Scale Wi-Fi to the IoT with embedded 802.11ax

July 16, 2018 Franz Dugand, CEVA

There’s a rush to “smarten” previously dumb appliances. In the smart home, we already see smart speakers, TVs, heating and A/C, security systems, video doorbells, and exercise machines. Where will it end? Probably a lot sooner than we think if we continue to depend on classic Wi-Fi, commonly 802.11n or 802.11ac. You may have had this experience: you stream a movie to the TV while your spouse is on the smart bike, the kids are browsing their phones or streaming another movie, and you want to turn down the A/C. Response slows to a crawl—not exactly what we pictured as our ideal smart home. The next generation of Wi-Fi, 802.11ax, will get us back on track, at least for those devices ready to support the standard.

[Figure 1 | As shown, the bandwidth for Wi-Fi has come a long way.]

Wi-Fi has been advancing steadily through a series of refinements at ever-increasing bandwidth, so what’s the problem? The issue is that the standard, as originally envisioned, never anticipated an access point (AP) needing to communicate simultaneously with the number of devices we now want to connect. The standard we commonly use assumes one active line of communication at a time; everyone else has to wait their turn. 802.11n improved bandwidth and data rate per station by supporting multi-input, multi-output (MIMO) antennae, but still only one station can talk to the AP at a time. When the number of devices you’re adding to your home network is rising faster than the bandwidth, those devices are going to be waiting/buffering a lot more commonly than you expected.

The problem is congestion. Even though there’s available bandwidth in any given transmission, only one station/AP link can be serviced at a time. Multi-user MIMO (MU-MIMO) improves this by dividing up MIMO operation between multiple users, for the first time allowing for simultaneous transmissions between multiple stations and the AP. The first generation of this capability appeared in the 802.11ac (Wave2) standard, supporting multiple users for downlink (but not uplink), allowing for up to four simultaneous data streams. That’s pretty good—you can support four devices talking at the same time without degradation, but most smart homes already have more than four devices. And uplink is still a problem.

Enter 802.11ax, a big step forward, even over 11ac. First, 11ax supports up to eight simultaneous data streams, doubling capacity. Then it borrows a trick from LTE (called OFDMA), dividing up the bandwidth within each stream into multiple sub-channels, each of which can be used to support a different station. And 11ax is symmetric; the uplink performance and efficiency are just as good as the downlink. In throughput alone, 11ax starts faster than 11ac and will be significantly less congested in environments with many active endpoints.

Another plus for battery-powered devices is lower-power operation than you’ll find in 11n. 11ax provides a feature called Target Wake Time (TWT) allowing an endpoint to go into deep sleep and wake up at a scheduled time rather than burning power on unnecessary listening. The standard also offers better co-existence with Bluetooth and 802.15.4 thanks to use of narrower sub-channels. And for those who were thinking of designing around the 11ah (HaLow) standard, market trends suggest that uptake for 11ah has been weak and is declining as most users with needs in that class are moving to 11ax for its greater flexibility.

Now, you’re planning your hot new home automation device, you want Wi-Fi connectivity, but you really don’t want the user experience compromised by congestion caused by other devices you don’t control. You should build for 11ax support (which incidentally is backward compatible). But you’re constrained by cost and power, so you really want a single-chip solution. And your design team is expert in your application, not in wireless standards and design. You need a turnkey 11ax solution.

CEVA supports Wi-Fi with its RivieraWaves family, with a considerable lead in licensable Wi-Fi, according to the Linley report. Solutions span from low power and cost up to Multi-Gig rates, all built as hardwired implementations on CEVA’s XC DSP platform, now also offering 11ax support across this entire range.

[Figure 2 | A single-chip implementation.]

The compact solution can be targeted to a single-chip turnkey Wi-Fi interface, allowing you to adapt the software protocol stack locally or on a host processor, if you need that flexibility. For cost-sensitive applications, the RISC-V CPU can manage both the full protocol stack and microcontroller-class application software, avoiding the need to add a third-party MCU. For more complex applications, the RivieraWaves solution works equally well as a turnkey Wi-Fi solution, embedded alongside a higher-performance application processor, memories and other IP.

Franz Dugand is a Director for the Connectivity Business Unit at CEVA.

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