Issues on 256-FFT per 20MHz - IEEE Standards Association

Issues on 256-FFT per 20MHz - IEEE Standards Association

January 2015 doc.: IEEE 802.11-15/0099 Payload Symbol Size for 11ax Date: 2015-01-12 Authors: 1 Name Ron Porat Matthew Fischer Sriram Venkateswaran Tu Nguyen Affiliation Address Phone email [email protected] [email protected] Broadcom Vinko Erceg 1 2111 NE 25th Ave, Hillsboro OR 97124, USA Robert Stacey +1-503-724-893 [email protected] Eldad Perahia

[email protected] Shahrnaz Azizi [email protected] Po-Kai Huang [email protected] Intel Qinghua Li [email protected] Xiaogang Chen [email protected] Chitto Ghosh [email protected] Rongzhen Yang [email protected] 1 Submission Slide 1 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Authors (continued) Name Address

Phone Fei Tong Innovation Park, Cambridge CB4 0DS (U.K.) +44 1223 434633 [email protected] Hyunjeong Kang Maetan 3-dong; Yongtong-Gu Suwon; South Korea +82-31-279-9028 [email protected] m Kaushik Josiam 1301, E. Lookout Dr, Richardson TX 75070 (972) 761 7437 [email protected] Mark Rison Innovation Park, Cambridge CB4 0DS (U.K.) +44 1223 434600 [email protected] Rakesh Taori 1301, E. Lookout Dr, Richardson TX 75070

(972) 761 7470 [email protected] Sanghyun Chang Maetan 3-dong; Yongtong-Gu Suwon; South Korea +82-10-88641751 [email protected] Lei Wang 5488 Marvell Lane, Santa Clara, CA, 95054 858-205-7286 [email protected] Hongyuan Zhang 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Yakun Sun 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Mingguan Xu

5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Jinjing Jiang 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Yan Zhang 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] 1 Affiliation Samsung 1 Liwen Chu 1 Submission Marvell Slide 2 email Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099

Authors (continued) Name 1 Affiliation Phone email Rui Cao 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Sudhir Srinivasa 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Saga Tamhane 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Mao Yu 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Edward Au 5488 Marvell Lane, Santa Clara, CA, 95054

[email protected] Hui-Ling Lu 5488 Marvell Lane, Santa Clara, CA, 95054 [email protected] Yasushi Takatori 1-1 Hikari-no-oka, Yokosuka, Kanagawa 239-0847 Japan [email protected] Marvell 1 Address Yasuhiko Inoue Yusuke Asai [email protected] NTT [email protected] Koichi Ishihara [email protected] Akira Kishida [email protected] Akira Yamada Fujio Watanabe NTT

DOCOMO 3-6, Hikarinooka, Yokosukashi, Kanagawa, 239-8536, Japan [email protected] m 3240 Hillview Ave, Palo Alto, CA 94304 [email protected] ns.com Haralabos Papadopoulos [email protected] vations.com 1 Submission Slide 3 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Authors (continued) Name 1 Affiliation Phillip Barber Address Phone

[email protected] ch.com The Lone Star State, TX Peter Loc [email protected] Le Liu F1-17, Huawei Base, Bantian, Shenzhen Jun Luo 5B-N8, No.2222 Xinjinqiao Road, Pudong, Shanghai Yi Luo F1-17, Huawei Base, Bantian, Shenzhen Yingpei Lin 5B-N8, No.2222 Xinjinqiao Road, Pudong, Shanghai Jiyong Pang Zhigang Rong Rob Sun David X. Yang Yunsong Yang Zhou Lan email Huawei +86-18601656691 [email protected]

+86-18665891036 Junghoon Suh 303 Terry Fox, Suite 400 Kanata, Ottawa, Canada Jiayin Zhang 5B-N8, No.2222 Xinjinqiao Road, Pudong, Shanghai [email protected] [email protected] 5B-N8, No.2222 Xinjinqiao Road, Pudong, Shanghai 10180 Telesis Court, Suite 365, San Diego, CA 92121 NA 303 Terry Fox, Suite 400 Kanata, Ottawa, Canada F1-17, Huawei Base, Bantian, Shenzhen 10180 Telesis Court, Suite 365, San Diego, CA 92121 NA F1-17, Huawei Base, Bantian, SHenzhen [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] +86-18565826350

[email protected] [email protected] +86-18601656691 [email protected] 1 Submission Slide 4 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Authors (continued) Name 1 Laurent cariou Thomas Derham Affiliation Address Phone [email protected] Orange 1 [email protected] 19, Yangjae-daero 11gil, Seocho-gu, Seoul 137-130, Korea

Wookbong Lee [email protected] Kiseon Ryu [email protected] Jinyoung Chun [email protected] Jinsoo Choi Jeongki Kim [email protected] LG Electronics [email protected] Giwon Park [email protected] Dongguk Lim [email protected] Suhwook Kim [email protected] Eunsung Park [email protected] HanGyu Cho 1 email

[email protected] Albert Van Zelst Straatweg 66-S Breukelen, 3621 BR Netherlands [email protected] Alfred Asterjadhi 5775 Morehouse Dr. San Diego, CA, USA [email protected] Bin Tian 5775 Morehouse Dr. San Diego, CA, USA [email protected] Carlos Aldana 1700 Technology Drive San Jose, CA 95110, USA [email protected] George Cherian 5775 Morehouse Dr. San Diego, CA, USA [email protected] Gwendolyn Barriac 5775 Morehouse Dr. San Diego, CA, USA

[email protected] Qualcomm 1 Submission Slide 5 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Authors (continued) Name 1 Affiliation Address Phone email Hemanth Sampath 5775 Morehouse Dr. San Diego, CA, USA [email protected] m Menzo Wentink Straatweg 66-S Breukelen, 3621 BR Netherlands

[email protected] m Richard Van Nee Straatweg 66-S Breukelen, 3621 BR Netherlands [email protected] Rolf De Vegt 1700 Technology Drive San Jose, CA 95110, USA [email protected] 5775 Morehouse Dr. San Diego, CA, USA [email protected] m Simone Merlin 5775 Morehouse Dr. San Diego, CA, USA [email protected] Tevfik Yucek 1700 Technology Drive San Jose, CA 95110, USA [email protected] VK Jones 1700 Technology Drive San Jose, CA 95110, USA [email protected]

Youhan Kim 1700 Technology Drive San Jose, CA 95110, USA [email protected] m Sameer Vermani Qualcomm 1 Submission Slide 6 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Authors (continued) Name 1 Affiliation James Yee Address No. 1 Dusing 1st Road, Hsinchu, Taiwan Phone +886-3-567-0766 Alan Jauh email [email protected]

[email protected] Mediatek 1 Chingwa Hu [email protected] Frank Hsu [email protected] 2860 Junction Ave, San Jose, CA 95134, USA ` Thomas Pare +1-408-526-1899 [email protected] m ChaoChun Wang James Wang Jianhan Liu 1 [email protected] [email protected] Mediatek USA [email protected] Tianyu Wu [email protected]

Russell Huang [email protected] ` Submission Slide 7 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Outline The issue of symbol size for 11ax payloads has been discussed previously Eg: [1] investigated the impact of CFO estimation on symbols with larger FFT sizes (256 and 512 FFT) We have investigated several symbol durations for the payload and propose a new symbol duration We also follow up with a proposal for the choices of CP Provide simulations with real processing that show Significant goodput gains over current choice of (64 FFT, 0.8 us CP) Robust performance in outdoor UL OFDMA Submission Slide 8 Ron Porat, Broadcom

January 2015 doc.: IEEE 802.11-15/0099 Proposal on payload symbol size & CP size We propose to replace the current symbol duration (3.2 us) with longer symbols of duration 12.8 us in the payload to meet the following 11ax requirements Robustness in outdoor channels: both point to point and point-to-multipoint (UL MU/UL OFDMA) Greater tolerance to timing jitter across users in UL MU/OFDMA Efficient indoor operation by reducing CP overhead We also propose the three following natural CP sizes for the proposed longer payload symbols 0.8 us: similar to 11ac long GI, in particular serving high efficiency in indoor settings 1.6 us: percentwise 11ac short GI, in particular serving high efficiency in outdoor channels and indoor UL MU/OFDMA 3.2 us: percentwise 11ac long GI, in particular providing robustness in the challenging case of outdoor UL MU/OFDMA Submission Slide 9 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Channel models & implications Outdoor channels UMi-LoS, UMi-NLoS, UMa-NLoS NLoS channels have large delay spreads with significant probability

Intersymbol interference leads to high error floors ITU RMS Delay Spread CDF 1 UMi NLOS UMi LOS UMa NLOS UMa LOS 0.9 0.8 0.7 0.8 us CP leads to error floors Need longer CPs 0.6 0.5 0.4 0.3 0.2 0.1 0 Submission 0 0.5 1 1.5 2 2.5 [uS] 3 3.5 4

4.5 Slide 10 5 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Spectral efficiency Assume a fixed transmission bandwidth and choose an MCS R = code rate, bps = bits/sample (in the frequency domain. Eg. 256 QAM, bps = 8) Nfft = symbol FFT size, Ndata = #data tones/symbol, Ncp = #CP samples Spectral Efficiency Increases as Ncp increases Decreases as Ncp increases As PER decreases for ISI channels Unless we increase Nfft Tone utilization Depends only on MCS For a given Ncp (dictated by channel length), increase Nfft for smaller overheads and greater spectral efficiency Submission Slide 11

Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Simulation setup L-STF L-LTF L-SIG 11ax Preamble 11ax-LTF Payload Packet structure Payload 1000 bytes FFT sizes: 64, 128, 256 FFT Data tones as defined for corresponding FFT sizes in 11ac CP sizes: 0.8 us, 1.6 us 11ax-LTF: 1 symbol, same (FFT size, GI) as payload 11ax-Preamble: 3 symbols, 64 FFT (precise structure undecided now, but # guided by 11ac) How is the preamble relevant? Pilots used for phase tracking, reduce CFO estimation error 20MHz bandwidth, SISO, BCC Real processing

Channel estimation, timing, frequency offset estimation, phase tracking, phase noise: all real Submission Slide 12 Ron Porat, Broadcom ? January 2015 doc.: IEEE 802.11-15/0099 UMi-LoS: PER for MCS 0-4 PERs with 1.6 us GI (right figure) smaller than PERs with 0.8 us GI (left figure) Even for a given GI, increasing FFT size reduces PER (ICI corrupted samples is a smaller fraction of the total number of samples) Submission Slide 13 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 UMi-LoS: PER for MCS 5-9 PERs with 1.6 us GI (right figure) smaller than PERs with 0.8 us GI (left figure) Even for a given GI, increasing FFT size reduces PER (ICI corrupted samples is a smaller fraction of the total number of samples) Submission Slide 14 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 UMi-NLoS: PER for MCS 0-4

PERs with 1.6 us GI (right figure) smaller than PERs with 0.8 us GI (left figure) Even for a given GI, increasing FFT size reduces PER (ICI corrupted samples is a smaller fraction of the total number of samples) Submission Slide 15 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 UMi-NLoS: PER for MCS 5-9 PERs with 1.6 us GI (right figure) smaller than PERs with 0.8 us GI (left figure) Even for a given GI, increasing FFT size reduces PER (ICI corrupted samples is a smaller fraction of the total number of samples) Submission Slide 16 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput metric used for comparison Goodput = max spectral efficiency obtained by picking the best MCS (for each SNR) Spectral Efficiency For a given CP size Ncp, choose largest possible Nfft CP overhead decreases, tone utilization also improves For a given FFT size Nfft , there is a tradeoff in choosing Ncp

Submission Small Ncp : small overhead, but PER may be large Large Ncp : PER is small, but overhead large Choose the sweet spot for Ncp Slide 17 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput: AWGN Absolute Goodput Goodput relative to (64 FFT, 0.8 us CP) For best results, pick as large an FFT as possible and then pick the smallest CP Increasing CP has no PER benefit in AWGN, increasing FFT reduces overhead Using (256 FFT, 0.8 us CP) gives 1.32x goodput of (64 FFT, 0.8 us CP) Submission Slide 18 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput: 11nD Absolute Goodput Goodput relative to (64 FFT, 0.8 us CP)

Using (256 FFT, 0.8 us CP) gives ~1.3x goodput of (64 FFT, 0.8 us CP) Since channels have small delay spreads, 0.8 us CP has 6-7% better throughput than 1.6 us CP (256 FFT, around 15-20 dB) Submission Slide 19 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput: UMi-LoS Absolute Goodput At high SNR Goodput relative to (64 FFT, 0.8 us CP) Best to use large FFT with longer CP (256 FFT, 1.6 us CP) (256 FFT, 1.6 us CP) gives nearly 2.2x goodput of (64 FFT, 0.8 us CP) At small SNR Submission Thermal noise dominates ISI: increasing CP doesnt give substantial PER gains Stick to smaller CPs, but use larger FFTs: (256 FFT, 0.8 us CP) works best Slide 20

Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput: UMi-NLoS Goodput relative to (64 FFT, 0.8 us CP) Absolute Goodput Large delay spreads leads to high ISI Submission Best to use large FFT with longer CP (256 FFT, 1.6 us) (256 FFT, 1.6 us CP) gives ~2.5x goodput of (64 FFT, 0.8 us CP) at 25 dB SNR Slide 21 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Challenge of UL-OFDMA Timing jitter across users on the UL effectively increases channel delay spread. What is the impact on performance? Impact of intended user delay on himself Impact of delay of interfering users on intended user Sources of timing jitter Different round trip delay Different timing acquisition points due to different channel delay spreads and noise Net timing jitter ~1.3 us (details in Appendix) Submission

Slide 22 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Simulation setup UL OFDMA with 4 users Each user has one antenna, AP has 4 Rx antenna 20MHz, 256 FFT Each user is allocated a contiguous block of 56 tone. User allocations are fixed, and the second user (middle one) is the desired user (PER/Goodput are calculated for this user) GI values considered: 1.6us, 3.2us ITU UMi NLOS channel 1000 bytes packets Real channel estimation from one LTF Submission Slide 23 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099

Results Interfering users have no jitter For intended user delay of 1.2 us Goodput with 3.2 us GI = 1.16x Goodput with 1.6 us GI Submission Slide 24 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Discussion Summary 256 FFT consistently outperforms current symbol choice of (64 FFT, 0.8 us CP) Goodput gains range from 1.3x-2.5x depending on channel Use 0.8 us CP with 256 FFT for high efficiency in indoor channels Use 1.6 us CP with 256 FFT for greater robustness to long outdoor channels and indoor UL OFDMA/MU Use 3.2 us CP with 256 FFT for robust performance in outdoor UL OFDMA/MU Why not even higher FFT sizes, say 512 FFT over 20 MHz? Implementation complexities increase with increasing FFT sizes and bandwidths Tones are more narrowly spaced , CFO correction needs to be very precise: challenging task [1] Incremental gain over 256 FFT (3-6% depending on CP size) too small for increased complexity

256 FFT size in 20 MHz seems to be the sweet spot between performance and implementation complexities Submission Slide 25 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Proposal We propose that 11ax shall have one longer payload symbol size of duration 12.8 us based on a 256 FFT in 20 MHz And correspondingly 512 FFT in 40 MHz, 1024 FFT in 80 MHz/80+80 MHz and 2048 FFT in 160 MHz We also propose to use the following CP sizes: 0.8 us, 1.6 us and 3.2 us Submission Slide 26 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 References [1] 11-14-0801-00-00ax-envisioning-11ax-phy-structure-part-ii Submission Slide 27 Ron Porat, Broadcom

January 2015 doc.: IEEE 802.11-15/0099 Straw Poll #1 Do you support that 11ax payload symbols use symbol size based on a 256 FFT in 20 MHz? And correspondingly 512 FFT in 40 MHz, 1024 FFT in 80 MHz/80+80 MHz and 2048 FFT in 160 MHz Yes No Abstain Submission Slide 28 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Straw Poll #2 Do you support the following CP choices for the proposed payload symbols: 0.8 us, 1.6 us and 3.2 us? Yes No Abstain Submission Slide 29 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Appendix

Submission Slide 30 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 UMa-NLoS: PER for MCS 0-4 Submission Slide 31 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Goodput: UMa-NLoS Goodput relative to (64 FFT, 0.8 us CP) Absolute Goodput Submission Slide 32 Ron Porat, Broadcom January 2015 doc.: IEEE 802.11-15/0099 Sources of timing jitter Different round trip delays can contribute 0.6 us (~ 200 m)

Timing acquisition on DL can contribute 0.7 us jitter in UMi-NLoS channels. For example, at 10 dB in figure below, 14 samples @ 20 MHz = 0.7 us AP has 4 antennas, STA has 1 antenna Timing acquired from L-LTF Submission Slide 33 Ron Porat, Broadcom

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