As global demand for mobile data continues to grow and users’ performance expectations increase, manufacturers of smartphones and other mobile devices are responding by delivering faster Wi-Fi performance.
The industry is rapidly shifting to the IEEE 802.11ac Wi-Fi standard, which offers significant performance improvements over 802.11n, the previous-generation standard, with enhancements including a greater number of spatial data streams (up to 8X MIMO), wider channels (up to 160 MHz), and higher-order modulation (up to 256 QAM). In handsets, manufacturers are increasingly implementing 2×2 MIMO, which provides up to twice the single-stream performance for a theoretical maximum data rate of 1.69 Gbps with 802.11ac.
For engineers working on mobile device designs, MIMO and 802.11ac present new challenges. Designs must shoehorn an additional Wi-Fi chain into the already crowded space within slim handsets, while managing design complexity and supporting more-stringent performance requirements.
To illustrate the difficulties, around 60 percent of the internal area of a smartphone is typically dedicated to the battery to ensure that the device provides adequate battery life. All other components must squeeze into the remaining 40 percent, including the processor, modem, sensors, and cellular and Wi-Fi front ends. As device complexity increases, so does the problem of fitting all these components into the available area. For example, each generation of flagship smartphones supports a greater number of LTE bands than the previous generation. Today’s 4G smartphone models may need to accommodate three times as many RF bands as the 3G phones of a few years ago, in a similar form factor.
In addition to working within tight space constraints, engineers must contend with the higher performance requirements imposed by the 802.11ac standard. Compared with previous Wi-Fi standards, 802.11ac front ends must offer greater linearity, while maintaining low insertion loss and low current consumption. The increase in maximum modulation complexity, from 64 QAM in 802.11n to 256 QAM in 802.11ac, provides a 33 percent throughput increase. However, it also requires greater linearity (lower constellation error), as measured by a tighter dynamic error vector magnitude (EVM) requirement. In 802.11ac, the maximum allowed system EVM for a transmitting device is -32 dB when using a 5/6 coding rate, compared with -28 dB for 802.11n. The 802.11ac Wi-Fi front end must achieve EVM that is even lower, since the system requirement includes contributions from the Wi-Fi chipset.
Integrating the Wi-Fi Front End
Increasing integration of Wi-Fi front end components can help to address both the space constraints and the performance requirements of 802.11ac MIMO mobile designs. Advanced wafer-level packaging and flip-chip manufacturing technologies now enable the manufacture of highly integrated front end modules that include amplifiers, filters and switches. The replacement of multiple discrete components with this integrated architecture can play a significant role in helping engineers conserve valuable PCB real estate. A typical dual-band integrated front end module (FEM) for example, integrates 2.4GHz and 5GHz power amplifiers and low noise amplifiers along with Wi-Fi/LTE coexistence filter, diplexer, coupler and switches. In a 2X2 MIMO configuration, this occupies less space than would be required using discrete components.
Besides saving space, integration also translates into other tangible benefits for handset manufacturers. These benefits include simplified handset design, improved performance and lower power consumption. The reduction in the overall number of front end components simplifies design and decreases the potential for yield fallout during the handset manufacturing process. The components within the module have already been tested and tuned to optimize performance, making it easier for the device to meet 802.11ac system requirements.
To maximize iFEM performance and power efficiency, it is necessary to combine components based on different process technologies. Power amplifiers based on GaAs processes deliver high power output and linearity. Temperature-stable BAW filters are required for precise selectivity under all operating conditions, and to enable coexistence with adjacent LTE bands.
A shared-antenna architecture can also help engineers more efficiently use the limited space within the smartphone, while delivering higher Wi-Fi performance through MIMO. This architecture takes advantage of the smartphone’s diversity antenna, which is used to improve the quality and reliability of LTE signals. By including a high-performance diplexer, the diversity antenna can also act as a second Wi-Fi antenna, enabling 2×2 MIMO for faster Wi-Fi throughput. The diplexer enables a cellular path and a filtered Wi-Fi path to share a single antenna port. This shared-antenna architecture is particularly attractive for high-end phones, where space is at a premium. Diplexers designed for this purpose must provide low insertion loss and good rejection for both LTE and Wi-Fi under a range of operating conditions, including temperature variation.
Engineers working on mobile devices are under constant pressure to deliver higher performance within small form factors. Highly integrated Wi-Fi front end modules can help solve these design challenges. As complexity increases with each device generation, higher levels of integration will be critical to enabling smartphone manufacturers to respond to ever-growing demands for mobile data.
Kevin Gallagher is Senior Product Marketing Manager at Qorvo Inc., an American semiconductor company that designs and manufactures radio frequency solutions for mobile.