Solving 5G Interconnect Design Challenges

As 5G installations continue to roll out, momentum around the fifth-generation broadband brings design challenges and opportunities in equal measure.

The proliferation of data-intensive new technologies that rely on high-speed data delivery means the arrival of 5G connectivity is right on time. These technologies transmit and receive huge volumes of data at speeds we’ve never seen before. In the world of interconnect, RF, optical, and copper all play a role in making these connections. Much of this data traffic is moving into the optical domain, due in large part to the benefits it brings, like EMI resistance and high speeds. However, only copper can deliver both power and data. 5G infrastructure requires a combination of both modes.

“The old way was the passive antenna, with coax cables running down to a large baseband unit, where it would be demodulated and converted to fiber for the long backhaul. With 4G, operators started putting the remote radio heads up onto the pole, right below the antennas. That’s when fiber down to the baseband unit started becoming popular,” said Molex Global Product Manager Mike Hansen. “With 5G there are a lot more antennas with massive MIMO, there are new frequency bands, and it’s all a lot faster. So, in addition to bringing the remote radio head up into the antenna and it becoming an active antenna unit, there’s just a lot going on in a relatively small box.”

To prepare networks for 5G, these massive arrays of antennas now need to be demodulated, converted from analog to digital, and then switched or routed until it gets to the fiber. “Of course, the fiber is great for that long distance down the pole to the baseband unit, but we’re still seeing quotes coming in for board-to-board and high-speed mezzanine connectors because there are so many boards being packed into the active antenna unit (AAU),” Hansen explained.

All the traditional copper-versus-fiber arguments still apply, but according to Hansen, it’s becoming more of a hybrid solution. “Inside the box, we’re seeing copper, and fiber outside the box over long distances. It’s a trade-off between speed, density, and cost,” he said. In terms of density, copper still wins.

The trend for higher speed is now to route over twinax rather than use PCB traces because it avoids a lot of signal loss. Some of the companies designing and using high-speed switching chips, SerDes and ASICs don’t want to route through a board and introduce a lot of loss. Using twinax is a good solution to that.

"We might see a NearStack connector being right next to the chip on the board and going right out to a QSFP [quad small form factor] connector, so you have very little loss in the board because the traces are so short," Hansen said.

While PCB manufacturers are constantly improving their own technologies, flame retardant 4 (FR4) remains the dominant material. Although options including Mooring Equipment Guidelines 4 (MEG4), MEG6, and MEG7 are aimed at high-speed signaling, they are less common and more expensive. Using twinax to bypass the PCB is becoming popular as it effectively creates an overpass for data that can travel in a relatively straight line from point to point, instead of being routed using orthogonal PCB traces and perhaps moving between multiple layers through vias.

Moving from non-return to zero (NRZ) to pulse amplitude modulation 4-level (PAM-4) and 56 Gb/s to 112 Gb/s is also an important design consideration. The move to 5G will soon rely more on routing these high-speed signals and system designers are planning for that future. Specifying connectors that are ready for it is key, as connectorizing a system provides inherent modularity. Future upgrades such as new cards or better silicon can easily be added to a system.