Get Your Kicks with WiFi 6

Image Source: Panchenko Vladimir/Shutterstock.com

By Marcel Consée, Mouser Electronics

Published September 1, 2021

For the first time in the history of wireless LAN (802.11), the new ax version doesn’t focus on higher client data rates but infrastructure efficiency. WiFi 6 allows for central coordination of the radio cell and introduces OFDMA and Coloring—functions that are supposed to raise the speed in systems with many clients working simultaneously. A higher data rate is only the byproduct.

The original standard IEEE802.11 from 1997 with a maximum data rate of 2Mbit/s treated each data packet equally. The same was valid for 802.11b (11Mbit/s), 802.11g (54Mbit/s at 2.4GHz) and 802.11a (5GHz). Every access point (AP) could only send one packet per transmission window (TX Opportunity, TXOP). In a/g products, OFDM (Orthogonal Frequency-Division Multiplexing) was introduced. OFDM splits every available frequency block into many sub-carriers. Data streams are modulated as so-called Symbols (time-limited signals) onto the sub-carriers.

802.11n (600Mbit/s) introduced MIMO antennas (Multiple Input Multiple Output). Combining signals of multiple antennas allows the system to transmit multiple data streams, multiplying the capacity of each channel. The device with a smaller number of antennas determines the upper limit. This means a two-stream client receives a maximum of 300Mbit/s, even if the four-stream AP can transmit at 600Mbit/s.

The standard 802.11ac (6900Mbit/s at eight streams) made use of this situation. A base station with many antennas can serve multiple receiving clients with fewer antennas—DLMU-MIMO (Downlink Multi-User MIMO). An AP could ideally transmit to two clients at 300Mbit/s each, summarized 600Mbit/s. In this standard, one Symbol is 3.6µs long and carries up to 8-Bits using 8-Bit Quadrature Amplitude Modulation 256-QAM. With a 160MHz frequency block divided into 468 sub-carriers using an error-tolerant coding with a 5/6 rate, the resulting data stream per antenna is (468*8Bit*5/6)/3.6µs = 866Mbit/s.

IEEE802.11ax uses a higher modulation with 10 Bit per Symbol (1024-QAM), raising the data rate by 25 percent over ac. More gain results from the use of fewer empty carriers and a longer Symbol duration of 13.6µs, allowing for a division into 1960 sub-carriers. The maximum gross data rate per antenna is 1201 Mbit/s. This is a theoretical maximum that can hardly be reached under real-world conditions because Wireless LAN does not make much sense when the client device has to be placed directly beside the AP.

Reducing Overhead

Up to and including 802.11ac, one data packet always occupies the entire radio channel. Because each packet starts with a header or preamble, the relation between durations of the header and the actual data degrades with rising transmission speed. The preambles can’t get much shorter, so it makes sense to transmit less of them for higher efficiency. The ax standard imposes that one preamble is sufficient if multiple data packets are sent simultaneously in different areas of the radio channel. If this is implemented correctly, the ratio of payload to overhead improves significantly. A base station using OFDMA divides the radio channel into Resource Units (RU) of 2, 4, 8, 20, 40, and 80MHz. A 20MHz channel can be subdivided, for example, so that the AP simultaneously transmits to five clients for 1ms. One station pulling a large download can then be assigned an 8MHz RU, another a 4MHz RU for its video stream, and the others each get a 2MHz RU. The remaining 2MHz are used for protection ratios.

On the receiving end, something similar happens: Stations that receive data in parallel via OFDMA are supposed to acknowledge them simultaneously.

Centralized control

According to the standard, a WiFi 6 base should use OFDMA for transmitting and  receiving. The AP first collects information concerning the expected sending data rates of the clients and the data packets waiting in the stations. Then it sends trigger frames to the stations that assign them their RU. Thus, the Access Point coordinates access to the radio channel also while receiving.

This base station also designates Resource Units for random access, with the stations competing for them like in earlier WiFi standards. They wait for an arbitrary number of those RUs before transmitting (Backoff). Collisions are possible here, but the AP can lower the chance of them happening by limiting the random number. This type of random access is essential for two reasons:

• The stations inform the AP about the amount of data waiting. This allows for better scheduling,
• New stations need this access to log on.

Another advantage of OFDMA is the ability of the WiFi base station to transmit around, possibly interfering with neighboring frequencies. The smallest WiFi radio channels usually use 20MHz. Larger channels are twice, quadruple, or even eightfold of this, allowing for bigger capacity. As WiFi 5 typically uses an 80MHz channel, the AP must check whether four adjacent 20MHz blocks are free. If even one is blocked, the AP has to wait. The WiFi 6 base station, on the other hand, can bundle up the three free blocks.

Since 2019, the WiFi Alliance (WFA) certifies 802.11ax as Wi-Fi 6, and all devices that came out since then have already implemented the main functions in addition to WPA3 encryption.

One example is the Intel product family AX200/AX201. Those modules support 2x2 WiFi 6, including features such as UL and DL, OFDMA, and 1024QAM, delivering data rates of up to 2.4GBit/s with increased network capacity and BLUETOOTH® 5 support. These features significantly improve user experience in dense deployments, supporting fast uploads and downloads, lower latency, and longer battery life than solutions supporting 802.11ac. The AX201 adapter is a CRF (Companion RF) module. Combined with Intel Core processors, the WiFi 6 AX200/AX201 module can provide Gigabit wireless speed and improve the connected experience at home, work, or on the go.

Figure 1: The Intel AX200/AX201 Modules (Source: Mouser Electronics)

The module is also included in the Intel WiFi 6 (Gig+) Desktop Kit, with two optimized antennas and mounting brackets.

Qorvo offers WiFi 6 development tools,  such as the QPF4228 Evaluation Board operating in the 2412MHz to 2484MHz frequency range. The corresponding Front-End Modules QPF4228 integrate a 2.4GHz Power Amplifier (PA), regulator, Single Pole Three Throw (SP3T) Switch, Low Noise Amplifier (LNA), and coupler.

Figure 2: The QPF4228 Evaluation Board from Qorvo (Source: Mouser Electronics)

Dedicated WiFi 6 antennas are supplied by Linx Technologies. The ANT-W63-FPC-UFL-100 is a flexible embedded multi-band antenna for WiFi 6E applications in the 6GHz band. The flexibility and adhesive backing make the antenna easy to mount in unique and custom enclosures while enabling protection from tampering or accidental damage. This antenna provides a ground plane independent dipole embedded antenna solution compared with performance to an external antenna.

Figure 3: The ANT-W63-FPC-UFL-100 WiFi 6 Antenna by Linx Technologies (Source: Mouser Electronics)

It operates in a frequency range from 5925MHz to 7125MHz and connects with the radio via a 100mm long and 1.13mm coaxial cable terminated in an MHF1/U.FL-compatible plug connector. Applications include complete WiFi/WLAN coverage, Internet of Things (IoT) devices, smart home networking, and sensing and remote monitoring.

What’s next?

Not every manufacturer obeys the WFA certification in every detail. Some modules insist that the same hardware is at the other end of the communication, keeping some ax features reserved for them. Indeed, Wi-Fi6 is rather complex, especially in comparison to older versions of the standard. Nevertheless, IEEE is already working on future standards building on ax. To name one, 802.11be will introduce Distributed MIMO, optimized for IoT networks with many clients and multiple APs. The complexity will increase, and we will have to wait and see whether the advantages are worth the effort.

OFDM(A) and QAM

• Orthogonal Frequency-Division Multiplexing (OFDM) is a common way of encoding data on multiple carrier frequencies. It is used not only in WiFi but also in LTE and 5G networks and in wired communications such as DSL or Power Line.
• Orthogonal Frequency-Division Multiple Access (OFDMA) is OFDM for multiple clients. It assigns subsets of subcarriers to individual clients.
• Quadrature Amplitude Modulation (QAM) modulates the amplitudes of two carrier waves 90° out of phase. 256-QAM encodes 8 Bit per symbol, 1024-QAM 10 Bit per Symbol. The higher the order, the higher the necessary transmission power and the susceptibility to errors.