VHT Frame Padding

VHT Frame Padding Authors: Date: 2010-05-19 Yong Liu, Marvell

Changes from r4 • Explain why PHY padding bits are added before tail bits • Add a straw poll on adding PHY padding bits before tail bits for VHT frames Yong Liu, Marvell

Outlines • 802.11n (HT) frame padding • VHT frame padding • 802.11n (HT) A-MPDU • VHT A-MPDU • Early End-of-Frame indication • Conclusions Yong Liu, Marvell

802.11n (HT) frame padding • L-SIG length/rate indicate the end of the PPDU • A length field (16 bits) is included in HT-SIGs to indicate the end of PSDU • Tail bits and PHY Pad are appended to fill in the last symbol Yong Liu, Marvell

VHT frame padding - motivations • Bits in VHT-SIG are very precious resource to signal important system parameters • Length field and rate field in L-SIG already provide sufficient information to signal the duration of a VHT frame (11-10-0534-00-00ac-Duration-in-L-SIG) • No need to signal PPDU length or duration again in VHT signal fields • Simplify design and save processing power Yong Liu, Marvell

VHT frame padding • L-SIG length and rate indicate PPDU duration (number of symbols) • No per user length (number of octets) indication in preamble • MAC provides an A-MPDU that fills the frame to the last byte for each per-user stream • Same preamble structure is used for both SU and MU VHT frames • Require that A-MPDU always be used with both SU and MU VHT frames • PHY provides 0-7 bits of padding • PHY padding bits are added before tail bits Yong Liu, Marvell

802.11n (HT) A-MPDU • Except when it is the last A-MPDU subframe in an A-MPDU, padding octets are appended to make each A-MPDU subframe a multiple of 4 octets in length. • Any subframe may be preceded by one or more zero-length delimiters to meet the minimum MPDU Start Spacing requirement, for example Yong Liu, Marvell

VHT A-MPDU • VHT A-MPDU subframes have the same structure as the subframes of an 802.11n A-MPDU • The QWord MAC Pad (0-3 octets) is added to reach the next 32-bit boundary of MAC octets, if such octets fit into the last PHY symbol (Note that normally, the LAST subframe is NOT padded) • Zero or more Null subframes and a Final MAC Pad (0-3 octets) are appended to extend the A-MPDU to the last octet of the last symbol of a VHT frame • Null subframes are absent if last PHY symbol is full with 0-3 octets of Final MAC Pad after the QWord MAC Pad Yong Liu, Marvell

Early EOF indication • The Null subframes appended to the end of a VHT A-MPDU can be special Padding Delimiters each with an EOF flag • When RX MAC detects the EOF Padding Delimiter, it may inform RX PHY to stop receiving to conserve power Yong Liu, Marvell

Add Tail, Encoding & Puncturing Encoder Parser Stream Parser AMPDU (with MAC pad) Add 0-7 PHY padding bits Scrambler PHY pad before tail bits • With multiple encoders there is no need to separately calculate the number of PHY padding bits to be added at each encoder, i.e., number of padding bits can be calculated once and added before the encoder parser. Yong Liu, Marvell

Conclusions • L-SIG length and rate indicate PPDU duration (number of symbols) • No per user length (number of octets) indication in preamble • Zero or more Null subframes and a Final MAC Pad can be appended to a VHT A-MPDU to extend the A-MPDU to the last byte of the VHT frame • In order to maintain the same preamble structure for both SU and MU frames, all VHT frames use A-MPDU structure • The Null subframes appended to the A-MPDU can include an EOF indication in the delimiter so that the receiver knows it can stop receive processing to conserve power • PHY padding bits are added before tail bits Yong Liu, Marvell

Reference • 11-10-0534-00-00ac-Duration-in-L-SIG.ppt • 11-10-0070-02-00ac-802-11ac-preamble.ppt • 11-10-0548-00-00ac-80MHz Transmission Flow.ppt Yong Liu, Marvell

Straw Poll 1 • Should the spec framework document (09/992) be updated to include the following change? • 3.2.x The Data Field • R3.2.x.A: When BCC encoding is used, the Data field shall consist of the 16-bit SERVICE field, the PSDU, the pad bits and the tail bits, in that order. • Yes: • No: • Abstain: Yong Liu, Marvell

Straw Poll 2 • Should the spec framework document (09/992) be updated to include the changes shown on the following two slides of this presentation? • Yes: • No: • Abstain: Yong Liu, Marvell

Spec framework changes (1) Add a section on A-MPDU format under Frame Formats: 6.3 Frame formats 6.3.1 A-MPDU format The VHT A-MPDU format is an extension of the 802.11n A-MPDU as shown in Figure YY. The extension (shaded in the figure) consists of zero or more delimiters with MPDU length zero and a possible final MAC Pad of less than 4 octets. The A-MPDU of a VHT PPDU fills the available octets in the payload. Figure YY – A-MPDU format for VHT PPHU Yong Liu, Marvell

Spec framework changes (2) Add a requirement to the power saving section: 6.1 Power saving R6.1.A: The delimiters with MPDU length zero that pad through the end of the frame shall include an indication that no additional MPDUs are present in the frame. This would allow the receiver to stop receive processing to conserve power. Yong Liu, Marvell

VHT Frame Padding