This seems to happen all the time. Regardless of how good my service actually is, I’m always at a few bars of LTE.
In: Technology
There are a number of reasons, but one common one that is not obvious is multipath. Multipath is where a signal is reflected off of nearby buildings/mountains/ground/etc, and results in the receiver seeing multiple competing signals. Imagine you were in a cave, and someone is trying to talk to you across a distance. If you were in an open field, you could easily understand them at that distance. In the cave though, the echoes of their voice reach your ears at slightly different times. Even though you are getting plenty of volume, it is hard to discriminate the message.
With radio signals, it can be easier in some ways and harder in others. If one “echo” is significantly weaker strength, modern receivers can determine that the echo can be ignored, AND they can cancel out some of that echo to make it easier to hear the main signal. As more echoes are added, as long as one is significantly stronger than the others, it just takes more processing power and all the others can be filtered out. (This is one reason why newer phones seem to have fewer problems connecting to cell service. Newer CPUs and signal processors are smarter at filtering/discriminating, faster at detecting and correcting, able to handle more signals at the same time, and overall better at dealing with multipath.) The problem comes when there are two paths that are very similar, resulting in two paths that are about equal.
Two paths that are about equal can result in about the same signal strength, and small changes (like the wind blowing tree limbs along one of the paths) can make one better than the other for just a moment. Plus, the same signal (same frequency) that takes different paths can actually cancel itself out in some places. (This is a bit complicated, but if you think about a lake, and there are two waves that meet, in some places the waves cancel out and the water does not go up or down.) The signal will essentially never completely cancel out, but it can cause enough of a problem to keep modern devices from having a good connection (again, even though it is getting plenty of “volume”, ie., overall signal strength).
In theory, cell phones could do periodic check-ins to determine how good their connections are, and display that instead of the signal “strength” bars. In practice though, that would mean a great deal more traffic for a small bit of extra information.
One thing to try if you are having this issue is to turn on airplane mode for ~15-30 seconds, then turn it back off. This will force your phone to check back in, and may allow it to connect to a “better” tower. (“Better”, not necessarily closer. Due to multipath, one tower might be closer but go through/past obstructions that cause problems.)
One hope for 5G is that there will be more “towers”/micro-cells, and less distance means less multipath. In practice, that may or may not help.
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