4G LTE may hit a frequency cliff
With the rollout of a global mobile standard like 4G LTE there are 20 or more frequency bands and operators have found themselves struggling with RF front end circuit technologies not up to the task, writes Jeremy Hendy, v-p sales and marketing, Nujira.
The problems facing designers of RF front-ends for handsets and other cellular terminals have become increasingly challenging. For the last decade, engineering teams have been firmly focused on optimising and differentiating their digital modem chipsets, perhaps at the expense of RF front end development.
In the 3G era, 3 to 5 different frequency bands provided worldwide coverage, and the RF front-end has evolved to a relatively mature state.
With the rollout of 4G LTE technology, already deployed in almost 20 different bands, mobile operators have found themselves struggling with an immature, lagging RF front end. These fundamental limitations cannot be overcome with existing RF technology, and until they are it will not be possible to produce a truly global LTE handset.
Fundamental RF engineering limitations
The need to simultaneously support different systems (TD-LTE, FD-LTE, WCDMA, HSPA, GSM etc.) in one handset, while supporting the increasing number of frequency bands has resulted in a significant increase in RF system complexity.
LTE is a highly fragmented spectrum deployed across 18 separate frequency bands globally. Operators demanding truly global roaming 4G handsets are requiring at least 14-15 of these bands (along with 3G and 2G bands) to be covered by ‘global’ LTE phones.
However, it is simply not feasible to deploy separate narrowband PAs in a smartphone to cover all of these bands – any potential narrowband solution introduces too much complexity in the RF front end. There is neither the space nor the power available to support so many discrete PAs in one handset.
Of course the obvious solution is to deploy broadband PAs to cover far more frequency bands in a single device or module. The problem is that the power efficiency of broadband PAs designed to cover broader frequency ranges suffers, making them unattractive for commercial deployment. Whereas narrowband PAs may achieve something in the region of 35% energy efficiency, today’s broadband PAs may achieve only 25% efficiency.
So whether using narrowband or broadband PAs, the greater demands to support multiple transmission modes and frequency bands being placed on cellular radios in LTE handsets force designers to devote more space and power consumption to the RF front end. 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, and their battery life.
To combat this, manufacturers have to date relied on a 2G/3G “world phone” configuration, plus a couple of narrowband “bolt on” LTE PAs which are band/region specific; these could either be PA-Duplexer modules, or separate PAs and Duplex filters.
Each of these lineups has to be highly tuned, and adds to the phone footprint and cost due to the PA and duplex filter. That is why we don’t see any single product covering all the LTE bands yet; instead we see two or three operator and region-specific PAs and filters added to cover a limited number of LTE bands for high-profile operators.
These region/operator specific handsets are a logistical challenge for manufacturers, as they highly complicate the design, validation, production and inventory management processes. The “frequency band lottery” can also result in frustration for operators and end-users alike, due to delayed rollout or introduction of particularly desirable handsets.
To combat this, the relatively mature RF front ends supporting all “world bands” for 2G and 3G have been integrated to some extent. Quad band GSM is generally integrated into a single PA module.
Due to the higher power level requirements for GSM signals, these are generally handled by a completely separate transmit chain, even though the frequency bands are often the same between 2G and 3G.
Although “Hybrid” multimode multiband PAs are starting to appear, which integrate GSM functionality into a single package, these are still generally multi-chip modules maintaining a separate GSM transmit chain. True “Converged” multimode, multiband PAs (which use the same chain for 2G and 3G) are starting to appear but it’s a difficult design compromise to combine the high power requirements of GSM with the high linearity needed for 4G.
What handset manufacturers and operators really want to do is adopt a single multimode/multiband PA solution that overcomes the space and power limitations of current RF approaches.
Achieving truly global 4G smartphones
Envelope Tracking (ET) technology is one way to enable this, by making multiband PAs significantly more power efficient, thus overcoming the cost, size and performance issues associated with supporting multiple modes and multiple bands.
ET improves PA efficiency by replacing the fixed DC supply with a very high bandwidth 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 4G performance together with 2G battery life.
By utilising ET, wasted energy in multiband PAs can be cut by more than 50% and efficiency boosted to 50% and beyond. This delivers significant improvements in power consumption and thermal management, and paves the way for multimode, multiband PAs that support global 4G coverage.
In 2013 we expect this to enable compact 4G smartphone RF implementations supporting up to 15 frequency bands, using just 2 wideband PA modules to cover the 700MHz to 2.6GHz frequency range. We can also expect further efficiency gains and integration in the future, allowing manufacturers to further optimize the RF front-end.
The adoption of Envelope Tracking in smartphones is enabling operators and device manufacturers to overcome today’s fragmented LTE frequency band headache, and develop single-handset LTE devices to achieve true global roaming.4G, design, LTE, mobile, Nujira, power, wireless