The emergence of GaN on Si PAs provides wideband performance and superior power density and efficiency compared to LDMOS devices, meeting the exacting thermal specifications while preserving valuable PCB space for the tightly-clustered mMIMO antenna arrays. Space-saving multifunction MMICs and multi-chip modules (MCM) are supplanting discrete ICs and single-function devices, enabling integrated RFICs for 5G base stations. FEMs are benefiting from a similarly streamlined design approach using integrated assemblies incorporating PAs, T/R switches, matching circuits, low noise amplifiers, digital step attenuators, controllers and DPD couplers packaged in compact packages (see Figure 5). With drain efficiencies approaching 60 percent and optimized integration of the Tx and Rx components, as well as DPD feedback paths, using FEMs in mMIMO radios and TRx boards has many benefits:
- Reuse portions of the transceiver board layout.
- Optimize device-to-heat sink thermal management.
- Optimize power levels, feedback loops, VSWR and control circuitry.
- Manage isolation and noise within the FEM.
- Enable dynamic power saving modes.
- Improve final yields compared to discrete designs, since the integrated FEM is fully tested.
Using the FEM design approach, redesigning a mMIMO radio for a different number of antenna elements, frequency band or power level is simplified, as FEMs are “plug and play” modules, with standardized interfaces, control logic and RF levels part of the design methodology.
In all mMIMO designs, where the antenna and electronics are contained in one enclosure (see Figure 6), the majority of the product engineering focus is managing thermal performance. The engineering efforts for signal processing, RF design, digital design, board layout, power design are indeed complex, but ultimately the mechanical/thermal/design and product environmental requirements will determine the volume and weight. Conventional 4G radio heads are built with the radio inside the heat sink, fins surrounding the entire package. With a mMIMO design, the antenna and its radome are very poor conductors of heat, limiting thermal dissipation to the rear of the mMIMO radio.
By leveraging advanced packaging techniques for the MMICs and MCMs within the FEM, additional cooling and space-saving benefits can be achieved. Figure 7 illustrates a simplified mMIMO design, not including the power supplies and fiber interfaces. The case housing has extruded fins bonded into the housing to save casting weight and increase thermal efficiency. The TRx board integrates the FEMs and RFICs, with the FEMs conducting heat down through thermal vias, while the heat from the RFICs conducts out through the lid. This allows heat to be dissipated in multiple directions, rather than unidirectionally from the FEMs and RFICs. Heat can be removed through the top lid and the bottom of the package through the ground vias and baseplate, distributing heat more efficiently and enabling a cooler device in a smaller footprint. Alternatively, the FEM can channel heat through both the thermal vias and the lid, to dissipate as much heat as possible.
The impending proliferation of 5G base stations operating sub-6 GHz, later at mmWave, will undoubtedly strain tower and rooftop deployment flexibility and site acquisition options for the foreseeable future. By alleviating the signal processing and conversion workload and exploiting higher levels of integration, from discrete components to FEMs, significant reductions in base station size and weight can be achieved.
The expected roadmap for FEMs, SOCs and full single module solutions, from optical in to RF out, is a natural progression of technology. Integration of the optical interfaces, with direct sampling RF Tx and Rx and the required signal conditioning, will define a true SOC. These evolving capabilities will enable 5G mMIMO base stations to become ubiquitous, fitting comfortably in the contours of our metro and suburban landscapes.