WiFi-6技术详解：IEEE 802.11ax标准规范
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"IEEE Standard for 802.11ax - Part11 WLAN MAC & PHY Specification，2021年版本，是国际通信协会（IEEE）制定的WiFi-6技术的原版协议规范，适用于本地和城域网络的无线局域网介质访问控制（MAC）和物理层（PHY）的特定要求。此修订版（Amendment 1）旨在提升高效率WLAN的功能。"
802.11ax是IEEE 802.11标准的最新版本，通常被称为WiFi-6，是第六代无线局域网技术，它带来了显著的性能提升和效率优化。这一标准的目标是在拥挤的无线环境中提供更高的数据速率、更低的延迟以及更高效的频谱利用率。
1. **高密度接入**：802.11ax引入了一种名为Orthogonal Frequency Division Multiple Access (OFDMA)的技术，允许多个设备同时在相同信道上通信，从而提高了网络的容量和效率。这在高用户密度的场景，如体育馆、机场或办公区，尤其有用。
2. **多用户MIMO**：除了OFDMA，802.11ax也扩展了Multi-User Multiple Input Multiple Output (MU-MIMO)功能，不仅支持下行链路，还支持上行链路的多用户传输，使得路由器可以同时与多个设备进行双向通信。
3. **1024-QAM调制**：与前代的256-QAM相比，802.11ax使用了更高的调制阶数1024-QAM，这使得每个信号载波可以携带更多信息，从而提高了数据传输速率。
4. **Target Wake Time (TWT)**：802.11ax引入了TWT机制，允许设备与接入点协商唤醒时间，减少了不必要的能量消耗，对电池寿命敏感的移动设备特别有益。
5. **基本服务集（BSS）颜色**：为了更好地区分和管理多个并存的无线网络，802.11ax引入了BSS颜色，这有助于降低同频干扰。
6. **性能提升**：802.11ax标准的目标是在相同频谱资源下，比802.11ac实现至少四倍的数据速率提升，并且在密集部署环境下，每个用户的平均吞吐量也有显著提升。
7. **向下兼容性**：802.11ax标准设计时考虑了向后兼容，能够与早期的802.11a/b/g/n/ac标准设备无缝协作。
IEEE 802.11ax标准通过技术创新提升了无线网络的性能和效率，为用户提供了更快、更稳定的上网体验，同时也优化了网络资源的管理和使用。对于网络工程师、IT专业人员和设备制造商而言，深入理解这一标准对于设计、部署和维护现代无线网络至关重要。
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10.6.5 Rate selection for Data and Management frames ...................................................... 244
10.6.5.1 Rate selection for non-STBC Beacon and non-STBC PSMP frames...... 244
10.6.5.3 Rate selection for other group addressed Data and Management frames 245
10.6.6 Rate selection for Control frames .............................................................................. 245
10.6.6.1 General rules for rate selection for Control frames ................................. 245
10.6.6.6 Channel Width selection for Control frames ........................................... 245
10.6.10 Modulation classes..................................................................................................... 246
10.6.11 Non-HT basic rate calculation ................................................................................... 247
10.6.12 Channel Width in non-HT and non-HT duplicate PPDUs ........................................ 248
10.6.13 Rate selection constraints for VHT STAs.................................................................. 248
10.6.13.3 Additional rate selection constraints for VHT PPDUs ............................ 248
10.8 HT Control field operation ...................................................................................................... 249
10.9 Control Wrapper operation ...................................................................................................... 251
10.11 A-MSDU operation.................................................................................................................. 251
10.12 A-MPDU operation.................................................................................................................. 252
10.12.1 A-MPDU contents ..................................................................................................... 252
10.12.2 A-MPDU length limit rules ....................................................................................... 252
10.12.3 Minimum MPDU Sstart Sspacing field rules ............................................................ 254
10.12.4 A-MPDU aggregation of group addressed Data frames ............................................ 254
10.12.6 A-MPDU padding for VHT PPDU, HE, or S1G PPDU............................................ 256
10.12.7 Setting the EOF/Tag field of the MPDU delimiter.................................................... 256
10.13 PPDU duration constraint ........................................................................................................ 256
10.15 Low-density parity check code (LDPC) operation .................................................................. 256
10.22 Operation across regulatory domains ...................................................................................... 257
10.22.3 Operation with operating classes ............................................................................... 257
10.22.4 Operation with the Transmit Power Envelope element ............................................. 258
10.23 HCF.......................................................................................................................................... 259
10.23.1 General....................................................................................................................... 259
10.23.2 HCF contention based channel access (EDCA) ........................................................ 259
10.23.2.2 EDCA backoff procedure ........................................................................ 259
10.23.2.4 Obtaining an EDCA TXOP ..................................................................... 260
10.23.2.5 EDCA channel access in a VHT, HE, or TVHT BSS ............................. 261
10.23.2.7 Sharing an EDCA TXOP......................................................................... 262
10.23.2.8 Multiple frame transmission in an EDCA TXOP.................................... 263
10.23.2.9 TXOP limits............................................................................................. 265
10.23.2.10 Truncation of TXOP ................................................................................ 267
10.23.2.11 Termination of TXOP.............................................................................. 267
10.23.2.12 Retransmit procedures ............................................................................. 267
10.23.3 HCF controlled channel access (HCCA) ................................................................... 268
10.23.3.5 HCCA transfer rules ................................................................................ 268
10.25 Block acknowledgment (block ack) ........................................................................................ 268
10.25.2 Setup and modification of the block ack parameters................................................. 268
10.25.5 Selection of BlockAck and BlockAckReq variants................................................... 268
10.25.6 HT-immediate block ack extensions.......................................................................... 269
10.25.6.1 Introduction to HT-immediate block ack extensions............................... 269
10.25.6.3 Scoreboard context control during full-state operation ........................... 270
10.25.6.5 Generation and transmission of BlockAck frames by an HT STA,
DMG STA, or S1G STA.......................................................................... 270
10.25.6.6 Receive reordering buffer control operation............................................ 270
10.25.8 GCR and GLK-GCR block ack ................................................................................. 271
10.25.8.1 Introduction General ................................................................................ 271
10.25.8.4 GCR block ack BlockAckReq and BlockAck frame exchanges ............. 271
10.27 Protection mechanisms ............................................................................................................ 273
10.27.6 Protection rules for HE STAs .................................................................................... 273
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