Issue link: https://resources.mouser.com/i/1207833
23 ©2018 Qorvo, Inc. OFDMA PPDU Formats 5 Formats for 802.11ax (High-Eciency Wireless) Field Description L-STF Legacy Short Training Field L-LTF Legacy Long Training Field L-SIG Legacy Signal Field RL-SIG Repeated Legacy Signal Field HE-SIG-A HE Signal A Field HE-SIG-B HE Signal B Field HE-STF HE Short Training Field HE-LTF HE Long Training Field DATA Data PE Packet Extension Field GI Guard Interval LTS Legacy Training Sequence ©2018 Qorvo, Inc. OFDMA PPDU Formats 5 Formats for 802.11ax (High-Eciency Wireless) Field Description L-STF Legacy Short Training Field L-LTF Legacy Long Training Field L-SIG Legacy Signal Field RL-SIG Repeated Legacy Signal Field HE-SIG-A HE Signal A Field HE-SIG-B HE Signal B Field HE-STF HE Short Training Field HE-LTF HE Long Training Field DATA Data PE Packet Extension Field GI Guard Interval LTS Legacy Training Sequence OFDMA PPDU Formats 5 Formats for 802.11ax (High-Eciency Wireless) Field Description L-STF Legacy Short Training Field L-LTF Legacy Long Training Field L-SIG Legacy Signal Field RL-SIG Repeated Legacy Signal Field HE-SIG-A HE Signal A Field HE-SIG-B HE Signal B Field HE-STF HE Short Training Field HE-LTF HE Long Training Field DATA Data PE Packet Extension Field GI Guard Interval LTS Legacy Training Sequence • 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 47dB 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 2.5-3dB. In 802.11ax, system sensitivity targets drive new LNA targets of 1.5-1.8dB NF. • Gain expansion/droop. Ten years ago, the gain imbalance target was 1dB. Now it has decreased to 0.3-0.5dB. As shown in the following figure, gain and phase imbalance are being pushed to the lower left to attain -47dB EVM. • The overall system margin. From a design perspective, the target PA specification is -47dB EVM, but the actual system spec is 35dB EVM. Chipset partners will typically drive for system margin. 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 circuit current (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 ( 47dB 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. 802.11ax FEM/iFEM vs. System Requirements ©2018 Qorvo, Inc.