The FAA is concerned that 5G cell phone signals could adversely affect radar altimeters on aircraft. This is because 5G signals are in the 3.7 to 3.98 GHz range, while radar altimeters operate in the 4.2-4.4 GHz frequency range. The worry is that the nearby cell signals will cause interference with avionics. However the 5G roll-out in Europe is also under way but I haven't noticed any concerns raised by EASA or the CAA. Why not? Is there a difference in frequencies/equipment between the US and Europe? Or have I just missed the news stories about potential problems outside the USA?
If you read the FAA’s statements (not media reports) closely, you will find their complaint is merely that it hasn’t been proven not to be a problem, not that they actually expect a problem or have any rational basis to suspect it would be.
This follows the general hyper-conservative stance of the FAA: assume everything is unsafe until proven safe (at someone else’s expense), which has not been done in this case.
As a result, operators are prohibited from relying on radioaltimeters in areas where the 3.7-3.98GHz band is being rolled out, but they are still to monitor it. If no problems are reported after some period, they will rescind the order.
Since you are asking specifically about the 3.7–3.98 GHz band, the answer is that most of the world uses the 3.3–3.8 GHz band. The 3.7–3.98 GHz band was seen as much less important until US operators (somewhat surprisingly) paid over $90 billion in an auction this year.
So, the US are the first (and at the moment only) ones to use this band, which is why it makes sense that they are also the first ones to worry about interference in this band.
However, the French DGAC has also voiced concerns.
The FAA wants to draw attention to and monitor any effect of 5G on aviation safety so it detects issues now, while they are minor, and not later, as the number of 5G devices per square mile grows and the effects get stronger.
The rest of this is just my theory on why it might be different by nation. In the USA, there has always been an abundance of extreme caution about allowing radio equipment to proliferate in the market before ruling out the potential that it could later have to be banned or heavily restricted. You want to know up front. This eliminates waste in producing such equipment. The national difference here is the "regulatory takings" doctrine, which I don't think much of Europe has. That is where, if a person or company bought a perfectly legal thing that had a perfectly legal use, and that use is later banned or restricted in a way that decimates its value, it's considered "taken" by the government, even if they didn't physically take hold of it. This means it must be for the public good, and even then, they have to pay. If you ban beef, you've "taken" the full value of all the cows currently being raised for beef, and would have to compensate farmers their full value under the Constitution. Ban driving, pay for all the cars. There are still exceptions to seat belt laws for extremely old cars that existed before the seat belt law - Congress keeps those exceptions because if they didn't have that exception, the law would be a "taking" of antique cars and they'd have to pay for them. Ban 5G later when problems are discovered, pay for all the towers (and all the phones' price difference from the 4G models most likely). Because the financial risk of proliferating infrastructure that will have to be banned falls on the agency banning it, they have an incentive to actually look ahead before more is built. If you are a European leader, ignore the problem until it is a crisis, then ban 5G, and make the telco's take the loss.