Marvell's 50G PAM-4 DSP for 5G optical fronthaul

• Marvell has announced the first 50-gigabit 4-level pulse-amplitude modulation (PAM-4) physical layer (PHY) for 5G fronthaul.

• The chip completes Marvell’s comprehensive portfolio for 5G radio access network (RAN) and x-haul (fronthaul, midhaul and backhaul).

Marvell's wireless portfolio of ICs. Source: Marvell.

Marvell has announced what it claims is an industry-first: a 50-gigabit PHY for the 5G fronthaul market.

Dubbed the AtlasOne, the PAM-4 PHY chip also integrates the laser driver. Marvell claims this is another first: implementing the directly modulated laser (DML) driver in CMOS.

“The common thinking in the industry has been that you couldn’t do a DML driver in CMOS due to the current requirements,” says Matt Bolig, director, product marketing, optical connectivity at Marvell. “What we have shown is that we can build that into CMOS.”

Marvell, through its Inphi acquisition, says it has shipped over 100 million ICs for the radio access network (RAN) and estimates that its silicon is in networks supporting 2 billion cellular users.

“We have been in this business for 15 years,” says Peter Carson, senior director, solutions marketing at Marvell. “We consider ourselves the number one merchant RAN silicon provider.”

Inphi started shipping its Polaris PHY for 5G midhaul and backhaul markets in 2019. “We have over a million ships into 5G,” says Bolig. Now Marvell is adding its AtlasOne PHY for 5G fronthaul.

Mobile traffic

Marvell says wireless data has been growing at a compound annual growth rate (CAGR) of over 60 per cent (2015-2021). Such relentless growth is forcing operators to upgrade their radio units and networks.

Stéphane Téral, chief analyst at market research firm, LightCounting, in its latest research note on Marvell’s RAN and x-haul silicon strategy, says that while 5G rollouts are “going gangbusters” around the world, they are traditional RAN implementations.

By that Téral means 5G radio units linked to a baseband unit that hosts both the distributed unit (DU) and centralised unit (CU).

But as 5G RAN architectures evolve, the baseband unit is being disaggregated, separating the distributed unit (DU) and centralised unit (CU). This is happening because the RAN is such an integral and costly part of the network and operators want to move away from vendor lock-in and expand their marketplace options.

For RAN, this means splitting the baseband functions and standardising interfaces that previously were hidden within custom equipment. Splitting the baseband unit also allows the functionality to be virtualised and be located separately, leading to the various x-haul options.

How the RAN is being disaggregated includes virtualised RAN and Open RAN. Marvell says Open RAN is still in its infancy but is a key part of the operators’ desire to virtualise and disaggregate their networks.

“Every Open RAN operator that is doing trials or early-stage deployments is also virtualising and disaggregating,” says Carson.

RAN disaggregation is also occuring in the vertical domain: the baseband functions and how they interface to the higher layers of the network. Such vertical disaggregation is being undertaken by the likes of the ONF and the Open RAN Alliance.

The disaggregated RAN - a mixture of the radio, DU and CU units - can still be located at a common site but more likely will be spread across locations.

Fronthaul is used to link the radio unit and DU when they are at separate locations. In turn, the DU and CU may also be at separate locations with the CU implemented in software running on servers deep within the network. Separating the DU and the CU is leading to the emergence of a new link: midhaul, says Téral.

Fronthaul speeds

Marvell says that the first 5G radio deployments use 8 transmitter/ 8 receiver (8T/8R) multiple-input multiple-output (MIMO) systems.

MIMO is a signal processing technique for beamforming, allowing operators to localise the capacity offered to users. An operator may use tens of megahertz of radio spectrum in such a configuration with the result that the radio unit traffic requires a 10Gbps front-haul link to the DU.

Leading operators are now deploying 100MHz of radio spectrum and massive MIMO - up to 32T/32R. Such a deployment requires 25Gbps fronthaul links.

“What we are seeing now is those leading operators, starting in the Asia Pacific, while the US operators have spectrum footprints at 3GHz and soon 5-6GHz, using 200MHz instantaneous bandwidth on the radio unit,” says Carson.

Here, an even higher-order 64T/64R massive MIMO will be used, driving the need for 50Gbps fronthaul links. Samsung has demonstrated the use of 64T/64R MIMO, enabling up to 16 spatial layers and boosting capacity by 7x.

“Not only do you have wider bandwidth, but you also have this capacity boost from spatial layering which carriers need in the ‘hot zones’ of their networks,” says Carson. “This is driving the need for 50-gigabit fronthaul.”

AtlasOne PHY

Marvell says its AtlasOne PAM-4 PHY chip for fronthaul supports an industrial temperature range and reduces power consumption by a quarter compared to its older PHYs. The power-saving is achieved by optimising the PHY’s digital signal processor and by integrating the DML driver.

Earlier this year Marvell announced its 50G PAM-4 Atlas quad-PHY design for the data centre. The AtlasOne uses the same architecture but differs in that it is integrated into a package for telecom and integrates the DML driver but not the trans-impedance amplifier (TIA). 

“In a data centre module, you typically have the TIA and the photo-detector close to the PHY chip; in telecom, the photo-detector has to go into a ROSA (receiver optical sub-assembly),” says Bolig. “And since the photo-detector is in the ROSA, the TIA ends up having to be in the ROSA as well.”

The AtlasOne also supports 10-gigabit and 25-gigabit modes. Not all lines will need 50 gigabits but deploying the PHY future-proofs the link.

The device will start going into modules in early 2022 followed by field trials starting in the summer. Marvell expects the 50G fronthaul market to start in 2023.

RAN and x-haul IC portfolio

One of the challenges of virtualising the RAN is doing the layer one processing and this requires significant computation, more than can be handled in software running on a general-purpose processor.

Marvell supplies two chips for this purpose: the Octeon Fusion and the Octeon 10 data processing unit (DPU) that provides programmability and as well as specialised hardware accelerator blocks needed for 4G and 5G. “You just can’t deploy 4G or 5G on a software-only architecture,” says Carson.

As well as these two ICs and its PHY families for the various x-haul links, Marvell also has a coherent DSP family for backhaul (see diagram). Indeed, LightCounting’s Téral notes how Marvell has all the key components for an all-RAN 5G architecture.

Marvell also offers a 5G virtual RAN (VRAN) DU card that uses the OcteonFusion IC and says it already has five design wins with major cloud and OEM customers.