

Collision

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New Hidden Terminal ProblemsDue to missed out RTS/CTS

New Challenges II …

48

While node pairs communicate X misses D’s CTS to S No DNAV toward D

New Hidden Terminal Problem II

D

Y

S

X

DataData

49

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

New Hidden Terminal Problem II

D

Y

S

X

Data

RTS

Collision

50

Deafnesswith Directional MAC

New Challenges III …

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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

S D

N

Data

RTS

M

52

Channel Underutilized

Collision: N must attempt less often Deafness: N should attempt more often

Misclassification incurs penalty (similar to TCP)

S D

N

Data

RTS

M

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”

DATA

RTS

X

DS

Z

RTS

54

Impact on Backoff

Another possible improvement:

Backoff Counter for DMAC flows

Backoff Counter for ToneDMAC flows

time

Ba

cko

ff V

alu

es

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MAC-Layer CaptureThe bottleneck to spatial reuse

New Challenges IV …

56

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

Capture

Wastage because the receiver could accomplish useful communication

instead of receiving the unproductive packet

57

Capture Example

A B

C

D

Both B and D are omni whensignal arrives from A

A B

C

D

B and D beamform to receivearriving signal

58

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

Beamforming for transmission and reception only is not sufficient

Antenna control necessary during idle state also

Impact of Capture

A B

C

DA B

C

D

60

Capture-Aware MAC (CaDMAC) D monitors all incident traffic Identifies unproductive traffic

Beams that receive onlyunproductive packets are turned off

However, turning beams offcan prevent useful communication in future

MAC Layer Solution

A B

C

D

Idle Beam

61

CaDMAC turns off beams periodically Time divided into cycles Each cycle consists of

1. Monitoring window + 2. Filtering window

1 2

CaDMAC Time Cycles

1 12 2

cycle

time

All beams remain ON,monitors unproductive beams

Node turns OFF unproductivebeams while it is idle.

Can avoid capture

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CaDMAC Communication

Transmission / Reception uses only necessary single beam

When node becomes idle, it switches back to appropriate

beam pattern Depending upon current time window

A B

C

D

A B

C

D

63

During Monitoring window, idle nodes are omni

Spatial Reuse in CaDMAC

A B

C

EF

D

64

At the end of Monitoring window CaDMAC identifies unproductive links

Spatial Reuse in CaDMAC

A B

C

EF

D

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During Filtering window use spatial filtering

Spatial Reuse in CaDMAC

A B

C

EF

D

Parallel CommunicationsCaDMAC : 3 DMAC & others : ≤ 2Omni 802.11 : 1

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Network Transport Capacity

Transport capacity defined as:bit-meters per second

(like man-miles per day for airline companies)

Capacity analysis

∑∑= =

≥nT

bit

bith

h

hbit nTLr

λ

λ1

)(

1

∑∑= =

≤nT

bit

bith

h

AWTkrλ

1

)(

1

2 .

⎟⎠

⎞⎜⎝

⎛∞→

n

WOLimn

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

⎟⎟⎠

⎞⎜⎜⎝

⎛

θβπ2

O

α

α

α

βπ

2

1

1

21⎟⎟⎠

⎞⎜⎜⎝

⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

s

m

g

g

n

W

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛ α2

s

m

g

gO

CaDMAC still below achievable capacity

68

CaDMAC cannot eliminate capture completely

Happens because CaDMAC cannot choose routes Avoiding capture-prone links A routing problem

Discussion

YX

AB

69

Routing using Beamforming AntennasIncorporating capture-awareness

70

Motivating Capture-Aware Routing

YX

AB

YX

AB

D

S

Z DZ

S

Find a route from S to D, given AB exists Options are SXYD, SXZG

Capture No Capture

71

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

Protocol Design

X

DS

C1

C2

C3

C5

USX

USD = USX + C2 + C5 + PD + 1

72

Uroute = Weighted Combination of1. Capture cost (K)2. Participation cost (P)3. Hop count (H)

Weights chosen based on sensitivity analysis

Unified Routing Metric

ijipijrouteij

kroute HPU ++= ∑∈

ωκω

73

CaRP Vs DSR

1

2

34

74

CaRP Vs DSR

75

CaRP Vs DSR

76

CaRP Vs DSR

77

CaRP Vs DSR

78

CaRP Vs DSR

79

CaRP Vs DSR

80

CaRP Vs DSR

81

CaRP Vs DSR

DSR CaRP

CaRP prefers a traffic-free direction“Squeezes in” more traffic in given area

82

Performance of CaDMAC

CaDMAC

DMAC

802.11

CBR Traffic (Mbps)

Ag

gre

gate

Th

rou

gh

pu

t (M

bp

s)

CMAC

83

Throughput with CaRP CaRP +CaDMAC

DSR +CaDMAC

DSR +802.11

Ag

gre

gate

Th

rou

gh

pu

t (M

bp

s)

Topology Number

Random Topologies

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 don’t 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 …

0

100

200

300

400

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Aggregate Throughput (Kbps)

802.11

DMAC

0

200

400

600

0 500 1000 1500 2000 2500

Sending Rate (Kbps)

Aggregate Throughput

802.11

DMAC

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

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 ?

Why adopt new antenna technology ?

97

But first, …Some basic antenna concepts

98

Shifting from WLAN to Multihop

99

Intuitive Benefit (1) – Spatial Reuse

Omni CommunicationOmni Communication Directional CommunicationDirectional Communication

a b c

d

Silenced NodeSilenced Node

a b c

d

e

f

Simultaneous CommunicationNo Simultaneous CommunicationOk

f

100

Intuitive Benefit (2): Range Extension

Omni CommunicationOmni Communication Directional CommunicationDirectional Communication

a b c

d

a b c

d

Link Between Nodes b and dNo Link Between Nodes b and dOk

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

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

Why Carrier Sense ?

B CM DataCarrier Sense 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

Today’s 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

Sum capture costs of all beams on the route Capture cost of a Beam j =

how much unproductive traffic incident on Beam j

Route’s hop count Cost of participation

How many intermediate nodes participate in cross traffic

Measuring Route Cost

X

DS