1. Directional Antennas for Wireless Networks
Romit Roy Choudhury

2. Several Challenges, Protocols
Internet
Applications
Several Challenges, Protocols

3. Omnidirectional Antennas
Internet
Omnidirectional Antennas

4. IEEE 802.11 with Omni Antenna RTS = Request To Send CTS = ClearY S
RTS D
CTS X K

5. IEEE 802.11 with Omni Antenna silenced M Y silenced S Data D ACKX K
silenced

6. IEEE 802.11 with Omni Antenna `` Interference management ``
silenced
silenced M A
silenced
`` Interference management ``
A crucial challenge for dense multihop networks F
silenced Y
silenced C
silenced S
Data D
ACK G
silenced
silenced X K B
silenced D
silenced
silenced

7. Managing Interference
Several approaches
Dividing network into different channels
Power control
Rate Control …
New Approach …
Exploiting antenna capabilities to
improve the performance
of wireless multihop networks

8. From Omni Antennas … E silenced silenced M A silenced F silenced YG
silenced
silenced X K B
silenced D
silenced
silenced

9. To Beamforming AntennasY C S D G X K B D

10. To Beamforming AntennasY C S D G X K B D

11. Today Antenna Systems  A quick look
New challenges with beamforming antennas

12. Antenna Systems Signal Processing and Antenna Design research
Several existing antenna systems
Switched Beam Antennas
Steerable Antennas
Reconfigurable Antennas, etc.
Many becoming commercially available
For example …

13. Electronically Steerable Antenna [ATR Japan]
Higher frequency, Smaller size, Lower cost
Capable of Omnidirectional mode and Directional mode

14. Switched and Array Antennas
On poletop or vehicles
Antennas bigger
No power constraint

15. Antenna Abstraction 3 Possible antenna modes
Omnidirectional mode
Single Beam mode
Multi-Beam mode
Higher Layer protocols select
Antenna Mode
Direction of Beam

16. Antenna Beam Energy radiated toward desired direction Pictorial Model
Main Lobe
(High gain) A A
Sidelobes
(low gain)
Pictorial Model

17. Directional Reception
Directional reception = Spatial filtering
Interference along straight line joining interferer and receiver C C
Signal
Signal A B A B
Interference D
Interference D
No Collision at A
Collision at A

18. Will attaching such antennas at the radio layer
yield most of the benefits ? Or
Is there need for higher layer protocol support ?

19. We design a simple baseline MAC protocol
(a directional version of )
We call this protocol DMAC and investigate
its behavior through simulation

20. DMAC Example Remain omni while idle
Nodes cannot predict who will trasmit to it Y S D X

21. DMAC Example
Assume S knows direction of D
RTS Y S D X

22. X silenced … but only toward direction of D
DMAC Example
RTS
CTS
RTS Y S
DATA/ACK D
X silenced … but only toward direction of D X

23. Performance benefits appear obvious
Intuitively
Performance benefits appear obvious

24. However …
Throughput (Kbps)
Sending Rate (Kbps)

25. Clearly, attaching sophisticated antenna hardware is not sufficient
Simulation traces revealed
various new challenges
Motivates higher layer protocol design

26. Antenna Systems  A quick look
New challenges with beamforming antennas

27. New Challenges [Mobicom 02]
Self Interference
with Directional MAC

28. Unutilized Range Longer range causes interference downstream
Offsets benefits
Network layer needs to utilize the long range
Or, MAC protocol needs to reduce transmit power
Data A B C D
route

29. New Hidden Terminal Problems
New Challenges II …
New Hidden Terminal Problems
with Directional MAC

30. New Hidden Terminal Problem
Due to gain asymmetry
Node A may not receive CTS from C
i.e., A might be out of DO-range from C
CTS
RTS
Data A B C

31. New Hidden Terminal Problem
Due to gain asymmetry
Node A later intends to transmit to node B
A cannot carrier-sense B’s transmission to C
CTS
RTS
Data
Carrier
Sense A B C

32. New Hidden Terminal Problem
Due to gain asymmetry
Node A may initiate RTS meant for B
A can interfere at C causing collision
Collision
Data
RTS A B C

33. New Hidden Terminal Problems
New Challenges II …
New Hidden Terminal Problems
with Directional MAC

34. New Hidden Terminal Problem II
While node pairs communicate
X misses D’s CTS to S  No DNAV toward D Y S
Data
Data D X

35. New Hidden Terminal Problem II
While node pairs communicate
X misses D’s CTS to S  No DNAV toward D
X may later initiate RTS toward D, causing collision
Collision Y S
Data D
RTS X

36. New Challenges III …
Deafness
with Directional MAC

37. Deafness Node N initiates communication to S
S does not respond as S is beamformed toward D
N cannot classify cause of failure
Can be collision or deafness M
Data S D
RTS N

38. Channel Underutilized
Collision: N must attempt less often
Deafness: N should attempt more often
Misclassification incurs penalty (similar to TCP) M
Data S D
RTS N
Deafness not a problem with omnidirectional antennas

39. Deafness and “Deadlock”
Directional sensing and backoff ...
Causes S to always stay beamformed to D
X keeps retransmitting to S without success
Similarly Z to X  a “deadlock” Z
DATA
RTS S D
RTS X

40. The bottleneck to spatial reuse
New Challenges IV …
MAC-Layer Capture
The bottleneck to spatial reuse

41. Capture Typically, idle nodes remain in omni mode
When signal arrives, nodes get engaged in receiving the packet
Received packet passed to MAC
If packet not meant for that node, it is dropped
Wastage because the receiver
could accomplish useful communication
instead of receiving the unproductive packet

42. Capture Example A B C D A B C D B and D beamform to receive
arriving signal
Both B and D are omni when
signal arrives from A

43. Take Away Message Technological innovations in many areas
Several can help the problem you are trying to solve
Although gains may not come by plug-and-play
New advances need to be embraced with care.
Directional/Beamforming antennas, a case study
Gains seemed obvious from a high level
Much revisions to protocols/algorithms needed
Some of the systems starting to come out today
Above true for projects you will do in class