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About 802.11ac Wave 2

In the second half of 2015, chip manufacturers such as Qualcomm and Broadcom released 802.11ac Wave 2 chips. WLAN and terminal vendors quickly followed their footsteps and released 802.11ac Wave 2 products and terminals. 802.11ac Wave 2 was put into commercial use at the end of 2015, indicating a new development stage of 802.11ac.

Since 1997, the Institute of Electrical and Electronics Engineers (IEEE) has launched five generations of Wi-Fi standards/protocols, which has continuously upgraded Wi-Fi technologies and improved Wi-Fi performance.

From 802.11 to 802.11n, the theoretical data transmission rate has increased from 2 Mbit/s, to 11 Mbit/s, 54 Mbit/s, and then 450 Mbit/s. In 802.11ac Wave 1, the theoretical data transmission rate reaches up to 1.3 Gbit/s, 650 times that in 802.11. However, the actual throughput is unsatisfactory. When only one user connects to an 802.11ac Wave 1 AP (maximum transmission rate: 1.3 Gbit/s), the network efficiency (ratio of the actual throughput to the theoretical transmission rate) is about 70% (910 Mbit/s throughput). As more users are connected, the air interface cost is higher and AP throughput decreases. When 20 users connect to the AP, the network efficiency is reduced to 38% (350 Mbit/s throughput).

Figure 1. Large Increase of the Data Transmission Rate in 802.11ac Wave 1, but the Concurrent User Access Capability Becomes a Bottleneck

Concurrent user processing capabilities instead of the maximum transmission rate becomes the bottleneck of Wi-Fi technologies. After Multi-User Multiple-Input Multiple-Output (MU-MIMO) technology was introduced to 802.11ac Wave 2, the bottleneck was broken.

MU-MIMO uses explicit beamforming technology to control signal transmission and receiving. The difference between the Single User Multiple-Input Multiple-Output (SU-MIMO) and MU-MIMO mode is similar to that of an Ethernet switch and hub. A hub can send data to only one port, while a switch can send data to multiple ports. Similarly, in SU-MIMO mode, an AP can send data to only one STA; in MU-MIMO mode, an AP can send data to four STAs concurrently.

After MU-MIMO is introduced, spatial streams of an AP can be flexibly distributed to multiple STAs for data transmission. This solves the problem that APs and STAs support different numbers of spatial streams, and improves AP performance. For example, three Xiaomi Note mobile phones (advanced edition, with a single spatial stream) supporting MU-MIMO are connected to an 802.11ac Wave 2 AP. In SU-MIMO mode, the average download rate is 330 Mbit/s; in MU-MIMO mode, the average download rate reaches up to 800 Mbit/s. The AP performance is improved by 240% after MU-MIMO is enabled on the AP.

Usually, 802.11n or 802.11ac Wave 1 APs support three spatial streams. However, due to limitations of dimensions and chip costs, mobile phones and tablets are usually designed with only one antenna and one spatial stream. Therefore, APs can only work in one spatial stream mode to communicate with these mobile phones and tablets. Because APs and STAs have different numbers of spatial streams, APs usually work with a light load, resulting in low performance. 802.11ac Wave 2 can use MU-MIMO technology to improve concurrent user processing capabilities, as well as AP performance and efficiency.

Table 1. Comparison between 802.11n, 802.11ac Wave 1, and 802.11ac Wave 2

Introducing New Technologies to Comprehensively Improve Performance

In addition to MU-MIMO, 802.11ac Wave 2 also introduces a more efficient signal modulation mode and supports higher channel bandwidth. The overall performance is improved to 3.47 Gbit/s from 1.3 Gbit/s in 802.11ac Wave 1.

802.11ac Wave 2 Supports 160 MHz Channel Bandwidth

802.11ac Wave 2 supports 160 MHz channel bandwidth. Flexible channel combination has high requirements on radio spectrum resources.

  • 160 MHz channels in the 5 GHz band are scarce resources. 802.11ac Wave 2 provides a higher theoretical transmission rate of up to 866 Mbit/s per spatial stream in 160 MHz mode. However, there are only a few available 160 MHz channels and the support for 160 MHz channel bandwidth has little significance in practical use.

In North America where abundant channel resources are provided, there are four channel ranges in the 5 GHz band: Unlicensed National Information Infrastructure 1 (U-NII1) (channels 36-48), U-NII2 (channels 52-64), U-NII2 extended (channels 100-144), and U-NII3/Industrial Scientific Medical Band (ISM) (channels 149-165). There are two contiguous 160 MHz channels (six 80 MHz channels). For details, see Figure 2.

Figure 2. Spectrum Resources in the 5 GHz Band

In China, the Ministry of Industry and Information Technology (MIIT) opened up channels 36-64 ranging from 5150 MHz to 5350 MHz in 2012 for use in indoor environments. After that, the number of non-overlapping 20 MHz channels in the 5 GHz band is increased to 13 from 5 (channels 149-165) ranging from 5735 MHz to 5835 MHz. However, only one contiguous 160 MHz channel (three 80 MHz channels) is available.

  • Deploying 160 MHz channels is difficult. 80 MHz and 40 MHz channels are more suitable. There are only a few 160 MHz channels in the 5 GHz band, so 160 MHz channels are not recommended, unless a network is planned in an isolated wireless environment (hotspot).

80 MHz channels can be deployed in offices, lecture halls, and auditoriums. Wireless networks in such areas are built by customers themselves and wireless environments are comparatively controllable. When more than three 80 MHz channels are deployed, they can provide continuous coverage in a large area through frequency reuse and effectively prevent signal interference.

Deploying 40 MHz channels is expected to become a mainstream choice. For example, multiple WLANs (built by carriers or third parties) exist in public areas such as commercial centers, train stations, airport waiting rooms, and exhibition centers. In these areas, wireless environments are complicated, inter-channel interference will occur if limited 80 MHz channels are deployed. As a result, network access becomes slow or even WLANs become unavailable. Therefore, 40 MHz channels are recommended during network planning to prevent signal interference between WLAN systems.

802.11ac Wave 2 Uses 256-Quadrature Amplitude Modulation (QAM)

802.11ac Wave 2 supports 256-QAM (8 bits) technology, which has 33% ([8-6] bit/6 bits)/6 bits = 33%) higher modulation efficiency than 64-QAM (6 bits) in 802.11n. 

In 40 MHz mode, 802.11 Wave 2 improves transmission rate per spatial stream to 200 Mbit/s from 150 Mbit/s in 802.11n. In 80 MHz mode, 802.11 Wave 2 improves transmission rate per spatial stream to 433 Mbit/s.

802.11ac Wave 2 Supports Four Spatial Streams

802.11ac Wave 2 supports four spatial streams, while 802.11ac Wave 1 and 802.11n support only three. 802.11ac Wave 2 increases the maximum transmission rate to 1.73 Gbit/s in 80 MHz mode and 3.47 Gbit/s in 160 MHz mode. 

Additionally, in 802.11ac Wave 2, four transmit and receive antennas are used, bringing high diversity gains. 802.11ac Wave 2 improves wireless transmission reliability and fault tolerance capabilities, increases signal coverage and quality, and enhances transmission performance.

802.11ac Wave 2 Deployment Suggestions

802.11ac Wave 2 applies to high-density scenarios where network access services are mainly provided to mobile terminals with a small number of spatial streams, such as mobile phones, ebooks, and tablets, such as airports, train stations, shopping malls, conference halls, and exhibition centers. 802.11ac Wave 2 can provide higher concurrent user processing capabilities, which is its true value.

802.11ac Wave 2 FAQ

Q1: What are MIMO and spatial stream?

Answer: Multiple-Input Multiple-Output (MIMO) technology uses multiple transmit and receive antennas to send and receive radio signals simultaneously to improve communication and transmission capacity. Space Diversity (SD) technology enables a receiver to receive data signals from multiple antennas, thereby improving data reception reliability and communication quality. Spatial multiplexing technology enables a sender to map serial data to parallel data and send the data from different antennas. In this way, the communication capacity is improved without increasing spectrum resources. Parallel spatial channels are called spatial streams. Generally, the number of spatial streams of a product is the same as that of antennas.

Q2: How is the transmission rate in 802.11ac computed?

Answer: Data transmission rate of Wi-Fi networks has improved continuously since Wi-Fi invention. However, transmission time of each carrier is always 4 μs. Using 802.11a/g as an example, when channel bandwidth is 20 MHz (48 data subcarriers), 64-QAM (6 bits) is used, and coding efficiency is 3/4, the data transmission rate is 54 Mbit/s (1s/4 μs x 6 bits x 48 x 3/4 = 54 Mbit/s). 802.11n optimizes modulation and coding technologies, and introduces channel bonding, Short Guard Interval (Short GI), and MIMO technologies. These technologies help improve the data transmission rate per spatial stream to 150 Mbit/s.

802.11ac uses 256-QAM and improves the data transmission rate per spatial stream to 200 Mbit/s in 40 MHz mode. In 80 MHz mode, the number of data subcarriers increases to 234 (2.16 times that in 40 MHz mode). In this case, the data transmission rate per spatial stream is improved to 433 Mbit/s (200 Mbit/s x 2.16 = 433 Mbit/s). In 160 MHz mode, the data transmission rate per spatial stream is improved to 866 Mbit/s.

Q3: What is explicit beamforming?

Answer: Explicit beamforming works as follows: An AP sends a channel estimation frame to a STA, and the STA then sends information required by the channel estimation frame back to the AP. Based on the information, the AP then estimates the downlink channel, processes downstream data based on the estimation result, and then sends the data. This function increases signal transmission quality. Explicit beamforming is an optional mode in 802.11n and has no unified implementation mechanism, which may result in incompatibility between devices with chips from different vendors. In 802.11ac Wave 2, explicit beamforming is mandatory and has a unified implementation mechanism. This makes interoperability possible between devices with chips from different vendors.

Q4: Is 802.11ac Wave 2 compatible with 802.11ac Wave 1 and other 5G STAs?

Answer: 802.11ac Wave 2 is backward compatible with 802.11ac Wave 1 and 802.11n terminals. Inventory terminals cannot support new Wave 2 features such as MU-MIMO, 256-QAM, and 160 MHz channel bandwidth. An 802.11ac Wave 2 AP can use four transmit and receive antennas to provide better coverage and improve overall performance of the terminals.

Q5: Are there any requirements on STAs for MIMO implementation? 

Answer: Implementation of MU-MIMO requires that both APs and STAs support MU-MIMO. Currently, 802.11ac Wave 2 mobile phones and laptops include Huawei Mate 8/P9, Xiaomi MI4 series/MI Note Pro, LeTV eMAX/LePro1, Google Nexus 5X, Microsoft Lumia 950/950XL, HTC One M8, ZTE Nubia Z9 series/Axon 7, Acer Aspire series, and Dell Allienware13/15/17.

Q6: Does MU-MIMO work in downstream direction only?

Answer: Yes. Currently, MU-MIMO implements spatial stream distribution based on downstream traffic, which matches the data model of networks where most traffic flows in the downstream direction. Spatial stream distribution based on upstream traffic is considered in the updated version of 802.11ac — 802.11ax. In 802.11ax, an AP can communicate with multiple end users in bilateral directions. It is expected that 802.11ax will be launched in about 2019.

Q7: Can 802.11ac Wave 1 products be software upgraded to support 802.11ac Wave 2?

Answer: No. 802.11ac Wave 2 requires hardware support. Therefore, 802.11ac Wave 1 products cannot be software upgraded to support 802.11ac Wave 2.

By Wei Yupeng and Ou Liyun