What’s inside Rakuten Mobile’s Open RAN radio?

5G Technology World spoke with Analog Devices’ Nitin Sharma on the massive MIMO technology deployed in 5G base stations from NEC.

Open radio access networks (open RAN) were the talk of the 5G town in 2020 and that continues. Deployment of these open networks, which disaggregates network functions between the radio and the transport network, has begun. Rakuten Mobile in Japan was early adopter of open RAN. The RAN’s radio unit (RU), supplied by NEC, transmits at 3.7 GHz using Massive MIMO and is based on a fourth generation wideband RF transceiver from Analog Devices.

What’s the underlying technology in the Massive MIMO cells? To find out, 5G Technology World spoke with Nitin Sharma, General Manager, Communications Business Unit at Analog Devices. For a backgrounder on open RAN, see Open RAN unlocks network functions, a video interview with Sharma.

5GTW: How many antennas does each cell support?
Sharma: We’ve been working with NEC very closely on this and other developments. The one we talked about in the press release is a massive MIMO system. With 5G, there are multiple types of base station radio configurations including indoor and outdoor picocells and small cells. Both have relatively limited range. Macro Cells can cover up to 20 km. Massive MIMO provides significant capacity and efficiency for high-density locations such downtown city environments, large multistory/multi-building apartment complexes, or areas with high traffic density. These massive MIMO systems have up to 64 transmit and 64 receive antennas. The Analog Devices technology is the heart of the massive MIMO system. It’s the software-defined transceiver that processes all the signal paths.

The antennas from an array of 128 elements. Behind each transmit antenna is a signal chain from the transceiver to a power amplifier, typically providing 5 W of transmit power. On the receive side, there is a signal path also leading from each antenna to the transceiver. In the image below (Figure 1), each blue or red line represents either a transmit or receive antenna.

5GTW: Because the radio units are using digital beamforming, each antenna element needs its own power amplifier. What issues arise in terms of power efficiency and heat?
Sharma: Analog beamforming is single stream, while digital beamforming engages multiple streams and drives significantly more spectral efficiency. While Analog beamforming uses less power, digital beamforming’s significant performance improvements outweigh the incremental power difference. Massive MIMO does not require a separate DAC/ADC for each element, but each stream needs a dedicated power amplifier (PA). Again, with beamforming and other algorithms, the efficiency of the system of bits per hertz is significantly better versus a traditional macro base station.

5GTW: What role does the transceiver play in the cell?
The heart of these Massive MIMO designs is the software-defined transceiver. It integrates up to 8-transmit and 8-receive signal chains on a single chip, significantly reducing total system size, weight, power consumption, and cost. In creating the initial design of these chips, our philosophy was to first eliminate the unnecessary, then integrate the rest. Recent generations of transceivers have expanded integration to include digital functionality, including power-amplifier algorithms, which significantly improve efficiency. These transceivers support RF frequencies from 600 MHz to 6 GHz. So, when you think of this system, these transceivers fit right into wireless infrastructure sweet spot, where most of the 5G mobile spectrum is located.

5GTW: Does Analog Devices provide any other components in addition to the transceivers?
Sharma: For RF systems, consistent, flicker free power is critical for these high-speed communication systems. Along with software defined transceivers, ADI also provides precision power with our Linear Technology power components. Finally, these systems rely on ADI’s precision clocks, which have the accuracy of losing less than a second every million years.

5GTW: Is the 3.7 GHz being used throughout Japan or does it depend on the area?
Sharma: It will depend because the frequencies are still quite sparse, but that’s changing. The specific designs that we did are around the 3.7 GHz band, but what’s important is that the transceiver technology is software programmable. We just need to adjust our front-end design to make the transceiver operate at other frequencies. Now that there are more frequencies relevant globally and across Europe and the US. If you think about the C-band auctions that are occurring, those are the different frequency bands operating across the nations differently.

5GTW: What was meant by high-efficiency in the press release? Spectrum efficiency?
Sharma: High-efficiency we think of it as a two-fold. Spectrum efficiency and then power efficiency. We previously spoke about that, but when you think of it as a massive MIMO perspective, it’s typically measured in bits per hertz. One of the ways 5G is improving spectrum efficiency is through massive MIMO base stations. They’re equipped with carrier efficiency and are cost efficient. Beamforming helps with that spectrum efficiency. We talk about it from both perspectives – base station itself is truly efficient and then the other part is new spatial aspects of advanced beamforming. Together they increase bits per hertz efficiency.

5GTW: Were there any unexpected technical issues that had to be resolved?
Any system with this level of complexity has its set of challenges. Massive MIMO in general is a base station with 32 or 64 transmit and receive paths. Typical design considerations include interference, power consumption, and heat load.

5GTW: Please provide some insight into these4 design issues.
Sharma: For interference, algorithms dynamically block interferers in real time by placing nulls in the antenna patterns, thus increasing system capacity. We’ve talked about improving power efficiency, but power also generates heat. While the PAs are spread across a relatively large area, the power density in the digital processes is more centralized. Managing those thermal issues isn’t trivial. The technology that ADI brings in terms of transceiver integration helps simplify design implementation to a significant degree. We talked about 64 antennas, but when you think about the transceiver technology, it is integrating dozens of signal chain functions multiplied by four separate Tx and Rx paths. This level of integration, combined with the programmable, layered software in the chip system, is what simplifies the system design and makes it easy to use. Working through those challenges and coming out with an efficient and successful system is a testament to the great people at every level of process, from the chip team through to the last person plugging in the wires.

5GTW: What kind of range is expected from each base station?
Sharma: That is more a function of the total radio power, the choice of PA, and the choice of how much power the base station radiates. These cells are generally being deployed in metro areas, so they have relatively long ranges unlike some of the other 5G base stations like small cells that typically have a shorter range.

5GTW: How is digital predistortion used and why is it needed?
Sharma: Digital predistortion (DPD) is a technique to improve power amplifiers. When you have massive MIMO, a station’s signal power dominates other stations. Anything that can be done to improve its efficiency is an improvement for wireless operators. DPD is a technique that we use in the transceivers to increase an amplifier’s power output and efficiency. The harder you run a PA, the more efficiency you get. So DPD is a commonly used technique. What is unique about what we’re doing is it being in the transceiver. It’s a very efficient, low power algorithm that we run. In the figure below (Figure 2), the blue line represents channel power without DPD, the yellow line is with DPD. Using DPD reduces a signal’s power that appears outside the channel of interest.

5GTW: Nitin, thank you for your time. We look forward to hearing from you again on 5G Technology World.