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IEEE 802.11ax An Overview Osama Aboul-Magd Huawei Technologies, Canada 1
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. 2
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) 3
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)
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. 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 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 6
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 7
Timeline RevCom & Publication SG Draft D1.0 Draft D3.0 Draft D5.0 Formation 1/14 1/15 1/17 1/19 1/20 1/13 1/16 1/18 Sponsor Ballot Draft D4.0 Draft D2.0 TG Formation 8
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 9
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 LTF SIG Data STF 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 SIG-A 1 SIG-A 2 LTF-1 AC LTF-n SIG-B 10
IEEE 802.11ax Main PHY Features RU L-Preamble General Frame Format HE-Data RL-SIG HE-LTF HE-STF HE-LTF HE-SIG-A HE-SIG-B 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 sec 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 11
802.11ax: A Quick Score Card 802.11ax 802.11ac 802.11n 20, 40, 80, and 160 MHz 20, 40, 80, and 160 MHz 20, 40 MHz Same Channel Bandwidth as in Wi-Fi 5 Channel Bandwidth OFDMA OFDM OFDM Achieve multiplexing gain and per User focus Waveform 2.4, 5, and 6 GHz 5 GHz 2.4 and 5 GHz Make use of the large spectrum available in the 6 GHz band Band 8 8 4 Same as in Wi-Fi 5 Number of Antennas Target Wake up Time (TWT) No No Efficient support of devices with power constraints Advanced Power Save 9.6 Gbps 6.9 Gbps 600 Mbps Modest rate increase compared to Wi-Fi 5 Aggregate Data Rate 4x improvement ? ? Focus is on user experience and per user throughput. User Experience Yes No Yes Allows support of IoT applications and eventual replacement of Wi-Fi 4 20 MHz-only operation Yes No No Efficient use of available spectrum Spatial Reuse DL MU MIMO and Triggered UL MIMO DL MU MIMO No Access point schedules users based on their traffic requirements, e.g. buffer size and delay requirements. MU MIMO Improved support No No Improved outdoor performance for open stadiums and hot spots. Outdoor 1024 QAM 256 QAM 64 QAM Improved throughput by packing more signals in the same space. QAM 12
Frame Format (I) Single User (SU) Frame Format SIG-A 2 SIG-A 1 L-LTF L-LTF STF LTF Data LTF L-SIG R-SIG L-STF L-STF Bit Field Name Bit Field Name Bit Field Name B0 Frame format (SU PPDU or Trigger-based PPDU) B19-B20 Bandwidth B11-B12 Pre-FEC padding Factor B1 Beam Change B21-B22 GI+LTF Size B13 BE Disambiguaty B2 UL/DL B23-B25 Nsts B14 Reserved SIG-A 1 And SIG-A2 B3-B6 MCS B0-B6 TXOP Duration B15 Doppler B7 DCM B7 Coding B16-B19 CRC B8-B13 BSS Color B8 LDPC Extra Symbol B20-B25 Tail B14 reserved B9 STBC B15-B18 Spatial Reuse B10 TxBF 13
Frame Format (II) SU Extended Range SIG-A 2 SIG-A 1 SIG-A 2 SIG-A 1 STF LTF Data L-LTF L-LTF LTF L-SIG R-SIG L-STF L-STF Repeated SIG-A 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 14
Frame Format (III)- Multi-User (MU) Frame Format SIG-A 2 SIG-A 1 L-LTF L-LTF L-SIG R-SIG STF LTF Data L-STF LTF SIG-B Bit Field Name Bit Field Name Bit Field Name B0 UL/DL B22 SIG-B Compression B15 PE Disambiguity B1-B3 SIG-B MCS B23-B24 GI+LTF Size B16-B19 CRC SIG-A 1 And SIG-A2 B4 SIG-B DCM B25 Doppler B20-B25 Tail B5-B10 BSS Color B0-B6 TXOP Duration B7 DCM B8-B10 Number of LTF Symbols B11-B14 Spatial Reuse B11 LDPC Extra Symbol B15-B17 Bandwidth B12 STBC B18-B21 Number of Sig-B symbols or MU-MIMO Users B13-B14 Pre-FEC padding Factor 15
Frame Format (VI) Trigger-Based Frame Format Bit Field Name Bit Field Name B0 Frame format (SU PPDU or Trigger-based PPDU) B0-B6 TXOP Duration B1-B6 BSS Color B7-B15 Reserved B7-B10 Spatial Reuse 1 B16-B19 CRC SIG-A 1 And SIG-A2 B11-B14 Spatial Reuse 2 B20-B25 Tail B15-B18 Spatial Reuse 3 B19-B22 Spatial Reuse 4 B23 reserved B24-B25 Bandwidth 16
OFDMA Tone Plan 20 MHz, 40 MHz, and 80 MHz Channel 7 1 3 5 DC 6 Edge 26 26 26 26 26 26 26 26 5 Edge 13 1 1 1 1 12 Edge 26 26 26 26 26 26 26 26 26 26 26 26 26 26 11 Edge 26 26 26 26 2 1 1 DC 2 2 1 1 1 1 2 1 1 1 3 7 DC 5 DC 52 52 52 52 5 Edge 13 6 Edge 52 52 52 52 52 52 52 52 26 1 1 26 1 1 11 Edge 1 2 1 1 2 1 1 2 1 12 Edge 1 2 1 5 DC 1 3 11 Edge 1 3 7 5 Edge 12 Edge 6 Edge 102+4 102+4 102+4 102+4 26 26 1 1 1 1 1 1 1 1 102+4 pilots 102+4 pilots DC 5 DC 11 Edge 242 242 12 Edge 5 Edge 6 Edge 242 + 3 DC HE20 with 7DC for OFDMA 12 Edge 484 usable tones +5 DC 11 Edge 20 MHz Tone Plan 40 MHz Tone Plan 2 6 1 3 1 3 2 6 1 3 1 3 7 DC 11 Edge 12 Edge 12 Edge 26 26 26 26 26 26 26 26 26 26 1 26 26 26 26 26 26 26 26 26 26 26 26 26 1 26 26 26 26 26 26 26 26 26 26 26 1 2 1 1 2 1 2 2 1 1 1 2 1 2 1 2 2 2 2 1 7 DC 11 Edge 52 52 52 52 52 52 52 52 52 52 52 52 52 52 52 52 26 26 1 26 1 1 1 2 26 2 1 2 1 1 2 2 2 1 2 1 2 1 1 2 1 2 1 3 1 3 11 Edge 7 DC 12 Edge 102+4 102+4 102+4 102+4 102+4 102+4 102+4 102+4 26 26 26 1 26 1 1 1 1 1 1 2 1 1 1 1 2 1 1 3 1 3 11 Edge 7 DC 12 Edge 242 242 242 242 12 Edge 996 usable tones +5 DC 11 Edge 80 MHz Tone Plan 17
Downlink (DL) Procedure STA-1 STA-2 STA-3 STA-4 H AP BA STA-1 BA STA-2 BA STA-3 Time STA BA STA-4 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. 18
Uplink (UL) Procedure BA STA-1 BA STA-2 BA STA-3 AP Trigger BA STA-4 STA-1 STA-2 STA-3 STA-4 Time STA H 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 19
Uplink (UL) Procedure AP Multi-STA BA Trigger STA-1 STA-2 STA-3 STA-4 Time STA H 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 20
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 21
Trigger Frame Variants Variant Trigger Dependent Common Info Trigger Dependent User Info Basic Trigger Not present MPDU MU Spacing TID Aggregation Limit Preferred AC AC Preference Level Beamforming Report Poll Not Present Feedback Segment Retransmission Bitmap MU-BAR Variant Not present BAR Information BAR Control MU-RTS Variant Not present Not present BSRP Variant Not present Not Present GCR MU-BAR Variant Not present BAR Information BAR Control BQRP Variant Not Present Not present 22
Spatial Reuse: The Concept OBSS signal@-82dBm OBSS signal@-72dBm CCA idle CCA busy A Intra-BSS OBSS CCA busy->idle; duration decoding correctly B CCA busy->idle; duration decoding error 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. 23
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 24
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: 25
CCA Sensitivity The 802.11ax hasn t changed the CCA levels on the primary 20 MHz 26
CCA Sensitivity The 802.11ax accounts for the introduction of the new parameter OBSS_Pdlevel o 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. o 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)). o 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. o 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)). 27
Operation in the 6 GHz Band- Channelization U-NII-5 U-NII-6 U-NII-7 U-NII-8 5945 5965 5985 6005 6025 6045 6065 6085 6105 6125 6145 6165 6185 6205 6225 6245 6065 6285 6305 6325 6345 6365 6385 6405 6425 6445 6465 6485 6505 6525 6545 6565 6585 6605 6625 6645 6665 6685 6705 6725 6745 6765 6785 6805 6825 6845 6865 6885 6905 6925 6945 6965 6985 7005 7025 7045 7065 7085 7105 7125 189 193 197 201 205 209 213 217 221 225 229 233 237 241 245 249 213 257 261 265 269 273 277 281 285 289 293 297 301 305 309 313 317 321 325 329 333 337 341 345 349 353 357 361 365 369 373 377 381 385 389 393 397 401 405 409 413 417 421 191 199 207 215 223 231 239 247 215 263 271 279 287 295 303 311 319 327 335 343 351 359 367 375 383 391 399 407 415 195 211 227 243 259 275 291 307 323 339 355 371 387 403 203 235 267 299 331 363 395 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 28
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). 29
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 30
