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Flexible RF front end designs for regional smartphones

24 Nov 2015  | EDN

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Rising levels of integration in mobile devices have resulted in flagship smartphone models that pack an extraordinary range of capabilities into slim designs, including highly integrated RF front ends that provide global coverage with support for 20 or more 4G LTE, 3G and 2G bands.

As the smartphone market continues to mature, however, much of the growth will be in less-expensive smartphones designed for regional use. This trend is evident worldwide, but is particularly evident in China and other Asian countries, where consumers are rapidly shifting from 2G and 3G feature phones to smartphones.

In these countries, falling prices are making phones affordable for more users; average smartphone prices have fallen 30% in Asia since 2008, based on research by GSMA, the mobile operators' industry group. Many manufacturers are already offering capable mid-tier smartphones at relatively low prices. In China, the market for smartphones costing less than $200 off contract is expected to grow nearly three times as fast as the market for phones costing $400 or more, according to ABI Research.

These mid-tier regional smartphones don't need to provide global coverage, but they do need to provide good performance and full support for the bands used by a specific country or region. They also must support LTE, which is rapidly spreading worldwide in tandem with smartphone adoption; the GSMA notes that LTE deployments across Latin America and the Asia-Pacific region will be the primary drivers of global 4G growth over the next five years. China alone added nearly 100 million LTE subscriptions in 2014 and is expected to overtake the US as the world's largest LTE market in 2015, ABI Research believes.

Designing mid-tier smartphones for regional use introduces several challenges that differ from those involved in designing flagship phones.

 • Cost is a top concern, since these devices must sell for much less than flagship phones. To control cost, it's important to limit the amount of hardware in the device. This means including only the RF front-end components necessary to support the required regional bands.
 • Flexibility is essential. Manufacturers need to be able to quickly customise a single handset design for use in different regional markets and to meet operator requirements. Newer smartphone makers, in particular, often operate in a fast-paced business environment in which there may be only a short time between winning a contract with an operator and the deadline for delivering a customised product.
 • To enable rapid customisation, manufacturers must be able to create high-performance regional designs while minimising labour-intensive and time-consuming engineering work. Typically, smartphone manufacturers want to use a single PCB layout that can be quickly adapted by adding appropriate RF hardware for each region.
These challenges call for lower-cost, flexible RF front-end designs that can be quickly customised for different countries or regions without burdensome engineering. One effective approach to meeting these requirements is to use region-specific RF modules to customise a single PCB design to meet different regional or operator specifications. These modules must provide a high degree of integration to optimise PCB space and simplify design, yet give manufacturers the ability to control cost by omitting components that are not needed for specific markets. Manufacturers of regional smartphones can then design the PCB to handle the most complex requirements that they will encounter, and depopulate the board as necessary when specific RF bands are not needed.

Design example
As an illustration, consider a typical example based on regional operator requirements in China. There are two common specifications:

 • Three modes, eight bands (3M8B). Typically, these are TD-LTE (Bands 39, 40 and 41); TDS-CDMA (Bands 34 and 39), and 2G (bands 2, 3, 5 and 8).
Five modes, 10 or more bands (5MXB). These are the modes and bands listed above, plus FDD-LTE (Bands 3, 5 and 7) and WCDMA (Bands 1, 2 and 5).

Figure: Solutions enable smartphone manufacturers to quickly customise smartphones for different regions, using a single PCB layout. (Source: Qorvo).

Both of these specifications can be met with a single PCB design by using region-specific RF front-end modules such as RF Flex from Qorvo Inc. RF Flex is a family of integrated front-end solutions consisting of transmit (TX) modules (which include 2G power amplifier, antenna switch and integrated coupler) and multi-band power amplifiers (MBPAs) covering the full LTE spectrum. In this example, manufacturers can design the PCB to support the more complex set of requirements (5MXB), then use the same board to support 3M8B at lower cost by swapping in simpler components and omitting some other hardware. One novel aspect of the RF Flex family is that the simpler part attaches to the same location on the PCB but uses a smaller area of the footprint, as shown in the diagram. The TX module concept has been widely used in 2G and 3G designs, so may offer the advantage of familiarity for manufacturers who are producing 4G LTE phones for the first time.

Design flexibility will become even more important as adoption of mid-tier smartphones grows worldwide and prices continue to fall. Solutions that support flexible customisation based on a single PCB, such as RF Flex, will be important to enable manufacturers to quickly produce mid-tier smartphones for regional markets.

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