For wireless access points or customer premises equipment (CPE), it can be hard to fully account for thermal management and the parameters affected by it prior to FCC certification. To save yourself the headaches of last-minute changes due to interference, coexistence or RF front-end (RFFE) linearity, be sure to design using component thermal parameters in mind. This blog post explains the biggest thermal challenges facing Wi-Fi front-end designs.
Increasing capacity for smart homes
In today’s wireless homes, communication operators and retailers have typically offered one large wireless router, using raw power to achieve coverage across the entire home. But with the sharp increase of household devices and the IoT, smart homes are pushing the capabilities of the single-router model.
As a result, new application models are evolving. Consumers are finding that placing more routers, or nodes, in the home helps service more clients and data backhaul to the home router/modem. This new mesh network model ensures wireless capability across a home using some techniques that are already present in office headquarters, hospitals and college campuses via enterprise-level systems.
The IoT challenge
It’s no surprise that the RF complexity within the access point increases because of this mesh networking model and as devices integrate more standards and capabilities.
The IoT brings several challenges:
- The addition of wireless radio needs. Access points today incorporate more than just Wi-Fi — they also support Zigbee, Bluetooth, Bluetooth Low Energy (BLE), Thread and narrowband IoT (NB-IoT). Operators are also finding ways to reach households who previously didn’t have access. Operator-supported LTE-M (the machine-to-machine version of LTE) is one example finding its way into some Wi-Fi gateways.
- More users within each home. Homes no longer have only one or two PCs and a few phones. Today, several computers, TVs, smartphones, wearables, security networks, wireless appliances and more all connect to Wi-Fi and the internet.
- Additional Wi-Fi bands. Units no longer have one 2.4 GHz band and one 5 GHz band. Now there are up to eight individual 2.4 GHz and eight 5 GHz paths. This change gives us the MIMO (multiple input / multiple output) and multiuser MIMO (MU-MIMO) paths within the Wi-Fi access point or node.
- Shrinking size and expanded functionality. Wi-Fi manufacturers are making Wi-Fi units smaller, sleeker, more decorative and less obtrusive. They’re also making some units all-weather or adding multifunctionality such as night-light capability.
Block diagrams of older vs. new access points highlight just how complex the RFFE design is now.
All these changes in the Wi-Fi front-end design increase the number of RF chains, contributing to the overall heat within the access point. This increase in unit temperature also increases the RF tuning challenges, especially when the size of the box is the same or even smaller.
In the Wi-Fi world, one of the most critical design challenges engineers need to address is product temperature. In today’s products, components are subject to average temperatures of 60°C or greater, while sitting in a room temperature environment of 25°C. It’s important to consider this fact early on in a design, to help minimize redesign issues or additional costs.
How heat challenges RF front ends to deliver capacity and range
Temperature affects three RFFE components:
- Power amplifiers
- RF switches and low noise amplifiers (LNAs)
Let’s examine the heat challenges and Wi-Fi design considerations for each category.
#1: How does the power amplifier factor in?
Engineers often balance between linearity, power output and efficiency in each of the RF chains. Using optimized, highly linear power amplifiers or front-end modules (FEMs) optimizes system efficiencies, creating less overall heat. It also reduces processing inefficiencies.
RF engineers should also consider several Wi-Fi design trends that affect power amplifiers:
- Use of time division duplexing (TDD). Wi-Fi networks use TDD, which means the PAs are pulsed on and off during operation — alternating transmit and receive functions. This increases PA transients, which attribute to high temperatures.
- More difficult error vector magnitude (EVM) specifications. EVM is a measure of modulation quality and error performance. In 802.11ac, the EVM specification was -35 dB, but in the next standard of Wi-Fi, 802.11ax, this specification increases to -47 dB — which is more difficult for Wi-Fi component designers to meet. Design engineers must design highly linear FEMs to optimize for EVM, which ultimately helps reduce overall product temperature.