Eli5:Why is it that cell phones , with their tiny antenaes are able to download files at LTE speeds inside concrete buildings from cell towers kilometers away yet getting wifi to work behind a corner is an exercise in frustration?
In: 2387
I am a telecoms engineer. I work with Radio Access Networks GSM/LTE/UMTS.
Most of the answers i see here tend to attribute the difficulty in communication on the WiFi case to a higher frequency of the WiFi signal hence a lower penetrability, but that’s only part of the answer.
The truth is that this is just one of the reasons.
There are several bands of LTE which are used on different environmental circumstances depending on what needs to be covered. Lower bands ( less than 1.8GHz) tend to have plenty of penetrability, more than WiFi’s 5GHz, so that’s great over large distances but it wouldn’t penetrate cement so well from many kilometers away.
We actually use higher frequencies (1,8GHz, 2.6GHz etc)in addition to lower bands to cover dense urban areas exactly because of their inability to penetrate, thus avoiding interference caused by over-shooting signals from kilometers away. This is called coverage overlay.
This means that while you may THINK that the LTE signal is coming from afar, depending on the coverage scenario, the station might actually be just few hundred meters away. This is further supported by the fact that mobile communications channels are actually uplink limited, meaning that even if you could get signal in a cement building from an antenna 20Kms away, your phone certainly does not have enough transmission power to reach that station from inside the building.
Which brings me to another reason. User u/alzee76 explains very well in his answer, that the effective power of an LTE antenna is thousands of times higher than your home router, therefore the signal is much much stronger than what you get from a router.
Another reason has to do with the radio wave’s physical properties too. Everytime the signal from the LTE antenna hits a tree, it is reflected into many tiny copies of that same signal which are then radiated into every which direction. This is called scattering. Depending on the environment there could be hundreds of tiny signals (called multipath components) that reach your phone, further increasing the probability that you receive the original message form the LTE antenna. Factor in all the reflections from the objects around you and the probability of getting a strong signal increases.
These multipath components carry very little energy so they dissipate reasonably easy but their effect is noticeable. That’s why you will notice that within a cement building, the best signal you can get is closest to the window.
Finally LTE antennas use some DSP trickery that can i crease the signal reception even further. One of these tricks is just copying the signal and sending it plenty of times from different antennas (also called spatial diversity) or sending it with some milisec delay (also called time-diversity) or automatically steering the signal radiation to the approximate location of the user (called beamforming).
There are more reasons that i may have missed but if you’re looking for a TLDR version just remember this:
Mobile Communications have been designed from the ground up to work over very large distances, utilizing VERY expensive and smart equipment, a lot of power and a multitude of locations.
Latest Answers