1、Gigabit Wireless LAN: Enhancements in 802.11acEldad Perahia, Ph.D., Intel Corporation, Robert Stacey, Apple, Dec 2012Outline Introduction History Usage models PAR Enhancements Channelization PHY Waveform design Packet structure PHY Transmitter flow Downlink multi-user MIMO Very High Throughput (VHT)
2、 waveform Preamble MAC Coexistence in wider channels Channel access in wider channels Dynamic bandwidth operation Aggregation DL MU-MIMO2Early History Very High Throughput Study Group (VHTSG) Began in May 2007 as a precursor to starting task group, in which purpose and scope of task group were defin
3、ed Started initially to address Very High Throughput for 6 GHz IMT-Advanced operation IMT-Advanced objective was dropped Focus for 6 GHz shifted to enhancing 802.11n in 5 GHz band 6 GHz PAR approved in Sept 20083Wi-Fi Alliance VHT Usage Models 6C at eg o ry # U sag e M o d el1. W ireless D isp lay 1
4、a D es k top Stor age 4 s s ( 5 G H z )1 1 a c ; 8 s s ( 5 G H z )2 0 /2 5 M H z4 0 M H z8 0 M H z1 6 0 M H z11New Features and Enhancements Proposed for IEEE 802.11ac Wider bandwidth 80 MHz channel width 160 MHz channel width Non-contiguous 160 MHz (80 MHz + 80 MHz) Modulation, coding, and spatial
5、streams 256 QAM, rate = 3/4 256 QAM, rate = 5/6 Up to 8 streams Downlink Multi-User MIMO (DL MU-MIMO) Increased aggregate size limits Enhancement to coexistence mechanisms12Mandatory vs. Optional 802.11n PHY FeaturesB asi c M I M O/ SD M20 M H z ; 64 QA Mr ate 5/ 6; 56 to n es1, 2* sp ati al str eam
6、 s 2* , 3, 4 sp ati al str eams40 M H z , 1 14 to n esT r a n s m i t B e a m fo r m i n gC o n vo l u ti o n a l C o d e L o w D e n s i ty Pa r i ty C h e c kM an d at o r y Op t i o n alSp a c e T i m e B l o c k C o d e Gu ar d I n ter valM i xed F o r mat Pr eamb l e Gr een F i el d Pr eamb l e
7、T hr oug hpu tE nhanc e m e ntInt e r ope r abi l i t yw/ L e gac yR obus t ne s sE nhanc e m e nt* 2 spa t i a l st re a m s m a n d a t o ry f o r A P o n l y 13Modifications in 802.11ac to 802.11n Features STBC only for 2x1, 4x2, 6x3, 8x4 No 3x2 or 4x3 as in 11n LDPC Added block-interleaving of c
8、onstellation symbols per stream, per OFDM symbol Transmit Beamforming Only Explicit feedback, no implicit feedback Only Compressed-V feedback, no Uncompressed-V, no CSI Only NDP sounding, no staggered sounding No unequal modulation14Mandatory vs. Optional 802.11ac PHY FeaturesB asi c M I M O/ SD M20
9、, 40, 80 M H z1 sp ati al str ea m 2 - 8 sp ati al str eams160 M H z , 80+ 80 M H zT r a n s m i t B e a m fo r m i n gC o n vo l u ti o n a l C o d e L o w D e n s i ty Pa r i ty C h e c kM an d at o r y Op t i o n alSp a c e T i m e B l o c k C o d e GI , 256 QA MVH T Pr eam b l eThroughputEnhance
10、mentInt e r ope r abi l i t yw/ L e gac yR obus t ne s sE nhanc e m e ntD L M U - M I M O15Channelization for 20/40/80 MHz 40/80 MHz channelization Consists of two adjacent IEEE 20/40 MHz channels Non-overlapping channelization 140136132128124120116112108104100 1651611571531496460565248444036I E E E
11、 c h a n n e l #2 0 MH z4 0 MH z8 0 MH z5170MH z5330MH z5490MH z5710MH z5735MH z5835MH z14416Channelization for Contiguous 160 MHz Apply the same rule as in 40 and 80 MHz channel construction Consists of two adjacent IEEE 80 MHz channels Non-overlapping channelization Not necessary to come up with c
12、oexistence rules for partially overlapping channels 140136132128124120116112108104100 1651611571531496460565248444036I E E E c h a n n e l #2 0 M H z4 0 M H z8 0 M H z5170MH z5330MH z5490MH z5710MH z5735MH z5835MH z1 6 0 MH z14417Noncontiguous 160 MHz (VHT80+80) BSS Any two nonadjacent 80 MHz channe
13、ls may be used in setting up a noncontiguous 160 MHz (VHT80+80) BSS Allows VHT80 STA to associate with the VHT80+80 BSS Allows contiguous-only devices to associate with the VHT80+80 BSS as a VHT80 STA140136132128124120116112108104100 1651611571531496460565248444036I E E E c h a n n e l #2 0 MH z4 0
14、MH z8 0 MH z5170M H z5330MH z5490MH z5710M H z5735M H z5835M H zE x a m p l e s o f V H T 8 0 + 8 0 B S S S e t u p1441880 MHz Sub-Carrier Design 14 Null tones: -128, -123, -1, 0, 1, 123, 127 242 Populated tones: -122, -2, 2, 122 8 Pilot tones: -103, -75, -39, -11, 11, 39, 75, 103 234 Data tones: Po
15、pulated tones Pilot tones-128 127-122-103 -39 -11-2 211 39 75122OFDM sub carrier number103-7519PHY Transmitter Flow Overview:Single User, 20-80 MHz Scrambler same as 11a/n BCC encoder / puncturing same as 11a/n LDPC fully optional Spatial Mapping same as 11nI n t e r l e a v e r ( f o r B C C )I n s
16、 e r t G I a n d W i n d o wA n a l o g a n d R FC S DC S DStreamParserC o n s t e l l a t i o n m a p p e rSTBCI n t e r l e a v e r ( f o r B C C )C o n s t e l l a t i o n m a p p e rI D F TSpatialMappingI n s e r t G I a n d W i n d o wA n a l o g a n d R FI D F TScramblerEncoderParsera p p e n
17、d t a i l ( f o r B C C ) , e n c o d i n g , p u n c t u r i n g ( f o r B C C )a p p e n d t a i l ( f o r B C C ) , e n c o d i n g , p u n c t u r i n g ( f o r B C C )A - M P D UA p p e n d M A C p a d d i n gA p p e n d P H Y P a d d i n g :0 - 7 b i t sP r e p e n d S e r v i c e F i e l d :S
18、 c r a m b l e r s e e d , R e s e r v e d , V H T - S I G - B C R C20160 MHz Sub-Carrier Design28 Null tones: -256, -251,-129,-128, -127, -5, -1,0,1 5, 127, 128,129,251, 255484 Populated tones: -250, -6, 6, 250 16 Pilot tones: +/-231, +/-203, +/-167, +/-139, +/-117, +/-89, +/-53, +/-25 468 Data ton
19、es: Populated tones Pilot tones-256 -250-231 -167 -139-130 -126-117 -89 -53-6OFDM sub carrier number-25-203255625 89 117126 130139 167 2032502315321PHY Transmitter Flow Overview:Single User, 160 MHz contiguous Code across 160 MHz, BCC interleaver per 80 MHz There may be 1 or more FEC encoders when B
20、CC encoding is used When using LDPC, BCC interleavers not used When using BCC, the LDPC tone mappers not usedI n t e r l e a v e rC S DSTBCB C C I n t e r l e a v e rSpatialMappingC o n s t e l l a t i o n m a p p e rC o n s t e l l a t i o n m a p p e rPHYPaddingScramblerFECEncoderI n s e r t G I a
21、 n d W i n d o wA n a l o g a n d R FFECEncoderEncoderParserStreamParserFECEncoderSegmentParserI D F TSegmentParserL D P C t o n e m a p p e rB C C I n t e r l e a v e rC S DSTBCB C C I n t e r l e a v e rSpatialMappingC o n s t e l l a t i o n m a p p e rC o n s t e l l a t i o n m a p p e rI D F T
22、I n s e r t G I a n d W i n d o wA n a l o g a n d R FL D P C t o n e m a p p e rL D P C t o n e m a p p t e r5 1 2 p t I D F T2 3 4s u b c a r r i e r s22PHY Transmitter Flow Overview:Single User, 80+80 MHz non-contiguousI n t e r l e a v e rC S DSTBCB C C I n t e r l e a v e rSpatialMappingC o n s
23、 t e l l a t i o n m a p p e rC o n s t e l l a t i o n m a p p e rPHYPaddingScramblerFECEncoderFECEncoderEncoderParserStreamParserFECEncoderSegmentParserSegmentParserL D P C t o n e m a p p t e rB C C I n t e r l e a v e rC S DSTBCB C C I n t e r l e a v e rSpatialMappingC o n s t e l l a t i o n m
24、 a p p e rC o n s t e l l a t i o n m a p p e rL D P C t o n e m a p p t e rL D P C t o n e m a p p t e rI n s e r t G I a n d W i n d o wA n a l o g a n d R FI D F TI n s e r t G I a n d W i n d o wA n a l o g a n d R FI D F TI n s e r t G I a n d W i n d o wA n a l o g a n d R FI D F TI n s e r t
25、G I a n d W i n d o wA n a l o g a n d R FI D F T2 5 6 p t I D F T2 3 4s u b c a r r i e r sFor 80+80 MHz sub-carrier design, each frequency segment follows the 80 MHz format23PPDU overview (SU) Illustrating 80 MHz bandwidth Parallel L-TFs, L-SIG, VHT-SIG-A, VHT-STF represents 20 MHz waveform replic
26、ated on each sub-channel MAC provides an A-MPDU that fills the frame to the last byte for each user L-SIG length and rate indicate PPDU duration (number of symbols) PHY Padding (0 7 bits) Tail after pad (in 11n, tail before pad)L - T F s L - S I G V H T - S I G AS e r v i c e L a s t S y m b o lV H
27、T A - M P D UP H Y P a dT a i lP P D U D u r a t i o n ( # o f s y m b o l s )M A C P a dL - T F s L - S I G V H T - S I G AL - T F s L - S I G V H T - S I G AL - T F s L - S I G V H T - S I G AV H T -S I G BV H T - S T FV H T - S T FV H T - S T FV H T - S T FV H T - L T F sF r e q24Preamble Overvie
28、w Legacy format the same as 11a/n VHT-SIG-A replaces HT-SIG VHT-STF and VHT-LTF similar to HT-STF and HT-LTF New VHT-SIG-BL - S T F L - L T FL -S I GV H T - S I G - AV H T -S T FV H T -L T FD a t aV H T f o r m a t P P D UV H T -L T F8 s 8 s 8 s4 s 4 sV H T - L T F s4 s p e r L T FV H T -S I G - B4
29、sL - S T F L - L T FL -S I GH T - S I GH T -S T FH T -L T FD a t aH T m i x e d f o r m a t P P D UH T -L T F8 s 8 s 8 s4 s 4 sH T - L T F s4 s p e r L T F25L-SIG Same number of subcarriers (data and pilot) as 11n for 20 MHz and 40 MHz For 80MHz and 160MHz: same number of subcarriers and positions a
30、s 11a/n L-SIG in each 20 MHz subchannel Same rate, length, reserve, parity and tail format As in 11n, 20 MHz waveform replicated in each 20 MHz sub-channel for 40, 80, and 160 MHz Major difference from 11n: Length field in L-SIG used to convey number of symbols in VHT packet No length field in VHT-S
31、IG-A See next slides26L-SIG Length Conveys Number of Symbols (1/2) Similar to 11n, use L-SIG spoof rate of 6 Mbps for 11ac packets 3 bytes / symbol Long GI packet 4 us / symbol Legacy spoof symbols = L-SIG length / 3 bytes per symbol VHT payload symbols = Legacy spoof symbols VHT preamble symbolsVHT
32、 Payloadlegacy spoof symbols = L-SIG length / 3 bytes per symbolLpreambleVHTpreambleL-SIG spoof rate is fixed at 6 Mbps (3 bytes / symbol)20 usecVHT payload symbols = legacy spoof symbols VHT preamble symbols27L-SIG Length Conveys Number of Symbols (2/2) Short GI packet 3.6 us / VHT symbol End of fr
33、ame may not be aligned to a 4 us boundary Legacy devices using L-SIG may find the end of the packet to occur up to 3.6 usec after the energy on the air has disappeared V H T P a yl oa d3 . 6 * V H T s y m b o l sL e g a c y s p o o f t i m e = 4 u s e c p e r s y m b o l * l e g a c y s p o o f s y
34、m b o l sL e g a c y s p o o f s y m b o l s = L - S I G l e n g t h / 3S h o r t G I s y m b o l t i m e = 3 . 6 u s e cL - S I G s y m b o l t i m e = 4 . 0 u s e cR e m a i n d e r = 3 . 6 u s e cLpr e a m bl eVHTpr e a m bl e28L-SIGAmbiguous End of Short GI Packets L-SIG can only indicate time i
35、n units of 4 us Two 3.6 us short GI boundaries may map to the same 4 us normal GI boundary used by L-SIG Addressed with extra short GI bit in VHT-SIG-A LSB set to 1 for short GI MSB set to 1 for short GI AND Nsym%10 = 93 .63 .63 .63 .6 3 .63 .63 .6444S h o r t G I p a c k e t w i t h N s y m b o l s
36、S h o r t G I p a c k e t w i t h N + 1 s y m b o l sL - S I G s p o o f w i t h M s y m b o l s29Length & Duration at Tx Tx MAC computes the number of OFDM symbols and padding, which includes A-MPDU (L) Service MAC Padding (to last byte boundary) PHY Padding (0-7 bits) PHY BCC tail (6 bits / encode
37、r) TXTIME Covers entire PLCP packet Short or long GI L_LENGTH Similar to 11n8 s e r v i c e t a i l E SS Y M S T B CS T B C D B P SL N N NNmmN 8P A D S Y M D B P S s e r v i c e t a i l E SN N N L N N N _T X T I M E f o r SG I L E G P R E A M B L E L S I G V H T S I G A V H T P R E A M B L ES Y M S S Y MV H T S I G B S Y MS Y MT T T TTNTTT 334 20T X T I M EL _ L E N G T H 30
