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IEEE 802.11ax – An Overview

IEEE 802.11ax – An Overview. Osama Aboul-Magd Huawei Technologies, Canada. Background. In mid 2012 discussions in IEEE 802.11 WG focused on the evolution of Wi-Fi to meet new use cases other than those related to consumers and enterprises.

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IEEE 802.11ax – An Overview

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  1. IEEE 802.11ax – An Overview Osama Aboul-Magd Huawei Technologies, Canada

  2. Background • In mid 2012 discussions in IEEE 802.11 WG focused on the evolution of Wi-Fi to meet new use cases other than those related to consumers and enterprises. • The discussion was initiated by network providers motivated by the increased volumes of data offloading • https://mentor.ieee.org/802.11/dcn/12/11-12-0910-00-0wng-carrier-oriented-wifi-cellular-offload.ppt • https://mentor.ieee.org/802.11/dcn/12/11-12-1063-00-0wng-requirements-on-wlan-celllular-offload.pptx • Additionally IEEE 802.11ac was in sponsor ballot and close to be published. • Work on IEEE 802.11ac started in May 2007 just on time for iPhone announcement.

  3. Evolution of Use Cases • IEEE 802.11ac was designed for the traditional Wi-Fi application. • Consumer market with emphasize on Internet access. • Enterprise application, e.g. an office building or a university with focus on connectivity. • IEEE 802.11ac design focus was on achieving higher aggregate throughput • Wider Channels (80 and 160 MHz) • DL MU MIMO • Higher order MCS (256 QAM)

  4. New Use Cases • The Wi-Fi landscape has been rapidly changing – more devices and bandwidth demanding applications Video Traffic is becoming Dominate Hotspots and Data offloading Multiple Interfering Devices (dense deployment)

  5. The IEEE 802.11ax Scope This amendment defines standardized modifications to both the IEEE 802.11 physical layers (PHY) and the IEEE 802.11 Medium Access Control layer (MAC) that enable at least one mode of operation capable of supporting at least four times improvement in the average throughput per station (measured at the MAC data service access point) in a dense deployment scenario, while maintaining or improving the power efficiency per station. This amendment defines operations in frequency bands between 1 GHz and 6 GHz. The new amendment shall enable backward compatibility and coexistence with legacy IEEE 802.11 devices operating in the same band.

  6. The IEEE 802.11ax Scope This amendment defines standardized modifications to both the IEEE 802.11 physical layers (PHY) and the IEEE 802.11 Medium Access Control layer (MAC) that enable at least one mode of operation capable of supporting at least four times improvement in the average throughput per station (measured at the MAC data service access point) in a dense deployment scenario, while maintaining or improving the power efficiency per station. This amendment defines operations in frequency bands between 1 GHz and 7.125 GHz. The new amendment shall enable backward compatibility and coexistence with legacy IEEE 802.11 devices operating in the same band. In December 2017 the IEEE-SA NesCom approved 802.11ax PAR Modification to include operation in the 6 GHz band

  7. Implications • Scenarios with dense deployments are the main focus of the new amendment. • Simulation Scenarios are developed to support dense environment: https://mentor.ieee.org/802.11/dcn/14/11-14-0980-14-00ax-simulation-scenarios.docx • Focus is on per-station performance improvement rather than aggregate throughput. • The new amendment focuses on 2.4 GHz, 5 GHz, and 6 GHz bands • Improving user experience in traditional WLAN bands. • Backward compatibility is still a strong requirement. • Operation in the 6 GHz band added with focus on discovery

  8. Timeline RevCom & Publication SG Formation Draft D1.0 Draft D3.0 Draft D5.0 1/15 1/17 1/14 1/19 1/20 1/13 1/16 1/18 Sponsor Ballot Draft D4.0 Draft D2.0 TG Formation

  9. IEEE 802.11ax Main Features – A Quick Summary • The use of Orthogonal Frequency Division Multiple Access (OFDMA) • Allows the multiplexing of multiple users in the frequency domain. • A departure from the use of the OFDM where all resources are assigned to a single user as in previous IEEE 802.11 amendments. • Support of OFDMA is both for the Uplink (UL) and the Downlink (DL) • Supporting Triggered UL MU MIMO • DL MU MIMO support is already in IEEE 802.11ac. • Allows multiplexing of multiple users in the spatial domain • The use of 256 FFT (20 MHz Channel) for the data portion of the 802.11ax PPDU. • A departure from the 64 FFT used in previous IEEE 802.11 amendments. • Pre-defined resource unit (RU) sizes • Four frame formats • Allows Spatial Reuse • MCS 10 and MCS 11 introducing 1024 QAM

  10. A Quick Summary of Previous Amendments (A/N/AC) – Frame Formats STF: Short Training Field LTF: Long Training Field SIG: Signal Field Legacy Preamble “A” STF LTF SIG Data STF LTF L-Preamble is included for backward compatibility SU-MIMO: As many LTF fields as number of Antennas Auto-Detection is achieved by changing the polarity of the signal “N” Data L-Preamble LTF-n LTF-1 LTF-2 SIG 1 SIG 2 L-Preamble is included for backward compatibility SU-MIMO and DL MU-MIMO SIG-B includes per user signal parameters Auto-Auto-Detection is achieved by changing the polarity of the signal Data L-Preamble “AC” LTF-1 LTF-n SIG-B SIG-A 1 SIG-A 2

  11. IEEE 802.11ax Main PHY Features RU General Frame Format • As in IEEE 802.11n/ac, HEW PPDU starts with a legacy preamble for backward compatibility. Legacy preamble is duplicated on every 20 MHz channel. • L-Preamble consists of L-STF, L-LTF, and L-SIG. • Repeated L-SIG (RL-SIG) is included for auto-detection. • HE-SIG-A is two-symbol long and is duplicated on every 20 MHz channel. HE-SIG-A is available in every PPDU. • HE-SIG-B is of variable length. It includes resource allocation information. HE-SIG-B is only present in the MU PPDU. • HE-Data uses DFT period of 12.8 msec and subcarrier spacing of 78.125 KHz. • Tone plan allowing 26-tone, 52-tone, 106-tone, 242-tone for OFDMA. 484-tone and 996-tone for non-OFDMA cases. • Mandatory support for LDPC coding in HE PPDU Data field for allocation sizes of 484 tones, 996 tones and 996*2 tones. • 1024-QAM is an optional feature for SU and MU using resource units equal to or larger than 242 tones in 11ax. • Dual sub-carrier modulation (DCM) is an optional modulation scheme for the HE-SIG-B and Data fields. DCM is only applied to BPSK, QPSK and 16-QAM modulations HE-SIG-A HE-Data … HE-SIG-B RL-SIG HE-LTF L-Preamble HE-STF HE-LTF

  12. 802.11ax: A Quick Score Card

  13. Frame Format (I) – Single User (SU) Frame Format SIG-A 2 SIG-A 1 STF LTF Data L-LTF LTF L-LTF L-SIG R-SIG L-STF L-STF SIG-A 1 And SIG-A2

  14. Frame Format (II) – SU Extended Range • Originally designed for out-door environment to increase SIG-A reliability • Results have shown a gain of 6 dbs allowing the signal to reach further • The SIG-A contents are the same as the SU Frame format SIG-A 2 SIG-A 1 SIG-A 2 SIG-A 1 STF LTF Data L-LTF LTF L-LTF L-SIG R-SIG L-STF L-STF Repeated SIG-A

  15. Frame Format (III)- Multi-User (MU) Frame Format SIG-A 2 SIG-A 1 L-LTF L-SIG L-LTF LTF STF Data R-SIG LTF L-STF SIG-B SIG-A 1 And SIG-A2

  16. Frame Format (VI) – Trigger-Based Frame Format SIG-A 1 And SIG-A2

  17. OFDMA Tone Plan – 20 MHz, 40 MHz, and 80 MHz Channel 5 DC 12 Edge 11 Edge 5 DC 11 Edge 12 Edge 5 DC 11 Edge 12 Edge 5 DC 11 Edge 12 Edge 2 2 2 2 26 26 1 1 1 1 1 1 1 1 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 1 1 1 1 2 2 2 2 1 1 1 1 52 52 52 52 52 52 52 52 26 26 12 Edge 484 usable tones +5 DC 11 Edge 1 1 1 1 26 26 102+4 102+4 102+4 102+4 1 1 1 1 20 MHz Tone Plan 40 MHz Tone Plan 242 242 7 DC 1 6 Edge 1 13 26 26 26 26 26 26 1 26 13 26 1 5 Edge HE20 with 7DC for OFDMA 7 DC 13 1 1 52 52 52 52 1 13 1 5 Edge 6 Edge 1 1 1 1 1 1 26 26 1 1 26 26 26 26 26 26 2 2 26 26 1 1 26 26 26 26 26 26 26 26 2 2 26 26 26 26 1 1 26 26 2 2 26 26 2 2 2 2 26 26 26 26 26 26 26 26 26 26 7 DC 13 11 Edge 12 Edge 13 1 1 1 1 1 1 1 1 52 52 52 52 52 52 26 26 2 2 52 52 2 2 2 2 52 52 52 52 52 52 2 2 1 1 1 1 52 52 2 2 26 26 7 DC 5 Edge 6 Edge 102+4 pilots 13 102+4 pilots 13 7 DC 11 Edge 13 13 12 Edge 1 1 102+4 102+4 102+4 102+4 1 1 1 1 1 1 26 26 1 1 26 26 2 2 102+4 102+4 102+4 102+4 1 1 5 Edge 11 Edge 7 DC 6 Edge 242 + 3 DC 13 13 12 Edge 242 242 242 242 11 Edge 7 DC 13 13 12 Edge 12 Edge 996 usable tones +5 DC 11 Edge 80 MHz Tone Plan

  18. Downlink (DL) Procedure STA-1 • ACKs from different stations are transmitted using trigger-based PPDU format • Unlike DL MU MIMO in 802.11ac where different stations are polled to transmit their ACKs to the AP. • ACK resources are indicated in either a Trigger frame or Resource Allocation A-Control (Aggregate Control) field. STA-2 H STA-3 AP STA-4 BA STA-1 Time STA BA STA-2 BA STA-3 BA STA-4

  19. Uplink (UL) Procedure BA STA-1 • AP initiates UL transmissions by mean of a trigger frame • Trigger frame includes information related to each STA participating in the UL transmissions. • Stations responds with a triggered-based PPDU SIFS time units after the reception of the trigger frame. • The AP may use the multi-STA block ACK to acknowledge UL transmissions from multiple stations. • The AP may send the • Padding BA STA-2 BA STA-3 AP Trigger BA STA-4 STA-1 Time STA STA-2 H STA-3 STA-4

  20. Uplink (UL) Procedure • AP initiates UL transmissions by mean of a trigger frame • Trigger frame includes information related to each STA participating in the UL transmissions. • Stations responds with a triggered-based PPDU SIFS time units after the reception of the trigger frame. • The AP may use the multi-STA block ACK to acknowledge UL transmissions from multiple stations. • The AP may send the • Padding AP Multi-STA BA Trigger STA-1 Time STA STA-2 H STA-3 STA-4

  21. Scheduled Trigger Frames • The main power save mechanism in 802.11ax • Makes use of the Target Wake Time (TWT) to establish trigger frame schedule with the AP • TWT was introduced in 802.11ah amendment to address requirements of low-power devices, e.g. sensors • Two TWT flavors are introduced: • Individual TWT • Broadcast TWT

  22. Trigger Frame Variants BAR Information BAR Information BAR Control BAR Control MPDU MU Spacing TID Aggregation Limit Preferred AC AC Preference Level Feedback Segment Retransmission Bitmap

  23. Spatial Reuse: The Concept OBSS signal@-82dBm OBSS signal@-72dBm • Pre 802.11 NAV rule: A station updates its NAV based on the Duration field in any valid frame. • Setting OBSS PD level to -72dBm, an intra-BSS device A located in the OBSS yellow ring with receiving OBSS signal strength from (-82, -72)dBm can change from CCA busy to idle. • However, if device A decodes the duration field correctly from OBSS signal, device A can’t transmit for spatial reuse due to the higher NAV value, following 11ac NAV rule. • When a STA uses its OBSS PD level(e.g. -72dBm) for OBSS signal, it should not update its NAV when receiving a valid duration field from OBSS signal, if the measured RSSI of OBSS signal is less than the OBSS PD level. • A station will need to maintain two NAV timers. CCA idle CCA busy A Intra-BSS OBSS CCA busy->idle; duration decoding correctly B CCA busy->idle; duration decoding error

  24. Spatial Reuse – BSS Color and 2 NAV Timers • BSS color in SIG-A field allows devices to differentiate between Intra-BSS frames and Inter-BSS frames. • An IEEE 802.11ax station maintains two NAV timers (Network Access Vector): Basic NAV and Intra-BSS NAV • Pre IEEE 802.11 devices maintain a single NAV. The value of the NAV is updated according to the Duration/ID field in the Frame Control. • The medium is idle when the two NAV timers are zero. • Two types of Spatial Reuse are defined: • OBSS PD-based Spatial Reuse • Spatial Reuse Parameters

  25. OBSS_PD Adjustment • If using OBSS PD-based spatial reuse, an HE STA shall maintain an OBSS PD level and may adjust this OBSS PD level in conjunction with its transmit power and this adjustment shall be made in accordance with Equation: 

  26. CCA Sensitivity • The 802.11ax hasn’t changed the CCA levels on the primary 20 MHz

  27. CCA Sensitivity • The 802.11ax accounts for the introduction of the new parameter OBSS_Pdlevel • Any signal within the any 20 MHz subchannel of secondary 20 MHz, secondary 40 MHz or second-ary 80 MHz at or above a threshold of –62 dBm within a period of aCCATime after the signal arrives at the receiver's antenna(s); then the PHY shall not issue PHY-CCA.indication(IDLE) primitive while the threshold continues to be exceeded. • An 80 MHz non-HT duplicate, VHT PPDU or HE PPDU detected in the secondary 80 MHz channel at or above max(–69 dBm, OBSS_PDlevel + 6 dB) with > 90% probability within a period aCCAMidTime (see 27.4.4 (HE PHY)). • A 40 MHz non-HT duplicate, HT_MF, HT_GF, VHT or HE PPDU detected in any 40 MHz sub-channel of the secondary 40 MHz or the secondary 80 MHz channel at or above max(–72 dBm, OBSS_PDlevel + 3 dB) with > 90% probability within a period aCCAMidTime. • A 20 MHz NON_HT, HT_MF, HT_GF, VHT, or HE PPDU detected in the any 20 MHz subchannel of secondary 20 MHz, secondary 40 MHz or secondary 80 MHz channel at or above max(–72 dBm, OBSS_PDlevel) with >90% probability within a period aCCAMidTime (see 27.4.4 (HE PHY)). 

  28. Operation in the 6 GHz Band- Channelization • Starting frequency of 5940 MHz • Only 10 MHz of Guard band for U-NII-5 • Challenging filter design • Channels can cross U-NII boundaries • In case U-NII-5 and 6 work under different regulatory rules • No 80 MHz channel in U-NII-6 • Only one 40 MHz channel in U-NII-6 Center Frequency [MHz] 20 MHz Channels 40 MHz Channels 80 MHz Channels 160 MHz Channels

  29. Operation in the 6 GHz • An HE STA indicated its capability to operate in the 6 GHz band • An HE AP operating in the 6 GHz band shall indicate support for at least 80 MHz channel width  • A STA shall not transmit an HT PPDU (802.11n) in the 6 GHz band. A STA shall not transmit a VHT PPDU (802.11ac) in the 6 GHz band. A STA shall not transmit a DSSS, HR/DSSS (802.11b), or ERP-OFDM (802.11g) PPDU in the 6 GHz band.  • An HE AP may transmit an HE SU beacon in the 6 GHz band.  • Rules are defined for passive and active scanning and out-of-band discovery (for APs in the 2.4 and 5 Ghz and collocated with AP in the 6 GHz).

  30. Closing Notes • IEEE 802.11ax is the next PHY layer after the successful 802.11n and 802.11ac. • It is the first 802.11 amendment to introduce OFDMA to wireless LAN. • IEEE 802.11ax adds UL MU MIMO • Allows power save based on scheduled trigger frames

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