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There is something I would like to understand.
My question is, if a smartphone can only send a Wi-Fi signal up to 10 meters, how does it communicate with a router which is 40 meters away ?
Or in other words, if a smartphone has a transmitter powerful enough to send data 40 meters away, why is the range of its Wi-Fi mobile hotspot so short ?
Good question!
Basically the smartphone can't transmit a very powerful signal, but a router can "hear" a much weaker signal.
Wireless communication waves don't have a hard cutoff line, they just become weaker or more distorted over distance. The more powerful a transmitter is, the farther the signal is strong enough for other devices to pick up. That's why your not-very-sensitive phone's receiver can pick up wireless networks from farther away.
But for a two-way communication you need a reverse channel and the transmitter is not very powerful, so you need a more sensitive antenna that will pick up this signal even though it's much weaker over the distance.
This is why just boosting transmitter's signal doesn't improve WiFI access point's range, you need a more sensitive antenna too.
The range of a wireless link is defined by the link budget.
The link budget takes into account:
So the max distance depends on both the transmitter and receiver. Of course, in most cases (and definitely in the case of WiFi), you also need to do the link budget in both directions, as data is sent in both directions.
When you compare your phone "talking" to your router or to another device, some things are common (your phone's transmit power, antenna gain, sensitivity), but other parameters change:
When your phone is communicating with your router, the link budget equation uses your router's antenna gain and sensitivity for one direction, and your router's transmit power and antenna gain for the other direction.
When your phone is communicating with another device, all these parameters are replaced by the values for that other device.
Routers usually have less constraints than other devices, especially other phones: larger/better antennas, higher transmit power, and possibly better sensitivity.
This explains why the communication between a device and a router is usually better (i.e. has better range, or better speed at the same range) than the communication between two non-router devices.
Note however that when comparing, you should do so in the same exact conditions. Wireless transmission is affected a lot by all obstacles in the way, and 30-40 meters range with a WiFi router is in many cases just a wild dream.
Additional factor is power - a mains-powered router has no battery management requirements.
By comparison a cellphone has to balance power usage by the wireless radio, cellular radio, CPU and rest of system. The two different radios are in use at the same time, forwarding data received in one and sent out the other.
My phone's battery lasts a significantly shorter time with hotspot enabled, and reducing the radio's transmit power helps save that precious battery charge.
The range of radio communication is defined by both the transmitter and the receiver, not just the transmitter.
When using a router, you have a 30m range between a big router antenna, and a tiny smartphone antenna.
When using a hotspot, you have a 10m range between a tiny smartphone antenna and another tiny smartphone antenna.
Obviously a poor transmitter and a good receiver can talk over a greater distance than a poor transmitter and an equally poor receiver.
The ranges you are quoting are distances within which a typical device can connect. The wi-fi router doesn't have an exact range, the signals get weaker with distance and with interference from objeccts in the environment. The Wi-Fi router probably has antennas you can see which allow it to transmit and receive better, and it transmits at a higher power than a smartphone. This allows a typical device like a phone to communicate with it over say 30 meters. You can purchase directional antennas that use the same power but achieve greater distances by focusing the signals in a narrow angle.
The smartphone has a smaller antenna and uses less power. When used as a hot spot, a similar device probably needs to be within 10 meters to communicate reliably with it. The signal strength from the smartphone at 10 meters is probably about the same as you would see from the wi-fi router at 30 meters.
By Design.
Why should the Smartphone make more?
A mobile Hotspot is mainly for the Smartphone User. So by Design the WiFi User is nearby. Mostly the Smartphone lays directly near the Laptop of the Smartphone owner. So a strong Signal is not needed. So the Smartphone Hotspot Feature can be designed to save energy which is more important.
Another point I don't really see raised is that less WiFi range can be better from a cross-talk point of view, allowing greater density of WiFi devices within a certain area.
Imagine a hall full of people talking to each other. If everyone talks to their conversation partner at just above a whisper, then everyone in the hall should be able to chat easily with each other. But if someone in the hall starts talking very loudly to someone right next to them, it makes it harder for other nearby people to hear each other, so they in turn might try talking louder to each other, causing a cascading effect of everyone trying to be as loud as possible to be able to speak over each other.
In WiFi terms, speaking volume is akin to transmission power.
So offices/campuses with good WiFi setups actually lower the transmission power of their access points while at the same time installing more of them. Because each access point is less "loud", this allows a much greater amount of WiFi devices within the coverage area.
In houses it's often not so easy to have multiple access points, so a single WiFi router with high transmission power makes sense to try cover its entirety, also, as it's unusual to have more than a dozen WiFi devices in a house, device density is not a real concern.
But this changes in an apartment block where each resident potentially has their own WiFi router and if everyone has their transmission power on maximum, then the cross-talk becomes problematic. Ideally everyone in an apartment block would have their router on the lowest transmission power possible while still being able to get signal. Unfortunately though it takes only one resident not adhering to this principle to "pollute the airwaves" such that everyone else has no choice to compensate with increasing their own transmission power. I can't see any easy solution to this problem without some sort of rules with regular enforcement for the complex block, and that does not seem like something estate management companies would care to try worry about.
To get back to addressing the original question, an upside of a WiFi hotspot having low transmission power (and thus range), is that you can have more people using them in a public place before interference between each other becomes a problem.
Also, as it's mobile you can normally move the hotspot to be nearer to wherever you are (ignoring problems like 4G reception issues), which is not so easy with some sort of fixed line router.
