2. Outline What is an ad hoc network Smart Antenna Overview
Protocol description
Implementation of the protocol within OPNET
Performance study of the protocol
Summary

3. Ad Hoc Networks A B C D Formed by co-operating wireless nodes
No fixed network infrastructure
No centralized administration
- Each node acts as a router

4. MAC in Wireless networks
Uses MAC protocol of IEEE based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Basic channel access method can not combat hidden and exposed terminal problems
RTS and CTS are used to reserve the channel for the entire duration of the transmission of data (including ACK)
Physical and virtual carrier sensing is used for Collision Avoidance

5. Antenna in Wireless networks
Uses Omni-Directional Mode
Limited spatial reuse of the channel A B C D
A B
C D
If (C,D) are transmitting A & B cannot, with directional
antenna simultaneous sessions are possible

6. Smart Antennas
Schematic of a smart antenna (adaptive linear array)

7. Smart Antennas
Adaptive Antenna Arrays can direct the Radiation / receiving pattern (main lobe) towards the desired node
Signals received by multiple antennas are weighed and combined to maximize ‘SINR’ (Signal-to-Interference plus Noise ratio)
Weight Vectors obtained will give information about the desired node position
Weight Vectors can be computed to ‘Null’ undesired signals

8. Smart Antennas
Received Power 
(Transmit power) *(Tx Gain) * (Rx Gain)
Directional gain is higher, with Nulling Rx
Gain can be negligible

9. Example (Beamforming)
Antenna pattern with 8 antenna elements
Desired=45 deg, Nulls=20,70 deg Desired=45 deg, Nulls=10, 20, 30, 70, 80, 90 deg

10. Protocol Description
Direction-of-Arrival (DOA)-ALOHA is based on Slotted-ALOHA protocol
DOA Minislot ACK Minislot
DATA Transmission D A B C E F
Node ‘A’ receives max
power from node ‘B’ hence
places main lobe towards B
and Nulls towards D & F

11. Protocol Description The largest minislot is for the data transmission
Receiver rejects the packet if not an intended destination
Receiver sends ACK if data correctly received
Sender performs back-off if no ACK received (similar to Slotted-ALOHA)
Do not do Collision Avoidance (CA) but exploit Nulling!

12. Implementation of the protocol within OPNET
Adaptive Antenna Array is
implemented in Matlab and
antenna module calls the Matlab
routines
A node has no packet scheduled
for transmission issues a remote
interrupt to antenna to compute
weights for omni-direction mode
Transmitter MAC calls antenna
module with desired direction
which invokes Matlab routines
to determine weights

13. Implementation of the protocol within OPNET
During the duration of the DOA-Minislot, dra-power pipeline stage computes the direction and the received power of all the signals
Antenna module inserts (pw, dir) pair in a dynamic list
Max power direction is the desired direction and all the other received signals are interfereres
Antenna module invokes Matlab routine with input parameter (desired_DOA, interferers) and returns new weights
We use Minimum Mean Square Error ( MMSE) algorithm for Nulling

14. MUSIC Spatial Spectrum - Receiver
Power (in dB)
Direction of Arrival (DOA) deg

15. Implementation of the protocol within OPNET
a  c and b  d, ‘d’ mistakenly
Forms a beam towards ‘a’
If a node beamforms incorrectly
in a given timeslot, it remembers
that direction in single-entry cache a d b
During next slot node ignores maximum signal strength
direction, if same, it selects second strongest signal
Cache is not updated if a node correctly receives the packet
and cache is reset if no signal from that direction

16. Simulation Parameters
Performance Study
Simulation Parameters
Background Noise + ambient Noise
-143 dB
Propagation model
Free space
Bandwidth
1,000 kHz
Data Rate
2 Mbits/s
Min Frequency
2,402 MHz
Carrier Sensing Threshold
+3dB
Bit Error
Based on BPSK Modulation curve
Maximum radio range
250 mts

17. Some Aligned Routes in Grid
Sending rate (Tx) vs Aggregate Throughput
Aggregate Throughput (Kbps)
Sending Rate (Kbps)

18. Unaligned Routes in Grid
Sending rate (Tx) vs Aggregate Throughput
Aggregate Throughput (Kbps)
Sending Rate (Kbps)

19. “Random” Topology Sending rate (Tx) vs Aggregate Throughput
Aggregate Throughput (Kbps)
Sending Rate (Kbps)

20. Summary Power control Impact on Routing
Extend study to multipath environments

21. Thank You

22. References
J. C. Liberti and T. S. Rappaport. Smart Antennas for Wireless Communications. Prentice Hall, 1999.
Nitin H. Vaidya Romit Roy Choudhury, Xue Yang, and Ram Ramanathan. Using directional antennas for medium access control in ad hoc networks. In ACM/SIGMOBILE MobiCom 2002, 23 – 28 Sep 2002.
Mar_21_Ravi_Govindarajula.pdf