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VHT Packet Duration Signaling

VHT Packet Duration Signaling. Authors:. Date: 2010-07-12. 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]

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VHT Packet Duration Signaling

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  1. VHT Packet Duration Signaling Authors: Date: 2010-07-12 Youhan Kim, et al.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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 same4 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.

  8. 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.

  9. 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.

  10. 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.

  11. Straw Poll #2 • Do you support adding the following row into Table 1 in the specification framework document, 11-09/0992? Youhan Kim, et al.

  12. 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.

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