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Filter Technologies for 5G Wireless Communication Systems

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Filter Technologies for 5G Wireless Communication Systems A s millimeter-wave fre- quency (mmWave) bands are increasingly used for high-speed links in next-generation wireless systems, filters will be essential to minimize interference. Fifth-Generation (5G) wireless technology represents the next milestone in mobile wireless com- munications, targeting more signal traffic, increased capacity, reduced latency, and lower energy consumption than its pre- decessors. To achieve these goals, networks will need to increase bandwidths through carrier aggregation and a push into mmWave spectrum, all while improv- ing spatial efficiency with base station densification, massive multiple-in-mul- tiple-out (MIMO) antenna technology, and beam-form- ing antenna arrays. These enabling technologies will place new demands on the underlying RF front-end components, particularly the vast number of filter designs required across a heterogeneous network of base stations (of varied cell sizes) and mobile devices. Systems offering large bandwidths through carrier aggregation and ubiquitous coverage through mas- sive overlapping of micro-cells must manage both in-band and out-of- band interference. Likewise, imple- mentation of Massive MIMO will require compact filtering technology that mitigates the adverse impact of out-of-band interference on the uplink sum rate of maximum-ratio- combining (MRC) receivers. This Basics of Design (BoD) takes a look at the filter challenges brought on by moving into relatively uncharted fre- quency spectra and adopting these new technologies, as well as the fac- tors driving the physical, electrical, and cost restraints for 5G filters and the supporting simulation technolo- gy that will help designers physically realize these components. Current Mobile Device Filter Technology Today's Fourth-Generation (4G) Long Term Evolution (LTE) smartphones support in excess of frequency 30 bands, requiring over 60 filters, many in the form of multiplexers. This number of filters consumes significant space and commands the largest share of the RF expense budget in the mobile ecosystem, putting considerable cost pressures on component manufac- turers to meet performance goals for low costs. The majority of these filtering components are based on surface-acoustic-wave (SAW) or bulk-acoustic-wave (BAW) technol- ogy. At the lower frequency range, SAW filters meet the requirements for low insertion loss and excellent rejection, covering broad band- widths at a fraction of the size of traditional cavity and even ceramic filters. Meeting these requirements with the increase in frequency to 6 GHz and mmWave bands is prov- ing to be a challenge for these filter technologies. A conventional filter stores signal energy in the charge on capacitors and current in inductors, whereas BAW and SAW filters store the signal energy in acoustic resonators. As the name implies, surface acoustic waves propagate in the lateral direction with the shape and center frequency of the passband determined by the pitch, line width and thickness of the interdigital transducers (IDT) (Fig. 1). Because they are fabricated on wafers, SAW filters can be created in large volumes at low cost; filters/ duplexers for different bands can be integrated on a single chip with little or no additional fabrication steps. Their key advantages are low cost, wide relative bandwidth, and flexible port configurations. However, due to the degradation in selectivity at higher frequencies, SAW filters have limited use above ~2 GHz; at those higher frequencies, they are mostly used for applications with modest performance require- ments, such as global system for mo - bile communications (GSM), code 1. Basic structure of a SAW filter. A Supplement to Microwaves & RF Sponsored by Cadence

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