Mitigating Deafness in Multiple Beamforming Antennas

Vivek Jain and Dharma P. Agrawal

ECECS DepartmentUniversity of Cincinnati

{jainvk, dpa}@ececs.uc.edu

Outline Antenna System – MAC Layer Perspective Multiple Beam Antennas IEEE 802.11 DCF Deafness Problem Solutions Proposed in Literature

Single Beam Antennas Multiple Beam Antennas

Algorithm for Mitigating Deafness (AMD) Performance Evaluation Conclusions

Antenna System – MAC Layer Perspective

Omnidirectional Antenna – Low Throughput in Wireless Ad hoc networks due to poor spatial reuse

Omnidirectional Communication

A B

C

D

E

F

G

H

Directional Communication

Directional Antenna – Better Spatial reuse. But a node still unable to fully utilize “spatial bandwidth”

A B

C

DF

G

H

X

Nodes in Silent Zone

E

Antenna System – MAC Layer Perspective

Multiple Beam Antenna – Exploits spatial bandwidth fully A node can initiate more than one simultaneous

transmission (or reception)

DATA

DATA

DATA

A

B

C

D

E

F

G

DATADATA

DATA

Multiple Beam Antennas - Types

2

34

6

7

8

10

11

12

5

9

1

Adaptive array Switched array

top view (horizontal)

Interferer 1

User 1

User 3

User 2 Interferer 2

Interferer 3

Applications

Military NetworksCellular Communication NetworksMultihop Wireless Networks

2

34

6

7

8

10

11

12

5

9

1

Switched array

top view (horizontal)

Interferer 1

User 1

User 3

User 2 Interferer 2

Interferer 3

Adaptive array

top view (horizontal)

Interferer 1

User 1

User 3

User 2 Interferer 2

Interferer 3

Multiple Beam Antennas - Beam Forming

A node can either transmit or receive but not both simultaneously

… …

Direction of Arrival Estimation Beam Formation

IEEE 802.11 DCF

TimeRTSDIFS

SIFS

DIFS RTS

Defer access

aSlotTime

RandomBackoff

Source

Destination ACK

Other

CTS

SIFS Data

NAV (RTS)

NAV (CTS)

NAV (Data)

Physical Carrier Sensing

Virtual Carrier Sensing

De-facto medium access control for wireless LAN and ad hoc networks Originally designed for omnidirectional communication, its virtual

carrier sensing (VCS) mechanism is enhanced for directional communication to include directional of arrival also

SIFS

BeamformingAdvantages Longer Range

Better connectivity and lower end-to-end delay

Spatial Reuse Increased capacity and

throughput

Limitations Deafness and hidden

terminal problems Better connectivity and lower

end-to-end delay

1

23

4

8

7

Directional Coverage Area

Omnidirectional Coverage Area

5

6

Deafness Problem

Nodes X and Y do not know the busy state of node A and keep transmitting RTSs to A

RTS

RTSB

Y

X

DATAA

Deafness – Consequences

At transmitter Increases retransmission attempts after

doubling contention window for every unsuccessful attempt

At receiver Can increase collisions due to interference

with active RTS or data receptions Overall Network

Reduces throughput and increases end-to-end latency

Deafness – Proposed Solutions (Single Beam Antennas)

Omni-directional transmission of control messages Asymmetry in gain of

directional and omni-directional nodes leads to deafness

Circular sweeping of control messages Increases end-to-end delay

due to sweeping

Deafness – Proposed Solutions (Multiple Beam Antennas)

Proactive approach A node transmits control

messages in all free beams

Reactive approach A node transmits control

messages in all beams that are free and have potential transmitters

Proposed AlgorithmHybrid Approach Uses DVCS mechanism to

dynamically maintain two parameters for every beam

isRTSReceived: Set to true when a node receives a RTS intended for itself

isCTSReceived: Set to true when a node receives a CTS not intended for itself

Transmit control messages in all unblocked beams whose isRTSReceived is set to true

Transmit control messages in all unblocked beams if isCTSReceived is true for the beam engaged in actual data communication

SCH

SCH

SCH

CTS

Performance Evaluation

1

23

4

8

7

Directional Coverage Area

Omnidirectional Coverage Area

5

6

The Antenna Model

Packet generation at each source node is modeled as Poisson process with specified mean arrival rate

Each packet has a fixed size of 2000 bytes and is transmitted at a rate of 2Mbps

Each node has maximum buffer of 30 packets

Each packet has a lifetime of 30 packet durations

Each simulation is run for 100 seconds

Parameter Value

Data rate 2 Mbps

Data packet size 2000 bytes

Control Packet size 45 bytes

ACK size 38 bytes

DIFS duration 50 microseconds

SIFS duration 10 microseconds

Short retry limit 7

Long retry limit 4

Performance Evaluation

Sample Scenarios Scenario 1 – Omnidirectional communication of control

messages degrades the performance of the system by causing collisions at the receiver

Scenario 2 – Omnidirectional communication of control message is required to prevent the deafness problem

A

BC

D

A

BC

D

Performance Evaluation Throughput obtained in MMAC-NB is low due to collisions

occurring at node D from transmissions by nodes A and B The topology has no effect on ESIF as control messages are

sent only in routes with potential transmitters

A

BC

D

Performance Evaluation Increased collisions at node D in MMAC-NB leads to

increase in retransmissions by node B

A

BC

D

Performance Evaluation No effect of AMD on MMAC-NB and ESIF MMAC-NB and AMD-MMAC-NB yield optimal performance ESIF mechanism suffers from synchronization losses

A

BC

D

Conclusions By extending DVCS concept, a distributed

algorithm to mitigate deafness in beamforming antennas is proposed

Two more parameters are added to DNAV or ENAV table which are maintained dynamically by MAC layer

Simulation results shows better performance and simpler implementation for MMAC-NB and ESIF, respectively, the only two on-demand protocols currently available for multiple beam antennas

Can We Eliminate Deafness in Beamforming Antennas?

NO !!! Inherent limitation of directional

communication A node cannot receive control

messages from directions other than the ones in which it has currently beamformed This impairs the directional virtual carrier

sensing mechanism leading to deafness

Thank You!!!

Questions ???

MMAC-NB – IEEE 802.11 DCF Mechanism

ESIF – Explicit Synchronization via Intelligent Feedback Mechanism

MAC – IssuesConcurrent Packet Reception with IEEE 802.11 DCF

Conclusion: Eradicate the backoff after DIFS duration

RTS

RTS

RTS

RTS

RTS

RTS

RTS

RTS

A

B

C

D

E

F

G

DATA

DIFS

DIFS

DIFS

CTSACK

RTS

DIFSC

TS