5G is one of the major enabling technologies that pushes the limit on wireless data transmission rates. Greater channel bandwidths require higher frequencies that demand the use of more advanced technologies to provide service to end users. One of the major changes to 5G cellular network infrastructure is the use of many more base stations distributed near end users, especially in urban areas. These small base stations require more advanced designs at every level, including the RF front end (RFFE) architecture and the physical design of the product itself.
In December 2021, the Small Cell Forum published a set of guidelines for systems designers, entitled “5G NR FR1 Reference Design: The Case for a Common, Modular Architecture”. The report and its recommendations provide guidelines for systems designers to build 5G-capable hardware for small-cell base stations, including a modular front-end that enables multi-operator usage with a neutral host and architecture for a digital front-end with integrated DSP features. In this article, we’ll break down this reference design and its major sub-systems.
5G Small Cell Reference Design Overview
Before getting into this small cell reference design, readers can download the reference material from the Small Cell Forum download page at this link. The reference design material includes a bill of materials with a list of off-the-shelf components for each sub-system in the specification. There are a few areas to examine further to better understand this reference design and its sub-systems.
Primary Interfaces and Sub-systems
In the 5G NR FR1 Reference Design, the entire signal chain is intended to develop and transmit/receive 3GPP-defined RF signals with optimized power consumption and minimal form factor. There is no required coverage area for small cells, although typically systems design will attempt to weigh tradeoffs between broadcast coverage area, power consumption, and reliability. The following lists the subsystems implemented in the small cell reference design:
- Power inputs and regulation
- Data I/O interfaces for control
- Baseband unit (BBU)
- RF transceiver array with DAC
- Beamforming controller
- Transmitting/receiving antenna array that connects to the RFFE
Each of these portions of the system plays an important role in signal transmission and filtering as outlined in the block diagram below.
The digital interface and other I/Os provide user control or connection to the distributed unit/mid-haul. The role of the BBU is to generate 3GPP-compliant signals that will be sent to the RFFE for broadcast. The NR FR1 reference design only specifies implementation of digital beamforming by the controller, although hybrid beamforming will likely be the technology that enables a scalable small cell system without significant increases in system size.
RFFE Design and Functionality
The components in the RFFE envision integration through heterogeneous integration of diverse functionality into a single component package. By incorporating these functions into individual packages were possible, the system size and power consumption can be minimized. For most of the RF power components, GaN or GaAs platforms are preferable as they aid low power transmission of 3GPP-defined signals along a wireless link. For reception and signal recovery at 5G frequencies, amplification and filtering are respectively preferred to be implemented with PHEMT-based LNAs and ceramic components.
From Architecture to Physical Design
5G small cells operate near the network edge, with physical construction and antenna designs that are not standardized across all small cell base station designs. Companies that want to deploy their own small cells and connect these to a network need to design and build a physical design that can provide the required transmit and receive throughput and directivity. RF physical designs can be difficult, including products like 5G base stations that can implement the required beamforming for high data throughput.
To qualify system design and architecture, 5G-capable systems require simulations at the systems level that can examine the following performance aspects:
- Antenna array design qualification
- Beamforming capabilities with single and multiple users
- Data transmit and receive with arbitrary channel state information
- RFFE qualification (power output, intermodulation, constellations, etc.)
- Cooling strategies in device enclosures
Simulations at the system level are important verifying individual portions of the system, as well signal propagation throughout the entire signal chain. Full evaluation of base station design requires simulating signal generation, transmission, and reception at both ends of a wireless link. A systems-level simulator can use your component and sub-system models to build these simulations for full evaluation of your next 5G-capable system.
No matter what 5G-capable system you want to design, you can qualify system performance with the complete set of system analysis tools from Cadence. Only Cadence offers a comprehensive set of circuit, IC, and PCB design tools for any application and any level of complexity. To learn more about interconnect simulation in high-speed systems, watch our webinar, System Simulation for RF Link Budget Analysis.