Honey, I Shrunk the Smartphone RF Front-Ends!

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Over the past 15 years, we’ve witnessed a remarkable increase in the Radio Frequency content in smartphones.  There are now seven wireless interfaces in high-end mobile devices: FDD cellular, TDD cellular, Wi-Fi, Bluetooth, GNSS such as GPS and GLONASS, Near Field Communication, and wireless power battery charging.

The cellular RF front-end ​specifically is rapidly evolving in terms of complexity.  Today’s world phones must support at least three air interfaces (GSM, CDMA or UMTS, and LTE), and support as many as sixteen FDD frequency bands and four TDD frequency bands.  To optimize signal strength and downlink data rates, these phones generally utilize a diversity receiver architecture, which itself increases RF switch and filter content by roughly 50%. The Samsung Galaxy S6 (and likely the upcoming new iPhone) support both intraband and interband LTE-Advanced Carrier Aggregation.  The number of carrier aggregation configurations in these “Cat 6” LTE-A Release 10 smartphones is small, but set to grow dramatically in the future, from three configurations in Release 10, to 24 configurations in Release 11, to 95 carrier aggregation configurations in LTE-A Release 12.  The probable use of higher-order MIMO cellular and Wi-Fi transceivers in smartphones and phablets will drive further RF content addition in the coming years.

Companies like Avago, Peregrine, Qorvo and Skyworks have historically responded to the RF content “big bang” challenge with increased functional integration.  Discrete power amplifiers have evolved into multi-mode, multi-band PAs.  Single pole RF switches now have ten or more throws to handle the rising band count.  Increasingly, single band SAW and BAW duplexers are being replaced with quadplexers (two bands) and possibly hexplexers (three bands) in the foreseeable future.  Intel, Mediatek, Qualcomm and Samsung Semi and others have responded by offering cellular RF transceivers with escalating RF transmit and receive port counts.

There is an extraordinary mix of semiconductor base materials used in smartphone RF front-ends.  Amplifiers are fabricated in gallium arsenide, indium gallium phosphide, and sIlicon germanium.  Switches are fabricated in silicon on sapphire or other silicon on insulator processes.  Piezoelectric materials such as lithium tantalate and lithium niobate are typical base materials for SAW filters and duplexers.  This process technology mix makes monolithic integration essentially impossible, and drives suppliers to module-level integration strategies.  But as in New York and Shanghai, relentlessly increasing density pressure in a fixed footprint eventually expresses itself in the vertical dimension.

So-called “2.5D” and 3D packaging technologies have been under development for many years, mostly used in digital IC applications, such as processors plus stacked memories, both flash and DRAM.  The trend toward 3D RF front-end modules in smartphones seems to be inevitable.  These 3D RF modules are likely to leverage high resistivity polycrystalline silicon substrates to enable the integration of passive RF components.  Wafer-level packaging, RF functional through silicon vias and solder bump interconnects, as well as ultra thin chip stacking will likely be required to manage module thickness to within leading smartphone OEM’s expectations.

Because of the broad range of functional requirements and form factors, we can expect that 3D cellular RF front-end modules will be designed on a custom basis for large smartphone OEMs.  Success metrics for smartphone cellular RF front-end suppliers in 2016 will extend beyond basic ingredient amplifier, switch and filter circuit technologies, to full 3D RF module design tools and next-generation, low-cost and high-volume vertical integration assembly methods.

EJL Wireless Research is publishing a new report on smartphone RF front-end content trends this week.  See for http://shop.maravedis-bwa.com/ more information.