Apple Shows New Levels of RF Design Complexity

Our teardown of the iPhone 12 reveals how Apple crafted its 5G radio

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The iPhone 12 series is Apple’s first 5G design, but more importantly, it has the distinction of being the industry’s most complex and comprehensive 5G radio frequency (RF) front-end system to date. As the perpetual follower in adopting the latest wireless technology, Apple’s entry with the iPhone 12 design came some 17 months after the first 5G smartphones hit the market. But despite being late to the 5G party, its deliberate approach produced a more mature 5G RF front-end design that supports up to 20 global 5G frequency bands including millimetre-wave connectivity.

Apple was also able to put the same 5G capabilities of the high-end iPhone 12 Pro in the lower-priced 5.4-inch iPhone 12 mini, something that other competitive 5G designs haven’t been able to do. This helps to lower the price of 5G phones and boosts adoption rates for the technology.

When we look back at the evolution of the RF front end in iPhones, we see a steady rise in performance and support for global frequency bands. From an RF complexity perspective, Apple engineers have managed to keep the real estate of RF in the printed circuit board essentially fixed within 300 square millimetres from the iPhone 6 all the way to the iPhone 11. This design trend vanishes with the iPhone 12, with the component count and footprint of the RF front end more than doubling to nearly 800 square millimetres for the most sophisticated 5G RF front end in a smartphone.

Custom 5G Radio Frequency Design

Apple is one of only a handful of global smartphone makers with the smarts and resources to design and develop its own RF front end. Although early 5G smartphone designs used off-the-shelf RF solutions by industry leader Qualcomm, which has made huge strides in RF front-end technologies, Apple took the more difficult path of creating a new 5G RF front end from scratch. This approach gave Apple the freedom to create a design that’s future-proof enough to allow the iPhone 12 to remain relevant as 5G technology evolves in the coming years.

A very clear example of this customization in the iPhone 12 is in millimetre wave. Apple took a hybrid approach, using a Qualcomm second-generation QTM525 antenna module, which is provided in an antenna-in-package form, and applying another millimetre-wave radio directly onto the printed circuit board with dual antennas. Most flagship 5G designs supporting millimetre-wave frequencies available on the market fit up to three QTM525 modules around a device to provide enough spatial diversity to overcome the challenging millimetre-wave propagation in real-world use.

The design principle of the millimetre-wave modules is to avoid signal loss by bringing RF transceiver functions as close to the antenna as possible. But designing multiple antenna modules to improve millimetre-wave reception isn’t without compromise: these millimetre-wave antenna-in-package solutions are costly and exclusively sourced from Qualcomm. So, designers must strike a balance between 5G performance and cost considerations.

For the custom second millimetre-wave antenna module, Apple separated the Qualcomm millimetre-wave transceiver SDR525 and power-management chip PMR525 from its antenna-in-package form and placed it on the printed circuit board. Directly mounted on the flip side of that board position is an array of millimetre-wave antennas. The whole printed circuit board assembly is mounted so that the antenna array faces the back glass cover of the iPhone. This close placement is for obvious reasons. However, Apple fitted an additional uplink antenna on the front-facing glass screen. We believe this is to ensure antenna diversity, even if the iPhone is lying flat on its back and blocking signals from reaching the main millimetre-wave antenna array.

So why did Apple decide to use two millimetre-wave antenna modules instead of the three typically found in other flagship 5G devices? Clearly, cost is a major consideration as US carriers are likely to be subsidizing the $30 additional cost of the millimetre-wave components in the iPhone. To offer millimetre-wave 5G capabilities at cost in the US, Apple used only two antenna modules, but took pains to modify one of them to run more efficiently by adding an auxiliary antenna.

5G = New Radio + LTE

Most of the 5G deployments worldwide so far have been configured for non-standalone 5G. This means that the control plane lies in the 4G LTE anchor, while 5G New Radio provides the extra speed and capacity boost. In essence, most 5G services that are in use today are a layer of connectivity on top of 4G LTE coverage. This non-standalone requirement brings immediate complexity to the RF front end, as devices must operate the 4G and 5G radios simultaneously. Other 5G considerations making RF front-end design more complex include:

• Mandatory 4×4 MIMO on mid-to-high sub-6 GHz frequencies, resulting in many more front-end modules
• Dual-carrier uplink to improve uplink-limited network performance, for example at the edge of a cell, which calls for additional power amplifiers
• Wider bandwidth filters to address New Radio requirements as well as ones listed above
• Integrated low-noise amplifiers, to overcome system loss of 2 dB
• Specific high-power user equipment to boost signal output of time-division duplex bands N41/N78 (for mid-to-higher frequency portions of sub-6 GHz 5G) requiring higher-output power amplifiers
• Wide-band envelope tracking to manage the power consumption of the 5G radio

Managing Exponential Growth in Complexity

For Apple and other handset makers, 5G presents huge opportunities and a daunting set of technological challenges. Currently, advancements in integrated power amplifiers, front-end modules, low-noise amplifiers, switches and filters are helping to control the added complexity. But 5G RF requirements are outpacing the improvements in components. The result is an RF front end that’s expected to balloon in complexity, size and cost in an unsustainable manner.

As the industry brings 5G into the mainstream, these challenges will remain, which manufacturers will try to overcome by introducing design innovations that will cut costs and improve performance. Apple has shown us how to manage RF front-end complexity while supporting a comprehensive set of global 5G networks. But to push 5G technology down into the reach of most consumers worldwide, a lot more needs to be done in RF front-end design to democratize the technology and fulfil the promise of our 5G future.