1. A Cross Layer MAC with Explicit Synchronization through Intelligent Feedback for Multiple Beam Antennas
Vivek Jain, Anurag Gupta Dharma P. Agrawal
Dhananjay Lal
ECECS Department
University of Cincinnati
{jainvk, guptaag,
Research and Technology Center
Robert Bosch Corporation

2. Outline Introduction Multiple Beam Antennas MAC Protocol Design Issues
The ESIF Protocol
Performance Evaluation
Conclusions

3. Introduction
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
Omnidirectional Communication

4. Introduction Multiple Beam Antenna – Exploits spatial bandwidth fully
A node can initiate more than one simultaneous transmissions (or receptions). A E
DATA
DATA D B F C G

5. Multiple Beam Antennas - Types
User 1
Interferer 1
top view (horizontal)
User 3
User 2
Interferer 3
Interferer 2
Adaptive array
top view (horizontal)
Interferer 1
User 1 2 3 4 6 7 8 10 11 12 5
User 3 9
User 2
Interferer 2
Interferer 3 1
Switched array
Applications
Military networks
Cellular Communication Networks
Wireless Local Area Networks

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

7. 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. 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

9. MAC Issues – Backoff Removal
Multiple transmitters, located in the same beam of common receiver, always get the same receiver schedule and thus initiate communication at the same time - collision
A node with very high data generation rate will overwhelm its receiver, without giving latter a chance to forward this traffic - fairness issue
All classes of service get same priority – QoS issue
Use p-persistent CSMA
DIFS A
RTS X
RTS B C
DIFS A B C
DIFS
DIFS
CTS
ACK
RTS
DATA
Hold the transmitting node

10. ESIF – Assumptions
Nodes are equipped with multiple switched beam antenna array and can precisely calculate the Angle of Arrival (AoA) of the received signal
All nodes form non-overlapping multiple beams with equal gain so as to collectively span entire space
Beam shape is assumed as conical and benefits of nulling or the impact of side-lobe interference are not considered
A node can either transmit or receive data on multiple beams at the same time but not both
The channel is symmetric.

11. ESIF – ENAV Every node maintains an ENAV:
The beam a neighbor falls within
Neighbor’s schedule - the duration until this neighbor is engaged in communication elsewhere
Whether a neighbor’s schedule requires maintaining silence in the entire beam
Number of data packets outbound for the neighbor
The p-persistent probability to use when talking to this neighbor

12. ESIF – Cross Layer Data Management
Using network layer information along with ENAV a node determines:
Whether a beam contains an active route
The number of potential transmitters in each beam
Until what time the node needs to maintain silence in a particular beam
Each node has a store-and-forward buffer for relaying data packets
Available buffer is used dynamically to form different queues for each beam - prevents head-of-the-line blocking

13. ESIF – Design
ESIF piggybacks feedback onto control messages; RTS with Intelligent Feedback (RIF), and CTS with Intelligent Feedback (CIF), Schedule Update with Intelligent Feedback (SCH)
SCH identifier allows a neighbor to adjudge whether to defer transmission for only this node or for the entire beam
buffer-threshold to control priorities between receiver and transmitter modes
Reception gets priority as long as the buffer size remains under the threshold
If a node cannot actually initiate transmitter mode, the receiver still gets the priority
Priority switch solves problems of an overwhelmed receiver.
This also provides a mechanism to control the contribution of a node to end-to-end delays

14. ESIF – Control Message Packet Format
Control packet (RIF/CIF/SCH) format
Type field in Frame Control is used to identify a control message as RIF, CIF or SCH
Duration holds the estimated time of communication that the other nodes must backoff for;
Priority contains the priority of this request, and
p is the persistent probability which the other nodes should use when talking to this node.

15. Performance Evaluation
Generation of packets is modeled as a Poisson process with the equal mean arrival time
IEEE DCF based protocols are used for omnidirectional antenna (Omni), single beam directional antenna (Directional-NB) and MBAA (MMAC-NB), directional (Directional) and multiple-beam (Multibeam)
Directional-NB, MMAC-NB and ESIF protocols involve DVCS
ESIF is implemented with a buffer-threshold value of 1 1 2 3 4 8 7
Directional Coverage Area
Omnidirectional Coverage Area 5 6
Gains from spatial reuse only are considered
The Antenna Model

16. ESIF – Basic Operation

17. Simulation Parameters
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
Sensing power
0.07 mW
Reception power
1.45 mW
Transmission power
1.75 mW
Total beams 8
Simulation Time
100 seconds
Buffer
30 packets
Packet Lifetime
30 Packet Durations

18. Performance EvaluationB
Removal of contention window based backoff in ESIF does not affect long-term fairness
Both the transmitters get equal opportunity to transmit

19. Performance EvaluationB A C D
Performance Evaluation
ESIF enhances throughput by the priority switch between transmission and reception modes
ESIF is able to achieve concurrent data communications between node pairs A-B and C-D

20. Performance EvaluationB C D E G F
Performance Evaluation
ESIF is able to achieve CPR at common intermediate node D
Dynamic priority switch ensures data packets just received are transmitted (concurrently) in the next cycle, thus, maximizing throughput and minimizing delay

21. Conclusions
ESIF is the first attempt to achieve concurrent packet reception with on-demand protocols for MBAA
ESIF removes the contention window based random backoff in IEEE DCF based protocols and uses embedded feedback to synchronize neighboring nodes
Allows nodes to receive or transmit multiple packets simultaneously in different beams
Cross layer information is used to guarantee long-term fairness
ESIF is a hybrid of synchronous and asynchronous on-demand medium access control

22. Questions ???
Thank You!!!

23. Performance EvaluationB C D
Performance Evaluation
Deafness and route coupling do not affect omni-protocols, but directional protocols experience performance degradation at higher loads.

24. Performance EvaluationB C D
Performance Evaluation
Omnidirectional protocols overwhelms node C leading to data loss when the packet lifetime expires.
ESIF extracts the highest throughput among all protocols by using a proportional p value for p-persistent CSMA.

25. Performance EvaluationD H I J C G B F A E
Directional protocols perform better at higher loads because of better spatial reuse.
ESIF takes advantage of CPR and CPT to achieve optimal performance

26. Performance EvaluationB C D E
Performance Evaluation
Node-based backoff protocols for multiple beam antennas achieve maximum throughput due to gains from concurrent packet transmissions
On-demand protocols does not yields optimal results for Complete-k topologies due to synchronization losses

27. Performance Evaluation
Multiple beam omni-directional protocols expend more energy due to omni-directional transmission of control messages.
Energy expended in random and compete-5 topologies

28. Frame control field for control messages
IEEE – Packet Formats
Frame control field for control messages
IEEE
Type value (b3 b2)
Type description
Subtype value (b7 b6 b5 b4)
Subtype description 01
Control
1011
Request To Send (RTS)
1100
Clear To Send (CTS)
0000–1001
Reserved