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The launch of the iPhone 5 last month has highlighted a major issue for all 4G smartphones – that no one handset can cover all of the global LTE frequency bands.
This is a significant headache for operators and consumers alike. For consumers a lack of true global roaming is both confusing (forcing end users to find out which phone works with which operators in each region) and irritating. For operators and manufacturers it requires them to manufacture several region-specific versions of the same device, a complex and undesirable supply chain expense.
The fact is, LTE smartphones are currently hampered by fundamental limitations in the RF front end that cannot be overcome with existing RF technology. Until these issues can be overcome it will not be possible to produce a truly global LTE handset.
Today’s fundamental RF engineering limitations
The need to simultaneously support different systems (LTE, WCDMA, HSUPA, GSM, etc.) in one handset, while simultaneously supporting an increasing number of frequency bands (LTE will be deployed in 18 bands by the end of 2012), has resulted in a significant increase in RF system complexity for mobile handsets.
The traditional approach to these challenges has been to use a number of separate PAs (maybe up to eight in current 3G designs) within the handset RF subsystem. Each PA covers a fairly narrow band and is optimized for peak efficiency in that band. This discrete approach works well when only a few frequency bands are required, but quickly becomes very complex for multiband, multimode handsets designed to support LTE, with RF switching and thermal issues all making a compact, cost-effective solution extremely difficult to achieve.
LTE is a particular problem for handsets as it operates with around twice the peak power output from the RF PA compared to previous mobile standards. This is why today’s 3G PAs are struggling to achieve viable performance levels, particularly in multiband applications.
Indeed the poor power efficiency of broadband PAs designed to cover broader frequency ranges makes them unviable for commercial deployment. Whereas highly tuned narrowband PAs may achieve something in the region of 30% to 35% efficiency, broadband PAs are only achieving 20% to 25% efficiency.
So whether using narrowband or broadband PAs, the greater demands being placed on cellular radios in LTE handsets to support multiple transmission modes and frequency bands are forcing designers to devote more board space and power consumption to the RF system.
As a result it is currently extremely difficult to design in support for all LTE bands without having a dramatic effect on the cost and physical size of handsets or their battery life – early LTE handset battery life is already measured in mere hours rather than days.
The impact on LTE handsets
Without the ability to support all LTE frequency bands, most smartphones support all “world bands” for 2G and 3G – Bands 1, 2, 3, 4, 5 and 8. The RF front ends for these bands are relatively mature and have already been integrated to some extent. LTE support in these devices is usually achieved with a couple of narrowband “bolt on” LTE PAs which are band/region/operator specific; these could either be PA-duplexer modules, or separate PAs and duplex filters.
Each of these lineups has to be highly tuned today and adds to the phone footprint and cost due to the additional PA and the duplex filter. As a result we don’t yet see any single product covering all the LTE bands, instead we see two or three operator and region-specific PAs and filters added to cover a limited number of LTE bands.
For the iPhone 5, Apple has adopted three separate product SKUs for different regions/operators – one each for AT&T Mobility and Verizon Wireless in the United States, plus a “rest of the world” version covering the most common bands for international markets. This approach is similar to implementations we have seen from other handset manufacturers.
Frequency band gaps
However, these SKUs still have some notable gaps in which bands are supported. Even if we exclude Japan and China from the equation, the main missing capabilities are:
–Band 20, the European 800 MHz digital dividend band.
–Band 7, the European 2.6 GHz LTE band already deployed across Europe, Brazil, Russia and parts of Asia.
–Bands 38 and 41, the 2.5/2.6 GHz TD-LTE bands being deployed by China Mobile and Clearwire/Sprint Nextel in the United States.
A fourth product variant could potentially address this with another set of “bolt on” LTE PAs, covering bands 7 (2.6 GHz) and 20 (800 MHz).
However, what handset manufacturers and operators really want to do is adopt a single SKU multimode/multiband PA solution that overcomes the space and power limitations of current RF approaches.
Achieving truly global LTE smartphones
Envelope tracking technology is one way to overcome these cost, size and performance issues by making multiband PAs significantly more power efficient. ET improves PA efficiency by replacing the fixed DC supply with a dynamic supply voltage, which closely tracks the amplitude, or “envelope,” of the transmitted RF signal. This significantly reduces the wasted power experienced in traditional fixed supply PAs, enabling LTE performance with 2G battery life.
By utilizing ET, wasted energy in multiband PAs can be cut by more than 50%, boosting efficiency to 50% and beyond. This delivers significant improvements in thermal dissipation and power consumption, and paves the way for multimode multiband PAs that include global LTE coverage. In 2013 we expect this to enable compact LTE smartphone RF implementations supporting up to 15 frequency bands, using just 2 wideband PA modules to cover the 700 MHz to 2.6 GHz frequency range.
In the longer term, ET technology unlocks the performance potential of CMOS PAs (currently limited to 2G and low-end 3G applications) to deliver highly linear LTE waveforms, further shrinking the footprint and cost of multimode, multiband RF front ends.
The adoption of ET in smartphones is enabling operators, device manufacturers and consumers to overcome today’s fragmented LTE frequency band headache and develop single-SKU LTE platforms and devices, to achieve true global roaming.
