2. Outline Introduction Multiple Access Protocols Mode Selection Criteria
Mobile Ad hoc Networks (MANETS)
Antennas
Multiple Access Protocols
Mode Selection Criteria
Motivations
Assumptions
Node Model
Antenna Pattern
Simulation Parameters
Performance Evaluation
Applicability of Mode Selection Criteria to Multiple Beam Antennas
Conclusions
Future Work

3. Mobile Ad Hoc Networks (MANETs)
Peer-to-peer connectivity
Lack of fixed infrastructure relays
Absence of centralized authority
Multi-hop forwarding to ensure network connectivity
Applications
Military.. Combat Systems, reconnaissance
Rescue, medical emergency, telemedicine

4. Antenna Types Omni-directional antenna Directional antenna
Transmits power equally in all directions
Directional antenna
Concentrates power in a directed zone
Smart Antenna
Has the in-built intelligence to change direction according to requirement (steer the beam)
Multiple-Beam Smart Antenna
Simultaneous transmission/reception in more than one directions
Multiple Input Multiple Output (MIMO)
Multiple streams of data in same channel.

5. Smart Antenna System

6. Antennas and MANETs
Omni-directional communication suffers from poor spatial reuse
Directional communication leads to better spatial reuse, reduces co-channel interference and provides range extension

7. Multiple Access Protocols
MAC Proposals differ based on
How RTS/CTS transmitted (omni, directional)
Transmission range of directional antennas
Channel access schemes
Omni or directional NAVs
Antenna Model
Two Operation modes
Omni & Directional
Omni Mode:
Omni Gain = Go
Idle node stays in Omni mode
Directional Mode:
Capable of beamforming in specified direction
Directional Gain = Gd (Gd > Go) --> Range Extension
Directional Gain = Gd (Gd = Go) --> Spatial Reuse
Range Extension
Spatial Reuse

8. Directional vs. Omni-directional
The Problem of utilizing directional antennas to improve the performance of ad hoc networks is non-trivial
Pros
Higher gain (Reduced interference)
Spatial Reuse
Cons
Potential possibility to interfere with communications taking place far away
Hidden Terminal
Deafness

9. Motivations Which mode? Omni-directional or directional
Analyze various topologies involving neighboring transmissions or receptions
Formulate mode selecting criteria for medium access control (MAC) for MANETs with heterogeneous technologies

10. Assumptions Two modes of operation: omni and directional
Directional transmission of RTS/CTS/DATA/ACK in directional mode
Transmission range of directional antennas is same as that of omni-directional ==> Spatial Reuse
4-Way CSMA for medium access control
The channel is symmetric

11. Node Model
The node model of advance MANET available in OPNET is modified to facilitate directional mode of communication
In directional mode, the antenna (tx_rx_ant) points in the desired direction with the help of antenna pointing processor (tx_rx_point)

12. Antenna Pattern
Conical directional antenna pattern of main lobe having beam-width of 45 degrees and a gain of 0 dBi. The gain in remaining spherical side-lobe is confined to -20dBi

13. Simulation Parameters
Value
Data rate
2 Mbps
Data packet size
1500 bytes
Packets Inter-arrival time
Constant (0.005 seconds)
Directional gain
0 dBi (main lobe)
-20 dBi (side lobes)
Transmit Power
0.5 mW
Packet reception-Power Threshold
-95 dBm
Buffer size
32 Kbytes (~21 Packets)
Simulation Time
100 seconds
Packets generated = 200 packets/sec/transmitter
Maximum achievable throughput
~ 130 packets/sec/receiver (non-overlapping communication)
~ 65 packets/sec/receiver (two overlapping transmissions)

14. Performance Evaluation – Deaf
Deaf communicating pair scenario
Receivers in same beam of the transmitter
Transmitters in same beam of the receiver
Both the transmitters are deaf to each other communication
Omni-directional mode performs better

15. Performance Evaluation – Deaf
Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode

16. Performance Evaluation – Deaf
Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver. However, average delay is nearly same in both cases

17. Performance Evaluation – Common Receiver
Common receiver scenario
Two or more transmitters with common receiver
Usually both the transmitters are deaf to each other communication
Omni-directional mode performs better

18. Performance Evaluation – Common Receiver
Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode

19. Performance Evaluation – Common Receiver
Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver.

20. Performance Evaluation – Linear_Pair_SameBeam
Another communicating pair in the same beam of the transmitter
Throughput of C-D pair suffers due to interference from A-B ongoing communication in directional mode
For optimal performance C switches to omni mode while other remains in directional mode

21. Performance Evaluation – Linear_Pair_SameBeam
Switching C to omni-directional mode while remaining nodes in directional mode gives optimal throughput

22. Performance Evaluation – Linear_Pair_SameBeam
Delay is less in directional mode as all newly generated packets are transmitted while packets in queue are dropped after maximum retransmission attempts

23. Performance Evaluation – Linear_Pair_SameBeam
Retransmission attempts by node C are much higher in directional mode owing to higher BER (i.e. collisions) at node D

24. Performance Evaluation – Tx_0
Another node transmitting in same direction
Again switching the mode of intermediate transmitting node to omni-directional mode while remaining with directional mode yields optimal performance

25. Performance Evaluation – Tx_0
Average throughput in directional mode is about 15% lower than in omni-directional mode

26. Performance Evaluation – Tx_0
BER is much higher in directional mode due to interference from transmitters as they are deaf to each other

27. Performance Evaluation – Tx_90 and Rx_90
Another non-interfering transmitter or receiver in the communicating beams
Omni-mode restricts simultaneous transmissions, hence directional mode is recommended
Tx_90
Rx_90

28. Performance Evaluation – Tx_90 and Rx_90
Directional communication achieves maximum possible throughput in all cases owing to better spatial reuse

29. Performance Evaluation – Tx_90 and Rx_90
Delay is more in omni-directional communication due to increased media access delay at the transmitters

30. Performance Evaluation – Tx_90 and Rx_90
Due to increased channel contention at the transmitters packet retransmission attempts are more in omni-directional mode

31. Performance Evaluation – Linear, Parallel and X topologies
Only the intended receiver or transmitter in the communicating beams
Both the transmitters are deaf to each other communication
No other communicating node in those beams
Directional mode outperforms omni-directional mode of communication
Linear X
Parallel

32. Performance Evaluation – Linear, Parallel and X topologies
Traffic Received (packets/sec) vs. time

33. Mode Selection Criteria
All nodes in omni-directional mode in the following cases:
Deaf communicating pair scenario
Receivers in same beam of the transmitter
Transmitters in same beam of the receiver
Both the transmitters are deaf to each other communication
Common receiver scenario
Two or more transmitters with common receiver
Intermediate transmitting node in omni-directional mode while other nodes in directional mode for the following cases:
Another communicating pair in the same beam of the transmitter
Another node transmitting in same direction
All nodes in directional mode, in the remaining cases including:
Another non-interfering transmitter or receiver in the communicating beams
Only the intended receiver or transmitter in the communicating beams

34. Applicability of Mode Selection Criteria to Multiple Beam Antennas
Can either transmit or receive multiple packets simultaneously. This requires:
Packet receptions in different beams at the node to commence at the same time
Packet transmissions by a node in multiple beams to begin simultaneously
A node cannot both send and receive data at the same time
Can simulate omni-directional mode by transmitting in all possible beams simultaneously
Can multiple beam antennas achieve optimal performance by transmitting control packets in beams having transmitters and receivers only ???

35. Conclusions Directional mode
Better spatial reuse
Enhances system capacity
Deafness and hidden terminal problems
However there are some cases where omni-directional mode performs better
Deaf communicating pair scenario
Interference from side-lobes cannot be ruled out
Common receiver scenario
Mode Selection Criteria forms the basis of developing MAC protocols for MANETs using heterogeneous antenna technologies
Dynamically switching a node from directional to omni-directional or vice versa depending on the neighboring nodes

36. Future Work Work needs to be extended for multi-hop topologies
Extensive study needs to be done with more communicating pairs within the vicinity so that performance varies with the node density
Game Theoretic approach for mode selection criteria in such scenarios
Performance of multiple beam antennas transmitting control packets in beams having transmitters and receivers only, need to be evaluated

38. IEEE 802.11 IEEE 802.11 DCF – RTS/CTS access scheme
Physical Carrier Sense
Physical Carrier Sensing
Virtual Carrier Sensing

39. Antenna System
Phased Array Antenna

40. Direction of Arrival Estimation

41. Antenna Pattern of 7-element uniform equally spaced circular array.
Beam Formation
Beam Forming
Technique in which the gain pattern of an adaptive array is steered to a desired direction through either beam steering or null steering signal processing algorithms
Adaptive beam forming algorithms can provide substantial gains (of the order of 10log(M) dB, where M is number of array elements) as compared to omni directional antenna system
Antenna Pattern of 7-element uniform equally spaced circular array.

42. Smart Antenna System Switched Beam
Consists of a set of predefined beams.
Allows selection of signal from desired user.
Beams have narrow main lobe and small side-lobes.
Signals received from side-lobes can be significantly attenuated.
Uses a linear RF network, called a Fixed Beam-forming Network (FBN) that combines M antenna elements to form up to M directional beams.

43. General Smart Antenna Architecture
Source: Chris Loadman, Zhizhang Chen and Dylan Jorgensen, “An Overview of Adaptive Antenna Technologies For Wireless Communications,” In Proc. o Communication Networks and Services Research Conference (CNSR), pp 15-19, 2003.

44. Features and Benefits of Smart Antenna Systems
Source:

45. The global market for smart antennas growth
Source: US analyst firm Visant Strategies

46. A terminal with 16 antennas mounted on a laptop
Source: Alexiou, A.and Haardt, M., “Smart antenna technologies for future wireless systems: trends and challenges,” IEEE Communications Magazine, Vol. 42, pp , Sept. 2004

47. MIMO PC Card
Source: Group MIMO Comes of Age.pdf