Understanding IEEE 802.11 Sub-7 GHz PHY Training Fields
Explore the details of IEEE 802.11 Sub-7 GHz PHY Long Training Field components, TX parameters, and their impact on sensing measurements. Learn about the negotiation of Training Field components for Sensing Transmitters and the trade-offs involved. Discover the significance of PHY transmission variables, Long Training Fields for OFDM PHYs, and the different OFDM PPDU types available. Dive into the graphical view of OFDM PPDU preambles, durations, and legacy, VHT, and HE LTFs defined for channel estimation in IEEE 802.11 protocols.
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April 2021 doc.: IEEE 802.11-21/0753r1 Sub-7 PHY Long Training Field Selection Date: 2021-04-30 Authors: Name Chris Beg Affiliations Cognitive Systems Address 560 Westmount Road N. Waterloo ON, CA 560 Westmount Road N. Waterloo ON, CA Phone email chris.beg@cognitivesystems.com Mohammad Omer Cognitive Systems Submission Slide 1 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Abstract To identify existing sub-7 GHz PPDU Long Training Field Components and related TX parameters impacting sensing measurements. Having ability for a Sensing Session to negotiate which Training Field Components and related TX parameters that will be used for by the Sensing Transmitter, allows an application to determine tradeoffs. Submission Slide 2 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Background in PHY Transmission Variables A number of PHY transmission variables are available which sensing can benefit from. Some parameters have been highlighted in [1]: Long Training Field Components (highlighted in this contribution) Bandwidth Selection Number of Antenna (Spatial Streams) Spatial Mapping/Steering Q-Matrix TX Power Options, which if made available in designed MAC protocol, allows application flexibility in achieving use-cases [4][5] Submission Slide 3 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Long Training Fields For OFDM PHYs, Long Training Fields (LTF) already defined for Channel Estimation, and can be used for Sensing Different versions available depending on PPDU Type Mixed Mode PPDUs contain Legacy and PHY Specific (MIMO) LTF Each version offers properties which can benefit sensing # Spatial Streams, Channel Interpolation, Compatibility, Duration Submission Slide 4 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 OFDM PPDU Types (up to AX) Graphical view of available PPDU preambles and durations. L-LTF, HT-LTF, VHT-LTF, and HE-LTF defined for channel estimation Legacy Preamble Legacy Preamble HE (SU) Preamble 8us 8us 4us NON_HT 8us 8us 4us 4us 8us 4us N x [4us, 8us, or 16us] L-STF L-LTF L-SIG HE_SU RL- SIG HE- STF HE-SIG-A HE-LTF L-STF L-LTF L-SIG 10 short symbols @ T=0.8us 2 symbols @T=3.2us Guard 1.6us . . . HE LTF Symbol HE LTF Symbol HE LTF Symbol Legacy Preamble HT Preamble 8us 8us 4us 8us 4us N x 4us N=[1,2,4,6, or 8] HT_MF HT- STF Legacy Preamble HE (MU) Preamble HT-SIG HT-LTF L-STF L-LTF L-SIG 4us / Symbol 8us 8us 4us 4us 8us 4us N x [4us, 8us, or 16us] HE_MU . . . RL- SIG HE- STF HE-SIG-A HE-SIG-B HE-LTF L-STF L-LTF L-SIG HT LTF Symbol HT LTF Symbol HT LTF Symbol . . . N=[1,2, or 4] HE LTF Symbol HE LTF Symbol HE LTF Symbol Note: Optional [1, 2, or 4] HT-ELFTs can be added to sound higher dimensions. HT Greenfield Preamble N=[1,2,4,6, or 8] Legacy Preamble HE (ER SU) Preamble 8us 8us 8us (N-1) x 4us HT_GF 8us 8us 4us 4us 16us 4us N x [4us, 8us, or 16us] HT-SIG HT-LTF HT-GF-STF HT-LTF1 HE_ER RL- SIG HE- STF HE-SIG-A HE-LTF L-STF L-LTF L-SIG . . . HT LTF Symbol HT LTF Symbol HT LTF Symbol . . . HE LTF Symbol HE LTF Symbol HE LTF Symbol N=[1,2, or 4] Note: Optional [1, 2, or 4] HT-ELFTs can be added to sound higher dimensions. N=[1,2,4,6, or 8] Legacy Preamble HE (TB) Preamble Legacy Preamble VHT Preamble 8us 8us 8us 4us 4us 8us N x [4us, 8us, or 16us] HE_TB 8us 8us 4us 8us 4us N x 4us 4us RL- SIG HE-SIG-A HE-STF HE-LTF L-STF L-LTF L-SIG VHT- STF VHT- SIG-B VHT VHT-SIG-A VHT-LTF L-STF L-LTF L-SIG . . . . . . HE LTF Symbol HE LTF Symbol HE LTF Symbol VHT LTF Symbol VHT LTF Symbol VHT LTF Symbol N=[1,2,4,6, or 8] N=[1,2,4,6, or 8] Submission Slide 5 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Channel Measurement Excited subcarriers in LTF used to measure/interpolate channel Different LTF definitions determine subcarrier excitation LTF definition is a function of channel bandwidth MIMO LTF definition is a function of Number Spatial Streams L-LTF Definition [2] 20 MHz Measurement Points with L-LTF: 20 MHz (64-Points) 26-Points -32 -26 -1 0 1 26 31 64-Point FFT Bin Submission Slide 6 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Legacy LTF (L-LTF) L-LTF is defined over 20 MHz BW Higher channel bandwidth transmissions duplicate legacy preamble on each spanning 20 MHz channel (w/ phase rotation) Benefit is shortest duration PPDU may be used (NON_HT) 20 MHz (64-Points) 26-Points 26-Points -32 -26 -1 0 1 26 31 64-Point FFT Bin 40 MHz (128-Points) 20 MHz (64-Points) 20 MHz (64-Points) 26-Points 26-Points 26-Points 26-Points -64 -58 -33 -32 -31 -6 -1 0 6 31 32 33 58 63 128-Point FFT Bin 80 MHz (256-Points) 40 MHz (128-Points) 40 MHz (128-Points) -58 -70 -33-32-31 313233 959697 -128 -122 -97 -96-95 -6 0 6 58 70 122 127 256-Point FFT Bin Submission Slide 7 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 HT and VHT LTF HT and VHT LTF defined as function of BW, utilizing what were previously null subcarriers within L-LTF, providing additional points for channel interpolation Additional points useful to improve SNR of channel estimation NOTE: VHT-LTF-28,28 = HT-LTF-28,28 and VHT-LTF-58,58 = HT-LTF-58,58 Able to measure spatial streams, providing MIMO estimation Tradeoff is longer duration PPDU (more air-time required) 20 MHz (64-Points) [HT/VHT]-LTS 28-Points 28-Points -32 -28 -1 0 1 28 31 64-Point FFT Bin 40 MHz (128-Points) 20 MHz (64-Points) 20 MHz (64-Points) [HT/VHT]-LTS 57-Points 57-Points -64 -58 -2 0 2 58 63 128-Point FFT Bin 80 MHz (256-Points) 40 MHz (128-Points) 40 MHz (128-Points) VHT-LTS 57-Points 57-Points 57-Points 57-Points -128 -122 -66 -62 -6 0 6 62 66 122 127 256-Point FFT Bin 160 MHz (512-Points) 80 MHz (256-Points) 40 MHz (128-Points) 40 MHz (128-Points) 40 MHz (128-Points) 40 MHz (128-Points) VHT-LTS 57-Points 57-Points -256 -250 -194 -190 -134 -122 -66 -62 -6 0 6 62 66 122 134 190 194 250 255 512-Point FFT Bin Submission Slide 8 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 HE LTF HE LTF allows 1x/2x/4x resolution modes for 20/40/80/160 MHz Defined by HE_LTF_TYPE Result can further improve SNR of channel estimation Tradeoff is longer duration PPDU (more air-time required) HE-LTF[3] (1x 20MHz) HE-LTF[3] (2x 20MHz) HE-LTF[3] (4x 20MHz) 80 MHz (1024-Points) 40 MHz (512-Points) 40 MHz (512-Points) 20 MHz (256-Points) 498-Points 498-Points 121-Points -512 -500 -3 0 3 500 512 -128 -122 -2 2 0 122 255 1024-Point FFT Bin 256-Point FFT Bin 160 MHz (2048-Points) 40 MHz (512-Points) 80 MHz (1024-Points) 20 MHz (256-Points) 20 MHz (256-Points) 40 MHz (128-Points) 40 MHz (128-Points) 40 MHz (128-Points) 40 MHz (128-Points) 242-Points 242-Points 498-Points 498-Points 498-Points -256 -244 -3 0 3 244 255 -1024 -1012 -515 -509 -12 0 12 509 515 1012 1023 512-Point FFT Bin 2048-Point FFT Bin Submission Slide 9 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Summary Table 1. Long Training Field Presence in PPDU Table 2. Present Subcarriers PPDU L-LTF HT-LTF VHT-LTF HE-LTF LTF 20 MHz 40 MHz 80 MHz 160 MHz NON_HT Yes L-LTF 52 104 208 416 HT_MF Yes Yes HT-LTF 56 * Nss 114 * Nss N/A N/A HT_GF Yes VHT-LTF 56 * Nss 114 * Nss 228 * Nss 456 * Nss VHT Yes Yes HE-LTF (1x) 60 * Nss 122 * Nss 250 * Nss 502 * Nss HE_SU Yes Yes HE-LTF (2x) 122 * Nss 242 * Nss 498 * Nss 996 * Nss HE_MU Yes Yes HE_ER Yes Yes HE-LTF (4x) 242 * Nss 484 * Nss 996 * Nss 1992 * Nss HE_TB Yes Yes Table 3. PPDU Preamble Duration Spatial Streams PPDU Preamble Duration Tradeoffs available with LTF Selection NON_HT No 20us NSS = Number of Spatial Streams NHE-SIG-B = Number of HE-SIG-B symbols HT_MF Yes 32us +Nss*4us HT_GF Yes 24us + (Nss-1)*4us VHT Yes 36us + Nss*4us THE-LTF = 4us for 1x mode 8us for 2x mode 16us for 4x mode HE_SU Yes 36us + Nss*THE-LTF HE_MU Yes 36us + NHE-SIG-B*4us + Nss*THE-LTF HE_ER Yes 44us + Nss*THE-LTF HE_TB Yes 40us + Nss*THE-LTF Submission Slide 10 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 Remarks Multiple existing LTFs which can be used for Sensing have been identified. PPDU presence, preamble duration, and number of measurement points as a function of bandwidth and Nss has been summarized. Expanding on [1], tradeoffs are available in preamble duration (air time) vs measurement requirements, which can be utilized by applications if negotiation allowed by MAC protocol [5]. Future thinking: at which point does this tradeoff need to be negotiated? At Setup-Phase only, or can existing sessions modify parameters without termination and new session creation? Submission Slide 11 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 References [1] 11-21-0365-02-00bf-wi-fi-sensing-parameters [2] IEEE 802.11REVme [3] IEEE Draft P802.11ax 8.0 [4] 11-21-0391-00-00bf-minimizing-impact-as-design-goal [5] 11-20-1712-02-00bf-wifi-sensing-use-cases Submission Slide 12 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 SP #1 Do you agree that during the setup phase of a sensing session, the participants should have the ability to negotiate: PPDU format, Channel Bandwidth, Nss and HE_LTF_TYPE? Y/N/A? Results: 15/20/15 Submission Slide 13 Chris Beg, Cognitive Systems
April 2021 doc.: IEEE 802.11-21/0753r1 SP #2 Do you agree that during the measurement phase of a sensing session, the participants should have the ability to identify which PPDU format, Channel Bandwidth, Nss and HE_LTF_TYPE are used to produce a sensing measurement? Y/N/A? Discussed, but not run based on results of SP #1 Submission Slide 14 Chris Beg, Cognitive Systems
