Open-RAN puts more focus on emulation in testing programs

Until recently, the trend in cellular infrastructure has been for the market to coalesce around a small number of big players who, to a large extent, can take on the systems integration of a network on behalf of an operator.

A combination of the shift towards cloud computing in core telecom management and a desire among governments, particularly in the West, to restrict the activities of two of the major equipment suppliers has opened up the field for the Open-RAN set of standards that are likely to help open up the market to new players.

One of the consequences of the shift towards Open-RAN in 5G communications is that systems that were once expected to work in a single-vendor ecosystem now have to work with more third-party hardware. This is not unusual in design overall but 5G adds the complication of being a moving target, characterized by multiple successive releases. This is putting increased pressure on testing and verification of 5G SoCs and subsystems on top of the complexity that comes with what is a much tougher protocol to implement compared to its predecessors.

“With Open-RAN, when you have distributed units and radio units from different vendors, there can be interoperability problems and you need to debug those,” says Harri Valasma, director of 5G solutions at Mentor, a Siemens business.

Shift left to run more tests

Because of all the changes, Valasma says the technical requirement is growing rapidly but design teams need to complete projects in the same time as they did for 4G. To deal with this, design and verification teams have opted for a shift-left strategy in which they try to do more functionality testing in simulation and emulation before tapeout but also do extensive post-silicon testing in different environments and network conditions.

“The number of tests is growing exponentially,” Valasma says. “Many look at the latency issues, which are very important in 5G. Delays to packets as well as overall functionality and interoperability are all important. There are a lot of different tests that need to be run to check compatibility. That’s why you need to start running them early.”

The key to effective verification is test stimulus that reflects real-world conditions. A number of vendors, such as Keysight and Viavi, have produced network emulators that exercise board-level hardware and boxed systems. Keysight’s Open-RAN Studio, for example, works as a programmable distributed unit that will generate complex test vectors to exercise one or more radio units. X-Step, which was acquired by Siemens in 2018 in its acquisition of Sarokal, is designed for the fronthaul 5G environment but to operate in a variety of scenarios, from SoC verification on a Veloce emulator, through hardware prototypes and unit testing of finished products through to systems integration of multiple radio and distributed units.

“The industry is moving away from the traditional design flow where you first develop the boards and then test things together,” Valasma claims. “X-Step can take Open-RAN testing from the beginning to the end. It can be run from one vendor to another – they can exchange tests cases that may be run from simulation through emulation to FPGA prototyping.

“It means you can do more in the emulation to test radio setups and configurations. And when the real silicon is ready test on the hardware.”

The ability to test hardware at different phases of hardware readiness is leading to what may become the virtual equivalents of plugfests where instead of moving all the finished hardware to a single location, such as Samsung’s OTIC, vendors work with each other’s test data to see how their devices react. “It’s already happening. They are building up a common environment to exchange information. ‘We verified our set and here is the test setup for you’.”

At the moment, the interactions are primarily one vendor dealing with another, Valasma says, though it is possible the use-cases will expand to work in more complex multivendor exchanges. If independent systems integrators become more important to the rollout of 5G, they may call on hardware vendors to work with them to build experimental setups to try out configurations before taking them to their operator customers.

Easing emulation

A secondary effect of the shifts in 5G, Valasma adds, is the transition from analog I/O in the radio units to greater adoption of the digital JESD204 serial interface between front-end and baseband processing subsystems. “It creates a situation where you have digital inputs and outputs to test. If you have analog on the other side, it’s problematic to control particularly in systems that have to deal with massive MIMO and beamforming. It’s easier to copy the tests from emulation to the post-silicon environment with the digital interface.”

Emulation is also important post-silicon, Valasma says. “There are test scenarios where you have huge amounts of software that can’t run pre-silicon. If you find a problem post-silicon you can go back to simulation or emulation very easily, run the test case there and debug very deeply.”

To help deal with the multivendor situation that is building around 5G, Mentor joined the O-RAN Alliance in early June. As an alliance member, Mentor aims to focus on verification and validation requirements for standards-based, configurable network development.