VHT Packet Duration Signaling Month Year doc.: IEEE 802.11-yy/xxxxr0 July 2010 VHT Packet Duration Signaling Date: 2010-07-12 Authors: Youhan Kim, et al. John Doe, Some Company

July 2010 Introduction Bits in VHT-SIG A are very precious resource to signal important system parameters Length field in L-SIG already contains sufficient information to convey the duration of a VHT packet [1] More details are provided on how to signal the VHT packet duration using the L-SIG length field Youhan Kim, et al.

July 2010 11ac Packet Length A 4 ms long VHT packet using 80 MHz, 4 streams, 256-QAM 5/6, short GI can transport ~800 Kbytes Requires 20 bits if length in bytes was indicated in preamble as in 11n Number of bits required for length in bytes for VHT packets is actually higher since 11ac supports 160 MHz, max. 8 streams and 256-QAM 5/6 [2] 22 bits required to support 4 ms packet duration 2 symbol VHT-SIG A has 34 useful bits 48 bits – 8 CRC bits – 6 tail bits 22 bit length field would use 65% of the valuable VHT-SIG A resource And this is not even considering DL MU-MIMO Clearly, signaling the length of the packet in bytes in VHT-SIG A is not an efficient scheme Youhan Kim, et al.

11ac Packet Duration: Number of Symbols July 2010 11ac Packet Duration: Number of Symbols Alternative is to signal the number of OFDM symbols of the packet But L-SIG already has sufficient information for RX to determine the number of OFDM symbols See following slides Do not need to indicate VHT packet duration again in VHT-SIG A Very efficient usage of the valuable VHT-SIG A resources Youhan Kim, et al.

L-SIG Length Conveys Number of Symbols (1) July 2010 L-SIG Length Conveys Number of Symbols (1) 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 symbols legacy spoof symbols = L-SIG length / 3 bytes per symbol 20 usec L preamble VHT preamble VHT Payload VHT payload symbols = legacy spoof symbols – VHT preamble symbols L-SIG spoof rate is fixed at 6 Mbps (3 bytes / symbol) Youhan Kim, et al.

L-SIG Length Conveys Number of Symbols (2) July 2010 L-SIG Length Conveys Number of Symbols (2) Short GI packet 3.6 us / VHT symbol End of frame 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 But this is existing problem in 11n Short GI symbol time= 3.6 usec 3.6 * VHT symbols L preamble VHT preamble VHT Payload Remainder <= 3.6 usec Legacy spoof time = 4 usec per symbol * legacy spoof symbols L-SIG symbol time = 4.0 usec Legacy spoof symbols = L-SIG length / 3 Youhan Kim, et al.

L-SIG Length Conveys Number of Symbols (2) July 2010 L-SIG Length Conveys Number of Symbols (2) Short GI packet 3.6 us / VHT symbol L-SIG can only indicate time in units of 4 us End of frame may not be aligned to a 4 us boundary Two 3.6 us short GI boundaries may map to the same 4 us normal GI boundary used by L-SIG Short GI packet with N symbols 3.6 3.6 3.6 Short GI packet with N+1 symbols 3.6 3.6 3.6 3.6 L-SIG spoof with M symbols 4 4 4 Youhan Kim, et al.

Short GI Packet Duration Signaling July 2010 Short GI Packet Duration Signaling Use two bits for short GI indication in VHT-SIG A b00 : Long GI packet # symbols = ceil(L-SIG length / 3) b01 : Reserved b10 : Short GI packet # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) b11 : Short GI packet # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) – 1 L-SIG spoof with M symbols 4 4 4 Short GI packet with N+1 symbols Short GI: b10 3.6 3.6 3.6 3.6 Short GI packet with N symbols Short GI: b11 3.6 3.6 3.6 Youhan Kim, et al.

July 2010 Conclusions Do not need to indicate VHT packet duration again in VHT-SIG A Length field in L-SIG already has sufficient information to signal the duration of a VHT packet Propose to use 2 bits for short GI indication in VHT-SIG A One bit for long/short GI indication One bit for short GI packet length ambiguity mitigation Youhan Kim, et al.

July 2010 Straw Poll #1 Do you support adding the following item into the specification framework document, 11-09/0992? R3.2.X: The number of OFDM symbols in a VHT packet shall be computed using the length field in L-SIG. Youhan Kim, et al.

July 2010 Straw Poll #2 Do you support adding the following row into Table 1 in the specification framework document, 11-09/0992? Field Bits Description Short GI 2 – b00 : Long GI packet • # symbols = ceil(L-SIG length / 3) – b01 : Reserved – b10 : Short GI packet • # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) – b11 : Short GI packet • # symbols = floor(ceil(L-SIG length / 3) * 4 / 3.6 ) – 1 Youhan Kim, et al.

July 2010 References [1] Kim, Y. et al., Duration in L-SIG, IEEE 802.11-10/0534r1, May 2010 [2] Stacey, R. et al., Specification Framework for TGac, IEEE 802.11-09/0992r11, May 2010 [3] Liu, Y. et al., VHT frame padding, IEEE 802.11-10/0064r5, May 2010 Youhan Kim, et al.