In a previous blog post, we covered five key things
to know about 802.11ax, the next big standard for Wi-Fi. Let’s
examine some of the challenges that RF engineers will face when designing
for 802.11ax and some tips on how to overcome them.
Some background: 5 OFDMA PPDU
formats for 802.11ax
But first, let’s look at the foundational signal structure for
802.11ax — the physical layer protocol data units that Wi-Fi clients
and devices use to communicate.
802.11ax uses five formats for its OFDMA PPDU:
- Single User (HE-SU). For transmitting data from a
- Multi User (HE-MU). For transmitting data to one or
more users that isn’t in response to a trigger frame.
- Outdoor (HE-xSU). For outdoor transmission for a
single user. This format is new in 802.11ax.
- Trigger Response (HE-TRIG). For transmitting data in
response to a trigger frame. Used to coordinate uplink MU-MIMO or uplink
OFDMA transmissions with the access point.
Downlink Channel Sounding (HE-NDP). For beamforming and downlink
See the image at the end of this blog post for details of the frame
packets and fields within each PPDU format.
Glossary of Terms
Error vector magnitude
- HE: High efficiency
- HEW: High-Efficiency WLAN (or High-Efficiency
- MU-MIMO: Multi-user multiple input/multiple
- NDP: Null data packet
frequency-division multiple access
- PLCP: Physical Layer Convergence Procedure
- PPDU: PLCP Protocol Data Unit
- TWT: Target wait time
Wait or sleep times: What are the
challenges for the RF front end?
One thing 802.11ax adds is target wait time (TWT) — also known as
sleep times — which allows a device to stay in a sleep state longer
before transmitting data. This resource scheduling improves battery life and
means a better experience for a consumer.
However, latency in turn-on mode could be an underlying challenge. TWT also
brings the following:
High susceptibility to frequency and clock offsets in
OFDMA. Unlike LTE base station technologies, 802.11ax doesn’t have a
synchronized clock signal. As a result, devices will rely on the access point to keep all the
devices on the network synchronized. Additionally, 11ax uses longer OFDM
symbols than 11ac, which means more data comes through. In short, the
access point will have to work harder — and be more accurate —
than in the past.
- Flatness maintained over a longer time period. The
specs we’ve received from some of our chipset partners show that the
initial power amplifier (PA) turn-on time has not changed in 802.11ax;
it’s still 200-400 nanoseconds. However, the gain flatness has been extended, guaranteeing
the front-end module (FEM) has no gain expansion or gain droop for the duration of the packet.
Indoor vs. outdoor Wi-Fi: What are the
similarities and differences?
For 802.11ax to work across all environments, both indoor Wi-Fi and outdoor
base stations or small cells
will be required.
The front-end development is very similar for indoor and outdoor
environments. The coexistence strategy — out-of-band rejection,
harmonic filtering and frequency range — is similar.
The main differences between indoor and outdoor environments include:
A new data packet structure for outdoor. As we mentioned
earlier, 802.11ax adds an entirely new data packet format for outdoor Wi-Fi,
the HE-xSU PPDU format (shown in the PPDU figure at the end
of this blog post). The extended range of the outdoor PPDU format allows the
Wi-Fi signal to travel longer distances, as is typical for an outdoor Wi-Fi
Power levels and the resulting thermal considerations.
Although some customer premises equipment (CPE) applications have similar
power targets as mobile, there is also a high-power category, which means
management is even more important.
Read more design tips: How Not to Run
Hot: Overcoming Thermal Challenges in Wi-Fi Front-End Designs
Designing for tighter system
requirements in 802.11ax
scheme used in 802.11ax, 1024 QAM, quadruples the wireless speeds. But
it also means the system becomes more sensitive to internal and external
Here are some of the design challenges that engineers should be aware
Tighter linearity specs for the PA. The tighter
constellation density in 1024 QAM drives the PA
linearity requirement to approximately ‑47 dB EVM in
802.11ax. (However, there are efforts to relax the system EVM requirement
doc 11-17-1350.) Also, don’t forget to assess the test systems
required to measure these EVM levels for FEMs/iFEMs.
- LNAs must have a lower NF. Earlier reference designs
required low noise amplifiers (LNAs) to have a noise figure (NF) target
range of 2.5-3 dB. In 802.11ax, system sensitivity targets drive new
LNA targets of 1.5-1.8 dB NF.
- Gain expansion/droop. Ten years ago, the gain
imbalance target was 1 dB. Now it has decreased to 0.3-0.5 dB. As
shown in the following figure, gain and phase imbalance are being pushed to
the lower left to attain -47 dB EVM.
- The overall system margin. From a design perspective,
the target PA specification is -47 dB EVM, but the actual system spec
is ‑35 dB EVM. Chipset partners will typically drive for system
To address all these design challenges, engineers and marketing can
consider the following:
Increase current consumption to meet EVM targets. A system
will typically achieve better EVM if you increase Icc, but it will also
lower the power-added efficiency (PAE). To achieve a decent PAE and
linearity tradeoff, you need to optimize these major focus areas:
- Load line
- Interstage matching
- Bias circuit design
- Digital predistortion (DPD)
- Envelope tracking (ET)
- Design assumptions: Ask if the device needs to be best-in-class
for the premium tier or serve mass tier. The answer really depends
on the market, because requirements vary by customer and application. Early
adopters and flagship premium products may push for best-in-class performance
(‑47 dB EVM). In contrast, if the product is for mass tier or the
low-cost market, devices probably won’t be required to support 802.11ax
for another year or two after initial adoption in the premium tier.
A final thought: Designing for a
standard that’s still in flux
Above all, remember that the 802.11ax spec is still being defined, and you
should work with your applications team to maximize your product designs for
the emerging standard. Qorvo is committed to helping customers and providing
design expertise as this Wi-Fi standard takes shape. For technical guidance
and applications support, please visit our Technical
Support web page.
You can also read these resources from some of our hardware partners to
dive into technical details of this developing technology.
Technical Resources for 802.11ax