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ISSCC: RF CMOS on the edge of transceiver chain

Posted: 11 Mar 2016 ?? ?Print Version ?Bookmark and Share

Keywords:RF CMOS? SiGe? GaAs? analogue front end? base band?

At this year's ISSCC there was very little to suggest any competition remains between RF CMOS and other fabrication technologies like SiGe or GaAs in the construction of cellular radio ICs.

There was little of what UCLA's Asad Abidi famously described 10 years ago as a "Culture Clash" between CMOS base band integrators and microwave engineers looking at high-GHz transmission effects. Indeed, even foundry services vendors are touting RF CMOS solutions from 130nm to 28nm. RF IC companies are obviously taking advantage of the integration capability. One ISSCC panel asked, "Do we need to downscale our radios below 20nm?"

Why then are we seeing trade magazine articles still debating the trade-offs between CMOS power amplifiers (PAs) and those built from GaAs? One answer is that this represents a large market, better than $5 billion in revenues. The largest PA supplier, Skyworks Solutions, claims proficiency in both technologies. The company touts GaAs for its linearity and its power efficiency. CMOS is cited of its integration ability, its low cost, and its low power consumption. But Skyworks' "CMOS Power Amplifiers" web link generates the equivalent of a 404 error. With the leader in RF power amplifiers the message is clear: GaAs still rules.

After 10 years of debate, where we are with RF CMOS? A clue is available from the recent (2014) introduction of an analogue front end for mobile handsets: We're just getting started. Qualcomm introduced what it called "the world's first [emphasis added] multi-mode, multi-band chip featuring an integrated CMOS power amplifier (PA) and antenna switch." Qualcomm has been the world's largest supplier of cell phone chip setsbase band and applications processorsbut, apart from power management controllers, has had a smaller impact on the analogue front ends. A multi-chip module, Qualcomm's QFE2320 includes a multi-mode multi-band power amplifier (MMMB PA), a radio transceiver, an envelope tracking power supply, and an integrated antenna switch.

Inside a cellular radio front end

The analogue front end is designed to relieve the clutter of components in handsets enabling to multiple 3G/4G and LTE encoding standards (such as GSM, GPRS, EDGE and WCDMA). It accomplishes this two ways: First, carrier signal aggregation reduces the number of radios required to decode a complex signalintegrating up to 5 carriers with up to 100MHz of spectrum or 3 carriers with peak data rates up to 450MBit/seffectively increasing data rates by paralleling carrier channels. Secondly, a built-in antenna switch allows the front end match antenna and radio choices. (A block diagram of the Qualcomm front end is shown in figure 1.)

Qualcomm front end

Figure 1: Carrier aggregation and an on-board antenna switch enables Qualcomm's module to handle a wide variety of 3G and LTE modulation schemes with a small number of CMOS power amplifiers. (Source: Qualcomm Technologies).

The integrated switches enable the analogue front end to decode a variety of frequencies and transmission modes with a small number of power amplifiers. The integrated switches also decrease the footprint of the module e frequencies and transmission modes According to Qualcomm, the QFE2320 covers all major cellular modes (including LTE TDD/FDD, WCDMA/HSPA+, CDMA 1x, TD-SCDMA, and GSM/EDGE) and RF bands 700MHz to 2,700MHz. The analogue front end already supports a number of commercial smartphones, particularly ZTE's new flagship product, the Grand S II LTE.

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