1 Directional Antennas for Wireless Networks Romit Roy Choudhury

2 Applications Several Challenges, Protocols Internet

3 Omnidirectional Antennas

4 CTS = Clear To Send RTS = Request To Send IEEE with Omni Antenna D Y S M K RTS CTS X

5 IEEE with Omni Antenna D Y S X M K silenced Data ACK

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

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 … D S X M K silenced Y D E A C F B G

9 To Beamforming Antennas D S X M K Y D E A C F B G

10 To Beamforming Antennas D S X M K Y D E A C F B G

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 A Pictorial Model A Main Lobe (High gain) Sidelobes (low gain)

17 Directional Reception Directional reception = Spatial filtering  Interference along straight line joining interferer and receiver AB C D Signal Interference No Collision at A AB C D Signal Interference 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 Remain omni while idle  Nodes cannot predict who will trasmit to it DMAC Example D Y S X

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

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

23 Intuitively Performance benefits appear obvious

24 However … Sending Rate (Kbps) Throughput (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 Self Interference with Directional MAC New Challenges [Mobicom 02]

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 A BC Data D route

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

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 B CA Data RTS CTS

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 B CA Data Carrier Sense RTSCTS

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

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

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

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

36 Deafness with Directional MAC New Challenges III …

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 S D N Data RTS M

38 Channel Underutilized Collision: N must attempt less often Deafness: N should attempt more often  Misclassification incurs penalty (similar to TCP) S D N Data RTS M 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” DATA RTS X D S Z

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

41 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 Capture Wastage because the receiver could accomplish useful communication instead of receiving the unproductive packet

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

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

45 Recall RTS/CTS + Large CS Zone  Alleviates hidden terminals, but trades off spatial reuse C F AB E D CTS RTS

46 Recall Role of TDMA

47 Recall Beamforming Omni Communication Directional Communication ab c d Silenced Node ab c d e f Simultaneous Communication No  Simultaneous Communication Ok ff

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

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

52 Some Basics Friss’ Equation Shannon’s Equation Bit-energy-to-noise ratio C = B * log 2 (1 + SINR) E b / N 0 = SINR * (B/R) Leads to BER Varying with time and space How do we choose the rate of modulation

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 CE A D B Rate = 10

56 Rate and Range Rate = 20 There is no free lunch  talking slow to go far CE A D B Rate = 10

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

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

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 © Gavin Holland 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

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

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

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

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

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

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

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

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

70 Who should select the data rate? AA B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A BB

72 Previous Work PRNet  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) 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 BB AA  DAT A  2 Mbps  Effective Range  1 Mbps  Effective Range

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

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 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 Receiver-Based Autorate (RBAR) Protocol CC BB AA  CTS (1)  RTS (2)  2 Mbps  1 Mbps DD  DATA (1)  2 Mbps  1 Mbps

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

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 BC

81 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 BC A B C Timeshare 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 A B CD A C Data

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

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