From the early feature phones, which were primarily used for calls and texts, to today’s smartphones that can download content faster than many home Wi-Fi networks, the role of the RF front end has long been overlooked. While most smartphone users today might not even realize what the RF front end is, it remains an essential component of every mobile phone design. The RF Front End (RFFE) refers to the functional area between the RF transceiver and the antenna in a mobile phone. It mainly includes power amplifiers (PAs), low noise amplifiers (LNAs), switches, duplexers, filters, and various passive components. Without a properly designed RF front end, a device cannot connect to the mobile network, rendering it practically useless for modern users. A well-engineered RFFE is crucial for enhancing the performance, functionality, and industrial design of today's handsets.
As the smartphone market matures, the high-end segment continues to evolve. Early high-end models had smaller screens, shorter battery lives, and limited mobile network bandwidth, which hindered the transmission of HD videos or the downloading of large files. Fortunately, advancements in technology and user expectations have transformed these devices into multifunctional tools. The introduction of LTE-enabled phones was a game-changer, allowing users to immerse themselves in their devices. Over the last few years, the demand for smartphones has grown, partly due to the rise of social media apps like YouTube, Facebook, and Twitter. These platforms have fueled the creation and consumption of user-generated content, pushing for faster and more reliable upload and download speeds. Since the advent of LTE devices, the complexity of the RF front end has increased significantly. Although other features of the device have also improved, contributing to better user experiences, these enhancements have posed greater challenges for RFFE design.
Today, video streaming has become one of the most common activities among smartphone users. As a result, the average screen size of smartphones has increased, with models featuring screens of 5 inches or larger accounting for 73% of total shipments in 2016, up from 53% the previous year. Larger screens, however, tend to drain battery life, prompting designers to incorporate bigger batteries. These changes, along with other functional upgrades, have led to a reduction in the physical space available for critical RFFE components. Additionally, with the growing emphasis on power efficiency due to the impact of large screens on battery life, RFFE design now prioritizes energy efficiency more than ever before.
"Gigabit LTE and the RF Front End: A Complex Challenge"
With each successive generation of wireless wide area network (WWAN) technology, the complexity of the RF front end has increased. However, compared to previous generations, the latest flagship models represent a significant leap in terms of RF content and complexity. The transition from LTE-A to LTE-A Pro may be the most substantial advancement in RFFE design yet.
The complexity of RFFE designs increases with the number of transmit and receive channels within a single device. This typically correlates with the number of antennas used in the design and the number of supported spatial data streams. As seen in the Galaxy S6 Edge+ and S7 Edge, the antenna architecture remains relatively stable between Cat 6 and Cat 9/12 devices. However, in Cat 16 devices like the Galaxy S8 and S8+, there is a noticeable increase in the number of antennas.
With its advanced carrier aggregation capabilities, higher-order modulation, increasingly complex antenna architectures, more spatial streams, and LTE-U functionalities, the RFFEs of new high-end smartphones like the Galaxy S8 and S8+ are among the most intricate designs in smartphone RF engineering. These phones were the first to support Cat16 LTE, offering download speeds of approximately 1 Gbps—a significant improvement over the previous generation’s flagship modems, which supported LTE Cat12 with speeds of up to 600 Mbps. Faster download speeds benefit both end-users and mobile network operators, leading to extended battery life, more efficient network interactions, and the ability to utilize license-free spectrum through technologies like LTE-U.
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