A key objective of the open RAN movement is to define common, modular network elements that can be mixed and matched based on open interfaces. This allows a wide variety of vendors to develop components, boxes and software, and for deployers to access a wide range of innovations without lock-in. It is particularly important for neutral host and private network operators to be able to access a choice of affordable, deployable elements that meet the needs of their business model and industry sector.

However, the complexity and performance demands of a 5G network mean this can never be as simple as that sounds. Some elements cannot be commoditized or made completely uniform, and the closer one gets to the end user, the more this is a key factor in network design. The components that receive and transmit signals to the user devices, including the antennas and RF front end (RFFE) systems, have to deal with a rising number of different 5G spectrum bands and combinations, including very high frequencies, as well as the need to drive down cost and power consumption.

Small Cell Forum has been working on the challenges of an RFFE that maximizes performance and spectral efficiency while being open – using as many common elements as possible and, crucially, having an open interface to the baseband and transceiver (TRx) units so that developers of centralized and distributed units (CU/DU) can choose radio units from any partner, and swap them in or out as required. It has unveiled its latest reference design work in this area, in a ground-breaking paper entitled ‘5G NR FR1 Reference Design: The case for a common, modular architecture for 5G NR FR1 small cell distributed radio units.’

As the title makes clear, the focus is on the sub-6 GHz (FR1) spectrum bands, with follow-on work underway for millimeter wave (FR2). Such projects are very important to private and enterprise network operators and neutral hosts. They are likely to need to access a wider variety of solutions than public network providers. The bulk of small cell Open RAN networks in the first few years are expected to be rolled out in enterprise and industrial settings, often by these specialized deployers. But each industry has its own particular requirements for 5G applications and performance, and this is true of the radio unit as much as the baseband functions.

For instance, some sectors will see a near term case for using low band spectrum, perhaps to cover a large campus with 5G quickly, while others will focus on midbands such as, in the USA, CBRS, initially for 4G. It is essential that private network deployers can choose from a wide range of radio units to suit their particular spectrum and performance requirements, and be sure these will interoperate, without significant custom integration, with their basebands.

Over time, many enterprises will introduce additional spectrum bands – adding millimeter wave 5G for a very high bandwidth application to an existing CBRS 4G network, for instance. So the demands on the RFFE will only get more complex as new use cases are adopted that require new spectrum, and future iterations of the SCF’s work will reflect the increasing diversity of 5G bands.

Adding more bands, while lowering latency and raising data rates, introduce new challenges in terms of power consumption, cost, network optimization and encryption. All of these factors can be daunting for smaller vendors, in particular, to solve on a repeated basis as technology moves on. In this scenario, reference designs are a critical enabler of an open ecosystem, which in turn provides deployers with the largest possible choice of radio/baseband combinations and of RFFE solutions. The designs support a significant amount of the work needed to make interoperable network elements, by specifying common interfaces and providing pre-integration. This not only simplifies design for vendors, and roll-out for deployers, but allows diversity in the ecosystem while protecting the economies of scale that come from common platforms.

As noted above, this is particularly challenging in the RFFE because of the large number of variations – radio frequency, output power and so on. The common factor is that enterprise networks, particularly those supporting multiple operators, will need to tap into as many spectrum bands as possible to maximize the devices, users and applications they can enable. In some cases, this will lead to adoption of dual-band small cells with two different RFFE units, but that needs to be achievable without unacceptable increases in cost and power consumption.

This new paper provides a framework for understanding the components and interfaces that make up a 5G NR FR1 small cell distributed radio unit. Reference designs can be built within this framework to address different markets with different RFFE requirements. The authors believe this will encourage and enable a market in which vendors design products that can be easily reconfigured for different applications rather than designed from scratch in each case.

Of course, this is a rapidly changing market. The paper sets out foundations for a framework for flexible yet common RFFE, but also recognizes that there will be many more projects required to address new requirements. One is to extend the framework to FR2, but even the FR1 definitions may be extended, with the top band potentially being raised from 3.3 GHz to 7.125 GHz or even 24 GHz.

Over time, the Forum also recognizes that the RFFE will need to have more components fully integrated, in the way that is seen in baseband and TRx already, to increase efficiency and simplicity. And new interfaces will emerge. There is currently uncertainty over whether Serial Peripheral Interconnect, or its challenger, MIPI RFFE, will become the standard next generation interface for control of the RFFE.

Other trends will be important in shaping the next iterations of this work. For instance, quick turn on/off times for components will be key to meet energy efficiency demands. The introduction of high-efficiency gallium nitride (GaN) power amplifiers will be very important to power efficiency.

Another important emerging development is to enable higher bandwidth in FR1 bands by developing hardware in which the frequency span can be fully occupied, which would reduce fragmentation of small cell variants to support different band plans.

In light of all these evolutions, and others that may be less visible now, SCF will continue its work on a common, modular architecture at interface, component and reference design levels, driving forward the goal of a fully open, flexible and integrated RFFE.