December 19, 2017

    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 single user.
    • 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 channel sounding.

     
    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

    • EVM: Error vector magnitude
    • HE: High efficiency
    • HEW: High-Efficiency WLAN (or High-Efficiency Wireless)
    • MU-MIMO: Multi-user multiple input/multiple output
    • NDP: Null data packet
    • OFDMA: Orthogonal frequency-division multiple access
    • PLCP: Physical Layer Convergence Procedure
    • PPDU: PLCP Protocol Data Unit
    • QAM: Quadrature amplitude modulation
    • 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. 

    Target Wait Time (TWT) in 802.11ax

    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 environment.
    • 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 thermal management is even more important.
       

    Designing for tighter system requirements in 802.11ax

    The modulation scheme used in 802.11ax, 1024 QAM, quadruples the wireless speeds. But it also means the system becomes more sensitive to internal and external impairments.

    Here are some of the design challenges that engineers should be aware of:

    • 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 per IEEE 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 margin.

    802.11ax FEM/iFEM vs. System Requirements

    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.
     

     
    OFDMA PPDU Formats – 5 Formats for 802.11ax (High-Efficiency Wireless)

     

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    Jeff Jones

    About the Author

    Jeff Jones
    Senior Manager, Applications Engineering

    Jeff Jones has been with Qorvo since 2000 and held various roles in test engineering, product engineering, design engineering and applications engineering. He currently manages the mobile Wi-Fi applications team.

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