Vehicle automation is one of the most ambitious uses envisioned for 5G. Why? The new generation of radios and networks might help self-driving cars talk to each other, share large volumes of driving data with the cloud and receive guidance from traffic management systems.

But when it comes to the radio frequencies that these applications would use, the grand vision comes back down to earth. Physics and regulation will largely dictate which radio signals robocars can use, and exotic new frequencies probably won't be in the picture anytime soon, according to LightReading recent analysis report.

A not-quite-unified approach
Some spectrum has already been set aside for automotive wireless applications. In the US, the European Union, China, Australia and other jurisdictions, frequencies in the 5.9GHz band have been designated for vehicle-to-everything (V2X) communication. V2X can work without a carrier network, and these frequencies don't belong to any operator. V2X applications could include traffic and road hazard warnings from roadside transmitters, cars alerting each other to their location and trajectory, sharing of high-definition sensor data, and future smartphones signaling to cars that a pedestrian is crossing the street.

But V2X spectrum assignments date back years, and it's not clear where or when 5G will be used on those frequencies. China is strongly committed to cellular V2X (C-V2X), which uses cellular radios and is expected to become part of the 5G standard. Regulators in the US and EU are still debating how much to embrace C-V2X, versus an older, wireless, LAN-based technology called Dedicated Short-Range Communications (DSRC) or IEEE 802.11p. DSRC is available in a few cars today but has seen very limited use.

A recent proposal by Federal Communications Commission Chairman Ajit Pai might be a game-changer for C-V2X. Pai asked the FCC to consider reallocating the chunk of 5.9GHz that is set aside for DSRC in 1999. Saying DSRC had failed to reach its potential, he proposed handing over the bulk of that spectrum to unlicensed use, especially for Wi-Fi. But 20MHz of the band would then be assigned to C-V2X. The other 10MHz would be allocated to either C-V2X or DSRC, depending on public input.

Either way, 5.9GHz is one band that won't be available for self-driving applications that require a wireless WAN. And the fast, millimeter-wave frequencies newly assigned to 5G are years away from being able to reach moving vehicles with anything like the kind of reliability needed.

Staying low
That leaves familiar cellular bands below 6GHz for most automated driving applications, operators and vendors say. While leaving 5.9GHz to support V2X applications that don't run on its own network, AT&T expects to use existing cellular bands for infrastructure-based 5G automotive services that require ultra-reliable, low-latency communication (URLLC), said Cameron Coursey, vice president and CTO of IoT at AT&T. At the high end, cars can't even connect yet to millimeter-wave cells while driving, analyst said.

"Anything above about 2-3GHz is going to be a challenge for vehicle-to-infrastructure connections, because of coverage and handover reliability when a vehicle is moving down a highway at 60 miles an hour. It is different for vehicle-to-vehicle because they tend to be moving in unison, and therefore the relative speed between vehicles is pretty small." Analyst Marshall commented.

To reach moving cars with millimeter-wave signals, the propagation characteristics haven't even been worked out yet. Millimeter-wave has much more limited propagation than the lower cellular frequencies and often uses complex paths that involve bouncing signals off buildings. Getting cars to connect with cellular networks on the upper frequencies reliably would probably require sophisticated antennas on all sides of the vehicle, said Martin Beltrop, head of Nokia's mobile networks automotive business.

An upper limit of 2-3GHz would also prevent AVs from taking advantage of the C-band spectrum, located between 4GHz and 8GHz, which the US Federal Communications Commission is considering auctioning off for additional "mid-band" 5G spectrum that could offer high speed with greater reach than millimeter-wave frequencies.

Long-term prospects
Any solution to the high-frequency problem probably belongs on a roadmap beyond five years from now, said Maxime Flament, CTO of the 5G Automotive Association (5GAA), a cross-industry group backing automotive 5G.

But AVs should be able to use millimeter-wave frequencies for "data showers," or fast downloads and uploads in fixed locations, AT&T's Coursey said. While a car is off the road at a parking lot or charging station, it could take advantage of gigabit-speed 5G to get software updates or share trip information with the automaker.

Meanwhile, those high-speed data breaks might also be used for quick downloads of movies and music for AV passengers to enjoy while not driving.