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802.11ax Introduction

With the demand for new usage models and more applications, dense Wi-Fi deployments and more outdoor and public access, the High Efficiency WLAN Study Group (HEW SG) within IEEE 802.11 working group had started its activity to consider the improvement of spectrum efficiency to enhance the system throughput/area in high density scenarios of APs and/or STAs around 3 years ago. And the HEW SG has successfully started the project 802.11ax in May 2014 to enhance 802.11 PHY and MAC in 2.4 and 5GHz.

As the successor of 802.11ax, the main focus of 802.11ax study in HEW SG is

• Improving spectrum efficiency and area throughput

• Improving real world performance in indoor and outdoor deployments

  • in the presence of interfering sources, dense heterogeneous networks

  • in moderate to heavy user loaded APs

The Key requirement for 802.11ax is to provide high scalability, which can be realized through

1. Efficient multi-user access. CSMA/CA MAC protocol offers a reasonable trade-off between performance, robustness and implementation cost. New scheme will be adopted.

2. Higher throughput. It arms for a 4x throughput increase compared with 802.11ac without more power consumption.

3. High reliability and limited delay for some usages.

These features are required by new applications such as interactive and high-definition video, and scenarios with high density of Wifi users such as stadium and public transportation.

To achieve the goals, some new technologies are adopted in the 802.11ax system. The system architecture and functionalities are designed to meet these requirements.

The key technologies include

1. Channel coding and framing, QAM and OFDM. Up to 11 Modulation and Coding Scheme (MCS) defined.

   1. BCC and LDPC coding for forward error correction.

   2. More PPDU formats defined to support multi-user case. The overhead for multi-user case would be higher compared to single user case.

   3. Up to 1024QAM to increase the data rates.

   4. OFDM with reduced sub-carrier spacing (4x symbol length of 802.11ac) and more variable CPs for flexibility to support different scenarios, especially long outdoor channel, considering tradeoff between efficiency and robustness.

2. Spatial reuse by dynamic power control and beamforming to reach an optimal tradeoff between individual transmission rates and the number of concurrent transmissions that maximize the area throughput.

3. Multi-user operation by OFDMA and SDMA.

   1. OFDMA, where multiple-access is achieved by assigning subsets of subcarriers to different users, allowing simultaneous data transmission by several users, and each group of subcarriers is denoted as a resource unit (RU). The RUs can be allocated to stations depending on their channel conditions and service requirements, and an OFDMA system can potentially allocate different transmit powers to different allocations.

      

   2. MU-MIMO. Both downlink and uplink MU-MIMO transmissions are supported on portions of the PPDU bandwidth (on resource units greater than or equal to 106 tones) and in an MU-MIMO resource unit, there is support for up to eight users with up to four space-time streams per user with the total number of space-time streams not exceeding eight.

   3. OFDMA+MU-MIMO, Potentially can be used.