设计Wi-Fi 6（802.11AX）：克服它们的共同挑战和提示
December 19, 2017
In a previous blog post, we coveredfive key things to know about Wi‑Fi 6 (802.11ax), the next big standard for Wi-Fi. Let’s examine some of the challenges that RF engineers will face when designing for Wi‑Fi 6 and some tips on how to overcome them.
Some background: 5 OFDMA PPDU formats for Wi‑Fi 6
But first, let’s look at the foundational signal structure for Wi‑Fi 6 (802.11ax) — the physical layer protocol data units that Wi-Fi clients and devices use to communicate.
802.11AX使用五种格式的OFDMA PPDU：
- Single User (HE-SU).For transmitting data from a single user.
- Multi User (HE-MU).用于将数据传输到一个或多个响应于触发器帧的用户。
- Outdoor (HE-xSU).对于单个用户的户外传输。此格式在802.11AX中是新的。
- 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.
- 下行链路通道声音（HE-NDP）。For波束成形and downlink频道听起来.
看看imageat the end of this blog post for details of the frame packets and fields within each PPDU format.
专业术语
- EVM:错误矢量幅度
- 他：High efficiency
- HEW:High-Efficiency WLAN (or High-Efficiency Wireless)
- mu-mimo：多用户多输入/多个输出
- 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?
Wi-Fi 6（802.11AX）的一件事是目标等待时间（TWT） - 也称为睡眠时间 - 这允许设备在传输数据之前长时间保持在睡眠状态。这种资源调度提高了电池寿命，这意味着消费者的更好体验。
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.与LTE基站技术不同，Wi-Fi 6没有同步时钟信号。因此，设备将依赖于接入点以使网络上的所有设备保持同步。此外，Wi-Fi 6使用比11ac更长的OFDM符号（Wi-Fi 5）, 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.我们从我们的一些芯片组合作伙伴接收的规范表明，Wi-Fi 6（802.11ax）中没有更改初始功率放大器（PA）接通时间;它仍然是200-400纳秒。然而，增益平整度已经扩展，保证前端模块（FEM）没有增益扩展或增益下降，以便在分组的持续时间内。
Indoor vs. outdoor Wi-Fi: What are the similarities and differences?
For Wi‑Fi 6 (802.11ax) to work across all environments, both indoor Wi-Fi and outdoor base stations or小细胞将被要求。
The front-end development is very similar for indoor and outdoor environments. Thecoexistence strategy- 带外抑制，谐波滤波和频率范围 - 类似。
室内和室外环境之间的主要差异包括：
- A new data packet structure for outdoor.As we mentioned earlier, Wi‑Fi 6 (802.11ax) adds an entirely new data packet format for outdoor Wi‑Fi, theHe-xsu.PPDU格式（所示PPDU figure在这个博客帖子的末尾）。户外PPDU格式的扩展范围允许Wi-Fi信号在户外Wi-Fi环境中典型的距离。
- 功率水平和产生的热考虑因素。Although some customer premises equipment (CPE) applications have similar power targets as mobile, there is also a high-power category, which meansthermal managementis even more important.
在Wi-Fi 6中设计更紧凑的系统要求（802.11AX）
Themodulation scheme used in Wi‑Fi 6，1024 QAM，四次无线速度。但它也意味着系统对内部和外部损伤更敏感。
Here are some of the design challenges that engineers should be aware of:
- Tighter linearity specs for the PA.1024 QAM中的较小星座密度驱动PA linearity requirement在802.11ax中约为-47 dB EVM。（但是，努力放宽系统EVM要求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 Wi‑Fi 6 (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.
To address all these design challenges, engineers and marketing can consider the following:
- 增加当前消费to meet EVM targets.如果增加ICC，系统通常会实现更好的EVM，但它也将降低增强效率（PAE）。为实现体面的PAE和线性权衡，您需要优化这些主要焦点领域：
- 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.答案确实取决于市场，因为需求因客户和应用而异。早期采用者和旗舰优质产品可能会推动级别的性能（-47 dB EVM）。相比之下，如果产品用于质量层或低成本市场，则在优质层初始采用后，设备可能不会需要在另一年内提供Wi-Fi 6。
A final thought: Designing for a standard that’s still in flux
最重要的是，请记住，Wi-Fi 6（802.11ax）规范仍在定义，并且您应该与您的应用团队合作，以最大限度地为新兴标准设计产品设计。Qorvo致力于帮助客户提供设计专业知识，因为此Wi-Fi标准呈现。有关技术指导和应用支持，请访问我们的118金宝app 网页。
您还可以从一些硬件合作伙伴阅读这些资源，以潜入此开发技术的技术细节。
Technical Resources for 802.11ax
- Introduction to 802.11ax High-Efficiency Wireless，国家文书
- IEEE 802.11ax Technology Introduction White Paper, Rohde & Schwarz
- Eye在802.11ax：它是什么以及如何克服它创造的设计和测试挑战, Keysight Technologies
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