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?
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3 main reasons:
1) you are comparing a use case (wifi) with two small devices with relatively bad antennas transmitting in all directions at low power with another case (4G) where on one side you still have the bad device, but on the other side you have a very effective antenna, transmitting just over a narrow angle and at a power orders of magnitude higher. Obviously the second case works better.
2) 4G frequency is usually lower than the WiFi frequency. And the lower the frequency, the better it travels through stuff. The drawback is that the lower it is, the less data you can transmit (yes, I know, that sentence is almost wrong, it has a lot of shortcuts).
3) 4G frequencies are used by a handful of actors with properly dedicated frequencies so it is usually a fairly clean environment. On the other hand, the WiFi frequency is basically a free for all with anyone being able to transmit over it (under specific conditions of course). This means you would have a lot more devices trying to talk over each other. Your phone is talking to your router but your neighbours phone talking to their router is also using the same frequency and everyone have to share it. There are WiFi mechanisms for this but if it gets too crowdy, then it impacts the speed.
Simplified version: you can either have *faster* speeds or a *farther* connection. You sacrifice one for the other.
As you’ve noticed yourself, with cellular you get much farther distance but at the cost of speed.
For the same reason you can sit in the shade at the park and still be able to read a book by sunlight but if you were trying to read in a cave with a lantern you would need it right next to you rather than around the nearest corner.
One of those sources is MASSIVELY more powerful despite being quite a bit further away
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.
Cell towers are way closer than you think. Most of them won’t be several kilometers away, but actually within 1 kilometer and less.
NASA uses ZigBee wifi (900mhz) for communicating between their rover and their drone copter. With the lack of other signals and thinner atmosphere they’re theoretically able to connect over distances of over 2700 feet.
https://www.theverge.com/2021/5/20/22445330/zigbee-on-mars-ingenuity-helicopter-perseverance-rover
Lucky you if you get High speed downloads indoors at work.
I get zero data connectivity while in my office.
Often, larger building will have a DAS (distributed antenna system) installed to provide coverage throughout the whole building. These are small, low powered, wide band antennae forming an array inside, for example, large commercial buildings or highrises. Many malls also have these installed. I have worked on a handful of these installations.
As to why cell providers do it is by pressure from large corporate clients, who insisted coverage improve, or they would switch their entire company’s provider, to someone who would provide coverage, which would be millions in lost revenue a year.
Power and contention.
1. A cellular tower transmits with a power level several thousands of times higher than your wireless access point. A typical cellular tower will transmit an ERP (effective radiated power — the actual transmission power, with antenna gain factored in) of 100-500W while maximum power for a 2.4GHz wifi access point is 100mW; the transmission power of the cellular tower is 1000-5000 times higher. At a range of 1km the signal from a 500W cellular tower will fall to 0.5mW thanks to the inverse square law. Your wifi access point falls to the same level at a distance of just 30m. This is all in open air with no obstructions.
2. It’s likely there are dozens if not hundreds of wifi hotspots and a few times that many client devices around you fighting for space in the radio spectrum, unless you’re well out into the countryside. There will be a half of that number or less of cellular clients though, and less than a dozen cellular towers, and everyone on the cellular network is cooperating to share the space through the protocol. Meanwhile wifi clients on the same channel but talking to different access points are fighting with one another, causing collisions and retransmits that slow down throughput.
The real answer here is because they were designed that way. Wifi is designed to use public frequency bands and for short ranges so they won’t have a large negative effect on other people using those same frequencies for their own use. Most people don’t want or need their WiFi to span a mile from their house. Cellular technology is built to use reserved frequencies that won’t conflict and is built to travel long distances.
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