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 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 • 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 • 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 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 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 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
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 #
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 #
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
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 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 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 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 • 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 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 • 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 • 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 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 Detection Error differences in the upper and lower 40MHz channels is indiscernible
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 detection Error differences in the upper and lower 40MHz channels is indiscernible
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 • 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 • 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.