1. Mitigating Deafness in Multiple Beamforming Antennas
Vivek Jain and Dharma P. Agrawal
ECECS Department
University of Cincinnati
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2. Outline Antenna System – MAC Layer Perspective Multiple Beam Antennas
IEEE DCF
Deafness Problem
Solutions Proposed in Literature
Single Beam Antennas
Algorithm for Mitigating Deafness (AMD)
Performance Evaluation
Conclusions

3. Antenna System – MAC Layer Perspective
Omnidirectional Antenna – Low Throughput in Wireless Ad hoc networks due to poor spatial reuse 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 D F G H X
Nodes in Silent Zone E
Omnidirectional Communication

4. Antenna System – MAC Layer Perspective
Multiple Beam Antenna – Exploits spatial bandwidth fully
A node can initiate more than one simultaneous transmission (or reception) A E
DATA
DATA D B F C G

5. Multiple Beam Antennas - Types
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 2 3 4 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 2 3 4 6 7 8 10 11 12 5 9 1
Switched array
Adaptive array
Applications
Military Networks
Cellular Communication Networks
Multihop Wireless Networks

6. Multiple Beam Antennas - Beam Forming
… …
Direction of Arrival Estimation
Beam Formation
A node can either transmit or receive
but not both simultaneously

7. Physical Carrier Sensing Virtual Carrier Sensing
IEEE DCF
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
Physical Carrier Sensing
DIFS
SIFS
RTS
Data
Time
Source
SIFS
SIFS
CTS
ACK
Destination
DIFS
NAV (RTS)
RTS
Other
NAV (CTS)
aSlotTime
NAV (Data)
Virtual Carrier Sensing
Defer access
RandomBackoff

8. Directional Coverage Area Omnidirectional Coverage Area
Beamforming
Advantages
Longer Range
Better connectivity and lower end-to-end delay
Spatial Reuse
Increased capacity and throughput
Limitations
Deafness and hidden terminal problems 1 2 3 4 8 7
Directional Coverage Area
Omnidirectional Coverage Area 5 6

9. Deafness Problem X
RTS A B
DATA
RTS Y
Nodes X and Y do not know the busy state of node A and keep transmitting RTSs to A

10. 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

11. 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

12. 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

13. Proposed Algorithm Hybrid 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
CTS

14. Performance Evaluation
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
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 1 2 3 4 8 7
Directional Coverage Area
Omnidirectional Coverage Area 5 6
The Antenna Model

15. Performance Evaluation
Sample Scenarios
Scenario 1 – Omnidirectional communication of control messages degrades the performance of the system by causing collisions at the receiver A C B D
Scenario 2 – Omnidirectional communication of control message is required to prevent the deafness problem A C B D

16. Performance EvaluationB D
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

17. Performance EvaluationB D
Increased collisions at node D in MMAC-NB leads to increase in retransmissions by node B

18. Performance EvaluationB D
No effect of AMD on MMAC-NB and ESIF
MMAC-NB and AMD-MMAC-NB yield optimal performance
ESIF mechanism suffers from synchronization losses

19. 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

20. 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

21. Questions ???
Thank You!!!

22. MMAC-NB – IEEE 802.11 DCF Mechanism

23. ESIF – Explicit Synchronization via Intelligent Feedback Mechanism

24. MAC – Issues Concurrent Packet Reception with IEEE 802.11 DCF
DIFS E
RTS
RTS
RTS
DATA
ACK
CTS
DIFS D B F
RTS
RTS
RTS
RTS
CTS
RTS C G
Conclusion: Eradicate the backoff after DIFS duration