It has long been known that the market for 5G networks depends heavily on the ability to get high frequency signals directly to users, which becomes more difficult as newer generations graduate to higher bands. The move to higher frequencies requires bringing base stations closer to the end user, as well as channel aggregation, MIMO and beamforming. In addition, a larger number of base stations is needed to enable service delivery that cannot be achieved with a highly centralized network.
As a result of this need for small cells closer to users, the industry has expected huge growth in small cell deployments, but implementation has been slow despite the emergence of 5G deployments from telecoms. To help speed up deployment of 5G small cells, the Small Cell Forum recently published a report titled “5G NR FR1 Reference Design: The Case for a Common, Modular Architecture.” This report provides design guidelines for small cells, including design outlines for a modular RFFE architecture that can be implemented in small cells.
Since the original publication in December 2021, the Small Cell Forum has continued to produce specifications on the functional application platform interface (FAPI), neutral host/multi-operator requirements, and the digital front end in small cells. In this article, we’ll look at this reference design and the development path it illustrates going forward.
5G small cells take a modular approach
The specification provided in the report illustrates a modular path forward for network operators and equipment vendors by standardizing small cell construction and interfaces between subsystems. The approach makes small cell construction modular such that components could conceivably be mixed and matched to build a serviceable small cell. The principal areas to look at deeper are the components and interfaces used in a small cell assembly, as well as the RFFE for transmission/reception of data.
Components and interfaces
A 5G small cell, as specified in the 5G NR FR1 Reference Design, is designed to receive 3GPP-defined RF signals with small power and small form factor while servicing a small coverage area. These units have multiple functions to perform and act as a middleman between the end-user and the mid-haul section of the network. A typical 5G small cell contains the following components and interfaces:
- Power and data interfaces for connecting to the distributed unit (DU).
- Baseband unit (BBU) that implements 3GPP protocols and waveform processing.
- RF transceiver (could be an array) that implements modulation and DAC with translation to RF frequencies.
- Beamforming controller to implement a beam steering; only digital beamforming is specified in the reference design and this could be built into the BBU.
- RF front end (RFFE) that incorporates the entire duplexing/amplification/filtering; more than one of RFFE may be present in a small cell.
- Transmitting/receiving antenna array that interfaces with the RFFE.
The components listed here provide all the functionality needed to transmit, receive, modulate/demodulate, and send data back to the core network. This architecture specifies and standardizes each of the above items as a unique sub-system with a set of required interfaces and data/signal formats.
Through this standardization, network operators and equipment vendors now have a foundation for building interoperable hardware, both in terms of system-level design and component selection, as well as the RFFE design as a highly integrated package. To aid interoperability between off-the-shelf components and vendor equipment, the specification demands standard digital interfaces (SPI, MIPI, I2C and JESD204C) as the main communication protocols between each subsystem in the small cell.
RFFE design
The specification for the RFFE design lists multiple functional requirements that must be met within the transceiver, power amplifier and filtering sections:
- Provide transceiver output power amplification with minimal intermodulation.
- Filter excess harmonic content on the transmit path.
Today’s RFFE components are designed and manufactured following heterogeneous integration concepts and practices, where multiple modules on separate dies are integrated into a single package. GaN or GaAs FETs are the material platform of choice for these portions of system design as they can provide the required power output with high efficiency and withstand high temperatures.
For filtering on the receive path, acoustic wave filters, including surface acoustic wave and bulk acoustic wave, are currently the most preferred option. PHEMT-based LNAs are popular because of their low noise figure (NF) and adequately high operating voltage. Typically, a ceramic filter is used to connect the RFFE to the antenna since its power handling capacity is better than that of acoustic filters.
Getting involved in small cell construction
Although the small cell specification demands certain aspects of system construction be implemented in these systems, the specification is not intended to limit designer creativity or innovation. Instead, it sets expectations between systems designers and component designers. Standardization provides a path forward for building and deploying 5G small cells without demanding every system be custom-designed by each telecom operator or an external vendor. This also sets expectations for semiconductor vendors as they can now tailor new product lines specifically to this market.
Systems designers, PCB designers and component designers that want to get involved in 5G small cell construction are recommended to use the specification as a starting point for product development. A look through the specification shows block-level reference design data for a 2T2R small cell that designers can use to select commercial components.
The reference design also includes a bill of materials with a list of the major off-the-shelf components for each item in the above block diagram. To download the specification, visit the Small Cell Forum download page at this link.
