New features make co-packaged optics suitable for future radio networks

Why we are considering co-packaged optics in RAN

Co-packaged optics (CPO) is emerging as a technology to meet the demand for high interconnection speed at low power consumption in data centers and AI compute clusters. It is not currently used in radio access networks (RAN), where pluggable optical transceivers are instead widespread. The reasons for the success of pluggable transceivers are their operational flexibility and a consolidated vendor ecosystem supported by standards and multi-source agreements (MSAs). However, as the bit rate increases, pluggable optics struggle to dissipate the corresponding increase in power. The next 200 Gbit/s generation of pluggable optics may approach or even exceed 1.25 W/cm2 dissipated power per area unit, which is considered, in the industry, as the threshold where passive thermal dissipation may become prohibitive. Compared to pluggable optics, CPO dramatically improves bandwidth density (transmission capacity per area unit) and energy efficiency. First conceived for high-speed interconnects in AI clusters and data center switches, CPO is also appealing for RAN applications, but as explained in a recent Ericsson Technology Review article, creating a version of the technology that is suitable for radio applications will require some dedicated development. Together with Nubis Communications, we have developed a demo showing three critical features that will enable CPO use in future networks:

• CPO capability to operate at the high temperatures that may occur inside an outdoor radio unit,
• its compatibility with the standard optical fibers used in radio access network (RAN) sites, and 
• automated eye-safety mechanisms. 

How CPO can deal with high operating temperatures

The most important of the above three features is CPO’s capability to work at high temperatures (>100°C) that may occur in a passively cooled outdoor radio unit. Unlike data centers, active cooling means, such as fans or liquid cooling, are difficult to use in RAN, due to their impact on power consumption, equipment weight, and lifetime. This situation may lead to a high failure rate of the optical transceivers plugged into the radio unit. Since the laser in the optical transmitter is the component most sensitive to high temperatures, a solution would be to remotize it, placing it far from the hotspots of the hardware board. This is the solution adopted by most CPO implementations, that uses an external laser source (ELS) connected via optical fiber with the rest of the optical transceiver (optical modulator and receiver), often referred to as optical engine. Apart from the laser, the optical transceiver can be realized in silicon photonics, a technology that can operate at high temperatures. This way, the transceiver can be miniaturized and placed in the same package as one of the integrated circuits in the radio boards. In the demo, a CPO optical engine, provided by Nubis Communications, is shown to be small compared to an Ericsson radio unit. The unit will operate without the data-center style fan cooling to emulate the harsh environmental conditions that may occur in an outdoor radio site. As expected, the optical engine continued to work.

CPO impacts on RAN site solutions: Enabling operation over standard optical fibers

One of the issues with the current CPO designed for the data center is the need to use a special kind of fiber, called polarization maintaining fiber (PMF) to connect ELS and the optical engine. The performance of the silicon photonics optical modulator is indeed very sensitive to the state of polarization of the incoming ELS light. The PMF ensures that the polarization state at the modulator input is the right one and does not vary over time. However, compared to the standard single-mode fibers (SSMF) used in current radio installations, PMF is more expensive and less robust to mechanical stress. This is not a big issue in data centers, where the typical link distances are shorter, and the fiber is placed in a controlled indoor environment. However, in RAN, SSMF would be highly preferred. To solve this problem, the demo utilized a solution invented at Ericsson Research based on a frequency and polarization multiplexed laser source to achieve polarization insensitiveness. No appreciable performance difference was noted connecting the ELS and optical engine with a PMF or an SSMF fiber. 

CPO impacts on RAN site solutions: Solving eye safety issues

6G will push for higher transmission speeds across all the interconnection links within a RAN. The highest interconnection speed occurs between the digital and the radio frequency integrated circuits of the radio unit. Its value spans from several hundred Gbit/s to several Terabit/s, in extreme cases. To achieve these high bit rates, pluggable and CPO transceivers use parallel lower-speed optical lanes (for example, 16 optical lanes, 50 Gbit/s each, for an 800 Gbit/s transceiver). With a remote light source, this would require either several ELSs equal to the number of optical lanes or a single ELS split among all optical lanes. The first solution is too expensive while the second one needs a laser with a high emission power, whose value is further increased by the need to compensate for the coupling loss of the fiber with the silicon photonics optical engine (fiber and silicon photonics waveguides have very different spot sizes). The maximum emission power of a light source is regulated by standards that ensure it is not dangerous for the human eye. When a certain hazard threshold is exceeded, the same standards recommend using automated mechanisms to swiftly deactivate the laser, preventing potential damage to the eye. Common instances of risk include a fiber break or an incautious disconnection of the ELS. The demo implemented a simple automatic laser shutdown mechanism, invented by experts at Nubis, where probe light is transmitted together with the ELS light, at a different wavelength. At the optical engine, the probe light is looped back to the ELS. When the probe wavelength is not detected at the ELS, this means that an unintended disconnection occurs, and the automatic laser shutdown mechanism is triggered.

Summary

CPO can meet the parallel demand for high capacity and high energy efficiency in future radio access networks while also managing the high operating temperature that may occur inside an outdoor radio unit. Most of the technologies developed for CPO in data centers can be reused in RANs, making it possible to leverage an already established ecosystem, with obvious benefits in terms of costs.

However, the adoption of CPO potentially impacts the RAN site building practices, particularly the use of standard optical fiber and the implementation of eye safety mechanisms. Through collaboration with Nubis Communications, we have developed a demonstration showcasing that this impact can be minimized. The demonstration was run at the European Conference on Optical Communications (ECOC) 2024 (September 22 to 26, 2024, Frankfurt, Germany), which is among the premier conferences and exhibitions on optical communications worldwide, attracting numerous experts from academia, industry, and service providers.

The Nubis Communications and Ericsson Research demo team at ECOC 2024

Although there is ongoing work to validate CPO as a fully-fledged technology for RAN, the demonstration marks a significant stride towards achieving the same level of success as pluggable optics.