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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?

What has happened in the past ten years is not a replacement of GaAs power amplifiers by RF CMOS amplifiers. The RF power amplifier functionthe launching of radio signals into the etheris performed at the edge of a radio transceiver chain. What happens in that chain is increasingly fabricated in RF CMOS. It is also a definitional issue: When marketers refer to an "all CMOS" radio, they're typically overlooking the analogue component. The antenna driver could be a single transistor, or Darlington pair. It may require a special technology, other than CMOS. PAs could be fabricated with SiGe bipolar on a BiCMOS substrate, as well as stand-alone component in GaAs. But this makes the RF power amplifier the interface between the digital radio and the physical world.

A modern 4G phone will have to address multiple encoding schemes and frequency bands if it is to be truly useful worldwide. The architectural choices are basically two: You build multiple radios into the mobile handset, one for each modulation scheme you hope to decode. Or you build an integrated transceiver with multiple antenna drivers (ie, power amplifiers), and include an RF switch to bump between a single transceiver and multiple PAs).

What is clear, looking into the future, is that there will be a proliferation of RF frequencies and modulation schemes to which the PAs must be responsive. This will require a multiplication of the radios within each handset, if not a multiplication of antenna drivers. While 5G has not been defined, it is clear that the handset must operate at very high frequencies and incorporate multiple radio architectures.

What is 5G?

The inclusion of multiple radios, frequency bands and modulations schemes is significant as manufacturers continue to ask whatbesides higher bandwidthscharacterizes 5G. What we DO know about the as-yet-undefined fifth generation?

In a presentation at this year's ISSCC, NTT Docomo's chief technology office, Seizo Onoe, projected that 5G RF devices must function up to 100GHz. 5G will require a high density of power devices but consume very little battery power. The 5G network, due to rollout in 2020, will not necessarily supersede 4G, but will rely on a combination of macro cells and pico cells (what Docomo calls "semi-macro cells"). Docomo projects reliance on active power arrays, Massive MIMO (multiple-input/multiple-output) antennas and advanced cloud-based radio access networks ("C-RAN enhancements") to ensure seamless handoffs for devices operating between macro-cells and smaller pico cells.

There is a consensus that 5G transmission will require a higher RF spectrum to effect GBit/s transmission, although radiated power tends to fall off at elevated frequencies. Existing phones utilise frequency bands operating in the 800MHz and 1.4GHz region. New spectrum will be available in the 3-10GHz range, the 10-to-30GHz range, and above 30GHz. MIMO experiments by Ericsson in the 15GHz range enabled 11.0GBit/s data rates. Similar experiments a Nokia in the 70GHz range enabled a 2075MBit/s data rate.

Even with 4G, the operating frequencies and encoding schemes can be complicated. For CDMA modulation schemes, for example, the radio must work in the 824-to-849MHz band, and the 1,850-to-1,910MHz band. For LTE, the PAs must operate from 2,500 to 2,570MHz, 2,305 to 2,315, 2,496 to 2,690, 2,300 to 2,400, and 2,545 to 2,575. Implementing 5G will likely expand the frequencies and modulation schemes to which the smartphone must respond (see Figures 2 and 3).

Convergence of standards

Figure 2: One 5G scenario assumes a convergence for Wireless Transmission Standards. (Source: NTT Docomo).

Divergence of standards

Figure 3: Another 5G scenario assumes a divergence for Wireless Transmission Standards. (Source: NTT Docomo).

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