A major change in smartphone RF filters and front ends as 5G approaches

-September 08, 2016

The ongoing challenge of multiband/multistandard RF solutions for smartphones requires the addition of more bands into the same or smaller physical space in the handset. In addition, performance must improve in the next generation of smartphones.

In long-term evolution (LTE) carrier aggregation (CA) and beyond, the need for multiple bands operating simultaneously through one antenna necessitates so many added challenges for filters and duplexers. Isolation loss and linearity are probably the most difficult to achieve. Reconfigurable radios are also another path which can be investigated going forward. With the radio spectrum becoming more crowded, smart cognitive radios are being looked at. The problem is that mobile phone manufacturers do not like having to add new models to keep up with bandwidth needs. This is not very cost effective.

The coming carrier aggregation and multiple-in-multiple-out (MIMO) designs will be needed to meet the needs of both interim LTE-Advanced and the ultimate arrival of 5G. Smaller and lower cost filters are a necessity in these new systems (Figure 1).


Figure 1
RF Front-end architecture for the low-band frequencies (700-900MHz), demonstrating the complexity of the RF front-end. (Image courtesy of Reference 1)
Click to enlarge


Tunable filters might be able to alleviate the design problems facing engineers that will have a good fit in a small, low power handset. I want to discuss the major innovations in this area by a company called Resonant. With 70 patents under their belt, they are out to change the direction of RF front ends (RFFEs) in the handset.

Filter architectures

Resonant has very unique and robust surface acoustic wave (SAW) with equal performance of the bulk acoustic wave (BAW) types of filters.



Figure 2 This image shows the different ways to construct an acoustic resonator (Image courtesy of Reference 1)


Their designers us an infinite synthesized networks (ISN) design method in developing a Band 3 duplexer by using low cost SAW processes that equal or exceed the performance of the BAW Band 3 duplexer--a more costly solution for the handset.

Power amplifiers

In today’s RF design community, engineers have been able to design a single power amplifier (PA) which is capable of handling multiple technology modes like CDMA, LTE, W-CDMA and multiple frequencies and bands. This is the multi-mode/multi-band (MMMB) PA. A filter is needed for each RF path, so this adds to the extra cost in the handset.

The infinite synthesized networks

Resonant has managed to combine modern filter theory, finite element modeling for both electro-magnetic and acoustic types, and an innovative set of optimization algorithms. Resonant’s very accurate filter models reflect the physical details of the filter structure which offers the actual filter performance in the areas of loss, isolation as well as in power handling capability and linearity.

Now designers are able to greatly reduce development time and complexity since the optimization can now be done on a computer instead of using high costs of multiple iterations in the fab. Another added benefit is the design and simulation of optimization of performance over many temperature ranges before producing a piece of hardware.

Designs can be optimized at higher power and higher temperatures such as that required by LTE, which operates at higher power than CDMA. Ultimately yield will be improved which in turn lowers the cost of production and brings a faster return-on-investment (ROI).

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