1. AccuRate: Constellation Aware Rate Estimation in Wireless Networks
Souvik Sen,
Naveen Santhapuri, Romit Roy Choudhury, Srihari Nelakuditi

2. Bit-rate in Wireless Networks
Wireless link throughput depends on transmission bit-rate
6 Mbps
24 Mbps
Choosing the optimal bit-rate is an important problem

3. Bit-rate in Wireless Networks
Optimal bit-rate selection is challenging because the wireless channel varies over:
Space
Time
6 Mbps
Channel
Time
24 Mbps
Channel
Time

4. Current Wireless Rate Selection
Frame Based
Remember History
SampleRate, RRAA
SNR Based
Data
RBAR, CHARM
Data
SNR
ACK
✦ Recently PHY-based: ✦
SoftRate [SIGCOMM ʼ09] •
Uses a BER heuristic to estimate bit rate
BER accurately identifies when to increase/decrease rate However, may not be able to jump to optimal rate

5. Ideally Tx wants optimal rate for next packet

6. Function of optimal rate of the previous packet
Ideally Tx wants optimal rate for next packet
Function of optimal rate of the previous packet

7. Function of optimal rate of the previous packet
Ideally Tx wants optimal rate for next packet
Function of optimal rate of the previous packet

8. Ideally Tx wants optimal rate for next packet
Function of optimal rate of the previous packet
In other words,
Given a transmission at rate R, what would have been the max rate R*,
at which that transmission would have been successful

9. Ideally Tx wants optimal rate for next packet
Function of optimal rate of the previous packet
In other words,
Given a transmission at rate R, what would have been the max rate R*,
at which that transmission would have been successful
We propose AccuRate

10. Background: Symbols, Modulation, Bit-rate

11. Physical Layer Symbols=
Data 11 01 00 10
Tx 4QAM Symbol

12. Physical Layer Symbols=
Data
2 bits together 11 01 00 10
Tx 4QAM Symbol

13. Physical Layer Symbols=
Data
2 bits together 11 01 00 10
Tx 4QAM Symbol

14. Symbols to Modulation =
Data 11 01 00 10
Tx 4QAM Symbol

15. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol 2 bits together
Data
2 bits together 11 01 00 10
Tx 4QAM Symbol

16. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol Rx 4QAM Symbol
Data
2 bits together 11 01 11 01 11 11
Channel 01 01 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol

17. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol Rx 4QAM Symbol
Data
2 bits together 11 01 11 01
Dispe
rsion 11 11
Channel 01 01 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol

18. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol Rx 4QAM Symbol
Data
2 bits together 11 01 11 01
Dispe
rsion 11 11
Channel 01 01 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol

19. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol Rx 4QAM Symbol
Data
2 bits together 11 01 11 01
Dispe
rsion
Channel 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol

20. Symbols to Modulation = 01111001 .... Tx 4QAM Symbol Rx 4QAM Symbol
Disper
sion =
Data
2 bits together
Channel 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol

21. Different Modulations in 802.1101 11 01
Dispe
rsion =
Data
2 bits together
Channel 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol
011
Dispersion 1 =
Data
Channel
4 bits together
Tx 16QAM Symbol
Rx 16QAM Symbol

22. Different Modulations in 802.1101 11 01
Dispe
rsion =
Data
2 bits together
Channel 00 10 00 10
Tx 4QAM Symbol
Rx 4QAM Symbol
01111
Dispersion =
Data
Channel
6 bits together
Tx 64QAM Symbol
Rx 64QAM Symbol

23. Why not always transmit many bits per symbol?
e.g., 64QAM or 54Mbps

24. Weak Channel Induces Errors=
Data
Tx 16QAM Symbol

25. Weak Channel Induces Errors=
Data
Weak Channel
Tx 16QAM Symbol

26. Weak Channel Induces Errors
High Dispersion =
Data
Weak Channel
Tx 16QAM Symbol

27. Weak Channel Induces Errors
High Dispersion =
Data
Weak Channel
Tx 16QAM Symbol
Wrongly demodulated symbol

28. In General ...
Weak Channel
Moderate Channel 01
0111
Strong Channel

29. In General ... 01 0111 Weak Channel Moderate Channel Strong Channel
6 Mbps
Moderate Channel 01
24 Mbps
0111
Strong Channel
36 Mbps

30. In General ... Smaller dispersion permits higher rate 01 0111
Weak Channel
6 Mbps
Moderate Channel 01
24 Mbps
0111
Strong Channel
36 Mbps
Smaller dispersion permits higher rate

31. AccuRate Design and Implementation

32. AccuRate Design and Implementation
Hypothesis:
Symbol dispersion is independent of modulation

33. Dispersion Independent of Modulation?11 01 11 01 00 10 00 10
Channel
Tx 4QAM
Rx QPSK
Tx 16QAM
Rx16QAM

34. Dispersion Independent of Modulation?
Testbed 1
0.8
0.6
Fraction of symbols
0.4
BPSK QPSK 16QAM
64QAM
0.2
Symbol dispersion magnitude

35. Dispersion Independent of Modulation?
Testbed 1
0.8
0.6
Fraction of symbols
0.4
BPSK QPSK 16QAM
64QAM
0.2
Symbol dispersion magnitude
McKinley et. al., 2004,“EVM calculation for broadband modulated signals”

36. Symbol dispersion is independent of modulation
Hypothesis:
Symbol dispersion is independent of modulation
Selection of optimal modulation

37. Data

38. BPSK
Data
4QAM
16QAM

39. BPSK
Data
4QAM
16QAM

40. BPSK
Data
4QAM
16QAM

41. BPSK
Data
4QAM
16QAM

42. BPSK
Data
4QAM
16QAM

43. BPSK
Data
4QAM
16QAM

44. BPSK
Data
4QAM
16QAM

45. We call it Virtual Channel Replay
BPSK
Data
4QAM
16QAM
We call it Virtual Channel Replay

46. Channel Replay Vector V = {d1, d2, ...., dn}
✦ AccuRate records dispersion for every symbol in a packet ✦
Creates a vector: Channel Replay Vector (V) d1
V = {d1, d2, ...., dn}

47. ✦ When packet succeeds ✦ When packet fails Channel Replay Vector
✦ AccuRate records dispersion for every symbol in a packet ✦
Creates a vector: Channel Replay Vector (V) d1
V = {d1, d2, ...., dn}
✦ When packet succeeds ✦
All dispersions are known
✦ When packet fails ✦
Approximates V from (known) preamble/postamble

48. Receiver
Channel
Demodulator
Packet

49. Channel Replay CRC Check BPSK Demodulator Receiver Demodulator Packet

50. Channel Replay CRC Check BPSK Demodulator Receiver Demodulator Packet

51. Channel Replay CRC Check BPSK Demodulator Channel Replay CRC Check
Receiver
Channel
Demodulator
Channel Replay
CRC
Check
Packet
BPSK
Demodulator
Channel Replay
CRC
Check
4QAM
Demodulator

52. Channel Replay CRC Check BPSK Demodulator Channel Replay CRC Check
Receiver
Channel
Demodulator
Channel Replay
CRC
Check
Packet
BPSK
Demodulator
Channel Replay
CRC
Check
4QAM
Demodulator

53. Channel Replay CRC Check BPSK Demodulator Channel Replay CRC Check
Receiver
Channel
Demodulator
Channel Replay
CRC
Check
Packet
BPSK
Demodulator
Channel Replay
CRC
Check
4QAM
Demodulator
Channel Replay
CRC
Check
16QAM
Demodulator

54. Channel Replay CRC Check BPSK Demodulator Channel Replay CRC Check
Receiver
Channel
Demodulator
Channel Replay
CRC
Check
Packet
BPSK
Demodulator
Channel Replay
CRC
Check
4QAM
Demodulator
Channel Replay
CRC
Check
16QAM
Demodulator

55. Best Rate Channel Replay CRC Check BPSK Demodulator Channel Replay CRC
Receiver
Channel
Demodulator
Channel Replay
CRC
Check
Packet
BPSK
Demodulator
Best Rate
Channel Replay
CRC
Check
4QAM
Demodulator
Channel Replay
CRC
Check
16QAM
Demodulator

56. Bit-rate is a function of both modulation and coding

57. <modulation, coding> for a received packet?
Can we find the optimal
<modulation, coding> for a received packet?

58. CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2 Demodulator
Receiver
Channel
Demodulator
Decoder
CRC
Check
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
CRC
Check
Channel Replay
BPSK
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM4
1/2
Demodulator
Decoder
CRC
Check
Channel Replay
QAM4
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM64
3/4
Demodulator
Decoder

59. CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2 Demodulator
Receiver
Channel
Demodulator
Decoder
CRC
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
6 Mbps
Check
CRC
Check
Channel Replay
BPSK
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM4
1/2
Demodulator
Decoder
CRC
Check
Channel Replay
QAM4
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM64
3/4
Demodulator
Decoder

60. CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2 Demodulator
Receiver
Channel
Demodulator
Decoder
CRC
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
6 Mbps
Check
CRC
Channel Replay
BPSK
3/4
Demodulator
Decoder
9 Mbps
Check
CRC
Check
Channel Replay
QAM4
1/2
Demodulator
Decoder
CRC
Check
Channel Replay
QAM4
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM64
3/4
Demodulator
Decoder

61. CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2 Demodulator
Receiver
Channel
Demodulator
Decoder
CRC
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
6 Mbps
Check
CRC
Channel Replay
BPSK
3/4
Demodulator
Decoder
9 Mbps
Check
CRC
Channel Replay
QAM4
1/2
12 Mbps
Demodulator
Decoder
Check
CRC
Check
Channel Replay
QAM4
3/4
Demodulator
Decoder
CRC
Check
Channel Replay
QAM64
3/4
Demodulator
Decoder

62. CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2 Demodulator
Receiver
Channel
Demodulator
Decoder
CRC
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
6 Mbps
Check
CRC
Channel Replay
BPSK
3/4
Demodulator
Decoder
9 Mbps
Check
CRC
Channel Replay
QAM4
1/2
12 Mbps
Demodulator
Decoder
Check
CRC
18 Mbps
Channel Replay
QAM4
3/4
Demodulator
Decoder
Check
CRC
54 Mbps
Channel Replay
QAM64
3/4
Demodulator
Decoder
Check

63. Best Rate CRC Check CRC Check CRC Check CRC Check CRC Check BPSK 1/2
Receiver
Channel
Demodulator
Decoder
CRC
Data
Channel Replay
BPSK
1/2
Demodulator
Decoder
6 Mbps
Check
CRC
Channel Replay
BPSK 3/4
Demodulator
Decoder
9 Mbps
Check
Best Rate
CRC
Channel Replay
QAM4
1/2
12 Mbps
Demodulator
Decoder
Check
CRC
18 Mbps
Channel Replay
QAM4
3/4
Demodulator
Decoder
Check
CRC
54 Mbps
Channel Replay
QAM64
3/4
Demodulator
Decoder
Check

64. Performance Evaluation
✦ Used like Tx and Rx design on USRP/GnuRadio ✦
Modulation: BPSK, QPSK, 16QAM, 64QAM
Coding: Convolution coding with puncturing with rate 1/2, 3/4 Compare with Softrate, SNR-based
✦ Testbed ✦
10 traces at walking speed Trace based evaluation
Channel
Simulator
✦ Simulation ✦
Characterize AccuRateʼs performance under high mobility Raleigh fading channel simulator ported to GnuRadio

65. What is the True Optimal Rate?
✦ Testbed ✦
Using train of packets (Virtual Packet)
Each Virtual Packet consists of data packets at all bit-rates Similar method as Softrate
Virtual Packet
6Mbps
9Mbps
12Mbps
18Mbps
24Mbps
36Mbps
54Mbps

66. What is the True Optimal Rate?
✦ Testbed ✦
Using train of packets (Virtual Packet)
Each Virtual Packet consists of data packets at all bit-rates Similar method as Softrate
Virtual Packet
6Mbps
9Mbps
12Mbps
18Mbps
24Mbps
36Mbps
54Mbps

67. What is the True Optimal Rate?
✦ Testbed ✦
Using train of packets (Virtual Packet)
Each Virtual Packet consists of data packets at all bit-rates Similar method as Softrate
Virtual Packet
6Mbps
9Mbps
12Mbps
18Mbps
24Mbps
36Mbps
54Mbps
Optimal
Optimal-1
Optimal+1

68. Can we estimate the optimal rate?
Simulation Testbed
1 1
Simulation
Testbed
Fraction of Packets
0.8
Fraction of Packets
0.8
0.6
0.6
0.4
0.4
AccuRate correct-packets AccuRate using preamble AccuRate pre+postamble
AccuRate correct-packets AccuRate using preamble AccuRate pre+postamble
0.2
0.2 -1 -1
Optimal 1
Optimal 1
(AccuRate Rate) minus (Optimal Rate)
(AccuRate Rate) minus (Optimum Rate)
For correctly received packets,
100% in Simulation,
95% in Testbed

69. AccuRate needs to detect Interference

70. Rate selection needs to be independent of interference
AccuRate needs to detect Interference b
Rate selection needs to be independent of interference

71. How to Detect Interference?
✦ Interference causes substantial symbol dispersion
Interference

72. How to Detect Interference?
✦ Interference starts first: Preamble with high dispersion
Data
Interference
✦ Interference starts second: Postamble with high dispersion
Data
Interference

73. How to Detect Interference?
✦ Interference starts first: Preamble with high dispersion
Data
Interference
✦ Interference starts second: Postamble with high dispersion
Data
Interference
Compare preamble with postamble dispersion

74. Interference Detection Accuracy
Testbed
Testbed
1.2
AccuRate Softrate
Fraction of Lost Packets 1
0.8
0.6
0.4
0.2
BPSK 1/2
BPSK 3/4
QPSK 1/2
QPSK 3/4
QAM16 1/2 QAM16 3/4
Bit Rate

75. Interference Detection Accuracy
Testbed
Testbed
1.2
AccuRate Softrate
Fraction of Lost Packets 1
0.8
0.6
0.4
0.2
BPSK 1/2
BPSK 3/4
QPSK 1/2
QPSK 3/4
QAM16 1/2 QAM16 3/4
Bit Rate
Detection Accuracy is better at higher rates (95%)

76. Estimation Performance with Interference
Testbed
Testbed-AccuRate
1.2
Overselect Accurate Underselect
Over select 1
Fraction of Lost Packets
0.8
0.6
Correct
0.4
0.2
Under select
BPSK 1/2
BPSK 3/4
QPSK 1/2 QPSK 3/4
Bit Rate
QAM16 1/2
QAM16 3/4

77. Estimation Performance with Interference
Testbed
Testbed-AccuRate
1.2
Overselect Accurate Underselect
Over select 1
Fraction of Lost Packets
0.8
0.6
Correct
0.4
0.2
Under select
BPSK 1/2
BPSK 3/4
QPSK 1/2 QPSK 3/4
Bit Rate
QAM16 1/2
QAM16 3/4
91% accuracy in Optimal rate selection

78. AccuRate estimates the optimal rate for an already received packet
What is the performance if the next transmission uses this rate?

79. Throughput at Walking Speeds
Testbed
AccuRate Softrate
Testbed
Normalized Throughput
SNR based 1
0.8
0.6
0.4
0.2
Walking Trace Number

80. Throughput at Walking Speeds
Testbed
AccuRate Softrate
Testbed
Normalized Throughput
SNR based 1
0.8
0.6
0.4
0.2
AccuRate achieves 87% of the optimal throughput
Walking Trace Number

81. Throughput under Mobility
Simulation 18 16 14 12 10 8 6 4 2
AccuRate Softrate
Throughput (Mbps)
SNR based
1ms
.5ms .2ms
Channel Coherence Time
.1ms
AccuRate performs well even under high mobility

82. ✦ Rate estimation sub-optimal under packet failure
Limitations
✦ Hardware Complexity ✦
AccuRate targets optimal rate estimation Does not consider implementation cost
✦ Rate estimation sub-optimal under packet failure ✦
Pre/Post amble based estimation achieves 93% accuracy Improvements possible with midamble
✦ Interfering packet may engulf or be engulfed by data ✦
AccuRate unable to detect such cases

83. ✦ AccuRate uses symbol dispersion to estimate bit-rate
Summary
✦ AccuRate uses symbol dispersion to estimate bit-rate ✦
Symbol dispersion is a measure of channel behavior
✦ AccuRate replays this channel on different bit-rates ✦
The max rate that “passes” this replay is declared optimal
✦ The optimal rate is prescribed for subsequent transmissions ✦
USRP testbed results show 87% of optimal throughput
✦ SoftRate capable of choosing very good bit-rates ✦
AccuRate pushes rate estimation towards optimality