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OFDMA spectrum plot

OFDMA spectrum plot
Credit: Gjermund Raaen

I feel like I've been on hold for the past year.

As 2018 drew to a close, the collapse of advertising support caused me to shift gears into paid consulting. While this has been better for me financially, the primary focus on consulting work has meant virtually no product reviews have been posted on SmallNetBuilder in 2019.

I've really, really been trying to change that. But I continue to run into obstacles while trying to develop new Wi-Fi test methods to move beyond the focus on speed tests to measure the quality, robustness and efficiency of Wi-Fi products.

After finishing the automation of the TR-398 Wi-Fi Performance Test suite on the octoScope platform, I've been mainly trying to demonstrate that OFDMA, a key Wi-Fi 6 feature, can produce higher total throughput. TL;DR: I have seen higher total throughput with OFDMA enabled, but gains are highly variable.

OFDMA is beginning to look at lot like MU-MIMO.

As I noted in Wi-Fi 6 Performance Roundup: Five Routers Tested, one of the chief obstacles in the quest to test OFDMA has been the fact that most of the consumer Wi-Fi 6 products on the market to date have not had OFDMA enabled. Time and time again, I was told that new firmware enabling OFDMA would be released in a month or so, only to have that month come and go without new firmware. It's kinda hard to develop test methods when you don't have something to test.

As of end of 2019, there are four Wi-Fi 6 routers with OFDMA enabled that you can buy in the U.S., shown in Table 1. This is out of almost two dozen Wi-Fi 6 routers or mesh systems announced.

Product Wi-Fi 6 Certified Platform OFDMA Verified Notes
2.4 GHz
2.4 GHz
5 GHz
5 GHz
ASUS RT-AX88U Yes Broadcom No No Yes Yes Yes See below
NETGEAR RAX120 No Qualcomm Yes No Yes No Yes Chipset not capable of UL OFDMA
Amplifi Alien No Qualcomm Yes Yes Yes Yes No Likely uses new Qualcomm chip version
Linksys MX5300 (Velop AX) Yes Qualcomm Yes Yes Yes Yes No Likely uses new Qualcomm chip version
Buffalo AirStation WXR-5950AX12 Yes Qualcomm? Yes Yes Yes Yes No Japan only. Likely uses new Qualcomm chip version
Buffalo AirStation WXR-5950AX12R Yes Qualcomm? Yes Yes Yes Yes No Japan only. Likely uses new Qualcomm chip version
Table 1: OFDMA support summary

The Verified column in Table 1 means I've seen the product actually transmit HE-MU data frames, which are required for OFDMA operation. For the others, you'll have to take the manufacturer's word for it, which I would not recommend. Because Wi-Fi 6 Certification, which requires supporting Downlink (DL) and Uplink (UL) OFDMA on both bands, isn't a guarantee you'll actually get OFDMA in the router you buy.

Case in point, the ASUS RT-AX88U. ASUS started by releasing a beta firmware back around August, which enabled DL OFDMA in 5 GHz only. Then, in November, ASUS announced the router was now Wi-Fi 6 Certified. When updated firmware was posted a week or so later, however, there was no OFDMA enable for the 2.4 GHz radio. ASUS explained this was due to compatibility problems with "legacy" devices.

So Wi-Fi 6 Certification doesn't guarantee that OFDMA will be fully enabled, either. And key features like 160 MHz channel width, DL (AX) MU-MIMO, Target Wake Time (TWT) and many OFDMA-related features are optional Wi-Fi 6 Certifications, as indicated by the search filters shown below in the Wi-Fi CERTIFIED product finder.

Wi-Fi 6 Cerficiation option

Wi-Fi 6 Cerficiation options

What's The Holdup?

So here we are moving into year two of the Great Transition to Wi-Fi 6 with high-priced, top-of-line routers still shipping without support (or guarantee of support) for key Wi-Fi 6 features. The story has been different on the client side, however, where there has been no lack of OFDMA-enabled devices looking for APs and routers to work with.

The Samsung Galaxy S10 line started to ship shortly after Wi-Fi 6 routers hit store shelves. And Intel had its AX200 M.2 format card ready for laptops. Hell, even Apple released the AX-enabled iPhone 11 in a relatively timely manner, although about a year behind Samsung.

All these devices support OFDMA and DL AX MU-MIMO as near as I can tell, looking at packet captures. So the obstacle is clearly on the router / AP side. Why?

The answer is that the folks writing the code for the AP airtime scheduler have a hellaciously difficult job. In the old days, all the scheduler had to do was send out packets using simple methods like round-robin. Then came packet aggregation in 11n and even more in 11ac, which meant packets didn't always get sent out right away, but were queued and combined into larger frames. AC MU-MIMO complicated things even further, requiring decisions on when and how to hold packets and transmit them simultaneously using MU-MIMO beamforming.

Now with AX, the scheduler has all the previous methods at hand, plus OFDMA. OFDMA is another simultaneous-transmission method like MU-MIMO. But instead of using beamforming techniques, which stagger signals in the time domain (phase angle), OFDMA operates in the frequency domain, divving up a single 20, 40, 80 or 160 MHz channel into sub-channels, called RUs (Resource Units). Each of these RUs can be assigned to a AX device (STA), usually when it associates.

To complicate the scheduler's task even further, OFDMA also works in the uplink direction. So the scheduler must also decide if it wants to receive data simultaneously from multiple STAs. UL OFDMA requires precise synchronization between AP and STA, which, of course, isn't easy. Whether it works reliably enough in the real world to provide the higher bandwidth efficiency and lower latency benefits that OFDMA promises has yet to be determined.

Of course, all these decisions must be made for each transmit opportunity (TXOP). As I said, this is complex stuff.

A Little More About RUs

Since RUs are at the heart of OFDMA, they merit a closer look. RUs come in six different channel widths, with each width supporting a maximum data / link rate per stream. The table below, taken from the IEEE 802.11ax draft spec, shows the maximum number of RUs for each channel width.

20 MHz Bandwidth RU locations

20 MHz Bandwidth RU locations (from IEEE draft 802.11ax)

The next diagram shows how a 20 MHz wide channel can be split into different RU widths. The wider the RU, the greater its data / link rate and the fewer available. In other words, if an AP wants to cram more STAs into an OFDMA frame, each one gets a smaller data rate.

Note that all RUs don't have to be the same. Assuming 20 MHz channel width, a frame could have four STAs using 26-tone RUs and one STA using a 106-tone RU. This assignment can be dynamic, so the next frame could have all five STAs using 26-tone RUs. The AP keeps track of which STA gets what RU by assigning each STA an AID (Association ID) when it associates.

20 MHz Bandwidth RU locations

20 MHz Bandwidth RU locations (from IEEE draft 802.11ax)

Note that an RU's width doesn't change with Wi-Fi channel width. All that happens with wider channels is that more RUs are supported, as shown in the next diagram. Wider channels also bring two more RU widths; the 484-tone RU is only available with 40 and 80 MHz channel widths and the highest value 996-tone RU requires an 80 MHz wide channel.

RUs in varying channel widths

RUs in varying channel widths (from IEEE draft 802.11ax)

What can increase an RU's throughput is the number of spatial streams. Since most STAs are two-stream, let's look at the MCS table (which shows data / link rates for our ol' pal the 26-tone RU, with two streams. For one stream, divide the data rate values in half.

26-tone RU, 2 stream MCS table (from IEEE draft 802.11ax)

26-tone RU, 2 stream MCS table (from IEEE draft 802.11ax)

This table brings home the reality of the tradeoffs inherent in OFDMA. To support the highest number of STAs in a single frame, the highest data rate each could get is just shy of 30 Mbps. But since this is an MCS table, that number really is link rate, with actual delivered data rate being lower. Keep this in mind the next time you see claims that 11ax can support hundreds of simultaneous clients.

So what data rate does the single 996-tone RU supported in a two-stream 80 MHz channel provide? Table 27-97 (not shown) yields 1201 Mbps. This is also the maximum data rate supported in an AX non-OFDMA 80 MHz channel.

For one final example, let's go back to the maximum RU per channel width table to see what we could get using four two-stream OFDMA STAs in an 80 MHz channel. Table 27-6 above says we could use 242-tone RUs and the Table 27-81 below says each of our four STAs would get at best 287 Mbps data rate. Not bad, but not the 1201 Mbps a single two-stream 80 MHz bandwidth STA would get. It's also worth noting that 4 x 287 = 1148. So the benefit of getting four STAs into one transmit frame incurs a 4% data rate loss overhead.

242-tone RU, 2 stream MCS table (from IEEE draft 802.11ax)

242-tone RU, 2 stream MCS table (from IEEE draft 802.11ax)

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