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

44. Rate Control in Wireless Networks
Romit Roy Choudhury
Rate Control in Wireless Networks

45. Recall 802.11 RTS/CTS + Large CS Zone
Alleviates hidden terminals, but trades off spatial reuse E
RTS F
CTS A B C D

46. Recall Role of TDMA

47. Recall Beamforming Ok No e Silenced Node f f a b c a b c d d
Omni Communication
Directional Communication e
Silenced Node f f a b c a b c d d
Simultaneous Communication No
Simultaneous Communication Ok

48. Also Multi-Channel Current networks utilize non-overlapping channels
Channels 1, 6, and 11
Partially overlapping channels can also be used

49. Also Data Rates Benefits from exploiting channel conditions
Rate adaptation
Pack more transmissions in same time

50. What is Data Rate ? Number of bits that you transmit per unit time
under a fixed energy budget
Too many bits/s:
Each bit has little energy -> Hi BER
Too few bits/s:
Less BER but lower throughput

51. 802.11b – Transmission rates 1 Mbps 2 Mbps 5.5 Mbps 11 Mbps Time
Highest energy per bit
Lowest energy per bit
1 Mbps
2 Mbps
5.5 Mbps
11 Mbps
Time
Optimal rate depends on SINR:
i.e., interference and current channel conditions

52. Some Basics C = B * log2(1 + SINR) Eb / N0 = SINR * (B/R)
Friss’ Equation
Shannon’s Equation
Bit-energy-to-noise ratio
C = B * log2(1 + SINR)
Eb / N0 = SINR * (B/R)
Varying
with time
and space
How do we choose the rate of modulation
Leads to BER

53. Static Rates SINR Time # Estimate a value of SINR
# Then choose a corresponding rate that would transmit packets
correctly (i.e., E b / N 0 > thresh) most of the times
# Failure in some cases of fading  live with it

54. Adaptive Rate-Control
SINR
Time
# Observe the current value of SINR
# Believe that current value is indicator of near-future value
# Choose corresponding rate of modulation
# Observe next value
# Control rate if channel conditions have changed

55. Rate and Range
Rate = 10 B C E A D

56. Rate and Range There is no free lunch  talking slow to go far B C E A

57. Will carrier sense range vary with rate
Any other tradeoff ?
Will carrier sense range vary with rate

58. Total interference Carrier sensing estimates energy in the channel.
Rate = 10 B C E A D
Rate = 20
Carrier sensing estimates energy in the channel.
Does not vary with transmission rate

59. Bigger Picture Rate control has variety of implications
Any single MAC protocol solves part of the puzzle
Important to understand e2e implications
Does routing protocols get affected?
Does TCP get affected? …

60. A Rate-Adaptive MAC Protocol for Multi-Hop Wireless Networks
Gavin Holland
HRL Labs
Nitin Vaidya Paramvir Bahl
UIUC Microsoft Research
MOBICOM’01 Rome, Italy
© Gavin Holland

61. Background Current WLAN hardware supports multiple data rates
802.11b – 1 to 11 Mbps
802.11a – 6 to 54 Mbps
Data rate determined by the modulation scheme

62. Problem Modulation schemes have different error characteristics
8 Mbps
1 Mbps
BER
But, SINR itself varies
With Space and Time
SNR (dB)

63. Mean Throughput (Kbps)
Impact
Large-scale variation with distance (Path loss)
8 Mbps
Path Loss
SNR (dB)
Mean Throughput (Kbps)
1 Mbps
Distance (m)
Distance (m)

64. Small-scale variation with time (Fading)
Impact
Small-scale variation with time (Fading)
Rayleigh Fading
SNR (dB)
2.4 GHz
2 m/s LOS
Time (ms)

65. Question Which modulation scheme to choose?
SNR (dB)
SNR (dB)
2.4 GHz
2 m/s LOS
Distance (m)
Time (ms)

66. Answer  Rate Adaptation
Dynamically choose the best modulation scheme for the channel conditions
Desired
Result
Mean Throughput (Kbps)
Distance (m)

67. Design Issues How frequently must rate adaptation occur?
Signal can vary rapidly depending on:
carrier frequency
node speed
interference
etc.
For conventional hardware at pedestrian speeds, rate adaptation is feasible on a per-packet basis
Coherence time of channel higher than transmission time

68. Adaptation  At Which Layer ?
Cellular networks
Adaptation at the physical layer
Impractical for in WLANs
Why?

69. Adaptation  At Which Layer ?
Cellular networks
Adaptation at the physical layer
Impractical for in WLANs
For WLANs, rate adaptation best handled at MAC
Why?
RTS/CTS requires that the rate be known in advance D C B A
CTS: 8
RTS: 10 10 8
Sender
Receiver

70. Who should select the data rate?B

71. Who should select the data rate?
Collision is at the receiver
Channel conditions are only known at the receiver
SS, interference, noise, BER, etc.
The receiver is best positioned to select data rate A B

72. Previous Work PRNet Pursley and Wilkins
Periodic broadcasts of link quality tables
Pursley and Wilkins
RTS/CTS feedback for power adaptation
ACK/NACK feedback for rate adaptation
Lucent WaveLAN “Autorate Fallback” (ARF)
Uses lost ACKs as link quality indicator

73. Lucent WaveLAN “Autorate Fallback” (ARF)
DATA
2 Mbps
Effective Range
1 Mbps
Sender decreases rate after
N consecutive ACKS are lost
Sender increases rate after
Y consecutive ACKS are received or
T secs have elapsed since last attempt

74. Performance of ARF SNR (dB) Time (s) Rate (Mbps) Time (s)
Dropped Packets
Rate (Mbps)
Time (s)
Failed to Increase
Rate After Fade
Attempted to Increase
Rate During Fade
Slow to adapt to channel conditions
Choice of N, Y, T may not be best for all situations

75. RBAR Approach Move the rate adaptation mechanism to the receiver
Better channel quality information = better rate selection
Utilize the RTS/CTS exchange to:
Provide the receiver with a signal to sample (RTS)
Carry feedback (data rate) to the sender (CTS)

76. Receiver-Based Autorate (RBAR) Protocol
CTS (1)
RTS (2)
2 Mbps
1 Mbps D
DATA (1)
RTS carries sender’s estimate of best rate
CTS carries receiver’s selection of the best rate
Nodes that hear RTS/CTS calculate reservation
If rates differ, special subheader in DATA packet updates nodes that overheard RTS

77. Performance of RBAR SNR (dB) Time (s) Rate (Mbps) Time (s) Rate (Mbps)
ARF
Time (s)

78. Question to the class There are two types of fading
Short term fading
Long term fading
Under which fading is RBAR better than ARF ?
Under which fading is RBAR comparable to ARF ?
Think of some case when RBAR may be worse than ARF

79. Rate Selection and Fairness

80. Motivation
Consider the situation below A B C

81. Throughput Fairness vs Temporal Fairness
Motivation
What if A and B are both at 56Mbps, and C is often at 2Mbps?
Slowest node gets the most absolute time on channel? A B C
Timeshare A B C
Throughput Fairness vs Temporal Fairness

82. Rate Selection and Scheduling
Goal
Exploit short-time-scale channel quality variations to increase throughput.
Issue
Maintaining temporal fairness (time share) of each node.
Challenge
Channel info available only upon transmission

83. Approaches RBAR picks best rate for the transmission
Not necessarily best for network throughput
Idea: Wireless networks have diversity
Exploiting this diversity can offer benefits
Transmit more when channel quality great
Else, free the channel quickly

84. Opportunistic Rate Control Idea (OAR)
Basic Idea
If bad channel, transmit minimum number of packets
If good channel, transmit as much as possible D C A B
Data
Data
Data
Data A
Data
Data
Data
Data C

85. Why is OAR any better ? 802.11 alternates between transmitters A and C
Why is that bad D C A B
Data
Data
Data
Data A
Data
Data
Data
Data C
Is this diagram correct ?

86. Why is OAR any better ? Bad channel reduces SINR  higher Tx time
Fewer packets can be delivered D C A B
Data
Data
Data A
Data
Data C

87. These are only first cut ideas … Much advanced research done after these, covered in wireless courses However, important to understand these basics, even for mobile application design