Ieee802 11ac preamble with legacy 802 11a n backward compatibility
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IEEE802.11ac Preamble with Legacy 802.11a/n Backward Compatibility. Authors:. Date: 2009-11-18. Outline. Introduction Proposed Preamble Format (modified) Preamble Waveform Compatibility Simulations (added) PAPR (modified) Preamble Efficiency Conclusion. I. Introduction.

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IEEE802.11ac Preamble with Legacy 802.11a/n Backward Compatibility

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Ieee802 11ac preamble with legacy 802 11a n backward compatibility

IEEE802.11ac Preamble with Legacy 802.11a/n Backward Compatibility

Authors:

Date: 2009-11-18


Outline

Outline

  • Introduction

  • Proposed Preamble Format (modified)

  • Preamble Waveform

  • Compatibility Simulations (added)

  • PAPR (modified)

  • Preamble Efficiency

  • Conclusion


I introduction

I. Introduction

Functional requirements for TGac.

  • System Performance

    • Shall operate in 5GHz frequency band

    • At least 1Gbps at MAC SAPs utilizing 80MHz

    • At least 500Mbps for Single STA

    • Spectral efficiency at least 7.5 bps/Hz

  • Shall ensure backward compatibility with IEEE 802.11a/n.

  • Enable coexistence and spectrum sharing with IEEE 802.11a/n.

  • Compliance to PAR

Doc.IEEE802.11-09/0304r0 March 2009


Ii proposed preamble

II. Proposed Preamble

  • Main Features

    • Uses 80MHz channel and meets all functional requirements for a VHT preamble

    • Three preamble modes to minimize overhead

    • Seamless upgrade from 802.11n preamble

    • Almost identical PAPR characteristics with 802.11n

    • Comparable Preamble Efficiency to 802.11n


Proposed preamble modes

Proposed Preamble Modes

  • Mixed 11a/n Mode

    • 56us duration

    • Backward compatible with 802.11a/n

  • Mixed 11n Mode

    • 44us duration

    • Backward compatible with 802.11n

  • VHT Greenfield Mode

    • 36us duration

    • VHT only mode


A mixed 11a n mode frame format

A) Mixed 11a/n Mode Frame Format

Mixed 11a/n Mode: Backward compatible with IEEE802.11a/n

8μs

8μs

4μs

8μs

8μs

4μs

4μs per LTF

4/3.6μs per Data

L-STF

L-LTF

L-SIG

HT-SIG

VHT-SIG

VHT STF

VHT LTF1

VHT

LTF4

Data

Data

Duration=56μs *

A new SIG field is defined to accommodate changes in VHT while maintaining compatibility

802.11n Mixed Mode Preamble

8μs

8μs

4μs

8μs

4μs

4μs per LTF

4/3.6μs per Data

L-STF

L-LTF

L-SIG

HT-SIG

HT STF

HT LTF1

HT

LTF4

Data

Data

Duration=48μs *

* 4 spatial streams


Subcarrier extension for 80 mhz channel

Subcarrier Extension for 80 MHz Channel

S = subcarrier pattern for 20MHz system

S

jS

-64

0

63

802.11n 40MHz case

S

jS

ejθ(S)

ejθ(jS)

-128

-64

0

64

127

Direct Extension 80MHz

-Direct Extension of subcarriers results in best compatibility with 11a/n devices

-The phase shift vector [1 j ejθ jejθ] results in a high PAPR preamble for any value of θ


Proposed phase rotation for 80 mhz channel to maintain low papr

Proposed Phase Rotation for 80 MHz Channel to Maintain Low PAPR

S

jS

ejθ(S)

ejθ(jS)

-128

-64

0

64

127

Direct Extension 80MHz

S

jS

S

-jS

-128

-64

0

64

127

Proposed scheme for 80MHz

-The phases in the proposed scheme [1 j 1 –j] were chosen to give low PAPR properties

-All 80MHz symbols need to be phase shifted by the same scheme either for compatibility or for PAPR reduction


Ieee802 11ac preamble with legacy 802 11a

L-STF

L-LTF

L-SIG

HT-SIG

VHT-SIG

VHT STF

VHT LTF1

VHT

LTF4

Data

Data

Legacy – Short Training Field

L-STF 20MHz

-128

127

-64

0

64

Subcarriers #


Legacy long training field

L-STF

L-LTF

L-SIG

HT-SIG

VHT-SIG

VHT STF

VHT LTF1

VHT

LTF4

Data

Data

Legacy - Long Training Field

L-LTF 20MHz

-128

127

-64

0

64

Subcarriers #


Legacy signal field

L-STF

L-LTF

L-SIG

HT-SIG

VHT-SIG

VHT STF

VHT LTF1

VHT

LTF4

Data

Data

Legacy - SIGNAL Field

L-SIG 20MHz

-128

127

-64

0

64

Subcarriers #

GI

L-SIG

0.8

m

m

3.2

m

m

s

s

s

s


Vht signal field

L-STF

L-LTF

L-SIG

HT-SIG

VHT-SIG

VHT STF

VHT LTF1

VHT

LTF4

Data

Data

VHT – SIGNAL Field

BW

BW

MSB

LSB

STBC

LENGTH should be increased by one bit to maintain preamble efficiency lost with the use of 80MHz channel

Reserved one

STBC

Short GI

  • 64 options of MCS.

  • 131072 octet of data LENGTH = 2 X 802.11n.

  • CRC protected.


B mixed 11n mode frame format

B) Mixed 11n Mode Frame Format

Mixed 11n Mode: Backward compatibility with IEEE802.11n

8μs

8μs

8μs

8μs

4μs per LTF

4/3.6μs per Data

HT-STF

HT-LTF

HT-SIG

VHT-SIG

VHT LTF2

VHT

LTF4

Data

Data

The difference with 802.11n greenfield preamble is the additional SIG symbol

Duration=44μs *

802.11n Greenfield Preamble

8μs

8μs

8μs

4μs per LTF

4/3.6μs per Data

HT-STF

HT-LTF1

HT-SIG

HT LTF2

HT

LTF4

Data

Data

Duration=36μs *

* 4 spatial streams


C vht greenfield mode frame format

C) VHT Greenfield Mode Frame Format

VHT Greenfield Mode: No backward compatibility with IEEE802.11a/n

8μs

8μs

8μs

4μs per LTF

4/3.6μs per Data

VHT-STF

VHT-LTF1

VHT-SIG

VHT LTF2

VHT

LTF4

Data

Data

Duration=36μs*

-Same purpose as the 802.11n greenfield format preamble


Iii preamble waveform

III. Preamble Waveform

VHT-

LTF2

VHT-

LTF4

L-

SIG

HT-

SIG1

HT-

SIG2

VHT-

SIG1

VHT-

SIG2

VHT-

STF

VHT-

LTF1

VHT-

LTF3

L-STF

L-LTF

Time ( )

m

s


Iv co existence simulations

IV. Co-existence Simulations

  • Co-existence with 11a/n device at any 20MHz channel

jS

-jS

jS

-jS

S

S

S

S

80MHz

20MHz

20 MHz device

80 MHz device

11a/n devices at any 20MHz channel can properly detect the preamble

Proposed 80MHz format

a=legacy 802.11a/HT STS, LTS, etc…


Iv co existence simulations1

IV. Co-existence Simulations

  • Co-existence with 11n device at 40MHz channel

jS

-jS

S

S

jS

-jS

S

S

80MHz

40MHz

40MHz

40 MHz device

40 MHz device

80 MHz device

11n devices at the UPPER 40 MHz channel expects [1 j] subcarrier rotation and has a possible compatibility issue.

Proposed 80MHz format

a=legacy 802.11a/HT STS, LTS, etc…


Simulation setup

Simulation Setup

  • All receivers (40MHz HT) and transmitters (80MHz VHT) have 4 antennas

  • Channel Model

    • AWGN

    • TGn channel model ‘B’

    • TGn channel model ‘D’

  • Simulation tests

    • Frame Synchronization/Preamble Detection

      • Auto-correlation

      • Cross-correlation

    • Channel Estimation and SIG field Detection


Frame synchronization preamble detection

Frame Synchronization/Preamble Detection

  • SISO Case cross-correlation

  • Autocorrelation Based Frame Synchronization algorithm is not affected because it doesn’t use an expected reference sequence.

Cross-correlation of 40Mhz 11n STS

Cross-correlation of 40Mhz 11n STS when a [1 –j] phase rotated STS symbol is received


Frame synchronization preamble detection1

Frame Synchronization/Preamble Detection

-Error differences in the upper and lower 40Mhz channels

is indiscernible

-Performance only differs whether one uses auto-correlation or cross-correlation algorithm.


Frame synchronization preamble detection2

Frame Synchronization/Preamble Detection

Error differences in the upper and lower 40MHz channels is indiscernible


Channel estimation and sig field detection

Channel Estimation and SIG field detection

  • Because Data symbols also undergo the same subcarrier phase shifts, SIG detection is unaffected by our proposed phase shift method

  • Synchronization errors were also counted as SIG-field symbol error

Error differences in the upper and lower 40MHz channels is indiscernible


Channel estimation and sig field detection1

Channel Estimation and SIG field detection

Error differences in the upper and lower 40MHz channels is indiscernible


V papr

V. PAPR


Vi preamble efficiency

VI. Preamble Efficiency

The preamble efficiency is defined as

95.29

48

96.43

36

131072

94.26

230

56

131072

44

95.43

230

131072

230

96.23

36


Conclusion

Conclusion

  • We have been developing an 80MHz BW WLAN preamble which has backward compatibility with IEEE802.11a/n system.

  • Our simulation results confirm our proposed preamble’s backward compatibility with 802.11n 40MHz devices

  • The proposed preamble has comparable efficiency compared to IEEE802.11n’s preamble.

  • The proposed preamble has lower PAPR compared with IEEE802.11n’s preamble.


References

References

  • E. Perahia, ”IEEE P802.11 Wireless LANs: VHT below 6GHz PAR Plus 5C’s”, doc.:IEEE 802.11-08/0807r4.

  • ”Supplement to IEEE STANDARD for Information Technology – Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks - Specific Requirements”, IEEE Std 802.11a- 1999(R2003), June 2003.

  • ”Draft STANDARD for Information Technology – Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks - Specific Requirements”, IEEE P802.11n./D9.0, March 2009.

  • Peter L and Minho C, ”TGac Functional Requirements Rev.0”, doc.:IEEE 802.11-09/0304r0.

  • Rolf de V, ”802.11ac Usage Models Document”, doc.:IEEE802.11- 09/0161r2.

  • Minho C and Peter L, ”Proposal for TGac Evaluation Methodology”, doc.:IEEE802.11-09/0376r1.


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